Clinical Pathology

I created these curated notes as part of my board review studying process. I thought they might be helpful for others preparing for similar exams or just looking to review clinical pathology concepts.

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PART I: BLOOD BANKING AND TRANSFUSION MEDICINE

Transfusion medicine sits at the intersection of immunology, genetics, and clinical practice. Red blood cells carry hundreds of surface antigens capable of triggering immune responses, and the field developed largely from learning - often the hard way - which mismatches matter and why. This section covers the immunologic basis of blood groups, compatibility testing, component therapy, and transfusion reactions.

Chapter 1: The Immunological Basis of Blood Groups

This chapter covers the immunologic foundation of transfusion medicine: what blood group antigens actually are, how they are built, which antibodies they elicit, and which of those antibodies will hurt your patient. Master this chapter and the rest of transfusion medicine falls into place. Every downstream topic (compatibility testing, HDFN, hemolytic reactions, antibody identification) is ultimately a consequence of the biology laid out here.

1.1 What Are Blood Group Antigens?

Blood group antigens are molecules expressed on the surface of red blood cells that can be recognized as “foreign” by another individual’s immune system. These antigens fall into two major biochemical categories: carbohydrate antigens (like ABO, Lewis, I/i, MNS, P/GLOB) and protein antigens (like Rh, Kell, Duffy, Kidd).

This is the highest-yield distinction in transfusion medicine. It drives everything else.

Carbohydrate antigens are sugars attached to precursor chains by enzymes called glycosyltransferases. Because carbohydrate structures are everywhere in nature (bacteria, plants, food), humans encounter them early in life and make antibodies against the ones they lack without ever having seen foreign red cells. These antibodies are naturally occurring and IgM. A type A person has anti-B from early childhood because gut bacteria display B-like sugars that drove an immune response.

Protein antigens are structural proteins encoded directly by the gene. Antibodies against them are only made after actual red cell exposure - transfusion or fetomaternal hemorrhage. These antibodies are IgG and require sensitization. Protein antigens require a T-cell dependent response, which is why they take time and prior exposure to develop.

That single distinction explains a long chain of downstream facts. IgM activates complement efficiently, so ABO mismatch causes immediate intravascular hemolysis. IgG crosses the placenta, so anti-D and anti-K cause HDFN (IgM does not cross, which is why ABO-HDFN is usually mild and limited to the IgG anti-A,B component from type O mothers). This is also why we do both forward and reverse typing for ABO: the expected IgM antibodies are so reliable that their absence is itself a discrepancy worth investigating.

One more corollary of the IgM/IgG split: hemolysis mechanism. IgM ABO antibodies drive immediate intravascular hemolysis because the IgM pentamer is a superb complement activator. Most IgG alloantibodies cause extravascular hemolysis - splenic macrophages recognize the Fc portion and remove coated RBCs gradually. The notable exception is Kidd (anti-Jka and anti-Jkb), which is IgG but fixes complement efficiently enough to produce an intravascular component. More on that in Section 1.6.

There is one more wrinkle embedded in this distinction. For carbohydrate antigens, the gene encodes an enzyme, not the antigen itself. The relationship is indirect: gene -> enzyme -> adds sugar -> antigen. The O allele is “null” not because O is a thing, but because a single nucleotide deletion at position 261 causes a frameshift and produces a truncated, nonfunctional transferase. H stays unmodified. This enzyme-based logic is why Bombay is possible: if you can’t build the H scaffold, you can’t build A or B regardless of ABO genotype.

Classic pathogen connections worth memorizing: blood group antigens are functional molecules that pathogens exploit as receptors.

Lewis antigens: receptors for Norwalk virus (Norovirus) and H. pylori
Duffy antigen (DARC): receptor for Plasmodium vivax
MNS (glycophorins): receptors for Plasmodium falciparum
P antigen: receptor for Parvovirus B19 (and the target in PCH)

Each of these explains why the null phenotype is enriched in endemic regions - natural selection.

1.2 The ABO Blood Group System: Foundation of Transfusion Medicine

The ABO system remains the most clinically important blood group system because ABO antibodies are:

Universally present when the corresponding antigen is absent (expected antibodies, reverse typing works)
Capable of causing immediate, life-threatening intravascular hemolysis
Predominantly IgM, which efficiently activates complement

The Biochemistry of ABO

The ABO antigens are built upon a precursor structure called the H antigen. Think of H as a foundation on top of which A and B are built. But H itself isn’t the true starting point - below H sits the I/i precursor. These are oligosaccharide chains that serve as the substrate for H production and also define the I/i blood group. Type 1 and Type 2 chains are structurally similar (both are I/i antigens, with i being linear/unbranched and I being branched), but they differ in location: Type 2 chains sit on the surface of red cells, and Type 1 chains circulate in plasma and secretions. So the hierarchy of layers is: precursor oligosaccharide (I/i) -> fucose added -> H -> sugar added -> A or B. Each layer depends on the one beneath it, which is why losing any upstream enzyme kills everything downstream.

Two separate fucosyltransferases build H on these two chains:

H gene (FUT1) fucosylates Type 2 chains on the RBC surface to create surface H
Secretor gene (Se, FUT2) fucosylates Type 1 chains in secretions and plasma to create secreted H

These are different genes, on chromosome 19, acting on different substrates in different compartments. You can lose one without losing the other, and that’s what makes Bombay vs. Parabombay possible.

Once H is built, glycosyltransferases encoded by the ABO locus on chromosome 9 can modify it:

The A gene encodes an N-acetylgalactosaminyltransferase that adds N-acetylgalactosamine (GalNAc) to H, creating the A antigen
The B gene encodes a galactosyltransferase that adds galactose to H, creating the B antigen
The O gene is a null allele (frameshift mutation) producing a nonfunctional enzyme, so H remains unmodified

The A and B alleles differ from each other by only 7 nucleotides (4 amino acid substitutions) that change sugar specificity.

So the full biosynthetic pathway is: I/i precursor + fucose (FUT1/FUT2) -> H + GalNAc (A transferase) -> A, OR + galactose (B transferase) -> B. “Type O” just means H stayed unmodified. Type O cells therefore have the most H antigen - none of it got consumed by downstream enzymes.

This yields a hierarchy of H antigen quantity on red cells that’s worth knowing cold:

H antigen amount: O > A2 > B > A2B > A1 > A1B

A1B has the least H because both transferases are strong and consuming H together. A2 has more than B because the A2 enzyme is a less efficient transferase (more on that below). This hierarchy directly correlates with reactivity to Ulex europaeus lectin (which has anti-H activity).

ABO Genetics and Genotype vs. Phenotype

Each person inherits two ABO alleles, one from each parent. A and B are codominant; O is recessive.

Type A can be AA or AO (indistinguishable serologically)
Type B can be BB or BO (indistinguishable serologically)
Type AB is AB
Type O is OO

Because AA and AO look identical on forward and reverse typing, you cannot determine ABO zygosity from serology alone. Molecular genotyping can, and this matters in multiply-transfused patients, chimeras, and post-transplant cases where serologic typing gets unreliable. Homozygous AA may have slightly stronger A antigen expression on RBCs than heterozygous AO, but this dosage effect is not reliably distinguishable serologically in ABO (it matters more in Rh, Duffy, and Kidd).

AB is the universal plasma and platelet donor (no anti-A or anti-B in plasma). O is the universal RBC donor. This flips because transfused RBCs bring antigens, while transfused plasma brings antibodies. AB is also the rarest ABO type (~4% of the population).

Type O Produces Three ABO Antibodies

Type A patients make anti-B (IgM). Type B patients make anti-A (IgM). Type O patients are different - they make three ABO antibodies: anti-A (IgM), anti-B (IgM), and anti-A,B (IgG). Anti-A,B recognizes an epitope shared by A and B and cannot be separated into component specificities.

The clinically critical feature is that anti-A,B is IgG and crosses the placenta. This is the dominant mechanism of ABO-HDFN: a type O mother carrying a type A or B baby. Because ABO antigens are broadly distributed across fetal tissues (not concentrated on red cells), the antibody is effectively diluted and ABO-HDFN is usually mild. But unlike anti-D HDFN, ABO-HDFN can occur in the first pregnancy because the antibodies are naturally occurring and don’t need sensitization. Anti-A,B also can’t be separated into anti-A and anti-B by adsorption - it recognizes a shared A/B epitope as one specificity.

ABO Antibodies and Age

ABO antibodies are naturally occurring, but they take time to develop. They are driven by exposure to ABO-like antigens on gut flora and are typically detectable in infants by 3-6 months. Before that window, reverse typing is unreliable - infants just lack the antibodies. For this reason, ABO typing of infants <4-6 months old uses forward typing only, and both baby and mother are typed to select safe products.

The same logic applies at the other end of life. Elderly patients and immunocompromised patients (hypogammaglobulinemia, CLL, post-plasma-exchange, chronic steroids) can have weakened isohemagglutinins, causing false-negative reverse typing and apparent ABO discrepancy. CLL is the classic cause to mention on a question stem - the expanded but monoclonal B-cell population crowds out normal antibody production.

The Bombay Phenotype: When the Foundation Is Missing

The Bombay phenotype (Oh) occurs when someone is homozygous hh (no functional FUT1) AND sese (non-secretor). No FUT1 means no H antigen on red cells. No FUT2 means no H in secretions either. Without H, neither A nor B can be built, regardless of ABO genotype.

Bombay individuals have:

No A, B, or H antigens on red cells (only bare Type 2 chains)
No H in secretions (only bare Type 1 chains)
Anti-A, anti-B, AND anti-H in plasma

The anti-H is the critical danger. Type O blood is loaded with unmodified H, which is precisely the antigen Bombay patients make antibodies against. If a Bombay patient is given type O blood, they experience severe intravascular hemolysis. Bombay patients can only receive blood from other Bombay donors. Prevalence is about 1:10,000 in India and much rarer elsewhere.

Bombay is a classic ABO discrepancy: forward typing looks like O (no A, no B), but reverse typing reveals an extra reaction - the anti-H reacts with every group O reagent cell.

The Parabombay Phenotype

Parabombay is an important near-miss. These patients are hh (no FUT1) but Se+ (SeSe or Sese, functional FUT2). They can’t make H on red cells because FUT1 is absent, but they do make H (and A or B) in secretions and plasma via FUT2. A small amount of this secreted H/A/B antigen is adsorbed onto red cell membranes from plasma, producing weak surface expression.

The common form of Parabombay (hh/SeSe) produces anti-H that is usually clinically insignificant - it reacts at room temperature but not at 37°C, and it’s IgM. Because secreted H maintains immune tolerance and some H ends up on RBCs via adsorption, these patients don’t make a dangerous anti-H.

The less common form is different. Some Parabombay variants carry a weak H allele (Hw/h or Hw/Hw) with sese. Tiny amounts of H are made on RBCs but no H is present in secretions. Without secreted H to maintain tolerance, and with very low surface H, these patients can produce clinically significant anti-H.

The distinction matters: standard Parabombay (hh/Se+) is usually safe to transfuse with ABO-compatible non-Bombay blood, while the Hw variants with non-secretor status are not. Compatibility testing is essential in all Parabombay cases.

ABO Subgroups: A1 vs. A2

Not all type A individuals are created equal. A1 and A2 differ both quantitatively and qualitatively:

A1 (~80% of type A individuals):

Expresses roughly 1 million A antigen sites per red cell
Expresses both the linear A epitope AND an additional branched A1-specific epitope
Strong reactions with anti-A reagents

A2 (~20% of type A individuals):

Expresses roughly 250,000 A antigen sites per red cell (about 25% as many)
Only the linear A epitope (no A1-specific branched form)
More residual H antigen on the cell surface (less consumed)
~1-8% of A2 individuals and ~25% of A2B individuals develop anti-A1

Anti-A1 is almost always IgM, cold-reactive, and clinically insignificant. Occasionally it reacts at 37°C and can cause hemolysis, but that’s the rare exception.

A1 vs. A2 is resolved in the lab with lectins:

Dolichos biflorus lectin has anti-A1 activity and agglutinates A1 cells
Ulex europaeus lectin has anti-H activity and agglutinates A2 cells more strongly (because A2 has more residual H)

Classic ABO discrepancy scenario: an A2 or A2B patient with anti-A1. Forward types as A or AB, but reverse typing shows an unexpected reaction with A1 reagent cells.

Secretors vs. Non-Secretors

About 80% of people are secretors (at least one functional Se/FUT2 allele). Non-secretors (sese) cannot make H antigen in secretions - they only have bare Type 1 chains in saliva, plasma, and GI mucus.

A few critical points about this:

Secretor status does NOT affect RBC antigen expression. RBC antigens depend on FUT1 (H gene). Secretions depend on FUT2 (Se gene). Different genes, different compartments.
Secretor status is tested in the lab by examining saliva for A, B, and H substances. A non-secretor’s saliva is negative for all three regardless of ABO type. This was historically used for forensic identification.
Lewis typing is a quick surrogate: Le(a+b-) = non-secretor, Le(a-b+) = secretor (see the Lewis section below).
Non-secretor status correlates with increased susceptibility to H. pylori, Candida, and UTIs.
Secretor status is also relevant for solid organ transplantation. ABH antigens are present on vascular endothelium via FUT1, regardless of secretor status, so ABO compatibility matters for any vascularized graft.

For completeness, here is how genotype maps to phenotype across the common scenarios. In every case, the key rule is: RBC phenotype is determined by ABO + H (FUT1), while secretion phenotype is determined by ABO + Se (FUT2).

ABO	H (FUT1)	Se (FUT2)	RBC membrane	Secretions	Phenotype
OO	HH	SeSe	H only	H only	Group O secretor
AO/AA	HH	SeSe	A	A	Group A secretor
BO/BB	HH	SeSe	B	B	Group B secretor
AB	HH	SeSe	A and B	A and B	Group AB secretor
OO	HH	sese	H only	no H (bare Type 1)	Group O non-secretor
AO/AA	HH	sese	A	no H	Group A non-secretor
BO/BB	HH	sese	B	no H	Group B non-secretor
AB	HH	sese	A and B	no H	Group AB non-secretor
any	hh	sese	no H (bare Type 2)	no H	Bombay
any	hh	SeSe/Sese	trace A/B/H adsorbed from plasma	A/B/H present	Parabombay

The Acquired B Phenotype

Acquired B is a fascinating and testable ABO discrepancy. Patients who are genetically type A can transiently acquire a B-like antigen during certain bacterial infections. The mechanism: bacterial deacetylases remove the acetyl group from N-acetylgalactosamine (A antigen), converting it to galactosamine, which resembles galactose (B antigen). The result is an A patient who:

Forward types as AB (reacts with both anti-A and anti-B)
Reverse types as A (still makes anti-B, because the patient isn’t really AB)

The patient-plasma anti-B doesn’t lyse their own cells because the acquired B antigen reacts weakly with anti-B and only at alkaline pH. Acidifying the reagent anti-B eliminates the reaction - that’s diagnostic. Associated conditions are predominantly gram-negative bacteremia (especially GI-source) and colorectal carcinoma (bacteria accessing RBCs through tumor invasion).

Differential for the forward-AB/reverse-A pattern: acquired B (bacteremia, colorectal cancer) or true AB with anti-A1 (think A2B). Clinical history decides.

ABO Discrepancies: When Forward and Reverse Don’t Match

In ABO typing, we run two complementary tests:

Forward typing: patient’s red cells mixed with known anti-A and anti-B reagents
Reverse typing: patient’s plasma mixed with known A1 and B reagent red cells

These must agree. When they don’t, you have an ABO discrepancy, and you must resolve it before transfusion.

Extra reactions in forward typing (cells appear to type as more than one blood type):

Polyagglutination (T-activation from bacterial neuraminidase exposure)
Autoantibodies coating red cells
Recently transfused with non-identical ABO type (mixed field)
Rouleaux (protein coating causing stacking - seen in multiple myeloma, hyperglobulinemia)
Acquired B

Weak or missing reactions in forward typing:

ABO subgroups (A2, A3, Ax, Ael, Bx, etc.)
Disease states reducing antigen expression (leukemia)
Heavily transfused patients
Bone marrow transplant recipients (mixed field during engraftment)
Bombay or Parabombay (forward looks O, reverse shows anti-H)

Extra reactions in reverse typing:

Cold autoantibodies (often anti-I)
Cold alloantibodies
Rouleaux
High-titer, low-avidity (HTLA) antibodies
Anti-A1 in A2 or A2B patients
Anti-H from Bombay

Weak or missing reactions in reverse typing:

Age extremes (infants don’t make isohemagglutinins until ~3-6 months; elderly have diminished production)
Immunocompromised patients (hypogammaglobulinemia, CLL)
Recent plasma transfusion or plasma exchange

Mixed field agglutination deserves its own note. A positive autocontrol (patient RBCs + patient plasma) with mixed field reactivity means only a subset of cells is coated. The classic cause is an alloantibody against recently transfused non-self RBCs: the donor cells get coated (positive DAT), the patient’s own cells don’t, and you see two populations. Other causes of mixed field include ABO-mismatched stem cell transplant engraftment, the A3 subgroup (inherent mixed field), and antibodies in the Lutheran and Sid (Sda) systems.

1.3 The Rh Blood Group System: Complexity Beyond D

The Rh system is the second most clinically important blood group system and the most complex genetically, with over 50 antigens described. For practical purposes and board exams, five antigens matter: D, C, c, E, and e. Rh antigens are protein antigens, which means the usual rules apply: antibodies require sensitization, are IgG, and are clinically significant.

Rh Genetics: Haplotypes, Not Alleles

Rh antigens are encoded by two closely linked genes on chromosome 1:

RHD encodes the D antigen
RHCE encodes the C/c and E/e antigens

These genes are so close together that they rarely recombine during meiosis. They are co-inherited as a 3-antigen haplotype packet - one from each parent. This is why the Weiner shorthand (R1, R2, r, etc.) works: each symbol represents one inherited haplotype.

There is no “d” antigen. Lowercase d just indicates absence of D. You cannot make anti-d because there’s nothing to make an antibody against. Contrast with c and e, which are real proteins encoded by alternative alleles of RHCE and absolutely can elicit anti-c and anti-e.

The C/c and E/e antigens are antithetical pairs:

C and c differ by four amino acids in the second extracellular loop of the RhCE protein
E and e differ by a single amino acid (proline vs. alanine) at position 226

Because people inherit one RHCE allele from each parent, most people express both members of each antithetical pair. Someone who is Ce/cE (inheriting different RHCE alleles) expresses all four: C, c, E, and e.

The RhD and RhCE proteins don’t sit alone in the membrane. They form a large complex on the red cell surface and associate with additional proteins, notably Duffy (Fy5) and Landsteiner-Wiener (LW). This matters for Rh null (below).

Weiner and Fisher-Race Nomenclature

Two nomenclature systems coexist. Fisher-Race spells out each antigen in the haplotype. Weiner uses shorthand symbols. Memorize the mapping:

Weiner	Fisher-Race	Antigens	Notes
R1	DCe	D, C, e	Most common D+ haplotype in individuals of European ancestry
R2	DcE	D, c, E
R0	Dce	D, c, e	Most common D+ haplotype in individuals of African ancestry
Rz	DCE	D, C, E	Rare; ~6% in Indigenous American populations
r	dce	c, e	Most common D- haplotype
r’	dCe	C, e	D- with C
r”	dcE	c, E	D- with E
ry	dCE	C, E	Extremely rare

A quick trick to read Weiner notation: capital R = D+, lowercase r = D-. Subscript 1 or ’ = Ce (C uppercase, e lowercase). Subscript 2 or ” = cE. Subscript 0 or no mark = ce (both lowercase). z or y = CE (both uppercase).

Haplotype frequencies (approximate, worth memorizing the big ones):

Haplotype	European ancestry	African ancestry
R1 (DCe)	40%	15%
R2 (DcE)	10%	10%
R0 (Dce)	5%	45%
Rz (DCE)	<1%	<1%
r (dce)	40%	30%
r’ (dCe)	3%	3%
r” (dcE)	2%	<1%
ry (dCE)	<1%	<1%

(Rz is about 6% in Indigenous American populations.)

Knowing the big haplotypes lets you derive the common genotypes:

Most common D+ genotype in individuals of European ancestry: R1/R1 or R1/r (DCe/DCe or DCe/dce)
Most common D+ genotype in individuals of African ancestry: R0/R0 or R0/r (Dce/Dce or Dce/dce)
Most common D- genotype in both populations: r/r (dce/dce)

Note the critical clinical consequence. Donors of European ancestry are often R1/something, so they carry C and e. Donors of African ancestry are often R0/something, so they carry c and e but not C or E. When a sickle cell patient with an R0-predominant Rh background receives blood from an R1-predominant donor pool, they may be exposed to C and E antigens they do not express - and they alloimmunize. This is why extended Rh matching (C, c, E, e, K) is recommended for chronically transfused sickle cell patients.

The D Antigen: Immunogenicity and RhIG

D is highly immunogenic. Approximately 80% of D-negative people who receive a single unit of D-positive red cells will form anti-D. This makes D the most immunogenic blood group antigen after A and B. Anti-D is almost always IgG, crosses the placenta, and is the most common cause of severe HDFN.

Because of this, everyone is classified as Rh-positive or Rh-negative based solely on D status, and D-negative pregnant women receive RhIG to suppress alloimmunization.

Prevalence of D-negativity varies by population:

~10-15% of people of European ancestry
~8% of people of African ancestry
~1% of people of East Asian ancestry
Basques: 25% (highest worldwide) - a population-genetics board fact reflecting genetic isolation

The genetic mechanism also differs by population:

People of European ancestry: complete deletion of the RHD gene (RHD*01N.01) is the most common mechanism
People of African ancestry: silenced but present RHD allele (RHD pseudogene, RHD*Ψ) is more common

This matters for molecular typing: PCR for RHD is negative in most D-negative individuals of European ancestry (no gene to amplify), but can be positive in many D-negative individuals of African ancestry (silenced gene still amplifies), which can cause false-positive D typing by molecular methods if population-informed interpretation algorithms are not used.

Rh Immunogenicity Hierarchy

After D, the order of immunogenicity is: D > c > E > C > e.

Anti-c is the second most common Rh antibody and a significant cause of HDFN
Anti-E is relatively common and usually clinically significant
Anti-C and anti-e are less common because most C- or e-negative people also lack D, and anti-D formation often overshadows these

Critical clinical pearl: patients with anti-c or anti-e should NOT be given routine D-negative blood. Why? Because most D-negative blood is r/r (dce/dce), which expresses both c and e. Ordering “D-negative” does not protect against anti-c or anti-e. You have to specifically request c-negative or e-negative units.

Weak D: When D Testing Gets Complicated

Some patients have red cells that react weakly or not at all with anti-D on immediate spin but react when the test is carried through the indirect antiglobulin test (IAT). This is weak D.

Weak D has two main causes:

RHD mutations producing a structurally intact but reduced-expression D protein - fewer D antigens per cell, all D epitopes present
C in trans to D (position effect) - C encoded on the opposite chromosome somehow dampens D expression from the D-carrying chromosome. This happens when a D-carrying haplotype (R0 or R2) is paired with a C-carrying D-negative haplotype (r’ or ry). Example: R0/r’ = Dce/dCe. D is structurally normal in these cases, just reduced in quantity.

Either way, weak D individuals express all D epitopes - they just have fewer D antigens per cell. They do not make anti-D.

Weak D testing is indicated only for:

Donors - their blood enters recipients, so a missed D-positive donor could sensitize a D-negative recipient
Newborns of D-negative mothers - fetal blood may enter maternal circulation, so a weak D newborn means mom needs RhIG

In both cases, the question is: “could this person’s blood sensitize someone else?” If yes, you must detect weak D. For a patient receiving transfusion, weak D doesn’t matter - you call them D-negative on immediate spin and give them D-negative blood. Safer.

Weak D vs. Partial D:

Weak D = quantitative (fewer D antigens, all epitopes present). Cannot make anti-D. Safe to receive D-positive blood.
Partial D = qualitative (missing epitopes). CAN make anti-D against the epitopes they lack. Should receive D-negative blood.

Partial D individuals look like weak D serologically, and you need molecular typing to distinguish them. Common partial D types:

DVI: clinically significant; most make anti-D and should be treated as D-negative
DIIIa, DAR: more common among people with African ancestry, including African American populations; less likely to make anti-D

Modern practice is moving toward RHD genotyping, especially in pregnant women, to avoid unnecessary RhIG for certain weak D types while protecting true partial D patients from alloimmunization.

Rh Null: When the Entire Complex Is Missing

Rh null patients have complete absence of all Rh antigens (D, C/c, E/e). Two mechanisms:

Amorph type: homozygous deletion or inactivation of both RHD and RHCE
Regulator type: mutation in RHAG (Rh-associated glycoprotein), which is required for Rh complex assembly on the membrane

Because the Rh complex is a structural scaffold stabilizing other membrane proteins, Rh null patients also have diminished expression of LW, Fy5, and glycophorin B antigens (S, s, U). The Rh complex isn’t just immunologic - it’s essential for membrane integrity.

Without it:

Stomatocytosis on peripheral smear
Increased osmotic fragility
Chronic hemolytic anemia

Rh null patients can form anti-Rh29 (anti-total Rh), an antibody against an epitope present on all Rh proteins. This is a disaster for transfusion. Rh null patients should only receive Rh null blood. A classic board trap: an Rh null patient is mistakenly typed as D-negative, receives r/r blood (which expresses c and e), and forms anti-Rh29 that reacts with virtually all donors. Rh null patients should be enrolled in rare donor registries and autologous programs.

There’s also a related entity, Rh mod, where Rh expression is markedly reduced but not absent. Distinct from Rh null but with a similar clinical picture.

1.4 The Kell Blood Group System: High Immunogenicity and a Unique HDFN Mechanism

The Kell system contains over 35 antigens. The ones that matter:

K (K1, Kell) and k (K2, cellano) - antithetical pair
Kpa and Kpb - Kpa is low frequency
Jsa and Jsb - Jsa is low frequency, found predominantly in individuals of African ancestry

K is the third most immunogenic red cell antigen after D and c.

K Antigen Frequency and Alloimmunization

K frequency is the one number to memorize: ~9% in individuals of European ancestry, ~2% in individuals of African ancestry. Its antithetical partner k is >99.8% - essentially universal. Because K is uncommon but highly immunogenic, K-negative recipients frequently get exposed to K-positive blood and form anti-K. Many institutions provide K-negative blood to females of childbearing age to prevent future anti-K HDFN - a cheap intervention given that only ~9% of donors need to be excluded.

Kell Antigens and Their Structural Biology

Kell antigens are held together by disulfide bonds, both between Kell proteins and between Kell and the adjacent Xk protein. This is functionally important:

Kell antigens are destroyed by reducing agents (DTT, 2-mercaptoethanol, ZZAP) that break disulfide bonds
Kell antigens are NOT destroyed by proteolytic enzymes (ficin, papain)

DTT treatment has two big practical uses:

Antibody ID: if panel reactivity disappears after DTT treatment of panel cells, suspect anti-K.
Daratumumab interference: daratumumab (anti-CD38) causes pan-reactive antibody panels. DTT destroys CD38 (and incidentally Kell) on panel cells, resolving the interference. Remaining specificity after DTT reveals a real alloantibody.

DTT also destroys IgM antibodies (by breaking the J-chain disulfide). ZZAP combines DTT with papain, destroying Kell plus enzyme-sensitive antigens. These reagents are workhorses of the antibody ID bench.

Anti-K HDFN: Suppressed Erythropoiesis, Not Just Hemolysis

Anti-K causes severe HDFN through a uniquely ugly mechanism. Most alloantibody-driven HDFN kills mature fetal RBCs (hemolysis). Anti-K targets erythroid precursors in fetal bone marrow and suppresses erythropoiesis. Kell antigens are expressed early in erythroid development, so precursors are destroyed before maturing.

The consequence: fetal anemia with disproportionately low bilirubin and low reticulocyte count. The problem is production failure, not destruction, so the usual bilirubin-based monitoring underestimates severity. Middle cerebral artery Doppler peak systolic velocity (MCA-PSV) is the better measure of fetal anemia in anti-K HDFN.

P antigen behaves similarly: it’s expressed on erythroid precursors, which is why Parvovirus B19 infection and maternal anti-P antibodies can both suppress fetal/neonatal erythropoiesis and cause aplasia rather than just hemolysis.

Kell Null (K0) vs. McLeod Syndrome

These are two different diseases and the distinction is tested.

Kell null (K0):

Homozygous amorph (silent) KEL gene - no Kell protein at all
Xk protein and Kx antigen are still present and actually overexpressed
Can make anti-Ku, which reacts with all Kell antigens
RBC morphology is essentially normal

McLeod syndrome:

Mutation in the XK gene on the X chromosome - no Xk protein, no Kx antigen
Because Xk is required for Kell complex stability, Kell antigens are markedly reduced (but not absent)
X-linked recessive (affects males; female carriers may have mild acanthocytosis)
Xk is essential for membrane integrity, so RBCs are acanthocytic with shortened survival
Multi-system disease: acanthocytosis on smear, chronic hemolytic anemia, late-onset neuromuscular disease (chorea, dystonia, myopathy, Huntington-like), retinitis pigmentosa, muscular dystrophy-like features
Often co-occurs with chronic granulomatous disease (CGD) and Duchenne muscular dystrophy due to contiguous gene deletions on Xp21 (the XK, CYBB/CGD, and DMD genes are all clustered nearby)

Quick differentiator:

Kell null: no Kell, Kx overexpressed
McLeod: no Xk, Kell markedly reduced, Kx absent, acanthocytes + neurologic disease

Classic board vignette: male patient with acanthocytes, chorea, and hemolytic anemia -> think McLeod. Male patient with acanthocytes, neurodegeneration, and liver disease -> think abetalipoproteinemia instead.

1.5 The Duffy Blood Group System: Malaria Resistance

The Duffy system has five antigens total - Fya, Fyb, Fy3, Fy5, Fy6 - but only Fya and Fyb matter for routine antibody identification. They are encoded by codominant alleles of the ACKR1 gene (formerly DARC).

The Duffy-Malaria Connection

The Duffy glycoprotein (DARC / ACKR1) is a chemokine receptor expressed on RBCs and other cells. Plasmodium vivax merozoites bind DARC to invade red cells. No DARC on RBCs = no P. vivax invasion.

In West Africa, a GATA-1 promoter mutation arose that silences Duffy expression specifically on erythroid cells, while preserving expression in other tissues. This is evolutionary elegance: the receptor is gone from the one cell type the parasite cares about, but its physiologic role elsewhere is preserved. The result:

Fy(a-b-) phenotype in ~70% of African American populations and approaching 100% in West African populations
Complete resistance to P. vivax
P. vivax malaria is therefore rare in sub-Saharan Africa

This is one of the cleanest examples of a balanced polymorphism in human genetics - the same selective pressure (malaria) also drove the sickle cell trait, G6PD deficiency, and the thalassemia alleles to high frequency in endemic regions.

By contrast, Fy(a-b-) individuals of European ancestry are very rare and usually have true coding mutations with no Duffy expression anywhere. This distinction has an important immunologic consequence:

Duffy-null phenotype common in African ancestry populations: Duffy protein still expressed in other tissues -> immune tolerance -> generally cannot form anti-Fy antibodies
True Duffy-null phenotype more typical of rare European-ancestry cases: true null -> no Duffy anywhere -> can form anti-Fy and anti-Fy3

Duffy Antigen Frequencies

European ancestry: Fy(a+b+) ~48%, Fy(a-b+) ~32%, Fy(a+b-) ~20%, Fy(a-b-) very rare

African ancestry: Fy(a-b-) ~70%, with the rest distributed among Fy(a+b-), Fy(a+b+), and Fy(a-b+)

Duffy Antibodies

Duffy follows the protein-antigen rulebook:

IgG
Acquired after sensitization
Clinically significant - cause both acute and delayed hemolytic transfusion reactions
Can cause HDFN, usually mild to moderate
Duffy antigens are destroyed by enzymes (ficin, papain) - key lab clue

Enzyme Treatment of Panel Cells: The Pattern

This is one of the most useful patterns in antibody identification. Treating panel cells with proteolytic enzymes (ficin, papain) cleaves surface proteins. Some antigens get destroyed and lose reactivity; others become more accessible and gain reactivity.

Destroyed by enzymes: MNS, Duffy, Lutheran, Chido, Rodgers, Yta

Enhanced by enzymes: Rh, Kidd, Lewis, P1, I

Unchanged by enzymes: Kell, Ss (some sources group these separately)

So if an antibody reacts in a standard panel but disappears after enzyme treatment - think Duffy or MNS or Lutheran. If reactivity gets stronger - think Rh or Kidd. If unchanged - think Kell.

1.6 The Kidd Blood Group System: Delayed Hemolysis and Evanescence

The Kidd system has two principal antigens, Jk(a) and Jk(b), encoded by the SLC14A1 gene - a urea transporter.

Why Kidd Antibodies Are Notorious

Kidd antibodies are the villains of delayed hemolytic transfusion reactions. Several features make them dangerous:

Delayed hemolytic transfusion reactions (DHTR): the classic scenario is a patient transfused without problems, then 3-14 days later develops anemia, jaundice, and a positive DAT. Workup reveals a new Kidd antibody.
Evanescence: anti-Jka and anti-Jkb often fall below detectable levels between exposures. An antibody screen can be negative today even though the patient had anti-Jka five years ago. Re-exposure triggers a rapid anamnestic response producing high titers within days.
Dosage effect: Kidd antibodies react more strongly with homozygous cells (Jk(a+b-) or Jk(a-b+)) than heterozygous cells (Jk(a+b+)). Screening cells are sometimes heterozygous, so anti-Jka in a patient can be missed entirely.
Fix complement efficiently: along with ABO antibodies, Kidd antibodies cause intravascular hemolysis when they react. This is why DHTR from anti-Kidd often has an intravascular component - hemoglobinuria, elevated LDH, dropped haptoglobin.

The dosage effect is a pitfall to internalize. If the screening cells used for the antibody screen are heterozygous Jk(a+b+), a weak anti-Jka may not react strongly enough to be detected, yet the patient absolutely still has the antibody and will react to homozygous Jk(a+b-) donor cells. This is how a “negative” antibody screen can miss anti-Kidd.

The clinical lesson: if a patient has a history of anti-Jka or anti-Jkb, always provide antigen-negative blood regardless of current antibody screen. Historical records trump current serology. The board mnemonic: “Kidd will KILL you.” Anti-Jka is more common than anti-Jkb.

Intravascular vs. extravascular hemolysis, in general: ABO alloantibodies and Kidd antibodies cause intravascular (complement-mediated). Most other IgG alloantibodies cause extravascular (splenic sequestration via Fc receptors).

Kidd Genotype Frequencies

European ancestry: Jk(a+b+) 50.3%, Jk(a+b-) 26.3%, Jk(a-b+) 23.4%, Jk(a-b-) rare

African ancestry: Jk(a+b-) 51.1%, Jk(a+b+) 40.8%, Jk(a-b+) 8.1%, Jk(a-b-) rare

Jk(a-b-) (Kidd Null) and Anti-Jk3

The Jk(a-b-) phenotype is the Kidd null, rare overall and found mainly in Polynesian and Finnish populations. These individuals can produce anti-Jk3, an antibody that reacts with any Jka- or Jkb-positive cell (i.e., >99.9% of donors).

Because the Kidd glycoprotein is a urea transporter (SLC14A1), Jk(a-b-) RBCs have impaired urea permeability - they resist lysis in 2M urea, which is a useful confirmatory lab test (normal RBCs lyse quickly in hypertonic urea because urea enters and brings water with it; null cells don’t let urea in). Clinically, Jk(a-b-) patients have mildly impaired urine concentrating ability but are otherwise healthy.

1.7 The MNS Blood Group System

MNS antigens sit on glycophorins. M and N are on glycophorin A (GYPA); S, s, and U are on glycophorin B (GYPB). Glycophorins serve as receptors for Plasmodium falciparum, which is how MNS connects to another malaria story.

Antigen Frequencies

M and N distribution: ~25% M+N-, ~50% M+N+, ~25% M-N+. Nearly 0% are M-N- because that would require null alleles at both GPA loci.

S: ~50% in individuals of European ancestry, ~30% in individuals of African ancestry s and U: high frequency, >98% of the population

S(-), s(-), U(-) blood is extremely difficult to find and patients negative for all three are found predominantly among people of African ancestry - roughly 0.6% in African ancestry populations and essentially absent in European ancestry populations. Finding compatible blood requires rare donor registries.

MNS Antibodies and Clinical Significance

Antibody	Class	Clinical Significance
Anti-M	Usually IgM, cold	No (usually); rare IgG form can cause mild HDFN
Anti-N	Usually IgM, cold	No (very rarely significant)
Anti-S	IgG	Yes
Anti-s	IgG	Yes
Anti-U	IgG	Yes, causes severe HDFN

Anti-N is unusually rare because glycophorin B carries an N-like epitope. Even people who lack N on GPA still have this N-resembling epitope on GPB, which maintains immune tolerance. So anti-N alloimmunization rarely occurs.

The exception is historical: anti-Nf - dialysis patients exposed to formaldehyde used to sterilize dialysis membranes. Formaldehyde modified the N antigen on patient RBCs, creating a neo-antigen. The resulting anti-Nf is clinically significant (unlike regular anti-N). Modern dialysis doesn’t use formaldehyde, so this is mostly historical, but it still shows up as a board trick.

MNS antigens are destroyed by enzymes (ficin, papain).

1.8 The Lewis Blood Group System

Lewis antigens are structurally unique among blood group antigens. They are carbohydrate antigens built by the Le gene (FUT3), but:

They are not synthesized by the red cell
They are produced by secretory epithelial cells, released into plasma, and passively adsorbed onto the RBC membrane
They are not present at birth; they develop over the first two years of life

Because Lewis antigens are plasma-adsorbed rather than intrinsic, they can transfer between cells in vitro (incubating Le(a-b-) RBCs in Le(b+) plasma makes them Le(b+)) and they redistribute during pregnancy. Pregnant women often appear Le(a-b-) because plasma lipoprotein changes strip Lewis antigens from their RBCs.

Lewis Biosynthesis

FUT3 can fucosylate two different substrates:

FUT3 + unmodified Type 1 chain = Lea
FUT3 + Type 1 H antigen (1H) = Leb

Because the Se gene (FUT2) makes Type 1 H from Type 1 chains, Leb production requires both Se (FUT2) and Le (FUT3) to be functional.

This leads to the Lewis-Secretor genetics:

Le(a+b-): Le+, Se- (non-secretor). FUT3 acts on bare Type 1 chains -> Lea. No Type 1 H to make Leb.
Le(a-b+): Le+, Se+ (secretor). FUT2 converts most Type 1 chains to Type 1 H before FUT3 acts, so FUT3 primarily produces Leb. A tiny amount of Lea is still made (not enough to type as +, but enough for immune tolerance - so Le(a-b+) individuals do not make anti-Lea).
Le(a-b-): Le- (no functional FUT3). Neither Lewis antigen made. Common in people of African ancestry (~30%) and ~6% of people of European ancestry.
Le(a+b+): doesn’t normally exist in adults because FUT2 consumes Type 1 chains before FUT3 can make Lea. Exception: partial secretors (more common in East Asian and Polynesian populations) can show both. Transient in infancy.

Bombay individuals and non-secretors cannot produce Leb because they can’t make Type 1 H (the substrate for FUT3 to produce Leb). They make Lea only if Le+.

Lewis Development in Infants

Le gene expression increases with age. Lewis phenotype cannot be reliably determined until age 2. Infants destined to be Le(a-b+) go through a stereotyped sequence as enzymes come online:

Le(a-b-) at birth -> Le(a+b-) -> Le(a+b+) -> Le(a-b+) by ~2 years

The logic: FUT3 (Le) expression develops first but slowly; FUT2 (Se) lags. Early on, Le is present but Se isn’t producing much Type 1 H, so FUT3 fucosylates bare Type 1 chains -> Lea. As Se catches up, both substrates become available -> transient Le(a+b+). Eventually Se becomes robust and consumes nearly all Type 1 chains before FUT3 can act on them -> Le(a-b+).

Because infants are transiently Le(a-b-), Lewis antibodies are not clinically significant in neonates, and Lewis compatibility is not required for neonatal transfusions.

Lewis Antibodies

Naturally occurring (carbohydrate antigen rule)
Almost always IgM, cold-reactive
Clinically insignificant - do not cause HDFN (IgM doesn’t cross placenta), rarely cause hemolytic reactions
Transfused RBCs acquire the Lewis phenotype of the recipient, so anti-Lea against donor cells quickly becomes irrelevant as donor cells shed and re-adsorb Lewis antigens from recipient plasma

Who can make which Lewis antibody follows the phenotype logic. Le(a+b-) individuals (Le+, Se-) can make anti-Leb. Le(a-b+) individuals do not make anti-Lea because they still produce a small amount of Lea before FUT2 consumes the substrate - enough to maintain immune tolerance. Le(a-b-) individuals (lack functional FUT3) can make both anti-Lea and anti-Leb. None of these antibodies are usually clinically significant.

Lewis antibodies are the prototypical “nuisance” antibodies in blood banking. They show up on panels, you work them up, and then they don’t matter.

1.9 The I/i Blood Group System

I and i are not antithetical. They represent developmental differences in carbohydrate chain branching:

i antigen: linear (unbranched) oligosaccharide chains. Predominant on fetal/cord blood cells, then decreases with age.
I antigen: branched oligosaccharide chains. Predominant on adult cells.

Both i and I serve as the substrate and scaffold for ABH and Lewis antigen synthesis. The i -> I transition is developmental, completed by around age 2 as branching enzymes mature.

Abnormally elevated i in an adult signals defective red cell maturation, analogous to elevated HbF in thalassemia. Causes:

Congenital dyserythropoietic anemia (CDA) type II
Diamond-Blackfan syndrome
High RBC turnover (hemolytic anemias - reticulocytes released before full branching)
iadult phenotype - rare inherited congenital absence of the branching enzyme, reported predominantly in people with East Asian ancestry

I/i Antibodies

Anti-I and anti-i are IgM autoantibodies (the body attacking its own RBC antigens). They are cold-reactive and usually benign. They become clinically significant in cold agglutinin disease when the thermal amplitude extends near 37°C, at which point IgM binds RBCs in the cool periphery, fixes complement, and the RBCs are cleared extravascularly (complement-coated cells picked up by hepatic macrophages via C3b receptors). DAT is C3d positive, IgG negative - because IgM dissociates during washing even though complement remains bound to the cell.

Classic associations (frequently tested):

Anti-I: Mycoplasma pneumoniae infection, lymphoproliferative disease (CLL, lymphoma), Raynaud
Anti-i: Epstein-Barr virus infection (infectious mononucleosis)

Mnemonic: Mycoplasma = big M = big I. EBV = little virus, little i.

Anti-I reacts best with adult (I-positive) cells. Anti-i reacts best with cord blood (i-positive) cells - a useful diagnostic distinction in cold agglutinin workups.

1.10 The P/GLOB System

The P/GLOB story is confusingly organized across two blood group systems. Just remember:

P blood group system: only P1
GLOB (globoside) blood group system: P and Pk

Three carbohydrate antigens, two systems. P1, P, and Pk are biosynthetically related but separately classified.

Phenotypes and Testing

P/GLOB phenotypes are determined using four antisera: anti-P1, anti-P, anti-Pk, and anti-PP1Pk. Anti-PP1Pk reacts with any P/GLOB antigen (P1, P, or Pk), so it’s positive in anyone except the rare p phenotype.

Phenotype	anti-P1	anti-P	anti-Pk	anti-PP1Pk
P1	+	+	+/-	+
P2	-	+	+/-	+
p	-	-	-	-

Quick pattern recognition:

anti-P1 positive -> P1 phenotype
anti-P1 negative but anti-P positive -> P2 phenotype
everything negative -> rare p phenotype

75% of the population is P1. P2 individuals lack P1 antigen, and ~30% develop anti-P1.

Anti-P1

Anti-P1 is almost always IgM, cold-reactive, and clinically insignificant. But it can have elevated titers in some clinically interesting settings:

Echinococcal (hydatid cyst) infection - hydatid cyst fluid contains P1-like antigens that stimulate antibody production. Hydatid cyst fluid can also be used in the lab to neutralize anti-P1 to confirm specificity.
Bird handlers (pigeon/turtledove) - pigeon and dove eggs contain P1-like antigens. Occupational exposure drives anti-P1 production.

Titers may be impressive in these patients, but the antibody remains clinically insignificant.

While we’re on neutralization: it’s a recurring tool for confirming antibody specificity when the panel pattern is ambiguous. You add soluble antigen to the patient’s plasma and re-test. If reactivity disappears, the antibody was specific for that antigen. Useful pairings to know: hydatid cyst fluid for anti-P1, saliva (containing A/B/H or Lewis substances) for Lewis antibodies, and pooled human plasma for Chido/Rodgers (which are complement-associated antibodies that bind soluble C4 in plasma).

Anti-PP1Pk and the p Phenotype

The rare p phenotype (formerly Tj(a-)) lacks all three P/GLOB antigens and produces anti-PP1Pk. This antibody is a broad carbohydrate-system antibody that:

Reacts with >99.9% of donor cells
Is clinically significant (hemolytic transfusion reactions)
Is associated with recurrent spontaneous abortions because P antigens are expressed on placental trophoblast tissue

Anti-PP1Pk is one of the few carbohydrate-system antibodies that is clinically significant. Differential for an antibody reacting with all panel cells: anti-PP1Pk, anti-Ku (Kell null), anti-Rh29 (Rh null), or warm autoantibody. Clinical history and DTT/enzyme panels help distinguish them.

Parvovirus B19 and Paroxysmal Cold Hemoglobinuria

Two high-yield connections to P antigen:

Parvovirus B19 uses P antigen as its receptor to infect erythroid precursors -> transient aplastic crisis (especially in sickle cell disease, hereditary spherocytosis, and in utero causing hydrops fetalis). Parvovirus B19 also causes erythema infectiosum / fifth disease / slapped cheek disease.
Paroxysmal cold hemoglobinuria (PCH): the Donath-Landsteiner antibody is an IgG anti-P that binds RBCs in the cold, fixes complement there, and then causes intravascular hemolysis when cells warm back to body temperature (biphasic hemolysis). Note the pattern - binding is cold, lysis is warm. That’s the defining feature and the reason the diagnostic test uses a two-temperature incubation. PCH is the most common cause of autoimmune hemolytic anemia in children and is often post-viral. The Donath-Landsteiner test is diagnostic: patient serum + RBCs incubated at 4°C then warmed to 37°C -> hemolysis confirms biphasic anti-P. PCH is discussed in more detail in Chapter 6.

1.11 The Lutheran Blood Group System

Lutheran antigens are glycoproteins that serve as laminin receptors, mediating RBC adhesion to extracellular matrix.

Antigen Frequencies

Lua: low frequency, present in ~7% of the population
Lub: high frequency, present in ~99% of the population

Most people are Lu(a-b+); about 7% are Lu(a+b+); Lu(a+b-) is very rare.

Lutheran in Sickle Cell Disease

Lutheran (BCAM) expression is increased on sickle cells, contributing to abnormal RBC adhesion to vascular endothelium. This is one of the mechanisms driving vaso-occlusive crises - a nice bridge between blood group immunology and hemoglobinopathy pathophysiology.

Lutheran Antibodies

Usually IgM, clinically insignificant
Characteristically show mixed field agglutination - a distinctive feature that helps identify them on panels
Anti-Lub can occasionally be IgG and clinically significant
Lutheran antigens are destroyed by enzymes - same enzyme-sensitivity category as Duffy and MNS

Other antibodies showing mixed field agglutination: Sid (Sda) antibodies, and the A3 subgroup on forward typing.

1.12 HLA Antigens and Transfusion Medicine

HLA (human leukocyte antigen) is a bridge between transfusion medicine and transplant immunology, but it’s taught in transfusion because HLA antibodies matter for platelet support, TRALI, and febrile reactions.

The MHC Locus

HLA antigens are encoded on the major histocompatibility complex (MHC) on chromosome 6p. The MHC has three classes:

MHC Class I: HLA-A, HLA-B, HLA-C. Expressed on all nucleated cells and platelets. Present peptides to CD8+ T cells.
MHC Class II: HLA-DP, HLA-DQ, HLA-DR. Expressed on professional antigen-presenting cells (B cells, macrophages/dendritic cells, thymic epithelial cells; also activated T cells and some endothelium). Present peptides to CD4+ T cells. Thymic epithelial cells are the notable non-bone-marrow-derived Class II-expressing cell - essential for positive selection of T cells.
MHC Class III: complement proteins (C2, C4, factor B) and TNF-α. Not antigen presentation at all - inflammatory mediators.

The MHC locus also contains other important genes:

HFE gene (mutated in hereditary hemochromatosis - C282Y, H63D)
CYP21A2 gene (mutated in congenital adrenal hyperplasia - 21-hydroxylase deficiency)

Because these genes are in the MHC region, they are linked to specific HLA haplotypes. HFE C282Y is linked to HLA-A3, for example.

MHC Inheritance as Haplotypes

MHC loci are so close together that recombination essentially doesn’t occur within the MHC. Each parent has two MHC haplotypes (one per chromosome 6), and children inherit one complete haplotype from each parent unchanged. This means siblings have a 25% chance of being HLA-identical (inheriting the same haplotype from each parent).

Probability of having at least one HLA-identical sibling with N siblings = 1 - (0.75)^N:

1 sibling: 25%
2 siblings: ~44%
3 siblings: ~58%

This is the math behind bone marrow transplant donor searches. With no matched siblings, you need an unrelated donor (MUD) from a registry.

HLA on RBCs vs. Platelets

Medical school teaches that RBCs don’t express HLA. That’s not quite right. Young RBCs do express MHC Class I antigens (HLA-A, -B, -C), but they lose them within the first few weeks of the ~120-day RBC lifespan. So mature RBCs are mostly HLA-deficient.

A few HLA subtypes persist on mature RBCs. These are the Bennett-Goodspeed (Bg) antigens:

Bga = HLA-B7
Bgb = HLA-B17
Bgc = HLA-A28

Bg antibodies arise in multiparous women and multiply-transfused patients who have formed anti-HLA antibodies. They can cause weak, inconsistent reactivity on panels that doesn’t fit any standard blood group pattern - usually clinically insignificant, but they need to be identified so transfusion isn’t unnecessarily delayed. Some cross-reactivities exist: anti-HLA-B27 often cross-reacts with Bg(a+) cells.

Platelets are different. Platelets express abundant HLA Class I because their ~8-10 day lifespan is too short to lose it (and megakaryocytes are nucleated). This is why HLA antibodies cause platelet refractoriness - treatment is HLA-matched (at HLA-A and HLA-B) platelets. Leukoreduction of platelets reduces but doesn’t eliminate this.

HLA-Mediated Transfusion Reactions

Mnemonic: “Transfusion Problems Follow Treatments” (TRALI, Platelet refractoriness, FNHTR, TA-GVHD). Each has a different mechanism.

Reaction	Mechanism	Prevention
TRALI	Donor anti-HLA or anti-neutrophil antibodies activate recipient neutrophils in pulmonary vasculature	Male-predominant plasma donors
Platelet refractoriness	Recipient anti-HLA destroys donor platelets	HLA-matched platelets
FNHTR	Recipient anti-HLA/anti-leukocyte + donor leukocytes -> cytokine release	Leukoreduction
TA-GVHD	Donor T cells attack immunocompromised recipient	Irradiation

For transplant: HLA-DR matching is most important for renal transplant outcomes; HLA-A and -B matter most for stem cell transplant.

Chapter 2: Antibody Identification and Compatibility Testing

This chapter is about how the blood bank decides whether a specific unit of red cells is safe for a specific patient. That decision chain runs from the moment blood is drawn at the bedside through the antiglobulin test, the antibody screen, the identification panel, and the crossmatch. Everything builds on the antiglobulin (Coombs) reagent, so start there. Later chapters on transfusion reactions, HDFN, and AIHA all assume you know how the DAT and IAT work - we put the mechanics here so we don’t repeat them.

2.1 The Antiglobulin Test: The Foundation of Modern Blood Banking

The antiglobulin test (Coombs test) is perhaps the single most important tool in transfusion medicine. Understanding it deeply is essential.

The Principle

Human antibodies (IgG) and complement fragments (C3d) bound to red blood cells are invisible to the naked eye - they don’t directly cause agglutination. The antiglobulin test makes them visible:

Anti-human globulin (AHG) is an antibody (made in animals) directed against human IgG and/or human complement components. When AHG is added to red cells coated with IgG or C3d, it cross-links the antibodies/complement, creating visible agglutination.

Why IgG alone doesn’t agglutinate: IgG is a small monomer with two antigen-binding sites fairly close together. Red cells carry a net negative surface charge (the zeta potential) that pushes neighboring cells apart. A single IgG molecule can bind antigen on one cell but can’t stretch to a second cell across that gap. IgM, a pentamer with ten binding sites and a much longer span, has no such problem - it agglutinates directly. The AHG reagent solves the IgG span problem by physically bridging IgG molecules on adjacent cells.

Direct Antiglobulin Test (DAT)

The DAT detects antibodies or complement already bound to the patient’s red cells in vivo. The procedure:

Wash the patient’s red cells (to remove unbound proteins)
Add polyspecific AHG (contains both anti-IgG and anti-C3d)
If positive, test with monospecific AHG to determine what’s on the cells

Polyspecific AHG contains both anti-IgG and anti-C3d. It tells you something is coating the cells but not what. If the polyspecific reagent is positive, you follow up with monospecific anti-IgG and monospecific anti-C3 to separate the coating into its components.

Clinical interpretation of DAT results:

DAT Result	IgG	C3d	Possible Causes
Positive	+	-	Warm autoimmune hemolytic anemia, drug-induced (methyldopa type), HDFN, alloantibody-coated cells
Positive	-	+	Cold agglutinin disease (IgM elutes during washing, leaves complement), paroxysmal cold hemoglobinuria, some drug-induced
Positive	+	+	Warm AIHA with complement activation, drug-induced (immune complex), some alloantibodies
Negative	-	-	Normal; IgM-only sensitization; low-level sensitization below test threshold

Why cold antibodies typically show C3 only: IgM binds RBCs in the cold periphery and fixes C1, C4, C2, C3 to the cell surface. As blood returns to the warm core (and during the warm wash step before AHG is added), the IgM dissociates from the cell - but complement (C3) is covalently bound and stays. So the DAT detects C3 alone. The same pattern shows up in paroxysmal cold hemoglobinuria, which uses a biphasic IgG against P - the IgG binds cold, fixes complement, then falls off when warmed. Different antibody class, same DAT pattern: C3+, IgG-.

Important nuances:

A positive DAT doesn’t always mean hemolysis is occurring. Up to 15% of hospitalized patients have a positive DAT without clinical hemolysis (versus ~1% of outpatients). Causes include recent transfusion, IVIG, anti-thymocyte globulin, and nonspecific protein coating from medications. Correlate with actual evidence of hemolysis (anemia, reticulocytosis, elevated LDH, low haptoglobin, spherocytes) before you chase the DAT.
A negative DAT doesn’t rule out immune hemolysis - some antibodies are low-affinity and wash off
Drug-induced antibodies may require special techniques (including the drug) to detect
In warm AIHA, 30-50% of warm autoantibodies show C3 on the DAT, and the presence of C3 correlates with more severe hemolysis (complement adds an intravascular component on top of extravascular clearance).

Three clues that point toward an autoantibody when you’re working up a patient: a positive DAT, a positive autocontrol on the panel, and an anemia with reticulocytosis or microspherocytes (splenic macrophages nibble IgG-coated membrane, leaving spheres). Any one of these alone can have other explanations, but together they strongly suggest autoantibody coating.

Check Cells (Coombs Control Cells)

Every negative AHG reaction needs a control. Check cells are IgG-coated RBCs added to any tube that read negative at AHG. They must agglutinate. If they don’t, the anti-IgG reagent was either omitted, neutralized, or washed away, and your negative result is invalid. The tube gets repeated. You don’t add check cells to positive tubes - the reaction already proved the reagent was active. This is a mandatory quality-control step, not optional.

Indirect Antiglobulin Test (IAT)

The IAT detects antibodies in the patient’s serum/plasma that bind to reagent red cells in vitro. The procedure:

Incubate patient serum with reagent red cells at 37°C
Wash cells (to remove unbound proteins)
Add AHG
Read for agglutination

The AHG phase of an antibody screen or panel IS the IAT. Different name, same test. DAT = antibodies already bound in the patient (in vivo). IAT = antibodies in the serum that we make bind to test cells (in vitro). Same Coombs reagent, different starting material.

The IAT is used in:

Antibody screening
Antibody identification
Crossmatching
Phenotyping with certain reagents

A practical pitfall: weak, complement-dependent antibodies can be missed in EDTA (purple-top) plasma because EDTA chelates calcium and blocks complement activation. Weak anti-Jka is the classic example - if it relies on complement to produce visible agglutination, the EDTA specimen can read negative. Serum (red-top or gold-top) preserves active complement, which is why some reference labs prefer serum for antibody workup.

2.2 Pre-Transfusion Specimen and Workflow Basics

Before the first drop of plasma touches a reagent cell, there’s a chain of custody and paperwork that has to be airtight. Most fatal transfusion events don’t come from exotic antibodies - they come from somebody hanging the wrong unit on the wrong patient.

Specimen Collection

Blood samples must be labeled at the bedside, and the label must carry two unique patient identifiers (name + MRN, or name + DOB), the date and time, and the identity of the phlebotomist. Any discrepancy or doubt means the sample is rejected and redrawn. You can’t relabel or “fix” a mislabeled tube. Specimen labeling errors are the #1 cause of ABO-mismatched transfusions, the most lethal transfusion error. Zero tolerance.

Specimen Validity

For most patients, type and screen samples are redrawn every 3 days and retained for 7 days after transfusion. The 3-day window catches new alloantibodies that could form after recent sensitization. The 7-day post-transfusion hold exists to support workup of delayed hemolytic reactions, which typically present 3 to 14 days post-transfusion.

Exception: if the patient has no history of alloantibodies AND no transfusion or pregnancy in the past 3 months, the sample is valid for up to 1 month. The logic is that new alloantibody formation requires a sensitizing event, and without one the antibody status won’t change. This is useful for elective surgical planning.

Donor Unit Arrival

When a donor unit arrives at the transfusion service from the collection center, the hospital repeats:

ABO: forward AND reverse typing
RhD: forward only (no need to repeat weak D - that was done at the donor center)

Reverse typing isn’t really necessary on the donor confirmation for Rh because everyone is expected to lack anti-D. Any discrepancy between the unit label and the hospital’s retype quarantines the unit and the donor center gets notified.

Recipient testing is slightly different: ABO forward + reverse, RhD forward only, no weak D. Why the asymmetry? A recipient who is weak D can safely receive D-positive blood because they have all the D epitopes and won’t make anti-D. But a donor who is weak D must be labeled D-positive because their blood will sensitize a D-negative recipient. It comes down to what enters someone else’s circulation.

Visual Inspection

Before any unit is transfused, visually inspect it. Discoloration (purple/brown), a zone of hemolysis (pink supernatant), or visible clots all suggest bacterial contamination or mechanical failure - don’t transfuse, culture the unit, notify the blood bank. Visibly lipemic units (milky from high triglycerides) should also not be transfused; they interfere with testing and can clog filters.

Unit labels must carry the ABO/Rh, two unique identifiers of the intended recipient (not donor), the donation identification number, and an interpretation of compatibility testing if applicable. The unique donation ID is the barcode that ties the unit back through every step of collection, testing, and processing - critical for traceback if a reaction occurs.

Authorization, Identification, and Transfusion Workflow

Four things must happen before blood enters a patient:

A physician order - transfusion is a medical decision
Written informed consent (risks, benefits, alternatives, right to refuse)
Patient identified at bedside using two unique identifiers
Unit verified to match the recipient

The bedside identification step is where most fatal ABO-mismatched transfusions originate. The nurse compares the unit tag to the patient’s wristband. Skip or rush this step and people die.

Oversight: transfusion protocols are usually written into nursing policy, but the blood bank medical director is responsible for formulating them, reviewing them, and ensuring hospital compliance. They also review transfusion reactions and approve emergency-release procedures. This is a Medical Directorship topic that overlaps with Transfusion Medicine on boards.

Filters and Tubing

Standard blood infusion sets use a 170 µm filter to remove clots and debris. This filter does NOT leukoreduce - if the product needs leukoreduction and wasn’t pre-storage leukoreduced, a separate finer filter is required. One standard filter set can be used for up to 4 hours and for multiple sequential units to the same patient, after which bacterial growth in the tubing mandates a new set.

Neonatal transfusions usually skip the drip-chamber filter because the volumes are too small (10-20 mL/kg). Instead, the blood bank pre-filters the product and dispenses it in a syringe for precise volume control.

Timing

Transfusion must start within 30 minutes of issuance and complete within 4 hours. Once blood leaves the controlled temperature of the blood bank, bacterial growth risk begins. If two units are released simultaneously to the same patient, only the first must start within 30 minutes, but both must finish within 4 hours of issue.

Monitoring

Vitals are taken at three timepoints: before the transfusion, at 15 minutes, and at the end. The first 15 minutes run at a deliberately slow rate (~2 mL/min) because most severe acute reactions (ABO mismatch, anaphylaxis, TRALI) manifest early, and limiting the incompatible volume limits the damage.

Compatible Fluids

Only 0.9% normal saline (and certain FDA-approved crystalloids) can co-infuse with blood products on the same line.

Lactated Ringer’s is contraindicated - the calcium overwhelms the citrate anticoagulant and causes clotting in the tubing.
Dextrose (D5W) and hypotonic solutions (0.45% NaCl) are contraindicated - they’re hypotonic relative to RBC cytoplasm and cause osmotic hemolysis. You’ve destroyed the product before it hits the patient.

ABO/Rh Typing: Forward and Reverse

Forward typing tests the patient’s RBC antigens against reagent antisera:

Anti-A reagent is colored blue
Anti-B reagent is colored yellow
Anti-D is typically clear or lightly tinted

Color-coding prevents the worst possible mistake: accidentally swapping reagents between tubes. Agglutination in blue = A antigen present; yellow = B; both = AB; neither = O.

Reverse typing tests the patient’s serum antibodies against reagent A1 and B cells. A1 specifically (not just “A”) because some A2 and A2B individuals have anti-A1, and you need the A1 cells to detect it. Agglutination with A1 cells → patient has anti-A → patient is not type A.

If forward and reverse disagree, use type O blood until the discrepancy is resolved. O cells lack A and B antigens, so they won’t react with the patient’s antibodies regardless of which type they truly are.

Common Causes of ABO Discrepancies

Detailed resolution belongs in Chapter 1, but the high-yield causes pattern out like this:

Extremes of age: neonates don’t yet produce isohemagglutinins (they develop at 3-6 months), elderly/immunosuppressed patients (CLL, myeloma, rituximab) have weak antibody production. Both disrupt reverse typing.
Cold reactive antibodies: interfere with reverse typing by producing unexpected agglutination at room temperature.
Hematolymphoid neoplasms: can weaken A or B antigen expression on RBCs, producing mixed-field agglutination on forward typing.
Subgroups (A2B with anti-A1, which is present in 22-35% of A2B individuals, usually a cold-reactive IgM, typically clinically insignificant).
Acquired B phenotype: in a type A patient with gram-negative bacteremia or GI disease (colon cancer, obstruction), bacterial deacetylases convert N-acetylgalactosamine (A) to galactosamine (which resembles galactose, B). Forward types as AB, reverse correctly types as A. The acquired B antigen is pH-dependent - acidifying the serum removes its reactivity. Acetic anhydride can re-acetylate it back to the A antigen. A different monoclonal anti-B reagent may not react with it at all. The DAT may be positive because the patient still has anti-B in circulation that coats the acquired B on their own cells.
B(A) phenotype: a rare autosomal dominant mutation of galactose transferase that adds mostly galactose but some GalNAc to H. Genetically group B, but expresses a small amount of A antigen. Forward may appear AB; reverse shows anti-A (they’re truly B immunologically).
Bombay phenotype: lacks the H antigen. Forward and reverse both type as O, but antibody screen is pan-positive because all reagent cells are group O and express abundant H. Anti-H from Bombay patients is clinically dangerous - high-titer and broadly thermally reactive.
Gastric adenocarcinoma shedding soluble A or B antigen: the free antigen neutralizes reagent antisera in forward typing, so a type A patient appears as type O on forward but still shows anti-B (and no anti-A) on reverse.
Antibody to a high-frequency antigen: reverse is fine, but any antibody panel run on the sample will be pan-positive.

For most of these discrepancies, the diagnostic approach involves the confirmation tests above, lectin studies (Dolichos biflorus for A1, Ulex europaeus for H), and occasionally genotyping.

2.3 Antibody Screening: The First Line of Defense

Before transfusion, every patient receives an antibody screen to detect clinically significant unexpected alloantibodies. “Unexpected” means non-ABO - anti-A and anti-B are the expected antibodies and are handled by ABO typing itself.

The Method

The antibody screen tests patient plasma against 2 to 4 reagent screening cells with known antigen profiles. These cells are group O (to avoid ABO reactions) and are selected to express all common clinically significant antigens between them. Because they’re O, any reactivity you see must be from a non-ABO antibody.

The standard panel detects 18 antibodies across 7 blood group systems. Mnemonic: “Really Kind Ducks Keep Making Pretty Lakes”:

Rh: D, C, E, c, e
Kidd: Jka, Jkb
Duffy: Fya, Fyb
Kell: K, k
MNS: M, N, S, s
P: P1
Lewis: Lea, Leb

Some institutions add Lutheran (Lua, Lub) and additional Kell antigens (Kpa, Kpb, Jsa, Jsb).

The screen uses 2-4 cells because that’s enough to detect whether any of the 18 antibodies is present. Once you know something is there, you move to an antibody panel (10-14 cells) which has enough antigen variation to identify the specific antibody by rule-in/rule-out pattern analysis.

The Three Phases

A tube-based antibody screen or panel runs through three phases:

Immediate spin (room temperature) - detects IgM
37°C incubation - allows IgG binding
AHG (antihuman globulin) phase, also called IAT - detects IgG

IgM antibodies (cold-reactive, mostly clinically insignificant - anti-Lewis, anti-M, anti-P1, anti-I) agglutinate at immediate spin. IgG antibodies (clinically significant - anti-D, anti-K, anti-Fya, anti-Jka, anti-S, etc.) require warm incubation for binding and AHG to make that binding visible. The AHG phase is the most important - it catches all the antibodies that actually cause hemolytic transfusion reactions.

Two types of positive reactions: agglutination (graded microscopic through 4+) and hemolysis (pink supernatant from complement-mediated lysis of test cells). Both count as positive. Hemolysis on the panel means complement activation and is a strong indicator of clinical significance.

Along with the screening/panel cells, an autocontrol (patient serum + patient RBCs) is run in parallel. Autocontrol negative + panel positive = alloantibody. Autocontrol positive = autoantibody (or recent transfusion with circulating incompatible donor cells).

Enhancement Media

To increase sensitivity, most modern screens use enhancement reagents:

LISS (Low Ionic Strength Saline): Reduces the ionic cloud (zeta potential) around red cells, bringing antibody and antigen closer together. Increases antibody uptake, reduces incubation time. LISS enhances all antibodies - both clinically significant and insignificant.

PEG (Polyethylene Glycol): Concentrates antibodies by excluding water, forcing antibody-antigen interactions. Highly sensitive but can cause false positives; must wash before adding AHG.

Gel or solid-phase technology: Modern platforms that capture antibody-antigen reactions in a gel column or on a solid surface. More standardized and amenable to automation. Antibodies against test reagents most frequently occur with gel or solid-phase methods - patients occasionally develop antibodies to the matrix proteins themselves, creating a pan-reactive pattern. Switch to a different enhancement medium or wash the test cells to resolve.

Historical Antibody Records and Evanescence

Always compare current results with previous results. Some alloantibodies fade below detection threshold over time, a phenomenon called evanescence. Kidd, Duffy, and Kell antibodies are the classic evanescers (“Kidd will kill you” - anti-Jka is the most notorious for dropping below detection and then causing a delayed hemolytic reaction on re-exposure).

Treat patients as if they have the antibody for life. If the chart says “anti-K” from 4 years ago and today’s screen is negative, you still give K-negative blood. The memory B cells are still there; re-exposure triggers an anamnestic response in days, and a delayed hemolytic transfusion reaction follows. Blood banks maintain antibody histories indefinitely for this reason.

Corollary: if the screen is negative but the chart documents an alloantibody, the cause of the discrepancy is evanescence and historical records override current results.

Fundamental immunology principle, phrased for the blood bank: alloantibodies form only against antigens the patient does not have (foreign). Autoantibodies form only against antigens the patient does have (self). This distinction is why once you have an antibody, phenotyping or genotyping the patient confirms whether it’s an allo- or auto-antibody.

2.4 Antibody Identification: The Art of Pattern Recognition

When the antibody screen is positive, you must identify the antibody. This is both a science and an art.

The Panel

An antibody identification panel consists of 10-14 group O reagent cells with known antigen profiles. You test the patient’s plasma against each cell and record reactivity in all three phases.

The Rule-Out Process

For each antigen on the panel, you can potentially “rule out” the corresponding antibody if:

At least one cell that is positive for the antigen is non-reactive with the patient’s plasma

For example, if cell #3 is K+ (Kell positive) and the patient’s plasma does not react with cell #3, you can rule out anti-K.

The standard of practice is the “rule of three” - for statistical confidence, you need:

At least 3 antigen-positive cells reacting with the plasma
At least 3 antigen-negative cells NOT reacting with the plasma

This gives statistical significance (p < 0.05) that the association is real.

Dosage Effect

Some antigens show dosage: antibody reacts more strongly when the antigen is expressed in double dose (homozygous) than single dose (heterozygous). If your panel cell is heterozygous for the antigen, a real antibody might react only weakly or not at all, and you’d falsely rule it out.

Mnemonic: “My Dosage Kills Circulating Erythrocytes” - MNS, Duffy, Kidd, C/c, E/e all display dosage. Note that D and K do NOT show dosage - anti-D and anti-K react equally whether the antigen is homozygous or heterozygous.

The practical rule: for dosage-effect antigens, rule-out requires a homozygous-negative cell. If the only non-reactive cell for a particular antigen is heterozygous, you cannot rule out that antibody safely.

Reading the Pattern

After running the panel, you look at which cells react and which don’t. Correlate with the antigen profile:

Which antigens are present on all reacting cells?
Which antigens are absent on non-reacting cells?

The antibody specificity should explain the reactivity pattern.

Complications in Antibody Identification

Real-world cases are often more complex:

Multiple antibodies: The patient has more than one antibody. The pattern won’t fit a single specificity. Solutions:

Selected cell panels with cells positive for one suspected antigen and negative for another
Antibody removal techniques (adsorption) to separate and identify each antibody

Cold antibodies interfering: Cold-reactive IgM antibodies (anti-I, anti-M, anti-Lewis) can cause positive reactions at room temperature, confusing the pattern. Solutions:

Test strictly at 37°C
Pre-warm technique (warms the specimen to 37°C before adding reagent cells, so cold IgM can’t bind)
Enzyme treatment (destroys some cold-reactive antigens)

Pre-warming is also the standard technique for the immediate-spin crossmatch in a patient with a known clinically insignificant cold antibody (anti-M, anti-Lewis) - if any reactivity persists after pre-warming, it’s a warm-reactive antibody and IS clinically significant.

Autoantibody: If the DAT is positive and the patient appears to have a pan-reactive pattern, they may have an autoantibody. Autoantibodies coat the patient’s own cells and react with all panel cells.

Classic Panel Patterns and What They Mean

These are the pattern-recognition scenarios you’ll see on boards:

All panel cells positive at AHG + autocontrol positive = warm autoantibody. Next step: adsorption, or run the test on less-sensitive saline media to see through the autoantibody.
All panel cells positive at AHG + autocontrol negative = antibody to a high-frequency antigen. The patient lacks the antigen that nearly everyone has. DTT treatment that doesn’t resolve the pan-reactivity rules out daratumumab interference (which DTT does resolve).
All panel cells positive at immediate spin only + autocontrol positive = cold autoantibody (IgM). Classic anti-I (Mycoplasma) or anti-i (EBV). Usually clinically insignificant.
All panel cells positive at IS and 37°C but NOT at AHG + autocontrol positive = cold autoantibody with broad thermal amplitude, OR antibody to test reagents. The first is clinically significant (IgM that binds up to body temperature); the second resolves by switching enhancement media.
Only one panel cell positive + standard antigens don’t explain it = antibody to a low-incidence antigen (Wra, Kpa, Cw, Jsa, Lua). Check the reagent cell package insert for the extended antigen list - the standard panel sheet only shows the 18 mandatory antigens.
Pattern matches anti-D but something’s off (patient is D+ or pattern is weaker) = consider anti-LW (expressed more highly on D+ cells, mimics anti-D), anti-D autoantibody, or alloanti-D in a partial D patient.

Partial D deserves its own note. Partial D = a qualitative defect where some D epitopes are missing. These patients type as D-positive because standard anti-D reagents detect the epitopes they do have, but when they’re exposed to wild-type D (transfusion, pregnancy), they recognize the missing epitopes as foreign and make anti-D. This is different from weak D, which is a quantitative reduction (all epitopes present, just fewer of them) and doesn’t make anti-D.

The LW (Landsteiner-Wiener) antigen is expressed on nearly all RBCs but at higher density on D-positive cells. Anti-LW reacts strongest with D+ cells → mimics anti-D on the panel. To distinguish auto-anti-D from anti-LW: perform alloadsorption using D-negative cells. Since LW is on all cells (just less on D-), D-negative cells will adsorb the anti-LW out. Then test the eluate from those D-negative cells (agglutinates → anti-LW was there) and the adsorbed serum against D+ cells (still reactive → anti-D was also there).

Polyagglutination and Cryptic Antigens

Polyagglutination is when a cryptic (hidden) RBC antigen is exposed and reacts with antibodies that most normal adult serum carries. All random donor serum will agglutinate the patient’s cells.

Adult plasma contains naturally occurring IgM antibodies against four cryptic carbohydrate antigens: T, Tn, Tk, Cad. These antibodies form because the carbohydrate antigens aren’t normally presented to the immune system during T/B cell negative selection, so tolerance never forms.

T-activation is the most common: bacterial neuraminidase cleaves sialic acid from RBC glycophorins, exposing the underlying T antigen. Because ~everyone over 6 months has anti-T IgM, the exposed T reacts with all adult serum.

In adults, polyagglutination is usually asymptomatic.
In infants, it’s clinically dangerous in two high-yield settings: (1) necrotizing enterocolitis with Clostridia neuraminidase, and (2) atypical hemolytic uremic syndrome with Streptococcus pneumoniae neuraminidase [TODO: verify - modern classification often separates pneumococcal HUS from atypical HUS, which is reserved for complement-mediated disease]. In both, bacterial neuraminidase exposes T antigen on the infant’s cells. Transfused plasma (which contains anti-T) can then hemolyze those cells. Pneumococcal HUS also hits platelets and glomerular endothelium, giving the microangiopathic hemolytic anemia + thrombocytopenia + acute kidney injury triad. Unlike typical HUS (Shiga-toxin), pneumococcal HUS is DAT-positive (C3+) because of the T-activation mechanism.
Treatment: give washed RBCs (plasma removed) or low-plasma products; avoid giving plasma-containing products to these infants.

T-activation is transient and resolves once the infection is treated. Tn-activation, in contrast, results from a somatic mutation in hematopoietic precursors (loss of the enzyme that adds galactose to GalNAc) and can be persistent.

Diagnostic test for T-activation: Arachis hypogaea (peanut) lectin. Peanut lectin mimics anti-T, so it agglutinates T-activated cells. If peanut lectin agglutinates the patient’s RBCs, T-activation is confirmed.

HTLA Antibodies (High Titer, Low Avidity)

HTLA antibodies are a subset of antibodies to high-frequency antigens. They bind weakly (low avidity → 1+ reactions) but at high titers (don’t dilute out). The result is low-level pan-reactivity (1+) across all panel cells, with a negative autocontrol (because the patient lacks the high-frequency antigen they’re targeting).

Most HTLA antibodies are clinically insignificant, but the dangerous thing is what they hide: a clinically significant alloantibody (e.g., anti-Jka at 2+) can be completely masked by the 1+ HTLA background.

Clinically significant HTLAs: Cartwright (Yta), Holley (Hy), Gregory (Gy). Mnemonic: “Yellow Hats Glide.”
Clinically insignificant HTLAs: Chido and Rodgers. These are actually adsorbed complement C4 fragments on RBCs, not intrinsic antigens. They can be neutralized by plasma (which is loaded with C4) or destroyed by ficin/papain.

Resolving HTLA interference: dilution studies (the significant alloantibody usually persists at higher titer), enzyme treatment (destroys some HTLAs while preserving the alloantibody target), neutralization (plasma for Chido/Rodgers).

Special Techniques

Enzyme treatment (ficin, papain, trypsin, bromelain): proteolytic enzymes cleave protein antigens off the RBC surface. Antigens on the cleaved portion are destroyed; antigens close to the membrane become more accessible.

Enhanced by enzymes: Rh, Kidd, Lewis, ABO, P, I/i
Destroyed by enzymes: MNS (glycophorin A/B), Duffy (DARC protein), Lutheran, Cartwright (Yta), Chido, Rodgers
Unaffected: Kell (disulfide-stabilized, resistant to protease)

Useful pattern: antibody goes up after enzyme treatment → think Rh or Kidd. Antibody disappears after enzyme treatment → think Duffy or MNS. This differential effect is a workhorse tool for unmasking Rh/Kidd antibodies behind Duffy antibodies.

DTT (Dithiothreitol): a reducing agent that breaks disulfide bonds. Uses:

Denatures IgM antibodies (eliminates IgM cold antibodies to see underlying IgG)
Destroys Kell antigens (Kell relies on disulfide bonds - resistant to enzymes but destroyed by DTT)
Destroys CD38 - resolves daratumumab (anti-CD38) interference. Daratumumab is a monoclonal antibody used in myeloma that binds CD38 on all RBCs and causes pan-agglutination on the panel. DTT treatment of the panel cells destroys CD38, eliminating the interference. After DTT, you can’t test for anti-K anymore (K is also destroyed), so give K-negative blood (91% of donors are K-negative) and rely on Rh matching.

ZZAP (DTT + papain): combined reagent for adsorption pretreatment. Strips already-bound antibodies off patient RBCs and exposes more antigen sites, making the cells a better “sponge” for soaking up autoantibody from serum.

Adsorption: Removing antibodies from serum by incubating with antigen-positive cells.

Autoadsorption: use the patient’s own RBCs (ZZAP-treated) to soak up autoantibody, leaving alloantibodies behind in the cleaned serum. Simple, but can’t be used if the patient was transfused or pregnant in the past 3 months - circulating donor or fetal cells could soak up a real alloantibody and make you miss it.
Alloadsorption: use carefully selected donor cells of known phenotype, negative for the suspected alloantibody target. Safe even with recent transfusion/pregnancy, but more labor-intensive (often requires multiple adsorptions with differently-phenotyped cells). The steps are otherwise identical to autoadsorption.

Elution: Recovering antibodies from the red cell surface for identification. Methods include acid elution (most common), heat elution, organic solvent, and freeze-thaw. The resulting eluate is tested against a standard panel like you would test serum. Elution should be performed on any positive IgG DAT to characterize the target: eluate shows anti-D → HDFN or auto-anti-D; eluate shows anti-A → ABO-HDFN; eluate is negative despite positive DAT → think non-immune protein adsorption (drug effect).

Hemagglutination inhibition and induction: identify antibodies by using a soluble substance that mimics either the target antigen (inhibition - saturates antibody → no agglutination) or the antibody itself (induction - a lectin that agglutinates cells expressing the antigen). Key neutralizing substances and lectins:

Substance	Mimics	Clinical Use
Guinea pig urine (or human urine)	Sda	Confirm anti-Sda
Hydatid cyst fluid, pigeon eggs	P1	Confirm anti-P1
Saliva (from secretor)	H and Lewis	Confirm anti-H / anti-Lewis; determine secretor status
Breast milk	I	Confirm anti-I
Plasma	Chido, Rodgers	Confirm (C4-derived)
Dolichos biflorus lectin	Anti-A1	Distinguish A1 from A2 subgroups
Bandeiraea simplicifolia lectin	Anti-B	Confirm B expression
Ulex europaeus lectin	Anti-H	Determine secretor status; distinguish A1 vs A2 (A2 has more H)
Arachis hypogaea lectin (peanut)	Anti-T	Diagnose T-activation
Vicia graminea lectin	Anti-N	Confirm N expression

H antigen density hierarchy (highest to lowest): O > A2 > B > A2B > A1 > A1B. A2 cells have more unmodified H than A1 cells because A2-transferase is less efficient at converting H to A.

Phenotyping vs Genotyping

Phenotyping = serologic testing: mix patient RBCs with known antisera, look for agglutination. Fast and cheap, but inaccurate if the patient was recently transfused (you’re testing a mix of donor and native cells).

Genotyping = molecular testing: analyze DNA from the patient’s WBCs to predict antigen expression. Unaffected by circulating donor RBCs.

Genotype (rather than phenotype) is preferred in: autoantibody workup, multiple alloantibodies, confusing serologic results, transfusion-dependent patients (sickle cell, thalassemia), hemolytic transfusion reaction workup, pregnant women with alloantibodies (for fetal antigen prediction).

Recipient phenotyping/genotyping confirms whether a detected antibody is allo- (patient negative for antigen) or auto- (patient positive for antigen). This matters because autoantibodies are handled differently (adsorption, steroids, don’t try to transfuse antigen-negative blood).

Fetal Genotyping

For a pregnant woman with an alloantibody, you need to know whether the fetus carries the target antigen. Three sources:

Cell-free fetal DNA (cffDNA) in maternal plasma - non-invasive, first trimester, preferred method. The go-to for RHD genotyping of fetuses from D-negative mothers.
Chorionic villus sampling - invasive, ~10-12 weeks
Amniocentesis - invasive, ~15-20 weeks

Alloantibody-Directed Transfusion and Antigen-Negative Unit Math

A patient with an alloantibody gets antigen-negative blood. Identify the antibody → find its target antigen → transfuse blood negative for that antigen. For multiple antibodies, you need blood negative for all targets simultaneously, which gets hard quickly.

The probability of finding antigen-negative blood uses basic independent-probability math: P(antigen-negative) = 1 - antigen frequency. For multiple independent antigens, multiply.

Worked example: D frequency 85%, E frequency 30%. P(D-negative) = 0.15. P(E-negative) = 0.70. P(both negative) = 0.15 × 0.70 = 10.5%. You’d test about 10 units to find 1 compatible, 20 units to find 2.

Worked example with two antibodies: anti-c + anti-S. c frequency 80%, S frequency 55%. P(c-neg) = 0.20. P(S-neg) = 0.45. P(both neg) = 0.09 = 9%. To find 2 compatible units: 2 / 0.09 ≈ 22 units need to be tested.

These calculations are frequently tested on boards. Formula: units to test = desired units ÷ P(compatible).

Rare Phenotypes

A handful of null phenotypes come up on boards:

Duffy null [Fy(a-b-)] is most common in African-origin individuals (~70%). Results from a GATA-1 promoter mutation that silences Duffy on erythroid cells but not other tissues. Provides resistance to P. vivax malaria. Because Duffy is still expressed elsewhere, these individuals are tolerized and generally don’t make anti-Fy antibodies.
MkMk phenotype: homozygous deletion of both glycophorin A and glycophorin B. Completely lacks M, N, S, s, and U. Can form antibodies against all of them; finding compatible blood is extremely difficult.
Lutheran null Lu(a-b-): three mechanisms. True null (LuLu, autosomal recessive) with no Lutheran antigens at all - these patients can make anti-Lu3. KLF1 suppressor (In(Lu) phenotype, autosomal dominant) - normal Lutheran genes but suppressed expression; trace antigen detectable by adsorption/elution; these patients generally don’t make Lutheran antibodies because they still express some antigen. GATA-1 mutation (X-linked).
Bombay (hh): no H, so no A or B can be built on top of H either. Makes potent anti-H.

Newborn-Specific Antigen Expression

Most blood group antigens develop gradually after birth (ABO, Lewis, Ii, P1). Kell antigens are strongly and fully expressed at birth, which is why anti-K HDFN can be severe even in early fetal life - the target is already there. Kell also suppresses erythropoiesis (not just hemolysis), compounding the damage.

Population-Associated Rh Haplotypes

The “big four” Rh haplotypes differ by population:

European ancestry: R1 > r > R2 > R0 (R1 = DCe dominates)
African ancestry: R0 > r > R1 > R2 (R0 = Dce dominates)

Clinical relevance: a sickle cell patient (often R0/R0) receiving blood from an R1-predominant donor pool is exposed to C and E antigens they lack → makes anti-C and anti-E. This is why chronically transfused patients get extended Rh and K antigen matching from the start, before they form antibodies.

2.5 Autoantibodies and Autoimmune Hemolytic Anemia (Blood Bank Perspective)

This is the blood bank’s angle on AIHA - the patterns you see, the DAT findings, and the decisions around transfusion. Chapter 6 handles the clinical syndrome in depth.

Warm-reacting antibodies react optimally at 37°C (IgG, clinically significant). Cold-reacting antibodies react optimally at 4°C (IgM, usually clinically insignificant unless high titer or broad thermal amplitude).

Cold IgM antibodies are significant only if titer > 1000 at 4°C or if the thermal amplitude reaches toward 37°C (meaning the antibody binds at body temperature in the patient, not just in the cold test tube).

Cold Autoantibodies (IgM)

Key cold autoantibodies and their targets:

Anti-I → adult RBCs (I antigen = branched chains). Classic associations: Mycoplasma pneumoniae, lymphoproliferative disorders (CLL, lymphoma), Raynaud/acrocyanosis. “Big M → big I.”
Anti-i → cord/neonatal RBCs (i antigen = unbranched chains). Association: EBV (infectious mononucleosis). Adults with high i include iadult phenotype (reported more often in East Asian populations), congenital dyserythropoietic anemia type II, Diamond-Blackfan, and stress erythropoiesis states.
Anti-Pr → Pr-positive RBCs. Usually monoclonal IgM. Can cause complement-mediated intravascular hemolysis in extremities.
Anti-H → group O RBCs (most unmodified H). Found in Bombay/para-Bombay. In Bombay patients, anti-H is high-titer with broad thermal amplitude and clinically devastating.
Anti-A1 → A1 RBCs. Found in 22-35% of A2B and some A2 individuals. Usually clinically insignificant. Compatible blood for an A2B with anti-A1 can often be found via prewarmed crossmatch.

IgM activates complement in vitro. This is why cold agglutinins often produce C3-only DAT patterns (IgM binds cold, fixes complement, dissociates during warm washing; complement stays).

Mixed-Type AIHA

Both warm (IgG) and cold (IgM) autoantibodies coexist. DAT shows IgG+ and C3+. Frequently in lupus (SLE). These patients have particularly severe hemolytic anemia and poorer response to standard therapy.

Paroxysmal Cold Hemoglobinuria (PCH)

PCH is the least common overall form of AIHA but the most common AIHA in children. Caused by the Donath-Landsteiner antibody: a biphasic polyclonal IgG against the P antigen.

Most commonly affects children after viral infection (URI, measles, mumps, chickenpox, EBV) or bacterial infection (otitis media).
Presentation: sudden-onset hemoglobinuria after cold exposure.
Usually self-limited.

Mechanism (the biphasic part): the IgG anti-P binds P antigen at 4°C in the cold periphery and fixes early complement (C1, C4, C2, C3). Blood returns to the warm core at 37°C, where the IgG dissociates from the cell but the complement cascade continues to completion (C5-C9 MAC) → intravascular hemolysis → hemoglobinuria.

PCH DAT: C3 positive, IgG negative. Same pattern as cold agglutinin disease, but a different antibody class. The IgG has dissociated by the time the sample is tested at room temp, leaving only the covalently bound C3 on the cells.

PCH antibody screen is usually negative. The standard phases (immediate spin at room temp, 37°C, AHG) don’t hit the 4°C-then-37°C sequence the DL antibody requires. The only way to detect it is the Donath-Landsteiner test.

Donath-Landsteiner test: recreates the biphasic hemolysis. The indirect version uses patient serum + donor RBCs in three tube sets - one kept at 0°C, one at 37°C, one biphasic (0°C then 37°C). Hemolysis only in the biphasic tube = positive = PCH confirmed. Direct version uses two vials of the patient’s own blood - vial 1 biphasic (4°C → 37°C), vial 2 at 37°C only. Hemolysis in vial 1 only = positive.

Treatment: keep the patient warm (prevents cold antibody binding) and transfuse warmed RBCs as needed. Steroids are less effective than in warm AIHA. P-negative blood isn’t really an option (P is near-universal), but warming solves the problem in practice.

Cold Autoantibody Workup Checklist

When you’re staring at a cold autoantibody:

Determine clinical significance: titer > 1000 at 4°C? Broad thermal amplitude (reactivity persisting toward 37°C)?
Screen for masked alloantibodies at 37°C only (skips the cold antibody’s optimal binding temperature). Use monospecific anti-IgG AHG (not polyspecific - avoids detecting complement deposited by the cold antibody). Consider cold autoadsorption at 4°C to remove the cold autoantibody from serum before panel testing.

Warm Autoantibody Workup Checklist

When the panel is pan-positive at AHG with a positive autocontrol:

Confirm warm autoantibody (adsorption can further verify).
Unmask underlying alloantibodies: adsorption (auto- or allo-) is the gold standard. Alternatively, run the test on less-sensitive saline media - the warm autoantibody binds weakly and falls below detection, while true alloantibodies remain detectable (faster, less reliable).
If transfusion is urgent and there’s no time for adsorption: release Rh- and Kell-matched blood (“least incompatible”). Rh and Kell are the most immunogenic non-ABO systems, so a hidden alloantibody is most likely to target one of them. Some institutions go further with full phenotype matching (Duffy, Kidd, MNS) when time allows. This is a non-standard release: the physician and medical director must acknowledge the risk.

Warm autoantibody patients are at greater risk of transfusion-related complications, both because no truly compatible unit exists (the autoantibody reacts with all donor cells) and because alloantibodies hide behind the autoantibody and can trigger an acute hemolytic reaction.

2.6 Drug-Induced Positive DAT: Four Mechanisms

Four mechanisms. Learn them as a table - the DAT findings + hemolysis pattern + eluate result distinguish them.

Mechanism	Pathophysiology	DAT	Eluate	Hemolysis	Classic Drug
Drug adsorption (hapten)	Drug non-covalently binds RBC → IgG antibody against drug → IgG binds drug on RBC	IgG+, C3-	Reacts with drug-treated cells only	Extravascular	High-dose IV penicillin
Drug-dependent antibody (immune complex)	Antibody targets drug-RBC membrane complex as a neoantigen	IgG±, C3+	Reacts with drug-treated cells	Intravascular (complement)	Quinidine, ceftriaxone
Drug-independent antibody (autoimmune)	Drug induces true autoantibody; indistinguishable from warm AIHA	IgG+, C3-	Reacts with untreated panel cells	Extravascular	Methyldopa, fludarabine
Non-immune protein adsorption	Drug causes nonspecific proteins (IgG, albumin, etc.) to adsorb to RBCs	IgG+, C3-	Negative	None	Cephalosporins (cefotetan, ceftriaxone)

Key distinguishing points:

Non-immune protein adsorption is the only mechanism with a positive DAT but a negative eluate (because the IgG stuck to the cells isn’t specific for anything). It’s also the only mechanism without hemolysis - a lab nuisance, not a disease.
Drug-dependent antibody is the most severe because it activates complement and causes intravascular hemolysis.
Drug-independent (methyldopa-type) is clinically indistinguishable from warm AIHA - you diagnose it by recognizing the drug association. Hemolysis can persist for weeks after stopping the drug.
For drug adsorption and drug-dependent, confirmation requires demonstrating that serum/eluate reacts with drug-treated cells but not untreated cells - proves the antibody’s target involves the drug.

2.7 Compatibility Testing: Putting It All Together

The Type and Screen

For most patients, a “type and screen” is sufficient before surgery:

ABO/Rh typing
Antibody screen

If the screen is negative and the patient has no history of antibodies, blood can be rapidly provided with an abbreviated crossmatch.

The Crossmatch

The crossmatch tests compatibility between the specific donor unit and the patient. Three types, ordered by complexity:

Electronic (computer) crossmatch: LIS verifies ABO/Rh compatibility digitally - no physical mixing of blood. Requirements:

Two separate ABO/Rh determinations on the recipient, at least one on a current sample
Donor ABO/Rh and donation ID on file
Validated computer system with proper logic to flag incompatibilities
Current negative antibody screen
No history of clinically significant antibodies

Computer crossmatch requires a fully negative antibody screen - any antibody (even clinically insignificant) disqualifies it. Fastest method when eligible.

Immediate-spin crossmatch: Mix recipient serum + donor RBCs, centrifuge, read. Detects ABO incompatibility only (IgM anti-A/B agglutinate at room temperature). Does NOT detect IgG alloantibodies. Appropriate when:

Current antibody screen is negative, OR
Patient has only clinically insignificant antibodies (anti-M, anti-N, anti-Lewis, anti-P1)
No history of clinically significant antibodies

If a patient has a clinically insignificant cold antibody, the immediate-spin crossmatch can be performed after prewarming the specimen to 37°C - this eliminates the cold IgM’s reactivity so you can actually interpret the result.

Full (antiglobulin, AHG) crossmatch: Includes 37°C incubation and AHG phase. Required when:

Current antibody screen is positive with a clinically significant antibody, OR
Patient has a history of clinically significant antibodies

This is essentially an IAT between the patient’s serum and the specific donor unit - the same principle as the antibody screen, but done against the actual unit about to be transfused. Takes ~45-60 minutes.

The electronic crossmatch is now standard at many institutions when eligible - it’s fast and eliminates the serological crossmatch.

Which Products Need a Crossmatch?

Only RBCs and granulocytes require crossmatching. RBCs because they carry the antigens. Granulocyte concentrates require ABO compatibility AND crossmatch because they contain a significant number of contaminating RBCs from the apheresis collection. Platelets, plasma, and cryoprecipitate don’t need crossmatching because they contain minimal RBCs (unless visibly bloody, in which case give ABO-compatible).

Infants Under 4 Months

Infants up to 4 months don’t require crossmatching for blood transfusions, provided there are no clinically significant maternal antibodies present. The infant’s antibody repertoire is maternal IgG that crossed the placenta, so the relevant check is maternal antibody status. If mom’s screen is negative, give ABO/Rh-compatible blood without a crossmatch.

RhIG for Rh-Negative Recipients of Platelets or Cryo

Platelet and cryo concentrates can contain small amounts of contaminating RBCs expressing D antigen. For a D-negative female of childbearing potential receiving a D-positive platelet or cryoprecipitate, offer Rh immune globulin (RhIG) to prevent sensitization. One vial of RhIG covers about 30 mL of D-positive whole blood. Without RhIG, the small D exposure could sensitize her → anti-D → future D-positive pregnancies at risk for HDFN.

Urgent Transfusion: When There’s No Time

In emergencies, uncrossmatched blood may be necessary:

O-negative red cells: The “universal donor” for red cells - no A, B, or D antigens to cause immediate hemolysis. Use for:

Unknown blood type
D-negative women of childbearing potential

O-positive red cells: Acceptable for males and post-menopausal females when O-negative is scarce

Type-specific, uncrossmatched: Once ABO/Rh is known (takes ~5 minutes), switch to type-specific blood

AB plasma: The “universal donor” for plasma - no anti-A or anti-B to cause hemolysis

Document the clinical urgency when releasing uncrossmatched blood.

Blood Product Compatibility Quick Reference

Red Blood Cell Compatibility (based on recipient’s ABO type) - ABO compatible:

Patient Blood Type	Can Receive RBCs From
O	O only
A	A, O
B	B, O
AB	A, B, AB, O (universal recipient)

Rh Consideration: Rh-negative patients should receive Rh-negative RBCs. Rh-positive patients can receive either. In emergencies, Rh-positive blood can be given to Rh-negative males and post-menopausal females when Rh-negative is unavailable.

Plasma Compatibility (opposite of RBCs - based on what antibodies the plasma contains) - ABO plasma compatible:

Patient Blood Type	Can Receive Plasma From
O	O, A, B, AB
A	A, AB
B	B, AB
AB	AB only

Whole blood: ABO identical only, or low-titer group O whole blood as a universal option in trauma/massive transfusion. Why ABO identical? Whole blood has both cells (antigens) and plasma (antibodies), so it must be ABO-compatible in both directions simultaneously - and identical is the only way to achieve that.

Platelets and cryoprecipitate: non-ABO-compatible is acceptable in most cases (minimal RBC content). Give ABO-compatible if visibly bloody. ABO-identical platelets give better post-transfusion increments because ABO antigens are expressed on platelet surfaces.

Key Principle: For RBCs, you avoid giving antigens the patient has antibodies against. For plasma, you avoid giving antibodies that will attack the patient’s RBCs. This is why O is the universal RBC donor but AB is the universal plasma donor.

Chapter 3: Blood Components and Their Clinical Use

This chapter covers what you actually hand to the nurse: the bags. How each component is made, what’s in it, how long it lasts, what it does when it hits the patient, and the specific clinical scenarios where you pick one over another. The board exam loves storage temperatures, shelf lives, and the specific indications for modifications (irradiated, leukoreduced, washed, frozen). Most of this is memorization anchored to a few core ideas: colder is better for everything except platelets and granulocytes, every modification to a unit trades shelf life for safety, and every product has a narrow set of indications you need to know cold.

Transfusion reactions that result from these products are covered in Chapter 4, and HLA/platelet-specific antibody work-up lives in Chapter 10.

3.1 Component Preparation and Anticoagulant-Preservative Solutions

Understanding how blood components are prepared and stored is essential for understanding their clinical properties.

Whole Blood Processing

Whole blood collection yields approximately 450-500 mL of blood mixed with 63-70 mL of anticoagulant-preservative solution. From this single donation, multiple components can be prepared:

Centrifugation steps:

Soft spin (2000g × 3 minutes) [TODO: verify exact g-force and duration; values vary by reference]: Separates packed RBCs from platelet-rich plasma (PRP)
Hard spin (5000g × 5 minutes) [TODO: verify exact g-force and duration; values vary by reference]: Separates platelets from plasma
Cryoprecipitate preparation: Slow thawing of FFP at 1-6°C causes cold-insoluble proteins to precipitate; a second hard spin separates cryoprecipitate from cryo-poor plasma

The density hierarchy (from least to most dense): Plasma < Platelets < Lymphocytes < Monocytes < Neutrophils < Red cells [TODO: verify exact ordering of lymphocytes vs monocytes; some references treat them as same-layer mononuclear]. This is why centrifugation effectively separates components.

Timing of processing matters. Whole blood must be processed within 8 hours of collection if the plasma is destined for FFP or if platelets are being made. The single exception: plasma processed between 8 and 24 hours becomes PF24 (plasma frozen within 24 hours). Cryoprecipitate can only be made from FFP (the 8-hour product), never from PF24, because the labile clotting factors degrade too much past 8 hours to produce useful cryo.

Anticoagulant-Preservative Solutions

Traditional solutions (used alone):

Solution	Key Features	RBC Shelf Life
CPD	Citrate + Phosphate + Dextrose	21 days
CP2D	Double dextrose compared to CPD	21 days
CPDA-1	CPD + Adenine	35 days

The role of each component:

Citrate: Anticoagulant - chelates calcium to prevent clotting
Phosphate: Buffer - maintains pH to support glycolysis
Dextrose: Fuel - metabolized via glycolysis to generate ATP. Every anticoagulant-preservative solution contains dextrose because mature RBCs lack mitochondria and depend entirely on glycolysis for ATP.
Adenine: ATP precursor - directly supports ATP synthesis via the salvage pathway (adenine → AMP → ADP → ATP), extending RBC viability by ~14 days over CPD alone

Both CPD and CPDA-1 can be used for RBCs AND whole blood. This matters because additive solutions (below) can only go into RBC units.

Additive solutions (AS-1, AS-3, AS-5, AS-7):

Added to RBCs after plasma removal (so cannot be used with whole blood, which retains its plasma)
Must be added within 72 hours of collection
Extend shelf life to 42 days
Allow more plasma to be harvested (better platelet and FFP yield)
Result in lower hematocrit (~55-65%) making transfusion easier (less viscous = faster flow)

Additive Solution	Brand Name	Key Components
AS-1	Adsol	Dextrose, adenine, mannitol, NaCl
AS-3	Nutricel	Dextrose, adenine, phosphate, citrate, NaCl (no mannitol)
AS-5	Optisol	Similar to AS-1, less mannitol
AS-7	SOLX	Newer formulation with improved RBC recovery [TODO: verify - SOLX/AS-7 status in US; was withdrawn/limited use]

Mannitol in AS-1 and AS-5 serves as a membrane stabilizer, reducing hemolysis during storage. Clinically, mannitol is an osmotic diuretic, so for neonates and very small children some institutions prefer AS-3 (no mannitol) or wash the unit to avoid a diuretic load. AS is the dominant preservative system in the US.

Why the shelf-life ladder works: CPD (21d) → CPDA-1 (35d, add adenine for ATP salvage) → CPD + AS (42d, add more dextrose, adenine, saline, ± mannitol). Each step provides more metabolic support to keep RBCs viable longer.

RBC Unit Composition

A standard RBC unit (post-processing, with additive solution) has a total volume up to 350 mL, containing:

RBCs: ~200 mL
Residual plasma: <50 mL (the plasma was mostly removed to make FFP)
Anticoagulant-preservative solution: ~50-100 mL

The hematocrit of the final unit ranges from 55-80%. Units with additive solution sit at the lower end (~55-65%, the extra fluid dilutes it). Units without AS run higher. Higher hematocrit means more viscosity and slower flow through tubing.

Iron content matters for chronic transfusion: each unit delivers ~200 mg of iron, or about 1 mg per mL of RBCs. The body has no active iron excretion mechanism, so chronically transfused patients (sickle cell, thalassemia, MDS) accumulate iron. After ~20 units, you start thinking about chelation (deferoxamine, deferasirox). Ferritin >1000 ng/mL is the usual trigger.

Opened (Spiked) Units

Once a unit is “spiked” - the tubing is pierced or a port is opened - the closed system is broken and bacterial contamination becomes possible. Shelf life drops immediately:

Refrigerated (1-6°C): 24 hours after opening
Room temperature: 4 hours after opening

This is why you never return a spiked unit to inventory.

Returning Unused Units to Inventory

RBC units sent to the floor but not transfused can sometimes be re-issued. Three criteria must ALL be met:

Unit has NOT been spiked (closed system intact)
Unit has not been outside 1-10°C for more than 30 minutes
At least one segment of sealed tubing remains attached (for future crossmatch)

The expiration of a returned unit depends on temperature history: if the cold chain was continuously maintained at 1-10°C during transport, the unit keeps its original expiration. If the cold chain was briefly broken (still under 30 minutes), the unit can be re-issued but expires within 4 hours of return. Outside 1-10°C for more than 30 minutes? The unit can’t go back to inventory at all.

Context: at most facilities, individual units are sent to the floor at room temperature, which is why they must be transfused within 4 hours. In massive transfusion protocols, whole coolers validated at 1-10°C go to the floor, and unused units can come back with the original expiration intact.

Quality Control Standards for Blood Components

Component	Quality Control Requirement
RBCs (additive solution)	Hct ≤80%; ≥75% recovery at 24 hours post-transfusion
Leukoreduced RBCs (WB-derived)	<5 × 10^6 WBCs; retain ≥85% original RBCs
Leukoreduced RBCs (apheresis)	<5 × 10^6 WBCs; retain ≥51 g hemoglobin
Leukoreduced WB-derived platelets	<8.3 × 10^5 WBCs
Apheresis platelets	≥3 × 10^11 platelets; pH ≥6.2 at outdate; 150 mg fibrinogen; ~100 mL
Whole blood platelets	≥5.5 × 10^10 platelets per unit; pH ≥6.2; 80 mg fibrinogen; ~50 mL
FFP	All clotting factors ≥1 IU/mL; ~2 mg/mL fibrinogen
Cryoprecipitate	≥80 IU Factor VIII; ≥150 mg fibrinogen per unit
Irradiated products	25 Gy to mid-plane; ≥15 Gy at all points

Why the leukoreduced RBC targets differ by source: whole blood-derived units are measured as percent retention (≥85%) because the starting RBC mass is standardized by the whole blood collection volume. Apheresis collections vary in volume, so they’re measured as hemoglobin mass (≥51 g, roughly equivalent to one standard unit’s content).

3.2 Red Blood Cell Products

Storage Temperatures at a Glance

Before getting into modifications, memorize the basic temperatures (these come back repeatedly on the exam):

Product	Storage	Transport
RBCs (standard and irradiated)	1-6°C	1-10°C
Whole blood	1-6°C	1-10°C
Frozen RBCs	<-65°C	<-65°C
FFP / Cryoprecipitate	<-18°C (1 year) or <-65°C (7 years)	frozen
Platelets	20-24°C with agitation	20-24°C
Granulocytes	20-24°C (no agitation)	20-24°C

The pattern: red cells like cold, platelets and granulocytes need room temperature, plasma lives in the freezer.

The Storage Lesion

During refrigerated storage, red cells undergo progressive changes - the “storage lesion”:

Decreased 2,3-DPG: Shifts the oxygen dissociation curve LEFT, reducing oxygen offloading to tissues. Transfused RBCs initially hold onto O₂ instead of releasing it. Levels regenerate within 12-24 hours post-transfusion.
Decreased ATP: Compromises membrane integrity; Na-K pumps fail, shape changes, cells become rigid
Increased potassium: RBCs leak K+ as the Na-K-ATPase fails. Fresh units have ~4 mEq/L supernatant K+; 42-day units may have 40-50 mEq/L
Decreased pH: Lactate accumulates from anaerobic glycolysis
Microvesicle formation: Membrane blebs form
Increased free hemoglobin: Some intrabag hemolysis releases hemoglobin

Clinical relevance:

Neonates and massive transfusion patients are most affected by high K+ and low pH. Fresh units or washed units are preferred in these scenarios.
Most changes are rapidly reversed after transfusion in stable adults
There’s ongoing debate about whether “fresh” blood improves outcomes; current evidence doesn’t support routine use of only fresh units in adults

A useful mental model: the storage lesion is the cost of shelf life. A 42-day unit has more accumulated damage than a 7-day unit, but it’s still clinically effective for most patients.

Modified RBC Products

Leukoreduced RBCs: Contains <5 × 10^6 white blood cells (>99.9% reduction). For WB-derived platelets, the threshold is <8.3 × 10^5 WBCs.

How it’s made: Filtration through specialized filters during collection (prestorage, preferred) or at the bedside. Prestorage is better because it removes WBCs before they release cytokines during storage.

Benefits (mnemonic: FACT):

Febrile non-hemolytic transfusion reactions - reduced (caused by WBC cytokines and recipient anti-HLA antibodies)
Alloimmunization to HLA - reduced (important for patients who may need platelet transfusions or transplant, since HLA antibodies drive platelet refractoriness)
CMV transmission - reduced (CMV lives in WBCs; leukoreduction is considered “CMV-safe”)
TRIM - reduced (transfusion-related immunomodulation: donor WBCs suppress recipient immunity, linked to increased infection and cancer recurrence risk)

Important: leukoreduction does NOT prevent TA-GVHD. Residual lymphocytes can still engraft. If TA-GVHD prevention is the goal, you need irradiation.

Currently, over 90% of RBCs in the US are universally leukoreduced.

Irradiated RBCs: Exposed to 25 Gy gamma radiation (or X-ray) to the mid-plane, with ≥15 Gy to any point of the product.

Purpose: Prevents transfusion-associated graft-versus-host disease (TA-GVHD), a near-uniformly fatal (>90% mortality) condition where donor T-lymphocytes engraft in an immunocompromised recipient and attack skin, liver, gut, and marrow.

How it works: Radiation destroys donor T-lymphocytes’ ability to proliferate. It does NOT sterilize the product, does NOT remove WBCs, and does NOT significantly affect plasma or platelet function.

Indications (mnemonic: IBHRIG - “Irradiated Blood Helps Recipients Inhibit GVHD”):

Immunocompromise (selected cases): T-cell defects (SCID, post-alemtuzumab, purine analog chemotherapy, mTOR inhibitors, CAR-T), Hodgkin and non-Hodgkin lymphoma (NOT leukemia), aplastic anemia. B-cell defects alone do NOT require irradiation.
Bone marrow / stem cell transplant recipients
HLA-matched or HLA-selected products (recipient may not recognize matched donor cells as foreign - “one-way HLA match”)
Relative / directed donations (partial HLA sharing between blood relatives is the exact setup for TA-GVHD)
Intrauterine transfusions and neonatal transfusions (especially premature or those receiving exchange transfusion)
Granulocyte transfusions (full of viable lymphocytes)

Side effect - accelerated K+ leak: Irradiation damages the RBC membrane, so potassium leaks out faster than normal.

Shelf life becomes 28 days from irradiation OR original expiration, whichever comes first
Storage temperature is unchanged: 1-6°C storage, 1-10°C transport
Example: a unit collected with AS (42-day expiration) irradiated on day 30 → expires on the original day 42 (original comes first). Same unit irradiated on day 5 → expires day 33 (28 days from irradiation comes first).
In neonates and patients with renal failure, use fresh irradiated units or wash before transfusion to minimize hyperkalemia

Washed RBCs: RBCs washed with normal saline to remove plasma proteins (the plasma is replaced with 0.9% NaCl).

Indications:

IgA-deficient patients with anti-IgA (prevents anaphylaxis from donor IgA)
Severe allergic reactions not controlled by premedication
T-activated cells (washing removes recipient plasma containing antibodies to T antigen)
Hyperkalemia prevention - especially important in neonates and renal failure, because stored RBC supernatant accumulates K+. Washing pulls that K+ out. Irradiated units have even more K+ leak, so washing irradiated units before neonatal transfusion is common practice.

Drawbacks: The washing process is open (breaks sterility), so shelf life drops to 24 hours. Some RBCs are also lost in the process. Plan ahead.

Frozen / Deglycerolized RBCs: Cryopreserved using 40% glycerol as a cryoprotectant, stored at <-65°C for up to 10 years.

Why glycerol, and why 40%: glycerol enters the RBC and replaces intracellular water, preventing ice crystal formation during freezing (which would lyse the cells). 40% glycerol (high-glycerol method) is the US standard because it allows freezing in a mechanical freezer at -65°C. The alternative 20% glycerol method requires liquid nitrogen (far colder) and is rarely used in the US.

Most common uses:

Storing rare blood types (Bombay, Jk(a-b-), Rh-null phenotypes - so rare that keeping liquid units is impractical)
Autologous donation (elective surgery with a long lead time)

Timing rules for freezing:

Without additive solution (CPD only): freeze within 6 days of collection, before major metabolic deterioration
With additive solution: freeze before the original expiration date

Post-thaw processing:

RBCs are washed in progressively less concentrated saline to remove glycerol without osmotic shock
Because the wash is an open process, shelf life is 24 hours after thawing
The product is considered both leukoreduced and washed (WBCs and plasma are removed during processing)
Storage after thaw: 1-6°C, same rules as fresh RBCs

Plan ahead - thawing and deglycerolization takes 1-2 hours.

Whole Blood

Whole blood = RBCs + plasma + platelets (mostly) together in one bag. Stored at 1-6°C (transport 1-10°C), same as PRBCs. Shelf life depends on the preservative: CPD (21d) or CPDA-1 (35d). Additive solutions cannot be used with whole blood because the plasma hasn’t been removed.

Whole blood requires ABO-identical matching (not just compatible) in most scenarios because it contains both RBC antigens AND plasma antibodies. Exception: low-titer group O whole blood is used in massive transfusion protocols when the patient’s type is unknown, because O RBCs are universally compatible and low-titer anti-A/B in the O plasma is less likely to cause hemolysis.

“Cold-stored whole blood” is making a comeback in trauma and combat medicine - it provides RBCs, plasma, and some platelets in a single product, faster and simpler than component therapy when you’re drowning in a hemorrhaging patient.

RBC Transfusion Dosing and Response

Adult dose:

1 unit of RBCs raises hemoglobin by ~1 g/dL and hematocrit by ~3% in a 70 kg adult
If the rise is less than expected, think ongoing bleeding, hemolysis, sequestration (splenomegaly), or volume overload diluting the effect

Pediatric dose:

10 mL/kg of PRBCs achieves a similar ~1 g/dL Hgb rise
4 mL/kg is approximately equivalent to 1 adult unit on a body-size basis
Neonatal transfusions often use 10-20 mL/kg, and small aliquots (“pedi-packs”) can be divided from a single adult unit to minimize donor exposure in premature infants who will need multiple transfusions

RBC Transfusion Contraindications

Absolute: None. If a patient is dying from anemia, you transfuse.
Relative:
- Warm autoimmune hemolytic anemia (see Chapter 6) - autoantibodies destroy transfused cells just as they destroy the patient’s own, so survival is shortened, but the transfusion still helps temporarily
- Hyperhemolysis syndrome in sickle cell disease - paradoxical destruction of BOTH donor AND recipient RBCs after transfusion. Post-transfusion Hgb drops BELOW pre-transfusion level. Devastating complication; usually treated with IVIG, steroids, and avoiding further transfusion if possible.

3.3 Platelet Products

Platelet Biology Relevant to Storage

Platelets are far more delicate than red cells:

Must be stored at 20-24°C (room temperature) - cold causes irreversible disc-to-sphere transformation and aggregation
Require continuous gentle agitation to allow gas exchange through the bag wall (O₂ in, CO₂ out) and maintain pH ≥6.2. Below pH 6.2, platelets are irreversibly activated and nonfunctional.
Can tolerate up to 24 hours without agitation (e.g., during shipping) without major functional loss
Have short shelf life: 5 days (extended to 7 days with pathogen reduction)
Are prone to bacterial contamination because room temperature supports bacterial growth - this is the main reason shelf life is capped at 5 days

Platelet lifespan in vivo: 8-10 days (for reference, RBC lifespan is 120 days). This is why chronically thrombocytopenic patients need platelet transfusions weekly while RBC transfusions are less frequent, and why platelet shelf life (5 days) is already half the natural lifespan.

Types of Platelet Products

Whole blood-derived platelets (random donor platelets): Made by centrifuging whole blood donations. The classic platelet-rich plasma method uses two-step centrifugation:

Slow (soft) spin first: separates heavy RBCs from lighter platelet-rich plasma
Fast (hard) spin next: pellets platelets out of the plasma; the supernatant becomes FFP

Alternative: buffy coat method (more common in Europe): hard spin to separate the buffy coat (WBCs + platelets), then light spin to separate platelets from WBCs.

Per-unit specs: ≥5.5 × 10^10 platelets, ~50 mL volume, ~80 mg fibrinogen, pH ≥6.2. Usually pooled (4-6 units) to provide a therapeutic dose (~3 × 10^11 platelets).

Apheresis platelets (single-donor platelets): Collected by apheresis from a single donor.

≥3.0 × 10^11 platelets (≈ 4-6 pooled WB-derived units)
~100 mL volume, 150 mg fibrinogen, pH ≥6.2
Single donor exposure = lower infection risk, lower HLA alloimmunization risk
No need to break the closed system for pooling
Can be HLA-matched if needed
Most US institutions now use apheresis platelets predominantly

Modifications to Platelets

Pathogen reduction (amotosalen + UV): Most US institutions that pathogen-reduce platelets use amotosalen, a synthetic psoralen, followed by UV irradiation to activate it.

Mechanism: amotosalen intercalates into the DNA/RNA double helix. UV light activates it, forming permanent covalent crosslinks between nucleic acid strands. Replication is blocked. This kills bacteria, viruses, parasites, AND donor T-cells. Platelets and RBCs (no nucleus) are unaffected.

Implications:

Decreases infectious disease transmission
Decreases TA-GVHD risk (T-cells are inactivated)
Because of the T-cell inactivation, pathogen-reduced platelets may not need separate gamma irradiation for TA-GVHD prevention

Don’t confuse UV activation of amotosalen (for platelets) with gamma irradiation (25 Gy for RBCs, granulocytes, and non-pathogen-reduced products). Different radiation, different purpose.

PAS (Platelet Additive Solution) platelets: 67% of plasma is replaced [TODO: verify - PAS replacement fraction varies 60-70% by formulation/reference] with platelet additive solution via a sterile connection (closed system).

Indications: allergic transfusion reactions; ABO-plasma-incompatible platelets (to reduce isohemagglutinin load)
No impact on shelf life (still 5 days, closed system preserved)
Not sufficient for IgA-deficient patients - 33% plasma remains, and that’s enough IgA to cause anaphylaxis

Washed platelets: 100% of plasma is replaced with normal saline.

Indications: allergic transfusion reactions; ABO-plasma-incompatible platelets; IgA deficiency with anti-IgA (this is the only option that fully removes donor IgA)
Shelf life drops to 4 hours after washing because the open process breaks sterility
Significant logistical constraint - must be prepared close to transfusion time

Quick comparison to remember:

Modification	% Plasma Removed	Shelf Life Impact	Covers IgA Deficiency?
PAS	67%	None (5 days)	No
Washed	100%	Reduced to 4 hours	Yes

Platelet Transfusion Thresholds

Adult prophylactic and treatment thresholds:

<10,000/μL: prophylactic transfusion in a stable patient (no bleeding, fever, sepsis, or coagulopathy)
<20,000/μL: often used when the patient has minor clinical risk factors (fever, sepsis)
<50,000/μL: with extracranial bleeding, or pre-procedure / pre-surgery (non-neurosurgery)
<100,000/μL: with intracranial bleeding or neurosurgery/ocular surgery

These are guideline values for adults. The <10,000/μL prophylactic threshold has good evidence from the TRAP and PLADO trials. [TODO: verify - TRAP studied alloimmunization/refractoriness; the 10K threshold evidence is more typically attributed to Rebulla, Wandt, and PLADO]

Patients with normal platelet counts may still need platelet transfusion if they have functional platelet defects:

Hereditary: Bernard-Soulier syndrome (absent GPIb → defective vWF adhesion), Glanzmann thrombasthenia (absent GPIIb/IIIa → defective aggregation)
Medications: aspirin (irreversibly inhibits COX-1 → no TXA2)
Organ dysfunction: uremia (renal failure) - but platelets are second line here. First line for uremic bleeding is desmopressin (DDAVP) + cryoprecipitate, both of which boost vWF-mediated adhesion. DDAVP binds V2 receptors on endothelium and platelets, releasing stored vWF from Weibel-Palade bodies. Cryoprecipitate provides vWF directly. Transfused platelets become dysfunctional in the uremic environment, which is why they’re not first line.

Platelet Transfusion Contraindications

Absolute:
- TTP (thrombotic thrombocytopenic purpura) - transfused platelets feed the ongoing thrombotic process and can precipitate fatal thrombosis. Treat with plasma exchange (see Chapter 7), not platelets. Reserved for life-threatening hemorrhage only.
- HIT (heparin-induced thrombocytopenia) - platelet-factor 4 complexes activate more platelets, worsening thrombosis. Use only for pre-procedure or severe bleeding.
Relative:
- ITP (immune thrombocytopenic purpura) - transfused platelets are destroyed by the same autoantibody. Reserved for life-threatening bleeding.

Board pearl: TTP = don’t give platelets unless hemorrhaging. ITP = emergency use only.

Platelet Transfusion Dosing and Response

1 WB-derived unit (5.5 × 10^10 platelets) → +5,000/μL
1 therapeutic dose (4-6 pooled WB units OR 1 apheresis unit, ~3 × 10^11 platelets) → +30,000/μL

If the increment is less than expected, the patient may be refractory (see below).

Corrected Count Increment (CCI): Used to assess platelet transfusion response, correcting for patient size and dose transfused.

CCI = (Post-transfusion count - Pre-transfusion count) × BSA (m²) / Platelets transfused (× 10^11)
Normal CCI at 1 hour: >7,500
Normal CCI at 24 hours: >4,500
CCI <5,000-7,500 at 1 hour after a standard dose (≥3 × 10^11 platelets), on two consecutive ABO-compatible fresh transfusions, defines refractoriness

Assessment requires pre- and post-transfusion platelet counts (post drawn 10-60 minutes after the transfusion completes).

Platelet Refractoriness

When a patient has repeated poor platelet increments, they’re refractory. Causes split into non-immune (more common) and immune.

Non-immune causes (~70-80%):

Splenomegaly (platelet sequestration)
Fever / sepsis (increased consumption)
DIC (consumption)
Medications (amphotericin B, vancomycin, and others)
Active bleeding (consumption)

Immune causes (~20-30%):

HLA class I antibodies (most common immune cause): recipient has antibodies to donor HLA Class I antigens expressed on platelet surfaces
HPA (Human Platelet Antigen) antibodies: less common

Management:

Non-immune: treat underlying cause; consider more frequent transfusions
Immune:
- HLA-matched platelets (requires HLA typing and an HLA-matched donor inventory)
- Crossmatch-compatible platelets (test recipient serum against donor platelets)
- ABO-matched platelets (sometimes helps)

HLA antibody workup and platelet antigen systems are covered in more detail in Chapter 10.

Transfusion Filters

Most blood products - including platelets - are transfused through a 170 μm standard filter that removes gross clots and debris.

Exception: neonatal transfusions may use syringe delivery with pre-filtering by the blood bank
Note: the 170 μm filter does NOT leukoreduce products - that requires separate, much finer filtration done at collection or bedside

ABO and Rh Considerations for Platelets

The critical distinction to have straight:

ABO compatibility: patient receives blood without foreign antigens on the donor RBCs (antigen matching - matters for RBC products)
ABO plasma compatibility: patient receives blood product without donor antibodies to their own antigens (antibody matching - matters for plasma and platelet products, because platelets are suspended in plasma)

Platelets are suspended in plasma that may contain anti-A and anti-B (isohemagglutinins). ABO-plasma-incompatible platelets are routinely transfused in adults because the small plasma volume is diluted quickly. Use caution in infants (small blood volume, less dilution) and patients with extensive transfusion histories (cumulative isohemagglutinin exposure).

Some platelet units have high-titer isohemagglutinins (especially type O donors, whose plasma contains IgG anti-A,B in addition to IgM anti-A and anti-B). Many blood centers test donors and label units accordingly. High-titer units can cause real hemolysis in non-O recipients. PAS platelets (67% plasma removed) mitigate this risk.

Rh matching for platelets:

Whole-blood-derived platelets should be Rh-matched to the recipient, because they contain more contaminating RBCs than apheresis platelets. Those RBCs carry D antigen and can sensitize D-negative recipients.
Apheresis platelets have negligible RBC contamination; Rh matching is less critical.
If an Rh-negative patient must receive Rh-positive whole-blood-derived platelets, administer RhIG to prevent sensitization. Most important for D-negative females of childbearing potential, where anti-D development would create HDFN risk in future pregnancies. One vial of RhIG (standard 300 μg dose) covers ~30 mL of D-positive whole blood (≈ 15 mL RBCs), which is far more than a single platelet dose contains.

3.4 Plasma Products

Fresh Frozen Plasma (FFP)

FFP is plasma separated from whole blood and frozen within 8 hours of collection at <-18°C (or up to <-65°C for longer storage).

Storage and shelf life:

<-18°C: shelf life 1 year
<-65°C: shelf life 7 years (used by reference labs for long-term storage)
Most hospital blood banks store at -18 to -30°C in standard freezers

Contents:

All coagulation factors, including labile Factors V and VIII (~1 IU/mL of each)
Fibrinogen (~2 mg/mL, or ~500 mg in a 250 mL unit)
Plasma proteins (albumin, immunoglobulins)
Volume: ~200-300 mL per unit (variable because it depends on donor hematocrit - higher hematocrit donors give less plasma per collection)

Thawing:

For clinical use: thaw at 30-37°C (body temperature, ~30-45 min in a water bath or microwave blood thawer)
For cryoprecipitate production: thaw at 1-6°C instead (this causes the cold-insoluble proteins to precipitate, which are harvested as cryo)

Expiration after thawing:

24 hours if labeled as FFP
5 days if relabeled as “thawed plasma” after the initial 24 hours. Factors V and VIII decline during those 5 days, but stable vitamin-K-dependent factors (II, VII, IX, X) remain adequate. Thawed plasma is useful for warfarin reversal but NOT for DIC (Factor V too low).

Clinical uses:

Replacement of multiple coagulation factors (liver disease, DIC, massive transfusion)
Reversal of warfarin (when 4-factor PCC / Kcentra is not available or contraindicated - PCC is actually first-line when you have it)
TTP therapeutic plasma exchange (as replacement fluid, often cryo-poor plasma is used instead)
Rare factor deficiencies without a specific concentrate available
Sole source of Factor V - there is no Factor V concentrate, so FFP is the ONLY replacement option

Dosing: 10-15 mL/kg (typically 3-4 units in a 70 kg adult) to achieve a 20-30% factor activity increase.

Timing before procedures: FFP should be given within 2 hours of an invasive procedure. Factor VII has a short half-life (~4-6 hours), and waiting longer means Factor VII levels are already declining by the time the surgeon cuts.

Board pearl: FFP is NOT effective for mild INR elevations (1.5-1.9). The volume required to correct a small INR elevation would cause circulatory overload. Use 4-factor PCC instead when you’re trying to rapidly reverse warfarin or vitamin K deficiency.

ABO compatibility for plasma - mirror image of RBC compatibility:

Type A recipients can receive A or AB plasma
Type B recipients can receive B or AB plasma
Type O recipients can receive O, A, B, or AB plasma
Type AB recipients can receive only AB plasma

AB plasma is the universal donor for plasma because it contains no anti-A or anti-B. Mirror image of RBC compatibility: type O is the universal RBC donor (no A or B antigens). In massive transfusion with unknown type: give O RBCs + AB plasma. This is why AB plasma is a scarce, actively managed resource in blood banks.

Thawed plasma: FFP thawed and stored at 1-6°C for up to 5 days. Has reduced levels of factors V and VIII but is suitable for most indications.

PF24 (Plasma Frozen Within 24 Hours)

PF24 is plasma frozen between 8 and 24 hours after collection (FFP is frozen within 8 hours).

Depleted in Factor V and Factor VIII (the labile factors, which degrade during the extended time between collection and freezing)
Interchangeable with FFP for most indications (warfarin reversal, liver disease, massive transfusion)
Not suitable for DIC - the reason is Factor V deficiency, not Factor VIII. Factor VIII is covered by cryoprecipitate, but Factor V has no concentrated replacement product. FFP is the only Factor V source. DIC consumes ALL factors, including V, so PF24 leaves the patient short on Factor V.
Similarly, cannot replace FFP in other situations where Factor V matters

Net: if you want FFP specifically (as opposed to PF24), the usual reason is Factor V replacement.

Massive Transfusion Protocols

Massive transfusion protocols (MTP) aim to approximate whole blood by giving balanced ratios of components:

1:1:1 ratio of RBCs : plasma : whole-blood-derived platelets
6:6:1 ratio of RBCs : plasma : apheresis platelets (equivalent - 1 apheresis ≈ 6 WB-derived units)

The PROPPR trial (2015) showed improved hemostasis and reduced 24-hour mortality with balanced ratios vs. RBC-heavy resuscitation in trauma. [TODO: verify - PROPPR’s primary endpoints (24-hr and 30-day mortality) did not reach statistical significance; benefit was seen in death from exsanguination by 24 hr] Reasons to use O RBCs + AB plasma at the start when the patient’s type isn’t yet known.

Cryoprecipitate

Cryoprecipitate is the cold-insoluble portion of plasma that precipitates when FFP is slowly thawed at 1-6°C. The cold-insoluble proteins collect at the bottom of the centrifuge; they’re harvested, and the supernatant (cryo-poor plasma) is saved separately for use as TTP replacement fluid.

Production sequence:

Thaw FFP at 1-6°C
Centrifuge → cryoprecipitate pellet at the bottom, cryo-poor plasma on top
Scoop off the cryo-poor plasma (used in TTP plasmapheresis as replacement fluid - see Chapter 7)
Freeze the cryo pellet at <-18°C within 1 hour

Storage:

Frozen: <-18°C, shelf life 1 year (same as FFP)
After thawing and pooling: 4 hours if pooled in an open system (non-sterile connection); 6 hours if pre-pooled in a closed system before freezing. Most institutions now pre-pool before freezing to maximize the post-thaw window.

Contents per unit (~15 mL volume - small volume is useful for fluid-restricted patients):

Fibrinogen ≥150 mg (usually ~250 mg) - the main indication driver
Factor VIII ≥80 IU
Factor XIII
von Willebrand factor
Fibronectin

The common theme: all the cold-insoluble plasma proteins. Everything else stays soluble in cryo-poor plasma.

Indications:

DIC - the most common indication. Cryo replaces fibrinogen (aim to keep >100-150 mg/dL) and Factor VIII. Give with FFP in DIC to also replace Factor V and the other consumed factors. Cryo alone can’t replace Factor V; FFP alone can’t efficiently raise fibrinogen.
Hypofibrinogenemia (congenital or acquired; target fibrinogen usually >100-150 mg/dL)
Uremic bleeding (provides vWF; second-line behind DDAVP)
Factor XIII deficiency (rare congenital deficiency with delayed bleeding; cryo is one of the few sources)
Hemophilia A / vWD (second-line, when recombinant Factor VIII or vWF concentrates aren’t available - common in hospitals without those products on formulary)
Biological adhesive (“fibrin glue”) - cryo (fibrinogen source) + thrombin (activator) → fibrin polymerizes into a surgical adhesive. Used for hemostasis, tissue sealing, graft fixation. Has some antibacterial activity. Can be prepared from autologous cryo to eliminate infection risk.

Dosing for fibrinogen replacement:

1 unit of cryo (150 mg fibrinogen) raises adult fibrinogen by ~7 mg/dL
Typical adult dose: 10 units (one “pool”) → raises fibrinogen ~70 mg/dL
If starting fibrinogen is 80 mg/dL and target is 150 mg/dL, you need ~10 units
Always check post-transfusion fibrinogen to confirm response

Cryoprecipitate-reduced plasma (cryo-poor plasma): Plasma remaining after cryoprecipitate is removed. Used specifically for TTP therapeutic plasma exchange (lacks the large vWF multimers that drive TTP pathology).

Factor Concentrates and Bypassing Agents

Beyond FFP and cryo, there’s a growing list of purified/recombinant factor products that often replace blood products as first-line therapy.

Recombinant Factor VIII: standard treatment and prophylaxis for hemophilia A (Factor VIII deficiency, X-linked recessive). Virus-free, precisely dosed, available in standard and extended half-life formulations.

Humate-P: a plasma-derived Factor VIII concentrate that uniquely contains sufficient vWF to treat von Willebrand disease. Most recombinant Factor VIII products do NOT contain vWF.

vWD treatment ladder: (1) DDAVP - first line for Type 1 vWD (releases stored vWF from endothelial Weibel-Palade bodies). (2) If refractory, or in severe Type 2 / Type 3: Humate-P or other vWF-containing concentrates. (3) Cryoprecipitate as a backup.

Recombinant Factor VIIa (NovoSeven / rFVIIa): indications include Factor VII deficiency and bypassing agent for Factor VIII or Factor IX inhibitors. Activates Factor X directly on the platelet surface, bypassing the need for VIII/IX. High cost and thrombosis risk limit its use to refractory bleeding in inhibitor patients.

Prothrombin Complex Concentrate (PCC): contains Factors II, VII, IX, X. Used for warfarin reversal (4-factor PCC / Kcentra is first-line) and for hemophilia B.

FEIBA (Anti-Inhibitor Coagulant Complex): contains Factors II, activated VII, IX, X. Used as a bypassing agent for patients with Factor VIII or Factor IX inhibitors. The activated Factor VII drives coagulation through alternative pathways.

Factor IX products: recombinant Factor IX (pure, for hemophilia B) vs. PCC (II/VII/IX/X, warfarin reversal or hemophilia B) vs. FEIBA (bypass agent).

Factor pharmacokinetics worth memorizing:

Factor VIII half-life: 12 hours → dose every 8-12 hours for acute bleeding
Factor IX half-life: 24 hours → dose every 12-24 hours
Factor VIII intravascular recovery: 100% - all infused Factor VIII stays in the vascular space
Factor IX intravascular recovery: 50% - half distributes to extravascular sites (binds endothelium and extracellular matrix). So you need to give twice the “units per kg” of Factor IX compared to Factor VIII to achieve the same plasma level.

Emicizumab (bispecific antibody mimicking Factor VIII) is now first-line prophylaxis for hemophilia A inhibitor patients - worth knowing as a newer class.

3.5 Other Blood Products

Granulocytes

Granulocyte transfusions are rare and reserved for the most desperate scenarios.

Indications:

ANC <500/μL with documented infection unresponsive to antibiotics
Neutrophil dysfunction syndromes (rare)

Collection: Apheresis with donor stimulation (G-CSF ± dexamethasone) to boost the donor’s neutrophil count before collection. Each unit contains ≥1 × 10^10 granulocytes.

Storage: 20-24°C (room temperature), NO agitation. 24-hour shelf life, the shortest of any blood product. Refrigeration kills neutrophils. Even at 20-24°C, chemotactic and bactericidal function decline within hours of collection, which is one reason efficacy has been hard to prove in trials.

Required modifications:

Must be ABO-compatible (significant RBC contamination in the product)
Must be crossmatched (same reason)
Must be irradiated (full of viable donor lymphocytes → TA-GVHD risk)
Cannot be leukoreduced (you’re transfusing white cells on purpose)
Cannot be made CMV-safe through leukoreduction for the same reason

Usually given daily until infection resolves or neutrophil recovery. Clinical efficacy is debated; recent trials (RING) showed modest benefit in certain settings but not universal.

3.6 Bombay and Para-Bombay Phenotypes in the Context of Component Selection

These are the rare cases where “give O RBCs and figure it out later” fails. The chapter on ABO biology (Chapter 1) covers the core genetics; included here is what matters for component selection and recognition in the blood bank.

Bombay (classical)

Genotype: hh, sese
Nonfunctional FUT1 (no H on RBCs) AND nonfunctional FUT2 (no H in secretions or plasma)
Because the A and B transferases modify H to make A and B antigens, no H means no A or B, regardless of ABO genotype
Forward-types as O (no A, no B on the surface)
BUT the immune system has never encountered H, so it produces potent naturally-occurring anti-H that reacts with ALL ABO types (including O, which has the most H of any group)
Only Bombay-to-Bombay transfusion is safe; giving any standard O unit will cause acute hemolysis
Diagnostic clue: Bombay RBCs do not agglutinate with Ulex europaeus lectin (which has anti-H activity and normally agglutinates O cells strongly). This distinguishes Bombay from true group O.
Prevalence: ~1:10,000 in India; rare elsewhere

Para-Bombay

Two flavors, with different clinical implications.

Most common Para-Bombay:

Genotype: hh, SeSe or Sese
No H on RBCs (FUT1 nonfunctional) but H IS made in secretions/plasma (FUT2 functional)
Secreted H maintains immune tolerance → anti-H is usually clinically insignificant

Less common Para-Bombay:

Weak H allele + sese
Small amount of H on RBCs, no H in secretions → no immune tolerance → clinically significant anti-H despite having some H on the cells
This form is actually more dangerous than the more common type because of that lack of secretor-derived tolerance

Secretor Status Refresher

~80% of people are secretors (SeSe or Sese) - FUT2 is functional, and they secrete ABH antigens in saliva, tears, breast milk, and plasma. Secretors also express Lewis b (Le^b).

~20% are non-secretors (sese) - no ABH in secretions. They express Lewis a (Le^a) instead. Secretor status doesn’t usually affect transfusion in normal ABO types but becomes critical in Bombay / Para-Bombay, where secreted H is the thing maintaining anti-H tolerance.

Leukocyte Adhesion Deficiency Type II (LAD II)

Worth knowing because it ties the Bombay phenotype to a broader fucosylation problem.

Mutation in the GDP-fucose transporter → global defect in fucose addition to glycoproteins
Neutrophils can’t make fucosylated selectin ligands → can’t roll / adhere to vessel walls → recurrent infections without pus (because neutrophils can’t migrate into tissues)
RBCs can’t make the fucose-containing H antigen → LAD II patients have the Bombay phenotype
The reverse is NOT true: not every Bombay patient has LAD II. Most Bombay cases have a local FUT1 problem, not a global fucosylation defect.

Practical Implications for Component Selection

Frozen RBCs are the backbone of rare-type inventory (Bombay, Jk(a-b-), Rh-null) - 10-year shelf life makes them practical to stockpile
When a Bombay patient needs transfusion, the blood bank calls the regional rare donor registry. Plan hours ahead for thawing and deglycerolization.
Autologous donation is a reasonable option for elective surgery in Bombay patients

Chapter 4: Transfusion Reactions

Understanding transfusion reactions requires integrating immunology, pathophysiology, and clinical medicine. Every reaction has a mechanism, and understanding that mechanism guides recognition and management.

A useful frame for the board exam: group reactions by timing and by chief complaint. Fever during or within a few hours of transfusion has a differential we teach with the mnemonic “Fevers Hate Swift Transfusions”: FNHTR, acute Hemolytic, Sepsis, and TRALI. Respiratory distress within 6 hours is TRALI vs TACO. Delayed fever (days to weeks) is DHTR vs TA-GVHD. Delayed thrombocytopenia is post-transfusion purpura. Keep that framework in mind as you work through the sections - the individual reactions are the cell types that live inside it.

By frequency: FNHTR is the most common transfusion reaction; allergic reactions are second. The dangerous reactions (AHTR, sepsis, TRALI, TACO, TA-GVHD) are far less common but carry most of the morbidity and mortality.

Universal first response to any suspected reaction

Before the section-by-section mechanisms, the immediate bedside protocol is identical across reaction types:

Stop the transfusion immediately
Keep the IV line open with normal saline (you may need vascular access for resuscitation)
Notify the physician and blood bank
Save the blood bag and tubing for workup

The first 15 minutes of any transfusion are the highest-risk window - this is when ABO mismatch, anaphylaxis, and severe TRALI tend to manifest. Most fatal reactions would have been prevented if the clerical check at the bedside had caught the error before the unit was hung.

The blood bank workup

When a reaction is suspected, the blood bank performs a standardized four-part workup:

Clerical check: verify patient wristband identifiers against the blood bag label, the original order, and blood bank records. If any discrepancy is found (wrong name, wrong MRN, wrong type on the bag), you have identified the root cause.
Visual inspection for hemolysis: compare pre- and post-transfusion serum. Pink/red serum = free hemoglobin = intravascular hemolysis.
Repeat ABO on both pre- and post-transfusion samples. This catches wrong blood given (post-transfusion sample may show mixed-field) and original sample mislabeling.
DAT on pre- and post-transfusion samples. Pre establishes baseline; a new post-transfusion DAT+ (often mixed-field) indicates recipient antibody coating donor cells.

A few workup pitfalls worth knowing:

Late workup (>8 hours): free hemoglobin is cleared quickly (bound by haptoglobin, cleared by liver). Serum may look normal. Check serum bilirubin, which remains elevated for 24-36 hours. LDH also stays elevated. The timeline: free Hgb peaks immediately then clears within hours; bilirubin rises as heme is metabolized and stays up 24-36 hours; LDH persists. If you’re late, bilirubin and LDH are your best markers.
Myoglobin false positive: free myoglobin from rhabdomyolysis (trauma, crush, compartment syndrome) makes serum look pink and mimics hemolysis. To distinguish: hemoglobin drops haptoglobin, myoglobin does not. CK is markedly elevated in rhabdo.
Urine dipstick for “blood”: positive in hematuria, hemoglobinuria, or myoglobinuria. Urine microscopy must be negative for RBCs before you call dipstick positivity evidence of hemolysis. The dipstick peroxidase reaction detects heme and does not distinguish intact RBCs from free hemoglobin or myoglobin. In the context of a transfusion reaction, dipstick+ with no RBCs on micro = hemoglobinuria.
Negative DAT in a clinically obvious acute hemolytic reaction: if the reaction is severe and fast enough, all antibody-coated donor cells have already been destroyed before the sample was drawn. No coated cells remain to detect. Don’t let a negative DAT overrule pink plasma, hemoglobinuria, rising LDH, and falling haptoglobin.

Regulatory reporting

Transfusion-related fatality or serious morbidity requires immediate notification to the FDA (phone), with a written report within 7 days. If the reaction is traced to a specific donor, the collecting facility must also be notified immediately so they can quarantine and recall any other products from that donation (RBCs, platelets, plasma, and cryo can all derive from one whole blood collection).

For confirmed or suspected transfusion-transmitted infection, “lookback” is a legal requirement: notify the donation center, all recipients of that donor’s products, each recipient’s physician, and the FDA. Every reaction, even a mild allergic reaction, also requires a medical note in the patient’s chart (for future transfusion planning, QI, regulatory compliance, and medicolegal protection).

4.1 Acute Hemolytic Transfusion Reaction (AHTR)

The Mechanism

AHTR occurs when the recipient’s antibodies destroy transfused red cells. The most common scenario is ABO incompatibility from clerical error - a unit labeled for one patient given to another. Errors happen at collection (mislabeled sample), in the lab (wrong unit selected), or at the bedside (wrong patient identified). Two-identifier verification at every step is designed to prevent this. Despite all the technology, human error remains the greatest threat in transfusion safety.

The pathophysiology:

Antibody binding: Pre-existing IgM antibodies (anti-A, anti-B) bind to incompatible donor red cells
Complement activation: IgM efficiently activates the classical complement pathway (C1 → C4 → C2 → C3 → C5-C9)
Membrane attack complex: C5-C9 creates pores in the red cell membrane, causing intravascular hemolysis
Cytokine storm: Complement fragments (C3a, C5a) and other mediators trigger a massive inflammatory response
End-organ effects:

Hypotension from vasodilation
DIC from procoagulant release (massive intravascular lysis spills phospholipids; tissue factor activity rises through the inflammatory response)
Acute kidney injury from hemoglobin and cytokine effects on renal vasculature

The two antibody classes that classically cause acute intravascular hemolysis are:

Anti-ABO (IgM, complement-fixing) - acute and severe, #1 cause of fatal AHTR
Anti-Kidd (Jka, Jkb) - IgG but unusually good at fixing complement; usually causes a delayed reaction, but with a high-titer anamnestic response it can produce acute intravascular hemolysis

Non-immune (“non-antibody mediated”) acute hemolysis

Not every case of hemoglobinemia after transfusion is antibody-driven. The DAT is negative in these scenarios, which is the key clue:

Storage lesion: old units with fragile RBCs
Small-bore needles or malfunctioning rapid infusers: mechanical shear stress
Wrong IV solutions co-infused: only normal saline is compatible. Hypotonic fluids or D5W cause osmotic lysis
Temperature extremes: freezing or overheating the unit
Bacterial contamination, particularly Clostridium spp.: lecithinases and other toxins lyse RBCs directly. The unit may appear discolored (purple/brown), and the patient develops overwhelming sepsis with intravascular hemolysis that mimics an ABO-mismatch reaction

Clinical Presentation

Symptoms can begin within minutes of starting transfusion. The classic mnemonic is HBIDC - Hypotension, Back pain (flank), Infusion site pain, DIC, Chills/fever:

Fever, chills (sometimes the only sign in anesthetized patients)
Chest pain, back pain, flank pain (renal vasoconstriction from free hemoglobin)
Pain at the infusion site
Hypotension
Dark urine (hemoglobinuria)
Anxiety, sense of doom
Bleeding from DIC

In anesthetized patients the only signs may be hemoglobinuria (dark urine in the Foley), hypotension, and diffuse oozing from DIC.

Laboratory Findings

Positive DAT (often mixed-field due to mix of patient and donor cells)
Hemoglobinemia (pink/red plasma)
Hemoglobinuria (pink/red urine with positive blood on dipstick but no RBCs on microscopy)
Decreased haptoglobin (bound by free hemoglobin)
Elevated LDH (released from lysed cells)
Elevated indirect bilirubin (delayed, from hemoglobin metabolism)
Elevated creatinine (acute kidney injury)
Coagulation abnormalities (prolonged PT/aPTT, low fibrinogen, elevated D-dimer) if DIC develops

Management

Stop the transfusion immediately
Maintain IV access with normal saline
Support blood pressure (fluids, vasopressors if needed)
Maintain urine output >1 mL/kg/hr (fluids, diuretics if needed; goal is to prevent renal tubular damage from hemoglobin)
Clerical check: Verify patient identification and unit labeling
Send blood bank samples: Post-transfusion patient sample, the unit with attached segment, and tubing
Monitor and treat DIC if it develops
Anticipate and treat acute kidney injury

Intravascular vs extravascular hemolysis - what the labs tell you

This distinction is heavily tested. The pattern of lab findings tells you which mechanism is operating, which in turn tells you which antibody system to suspect.

Finding	Intravascular	Extravascular
Positive DAT	Yes (may be negative if all donor cells destroyed)	Yes
Pink/red serum (hemoglobinemia)	Yes	No (Hgb contained in macrophages)
Hemoglobinuria	Yes	No
Hyperbilirubinemia	Possible	Yes (primary marker)
Decreased haptoglobin	Yes (dramatic, undetectable)	Yes (mild)
Elevated LDH	Yes (very high)	Yes (moderate)
Coagulopathy / DIC	Yes	No
Schistocytes	Yes (MAHA, DIC)	No
Spherocytes	No	Yes (splenic macrophage nibbling)

A few mechanism-level points that tie these markers together:

Free hemoglobin is only released into plasma when cells burst in the bloodstream. It binds haptoglobin first; once haptoglobin capacity is overwhelmed, free Hgb is filtered at the glomerulus → hemoglobinuria.
Haptoglobin <25 mg/dL with elevated LDH has high sensitivity for intravascular hemolysis.
Spherocytes form when splenic macrophages phagocytose partial membrane from antibody-coated RBCs. The cell loses surface area but keeps volume → sphere. Decreased deformability → trapped in splenic sinusoids → further destruction.
DIC follows intravascular hemolysis because lysed RBCs spill procoagulant phospholipids (and tissue factor activity from the cytokine-driven inflammatory response) [TODO: verify mechanism specifics]. Extravascular destruction happens inside macrophages and does not release enough procoagulant to drive DIC.

4.2 Delayed Hemolytic Transfusion Reaction (DHTR)

Definitions: DHTR vs DSTR

Two closely related entities:

Delayed hemolytic transfusion reaction (DHTR): transfusion reaction occurring >24 hours but <28 days after transfusion, with evidence of hemolysis
Delayed serologic transfusion reaction (DSTR): a newly detectable alloantibody after transfusion without hemolysis

DSTR is more common than DHTR. Both require documentation and antigen-negative blood for future transfusions, but DSTR is clinically benign.

The Mechanism

DHTR can occur through two distinct mechanisms:

Anamnestic response (most common): the patient was previously sensitized to the antigen, the antibody evanesced below the detection threshold, and re-exposure triggers memory B cells to produce a rapid, high-titer IgG response within days. This is the classic scenario of anti-Jka that was undetectable on the pre-transfusion screen. Memory responses are fast and vigorous - this is the more dangerous subtype of DHTR.
Primary response: the patient had never seen the antigen before. Naive B cells generate the antibody de novo, which takes 1-2 weeks. By the time the antibody is at significant titer, many transfused RBCs have already reached the end of their natural lifespan. Primary responses usually produce a DSTR (serologic only) rather than a full DHTR.

The steps of the anamnestic response:

Memory B cells recognize the antigen
Rapid proliferation and differentiation into plasma cells
IgG antibody production within days
Antibody-coated donor cells are removed primarily by the spleen (extravascular hemolysis)

Timing: 3-14 days post-transfusion (typically 5-7 days)

Common antibodies causing DHTR - the antigens notorious for evanescence:

Kidd (especially Jka): the classic culprit; “Kidd will Kill you.” Uniquely, anti-Kidd activates complement efficiently despite being IgG, so Kidd-mediated DHTR can be delayed AND severe, with intravascular hemolysis
Duffy (Fya)
Kell
Rh (C, c, E, e)

The serologic pattern of DHTR is stereotyped: negative antibody screen pre-transfusion, positive antibody screen post-transfusion, plus a positive post-transfusion DAT and the biochemical footprint of hemolysis (falling Hgb, rising LDH and bilirubin, dropping haptoglobin).

Clinical Presentation

Often subtle:

Falling hemoglobin or failure to achieve expected post-transfusion hemoglobin
Low-grade fever
Mild jaundice
Symptoms may be mistaken for other conditions (sepsis, bleeding)

Some patients have an intravascular component and present more dramatically (especially with Kidd antibodies).

Laboratory Findings

Positive DAT (usually IgG; may be mixed-field)
New alloantibody detected on antibody screen (wasn’t present before transfusion)
Evidence of hemolysis: Elevated LDH, elevated indirect bilirubin, decreased haptoglobin
Reticulocytosis (compensatory response)

Management

Usually supportive; most cases are self-limited
Transfuse if needed with antigen-negative blood
Ensure the new antibody is documented for future transfusions

The key lesson: Maintain historical antibody records. A patient who made anti-Jka once should receive Jk(a-) blood forever, even if the current screen is negative.

Delayed fever differential: Fever appearing days to weeks after transfusion (rather than during or within hours) narrows the differential to two main entities: DHTR (3-14 days, low-grade fever + falling Hgb + new positive antibody screen) and TA-GVHD (2 days to 6 weeks, fever + rash + hepatitis + pancytopenia). TA-GVHD is far more dangerous.

4.3 Febrile Non-Hemolytic Transfusion Reaction (FNHTR)

The Mechanism

FNHTR is the most common transfusion reaction overall. Two mechanisms contribute:

Recipient antibodies to donor WBCs/HLA: The recipient’s anti-HLA antibodies react with donor leukocytes, causing cytokine release from the donor cells
Cytokines accumulated during storage: Donor WBCs release IL-1, IL-6, IL-8, and TNF-α during storage; these are transfused and cause fever. This mechanism dominates with platelets (stored at room temperature, more cytokine accumulation)

The relative contribution depends on whether the product is leukoreduced:

Non-leukoreduced: Both mechanisms contribute
Prestorage leukoreduced: Mainly the second mechanism (fewer WBCs to make cytokines)
Universal leukoreduction has dramatically reduced FNHTR incidence (it also reduces CMV transmission, HLA alloimmunization, and transfusion-related immunomodulation). Leukoreduction is now standard in the US for virtually all RBC and platelet products.

Clinical Presentation

FNHTR is defined as a temperature rise ≥1°C (~1.8°F) during or within 4 hours of transfusion, with no other explanation:

Chills and rigors
No hemolysis (this is the key distinguishing feature)

It is a diagnosis of exclusion. You must rule out hemolytic reaction, sepsis, and TRALI before calling a febrile reaction “FNHTR.”

Differential Diagnosis

The challenge: Fever during transfusion could be:

FNHTR (benign)
AHTR (life-threatening)
Septic transfusion reaction (life-threatening)
Unrelated fever (patient’s underlying condition)

You must always consider AHTR and sepsis before diagnosing FNHTR.

Management

Stop or slow the transfusion
Evaluate for hemolysis (clerical check, visual inspection of plasma, DAT)
Consider sepsis workup if clinically indicated
Antipyretics (acetaminophen) for symptom relief
If FNHTR is confirmed and patient needs more blood: Premedicate with acetaminophen; ensure products are leukoreduced. For recurrent FNHTR, washed products can be considered.

Once hemolytic reaction, sepsis, and TRALI are excluded, it is usually safe to resume the transfusion, though many institutions simply discard the remainder out of caution.

4.4 Allergic Reactions

Allergic reactions are the second most common transfusion reaction after FNHTR. They are triggered by donor plasma proteins that the recipient is sensitized to. Severity ranges from mild urticaria to fatal anaphylaxis. Products with more plasma (platelets, FFP) carry higher risk than RBCs.

Allergic reactions are graded mild, moderate, or severe. The grading drives management.

Mild Allergic Reactions (Urticaria)

Mechanism: IgE-mediated reaction to plasma proteins.

Presentation:

Urticaria (hives)
Pruritus
Sometimes mild throat tightness
No respiratory or cardiovascular compromise

Management:

May slow or stop transfusion
Diphenhydramine (treats and prevents mild allergic reactions)
If symptoms resolve after antihistamine, may cautiously resume the transfusion

Routine premedication for all patients is not well supported - evidence that it reduces reactions in unselected populations is limited, and anticholinergic side effects of diphenhydramine matter in the elderly. Reserve premedication for patients with a documented history of allergic reactions.

Moderate Allergic Reactions

Moderate reactions have significant symptoms - airway swelling, facial edema, severe urticaria - but no anaphylactic shock (blood pressure is maintained).

Management:

Stop the transfusion
Diphenhydramine, +/- epinephrine depending on severity
Escalate to epinephrine if airway compromise progresses despite antihistamines

Prevention for patients with recurrent moderate reactions:

Diphenhydramine premedication
Washed or PAS-modified products to reduce plasma protein exposure
Product modification is more effective than premedication alone for recurrent reactions

Severe Allergic Reactions (Anaphylaxis)

Mechanism: Multiple possible causes, all converging on IgE- or IgG-mediated complement activation or mast cell degranulation.

IgA deficiency with anti-IgA: the classic teaching. IgA deficiency is the most common primary immunodeficiency, present in ~1:700 individuals. Most IgA-deficient people are asymptomatic, but a minority develop antibodies (IgG or IgE class) against IgA. When transfused with IgA-containing products, these patients develop severe anaphylaxis.
Haptoglobin deficiency: similar mechanism to IgA deficiency but targets haptoglobin. More common in East Asian populations (up to ~4% in Japanese individuals). Patient lacks haptoglobin, makes anti-haptoglobin, and transfused plasma containing haptoglobin triggers anaphylaxis.
Other plasma protein antibodies: anti-C4, etc.
IgE to other allergens in donor plasma

Presentation:

Rapid onset (often within minutes of starting the transfusion)
Hypotension, bronchospasm, stridor
Urticaria, angioedema
GI symptoms (nausea, vomiting, diarrhea)
Can be fatal

Management:

Stop transfusion immediately
Epinephrine IM (0.3-0.5 mg of 1:1000) is first-line
Maintain airway (intubation if needed)
IV fluids for hypotension
Antihistamines, steroids as adjuncts (not primary treatment)
Workup: check quantitative IgA level and anti-IgA antibody, consider anti-haptoglobin testing, particularly in patients from populations where haptoglobin deficiency is more prevalent, such as Japanese and other East Asian populations

Prevention for confirmed IgA deficiency with anti-IgA (or haptoglobin deficiency with anti-haptoglobin):

Washed cellular products (removes essentially all plasma)
Plasma products from IgA-deficient donors (rare; maintained in special registries)
Corticosteroids as premedication for patients with documented severe reactions

Product modifications: washed vs PAS

The two ways to reduce plasma in a cellular product:

Washed RBCs or washed platelets: the unit is spun, plasma removed, and cells resuspended in saline. Nearly 100% of plasma is removed. Downside: platelet shelf life drops to 4 hours after washing, RBCs to 24 hours.
PAS (platelet additive solution): replaces ~65-67% of plasma with a synthetic additive. Maintains normal shelf life but retains ~33% plasma. Adequate for most moderate reactions.

For IgA or haptoglobin deficiency, PAS is NOT sufficient - these patients need washed products (or products from matched deficient donors).

A simple way to remember the management ladder:

Severity	Prevention	Treatment
Mild (urticaria)	Diphenhydramine (often unnecessary)	Diphenhydramine
Moderate (edema, no shock)	Diphenhydramine +/- washed or PAS products	Diphenhydramine +/- epinephrine
Severe (anaphylaxis)	Corticosteroids + washed products	Epinephrine

4.5 Transfusion-Related Acute Lung Injury (TRALI)

TRALI has historically been the leading cause of transfusion-related mortality in the US, though TACO has surpassed it in some jurisdictions since TRALI mitigation strategies took effect. TRALI mortality is ~15%. Understanding it requires appreciating the “two-hit hypothesis.”

The Two-Hit Hypothesis

TRALI occurs when two conditions are present:

First hit: The recipient has “primed” neutrophils - neutrophils that are partially activated and adherent to pulmonary capillary endothelium. This occurs in conditions like:

Sepsis
Recent surgery
Trauma
Hematologic malignancy
Mechanical ventilation

Second hit: Something in the transfused product triggers full activation of these primed neutrophils:

Antibody-mediated: Donor plasma contains HLA antibodies (anti-HLA Class I or II) or anti-neutrophil antibodies (anti-HNA) that bind recipient neutrophils
Non-antibody mediated: Bioactive lipids accumulated during storage activate neutrophils

When neutrophils fully activate, they release reactive oxygen species and enzymes, causing capillary leak and non-cardiogenic pulmonary edema. Both hits are needed - healthy recipients rarely develop TRALI even when exposed to a high-antibody plasma unit.

The plasma-rich products - FFP and platelets - carry the highest TRALI risk. RBCs have less residual plasma and lower but still real risk.

Clinical Presentation

TRALI diagnostic criteria (all required):

Onset within 6 hours of transfusion (typically 1-2 hours into the transfusion)
Hypoxemia: SpO2 <90% on room air, or PaO2/FiO2 <300 mmHg
Bilateral infiltrates on chest X-ray
No pre-existing acute lung injury before the transfusion
No evidence of circulatory overload (normal BNP, normal wedge pressure, no response to diuretics)

Fever and hypotension are often present but are not part of the diagnostic criteria.

Distinguishing TRALI from TACO

This is a critical distinction because treatments are opposite:

Feature	TRALI	TACO
Mechanism	Capillary leak (non-cardiogenic)	Volume overload (cardiogenic)
BNP	Normal or mildly elevated	Significantly elevated
BP response	Hypotension common	Hypertension common
Jugular venous pressure	Normal/low	Elevated
Response to diuretics	Not helpful (may worsen)	Helpful
Fluid balance	May need more fluid	Needs less fluid

TRALI vs ARDS

ARDS and TRALI are essentially identical on imaging and labs - both are non-cardiogenic pulmonary edema with bilateral infiltrates and hypoxemia. The only way to distinguish them is temporal association with transfusion. Lung injury appearing within 6 hours of transfusion in a patient with no pre-existing ALI is TRALI. If lung injury preceded the transfusion, the patient has ARDS that happened to coincide with the transfusion.

Management

Stop transfusion
Supportive care: Oxygen, mechanical ventilation if needed
Most survivors recover within 48-72 hours with supportive care
Diuretics are not indicated and may worsen the patient (this is leaky capillaries, not volume overload)
Notify blood bank: the implicated donor should be tested for HLA/HNA antibodies and deferred from plasma/platelet donation if positive

Prevention

TRALI risk reduction has been achieved primarily through deferral of high-risk plasma donors:

Multiparous women have high rates of anti-HLA antibodies from exposure to fetal HLA during pregnancies. Their plasma is rich in the antibodies that trigger TRALI.
Most blood centers now use plasma predominantly from male donors, or test female donors for HLA antibodies before accepting their plasma
Multiparous women can still donate RBCs (minimal plasma), just not plasma/apheresis platelets
This has reduced TRALI incidence by approximately 50%

4.6 Transfusion-Associated Circulatory Overload (TACO)

The Mechanism

TACO is straightforward: giving more volume than the cardiovascular system can handle. It’s cardiogenic pulmonary edema from transfusion.

Risk factors:

Advanced age
Pre-existing cardiac dysfunction
Renal failure
Small body size
Rapid transfusion rate
Large volume transfused

TACO is increasingly recognized as possibly the most common serious transfusion reaction, especially in elderly hospitalized patients.

Clinical Presentation

Onset during or within 6 hours of transfusion
Dyspnea, orthopnea
Bilateral pulmonary edema
Elevated blood pressure (unlike TRALI)
Elevated jugular venous pressure
Elevated BNP (typically >3 times baseline or >500 pg/mL)

Management

Stop or slow transfusion
Upright positioning
Supplemental oxygen
Diuretics (furosemide)

Prevention

Transfuse slowly (1-2 mL/kg/hr for at-risk patients)
Consider one unit at a time with reassessment
Diuretic between units if needed
Avoid unnecessary transfusions

4.7 Transfusion-Associated Graft-versus-Host Disease (TA-GVHD)

The Mechanism

TA-GVHD occurs when viable donor T-lymphocytes in a blood product engraft in the recipient and mount an immune attack against the recipient’s tissues.

Normally, the recipient’s immune system would recognize and destroy donor lymphocytes. TA-GVHD occurs when:

The recipient is severely immunocompromised (can’t reject donor cells), OR
The donor is homozygous for an HLA haplotype that the recipient is heterozygous for (recipient immune system sees donor cells as “self” and spares them, while donor T-cells see recipient’s other haplotype as foreign and attack)

Concrete scenario: parent homozygous at HLA loci, child heterozygous. The child received half of those alleles from the parent. When the child receives the parent’s blood, the child’s immune system doesn’t reject the parent’s T-cells (they look like “self” at shared loci). The parent’s T-cells see the child’s unshared alleles (from the other parent) as foreign and attack. This is why directed donations from blood relatives must be irradiated, especially when consanguinity is possible.

Clinical Presentation

Onset: 2 days to 6 weeks post-transfusion (typically 8-10 days).

Classic features - four target organs (skin, liver, GI, marrow):

Skin rash is usually the earliest sign - erythematous maculopapular, often starting on face and trunk, can progress to desquamation
Hepatitis (elevated transaminases, hyperbilirubinemia, cholestasis)
Diarrhea (watery, may be bloody)
Bone marrow aplasia / pancytopenia - this is what makes TA-GVHD different from transplant GVHD and what makes it almost uniformly fatal

The bone marrow aplasia occurs because the donor T-cells attack the recipient’s hematopoietic stem cells (which, unlike in transplant GVHD, are recipient-origin and thus targets).

Prognosis

Mortality: >90%. Most patients die from infection secondary to pancytopenia. Unlike transplant GVHD, TA-GVHD is refractory to immunosuppression because the engrafted donor T-cells are extremely difficult to eliminate. There is no effective treatment. Prevention is critical.

Prevention

Two methods prevent TA-GVHD by inactivating donor T-cells:

Gamma irradiation of cellular blood products (25-50 Gy) damages T-cell DNA and prevents proliferation. This is the standard method and works for RBCs, platelets, and granulocytes.
Amotosalen (psoralen) + UVA crosslinks nucleic acids and prevents T-cell replication. Currently approved for platelets only. This is different from gamma irradiation - it is the same pathogen-reduction technology used to inactivate bacteria and viruses in platelet products.

Leukoreduction alone is NOT sufficient to prevent TA-GVHD - residual T-cells can still cause disease. You need irradiation or pathogen reduction.

Indications for irradiation:

Intrauterine transfusions
Exchange transfusions in neonates
Premature infants
Congenital immunodeficiency syndromes
Hematologic malignancies (especially Hodgkin lymphoma)
Recipients of hematopoietic stem cell transplants (before engraftment and often long-term)
Patients receiving purine analogs (fludarabine, cladribine) or other immunosuppressive chemotherapy
HLA-matched or HLA-selected products
Directed donations from blood relatives
Patients receiving granulocyte transfusions

4.8 Septic Transfusion Reactions

Bacterial contamination is the most common transfusion-transmitted infection - more common than any viral infection.

The Mechanism

Bacterial contamination of blood products can cause severe sepsis when transfused.

Sources of contamination:

Donor skin - the most common source. The venipuncture needle carries skin flora into the collection bag.
Donor bacteremia (asymptomatic transient bacteremia at time of donation)
Environmental contamination during processing

Risk varies by product:

Platelets: highest risk (stored at 20-24°C, warm enough for skin flora to grow)
RBCs: lower risk (stored at 1-6°C; but certain cold-loving organisms grow)
Plasma: lowest risk (frozen)

Common organisms - temperature drives which bugs grow:

==Platelets (room temp): Staphylococcus aureus== and coag-negative staph; skin flora (Streptococcus, Cutibacterium/Propionibacterium); occasional gram-negatives. S. aureus is the most clinically significant platelet contaminant.
==RBCs (cold): Yersinia enterocolitica== (classic board answer - psychrophilic gram-negative rod that replicates at 1-6°C). Also Serratia liquefaciens, Citrobacter, Pseudomonas.
Classic high-yield association: cold-stored RBC + post-transfusion sepsis = think Yersinia.

Clinical Presentation

High fever (often >39°C / 102°F)
Rigors
Hypotension (may be severe, septic shock)
Tachycardia
Nausea, vomiting
Symptoms often begin during or immediately after transfusion. Septic reactions often present ~1-2 hours after completion, while ABO reactions typically start within the first 15 minutes - a useful time clue.

A classic vignette: patient receives a transfusion, an hour after completion the temperature is 39°C, HR 120, RR 26, BP 86/70. That is septic shock from transfusion-related sepsis until proven otherwise. Because it overlaps clinically with AHTR, the key differentiator is Gram stain and culture of the product bag.

Visual clues of contamination

Visual inspection before transfusion can catch contaminated units:

Visible discoloration (purple, brown, green - bacteria metabolizing hemoglobin)
Visible hemolysis in the supernatant (some bacteria produce hemolysins; Clostridium, beta-hemolytic Strep)
Clots from bacterial procoagulant activity

Any of these findings: don’t transfuse, culture the unit, notify the blood bank.

Workup and Management

Stop transfusion
Blood cultures from the patient AND from the product bag/tubing. If the same organism grows from both, it’s a confirmed transfusion-transmitted infection.
Gram stain of product (rapid preliminary information while cultures incubate)
Broad-spectrum antibiotics immediately (cover gram-positive and gram-negative, don’t wait for culture results)
Supportive care for sepsis (IV fluids, vasopressors)
Narrow antibiotics once culture results return

Prevention

Diversion pouches: the first 30-45 mL of collection (“skin plug”) is diverted into a separate pouch and discarded. This contains the highest bacterial load from skin contamination.
Aseptic technique at phlebotomy
Bacterial testing of platelets: culture-based or rapid detection methods
Visual inspection of products before transfusion (discoloration, clumps, hemolysis)
Limiting platelet storage duration
Pathogen reduction technology (amotosalen + UVA): treats platelets with a photosensitizer and UV light to crosslink nucleic acids, inactivating bacteria, viruses, and parasites. Also prevents TA-GVHD and extends shelf life.

Platelet Bacterial Risk Mitigation Strategies:

LVDS (Large Volume Delayed Sampling): Secondary culture taken from platelet unit ≥36 hours after collection using BACT/ALERT bottles
PRT (Pathogen Reduction Technology): Intercept system uses amotosalen (psoralen) + UVA to crosslink nucleic acids, inactivating bacteria, viruses, and parasites; extends shelf life to 7 days and also reduces CMV and TA-GvHD risk
Rapid PGD (Platelet Pathogen Detection): Point-of-issue rapid testing using immunoassay or fluorescence-based detection as a final safety check before transfusion

4.9 Post-Transfusion Purpura (PTP)

The Mechanism

PTP is a rare but severe reaction occurring within ~2 weeks (typically 5-12 days) after transfusion of RBCs, whole blood, or platelets, characterized by sudden, profound thrombocytopenia. It most commonly follows RBC/whole blood transfusion (which contain some residual platelets) even though it is fundamentally a platelet antibody disorder.

The classic patient is a multiparous woman. HPA-1a is a high-frequency antigen present on platelets in ~98% of the population. Women who lack HPA-1a can become sensitized during pregnancy (fetal HPA-1a+ platelets enter maternal circulation via fetomaternal hemorrhage). Anti-HPA-1a titers rise with each subsequent pregnancy, so the risk compounds. Previously transfused individuals can also be sensitized.

The paradox: the patient’s own platelets are HPA-1a negative, so why do they get destroyed?

Several overlapping mechanisms:

Innocent bystander: soluble HPA-1a antigen from destroyed donor platelets adsorbs onto recipient platelets, which become targets
Immune complex adsorption: antigen-antibody complexes stick to recipient platelets non-specifically
Autoantibody-like cross-reactivity: at very high titers, anti-HPA-1a may cross-react with an epitope on HPA-1a negative platelets

Anti-HPA-1a: three clinical scenarios

The same antibody shows up in three syndromes:

Neonatal alloimmune thrombocytopenia (NAIT): maternal anti-HPA-1a crosses the placenta, causes fetal/neonatal thrombocytopenia, with intracranial hemorrhage risk
Post-transfusion purpura (PTP): destroys both donor and recipient platelets post-transfusion
Immune-mediated platelet refractoriness: less common than HLA-mediated refractoriness but can contribute

Clinical Presentation

Severe thrombocytopenia (often <10,000/μL)
Purpura, petechiae
Mucosal bleeding, intracranial hemorrhage in severe cases
Self-limited - usually resolves in ~3 weeks as antibody titers wane (if the patient survives the bleeding period)

Diagnosis

Platelet-specific antibodies detected (anti-HPA-1a most common)
Patient’s own platelets type negative for the corresponding antigen

Management

High-dose IVIG is first-line; shortens recovery to 3-5 days by blocking Fc receptors on splenic macrophages
Plasma exchange (if IVIG fails)
Platelet transfusions are generally ineffective - transfused platelets are also destroyed because of the bystander effect. For severe life-threatening bleeding, HPA-1a negative platelets from rare-donor registries may be tried.

4.10 Platelet Refractoriness

Platelet refractoriness is the repeated failure of platelet count to rise appropriately after transfusion. Formally diagnosed by inadequate corrected count increment (CCI) on two consecutive occasions with ABO-compatible, fresh (<72 hours old) platelets.

Calculating the CCI

\[CCI = \frac{(Post - Pre) \times BSA}{\text{platelets transfused (} \times 10^{11} \text{)}}\]

Where:

Post and Pre are platelet counts in cells/µL
BSA is body surface area in m²
Platelets transfused are expressed in units of 10^11

Worked example: patient with BSA 1.9 m², pre-transfusion platelet count 8,000/µL, receives a unit containing 3.0 x 10^11 platelets. Post-transfusion count is 18,000/µL.

\[CCI = \frac{(18{,}000 - 8{,}000) \times 1.9}{3.0} = \frac{19{,}000}{3.0} \approx 6{,}333\]

Post-transfusion count must be drawn 10-60 minutes after completion for a valid CCI. Too early, platelets haven’t equilibrated. Too late, consumption/sequestration/immune destruction may have already dropped the count, falsely lowering CCI.

A CCI <5,000-7,500 on two consecutive transfusions meets refractoriness criteria.

Causes: non-immune vs immune

Non-immune (secondary) refractoriness is 3-4x more common than immune (primary).

Non-immune causes (70-80% of refractoriness) - things that consume or sequester platelets regardless of compatibility:

Severe infection/sepsis (cytokines activate and consume platelets)
Splenomegaly (sequesters platelets; massive splenomegaly can sequester up to 90% of circulating platelets)
Medications (amphotericin B, vancomycin, heparin-induced thrombocytopenia)
DIC (consumptive coagulopathy)
Fever, bleeding, bone marrow transplant

Immune (primary) causes (20-30%):

Anti-HLA Class I antibodies (most common immune cause) - HLA-A, B, C are expressed on platelets. Repeated transfusions expose the patient to donor HLA, leading to sensitization.
Anti-HPA antibodies (less common) - platelet-specific antigen antibodies like anti-HPA-1a

Risk modulators

Splenectomy reduces risk (no sequestration) - these patients have the best increments
Palpable splenomegaly increases risk dramatically
Each transfusion is a sensitizing event, so risk of immune refractoriness rises with cumulative transfusions

Management

Approach depends on whether the cause is immune or non-immune:

Non-immune: treat the underlying cause (antibiotics for sepsis, address DIC, review medications). Changing platelet products won’t help if cytokines are consuming them.
Immune: provide HLA-matched platelets (matched for HLA-A and HLA-B) or crossmatched platelets when HLA-matched donors aren’t available.

Prevention

Minimize unnecessary transfusions (fewer sensitizing events)
Leukoreduction (removes donor WBCs that carry HLA antigens and drive HLA alloimmunization)
HLA-matched platelets from the start for patients expected to need long-term platelet support (aplastic anemia, AML induction, HSCT recipients)

4.11 Transfusion-Transmitted Infections

Residual risk for transfusion-transmitted viral infections has dropped to vanishingly low rates in the US. Still, donor screening panels - and the logic behind them - are board-testable.

Viral screening

Every donation is screened:

HIV: anti-HIV-1/2 antibodies + HIV-1 RNA by NAAT. Antibody testing catches established infections; NAAT catches the early window period (2-3 weeks of viremia before antibodies develop). Residual risk ~1:1.5 million.
HCV: anti-HCV antibodies + HCV RNA by NAAT. Same logic - antibody for established, NAAT for window period. Residual risk ~1:1.1 million.
HBV: HBsAg + anti-HBc + HBV DNA by NAAT. Three tests. HBsAg catches acute and chronic infection; anti-HBc catches past or present infection; NAAT catches early window. Triple testing covers carriers, resolved infections, and acute window simultaneously.
HTLV I/II: antibody
West Nile Virus: NAAT only (no donor antibody test). Seasonal mosquito-borne risk, tested year-round with minipool NAAT and individual-donation NAAT during local outbreaks.

HAV and HEV can be transmitted by transfusion during acute viremia but are NOT routinely screened. Both are RNA viruses, usually self-limited. HEV can cause chronic infection in immunocompromised patients.

Classical CJD has never been transmitted by transfusion. Variant CJD (from BSE) has been transmitted - four documented cases in the UK. No screening test exists; prevention relies on donor-history deferral (significant UK/European residence during BSE outbreak years).

Parasitic / bacterial screening

Babesia: NAAT. B. microti is the most common transfusion-transmitted parasite in the US. Intraerythrocytic parasite (survives RBC storage). Geographically concentrated - in Northeastern endemic areas, rate is ~1:1,000 transfusions [TODO: verify]. Universal NAAT testing in endemic states since 2019. Antibody testing was discontinued in 2018. Particularly dangerous in asplenic or immunocompromised recipients.
Trypanosoma cruzi (Chagas disease): NAAT. Causes chronic cardiomyopathy, megaesophagus, megacolon. Significant transfusion risk in Central/South America; in US, donors from endemic areas at risk. All first-time donors are screened.
Syphilis (Treponema pallidum): rarely transmitted - spirochete dies within ~72 hours at 1-6°C (refrigerated RBC storage). Serologic test is still performed, primarily as a surrogate marker for high-risk behaviors.

CMV

CMV is a special case worth understanding in detail:

CMV lives inside monocytes
Leukoreduction removes >99.9% of WBCs and therefore nearly all CMV-carrying monocytes. Leukoreduced products are considered “CMV-safe” and functionally equivalent to CMV-seronegative for most purposes.
CMV antibody testing is performed on a subset of donations (all platelet donations) to maintain an inventory of truly CMV-seronegative products.

CMV-seronegative (or CMV-safe / leukoreduced) products are indicated for:

Transplant candidates and recipients (primary CMV infection is devastating in immunosuppressed patients)
Low birth weight neonates (<1200 g) - immature immune system
Intrauterine transfusion
CMV-seronegative pregnant women
Severe congenital immunodeficiency

Seronegative = donor has no anti-CMV antibodies, implying never infected. In most institutions, leukoreduced products are used interchangeably with CMV-seronegative. The belt-and-suspenders option is to use leukoreduced AND CMV-seronegative products for the highest-risk patients.

4.12 Metabolic complications of massive transfusion

When a patient receives massive transfusion (commonly defined as ≥10 units RBCs in 24 hours, or replacement of one total blood volume), metabolic complications become their own class of problem.

Hypocalcemia (citrate toxicity)

Stored blood products contain citrate as anticoagulant. Citrate chelates calcium.

Citrate toxicity → hypocalcemia. Normally the liver rapidly metabolizes citrate, but massive transfusion overwhelms that capacity.
Also occurs in apheresis donors (platelet apheresis returns citrate-containing blood to the donor) - perioral tingling, muscle cramps, rarely cardiac arrhythmias.
Treatment: IV calcium gluconate; slow the transfusion or apheresis rate.

Hyperkalemia

During RBC storage, K+ leaks from inside cells into the supernatant. Older units and irradiated units have higher extracellular K+.

Neonates and patients with renal failure are at highest risk - neonates because of tiny blood volume (small K+ load has big effect), renal failure patients because they can’t excrete the extra K+.
Prevention: use fresh (<7 days old) units for at-risk patients, or washed units.

Hypokalemia

Paradoxically, hypokalemia can occur as a delayed complication. Transfused RBCs have been depleted of intracellular K+ during storage. Once rewarmed, they actively pump serum K+ back inside, dropping serum levels. Less common than hyperkalemia but worth recognizing.

Hypothermia

Blood products are stored cold (1-6°C for RBCs). Rapid transfusion of cold blood causes hypothermia.

Prevention: blood warmers that heat to <42°C. Above 42°C, hemolysis risk.
Hypothermia is part of the trauma lethal triad: hypothermia + acidosis + coagulopathy.
Hypothermia worsens both hyperkalemia (cold impairs Na/K-ATPase, K+ stays extracellular) and hypocalcemia (cold slows hepatic citrate metabolism, more citrate accumulates and chelates more calcium). Warming blood products helps break this cycle.

Iron overload (chronic transfusion)

Each unit of RBC contains ~200 mg of iron.
The body has no active mechanism to excrete iron.
Clinical iron overload develops around 500 mg/kg cumulative burden - relevant in chronically transfused patients (sickle cell disease, thalassemia, MDS).
Iron deposits in heart (cardiomyopathy), liver (cirrhosis), pancreas (diabetes), endocrine glands (hypogonadism).
Treatment: iron chelation with deferoxamine (IV/SC) or deferasirox (oral). Start when ferritin >1,000 ng/mL or liver iron concentration is elevated. Monitor with ferritin and cardiac MRI T2* for cardiac iron.

4.13 Other special situations

A handful of transfusion-medicine topics don’t fit neatly into reaction categories but are frequently tested.

Massive transfusion with DIC

Classic vignette: postpartum hemorrhage patient receives 8 units RBC, 4 units plasma, 1 unit platelets, then continues to ooze. Labs: Hgb 9, platelets 55k, INR 1.6, fibrinogen <25 mg/dL.

The critical deficit here is fibrinogen - the next step is cryoprecipitate. Each unit of cryoprecipitate provides ~150 mg of fibrinogen; a 10-unit pool raises fibrinogen by ~70 mg/dL. Normal fibrinogen in the third trimester is ~6 g/L (higher than in a non-pregnant patient, which is 2-4 g/L), so in postpartum hemorrhage a “normal-range” fibrinogen may already represent significant consumption.

Also continue RBCs, plasma, and platelets as needed, and treat the underlying cause of the bleeding.

TTP and transfusion

Thrombotic thrombocytopenic purpura (TTP): the classic pentad is FAT RN - Fever, microangiopathic hemolytic Anemia, Thrombocytopenia, Renal dysfunction, Neurological impairment. Not all five are present in every case. Caused by ADAMTS13 deficiency (congenital, or acquired anti-ADAMTS13 autoantibodies), which allows ultra-large vWF multimers to accumulate and form platelet microthrombi.

Treatment: emergent plasmapheresis (therapeutic plasma exchange). Do NOT delay for confirmatory testing - start empirically. Plasma exchange removes the anti-ADAMTS13 antibodies and the ultra-large vWF multimers, and replaces functional ADAMTS13. Also add corticosteroids and rituximab. Platelet transfusion is relatively contraindicated in TTP - it fuels thrombosis.

Pure red cell aplasia after ABO-mismatched stem cell transplant

After an ABO-mismatched stem cell transplant, the recipient’s pre-existing anti-A or anti-B antibodies (half-life ~3 months [TODO: verify - IgG half-life is ~21 days; the long persistence may reflect ongoing plasma cell production rather than antibody half-life]) persist after conditioning and can attack the donor-derived erythroid precursors in the new marrow.

Highest risk: group O recipient receiving an A, B, or AB donor stem cell graft. The O recipient has anti-A, anti-B, and anti-A,B. The graft produces A or B precursors. Until the recipient’s plasma cells die off and the antibodies wane, RBC production is blocked. Usually self-limited.

Daratumumab interference in the blood bank

Daratumumab is an anti-CD38 monoclonal antibody used in multiple myeloma. CD38 is expressed at low levels on ALL RBCs, so daratumumab in the patient’s plasma binds to every test cell during antibody screening.

The pattern:

Pan-reactive antibody screen and panel
Autocontrol often negative (CD38 on the patient’s own RBCs is downregulated by chronic daratumumab exposure) [TODO: verify mechanism; remove or check Nedumcheril 2021 citation]
DAT is frequently negative

This mimics an anti-high-frequency antigen pattern and will halt crossmatching unless recognized.

Resolution:

Treat test RBCs with dithiothreitol (DTT) - DTT breaks disulfide bonds, destroying CD38 on test cells and eliminating the daratumumab interference.
DTT also destroys Kell antigens (Kell has disulfide bonds), so after DTT you cannot detect anti-K. Solution: give Kell-negative blood (91% of the population is K-negative anyway).
If DTT resolves all reactivity, daratumumab was the sole cause. If reactivity persists, there’s a real underlying alloantibody to work up.

Chapter 5: Hemolytic Disease of the Fetus and Newborn (HDFN)

This chapter covers the immune destruction of fetal and neonatal red cells by maternal IgG (HDFN), the platelet equivalent (NAIT), and the neonatal transfusion rules that flow from both. The through-line: the maternal immune system only “sees” fetal antigens when fetal cells cross the placenta, and the resulting IgG is the only thing that can cross back. Everything else - RhIG dosing, antibody monitoring, exchange transfusion, irradiated fresh O-negative neonatal units - is engineering around that single biological fact.

The antibody ID and DAT mechanics live in Chapter 2; we reference them here but do not re-derive.

5.1 The Pathophysiology

HDFN occurs when maternal IgG alloantibodies cross the placenta and destroy fetal red blood cells. This requires:

The mother is alloimmunized (has IgG antibodies to a red cell antigen she lacks)
The fetus inherits the corresponding antigen from the father
The maternal IgG crosses the placenta (IgG is the only immunoglobulin class that crosses; IgM does not)
Maternal antibody binds fetal red cells
Antibody-coated fetal red cells are destroyed (primarily in the fetal spleen)

IgG subclass matters

Not every IgG is equally dangerous. Two factors determine whether a given IgG alloantibody actually causes clinical HDFN: how well it crosses the placenta, and how well it drives hemolysis once it binds.

Placental transport is active, not passive. IgG is transferred mother-to-fetus by the neonatal Fc receptor (FcRn) on syncytiotrophoblast. Binding efficiency ranks IgG1 > IgG4 > IgG3 > IgG2 [TODO: verify - some sources cite IgG1 > IgG3 > IgG4 > IgG2]. IgG2 crosses poorly, which is one reason anti-carbohydrate antibodies (often IgG2) rarely cause significant HDFN even when the titer is high.

Hemolytic activity depends on Fc receptor engagement on splenic macrophages and, for some antibodies, complement activation. Ranking for complement activation: IgG3 > IgG1 >> IgG2, IgG4. IgG3 is the most potent complement activator, which is also why its serum half-life is only ~7 days (vs. ~21 days for IgG1/2/4) - IgG3-coated targets are cleared rapidly and the IgG3 goes with them.

Net effect: IgG1 and IgG3 are the clinically significant subclasses for HDFN. IgG1 crosses best; IgG3 hits hardest once across. IgG3 anti-D tends to produce the most severe HDFN.

The consequences depend on severity:

Mild: Hyperbilirubinemia requiring phototherapy
Moderate: Significant anemia, hydrops fetalis (extramedullary hematopoiesis causes hepatosplenomegaly; severe anemia causes high-output cardiac failure)
Severe: Fetal or neonatal death

One thing to keep straight: in utero, the placenta clears bilirubin for the fetus, so severe hemolysis shows up mainly as anemia and hydrops. After birth, the placenta is gone and the neonatal liver cannot conjugate fast enough. Unconjugated bilirubin then crosses the blood-brain barrier and deposits in basal ganglia (kernicterus). This is why neonatal management shifts from “treat the anemia” (in utero) to “drop the bilirubin” (after birth).

5.2 Causes of HDFN

Current ranking of severe HDFN causes

Before RhIG, anti-D was the runaway #1. In the post-RhIG era, the ranking has shifted. For boards, memorize the current order of causes of severe HDFN:

Anti-K (Kell)
Anti-c
Anti-D

Anti-D is still common and still dangerous when it occurs - but RhIG prophylaxis has taken it out of the top spot. There is no RhIG-equivalent for K or c, which is why they have risen. Prevention for K in particular depends on transfusing K-negative blood to girls and women of childbearing age (see Chapter 2).

Anti-D (Rh HDFN)

Rh antigens are protein antigens. That matters: protein antigens drive T-cell-dependent IgG responses, so anti-D is IgG and forms only after sensitization - it is not naturally occurring. Contrast with ABO, where the antigens are carbohydrates and IgM antibodies appear without prior red cell exposure.

Before RhIG prophylaxis, anti-D was the leading cause of severe HDFN, causing thousands of fetal and neonatal deaths annually.

Why D is so problematic:

D is the most immunogenic non-ABO antigen. ~80% of D-negative individuals exposed to D-positive blood make anti-D. (Compare: K, the next most immunogenic, sensitizes only ~5% of K-negative recipients.)
Approximately 15% of people of European ancestry are D-negative, so mismatched pregnancies are common
Anti-D is almost always IgG (IgG1 ± IgG3)

Why first pregnancies are usually spared: the bulk of fetomaternal hemorrhage happens at delivery. By the time the primary immune response ramps up, delivery is over. The second pregnancy is where trouble appears - memory B cells produce a high-titer anamnestic IgG response early, and that IgG crosses the placenta. Exception: a D-negative woman previously sensitized by a D-positive transfusion can have anti-D during her first pregnancy.

Sensitizing events in D-negative mothers - any fetomaternal hemorrhage will do:

Normal delivery (most common; detectable FMH in roughly half of deliveries)
Chorionic villus sampling, amniocentesis, cordocentesis
Abortion (including threatened), miscarriage, ectopic
Placental abruption, antepartum hemorrhage
Abdominal trauma, external cephalic version

Without RhIG, ~15% of D-negative mothers carrying a D-positive fetus will become sensitized during or after the first pregnancy. That 15% is the number RhIG was designed to zero out.

With RhIG prophylaxis, Rh HDFN has become rare, but still occurs when:

RhIG is not given or is given inadequately
Massive fetomaternal hemorrhage exceeds RhIG coverage
Alloimmunization occurred before RhIG was available

Weak D vs. partial D [HIGH YIELD]

This distinction is board-testable and clinically load-bearing for pregnancy.

Feature	Weak D	Partial D
Quantity of D antigen	Decreased	Normal
Structure of D antigen	Normal	Mutated (missing epitopes)
Initial typing (immediate spin)	D-negative	D-positive
Confirmation	IAT with anti-D → reacts → weak D	Same typing, but can form anti-D
Can the patient make anti-D?	No (self)	Yes (foreign epitopes on wild-type D)
Needs RhIG in pregnancy?	No (for weak D types 1, 2, 3)	Yes - treat as D-negative
Can receive D+ blood?	Yes (for weak D types 1, 2, 3)	No - give D-negative

The easy way to keep them straight: weak D = less normal antigen; partial D = normal amount of mutated antigen. Weak D is a quantity problem; partial D is a quality problem. The partial-D patient types D-positive on routine reagents, so it is easy to miss - increasingly, RHD genotyping is used in women of childbearing age to catch partial D and ensure they get RhIG.

Anti-K (Kell HDFN)

Anti-K causes HDFN by a unique mechanism: in addition to hemolysis, anti-K suppresses fetal erythropoiesis. This is because Kell antigen is expressed on early erythroid progenitors.

Kell Paradox: Kell-affected fetuses can have severe anemia with surprisingly NORMAL bilirubin levels because the primary mechanism is suppression of erythropoiesis (not hemolysis). This means standard monitoring tools underestimate severity:

Anemia may be more severe than predicted by antibody titer or bilirubin
Reticulocyte count may be inappropriately low
Amniocentesis for bilirubin (ΔOD450) underestimates severity
MCA Doppler (middle cerebral artery peak systolic velocity) is preferred for monitoring

Because of that dual mechanism, Kell uses a lower critical titer of 1:8 (vs. 1:16 for anti-D and most others). At 1:8 or above, move to MCA Doppler monitoring.

Severity by antibody

Antibody	HDFN severity	Notes
Anti-K	Severe	Suppresses erythropoiesis; critical titer 1:8
Anti-c	Severe	#2 severe cause post-RhIG; may evanesce
Anti-D	Severe	Classic cause; now rare with RhIG
Anti-C	Moderate to severe
Anti-E	Usually mild	If anti-E present, always check for anti-c
Anti-Fya, anti-Fyb	Moderate
Anti-Jka, anti-Jkb	Mild / rare	Kidd poorly expressed on fetal RBCs
Anti-A,B (IgG, type O mother)	Mild	See ABO section

Anti-E and the hidden anti-c

Clinical rule: if you find anti-E, suspect anti-c as well. The common genotype that makes anti-E is R1R1 (DCe/DCe), which lacks both E and c. When such a patient is transfused with R2-containing blood (DcE), she is exposed to E and c simultaneously, and both antibodies can form. Anti-E is more readily detected; anti-c often lags but matters far more clinically because anti-c is a major cause of severe HDFN. Anti-c also has the annoying habit of evanescing, so a negative screen now does not rule out a prior anti-c - it is a classic cause of delayed hemolytic transfusion reaction.

Anti-G and the pregnancy-specific problem

G is a conformational antigen present whenever D or C is present. If you have D, you have G. If you have C, you have G. Only patients lacking both D and C (genotypes r/r, r/r’‘, or r’‘/r’’) can form anti-G.

The problem: anti-G looks like anti-D + anti-C on routine antibody panels. It reacts with every D-positive cell and every C-positive cell, indistinguishable serologically from the combination.

Why does the distinction matter? Not for transfusion - either way you transfuse D-negative, C-negative blood, which is also G-negative. The distinction matters in pregnancy:

Anti-G only: patient is not sensitized to D → still needs RhIG.
True anti-D (± anti-C): patient is already sensitized → RhIG is contraindicated (useless, and the point of RhIG - prevent sensitization - is moot).

Definitive distinction requires double adsorption studies:

Adsorb the serum with D+ C- cells (R2R2, DcE/DcE): these cells carry D and G but not C. Anti-D and anti-G stick to them; anti-C does not. Eluting off these cells gives you a mixture of anti-D and anti-G. If the supernatant still reacts with C-positive cells, anti-C is present in the original serum.
Adsorb with D- C+ cells (r’r, dCe/dce): these cells carry C and G but not D. Anti-C and anti-G stick; anti-D does not. If the supernatant still reacts with D-positive cells, true anti-D is present (and the patient is actually sensitized).

By checking what remains in each supernatant (and what elutes off), you can tell whether true anti-D is present. If neither supernatant has anti-D activity, the apparent “anti-D + anti-C” pattern was really anti-G alone, and the patient still benefits from RhIG.

Anti-f (ce compound antigen)

f is a compound antigen defined by c and e on the same haplotype (cis). Not “having both c and e” - they must be inherited together on a single chromosome. Haplotypes that carry ce in cis: R0 (Dce) and r (dce). Anyone with at least one R0 or r has f antigen and cannot make anti-f.

Common anti-f producers: R1R1, R1R2, R2R2 - these carry c and e, but on different chromosomes (trans), so f is not formed. Anti-f is clinically significant and can cause hemolysis and HDFN.

Kidd in HDFN

Although Kidd antibodies (anti-Jka, anti-Jkb) are IgG and clinically significant for transfusion, they rarely cause significant HDFN. Reason: Kidd antigens are weakly expressed on fetal RBCs. Not enough antigen on the cell surface to support meaningful antibody binding.

Kidd merits extra mention because it is the antibody system most likely to be missed on pre-transfusion testing and to cause delayed hemolytic transfusion reactions in mothers and neonates alike:

Evanesce rapidly - titers drop below detection between exposures
Lose potency in stored serum (complement-dependent activity degrades)
Show dosage - react much more strongly with homozygous cells (Jka/Jka) than heterozygous cells (Jka/Jkb)

Kidd glycoprotein is a urea transporter. It is also expressed in the renal medulla, where it participates in the urinary concentrating mechanism. Kidd-null individuals have a mild urine-concentrating defect as a result, which is rarely clinically symptomatic but is a classic board-style association.

The same urea-transport function drives the Kidd-null lysis test: Jk(a-b-) cells resist lysis in 2M urea. Normal cells take up urea via the transporter, water follows, cell swells and lyses; null cells exclude urea and stay intact. Jk(a-b-) is rare, most often seen in Finnish and Polynesian (Pacific Islander) ancestry, and in a small Japanese subset via a dominant In(Jk) inhibitor gene (analogous to In(Lu) for Lutheran). Kidd-null individuals can form anti-Jk3, an antibody to the underlying Kidd glycoprotein itself.

Kidd antigen frequencies vary by ancestry/population. Jk(b)-negative phenotypes are more common in people of African ancestry, while Jk(b)-positive phenotypes are more common in people of European ancestry. This matters for antibody identification and for sourcing antigen-negative units: anti-Jkb is more common in patients with African ancestry, and antigen-negative donors may need to be drawn from a matched-ancestry pool.

Full coverage lives in Chapter 1 and Chapter 2; the HDFN-relevant point is that Kidd is mostly a transfusion problem, not an HDFN problem.

Dosage also bites in another direction: serologic crossmatch can be falsely negative if the donor RBC is heterozygous for a dosage-dependent antigen. This is a reason some centers prefer electronic crossmatch (checking antigen typing directly) when a previously identified dosage antibody is in the patient history.

ABO HDFN

ABO incompatibility between mother and fetus is very common (about 20% of pregnancies), but ABO HDFN is usually mild because:

A and B antigens are not fully developed on fetal red cells
Anti-A and anti-B are mostly IgM (don’t cross placenta); the IgG component is relatively weak
A and B antigens are widely distributed in tissues (not just red cells), so antibody is “absorbed” by other tissues

ABO HDFN:

Most common cause of HDFN overall (but most cases are mild)
Typically presents as neonatal jaundice requiring phototherapy
Rarely requires exchange transfusion
Can occur in the first pregnancy (unlike Rh HDFN, which usually requires prior sensitization)
DAT may be only weakly positive or negative (low antibody density on cells)

The key setup: only group O mothers cause clinically meaningful ABO HDFN. Group A mothers make anti-B that is almost entirely IgM. Group B mothers make anti-A that is almost entirely IgM. IgM does not cross the placenta. Group O mothers, by contrast, produce an antibody known as anti-A,B - a single antibody with IgG component that recognizes a determinant shared by both A and B antigens. That IgG anti-A,B is the one that crosses. The corollary: the at-risk pairing is mother O, fetus A or B. AB fetuses happen but require either a non-O father with appropriate alleles or, in rare cases, the cis-AB or Bombay exceptions.

No prior sensitization is required, because anti-A,B IgG is already present in type O mothers - driven not by fetal exposure but by lifelong environmental exposure to A- and B-like carbohydrate structures on bacteria and dietary antigens. That is why ABO HDFN affects the first pregnancy just as readily as later ones.

5.3 Prevention of Rh HDFN: RhIG

Rh immune globulin (RhIG) is one of the great success stories of modern medicine. It prevents Rh sensitization through a mechanism called antibody-mediated immune suppression - when passively administered anti-D coats fetal D-positive cells in the maternal circulation, those cells are cleared before they can stimulate the maternal immune system.

Timing of Administration

Antepartum (routine):

At 26-28 weeks gestation (covers the period when fetomaternal hemorrhage becomes more common). Dose: 1 vial (300 μg).

Postpartum:

Within 72 hours of delivery of a D-positive or D-unknown infant. Standard starting dose: 1 vial (300 μg). More may be needed once FMH is quantified.
Can be given up to 13-28 days with partial protection. The 72-hour window is for maximum efficacy, not a hard cutoff. If you blew past 72 hours, still give it; partial protection is better than none.

After sensitizing events:

Amniocentesis, chorionic villus sampling, cordocentesis
Abdominal trauma
External cephalic version
Miscarriage, ectopic pregnancy, abortion
Antepartum hemorrhage, placental abruption

Dose for each of these: start with 1 vial (300 μg) within 72 hours; quantify FMH and add vials if needed.

When NOT to give RhIG

Three situations where RhIG is either wrong or pointless:

Mother already sensitized (anti-D on screen from true alloimmunization, not from a prior RhIG dose). RhIG cannot “un-sensitize” memory B cells. Giving it is useless.
Sensitizing event before 12 weeks gestation. D antigen is poorly expressed on fetal RBCs in the first trimester [TODO: verify - some sources cite earliest detection ~38 days post-conception], and the volume of FMH at that gestational age is small. Early miscarriage before 12 weeks does not require RhIG by most guidelines. (Some centers give it anyway for simplicity.)
Postpartum after cord blood confirms baby is truly D-negative (including weak D negative). If the cord blood weak-D test is also negative, the postpartum dose is not needed. Note: weak D positive cord cells still sensitize the mother, so weak D testing on cord blood is mandatory before skipping the dose.

A pragmatic rule for the 26-28 week dose: we give it because fetal D status is usually unknown at that gestational age. If the father is confirmed D-negative (or cell-free fetal DNA confirms a D-negative fetus), the routine dose is not required.

Jehovah’s Witnesses and RhIG: RhIG is a plasma-derived immunoglobulin, not a red cell or whole-blood product. Most Jehovah’s Witnesses will accept it, though individual preferences vary. Ask the patient directly.

Partial D and weak D in RhIG decisions

Partial D women must be treated as D-negative for RhIG purposes - they type D-positive on routine reagents but can be sensitized to the wild-type D epitopes they lack. Give RhIG at all standard timepoints. For transfusion, give them D-negative blood.

Weak D types 1, 2, 3 are treated as D-positive - their D protein is structurally normal, just reduced in quantity, so they cannot form anti-D. No RhIG needed. They can receive D-positive blood.

This distinction was covered in the weak vs. partial table above; it comes up repeatedly on exam questions and is the single most common RhIG trap.

Dosing

Standard dose: 300 μg (1500 IU) of RhIG

This covers approximately 15 mL of fetal red cells (30 mL of whole blood).

For larger fetomaternal hemorrhage (FMH), additional doses are needed.

Quantifying Fetomaternal Hemorrhage

The workflow is two-step: screen qualitatively, then quantify if positive.

Rosette test (qualitative screen):

Detects FMH of ≥10 mL of fetal red cells [TODO: verify - threshold often cited as ~15 mL fetal RBCs / 30 mL whole blood, the volume covered by a single vial]. Anything less than that is considered covered by a single 300 μg RhIG vial.
Principle: maternal blood is incubated with anti-D reagent, which coats any D-positive fetal cells present. D-positive indicator cells are then added. The anti-D on the fetal cells bridges them to the indicator cells, forming visible rosettes (clusters) around each fetal cell under the microscope.
Negative result: no large FMH - the standard 1-vial postpartum dose is sufficient.
Positive result: significant FMH present - proceed to quantitative testing.
Many centers also give a precautionary second vial (total 2 vials / 600 μg) immediately when the rosette is positive, while the KB or flow result is pending.

If rosette screen is positive, quantify with:

Kleihauer-Betke (acid elution) test: based on the principle that fetal hemoglobin (HbF) is resistant to acid elution, while adult hemoglobin (HbA) is not. HbF has α2γ2 globin chains vs. α2β2 for HbA; the γ-chain structure resists the acid.

A maternal blood smear is treated with acid (citric-acid phosphate buffer), then stained with eosin.
Adult RBCs appear as “ghosts” - pale, empty, hemoglobin washed out.
Fetal RBCs retain HbF, stain pink/orange, appear intact.
Count fetal cells per 2000 maternal cells to get a percentage.

Kleihauer-Betke pitfall: any maternal condition with elevated HbF causes false positives. Sickle cell trait, sickle cell disease, β-thalassemia, and hereditary persistence of fetal hemoglobin (HPFH) all drive up the count and overestimate FMH. In these patients, flow cytometry (which can target HbF-specific cells with additional markers) is preferred.

Flow cytometry: More precise method using anti-HbF antibodies (± anti-D to specifically identify fetal D-positive cells). More reproducible than KB, less subject to interobserver variability, and less affected by maternal HbF conditions. Increasingly used as primary quantification.

RhIG dose calculation after FMH

Four steps. Commit these to muscle memory; it is a board favorite.

Maternal blood volume (mL) = weight (kg) × 70
FMH volume (mL) = maternal blood volume × % fetal cells (expressed as decimal, from KB or flow)
Vials needed = FMH volume ÷ 30 mL, rounded (round down if decimal < 0.5, round up if decimal ≥ 0.5)
Add 1 vial as a safety margin

Each 300 μg vial covers 30 mL of fetal whole blood (equivalently 15 mL of packed fetal RBCs).

Worked example: 70 kg mother, KB shows 1.6% fetal cells, one vial of RhIG already given postpartum.

Step 1: Blood volume = 70 × 70 = 4900 mL
Step 2: FMH = 4900 × 0.016 = 78.4 mL
Step 3: 78.4 ÷ 30 = 2.61 → round up to 3
Step 4: 3 + 1 = 4 vials total
Already given 1 vial → 3 additional vials needed

Watch the wording on exam questions: they sometimes ask for “total vials” and sometimes “additional vials.” Read carefully.

5.4 Management of the Alloimmunized Pregnancy

Antibody Monitoring

Once alloimmunization is detected:

Determine antibody specificity and titer
Determine the father’s antigen status and zygosity (is the fetus at risk?)
If father is antigen-negative: Fetus is not at risk
If father is homozygous positive: Fetus is definitely at risk
If father is heterozygous: 50% chance fetus is at risk → consider fetal testing
Cell-free fetal DNA testing can determine fetal antigen status from maternal blood

Titer Monitoring

If the fetus is at risk:

Monitor antibody titers every 2-4 weeks
“Critical titer” (varies by laboratory, typically 16-32): Above this level, significant HDFN is possible
Below critical titer: Continue monitoring
At or above critical titer: Move to MCA Doppler monitoring

Fetal Monitoring

Middle Cerebral Artery (MCA) Doppler: Non-invasive assessment of fetal anemia

In anemic fetuses, cardiac output increases (to compensate) and blood viscosity decreases, resulting in increased blood flow velocity
MCA peak systolic velocity >1.5 MoM (multiples of the median for gestational age) suggests moderate to severe anemia
Serial MCA Doppler measurements guide decision-making about intervention

Amniocentesis: Historical method using ΔOD450 (optical density deviation at 450nm, indicating bilirubin level). Now largely replaced by MCA Doppler.

Fetal Intervention

Intrauterine transfusion (IUT): Direct transfusion of red cells to the fetus

Usually intravascular (into umbilical vein under ultrasound guidance)
Uses O-negative (or antigen-negative), irradiated, CMV-safe, leukoreduced, fresh RBCs
Hematocrit targeted to be high (80-85%) since volume is limited
May need to be repeated every 1-3 weeks until delivery

5.5 Neonatal Management

Assessment at Birth

Standard cord blood testing at delivery:

ABO/Rh type (drives maternal RhIG decision and product selection for the baby)
Direct antiglobulin test (DAT) (detects maternal IgG already bound to fetal RBCs)
Hemoglobin/hematocrit
Bilirubin (total and direct)
Evaluate for hydrops (edema, ascites, pleural effusions)

If cord blood is D-negative by routine testing, weak D testing is required before deciding whether the D-negative mother needs postpartum RhIG. A weak D positive baby will sensitize the mother; you must give RhIG in that case.

Interpreting the DAT-positive cord blood

DAT-positive cord blood means maternal IgG has attached to fetal RBCs. What you do next depends on the maternal antibody screen:

Cord blood DAT positive + maternal antibody screen positive: confirms immune-mediated HDFN.

Run a maternal antibody panel to identify the offending antibody (guides future pregnancies and, if severe, antigen-selection for exchange transfusion).
Serial neonatal bilirubin measurements. Unconjugated bilirubin rises as hemolysis continues; phototherapy or exchange transfusion thresholds depend on gestational age, age in hours, and risk factors.

Cord blood DAT positive + maternal antibody screen negative: three classic explanations:

ABO HDFN. Standard antibody screens use group O screening cells, so anti-A,B from a type O mother will not react on them. The DAT is positive because anti-A,B is bound to the baby’s A or B cells in vivo. A cord blood eluate tested against A1 and B cells confirms the diagnosis.
Passively administered RhIG. Antepartum RhIG at 28 weeks crosses the placenta and coats any D-positive fetal cells. The DAT picks this up. This is not maternal sensitization; it is the RhIG doing exactly what it is supposed to do. No treatment needed beyond standard neonatal monitoring.
Low-incidence antigen. Maternal antibody is directed against a paternal low-incidence antigen not represented on standard screening cells. Extend the antibody panel, test paternal and neonatal cells, and identify the antibody.

Neonatal and intrauterine transfusion requirements

The constraints are different from adult transfusion because neonates have tiny blood volumes, immature immune systems, and labile electrolytes. For IUT and neonatal transfusion specifically:

Fresh (<5 day old) red cells - minimizes extracellular potassium, which accumulates during storage. Even a small potassium load can cause cardiac arrhythmia in a neonate.
Irradiated - prevents transfusion-associated graft-versus-host disease (TA-GVHD). Neonates, especially premature or those receiving IUT, are functionally immunocompromised and cannot clear donor T cells.
CMV-safe - either CMV-seronegative or leukoreduced. CMV lives in leukocytes; neonatal CMV is a real hazard.
Leukoreduced.
Group O - avoids ABO incompatibility with any maternal antibodies still circulating in the neonate.
Antigen-negative for any maternal alloantibody - if mom has anti-K, transfuse K-negative cells. If mom has anti-c, transfuse c-negative cells. Otherwise the transfused cells get destroyed on arrival.

For IUT specifically, cells are transfused at a high hematocrit (80-85%) because the intravascular volume delivered is limited by the fetus’s size. Transfusion is intravascular (umbilical vein under ultrasound guidance) and may be repeated every 1-3 weeks until delivery.

Pediatric transfusion dosing (weight-based because total blood volume varies enormously: a 3 kg neonate has ~240 mL of total blood volume, compared to ~5000 mL in an adult):

Product	Dose	Expected increment
RBCs	10 mL/kg	Hgb up ~2-3 g/dL
FFP	10 mL/kg	Factors up ~15-20%
Platelets	10 mL/kg	Count up ~50-100K
Cryoprecipitate	0.1 unit/kg	Fibrinogen up ~60-100 mg/dL

Platelets for neonates: must be ABO plasma-compatible. This is the one place where plasma compatibility genuinely matters for platelet transfusion. Adults tolerate a unit of ABO-mismatched platelet plasma fine in most cases. Neonates receive a large plasma dose relative to their total blood volume - incompatible anti-A or anti-B in the platelet plasma can hemolyze neonatal red cells. Always use ABO plasma-compatible (or identical) platelets in neonates.

Treatment of Hyperbilirubinemia

Phototherapy: First-line treatment

Blue light (wavelength 460-490 nm) converts unconjugated bilirubin to water-soluble photoisomers that can be excreted without conjugation
Threshold for treatment depends on gestational age, age in hours, and risk factors

Exchange transfusion: For severe cases unresponsive to phototherapy, or when bilirubin levels approach dangerous thresholds

Removes antibody-coated red cells, free antibody, and bilirubin
Replaces with antigen-negative, compatible red cells
Usually “double volume exchange” (twice the infant’s blood volume, ~170 mL/kg)
Uses O-negative (or specifically antigen-negative), irradiated, CMV-safe red cells

IVIG: May reduce need for exchange transfusion by blocking Fc receptors on macrophages, reducing hemolysis

5.6 Neonatal Alloimmune Thrombocytopenia (NAIT)

NAIT is the platelet equivalent of HDFN - maternal antibodies against fetal platelet antigens cause fetal/neonatal thrombocytopenia. However, there are critical differences that make NAIT more dangerous in some ways.

Before getting into NAIT, distinguish it from the other cause of neonatal thrombocytopenia the exam likes to mix in: maternal ITP. The mechanism is different and the management is different. Keep them straight.

NAIT vs. neonatal thrombocytopenia from maternal ITP

Feature	Maternal ITP → neonate	NAIT
Antibody type	Autoantibody (mother’s own platelets also targeted)	Alloantibody (mother’s platelets unaffected)
Antigen target	Universal GP, usually GPIIb/IIIa	Specific HPA, usually HPA-1a
Mother’s platelet count	Low (she has ITP)	Normal
Severity in baby	Usually mild, self-limited	Severe; high ICH risk
Delivery	Vaginal usually fine	Cesarean often recommended
Platelet transfusion	Random donor, only if bleeding	Antigen-negative (or washed maternal)
First-line Rx	Supportive care	IVIG ± corticosteroids

Maternal ITP mechanism: mother has autoantibodies (most commonly against the universal platelet glycoprotein GPIIb/IIIa), those IgG autoantibodies cross the placenta, and they react with the baby’s platelets too because every platelet expresses GPIIb/IIIa. Usually self-limited - maternal antibody clears from neonatal circulation over weeks. Platelet transfusion is only indicated if there is active bleeding, and because the autoantibody targets a universal antigen, random donor platelets work (they will be destroyed just as fast as the native platelets, but they provide transient hemostatic support). In NAIT, by contrast, the mother’s platelets are normal, because her alloantibody targets an HPA antigen she herself lacks.

Pathophysiology

The mechanism parallels HDFN:

The fetus inherits a platelet antigen from the father that the mother lacks
Fetomaternal hemorrhage exposes the mother to fetal platelets
The mother develops IgG antibodies against the foreign platelet antigen
Maternal IgG crosses the placenta and destroys fetal platelets

The critical difference from HDFN: NAIT often affects the FIRST pregnancy. Unlike Rh HDFN, which requires prior sensitization, the amount of fetal platelets reaching maternal circulation during pregnancy is sufficient to cause sensitization and antibody production within the same pregnancy. About 50% of NAIT cases occur in first pregnancies.

The HPA System

Human Platelet Antigens (HPAs) are polymorphisms in platelet membrane glycoproteins. The most important are:

Antigen	Glycoprotein	Clinical Significance
HPA-1a (PlA1)	GPIIIa (integrin β3)	Causes ~80% of NAIT cases
HPA-1b (PlA2)	GPIIIa	Antithetical to HPA-1a
HPA-5b (Bra)	GPIa (integrin α2)	Second most common cause
HPA-3a	GPIIb	Less common
HPA-4a	GPIIIa	Rare

HPA-1a incompatibility is the classic scenario:

Mother is HPA-1a negative (HPA-1b/1b) - about 2% of people of European ancestry
Father is HPA-1a positive
Fetus inherits HPA-1a from father
Mother makes anti-HPA-1a

Clinical Presentation

Fetal/neonatal thrombocytopenia: Can be severe (<20,000/μL)

Intracranial hemorrhage (ICH): The devastating complication

Occurs in 10-20% of NAIT cases with severe thrombocytopenia
Can occur in utero (unlike HDFN, where severe anemia is the main fetal risk)
Leads to death or permanent neurological damage
ICH can occur as early as 16-20 weeks gestation

Petechiae, purpura, bleeding: At birth

Unlike HDFN: There is no readily available screening test or prophylaxis equivalent to RhIG. NAIT is usually diagnosed AFTER an affected baby is born.

Diagnosis

Clinical suspicion: Unexplained neonatal thrombocytopenia, especially if severe and in an otherwise healthy full-term infant

Laboratory confirmation:

Maternal platelet antigen typing (HPA genotyping)
Paternal platelet antigen typing
Detection of maternal anti-platelet antibodies (MAIPA - monoclonal antibody immobilization of platelet antigens)
Confirmation of incompatibility between maternal and fetal platelet types

Differential diagnosis:

Neonatal sepsis
Congenital infection (TORCH)
Maternal ITP (autoantibodies, not alloantibodies; affects mother too)
Congenital thrombocytopenia syndromes

Management

Affected neonate:

Platelet transfusion: Ideally with HPA-compatible platelets (antigen-negative for the implicated antigen)
If HPA-typed platelets unavailable, random donor platelets may provide partial, temporary increment
Maternal platelets can be washed (to remove antibody-containing plasma) and irradiated - they lack the offending antigen
IVIG: May raise platelet count by blocking Fc receptors on macrophages

Subsequent pregnancies (recurrence risk 75-90% with 50% chance of more severe disease):

Fetal blood sampling to monitor platelet count (high-risk procedure)
Maternal IVIG during pregnancy (may improve fetal platelet count)
Intrauterine platelet transfusions if severe thrombocytopenia
Early delivery if severe, to enable postnatal treatment
Cesarean section may be recommended to avoid birth trauma if platelet count is very low

Key Differences: NAIT vs. HDFN

Feature	NAIT	HDFN
Target cells	Platelets	Red blood cells
Most common antibody	Anti-HPA-1a	Anti-D
First pregnancy affected	Yes (~50%)	Rarely (requires prior sensitization)
In utero bleeding risk	High (ICH)	Low
Prophylaxis available	No	Yes (RhIG)
Screening	Not routine	Routine (blood type)
Severity in subsequent pregnancies	Often worse	Often worse

Chapter 6: Autoimmune Hemolytic Anemia (AIHA)

Before discussing the specific types of AIHA, it’s essential to understand how to distinguish intravascular from extravascular hemolysis in the laboratory - a distinction that has direct clinical implications.

Laboratory Differentiation of Hemolysis Types

Finding	Intravascular Hemolysis	Extravascular Hemolysis
Primary site	Bloodstream (complement-mediated)	Spleen/liver (RES macrophages)
Haptoglobin	↓↓ (absent - saturated by free Hgb)	↓ (moderate decrease)
LDH	↑↑ (marked elevation)	↑ (moderate elevation)
Indirect bilirubin	↑	↑
Hemoglobinemia	Present (pink/red plasma)	Absent
Hemoglobinuria	Present (dark urine, + blood dipstick, no RBCs)	Absent
Hemosiderinuria	Present (late finding)	Absent
Spherocytes	Variable	Often present (from partial phagocytosis)

Clinical correlation: Intravascular hemolysis releases free hemoglobin directly into plasma, which saturates haptoglobin (causing it to disappear), is filtered by kidneys (hemoglobinuria), and can cause renal tubular injury. Extravascular hemolysis occurs when macrophages remove antibody-coated or damaged cells intact; the hemoglobin is metabolized inside cells to bilirubin, never reaching plasma as free hemoglobin.

6.1 Warm Autoimmune Hemolytic Anemia

Warm autoantibody panel: Pan-reactivity (positive with all panel cells) at 37°C AHG phase. The autoantibody reacts with a high-frequency antigen present on all cells.

Pathophysiology

In warm AIHA, the patient’s immune system produces IgG autoantibodies that bind to their own red blood cells at body temperature (37°C). These antibody-coated red cells are recognized by Fcγ receptors on splenic macrophages and destroyed - primarily through partial phagocytosis, which creates spherocytes.

Why are spherocytes formed? When a macrophage “bites” a piece of antibody-coated red cell membrane but the cell escapes, the remaining cell has reduced surface area but the same volume. It becomes spherical - the shape with the lowest surface area-to-volume ratio. These spherocytes are rigid and become trapped in the splenic cords, leading to further destruction.

Causes

Primary (idiopathic): ~50% of cases; no underlying cause identified
Secondary:
Lymphoproliferative disorders (CLL, lymphoma)
Autoimmune diseases (SLE, rheumatoid arthritis)
Drugs (methyldopa, fludarabine)
Infections

Laboratory Findings

Evidence of hemolysis: ↓ Hgb, ↑ reticulocytes, ↑ LDH, ↑ indirect bilirubin, ↓ haptoglobin
Spherocytes on peripheral smear
DAT positive: Usually IgG positive (±C3)
Antibody screen and panel may be positive (autoantibody reacts with all cells)

Blood Bank Challenges

Finding compatible blood for warm AIHA patients can be difficult:

Autoantibody may react with all cells in the antibody screen
Must rule out underlying alloantibodies (patients with AIHA often have alloantibodies from prior transfusions)
Use autoadsorption or alloadsorption to remove autoantibody and unmask alloantibodies

Autoadsorption: Patient’s own cells (warmed to elute autoantibody, then used to adsorb remaining autoantibody from serum). Cannot be used if patient was recently transfused.

Alloadsorption: R1R1, R2R2, and rr cells used to adsorb autoantibody while preserving alloantibodies of those specificities.

Treatment

Corticosteroids: First-line; prednisone 1-1.5 mg/kg/day
Rituximab: Anti-CD20; often second-line
Splenectomy: Removes primary site of red cell destruction
Immunosuppressants: Azathioprine, mycophenolate, etc.
Transfusion: When needed, give “least incompatible” blood; don’t withhold blood for life-threatening anemia because you can’t find compatible blood

6.2 Cold Agglutinin Disease (CAD)

Cold autoantibody panel: Note reactivity at immediate spin (IS) and room temperature phases, with decreased or absent reactivity at 37°C AHG phase.

Pathophysiology

In CAD, the patient has IgM autoantibodies (cold agglutinins) that bind to red cells at temperatures below body temperature. The classic antibody is anti-I, which binds to the I antigen (a carbohydrate structure on adult red cells).

The mechanism of hemolysis:

Blood flows through cool peripheral tissues (fingers, toes, ears, nose)
IgM binds to red cells in the cold and activates complement (C1 through C3)
Blood returns to warm central circulation; IgM dissociates (poor binding at 37°C)
Complement activation stops at C3b (regulatory proteins prevent progression to MAC)
C3b-coated red cells are recognized by complement receptors on hepatic macrophages
Red cells are destroyed in the liver (extravascular hemolysis) or converted to spherocytes
Some intravascular hemolysis occurs if complement proceeds to C5-9

Causes

Primary: Clonal B-cell disorder producing monoclonal IgM cold agglutinin
Secondary:
Mycoplasma pneumoniae infection (produces anti-I; typically polyclonal, transient)
EBV infection (may produce anti-i, targeting fetal-type antigen)
Lymphoproliferative disorders

Laboratory Findings

DAT: C3d positive, IgG negative (IgM dissociates during washing)
May see red cell agglutination at room temperature (must warm sample to get accurate CBC)
Cold agglutinin titer (significant if ≥64 at 4°C)
Thermal amplitude (temperature range of reactivity) determines clinical significance - wider amplitude = worse disease

Blood Bank Challenges

Cold agglutinins interfere with testing:

May cause ABO discrepancies (agglutination in reverse typing)
May cause positive antibody screen at room temperature

Solutions:

Keep samples warm (37°C)
Pre-warm technique for crossmatching
Identify underlying alloantibodies using pre-warmed or enzyme-treated cells

Treatment

Avoid cold exposure (most important)
Corticosteroids are usually ineffective
Rituximab (targets the B-cell clone)
Splenectomy is not helpful (destruction is primarily hepatic)
Complement inhibitors (sutimlimab) recently approved
Transfuse through blood warmer if needed

6.3 Paroxysmal Cold Hemoglobinuria (PCH)

Pathophysiology

PCH is caused by the Donath-Landsteiner antibody, an unusual IgG that binds red cells at cold temperatures, fixes complement, and causes intravascular hemolysis when blood warms.

The antibody has anti-P specificity (reacts with P antigen, present on nearly everyone’s red cells).

The biphasic nature:

In cold (peripheral circulation): IgG antibody binds red cells and fixes complement through C3
Blood warms (central circulation): Complement cascade proceeds to MAC, causing intravascular hemolysis

Why doesn’t the IgG stay bound? The Donath-Landsteiner antibody has very low affinity at warm temperatures and rapidly dissociates.

Clinical Context

PCH occurs in two settings:

Children following viral infections (most cases today) - typically self-limited
Adults with syphilis (historical association; rare today)

Laboratory Findings

DAT: C3d positive only (similar to CAD)
Donath-Landsteiner test: Demonstrates biphasic hemolysis
Incubate patient serum with red cells at 4°C (antibody binds, fixes complement)
Warm to 37°C (lysis occurs)
Control tubes at only 4°C or only 37°C do not lyse

Treatment

Usually supportive; self-limited in children
Keep patient warm
Transfusion if needed (difficult to find P-negative blood; use standard compatible blood through warmer)

6.4 Drug-Induced Hemolytic Anemia

Drugs can cause a positive DAT and hemolytic anemia through several mechanisms:

Drug Adsorption (Hapten) Mechanism

Classic example: Penicillin
Drug binds tightly to red cell membrane
Antibody is directed against the drug (not the red cell)
Only drug-coated cells are destroyed
DAT: IgG positive
Requires high-dose drug therapy
Resolves when drug is stopped

Immune Complex (Innocent Bystander) Mechanism

Examples: Quinidine, quinine, certain NSAIDs
Drug-antibody immune complexes form in plasma
Complexes bind to red cells and activate complement
DAT: Usually C3 positive (IgG may be positive too)
Can cause severe intravascular hemolysis
Low drug doses can trigger reaction

Autoantibody Induction

Classic example: Methyldopa (also fludarabine, procainamide)
Drug induces true autoantibodies (not drug-dependent)
Autoantibodies react with red cells even without drug present
DAT: IgG positive
Mechanism unknown (possibly alteration of self-antigens or immune dysregulation)
Hemolysis may persist after drug is stopped (autoantibodies take time to disappear)

Membrane Modification (Non-immune)

Examples: Cephalosporins
Drug modifies red cell membrane to nonspecifically adsorb proteins (including immunoglobulins)
DAT: May be positive for IgG and/or C3
No true antibody-mediated hemolysis
Positive DAT without hemolysis

Chapter 7: Apheresis

7.1 Principles of Apheresis

Apheresis (from Greek “to take away”) refers to procedures that separate blood components, removing one component while returning the rest to the patient/donor.

Separation Technologies

Centrifugation-based: Most common. Blood flows into a spinning bowl or channel where components separate by density:

Plasma (least dense, innermost layer)
Platelets and buffy coat (middle)
Red cells (most dense, outermost layer)

Membrane filtration: Used for plasmapheresis. Plasma passes through a membrane while cells are retained.

Vascular Access

Apheresis needs high flow rates - blood has to be drawn out, processed, and returned fast enough to keep the circuit running. A standard peripheral IV won’t cut it for most procedures.

Large-bore, stiff-walled dialysis-type catheter is the standard.
Placed as a central line, ideally in the internal jugular vein.
Femoral and subclavian veins are acceptable alternatives if the IJ isn’t usable.
Peripheral IVs can work for single-needle procedures (some donor collections), but flow rates are suboptimal for therapeutic apheresis.

The catheter has to be stiff because the negative pressure on the draw side will collapse a soft-walled line. This is the same reason you use a dialysis catheter rather than a regular triple-lumen.

Pre-Procedure: Hold the ACE Inhibitor

ACE inhibitors must be stopped 24 hours before apheresis. This is one of the most heavily tested apheresis facts.

Mechanism: apheresis circuits (especially those using negatively charged filters or immunoadsorption columns) activate the contact/kallikrein system and generate bradykinin. ACE is the enzyme that normally degrades bradykinin. If ACE is inhibited, bradykinin accumulates, causing:

Hypotension
Flushing
Abdominal pain
Sometimes dyspnea

The picture can mimic anaphylaxis, but it’s a kinin reaction, not an IgE-mediated one. ARBs are generally considered safer in this context, but the classic board answer is hold the ACE-i 24 hours out.

What Actually Gets Removed - Drug and Substance Kinetics

Not every drug or toxin is removable by apheresis. Two properties predict removability:

High protein binding (>80%)
Low volume of distribution (Vd <0.2 L/kg)

These two travel together. If a drug is highly protein-bound (bound to albumin), it stays in the intravascular compartment - which is exactly where TPE can grab it. High protein binding drives low Vd, because the drug isn’t wandering off into tissue.

Drugs with high Vd (lipophilic, tissue-distributed) are poorly removed - most of the drug sits outside the blood, and whatever you pull out of the plasma gets replaced by redistribution from tissue within minutes.

Same logic applies to antibodies and paraproteins, which is why IgM (mostly intravascular) comes out more efficiently than IgG (see kinetics discussion below).

7.2 Therapeutic Apheresis

Therapeutic Plasma Exchange (TPE)

In TPE, the patient’s plasma is removed and replaced with a replacement fluid (albumin solution or FFP).

Mechanism of action: Removes pathogenic substances from the plasma:

Autoantibodies
Alloantibodies
Immune complexes
Paraproteins
Toxins bound to plasma proteins

Kinetics: Each 1 plasma volume exchange removes approximately 63% of an intravascular substance (exponential decay, fraction remaining = e^(-N) where N = plasma volumes exchanged):

1 PV exchanged: removes ~63%
2 PV exchanged: removes ~86% (cumulative)
3 PV exchanged: removes ~95% (cumulative)

Returns diminish quickly past 1-1.5 PV per session, which is why the standard exchange volume is ~1-1.5 PV. However, substances re-equilibrate from extravascular space between sessions, so serial exchanges are needed.

Replacement fluids: There are three replacement fluids used in therapeutic apheresis. You need replacement because you’re removing a volume of plasma and have to maintain circulating volume.

Normal saline (0.9% NaCl): Cheapest, readily available. Doesn’t maintain oncotic pressure, so not ideal for large-volume exchanges. Sometimes used to prime the circuit or as partial replacement, but rarely the sole replacement fluid for a full-volume TPE.
5% albumin: Standard replacement fluid for most plasmapheresis. Maintains oncotic pressure. No infectious risk. Lacks clotting factors, so repeated daily exchanges on albumin-only replacement cause a dilutional coagulopathy. Factor VII has the shortest half-life but also recovers fastest between sessions. Fibrinogen has the longest half-life and progressively depletes with serial exchanges, so it’s the one to watch on daily TPE.
FFP (or cryo-poor FFP): Required in TTP because it provides ADAMTS13, the enzyme the patient is missing. Cryo-poor FFP (supernatant after cryoprecipitate is removed) is depleted of vWF multimers and is sometimes preferred in TTP for that reason. FFP also replaces clotting factors, so it’s the choice if the patient is bleeding, needs an invasive procedure, or is getting daily exchanges. Downside: higher risk of allergic/anaphylactoid reactions, citrate load, and TRALI.

Indications (ASFA Category I - first-line treatment):

TTP (Thrombotic Thrombocytopenic Purpura): Removes anti-ADAMTS13 antibodies, provides functional ADAMTS13
Myasthenia gravis (acute exacerbation): Removes anti-acetylcholine receptor antibodies
Guillain-Barré syndrome: Removes pathogenic antibodies
ANCA-associated vasculitis (severe): Removes ANCA
Anti-GBM disease (Goodpasture): Removes anti-glomerular basement membrane antibodies
Hyperviscosity syndrome: Rapidly reduces paraprotein levels

Complications:

Citrate toxicity (hypocalcemia): Citrate anticoagulant in the circuit binds calcium. Paresthesias (classically perioral), muscle cramps, and in severe cases prolonged QT and arrhythmias. Treat with IV or oral calcium. Risk is higher with FFP replacement because FFP itself contains citrate.
Hypotension: From volume shifts, vasovagal response, or bradykinin-mediated reactions (see ACE inhibitor discussion in 7.1). Anaphylactoid reactions with FFP also drop pressure.
Coagulopathy: Dilution of clotting factors when albumin is the replacement fluid. Factor VII recovers fastest; fibrinogen is the slowest to rebound and is the one to watch on daily exchanges.
Allergic reactions: Urticaria through anaphylaxis, especially with FFP.
Line complications: Infection, thrombosis, pneumothorax (from central line placement).

TPE Removal Kinetics: IgM vs. IgG: TPE removes IgM more efficiently than IgG because IgM (pentamer, MW ~900 kDa) is predominantly intravascular (~80%), whereas IgG distributes ~45% intravascular / ~55% extravascular. A single 1.0 plasma volume exchange removes approximately 63% of intravascular substance. After TPE, IgM recovers more slowly because there is less extravascular reserve to redistribute. IgG re-equilibrates rapidly from extravascular stores, often requiring a series of 5-7 exchanges for conditions like myasthenia gravis or GBS.

Post-TPE Factor Recovery: After 1-volume TPE with 5% albumin replacement, coagulation factors recover to baseline within 24-48 hours. Factor VII recovers fastest (~24 hours, shortest half-life). Daily TPE can cause cumulative coagulopathy.

HPC Collection: The best predictor of successful peripheral blood stem cell collection is the peripheral blood CD34+ cell count. “Go” threshold = 10-20 CD34+ cells/µL. Target collection: 2-5 × 10⁶ CD34+ cells/kg recipient weight. ISHAGE protocol standardizes CD34+ enumeration by flow cytometry.

Cytapheresis Procedures

Leukapheresis: Removal of white blood cells

Indication: Hyperleukocytosis in leukemia (WBC >100,000/μL with symptoms)
Goal: Rapidly reduce WBC count while definitive chemotherapy takes effect
Symptomatic relief of leukostasis (pulmonary, neurologic)

Plateletpheresis: Removal of platelets

Indication: Symptomatic thrombocytosis (usually platelets >1 million with ischemic symptoms)
Provides temporary reduction while medications take effect

Erythrocytapheresis (Red Cell Exchange): Removal of patient’s red cells and replacement with donor red cells

Sickle cell disease: Rapidly reduces HbS percentage; first-line for acute chest syndrome, stroke
Severe malaria: Reduces parasitemia rapidly
Polycythemia vera: Alternative to phlebotomy

Photopheresis (Extracorporeal Photochemotherapy):

White cells are collected, treated with psoralen (photosensitizing agent) and UVA light, then returned
Mechanism: Induces apoptosis of treated lymphocytes, modulates immune response
Indications: Cutaneous T-cell lymphoma (mycosis fungoides/Sézary syndrome), chronic GVHD, solid organ transplant rejection

Chapter 8: Special Clinical Situations in Transfusion Medicine

This chapter covers the places where transfusion medicine gets weird - where the simple rules from Chapters 2 and 3 bend or break. Massive hemorrhage, sickle cell disease, platelet refractoriness, TTP, apheresis, emergency blood release, donor intervals. These are the clinical situations where a blood banker actually earns their paycheck and where the board exam likes to test judgment rather than rote facts.

8.1 Massive Transfusion

Definition: Replacement of one or more blood volumes within 24 hours (approximately 10 units of RBCs in a 70 kg adult), or transfusion of more than 4 units in one hour with ongoing bleeding. Alternate definitions you’ll see: half the blood volume replaced within 3 hours, or >4 units transfused within 4 hours with ongoing bleeding. Any of these triggers MTP activation.

Hemorrhage Classification (ATLS)

The American College of Surgeons ATLS scheme classifies acute blood loss by volume and physiologic response. Tachycardia is the first sign of acute blood loss - the heart compensates for lower stroke volume with a higher rate to maintain cardiac output. Hypotension is a late finding, because peripheral vasoconstriction keeps blood pressure up until ~30% volume loss.

Class	Blood loss (%)	Blood loss (mL, 70 kg)	Clinical picture
I	<15%	<750	Minimal signs, no intervention needed
II	15-30%	750-1500	Tachycardia, anxiety, narrow pulse pressure - crystalloids usually suffice
III	30-40%	1500-2000	Tachycardia + hypotension + confusion - needs blood
IV	>40%	>2000	Obtunded, life-threatening - MTP activation

Resuscitation Priority: Fluids First, Then Blood

Standard resuscitation starts with crystalloids (normal saline, lactated Ringer’s) to restore intravascular volume while blood is being prepared. After ~30 mL/kg of crystalloid (about 2 L in a 70 kg adult) with ongoing hemorrhage, transition to blood products.

Fluid distribution:

Crystalloids (NS, LR, D5W): only ~20% stays in the intravascular space - the rest distributes to the interstitium. You need 3 - 4x the volume of blood lost to restore circulating volume.
Colloids (albumin, dextran, blood products): ~100% stays intravascular for 24 hours - large molecules can’t cross capillary membranes easily.

Crystalloids and colloids impair hemostasis by dilution: they replace volume but not clotting factors or platelets. Excessive crystalloid -> dilutional coagulopathy -> worsens bleeding. This is one reason MTPs use balanced ratios rather than relying on fluids alone.

The Pathophysiology of Coagulopathy in Massive Hemorrhage

Massive hemorrhage creates a “deadly triad” that drives coagulopathy:

Hypothermia: Blood stored at 4°C, large-volume cold crystalloid resuscitation, and exposed body cavities all lower core temperature. At 34°C, enzymatic activity of coagulation factors is reduced by ~25%; at 32°C, by ~50%. Hypothermia also impairs platelet function.
Acidosis: Tissue hypoperfusion leads to anaerobic metabolism and lactic acidosis. At pH 7.2, the activity of the Xa/Va prothrombinase complex is reduced by 50%; at pH 7.0, by 70%.
Dilution: Resuscitation with crystalloids and RBCs (which contain no plasma or platelets) dilutes the patient’s remaining clotting factors and platelets. After replacement of one blood volume (~10 units RBCs), only ~35% of original plasma and platelets remain.

These three factors compound each other - hypothermia worsens acidosis, acidosis impairs clotting, and dilution exacerbates both problems. Together with free hemoglobin from partially hemolyzed stored units, massive transfusion classically lowers pH, lowers body temperature, raises potassium, and increases free hemoglobin.

Trauma-induced coagulopathy: In trauma patients, coagulopathy often begins before resuscitation, driven by tissue injury, shock, and activation of the protein C pathway. This “acute coagulopathy of trauma” causes fibrinolysis and factor depletion independent of dilution.

The Storage Lesion and Oxygen Delivery

Stored RBCs are not physiologically equivalent to fresh autologous blood. During storage, RBCs deplete 2,3-DPG and ATP. Without 2,3-DPG, hemoglobin binds oxygen more tightly - the oxygen dissociation curve shifts to the left and upward, meaning transfused Hgb holds onto O2 rather than releasing it to tissues. The effect is temporary: 2,3-DPG regenerates within 24 - 48 hours of transfusion. ATP depletion also impairs RBC deformability and Na/K pump function, which is part of why K+ leaks into the supernatant.

Massive Transfusion Protocols (MTP)

Modern MTPs aim to replace blood components in ratios approximating whole blood, preventing dilutional coagulopathy:

Ratio-based approach:

Typical ratios: 1:1:1 (RBC:FFP:platelets) or 1:1:2. With whole-blood-derived platelets the ratio is 1:1:1 per component; with apheresis platelets it’s effectively 6:6:1 (6 RBC : 6 FFP : 1 apheresis platelet unit, since one apheresis dose covers ~6 pooled whole-blood platelets).
The PROPPR trial (JAMA 2015) found 1:1:1 achieved hemostasis more often and reduced 24-hour exsanguination death versus 1:1:2; the primary 24-hour and 30-day all-cause mortality endpoints were not significantly different
Practical implementation: After every 4-6 units of RBCs, give 4-6 units of plasma and 1 apheresis platelet dose

Low-titer group O whole blood (LTOWB): Increasingly used as a single product in trauma. One unit contains RBCs + plasma + platelets + fibrinogen in physiologic ratios. Simpler logistics (one cooler, one tubing set), faster to deliver, and some studies suggest better outcomes. Group AB plasma is ideal (universal plasma donor) but AB is rare; many centers substitute group A plasma in MTP packs because A-plasma is abundant and the anti-B titer risk is manageable in the massive-transfusion setting.

Goal-directed approach (when labs are available):

Hematocrit <30%: Transfuse RBCs
INR >1.5 or aPTT >1.5× normal: Transfuse FFP (10-15 mL/kg)
Platelets <50,000/μL: Transfuse platelets
Fibrinogen <100-150 mg/dL: Transfuse cryoprecipitate (10 units)

Adjuncts:

Tranexamic acid: Antifibrinolytic; the CRASH-2 trial showed mortality benefit in trauma patients when given within 3 hours of injury (1g IV bolus, then 1g over 8 hours)
Calcium: Citrate in blood products chelates calcium; ionized calcium <1.0 mmol/L impairs coagulation. Replace with calcium gluconate.
Warm all products: Use blood warmers to prevent exacerbating hypothermia

Complications of Massive Transfusion

Citrate toxicity and hypocalcemia: Citrate anticoagulant chelates calcium. The liver normally metabolizes citrate quickly, but in massive transfusion (especially with liver dysfunction, hypothermia, or shock), citrate accumulates. Symptoms: perioral tingling, muscle cramps, prolonged QT, hypotension. Monitor ionized calcium and replace.

Hyperkalemia: RBCs leak potassium during storage (up to 50 mEq/L supernatant K+ in a 42-day unit). Usually clinically insignificant because the volume of supernatant is small and potassium is rapidly taken up by tissues. However, in massive transfusion of older units, rapid infusion, or renal failure, hyperkalemia can occur. Risk is higher with irradiated units (accelerated K+ leak).

Metabolic alkalosis: Counterintuitively, despite the acidic pH of stored blood, massive transfusion often causes alkalosis. Citrate is metabolized to bicarbonate by the liver. Each unit of blood contains ~17 mEq of citrate, which generates bicarbonate.

Emergency Release of Blood

When a patient will die before compatibility testing can be completed, the blood bank issues uncrossmatched type O blood under an emergency release. This is blood given WITHOUT a completed type and screen or crossmatch. The physician accepts the risk by signing a release form.

Which type O?

O-negative for all premenopausal females and for O+ patients with known anti-E or anti-C alloantibodies (D+ donor units commonly co-express E or C antigens due to Rh haplotype linkage - DcE and DCe are the common D+ haplotypes).
O-positive for males and postmenopausal females. D-sensitization isn’t a pregnancy concern for these groups, and O-negative is a scarce resource that must be reserved for those who need it most.

Even if a patient has a historical type and screen on file showing a specific blood type, emergency release still uses type O. The historical type can be wrong (rare lab errors) or outdated (ABO can change after ABO-incompatible stem cell transplant). Once a fresh type and screen is complete during the current admission, the patient can transition to type-specific blood.

Documentation requirements:

Within 24 hours, the treating physician must sign a release form acknowledging that compatibility testing was not completed.
The label on emergency-released blood must indicate that compatibility testing was not completed. This alerts every downstream handler (nurse, anesthesiologist) and satisfies regulatory and medicolegal requirements.
The blood bank completes the type and screen retroactively. If an incompatibility is discovered, the team is notified immediately.

Accidental D+ Transfusion in a D- Female of Childbearing Age

When a D-negative woman of childbearing age accidentally receives D-positive blood (emergency release, clerical error), she is at risk of D-sensitization - anti-D formation that could cause HDFN in future pregnancies. Management depends on the volume transfused.

If D+ blood is <20% of blood volume:

Give IV RhIg within 72 hours
Dose: ~90 - 100 IU/mL of transfused PRBCs or ~45 - 50 IU/mL of transfused whole blood [TODO: verify exact IU figure - common references cite 20 µg (100 IU) per mL of D+ RBCs]
Whole blood has ~half the RBC concentration of PRBCs, hence half the RhIg dose per mL
This is different from the pregnancy dosing (300 µg per 30 mL fetal whole blood) - here we use the IV formulation at weight-based doses

If D+ blood is greater than or equal to 20% of blood volume:

Do NOT rely on RhIg alone. The D+ cell burden is too large - the anti-D would cause a massive hemolytic reaction trying to opsonize that many cells.
Perform exchange transfusion with D-negative blood to physically remove the D+ cells, then give RhIg to mop up any remaining D+ RBCs.

8.2 Sickle Cell Disease and Transfusion

Patients with sickle cell disease require special transfusion considerations due to their unique pathophysiology and chronic transfusion needs.

Goals of Transfusion in Sickle Cell Disease

Simple transfusion: Adds normal (HbA) red cells without removing HbS cells. Increases oxygen-carrying capacity but also increases blood viscosity. Appropriate for:

Symptomatic anemia
Aplastic crisis
Splenic sequestration

Exchange transfusion (red cell exchange): Removes HbS-containing cells and replaces with normal cells. Reduces HbS percentage without increasing viscosity. Appropriate for:

Acute stroke (first-line treatment)
Acute chest syndrome (severe)
Pre-operative preparation for high-risk surgery
Chronic transfusion for stroke prevention

Target HbS percentage:

Acute stroke: <30%
Chronic stroke prevention: <30%
Pre-operative: <30% for major surgery
Acute chest syndrome: Clinician judgment

Calculating exchange volume: If the pre-exchange HbS percentage is unknown, assume HbS is 100% (worst case). The goal is to bring post-exchange HbS <30%. From these two values you can calculate what fraction of the patient’s blood volume needs to be exchanged. For a 70 kg patient with ~5000 mL blood volume going from HbS 100% to 30%, you need to exchange roughly 70% of the blood volume with HbS-negative donor blood. Automated red cell exchange (erythrocytapheresis) is preferred for precision and volume control.

Elective (chronic) transfusion indications:

Children with abnormal transcranial Doppler (TCD) velocities for primary stroke prevention. Chronic transfusion reduces stroke risk by ~90% in high-TCD children (STOP trial).
Progressive renal or cardiopulmonary disease
Complicated pregnancy

Chronic transfusion requires long-term commitment and iron chelation because each unit adds ~200 mg of iron and the body has no regulated excretion pathway.

Special Considerations

Alloimmunization: SCD has the highest alloimmunization rate of any transfusion-dependent population (reports range 20 - 50%, commonly cited ~30%) due to:

Chronic transfusion exposure
Antigenic differences between donor pools enriched for European-ancestry antigen profiles and many SCD patients with African-ancestry antigen profiles
Inflammatory state promoting immune responses

The most common alloantibodies in SCD patients reflect this mismatch: anti-K, anti-C, anti-E, anti-Fya, anti-Jkb.

Prevention strategies:

Prophylactic matching for Rh (C, E) and Kell (K) at minimum. This simple step reduces the per-unit alloimmunization rate from ~3% to ~0.5%.
Extended phenotype matching for Duffy, Kidd, and S/s in heavily transfused patients.
Recruitment and genotyping of donors with African ancestry when possible.
RBC genotyping (molecular testing) is increasingly used to provide more precise matching, especially for patients who already have multiple alloantibodies.

Iron overload: Each unit adds ~200 mg of iron. Chronically transfused SCD patients develop hemosiderosis affecting the heart, liver, and endocrine organs. Iron chelation (deferasirox, deferoxamine) is required for long-term transfusion programs.

Delayed hemolytic transfusion reaction (DHTR): More common and more severe in sickle cell patients. May present as “hyperhemolysis syndrome” - destruction of both transfused and autologous red cells, with hematocrit dropping below pre-transfusion baseline. Treatment: avoid further transfusion if possible, IVIG, steroids, possibly eculizumab.

ASPEN syndrome: A rare but severe complication occurring within ~11 days of exchange transfusion [TODO: verify exact timing window] in SCD patients. The mnemonic spells out the components: Association of Sickle cell disease, Priapism, Exchange transfusion, and Neurologic events. Mechanism: the sudden increase in HbA-containing RBCs improves oxygen delivery, which paradoxically shifts Hgb-oxygen dynamics in the remaining HbS cells and drives sickling -> priapism and neurologic complications (stroke, seizures).

Leukoreduction filter failure and sickle cell trait: The most common cause of leukoreduction filter failure is sickle cell trait (HbAS) in the donor. Under the low-oxygen conditions during processing and filtration, HbS polymerizes and RBCs become rigid, clogging the filter (or paradoxically letting too many WBCs through if the filter is bypassed). SCD patients aren’t eligible donors in the first place, so the problem is trait donors who don’t know their status - roughly 1 in 12 African American individuals in the United States. When repeated filter failures occur for a donor, check their hemoglobin electrophoresis.

8.3 Platelet Refractoriness

Definition: Failure to achieve expected platelet increment after transfusion. Operationally defined as corrected count increment (CCI) <7,500 at 10-60 minutes post-transfusion on two consecutive occasions.

CCI Calculation: CCI = (Post-transfusion count - Pre-transfusion count) × BSA (m²) / Platelets transfused (× 10^11)

Expected CCI at 1 hour: >7,500 (indicates platelet survival) Expected CCI at 24 hours: >4,500 (indicates platelet function)

Causes of Platelet Refractoriness

Non-immune causes (more common, ~80%):

Fever, sepsis (increased consumption)
Splenomegaly (sequestration)
DIC (consumption)
Bleeding (consumption)
Medications (amphotericin B, vancomycin)
TTP/HUS

Immune causes (~20%):

HLA alloimmunization (most common immune cause): Antibodies to HLA Class I antigens on donor platelets. Develops from pregnancy, prior transfusions with non-leukoreduced products, or transplantation.
HPA antibodies: Antibodies to platelet-specific antigens (rare)
ABO incompatibility: Contributes modestly to poor increments

Evaluation and Management

Step 1: Confirm refractoriness with ABO-identical platelets and measure CCI properly (1-hour post-transfusion count).

Step 2: Identify and treat non-immune causes (the most common causes).

Step 3: If immune cause suspected, test for HLA antibodies (panel reactive antibody, PRA).

Step 4: Management of HLA alloimmunization:

HLA-matched platelets: Match patient HLA type with donor HLA type. Graded by match quality (A = 4-antigen match; B = 3-antigen match with crossreactive group compatibility).
Crossmatch-compatible platelets: Test patient serum against donor platelets directly; use compatible units.
HLA antibody testing with single antigen beads: Identifies specific antibodies; select donors lacking those antigens.

Prevention: Universal leukoreduction significantly reduces HLA alloimmunization rates. The TRAP trial showed leukoreduction (filtration) of pooled platelets dropped HLA alloimmunization from ~45% to ~17 - 18%, comparable to apheresis platelets.

8.4 Therapeutic Apheresis

Therapeutic apheresis is the process of removing whole blood from a patient, separating out a component (plasma or cells), and returning the remainder with a replacement fluid. It’s a procedure, not a product - the goal is to physically remove an offending substance from the circulation.

Two main types:

Therapeutic plasmapheresis (also called plasma exchange, TPE): removes plasma. Used when the pathogenic substance is in plasma - autoantibodies, immune complexes, paraproteins, ADAMTS13 autoantibody. Classic indications: TTP, Guillain-Barré, myasthenia gravis.
Therapeutic cytapheresis: removes cells. Subtypes: leukapheresis (for hyperleukocytosis in AML with WBC >100k), erythrocytapheresis (red cell exchange for SCD), plateletpheresis (for symptomatic thrombocytosis).

The separation is done by centrifugation or membrane filtration. Replacement fluid depends on the component removed (albumin or FFP for plasma, saline or RBCs for cytapheresis).

ASFA Categories

The American Society for Apheresis (ASFA) publishes evidence-based guidelines categorizing the role of therapeutic apheresis across specific diseases, updated every few years. The categories are the standard reference for whether apheresis is appropriate and at what priority:

Category I: first-line / primary therapy. Strong evidence. Examples: TTP, Guillain-Barré syndrome, myasthenia gravis.
Category II: second-line therapy or accepted adjunct. Examples: ANCA-associated vasculitis, lupus nephritis.
Category III: role not established. Individualized decision-making based on the patient. Example: ABO-incompatible kidney transplant desensitization.
Category IV: published literature shows apheresis is ineffective or harmful. No role. Example: rheumatoid arthritis.

8.5 Thrombotic Thrombocytopenic Purpura (TTP)

TTP is the paradigm Category I apheresis indication. Untreated mortality approaches 90%; with prompt plasma exchange it drops below 20%. The transfusion medicine service is often involved both in the diagnostic workup and the treatment.

Pathophysiology

ADAMTS13 is the metalloprotease that cleaves ultra-large von Willebrand factor (UL-vWF) multimers into smaller functional forms. Without ADAMTS13, UL-vWF multimers accumulate, cause spontaneous platelet adhesion in the microvasculature, and produce microthrombi in arterioles and capillaries. The result is the classic pentad: thrombocytopenia, microangiopathic hemolytic anemia (schistocytes), fever, renal dysfunction, and neurologic changes. You rarely see all five on presentation - the diagnosis is often made on thrombocytopenia + MAHA alone.

Two forms:

Acquired TTP (more common): IgG4 autoantibody against ADAMTS13 inhibits its activity. Treatment is plasmapheresis + immunosuppression (steroids, rituximab, caplacizumab).
Congenital TTP (Upshaw-Schulman syndrome): inherited ADAMTS13 deficiency, presents in childhood. Treatment is plasma infusion - you’re just replacing the missing enzyme. No autoantibody to clear.

Diagnosis

Two main tools:

ADAMTS13 activity level: a level <10% confirms severe deficiency and supports the TTP diagnosis. The turnaround is often days, which is too slow to drive decisions for a dying patient.
PLASMIC score: a clinical prediction tool calculable at the bedside to estimate the probability of severe ADAMTS13 deficiency. High PLASMIC -> start plasmapheresis empirically while awaiting the ADAMTS13 result.

The PLASMIC mnemonic is P-MIC-HAT:

Platelet count <30k
MCV <90
INR <1.5 (above 1.5 favors DIC / coagulopathy)
Creatinine <2.0 (above 2.0 favors HUS)
Hemolysis present (reticulocytes >2.5%, low haptoglobin, unconjugated bilirubin >2)
Absence of active cancer
Transplant absence (no prior solid organ or stem cell transplant)

Each item scores 1 point (max 7). Score 6 - 7 = high probability of severe ADAMTS13 deficiency / TTP; score 5 = intermediate. Many centers start empiric plasma exchange at score ≥5 rather than waiting for the ADAMTS13 result, since the mortality cost of delayed treatment is too high.

8.6 Blood Donation Intervals and Deferrals

Donor eligibility rules exist to protect the donor. The intervals between donations are calibrated to how much volume and which components are lost.

Standard Donation Intervals

Donation type	Interval
Whole blood	8 weeks
Single-unit apheresis RBC	8 weeks
Double-unit apheresis RBC (2-RBC)	16 weeks
Single apheresis platelets	2 days (max 2/week, max 24/year)
Double or triple plateletpheresis	7 days (max 24/year)
Granulocyte donation	2 days (max 2/week, max 24/year)
Infrequent plasmapheresis	4 weeks
Frequent (paid) plasmapheresis	2 days

Why the intervals:

Whole blood and single-unit RBC apheresis: both remove ~200 mL of packed RBCs. RBC mass takes 4 - 8 weeks to regenerate (EPO-driven erythropoiesis). 8 weeks gives full recovery, maximum 6 donations/year.
Double-unit RBC apheresis: removes twice the RBC mass in one session. 16 weeks. Donors must meet stricter criteria (higher minimum Hgb, height/weight thresholds).
Platelets and granulocytes (single apheresis): platelets regenerate in 5 - 7 days, so 2-day intervals are safe. The 24/year cap prevents cumulative RBC loss from the small amount of RBCs that’s inevitably collected during apheresis.
Double/triple plateletpheresis: larger platelet depletion and greater RBC loss per session, so 7 days.
Plasmapheresis: plasma is acellular - no RBC or platelet loss. Plasma volume restores within 24 - 48 hours. Paid commercial plasmapheresis donors can donate twice per week (2-day interval). Infrequent non-commercial plasma donors follow the more conservative 4-week interval.

Plasma Replenishment After Donation

Fibrinogen: <1 day
Albumin: 2 - 3 days
IgG: ~4 days

Fibrinogen replenishes fastest (small pool, rapid hepatic synthesis). IgG is the slowest, which is why very frequent plasmapheresis donors can experience gradual IgG decline with continuous twice-weekly donation. Paid donors have routine total protein monitoring for this reason.

Deferrals Based on Cumulative RBC Loss

During platelet, plasma, or granulocyte apheresis, small amounts of RBCs are unavoidably collected and not returned. Over a rolling 8-week window:

Cumulative RBC loss (8 weeks)	Deferral
<200 mL	No deferral
200 - 299 mL	8 weeks (equivalent to one WB donation)
≥300 mL	16 weeks (equivalent to a double RBC donation)

This tracking prevents occult anemia in frequent apheresis donors.

Plasma Product Quality Requirements

Unlike RBCs (which have QC requirements for hemoglobin content and hematocrit) and platelets (which require minimum platelet counts per unit), plasma has no regulatory minimum for coagulation factor levels or total protein content. The assumption is that plasma from a healthy donor, collected and frozen within 8 hours of phlebotomy and stored at -18°C or colder, will contain adequate factor activity (~1 IU/mL of each factor). Quality is maintained through process controls (collection timing, freezing temperature) rather than release testing.

8.7 Hematopoietic Stem Cell Mobilization: Plerixafor

For autologous stem cell transplant, patients need to mobilize CD34+ hematopoietic stem cells (HSCs) from the bone marrow into peripheral blood so they can be collected by apheresis. Standard mobilization uses G-CSF. Some patients are “poor mobilizers” and don’t yield enough CD34+ cells on G-CSF alone.

Plerixafor (Mozobil) is a CXCR4 antagonist. Mechanism: HSCs are anchored to the bone marrow niche by the SDF-1 / CXCR4 axis. Plerixafor blocks CXCR4, disrupts that anchor, and HSCs are released into peripheral blood where apheresis can collect them. Think of it as unlocking stem cells from the marrow.

Indications: FDA-approved only for autologous transplant, in combination with G-CSF, in patients with multiple myeloma or non-Hodgkin lymphoma. Not approved for allogeneic donor mobilization (theoretical concerns about long-term effects on healthy donors).

8.8 IV RhIg for Immune Thrombocytopenic Purpura

IV RhIg can treat ITP in RhD-positive patients only. The patient must have the D antigen for the drug to work.

Mechanism: administered anti-D coats the patient’s D+ RBCs. The Fc-receptor-bearing splenic macrophages become busy clearing antibody-coated RBCs. This saturates macrophage Fc receptors and reduces their capacity to destroy autoantibody-coated platelets. Platelet count rises.

This is a “distraction” mechanism analogous to IVIG in ITP, except it’s antigen-specific and requires the D antigen to be present.

Black box warning: IV RhIg can cause severe (occasionally fatal) hemolytic anemia - the anti-D IS causing hemolysis, that’s the mechanism. Many centers avoid it after reports of severe reactions. It has essentially been superseded in practice by other ITP therapies but it is still board-testable. The black box is notably different from the situation in pregnancy, where RhIg is given to D-negative women (no D+ RBCs to hemolyze).

8.9 Antibody Panel Pattern Recognition

The antibody identification panel is the core diagnostic workup for alloantibodies. Before reading a panel, check two things: (1) the autocontrol (patient’s serum vs. patient’s own cells), and (2) whether the reactivity is pan-reactive (all cells positive) or selective (some cells positive).

Pan-reactive with Positive Autocontrol

The autocontrol tells you if the antibody also reacts with the patient’s own cells - i.e., if it’s an autoantibody or an antibody to a high-frequency antigen the patient expresses.

Cold autoantibody: pan-reactive at immediate spin and/or 4°C, positive autocontrol, reactivity weakens at 37°C and AHG phases. Classically IgM anti-I in cold agglutinin disease.
Warm autoantibody: pan-reactive at AHG phase (37°C), positive autocontrol. IgG autoantibody, often with broad Rh specificity (anti-e-like). Warm autoimmune hemolytic anemia. Watch out for alloantibodies hidden underneath the autoantibody - you need adsorption studies to unmask them before transfusion.
Antibody to a high-frequency antigen: pan-reactive at AHG phase, positive autocontrol (because the patient also expresses the high-frequency antigen on their own cells), and DTT does NOT resolve the pattern. Examples: anti-k (Cellano), anti-Kpb, anti-Jsb, anti-Colton, HTLA antibodies (Chido, Rodgers). Contrast with daratumumab - if DTT resolves the reactivity, it’s daratumumab, not a high-frequency antigen antibody.

Pan-reactive with Negative or Variable Autocontrol

Daratumumab (anti-CD38) interference: pan-reactive antibody screen, autocontrol often negative (CD38 is downregulated on patient cells from chronic exposure). DTT treatment of test cells destroys CD38 and resolves the reactivity - confirming daratumumab as the cause. Because DTT also destroys Kell antigens, it can mask an underlying anti-K. Standard precaution: give K-negative blood to daratumumab patients.

Selective Reactivity - Alloantibody Patterns

Single alloantibody with dosage: an antibody reacts more strongly with cells that are homozygous for the antigen (double dose of antigen on each cell) than with heterozygous cells (single dose). Example: anti-Jka gives 4+ agglutination with Jk(a+b-) cells [Jka homozygous] and only 2+ with Jk(a+b+) cells [Jka heterozygous]. Antigens showing dosage: MNS, Duffy, Kidd, Rh (E/e, C/c). Clinical risk: dosage can cause a false-negative crossmatch if the donor is heterozygous.
Multiple alloantibodies with additive reactivity: when a panel cell expresses BOTH antigens the patient has antibodies to, reactivity is stronger than when the cell expresses only one. Example: anti-D + anti-K patient shows 4+ reactivity with D+K+ cells and 2+ reactivity with D+K- or D-K+ cells.
Anti-D in a G1P1 woman with no transfusion history: the classic sensitization event. D-negative mother carried a D-positive fetus, RhIg was not given, and fetal-maternal hemorrhage at delivery sensitized her. Non-ABO alloantibodies only form after a sensitization event - transfusion or pregnancy. A persistent anti-D years after pregnancy just means the antibody (or memory B cells) stuck around.

8.10 Molecular Basis of Non-ABO Antigens

Most non-ABO blood group antigen differences derive from a single nucleotide polymorphism (usually a missense mutation) in the gene encoding the surface protein. This is why molecular genotyping (DNA-based testing) can accurately predict blood group phenotypes. Examples:

Kidd: Jka vs Jkb - one SNP
Duffy: Fya vs Fyb - one SNP
Kell: K vs k - one SNP

ABO is the notable exception: the A, B, and O alleles differ at multiple nucleotides and produce enzymes with substantially different glycosyltransferase activities. ABO is a “big change at the protein level” system; most other blood group polymorphisms are single-residue substitutions.

8.11 Miscellaneous Crossover Topics

HLA-A3 and hereditary hemochromatosis: ~70% of HH patients carry HLA-A3 (vs. ~25% of the general population). The HFE gene (C282Y mutation in classic HH) sits near the HLA complex on chromosome 6p. The association is driven by linkage disequilibrium - HFE and HLA-A3 segregate together because a shared ancestral haplotype is common in populations of Northern European origin. Classic genetics / transfusion medicine crossover question.

Xg blood group and CD99: The Xg blood group system includes CD99 (the MIC2 gene product). CD99 is a cell surface glycoprotein and a diagnostic marker for Ewing sarcoma - nearly all Ewing tumors show membranous CD99 on IHC. Another classic crossover between blood banking and surgical pathology.

Specific gravity of blood cellular components: The basis for centrifugal component separation.

RBCs: ~1.1 (heaviest cellular component, sinks to the bottom)
WBCs and platelets: intermediate
Platelets: ~1.04 (lightest cellular component, closest to plasma)
Plasma: ~1.03

During centrifugation, RBCs pack at the bottom, the buffy coat (WBCs + platelets) layers in the middle, and plasma floats on top. All apheresis and component preparation relies on this density gradient.

Chapter 9: Infectious Disease Testing in Blood Donors

This chapter covers everything that happens between “a person walks into a blood center” and “a labeled, released unit sits in inventory.” That includes donor eligibility screening, the physical collection process, donor adverse events, the mandatory infectious disease testing panel, and the large list of deferrals (behavioral, travel, vaccination, and medication-based). The title of the chapter is “infectious disease testing,” but the testing only makes sense in the context of the rest of the donation workflow, so we cover the whole pipeline here. Post-collection component preparation is in Chapter 3; apheresis-specific donation is in Chapter 7.

The big picture: blood is a biological product collected from a person you have no real way to fully vet, and transfused into patients who are often immunocompromised or critically ill. Every step of the donation process is a safety layer. Identification and history catch one set of risks. Physical exam catches another. Infectious disease testing catches a third. Deferrals catch a fourth. None of them alone is sufficient.

9.2 Donor Types

Allogeneic Donation

A donor gives blood for use by the general patient population. This is the default. All the standard eligibility criteria (physical exam, hemoglobin cutoffs, full infectious disease screen, full deferral questionnaire) apply.

Autologous Donation

Autologous donation is when an individual’s blood is collected for their own later use, typically before a planned surgery. The patient donates for themselves.

Why autologous exists: eliminates alloimmunization risk, eliminates transfusion-transmitted infection risk, eliminates hemolytic alloantibody reactions. Downsides: requires advance planning, units may expire if surgery is postponed or cancelled, still carries risk of bacterial contamination and clerical error (wrong unit, wrong patient). Autologous donation is less common now than it used to be, because the safety gap between autologous and allogeneic blood has narrowed considerably with modern screening.

Autologous donation eligibility (looser than allogeneic):

Physician order required (it’s a medical procedure ordered for the patient)
Hemoglobin ≥ 11 g/dL or hematocrit ≥ 33% (lower than the 12.5 / 13 allogeneic cutoffs)
Absence of bacteremia risk (active infection could contaminate the stored unit)
No upper age limit
No weight minimum

Collection must occur > 72 hours before anticipated surgery. The 72-hour rule exists so the donor has time to recover from blood loss before the procedure. Plasma volume restores within 24 to 48 hours; RBC mass takes weeks to fully recover. Donating the day before surgery would leave the patient acutely hypovolemic going into the OR, which defeats the entire purpose.

Directed Donation

A directed donor gives blood specifically for a named patient (classic example: family member donating for a surgical relative). Important points:

Directed donors must meet ALL standard allogeneic eligibility criteria
Directed donations from blood relatives must be irradiated (TA-GVHD risk from shared HLA - see Chapter 4)
Directed donations have NOT been shown to be safer than random-donor blood - family members may actually be less honest on the donor questionnaire due to social pressure

Autologous vs. Directed Units in General Inventory

Unit type	Can enter general inventory?	Why
Autologous	Cannot	Collected under relaxed criteria (lower Hgb, may skip some screening) - not safe for general recipients
Directed	Can	Collected under standard allogeneic criteria - safe for anyone

If an autologous unit is not used by the intended patient, it is discarded. A directed unit, if not used by the intended recipient, can be released into the general inventory.

Therapeutic Phlebotomy

Some patients get phlebotomy as treatment, not as donation. The key question: can their blood be used for transfusion?

Hemochromatosis: blood CAN be used for allogeneic donation (excess iron only, cells are normal - the iron is actually beneficial to an iron-replete recipient)
Polycythemia vera / MPN: CANNOT be used (blood contains clonal / malignant cells)
Porphyria cutanea tarda: CANNOT be used (blood contains toxic porphyrins)

The principle is simple: don’t transfuse diseased blood. Hemochromatosis blood is safe because the only abnormality is excess iron.

Rules for hemochromatosis donation:

Donor must be otherwise eligible
Medical director must examine the donor if less than 8 weeks have passed since last donation (hemochromatosis patients often need more frequent phlebotomy than the standard 8-week inter-donation interval)
Units must be labeled as derived from a hemochromatosis patient, OR the center must obtain an FDA variance for labeling exemption

Patients with hemochromatosis or on testosterone therapy may donate with Hgb > 20 g/dL under physician supervision (see Hgb max rule below). PV/PCT patients still get phlebotomy, but that blood is discarded.

9.3 Donor Physical Exam and Minimum Requirements

The physical exam is brief but specific. It exists for two reasons: protect the donor from a dangerous collection (can they safely lose ~450 mL of blood?) and protect the recipient (is this donor showing signs of active illness?).

Standard allogeneic donor minimums:

Parameter	Requirement
Temperature	≤ 37.5°C (99.5°F)
Pulse	50 to 100 BPM, regular rhythm (athletes: < 50 BPM accepted)
Blood pressure	90 to 180 systolic / 50 to 100 diastolic mmHg
Hgb / Hct (men)	≥ 13.0 g/dL or ≥ 39%
Hgb / Hct (women)	≥ 12.5 g/dL or ≥ 38%
Hgb maximum	20.0 g/dL (unless physician waiver)
Weight	≥ 110 lbs (50 kg)
Venipuncture site	Free of lesions, no evidence of IV drug use

Why these specific thresholds:

Temperature: fever suggests active infection, which could mean occult bacteremia and a contaminated unit
Pulse: tachycardia can reflect dehydration, anxiety, infection, or cardiac disease. Bradycardia is fine in athletes but can indicate heart block in others
Blood pressure: low BP means high risk of syncope during the draw. Very high BP means risk of hematoma and prolonged bleeding
Hgb minimum: protects the donor from being pushed into symptomatic anemia after losing a unit. The sex-based cutoffs reflect baseline differences in hemoglobin physiology
Hgb maximum: Hgb > 20 suggests polycythemia (PV, secondary polycythemia, testosterone use, hemochromatosis). PV blood is diseased; hemochromatosis and testosterone patients can donate with a physician waiver since their RBCs are normal
Weight: the max draw volume is 10.5 mL/kg (including test tubes). For a 50 kg donor: 50 × 10.5 = 525 mL maximum, which just accommodates a 450 mL collection plus ~30 to 45 mL of test tubes. Below 110 lbs, the collection would represent too large a fraction of total blood volume
Venipuncture site: track marks, scarring, or other evidence of IVDU → permanent deferral. IVDU carries extremely high risk of HIV, HBV, HCV

Double RBC apheresis donors have stricter requirements (not high yield but good to know):

Male: 17 years, 5’1”, 130 lbs
Female: 19 years, 5’3”, 150 lbs

These higher weight thresholds exist because double RBC collection removes twice the usual RBC mass.

Donor antibody screening: Donors are only screened for unexpected alloantibodies if they have a history of transfusion or pregnancy. These are the two events that sensitize people to non-ABO alloantigens (e.g., anti-K, anti-Jkᵃ). Alloantibodies in donor plasma could hemolyze recipient RBCs that carry the corresponding antigen. If the donor has no history of transfusion or pregnancy, alloantibodies are vanishingly unlikely and routine screening isn’t required.

9.4 ABO and Rh Typing of Donor Units

Every allogeneic unit is ABO and Rh typed. The methods mirror those used for recipients (covered in Chapter 1) but there are a few donor-specific points worth highlighting.

ABO typing requires both forward AND reverse (Landsteiner’s rule, confirming agreement):

Forward typing (cell typing): mix donor RBCs with known antisera (anti-A, anti-B). Agglutination means the corresponding antigen is present.
Reverse typing (serum typing): mix donor serum with known A1 and B cells. Agglutination means the corresponding antibody is present in the serum.

The two must agree. Landsteiner’s rule: you make antibodies against the antigens you lack. Discrepancies require investigation before the unit is released. Reverse typing must be interpreted with caution if the donor has recently received plasma or platelets (exogenous antibodies can confuse the picture), but this is mostly a concern in recipients, not healthy donors.

Expected ABO results:

ABO group	Forward: anti-A	Forward: anti-B	Reverse: A1 cells	Reverse: B cells
O	0	0	4+	4+
A	4+	0	0	4+
B	0	4+	4+	0
AB	4+	4+	0	0

Group O has no A or B antigens but has both antibodies. Group O RBCs are “universal donor” (no antigens to react with), but Group O plasma is NOT universal donor (it has anti-A and anti-B that can attack the recipient’s RBCs). Group AB is the opposite: universal plasma donor, rarest type (~4%).

Rh typing:

Mix donor blood with anti-D reagent (forward testing only)
No reverse typing needed, because anti-D is NOT naturally occurring - it forms only after sensitization
If initial D testing is negative on a donor, weak D testing is required (extended incubation with anti-D, then addition of antihuman globulin)
A weak D-positive donor must be labeled as D-positive to prevent D-negative recipients from being exposed to D antigen

Recipients do NOT undergo routine weak D testing - if their initial D typing is negative, they receive D-negative blood (the conservative/safe approach). This asymmetry is a classic board question.

9.5 The Infectious Disease Testing Algorithm

Every blood donation in the United States undergoes mandatory testing for the following:

HIV-1/2: Antibody screening AND nucleic acid testing (NAT)
Hepatitis B: HBsAg, anti-HBc, AND NAT
Hepatitis C: Anti-HCV AND NAT
HTLV-I/II: Antibody screening
Syphilis: Serological testing (RPR or treponemal assay)
West Nile Virus: NAT
Zika virus: NAT (in endemic areas / periods)
Trypanosoma cruzi (Chagas disease): Antibody screening (one-time test)
Babesia: NAT or antibody (in endemic areas)

Each agent is tested based on the kinetics of its detectability and its transmission risk through stored blood. For some (HIV, HCV, HBV) the antibody-NAT combo is designed to catch both chronic and window-period infections. For others (Chagas) one-time testing suffices because infection is lifelong once established.

Platelet Bacterial Contamination Testing

Platelets are stored at 20 to 24°C (room temperature), which is a good temperature for bacterial growth. Because of this, platelets must be tested for bacterial contamination before release, unless the unit has been pathogen-reduced.

Bacterial testing methods:

Culture: gold standard, but takes time
Rapid antigen / immunoassay testing: faster but less sensitive
Large volume delayed sampling (LVDS): a newer approach using larger sample volumes with delayed culture

Pathogen reduction (psoralen + UVA, also called “amotosalen treatment”) inactivates bacteria, viruses, and parasites in the unit. A pathogen-reduced platelet bypasses the bacterial culture requirement, which is a major operational advantage. This is also why pathogen-reduced platelets and plasma have looser travel-based deferrals (see malaria section below).

9.6 Nucleic Acid Testing (NAT)

NAT revolutionized blood safety by dramatically reducing the “window period” - the time between infection and detectability.

Window period comparison:

Agent	Antibody / antigen detection	NAT detection
HIV	~22 days (Ab); shorter with p24 Ag combo	~9 days
HCV	~70 days	~7 days
HBV	~38 days (HBsAg)	~20 to 25 days

NAT methodology:

PCR or transcription-mediated amplification (TMA)
Detects viral nucleic acid directly (no antibody response required)
Can be performed on individual donations (ID-NAT) or minipools (MP-NAT)
ID-NAT is more sensitive; MP-NAT is more cost-effective

The difference between ID-NAT and MP-NAT matters when viremia is low. Pooling dilutes the target viral load, so MP-NAT has a slightly longer window than ID-NAT. For agents with very high transmission risk at low viral loads (HIV, HCV), most US centers have moved to ID-NAT.

9.7 Understanding Each Test

HIV Testing

4th generation assay: detects both HIV-1/2 antibodies AND p24 antigen. p24 is the viral capsid protein, detectable before seroconversion. This shortens the serologic window significantly.
NAT: detects HIV RNA. Catches acute infection before antibody or even p24 develops.
If screening is reactive, supplemental testing (HIV-1/2 differentiation assay) is performed to distinguish true positives from false positives and to identify the subtype.

The combination (4th gen + NAT) is why transfusion-transmitted HIV is now vanishingly rare.

Hepatitis B Testing

The three-test combination is designed to catch different phases of infection:

HBsAg: surface antigen - indicates acute or chronic infection
Anti-HBc: antibody to core antigen - indicates ever infected (current or resolved)
NAT: detects HBV DNA - catches occult HBV infection (HBsAg-negative donors who are still low-level viremic)

Occult HBV is the reason NAT was added to an already robust serology panel. Some chronically infected individuals suppress HBsAg production to undetectable levels but remain viremic and infectious.

HBV deferral rule: if any of HBsAg, anti-HBc, or HBV NAT is persistently positive → indefinite deferral. Chronic HBV (especially HBsAg+ carriers) remains infectious for life.

Hepatitis C Testing

Anti-HCV: indicates ever infected
NAT: detects current viremia (active infection)

The combination catches both active infection (NAT positive) and window-period infections (antibody-negative but NAT-positive).

HCV deferral: repeatedly reactive HCV → indefinite deferral. HCV is now curable with direct-acting antivirals, but the antibody test remains positive for life after infection, even after SVR (sustained virologic response), so these donors remain repeatedly reactive on screening and are permanently deferred. Policy prioritizes blood safety over restoring donor eligibility.

Syphilis Testing

Blood centers use either:

Non-treponemal test (RPR): traditional screening; may miss very early infection
Treponemal test (EIA / CIA): more sensitive; used in reverse algorithm

Treponema pallidum is fragile. It’s killed within 72 hours at 1 to 6°C (refrigerated RBC storage), so transfusion-transmitted syphilis from RBC units is extremely rare. Testing is still required, largely because syphilis is a marker for high-risk sexual behavior - a person with syphilis has elevated likelihood of HIV, HBV, or HCV co-infection. The deferral (3 months after treatment completion) is really about those co-infections, not the syphilis itself.

HTLV, Chagas, Babesia

HTLV-1/2: lifelong infection, causes adult T-cell leukemia/lymphoma and HTLV-associated myelopathy. Transmitted efficiently by cellular blood products (lives in T cells). Positive test → indefinite deferral.
T. cruzi (Chagas): one-time antibody test (infection is lifelong once established). Positive → indefinite deferral. Parasites persist in cardiac and GI tissue for decades, survive in stored blood, and have no curative treatment in the chronic phase. Endemic in Central and South America, increasing US prevalence from immigration.
Babesia: NAT or antibody in endemic Northeast US states. Positive Babesia NAT → 2 years deferral (Babesia can persist asymptomatically for years; 2 years allows clearance in most cases). After 2 years, donor can re-attempt; if repeat NAT is negative, eligible.

Malaria - NOT Tested in the Lab

There is no FDA-approved blood donor screening test for malaria. Donor eligibility depends entirely on the Donor History Questionnaire (DHQ) - travel and residence history, and prior malaria diagnosis. This is why travel-based deferrals (next section) are load-bearing for malaria prevention in the blood supply.

9.8 Donor Deferral and Lookback

Not every adverse finding means “never donate again.” The deferral framework matches the risk:

Repeat reactive: If a donor tests reactive on repeat testing, the donation is discarded, the donor is notified, and the donor is deferred (duration depends on the agent and confirmatory results).

Supplemental testing: Determines true positive vs. false positive; guides donor counseling and deferral duration.

Lookback: If a donor is found positive, previous donations are traced and prior recipients are notified.

Emergency Bypass of Infectious Screening

In rare emergencies or critical blood shortages (e.g., mass casualty events), infectious disease screening can be bypassed. Required safeguards:

Blood product label must clearly indicate infectious disease testing was omitted
Medical director of donor center must sign a release
Medical director of transfusion service must sign a release
Testing must still be completed as soon as possible (retrospectively)

If a retrospective test comes back positive, the recipient is notified and tested. Saving the life of someone actively exsanguinating takes priority over the screening window, but the paper trail is strict.

9.9 Behavioral and Travel Deferrals

These are the classic board-testable deferrals. A few patterns help:

3 months is the default for bloodborne-pathogen exposure (covers seroconversion window for HIV, HBV, HCV)
12 months for incarceration (cumulative risk over time in a high-risk environment)
Indefinite for confirmed chronic infection or unscreenable risk
2 years or longer for agents / medications that persist exceptionally long

3-month deferrals (bloodborne pathogen exposure window):

Event	Deferral
Tattoo / piercing at unregulated facility (non-sterile needles, non-single-use ink)	3 months
Ear / body piercing with non-single-use equipment	3 months
Travel to malaria-endemic area	3 months after departure from endemic area*
Blood transfusion received	3 months
Allogeneic tissue transplant (except dura mater)	3 months
Allogeneic bone marrow / HSC transplant	3 months
Needlestick / blood exposure	3 months
Pregnancy	6 weeks postpartum (exception: transfusion TO the fetus is allowed)
Living with HBV/HCV-positive person	3 months after moving out
Sexual contact with person with symptomatic HBV/HCV	3 months from last contact
Sexual contact with HIV-positive person	3 months from last contact
Anal sex with a new partner or more than one partner (any person, any orientation)	3 months from last encounter
Sex exchanged for money / drugs / payment	3 months from last instance
Sex with someone who exchanged sex for payment	3 months from last contact
Diagnosed with syphilis	3 months after treatment completion
Diagnosed with gonorrhea	3 months after treatment completion

*No deferral for pathogen-reduced platelets or pathogen-reduced plasma from malaria-travel donors - pathogen reduction inactivates Plasmodium. So: 3 months for RBCs, whole blood, non-pathogen-reduced platelets, non-pathogen-reduced plasma; no deferral for pathogen-reduced products.

Longer deferrals:

Event	Deferral
Incarceration ≥72 consecutive hours	12 months from release
Positive Babesia NAT	2 years
Injectable HIV PrEP (cabotegravir)	2 years after last dose
Lived >5 years in malaria-endemic area	3 years
Tattoo in state-regulated shop + sterile needles + single-use ink	No deferral
Ear / body piercing with single-use equipment	No deferral
Autologous transfusion / transplant	No deferral (your own tissue)

Notes on the anal sex deferral: this is a gender-neutral criterion under the 2023 FDA individual donor assessment update, which replaced the previous MSM-specific deferral. The 3-month deferral is triggered by anal sex with a new partner or with more than one partner in the past 3 months, not by any anal sex with any partner. Anal sex carries the highest per-act risk of HIV transmission due to mucosal fragility and high density of susceptible cells.

Indefinite deferrals:

HIV repeatedly positive or history of HIV treatment medications
HCV repeatedly positive
HBV (HBsAg, anti-HBc, or NAAT) persistently positive
HTLV-1/2
Chagas (T. cruzi)
Leishmaniasis (persists in macrophages for years, transfusion transmission documented)
Family history of CJD (50% risk of carrying the prion mutation)
Allogeneic dura mater transplant (prion risk)
Xenotransplantation - with exceptions for porcine heart valves (glutaraldehyde-fixed, no live cells) and porcine insulin (purified protein, no live cells)
Parous female donors - high-plasma-volume products (plasma, apheresis platelets) are typically restricted to reduce TRALI risk from anti-HLA antibodies. Most centers screen any female donor with prior pregnancy for HLA antibodies and defer from plasma / apheresis platelet donation if positive (see Chapter 4). RBC donations remain acceptable. [TODO: verify whether any specific parity threshold (e.g., ≥4 pregnancies) is used by AABB / FDA standards vs. the more common “any prior pregnancy → HLA screening” model.]

Clinical logic behind CJD / dura deferrals: there is no blood test for prion diseases. Cadaveric dura mater grafts (used in neurosurgery) were a documented route of CJD transmission before prion disease was understood. Familial CJD is autosomal dominant, so family members have 50% mutation risk. vCJD transmission by transfusion has been documented in the UK. Since we can’t screen, we defer.

9.10 Medication Deferrals

Medications are deferred for one of three reasons:

Teratogenic drugs that could harm a fetus if a pregnant woman received the blood
Platelet-inhibiting drugs that would render donated platelets non-functional
Anticoagulant drugs that would render donated plasma / whole blood with inadequate clotting factors

The common theme for teratogens: deferral duration is driven by drug half-life and tissue persistence.

Teratogens (big list; pharmacokinetics drive the duration):

Drug	Deferral	Why
Finasteride	1 month	5α-reductase inhibitor, t½ ~6 hr
Isotretinoin (Accutane)	1 month	Retinoid, t½ ~21 hr
Thalidomide / lenalidomide	1 month	Potent teratogens, now used for myeloma
Upadacitinib	1 month	JAK inhibitor (rheum / autoimmune)
Mycophenolate mofetil	6 weeks	Immunosuppressant, teratogenic
Dutasteride	6 months	5α-reductase inhibitor, t½ ~5 weeks
Vismodegib / sonidegib	2 years	Hedgehog pathway inhibitors for BCC
Leflunomide	2 years	DMARD - active metabolite is teriflunomide
Teriflunomide	2 years	MS drug, t½ ~15 to 18 days + enterohepatic recirculation
Acitretin	3 years	Psoriasis retinoid; can convert to etretinate with alcohol
Etretinate	Indefinite	Terminal t½ ~120 days, accumulates in fat for years

Key board-testable comparison: dutasteride (6 months) vs. finasteride (1 month). Same drug class, wildly different deferrals. Dutasteride has a ~5-week half-life; finasteride is ~6 hours. Duration of deferral tracks drug persistence.

Acitretin and etretinate are linked: acitretin is the shorter-acting successor to etretinate, but in the presence of alcohol, acitretin can convert back to etretinate, hence the long 3-year deferral. Etretinate itself has an indefinite deferral because it can be detected in blood years after the last dose.

Antiplatelet agents (affect platelet donation specifically - platelets lose function if donor is on these):

Drug	Deferral	Mechanism
Piroxicam	2 days	NSAID with antiplatelet effect
Aspirin	2 days	COX inhibitor
Prasugrel (Effient)	3 days	Irreversible P2Y12 inhibitor, short offset
Ticagrelor (Brilinta)	7 days	Reversible P2Y12 inhibitor, persistent metabolite
Clopidogrel (Plavix)	14 days	Irreversible P2Y12 inhibitor - blocks platelets for full lifespan
Ticlopidine (Ticlid)	14 days	Same class as clopidogrel
Vorapaxar (Zontivity)	1 month	PAR-1 antagonist, very long effect

Memory trick for the P2Y12 inhibitors: prasugrel 3d, ticagrelor 7d, clopidogrel 14d - roughly doubling. Antiplatelet deferrals don’t affect RBC or plasma donation.

Anticoagulants (affect plasma / whole blood - donated product would have inadequate clotting factors):

Drug	Deferral
DOACs / NOACs (apixaban, rivaroxaban, dabigatran, edoxaban, fondaparinux)	2 days
Enoxaparin / dalteparin (LMWH)	2 days
Warfarin	7 days
Heparin (UFH)	7 days

Warfarin’s 7-day deferral makes sense physiologically: it inhibits vitamin K-dependent factors (II, VII, IX, X), and factor II (prothrombin) has a half-life of ~60 hours, so full factor recovery takes several days. UFH’s 7-day deferral is more conservative than its short half-life would suggest, but it likely reflects the fact that patients on heparin usually have an underlying condition (DVT, PE, post-op) that would independently defer them.

Antimicrobials:

Active infection on oral antibiotics → deferred until resolution of infection OR completion of antibiotics, whichever is later
Active infection on injection antibiotics → 10 days after last injection (per Red Cross) - reflects more severe underlying infection and longer tissue distribution

Other specific agents:

HIV PrEP (oral): emtricitabine + tenofovir → 3 months after last dose
HIV PEP (oral): emtricitabine + tenofovir + raltegravir → 3 months after last dose
HIV PrEP (injectable, cabotegravir): 2 years after last dose (long-acting injectable persists in tissue for months)
Hepatitis B immune globulin (HBIG): 3 months (HBIG is given after HBV exposure → covers seroconversion window)

The oral vs. injectable PrEP distinction is high-yield. The deferral difference is driven entirely by drug persistence: oral drugs clear in days; injectable cabotegravir persists for months and could mask an early HIV infection (suppressing viremia below NAT detection).

9.11 Vaccination Deferrals

Live attenuated vaccines have a brief period of viremia / viral shedding. Killed vaccines don’t.

Vaccine type	Examples	Deferral
MMR (contains rubella)	Measles-mumps-rubella combo	4 weeks
Varicella	Chickenpox (Varivax), shingles (Zostavax-live)	4 weeks
Other live attenuated	Oral polio (Sabin), oral typhoid (Ty21a), yellow fever, measles alone, mumps alone	2 weeks
Inactivated / recombinant / mRNA / toxoid	HPV, COVID (all types), pneumococcal, influenza (injectable), hepatitis A, hepatitis B, tetanus, DTaP, IPV (Salk), Vi polysaccharide typhoid, Shingrix (recombinant)	No deferral

Memory trick: rubella and varicella need 4 weeks; other live vaccines need 2 weeks; killed vaccines need 0. Rubella alone is 4 weeks, which is why MMR (which contains rubella) is 4 weeks. Measles alone or mumps alone → 2 weeks.

9.12 Post-Donation Intervals

How long before a donor can give again?

Donation type	Interval
Whole blood or single-unit RBCs	8 weeks (56 days)
Double-unit apheresis RBCs	16 weeks
Plateletpheresis	2 days, with additional limits (typically up to 24 times per year)
Plasmapheresis	Varies by frequency (typically up to 2x per week with ≥48 hours between)

The 8-week interval allows adequate RBC regeneration (plasma volume restores within 24 to 48 hours; RBC mass takes 4 to 8 weeks, driven by EPO-stimulated erythropoiesis). Double-unit RBC = 16 weeks because twice the RBCs were removed → twice the recovery time.

9.13 The Physical Collection Process

Once the donor is eligible and consented, the actual collection has its own set of rules.

Aseptic, closed sterile system:

Blood must be collected in an aseptic manner into a closed sterile system
All collection bags, satellite bags, and tubing are connected before sterilization and remain sealed throughout
If the system is “opened” (penetrated), sterility is compromised → shelf life drops dramatically: RBCs 42 days → 24 hours; platelets 5 to 7 days → 4 hours
Sterile connection devices (SCD) allow tubing to be joined by melting and fusing plastic under controlled heat → maintains sterility → no shelf life reduction. Used for pooling, aliquoting, and adding solutions.

Diversion pouch:

For platelets prepared from whole blood or apheresis, a diversion pouch must divert the first 30 to 45 mL of blood
This contains the skin plug (bacteria introduced during venipuncture)
Dramatically reduces platelet bacterial contamination
Especially critical for platelets because they’re stored at room temperature (ideal for bacterial growth)
The diverted blood is often used for the lab testing tubes

Test tubes:

Blood tubes for laboratory testing must be collected, properly labeled with donor information, and accompany the unit
Used for ABO/Rh typing, antibody screening (if indicated), and all infectious disease testing
Labeled at the time of collection - mislabeling at this step is a root cause of serious transfusion errors

Segments:

After collection, the tubing is pinched to create sealed blood-filled segments
These can be cut off and used for crossmatch, QC, or transfusion reaction investigation - without entering the main unit
Typically 6 to 8 segments per unit

Maximum draw volume: 10.5 mL/kg (including samples), approximately 15% of total blood volume. This is why the 110 lb weight minimum exists.

Collection time for component preparation: If platelets or plasma will be separated from whole blood, the collection must be completed within 15 minutes. Prolonged collection → platelet activation and clotting factor degradation → poor component quality. This doesn’t apply to apheresis collection, where components are separated in real time by the machine.

Bag sizes: 450 mL (±45 mL) (most common) or 500 mL (±50 mL). The bag has a proportional amount of anticoagulant (CPD, CPDA-1, or CP2D) based on expected blood volume. The ±10% tolerance keeps the anticoagulant-to-blood ratio acceptable.

“Red Blood Cells Low Volume” label: Required on units collected at 60 to 90% of the bag’s ideal volume (< 405 mL in a 450 mL bag; < 450 mL in a 500 mL bag). The anticoagulant-to-blood ratio is off, so plasma and platelets can’t be reliably prepared. These units are only usable as PRBCs - no further component preparation. Units < 60% are discarded entirely.

Processing timelines:

Within 8 hours of collection for frozen plasma (FFP) and platelet concentrate. Factors V and VIII degrade rapidly at room temperature.
Within 24 hours - plasma can still be frozen but labeled as FP24 (Frozen Plasma within 24 hours). FP24 has lower factors V and VIII than true FFP but adequate for most clinical uses.
Platelets cannot be made from whole blood held > 8 hours (quality too degraded).

Transport temperature:

1 to 10°C if only PRBCs will be made (keeps RBCs viable)
20 to 24°C if platelets will also be separated (cold activates / damages platelets → can’t yield good platelet concentrates)

The decision about which components to prepare must be made BEFORE transport, because the transport temperature locks in what’s possible.

9.14 Donor Adverse Reactions

Donation is generally safe, but a handful of reactions are common enough to know cold.

Vasovagal Reaction

Presentation: slow heart rate, low BP, dizziness, syncope / near-syncope
Mechanism: parasympathetic surge → bradycardia + vasodilation → hypotension → syncope
Treatment: elevate feet above heart level, cold compress on neck, loosen clothing
Risk factors: young age, low blood volume, fear / anxiety, first-time donor
Most common donor adverse event

Vasovagal usually resolves within minutes with supportive care. The bradycardia is the distinguishing feature - this is what separates vasovagal from hypovolemic reaction.

Hypovolemic Reaction

Presentation: increased heart rate, nausea, syncope / near-syncope
Mechanism: compensatory tachycardia from volume loss (heart tries to maintain cardiac output)
Treatment: fluids (IV or oral), Trendelenburg position
Most common in smaller donors who lose a larger percentage of their blood volume

Key diagnostic point: tachycardia distinguishes hypovolemia from vasovagal (bradycardia). Same general feeling of near-syncope, but very different mechanisms and treatments.

Hyperventilation

Presentation: rapid respirations, facial twitches, seizure-like activity
Mechanism: anxiety → hyperventilation → respiratory alkalosis → decreased ionized calcium → paresthesias, carpopedal spasm, muscle twitching
Treatment: breathe into paper bag (rebreathes CO₂ → corrects alkalosis → restores ionized calcium)

Important distinction from citrate toxicity: similar symptoms (perioral tingling, muscle twitching, potentially seizures), but hyperventilation occurs during whole blood donation from anxiety, while citrate toxicity occurs during apheresis from citrate anticoagulant return. Different cause, similar symptoms - but treatment is different.

Citrate Effect (Hypocalcemia)

Presentation: perioral tingling, nausea, vomiting, ± arrhythmias, ± seizures
Mechanism: citrate anticoagulant returned to the donor during apheresis chelates calcium → hypocalcemia
Treatment: slow down the infusion; give calcium (oral or IV)
When it occurs: apheresis (especially plateletpheresis), NOT whole blood donation. In WB donation, citrate stays in the collection bag; it never goes back to the donor.

During apheresis, blood is drawn, the target component is extracted, and the remaining components (with citrate) are returned to the donor. The liver metabolizes citrate, but if the return rate outpaces hepatic metabolism, ionized calcium drops. Treatment is simple: slow the return, give calcium.

Hematoma

Presentation: rapid swelling around the needle
Mechanism: blood extravasates into surrounding tissue (through-and-through puncture, inadequate post-donation pressure)
Treatment: remove tourniquet and needle, apply firm pressure ≥ 10 minutes, ice to reduce swelling

Not dangerous, but uncomfortable and can deter future donations. Common enough that every blood center trains staff on prevention (proper vein selection, steady needle technique).

Stem Cell Donation and GVHD Risk

One useful cross-reference for HSC donation: bone marrow-derived HSC transplant has lower risk of transfusion-associated GVHD than peripheral blood-derived HSC transplant. Peripheral blood stem cells (collected after G-CSF mobilization) contain more T lymphocytes than bone marrow → more donor T cells engraft → higher GVHD risk. Peripheral blood HSCs engraft faster though, so the choice involves balancing GVHD risk against engraftment speed. This matters for HSC donors but is also covered in the special clinical situations chapter (Chapter 8).

Chapter 10: HLA and Platelet/Leukocyte Immunology

The HLA (Human Leukocyte Antigen) system is the human major histocompatibility complex - the most polymorphic genetic system in the body. Understanding HLA is essential for blood banking because HLA antibodies cause platelet refractoriness, TRALI, and TA-GVHD. HLA is also the gatekeeper for solid organ and stem cell transplantation, and it drives a long list of disease associations that show up on boards.

10.1 HLA Biology: Why It Matters

What HLA molecules do: HLA molecules present peptide antigens to T cells, enabling the adaptive immune system to recognize foreign proteins. The extreme polymorphism (thousands of alleles) evolved to ensure that the population can respond to diverse pathogens. However, this polymorphism creates problems in transfusion and transplantation - foreign HLA molecules are highly immunogenic.

Location in the genome: The HLA genes live in the Major Histocompatibility Complex (MHC) on chromosome 6p21. The MHC is the most gene-dense and polymorphic region in the human genome. Beyond the classical HLA genes, the MHC locus also encodes complement proteins (C2, C4A, C4B, Factor B), TNF-alpha, lymphotoxin, heat shock proteins, 21-hydroxylase (CYP21A2), HFE, and NOTCH4. This co-location explains several classic associations: HFE mutations (C282Y, H63D) in hereditary hemochromatosis show linkage with HLA-A3; C4 null alleles are associated with SLE; and CYP21A2 deficiency causes congenital adrenal hyperplasia.

Class I vs. Class II HLA molecules: This distinction has direct clinical implications.

HLA Class I (HLA-A, HLA-B, HLA-C) molecules are expressed on virtually all nucleated cells and on platelets. They present intracellular (endogenous) peptides to CD8+ cytotoxic T cells. Antibodies to Class I HLA cause platelet refractoriness because platelets express Class I. Of the Class I loci, HLA-A and HLA-B are the most polymorphic and the most clinically important for transplantation matching.

HLA Class II (HLA-DP, HLA-DQ, HLA-DR) molecules are expressed primarily on professional antigen-presenting cells (dendritic cells, macrophages, B cells). “Monocyte-derived cells” here includes tissue macrophages, dendritic cells, Langerhans cells, Kupffer cells, and Hofbauer cells - so when a card asks about APCs expressing Class II, think monocyte-derived cells and B lymphocytes as the stem answer. They present extracellular (exogenous) peptides to CD4+ helper T cells. Class II expression can be induced on activated T cells, endothelial cells, and epithelial cells by IFN-gamma. In the thymus, thymic epithelial cells express Class II for positive and negative selection of CD4+ T cells. Unlike Class I (on all nucleated cells), Class II has restricted expression, which ensures CD4+ activation only happens in the context of a proper professional APC.

HLA Class III is a separate concept - Class III genes encode C2, C4, Factor B, and TNF-alpha, not cell-surface HLA molecules. The Class III region sits between Class I and Class II on 6p21. If a question asks “which HLA class encodes complement proteins,” the answer is Class III.

Structure of Class I and Class II: Class I and Class II look different, and the structural difference is worth knowing.

Class I: a polymorphic alpha chain (encoded in the MHC on chromosome 6) non-covalently associated with beta-2 microglobulin (encoded on chromosome 15, non-polymorphic). Only the alpha chain is polymorphic. The alpha chain has three extracellular domains (alpha-1, alpha-2, alpha-3). The peptide-binding groove is formed by the alpha-1 and alpha-2 domains and holds 8 to 10 amino acid peptides. The groove is closed at both ends, which restricts peptide length. Beta-2 microglobulin stabilizes the complex, and serum beta-2 microglobulin is a prognostic marker in myeloma and CLL.
Class II: an alpha chain and a beta chain, both encoded in the MHC, and both polymorphic. The peptide-binding groove is formed by alpha-1 and beta-1 and holds 13 to 25 amino acid peptides because the ends of the groove are open.

Cell distribution - important details: HLA Class I is on all nucleated cells and platelets. Mature RBCs lack Class I because they lose their nucleus and Class I expression during erythroid maturation. Early RBC precursors do express Class I, and a subset of people carry so-called Bennett-Goodspeed (Bg) HLA antigens (usually HLA-A28, HLA-B7, or HLA-B17) that persist on mature RBCs. This is a board trivia-level detail but explains rare cases of HLA-mediated RBC effects.

The practical consequence: RBCs themselves are not subject to HLA-mediated rejection, but RBC units cause HLA alloimmunization through contaminating white cells. This is why leukoreduction matters.

Antigen processing pathways: Understanding which pathway handles which kind of pathogen is how Class I vs. Class II becomes intuitive.

Class I presents intracellular antigens - viral proteins in infected cells, mutant proteins in tumor cells, self-proteins for tolerance. Cytoplasmic proteins are degraded by the proteasome, transported into the ER by TAP (transporter associated with antigen processing), loaded onto MHC I, and displayed for CD8+ T cell surveillance.
Class II presents extracellular antigens - bacterial proteins, parasitic antigens, allergens. Antigens are internalized and degraded in endosomes/lysosomes. MHC II in the ER is chaperoned by the invariant chain (CLIP fragment) until it reaches the endosome, where CLIP is removed and antigenic peptide is loaded.

Effector consequences: Cells displaying foreign peptide on Class I are destroyed by CD8+ cytotoxic T cells via either perforin/granzyme (perforin makes pores, granzymes activate caspases) or Fas/FasL (FasL on CTL binds Fas/CD95 on target) - both paths end in apoptosis. Class II engagement activates CD4+ helper T cells, which then license B cell antibody production and macrophage killing.

HLA inheritance: Each person inherits one HLA haplotype from each parent - a set of linked HLA genes on chromosome 6 that are inherited as a block. The genes sit close enough on chromosome 6 that significant meiotic recombination is rare, so the whole cluster of HLA-A, -B, -C, -DR, -DQ, -DP alleles from one parental chromosome travels together. Mechanistically: prophase I of meiosis involves crossover between homologous chromosomes, but genes clustered this tightly almost always segregate as a single unit. That is why offspring inherit one of four possible HLA haplotype combinations from two parents.

Because only 4 possible haplotype combinations exist across two parents, sibling pairs fall into three categories:

25% chance HLA-identical (share both haplotypes)
50% chance haploidentical (share 1 haplotype)
25% chance fully mismatched

This is why siblings are the preferred stem cell donors. The probability of finding at least one HLA-identical sibling among N total siblings is 1 - (0.75)^N. Practical numbers: 1 sibling gives 25%, 2 siblings give 44%, 3 siblings give 58%, 4 siblings give 68%. This math is board-testable.

Linkage disequilibrium: Certain HLA allele combinations appear together more often than chance predicts because recombination is suppressed across the MHC. The classic example is the HLA-A1/B8/DR3 “ancestral haplotype” in people of Northern European descent, which tracks with several autoimmune diseases.

10.2 HLA Antibodies: How They Form and What They Cause

Sensitizing events: HLA antibodies develop after exposure to foreign HLA antigens. The three main sensitizing events are:

Pregnancy: The fetus expresses paternal HLA antigens, which the mother’s immune system sees as foreign. About 30% of multiparous women have HLA antibodies. This is why female donors are more likely to have HLA antibodies that can cause TRALI.

Transfusion: Cellular blood products (RBCs before universal leukoreduction, platelets) expose recipients to donor HLA antigens. This is why leukoreduction reduces HLA alloimmunization.

Transplantation: Patients who reject a transplant typically develop strong HLA antibodies.

Clinical consequences of HLA antibodies:

Platelet refractoriness: Class I HLA antibodies bind to donor platelets and cause their rapid destruction. This is the most common immune cause of poor platelet increments. Non-immune causes (fever, infection, sepsis, DIC, splenomegaly, drugs like amphotericin and vancomycin) are actually more common overall, so the workup has to exclude those first. Management of HLA-mediated refractoriness requires HLA-matched platelets (selected to be compatible with the patient’s antibodies) or crossmatch-compatible platelets.

TRALI: Donor HLA antibodies (in plasma-containing products) can bind to recipient neutrophils and trigger the pulmonary capillary leak that causes TRALI. This is why plasma is now preferentially collected from male donors or HLA antibody-negative female donors.

TA-GVHD: When the recipient cannot reject donor T cells, those T cells attack recipient tissues. The risk is highest when the donor is HLA-homozygous for a haplotype the recipient is heterozygous for - the recipient’s immune system sees the donor cells as “self.”

Public vs. private antigens and cross-reactive groups: HLA antibodies are not always as specific as the allele nomenclature suggests. A cross-reactive antigen group (CREG) is a cluster of HLA antigens that share enough structural similarity to be targeted by the same antibody. The shared epitope is a public antigen; the unique epitope on each individual HLA molecule is the private antigen.

The two public antigens that matter most are HLA-Bw4 and HLA-Bw6. They are present on almost all HLA-B molecules (Bw4 is also found on a subset of HLA-A molecules). Every normal individual carries Bw4 or Bw6 - usually both across their two HLA-B alleles, occasionally homozygous for one, essentially never neither. Bw4 is the ligand for KIR3DL1, an NK cell inhibitory receptor, so Bw4 engagement tells NK cells not to kill. Bw6 has no known KIR partner. This matters for haploidentical stem cell transplantation, where NK alloreactivity is part of the graft-versus-leukemia effect.

Board-style pitfall: a patient who lacks both Bw4 and Bw6 is extremely rare and clinically dangerous. Any exposure (transfusion, pregnancy) can produce antibodies against Bw4 and/or Bw6, and those antibodies react against nearly every HLA-B molecule in the population. Finding compatible platelets or organs for such a patient is extraordinarily difficult. This scenario shows up as a “why is this a problem?” stem.

Understanding CREGs matters for three reasons: interpreting broad antibody reactivity (one antibody, many apparent specificities), selecting acceptable donor mismatches when a perfect match is not available, and explaining why a single sensitizing event can produce broadly reactive antibodies.

10.3 HLA Testing Methods

When do we test HLA? The three main clinical indications:

Pre-transplantation workup (solid organ and stem cell)
Platelet refractoriness workup
HLA-linked autoimmune disease diagnosis (HLA-B27 for ankylosing spondylitis, HLA-DQ2/DQ8 for celiac, HLA-B*57:01 before abacavir)

Serological typing (complement-dependent cytotoxicity, CDC): The historical method. Known anti-HLA sera are mixed with patient lymphocytes in the presence of complement. If the antibody binds, complement activates and lyses the cell; cell death is read by dye uptake (eosin, trypan blue). The same CDC technology is used three ways:

HLA antigen typing (known sera vs. patient cells)
HLA antibody detection (patient serum vs. panel of typed cells)
HLA crossmatching (patient serum vs. donor lymphocytes)

CDC identifies HLA antigens at the antigen level (e.g., HLA-A2) but cannot distinguish between allelic variants. It only detects complement-fixing antibodies, which is the main sensitivity gap.

Mixed lymphocyte culture (MLC): A functional assay that measures recipient T cell reactivity against donor HLA Class II antigens. Donor lymphocytes are irradiated (so they cannot proliferate) and co-cultured with recipient lymphocytes. If recipient CD4+ T cells recognize the foreign Class II, they proliferate, measured by tritiated thymidine uptake. MLC has been replaced by molecular typing and crossmatch methods in routine practice but remains conceptually important: it was the original functional readout of alloreactivity, and the Class II distinction is a testable point.

Molecular (DNA-based) typing: PCR-based methods have replaced serology for most applications because they are more accurate and can resolve allelic differences. Compared with CDC, molecular typing detects all alleles (serology misses some), provides higher resolution (allele-level vs. antigen-level), is not affected by cell viability, and can distinguish serologically identical alleles that carry different clinical significance.

Low-resolution typing (antigen level): Identifies the antigen group (e.g., HLA-A*02). Used for platelet matching and initial transplant workup.
Intermediate resolution: often sufficient for solid organ transplantation.
High-resolution typing (allele level, e.g., HLA-A*02:01): Required for unrelated donor hematopoietic stem cell transplantation, where even allelic mismatches increase graft-versus-host disease risk.

Common molecular techniques: PCR-SSO (sequence-specific oligonucleotide), PCR-SSP (sequence-specific primers), SBT (sequence-based typing), and NGS.

HLA antibody detection (solid-phase assays): Solid-phase assays using beads coated with purified HLA molecules have largely replaced CDC for antibody detection. Luminex single-antigen bead (SAB) assays can identify antibody specificity at the single-allele level, which is essential for:

Selecting compatible platelets for refractory patients
Identifying “unacceptable antigens” for solid organ transplant candidates
Virtual crossmatching in transplantation

Panel reactive antibody (PRA): The PRA quantifies how broadly a patient is sensitized. Traditional PRA: patient serum is tested against a panel of cells representing the HLA diversity of the donor population, and PRA equals the percentage of panel cells that react. Modern calculated PRA (cPRA) uses Luminex SAB data to identify which HLA antigens the patient has antibodies against, then calculates the percentage of actual donors in the registry that carry any of those antigens.

Higher PRA means more sensitized and harder to find a compatible donor. PRA >80% defines “highly sensitized.” The PRA is how you monitor a transplant candidate on the waiting list over time.

HLA crossmatch: The crossmatch asks a specific question: does this recipient have preformed antibodies against this specific donor? A positive crossmatch predicts hyperacute rejection and is generally a contraindication to transplantation. Three techniques, mnemonic “CFL” (Crossmatch For Life):

Complement-dependent cytotoxicity (CDC): traditional, detects complement-fixing antibodies, lowest sensitivity.
Flow cytometry crossmatch: more sensitive than CDC. Recipient serum is incubated with donor lymphocytes, then labeled with fluorescent anti-human IgG. T cells and B cells are gated separately. Because T cells express only Class I and B cells express both Class I and Class II, a positive T cell crossmatch = anti-Class I antibodies, and a positive B cell crossmatch = anti-Class II (or Class I) antibodies. Flow crossmatch also detects non-complement-fixing antibodies.
Luminex (solid-phase): highest sensitivity, identifies specific donor HLA antigen targets, enables virtual crossmatch (combining patient’s antibody profile with donor’s HLA typing without physically mixing cells).

Sensitivity trade-off: increasing sensitivity means fewer false negatives but more low-level positive results whose clinical significance is debatable. This is a real interpretation problem in solid organ transplantation.

10.4 Clinical Applications

Platelet matching for refractory patients: When a patient has documented HLA antibodies causing platelet refractoriness:

HLA type the patient (Class I antigens)
Identify antibody specificities using single-antigen bead testing
Select donors who lack the antigens the patient has antibodies against
Alternatively, perform crossmatch testing between patient serum and potential donor platelets

TRALI mitigation: Blood centers now preferentially collect plasma from:

Male donors (never pregnant)
Female donors tested negative for HLA antibodies

This strategy has significantly reduced TRALI incidence.

Solid organ transplant HLA requirements: The rules differ by organ, driven mostly by ischemic tolerance and organ scarcity.

Kidney: ideally ABO compatible and HLA matched, with a negative crossmatch. The core HLA loci for matching are HLA-A, HLA-B, and HLA-DR (2 alleles each, so matches are described as “6/6,” “5/6,” “4/6,” etc.). HLA-DR matching has the biggest impact on graft survival. A 0-antigen mismatch (6/6) has the best long-term outcomes. Deceased donor kidney allocation considers HLA match quality, PRA sensitization level, time on the waiting list, geographic proximity, and medical urgency. Living donor kidneys generally have better outcomes than deceased donor kidneys. Modern immunosuppression (calcineurin inhibitor + mycophenolate + steroids, with or without induction using anti-thymocyte globulin or basiliximab) allows successful transplants even with incomplete HLA matching - even 0/6 matched kidneys can have acceptable outcomes with current regimens. Still, better matching correlates with longer graft survival, lower rejection rates, and potentially lower immunosuppression doses (reducing downstream side effects like infection, malignancy, and nephrotoxicity).
Heart, lung, liver: ABO compatibility is required; HLA matching is NOT required. Short ischemic tolerance and organ scarcity make HLA matching impractical. A prospective crossmatch is generally not performed, though retrospective crossmatch and antibody screening are routine. Highly sensitized candidates may benefit from virtual crossmatch to avoid donor-specific antibodies.

Stem cell transplant donor selection: Allogeneic stem cell transplant has the most stringent HLA matching requirements. The transplanted immune system has to coexist with the recipient’s tissues long-term, and mismatches in either direction cause problems:

Host-versus-graft mismatch -> graft rejection
Graft-versus-host mismatch -> GVHD

Matching loci for stem cell transplant expand beyond the solid-organ set: HLA-A, -B, -C, -DRB1, and usually -DQB1.

Sibling donors: HLA-identical (matched at HLA-A, -B, -C, -DR) preferred
Unrelated donors: 8/8 or 10/10 allele-level match at HLA-A, -B, -C, -DRB1 (and often DQB1)
Cord blood: can tolerate more mismatching because of naïve T cell populations
Haploidentical: half-matched family donors, now feasible with post-transplant cyclophosphamide
HLA-DPB1 permissive mismatches are generally tolerated

Even single allele-level mismatches significantly increase GVHD risk, which is why high-resolution molecular typing is the standard for stem cell transplant, not antigen-level serology.

Key Concept: In solid organ transplants, ABO matching is the #1 priority. In hematopoietic stem cell transplants, HLA matching is the #1 priority - ABO matching is NOT required. Major ABO incompatibility (recipient has antibodies against donor RBCs) is managed by RBC depletion of the graft. Minor ABO incompatibility (donor has antibodies against recipient RBCs) may cause delayed immune hemolysis.

HLA Disease Associations

HLA Allele	Associated Disease	Notes
HLA-A3	Hereditary Hemochromatosis	HFE gene on chromosome 6 near HLA-A; diagnose by HFE mutation (C282Y/H63D)
HLA-B27	Ankylosing Spondylitis (>90%), Reactive Arthritis, Psoriatic Arthritis, IBD arthritis, Whipple disease	Seronegative spondyloarthropathies; “bamboo spine.” Whipple is an infection, not autoimmune, but also tracks with B27.
HLA-B51	Behçet Disease	Oral/genital ulcers + uveitis
*HLA-B57:01**	Abacavir hypersensitivity	Test before starting abacavir; positive = do not use
HLA-DR2	Multiple Sclerosis, Goodpasture Syndrome	Anti-GBM antibodies in Goodpasture
HLA-DR3 + DR4	Type 1 Diabetes Mellitus	Strongest genetic HLA association; DR3/DR4 heterozygote = highest risk
HLA-DR4	Rheumatoid Arthritis	Anti-CCP antibody more specific for diagnosis
HLA-DQ2/DQ8	Celiac Disease	Near-100% NPV: if negative for both, celiac essentially excluded

Chapter 11: Blood Bank Quality and Regulation

Transfusion medicine is one of the most heavily regulated areas in all of laboratory medicine. Blood is both a drug and a biologic, so oversight stacks from multiple directions: FDA, CLIA, CMS, state health departments, and voluntary accreditors like AABB and CAP. This chapter covers who regulates what, what QC you owe on reagents and components, how long you’re required to keep records, and the administration safety steps that prevent the single most common cause of fatal transfusion reactions - clerical error.

11.1 Regulatory Framework

FDA (Food and Drug Administration)

Blood is regulated by the FDA’s Center for Biologics Evaluation and Research (CBER). Products are treated as both drugs and biologics under federal law, which is why the oversight is stricter than for ordinary clinical lab tests.

Blood establishments must register with FDA and comply with Current Good Manufacturing Practice (cGMP)
FDA sets mandatory standards for donor eligibility, infectious disease testing, product labeling, storage conditions, and manufacturing practices
Reporting: transfusion-related deaths must be reported to FDA within 24 hours
Routine inspections of blood establishments (typically every 2 years)
Violations can escalate from warning letters to consent decrees to facility closure

AABB (Association for the Advancement of Blood and Biotherapies)

AABB accreditation is voluntary, not legally required
Standards often exceed FDA minimums
Biennial inspection cycle for accreditation
Most hospital blood banks and blood centers pursue AABB accreditation because it signals quality beyond the federal floor

CLIA and CMS

CLIA (Clinical Laboratory Improvement Amendments) certification is mandatory for any lab performing testing on human specimens
Covers the testing side of blood banking (typing, screening, crossmatch, component QC)
CMS enforces CLIA

CAP (College of American Pathologists)

Voluntary accreditation, commonly pursued
Proficiency testing program (external quality assessment) that most blood banks enroll in

State regulations

Some states layer on additional requirements
New York is the classic example of notably stringent state-level oversight

The practical hierarchy for a US blood bank: FDA and CLIA are mandatory, AABB and CAP are voluntary but effectively expected, and state rules add a wrapper on top.

11.2 Quality Control Requirements

Reagent QC:

Anti-A, anti-B, anti-D: Tested each day of use with known positive and negative cells
AHG (anti-human globulin): Tested each day of use with IgG-sensitized cells (Coombs control cells) to confirm reagent activity. Coombs control cells are also added to any negative AHG result to confirm the test was valid (i.e., reagent was actually added and remained active after washing)
Reagent red cells (screening and panel cells): Verified for expected antigen expression

Component QC (random sample testing to verify product quality):

Red cells: Hematocrit, residual WBC count if leukoreduced (must be <5 x 10^6 per unit)
Platelets: Platelet count, pH must be >6.2 at outdate, residual WBC count
Plasma: Factor VIII activity, volume

Temperature monitoring:

Continuous monitoring of storage equipment (refrigerators, freezers, platelet incubators)
Audible and visual alarms for out-of-range temperatures
Documentation of corrective action any time temperature deviates

11.3 Record Retention

Record retention is heavily board-tested because the rules are specific and the logic tracks a real safety concern: you might need to trace a transfusion reaction, identify recipients of a newly-infectious donor, or provide antigen-negative blood decades after an antibody was first identified. There are effectively three tiers.

The three-tier framework

Donor records (at collecting facility): 10 years
Recipient records (at transfusion service): 5 years or 10 years, depending on the record
Problematic donors or recipients: indefinitely

The logic: operational quality records get 5 years, patient-care records get 10 years, and anything that directly impacts future patient safety gets retained forever.

5-year retention (operational quality records)

These are records that support the quality system but don’t directly track individual donor-recipient relationships:

Quality control documents
Quality management reviews (audits, corrective actions, process improvements)
Proficiency testing (CAP or equivalent external QA)
Instrument QC and maintenance records
Retyping of donor units on receipt (the transfusion service confirms ABO on every RBC unit received - a catch-step for collection facility errors)
Inspection of donor units on receipt (visual check for labeling, color, hemolysis, clots, leaks, transport temperature)
Annual review of procedures and discontinued procedures

10-year retention (patient-care and donor records)

Donor records at collecting facilities: all retained 10 years (covers late-presenting transfusion-transmitted infections and traceback investigations)
Patient pretransfusion testing results (ABO/Rh, antibody screen, crossmatch)
Transfusion records (which units went to which patients - essential for lookback if a donor later tests positive for an infectious disease)
Transfusion reaction interpretation and evaluation
Therapeutic apheresis and phlebotomy records
Final unit disposition (transfused, returned, discarded, transferred)
Employee signatures, initials, and identification codes (for error investigation)

Indefinite retention

Retain forever because they directly affect future transfusion safety:

Permanently deferred donors (prevents them from donating at another facility)
Clinically significant or unexpected antibodies (an evanesced anti-K from 15 years ago still mandates K-negative blood because it can resurge on re-exposure)
Special transfusion requirements (irradiated blood for immunocompromised patients, washed for IgA deficiency, CMV-negative, etc.)
Transfusion reactions and transfusion problems

The memorable summary: 5 for quality, 10 for patient care, forever for safety-critical.

11.4 Blood Administration Safety

Critical steps to prevent transfusion to the wrong patient:

Positive patient identification: Two independent identifiers at sample collection and at transfusion
Sample labeling: At bedside, immediately after collection (not at the nursing station, not later)
Verification at transfusion: Confirm unit identification matches patient identification at the bedside before infusion starts
Clerical check: The most common cause of fatal transfusion reactions is clerical error, wrong blood to the wrong patient from a mislabeled sample or a bedside ID failure

This is the board-testable punchline of the chapter: sophisticated reagents and NAT screening catch vanishingly rare donor-side errors, but the dominant mode of a fatal outcome remains a human identifying a patient incorrectly. Every step in the administration pathway exists to force a redundant check at a point where that error could be caught.

PART II: CLINICAL CHEMISTRY AND IMMUNOPATHOLOGY

Clinical chemistry is the laboratory discipline that measures chemical substances in body fluids to aid in diagnosis, treatment, and monitoring of disease. It bridges biochemistry and clinical medicine, requiring understanding of both analytical methods and pathophysiology.

Chapter 12: Laboratory Principles and Quality Control

Clinical chemistry is where physics, biochemistry, and process engineering meet patient care. This chapter is the foundational toolkit for everything else in chemistry, toxicology, endocrinology, and therapeutic drug monitoring. The guts of “how does this analyzer know the glucose is 87” live here: the optical principles behind spectrophotometry, the electrochemistry behind blood gases, the mass spec behind steroid panels and immunosuppressants, and the immunoassay architecture behind hormones and tumor markers. We also cover the whole pre-analytical lifecycle (specimen collection through transport) and the QC scaffolding that tells you when results can be trusted.

The organizing idea: every clinical chemistry result is the end of a long chain - patient identification, tube choice, draw technique, transport, processing, analysis, calibration, QC review, release. Most errors happen before the sample ever touches the analyzer. Understanding the chain makes you a better pathologist, a better consultant, and a better test orderer.

12.1 Pre-analytical Variables: Where Most Errors Occur

Studies consistently show that 60-70% of laboratory errors occur in the pre-analytical phase, before the sample ever reaches the analyzer. Understanding pre-analytical variables is essential.

Patient Identification

Before any specimen is collected, the phlebotomist must confirm patient identity using two or three items of identification (name, medical record number, date of birth). Room number is never acceptable - patients move between rooms. Identification should be an active process: the patient states their name and the phlebotomist verifies, rather than asking a closed-ended “Are you John Smith?” (confused patients may nod to anything). For paternity testing, forensic, or other medicolegal specimens, chain of custody requires additional identification such as a photograph.

Specimen misidentification is the #1 cause of ABO-incompatible transfusion fatalities and a leading cause of laboratory errors overall. AABB standards require two independent type-and-screen collections before transfusion to catch clerical errors.

Venipuncture Technique and Physiologic Variables

A few common draw errors that change results:

Prolonged tourniquet (>3 min) causes water to shift out of the vascular space, concentrating large molecules. Protein-bound analytes (total calcium, total T4, cortisol) and proteins themselves increase by 5-15%. Free/ionized calcium is not affected because only the protein-bound fraction concentrates. Total calcium can rise 0.5-1.0 mg/dL from tourniquet stasis alone - the classic confounder of a workup for hypercalcemia.
Fist pumping causes local anaerobic glycolysis in forearm muscles, releasing K+, phosphate, and lactate. K+ can increase by 1-2 mEq/L - enough to falsely suggest hyperkalemia. The lactate drop in pH displaces Ca2+ from albumin, paradoxically raising ionized calcium. Best practice: loose fist without repetitive clenching, release tourniquet within 1 minute.
Small-gauge needles (23-25G) and butterfly needles cause shear-stress hemolysis. Standard adult venipuncture uses 21G. Excessive vacuum, difficult draws, and multiple attempts compound the risk.
Advance labeling of tubes is prohibited - it’s a leading cause of mislabeling. Label at the bedside immediately after collection, in the presence of the patient. Some institutions require showing the labeled tube to the patient for verification.

Specimen Collection Tubes

Each tube type serves a specific purpose. The stopper color tells you the additive, and the additive dictates what tests you can run.

Red top (no additive): Blood clots; after centrifugation, you have serum (liquid portion without clotting factors). Used for most chemistry tests and serology.

Gold top / SST (serum separator tube): Contains clot activator and an inert thixotropic polymer gel with density between serum and cells. On centrifugation, the gel migrates to form a stable barrier between serum and clot. This prevents glycolysis and K+ leakage from cells. Limitation: gel can adsorb certain lipophilic drugs (phenytoin, lidocaine, some immunosuppressants), falsely lowering measured drug levels if the sample sits on the gel. Gel particles can also clog analyzer probes if not properly centrifuged.

Light blue (sodium citrate, 3.2%): Anticoagulant for coagulation testing. Citrate reversibly chelates calcium. The 9:1 ratio of blood to citrate is critical - underfilling the tube changes this ratio (excess citrate, falsely prolonged PT/PTT). In polycythemia (Hct >55%) the reduced plasma volume effectively concentrates citrate relative to plasma, so the citrate volume should be adjusted down.

Lavender/purple (K3-EDTA): Anticoagulant that chelates calcium (and Mg2+, Fe2+). Used for CBC (preserves cell morphology), HbA1c, intracellular drug levels (cyclosporine, tacrolimus), DNA extraction, and PCR-based viral quantification. Not suitable for calcium, magnesium, or iron (chelated and falsely low), and not for electrolytes (K+ is part of the salt). EDTA also inhibits alkaline phosphatase and CK (chelates metal cofactors).

Green (heparin): The most widely used anticoagulant for stat chemistry. Works by activating antithrombin III, which neutralizes thrombin. Available as sodium, potassium, lithium, and ammonium salts. Lithium heparin is NOT acceptable for lithium measurement; ammonium heparin is NOT acceptable for ammonia measurement - use sodium heparin instead. Heparin also inhibits Taq polymerase and is therefore unsuitable for most PCR-based testing.

Gray (sodium fluoride / potassium oxalate): Fluoride inhibits enolase in the glycolytic pathway, preserving glucose and lactate. Oxalate is the anticoagulant (chelates calcium via insoluble calcium oxalate complexes). Without fluoride, glucose falls ~5-7% per hour at room temperature due to RBC and WBC glycolysis. Fluoride’s effect is delayed; some newer tubes combine fluoride with citrate or EDTA for immediate glycolytic arrest. Not suitable for enzyme assays (fluoride inhibits many enzymes) or electrolyte measurement. Iodoacetate is an alternative glycolytic inhibitor that blocks GAPDH and doesn’t interfere with most enzyme assays.

Royal blue (trace element tube): Special low-metal tube for trace element testing. May contain EDTA or no additive depending on the element.

Yellow (ACD - acid citrate dextrose): For HLA typing, DNA testing, cytogenetics, flow cytometry, and blood bank. Citrate anticoagulates, dextrose provides glycolytic substrate for continued RBC metabolism during storage. The acid slows metabolism. CPDA-1 is a derivative used for RBC storage (35-day shelf life).

Order of Draw

When collecting multiple tubes from a single venipuncture, the order matters to prevent additive carryover:

Blood culture bottles (sterile)
Light blue (citrate) - must be first anticoagulated tube; contamination with EDTA chelates calcium and falsely prolongs PT/PTT; clot activator carryover causes clotting
Red / Gold (serum / SST)
Green (heparin)
Lavender (EDTA)
Gray (fluoride/oxalate)

Mnemonic: Boys Love Ravishing Girls Licking Grapes (Blood cultures, Light blue, Red, Green, Lavender, Gray). Skin puncture / heel stick has a different order: EDTA tube first (for CBC), then other anticoagulants, serum last - because clotting starts quickly in a skin puncture specimen, and the CBC tube is most sensitive to microclots.

Classic board scenario: lab shows K+ 9.0 mEq/L and Ca2+ 4.0 mg/dL on a well-appearing patient. Answer: wrong tube - EDTA contamination or electrolytes drawn from a lavender top. K3-EDTA contains ~7.5 mg K+ per mL of blood (pseudohyperkalemia), and the EDTA chelates Ca2+ (pseudohypocalcemia). Impossibly deranged K+ and Ca2+ in opposite directions = think EDTA.

Hemolysis: The Most Common Preanalytical Problem

Hemolysis is rupture of red blood cells, releasing their contents into serum/plasma. Visible hemolysis starts around 50 mg/dL free hemoglobin.

Causes:

Traumatic blood draw (small needle, excessive vacuum, frothing)
Prolonged tourniquet time
Vigorous mixing of the sample
Transport problems (temperature extremes, pneumatic tube trauma; LDH and AST can rise noticeably after fast pneumatic transport, which is why platelet function testing is hand-delivered)
In vivo hemolysis (the patient actually has hemolysis - different clinical meaning)

Effects on laboratory tests:

Analyte	Effect of Hemolysis	Reason
Potassium	↑↑↑	RBC K+ ~105 mEq/L (vs. 4 in plasma)
LDH	↑↑↑	High LDH concentration in RBCs
AST	↑	Moderate AST in RBCs
Phosphorus	↑	RBCs contain organic phosphates
Magnesium	↑	RBC Mg higher than plasma
Iron	↑	Hemoglobin releases iron
Bilirubin	Interference	Hemoglobin absorbs at similar wavelength

Modern analyzers flag hemolyzed samples using a hemolysis index (H-index) based on spectrophotometric detection at 415/540/580 nm (the hemoglobin Soret band).

Lipemia and Icterus (the other two of “HIL”)

Lipemic samples appear turbid/milky due to high triglyceride levels (chylomicrons and VLDL). The interference mechanisms:

Light scattering interferes with spectrophotometric assays at all wavelengths
Volume displacement falsely lowers electrolytes in methods using indirect ISE (pseudohyponatremia - see below)
Can interfere with immunoassays

Solutions: ultracentrifugation (which physically separates lipoproteins but makes the specimen useless for a lipid panel), lipid-clearing agents, dilution, or switching to direct ISE for electrolytes.

Icteric (yellow-orange, bilirubin-rich) samples cause:

Spectrophotometric interference (bilirubin absorbs at 450-460 nm)
Negative interference on creatinine by the Jaffe method
Variable interference elsewhere depending on methodology

HIL indices (H, I, L) are automatically flagged by modern analyzers using bichromatic spectrophotometry. Hemolysis is the #1 cause of rejected chemistry specimens.

Specimen Processing and Transport

Serum or plasma should be separated from cells within 2 hours of collection for most analytes (CLSI recommendation). Delays cause glucose to fall (~5-7%/hour), K+ to rise (cell leakage), phosphate to rise, and LDH to rise. If centrifugation is delayed, specimens should stay at room temperature, not 4C - cold causes cell membrane leak of K+.

Centrifuge tubes with the original cap in place to prevent evaporation, aerosolization of infectious particles, and release of volatiles (ethanol). Cap-on centrifugation also preserves anaerobic conditions, which matters for CO2 and ionized calcium.

Tubes should not be re-centrifuged after SST gel has set - it disrupts the barrier. Refrigerated transport (wet ice) is used for thermally labile analytes (ammonia, ACTH, catecholamines, lactate). Dry ice is used for frozen shipping; containers must be vented to prevent CO2 pressure buildup. Polypropylene or polyethylene containers with Teflon-lined screw caps are used for shipping.

Body Fluid Specimens

Urine: random (convenient, variable concentration), first morning (most concentrated, best for protein/nitrite/microscopy), timed (2h, 8h, 12h, 24h). For 24-hour collections, completeness is verified by total creatinine excretion: ~14-26 mg/kg/day (men) and 11-20 mg/kg/day (women), roughly 1-2 g/day total. Low creatinine suggests an incomplete collection. Acidification (pH <3) preserves calcium, steroids, catecholamines, and VMA but causes urates to precipitate. A mild base (pH 8-9) preserves porphyrins, urobilinogen, and uric acid.

CSF: collected by LP into 3-4 sequentially numbered sterile tubes. Tubes 1-2 for chemistry/immunology (least affected by traumatic tap), middle tube for microbiology, last tube for hematology/cytology. Cell count comparison between tubes 1 and 4 distinguishes traumatic tap (count falls) from SAH (count stays constant). CSF xanthochromia (yellow supernatant) appears >2-4 hours after SAH and distinguishes SAH from traumatic tap. Artifactual xanthochromia causes: hyperbilirubinemia (serum >10 mg/dL), CSF protein ≥150 mg/dL, rifampin (turns all fluids orange), delayed analysis, hypercarotenemia. Spectrophotometry at 415 and 455 nm is more objective than visual inspection.

Pleural fluid (Light’s criteria): exudate meets any of - pleural/serum protein >0.5, pleural/serum LDH >0.6, pleural LDH >2/3 upper limit of serum. Transudates: CHF, cirrhosis, nephrotic syndrome. Exudates: infection, malignancy, PE, autoimmune. Sensitivity ~98%, specificity ~80% (diuretics in CHF can cause misclassification).

Peritoneal fluid (ascitic): uses SAAG (serum-ascites albumin gradient). SAAG ≥1.1 g/dL = portal hypertension (cirrhosis, CHF, Budd-Chiari). SAAG <1.1 = non-portal (peritoneal carcinomatosis, TB, pancreatic ascites, nephrotic syndrome). SBP (spontaneous bacterial peritonitis): ≥250 PMN/μL, usually monomicrobial, Gram stain typically negative. Secondary bacterial peritonitis: Gram stain usually positive, polymicrobial, suggests perforation.

Pericardial fluid: biochemical and tumor marker testing adds little value beyond cytology and culture. TB pericarditis: AFB stain, mycobacterial culture, and adenosine deaminase. Cytology is the gold standard for malignant effusion (examine at least 60 mL).

Synovial fluid: collect in heparin or EDTA (never oxalate - oxalate crystals mimic disease). Key categories:

Group	WBC	PMN%	Δ Glucose	Appearance
Noninflammatory	<3,000	<25%	<10	Clear-straw
Inflammatory	<75,000	30-75%	<40	Turbid
Septic/crystal/severe RA	>100,000	>90%	30-100	Yellow-purulent

Gout crystals: monosodium urate, needle-shaped, negatively birefringent (yellow when parallel to slow axis). Pseudogout: calcium pyrophosphate (CPPD), rhomboid, positively birefringent (blue parallel to slow axis). Mnemonic: “negative = needles.” Always examine crystals even in septic-looking fluid - crystal arthropathy and infection can coexist.

Amniotic fluid: safe removal volume ~1 mL per gestational week. First drops are discarded (contain maternal cells that can mimic mosaicism). Uses: karyotype/microarray, fetal lung maturity (L/S ratio, phosphatidylglycerol, lamellar body count), neural tube defects (AFP), hemolytic disease (Liley chart, ΔOD 450), infection (IL-6, glucose, culture). In maternal diabetes, the L/S ratio is unreliable (hyperglycemia delays surfactant); phosphatidylglycerol (PG) is preferred because it appears later, isn’t affected by blood/meconium, and isn’t affected by diabetes. Rh-negative mothers need RhIG post-amniocentesis.

Feces: small aliquots for occult blood testing. Guaiac-based FOBT detects heme peroxidase - false positives from red meat, raw vegetables with peroxidase (turnips, horseradish), aspirin/NSAIDs; false negatives from vitamin C (reducing agent). Fecal immunochemical test (FIT) uses anti-human Hb antibodies, no diet restrictions, specific for lower GI bleeding (globin is digested in upper GI). Quantitative fecal fat: 72-hour collection on 100 g/day fat diet; normal <7 g/day. Elevated = steatorrhea (CF, chronic pancreatitis, celiac). Meconium can screen for maternal drug use during gestation.

Hair and nails: forensic genomic DNA analysis, and limited trace metal/drug analysis (segment-based timing is theoretically possible with longer hair, but no clinical standards exist).

FFPE tissue: DNA can be extracted but is degraded to low-MW fragments. Neutral buffered formalin (no heavy metals) does not typically interfere with amplification. Traditional uses: breast malignancy (ER/PR), liver iron/copper (hemochromatosis, Wilson disease).

Heel sticks in infants: use the lateral or medial plantar surface, lancet depth ≤2 mm, avoid the posterior curvature (osteomyelitis risk). First drop is wiped away (tissue fluid). Capillary blood is a mixture of arteriolar, venular, capillary, and interstitial fluid - glucose runs a bit higher than venous because of the arterial contribution. Drop-by-drop into the microtube; avoid scooping along the skin (causes hemolysis).

Laboratory Water, Reagents, and Reference Materials

Routine clinical testing requires Clinical Laboratory Reagent Water (CLRW) by CLSI criteria: resistivity >10 MΩ·cm, microbial content <10 CFU/mL, low silicates and organics. Prepared by a combination of distillation, ion exchange, reverse osmosis, and UV oxidation (usually all of the above, with filtration first). DNA/RNA testing may add protease/nuclease-free specifications. Trace metal analysis requires minimal metal content.

Chemical grades, from high to low purity: Primary reference material (≥99.98% purity, IUPAC grade, used to calibrate primary standards), Analytical Reagent (AR) grade / ACS grade (highest commonly available, labeled maximum impurities), USP grade (pharmaceutical), technical/commercial grade (not for analytical use).

Reference materials have well-characterized properties used for calibration and validation. The hierarchy: primary reference materials (NIST SRMs) → secondary reference materials (calibrated against primary) → working calibrators. Metrological traceability ensures an unbroken chain back to SI units and is required by ISO 15189 for result comparability between labs.

12.2 Core Laboratory Techniques

Centrifugation, Dilution, and Basic Mechanics

Centrifugation separates by density. Force is measured as RCF (relative centrifugal force, g-force), not RPM. RCF = 1.118 × 10⁻⁵ × r × N² (r = radius in cm, N = rpm) - RPM alone is meaningless without knowing the rotor radius. Standard serum/plasma separation: 1000-2000 g for 10-15 minutes. Platelet-poor plasma (coag testing): 2500 g for 15 min, sometimes double centrifugation. Ultracentrifugation (>100,000 g) fractionates lipoproteins. Refrigerated centrifuges prevent heat-induced analyte degradation. Types of rotors: fixed-angle (routine), swinging-bucket (density gradients, blood bank), ultracentrifuge.

Gravimetry measures mass with an analytical balance (weight = mass × gravity). Reference method for calibrating volumetric glassware and preparing primary standards. CLIA requires regular gravimetric pipette calibration.

Dilution: dilution factor = total volume / sample volume. A “1:10” dilution = 1 part sample + 9 parts diluent. Serial dilutions multiply factors. Multiply the measured result by the dilution factor when reporting. Dilution linearity should be verified in method validation (it’s how you characterize the analytical measurement range).

Filtration separates particles from liquid by pore-size exclusion. Sterilizing filters (0.22 μm) remove bacteria. Ultrafiltration with defined molecular weight cutoffs separates free from protein-bound analytes - used for measuring free drugs, free thyroid hormones, and ionized fractions.

Evaporation concentrates samples or removes solvents. Nitrogen blowdown at 37-40C is the standard for drying extracts before MS reconstitution. Rotary evaporation and vacuum centrifuges (SpeedVac) are also used. Beware: uncapped specimens on open analyzers lose water to evaporation, falsely elevating analyte concentrations.

Buffer solutions contain a weak acid + its conjugate base (or weak base + conjugate acid) and resist pH change. Henderson-Hasselbalch: pH = pKa + log([A-]/[HA]). Maximum buffering capacity at pH = pKa. In the lab, Tris, HEPES, and phosphate buffers maintain optimal pH for enzymatic reactions. Buffer type and concentration can affect measured enzyme activity.

Electromagnetic Radiation Basics

Energy increases with frequency, decreases with wavelength: E = hν = hc/λ. Low to high energy: radio → microwaves → infrared → visible → UV → X-rays → gamma → cosmic. Clinical chemistry uses visible (400-700 nm) and UV (200-400 nm). NADH absorbs at 340 nm; bilirubin at 450 nm.

Spectra come in three flavors:

Atoms = line spectra (discrete transitions between quantized energy levels). Basis for atomic absorption spectrophotometry and flame emission.
Molecules = band spectra (additional vibrational and rotational levels). Basis for most clinical molecular spectrophotometry.
Solids (incandescent) = continuous spectra (tungsten for visible, deuterium for UV, xenon arc for broad spectrum).

Photometry and Spectrophotometry

Photometry measures light intensity. Spectrophotometry measures light intensity at selected wavelengths using a monochromator. Most clinical chemistry analyzers are spectrophotometers.

Basic setup: light source → monochromator → sample cuvette → detector. Modern single-beam instruments with computerized zeroing have replaced double-beam instruments.

Transmittance T = Iₛ/I_R (intensity through sample / intensity through reference blank), often expressed as %T (0-100%).

Absorbance A = -log(T) = log(I_R/Iₛ). Dimensionless. Logarithmic relationship with transmittance means absorbance is linear with concentration - which is why absorbance is preferred for quantitative measurements.

Beer’s Law (aka Beer-Lambert):

A = εbc

Where A = absorbance, ε = molar absorptivity (L/(mol·cm), constant for a given substance at a given wavelength), b = path length (usually 1 cm), c = concentration. Absorbance is directly proportional to concentration - the foundation of quantitative spectrophotometry. Deviations occur at high concentrations (molecular interactions), with stray light, polychromatic light, chemical equilibria shifts, and fluorescent samples.

Key components:

Monochromator: isolates a narrow band of wavelengths. Types: filters (inexpensive, fixed wavelength, bandpass 5-20 nm), prisms (refraction), diffraction gratings (most common, bandpass 0.5-5 nm).
Bandpass / spectral bandwidth: range of wavelengths at ≥50% of peak transmittance. Narrower = better. Should be ≤ natural bandwidth of the analyte’s absorption peak (otherwise Beer’s Law fails).
Stray light: any wavelength reaching the detector other than the intended one. Causes scratched optics, dust, aged filters, internal reflections. Sets the upper absorbance limit (~2-3 A). QC by measuring with cutoff filters.
Photomultiplier tube (PMT): photocathode emits electrons on photon strike; series of 9-16 dynodes each at higher voltage amplifies signal ~10⁶-10⁷-fold. ~200x more sensitive than a simple phototube. Used in fluorometers, luminometers, gamma counters. Being replaced in many instruments by solid-state photodiode arrays.
Diode arrays: hundreds-thousands of photodiodes measure multiple wavelengths simultaneously. Entire absorption spectrum captured in one flash. No moving parts. Common in automated chemistry analyzers. Enables bichromatic measurements and HIL index detection.

Reaction monitoring modes:

Endpoint reaction: reaction goes to completion, final absorbance is proportional to analyte concentration.
Kinetic (rate) reaction: rate of absorbance change is measured. Used for enzyme assays (substrate in excess → rate proportional to enzyme activity).
Bichromatic analysis: absorbance at two wavelengths (one specific for analyte, one for background) subtracts interference.

Calibration establishes the signal-to-concentration relationship using standard solutions (calibrators) measured under the same conditions as samples. Two-point (blank + one standard) for linear assays; multi-point for nonlinear methods like immunoassays. Calibration compensates for biases in pipetting, wavelength accuracy, bandpass, and stray light because they affect calibrator and sample equally. NIST SRMs for spectrophotometry include holmium oxide (wavelength accuracy), neutral density filters (absorbance accuracy), and didymium glass.

Atomic absorption (AA) spectrophotometry: for metals (aluminum in dialysis, lead screening, Ca, Cu, Zn, Li, Mg). The ion is converted to a ground-state atom by a flame or graphite furnace, then introduced into a spectrophotometer. Hollow cathode lamp emits element-specific line spectra; ground-state atoms absorb these characteristic wavelengths. Being replaced by ICP-MS (multi-element, better sensitivity).

Jendrassik-Grof (diazo) method is the reference method for total bilirubin. Bilirubin + diazotized sulfanilic acid → colored azobilirubin; measured at 598 nm after alkaline tartrate addition. Direct (conjugated) bilirubin reacts with diazo reagent directly. Indirect (unconjugated) bilirubin is tightly bound to albumin and hydrophobic; an accelerant (caffeine-sodium benzoate) displaces it from albumin and solubilizes it for reaction. Measure direct without accelerant, total with accelerant; indirect = total - direct.

Fluorescence and Related Luminescence

Fluorescence: molecule absorbs light at one wavelength and reemits at a longer wavelength (lower energy) as electrons return to ground state. The wavelength difference = Stokes shift. Fluorescence is ~1000x more sensitive than absorbance spectrophotometry.

Fluorometer optical layout: excitation source (mercury/xenon arc) → primary filter → sample → secondary filter at 90° to the excitation beam → PMT. The 90° geometry keeps transmitted excitation light out of the detector.

Fluorescence is linear only in dilute solutions. At high concentrations, inner filter effects, concentration quenching, and self-absorption cause nonlinearity - opposite of Beer’s Law where linearity holds at moderate concentrations. Concentration quenching happens when fluorophores are too densely packed on a labeled macromolecule and energy transfers between adjacent fluorophores dissipate as heat - relevant in flow cytometry when trying to increase sensitivity by over-labeling.

Sample matrix interference is significant at 260-290 nm (protein absorption - tryptophan, tyrosine, phenylalanine). Excitation >300 nm avoids this. Time-resolved fluorescence using lanthanide chelates with long-lived emission discriminates against short-lived background fluorescence. Lipemia increases scatter and background fluorescence.

Chemiluminescence: a chemical reaction produces excited-state intermediates that emit photons as they return to ground state - no external light source required. Labels: acridinium esters, luminol. Subpicomolar detection limits. Used on Roche Elecsys, Siemens Centaur, Abbott Architect, Beckman DxI.

Electrochemiluminescence (ECL): chemiluminescent reaction triggered electrically at the surface of an electrode. The label is typically ruthenium tris-bipyridyl [Ru(bpy)3]²⁺, oxidized at the electrode and reacting with tripropylamine to emit at 620 nm. Regenerable label (multiple photons per cycle), very low background, wide dynamic range. Basis of the Roche Elecsys/cobas platforms.

Bioluminescence: chemiluminescence catalyzed by enzymes in living organisms. Firefly luciferase + luciferin + ATP → light at 560 nm (quantum yield ~88%). ATP bioluminescence is used for microbial contamination monitoring.

Nephelometry: detects light scattered by particles (antibody-antigen complexes) at an angle (15-90°) to the incident beam. Signal increases with particle size/number. Used for quantifying IgG, IgA, IgM, C3, C4, CRP, and other serum proteins. Rate (kinetic) nephelometry measures the peak rate of immune complex formation, which is faster and less affected by pre-existing turbidity or non-specific scatter.

Turbidimetry: detects the decrease in transmitted light caused by particles - detector is in line with the light beam (standard spectrophotometer geometry). Less sensitive than nephelometry but can be performed on any chemistry analyzer with no specialized equipment. Used for CRP, RF, ASO titers.

The four major optical methods summary: absorption (spectrophotometry), emission (fluorescence, chemiluminescence, flame emission), polarization (FPIA), scattering (nephelometry, turbidimetry).

Enzyme Kinetics and Measurement

Enzymes are biological catalysts - proteins that increase reaction rate without being consumed. They lower activation energy (Ea) without changing ΔG. Rate = f([enzyme], [substrate]).

In clinical chemistry, enzymes serve two roles:

As analytes (measure enzyme activity in the sample - AST, ALT, CK, amylase, lipase, LDH, ALP, GGT)
As reagents (use enzyme to catalyze a reaction with an analyte of interest - glucose oxidase for glucose, urease for BUN, uricase for uric acid, cholesterol oxidase/esterase for cholesterol, creatininase for creatinine)

Enzyme classes (EC system), six groups: (1) Oxidoreductases (redox - LDH, G6PD), (2) Transferases (transfer groups - AST, ALT, GGT, kinases), (3) Hydrolases (hydrolysis - amylase, lipase, ALP, ACP, cholinesterase), (4) Lyases (remove groups forming double bonds - aldolase, carbonic anhydrase), (5) Isomerases (structural rearrangement), (6) Ligases (join molecules using ATP - DNA ligase). Mnemonic: “Old Teachers Hate Lazy Intelligent Learners.”

Holoenzyme = apoenzyme + prosthetic group (cofactor). Example: AST and ALT require pyridoxal-5’-phosphate (P-5’-P, vitamin B6) as a cofactor. In B6 deficiency (alcoholism), ALT activity falls more than AST - which can exaggerate the De Ritis ratio and mimic alcoholic hepatitis. Some labs add exogenous P-5’-P to ensure full enzyme activation.

Isoenzymes: different structural forms of the same enzyme that catalyze the same reaction but differ in sequence, mobility, regulation, or tissue distribution. CK (MM, MB, BB), LDH (1-5), ALP (liver, bone, intestinal, placental). Isoforms are post-translational variants of the same isoenzyme. Preferred modern isoenzyme measurement is solid-phase sandwich ELISA; older methods used electrophoresis, chemical inactivation, or differential substrate specificity.

Michaelis-Menten kinetics:

V = Vmax[S] / (Km + [S])
Km = substrate concentration at which V = Vmax/2
Km is inversely proportional to enzyme affinity: low Km = high affinity (hexokinase, Km ~0.1 mM for glucose); high Km = low affinity (glucokinase, Km ~10 mM, acts as a glucose sensor in the liver)
Vmax is directly proportional to enzyme concentration: doubling [enzyme] doubles Vmax. Km is an intrinsic property and does NOT change with [enzyme].

Zero-order kinetics: substrate is saturating, rate is constant (= Vmax), independent of [substrate]. Clinical assays are designed to operate in zero-order conditions (substrate in excess, typically 10-20× Km) so that measured rate reflects only enzyme concentration. Drugs with zero-order kinetics (ethanol, phenytoin, aspirin at high doses) are clinically dangerous because small dose increases cause disproportionate concentration rises.

First-order kinetics: substrate is limiting, rate is proportional to [substrate]. Most drugs are eliminated by first-order kinetics (constant fraction cleared per half-life). Clinical assays avoid first-order conditions because rate depends on both enzyme and substrate.

Lineweaver-Burk plot (double reciprocal, 1/V vs 1/[S]):

X-intercept = -1/Km (more negative = smaller Km = higher affinity)
Y-intercept = 1/Vmax (higher Y-intercept = lower Vmax)
Slope = Km/Vmax

Enzyme inhibition on Lineweaver-Burk:

Inhibitor	Km	Vmax	Resembles substrate?	Binds active site?	Overcome by more [S]?	Lines on L-B
Reversible competitive	↑	unchanged	Yes	Yes	Yes	cross at Y-axis (same Vmax)
Noncompetitive	unchanged	↓	No	No (allosteric)	No	intersect at X-axis (same Km)
Irreversible competitive	unchanged	↓	Yes	Yes (covalent)	No	same as noncompetitive

Mnemonic: “Lines Cross = Competitive”. Classic clinical examples:

Competitive inhibition: methanol/ethylene glycol poisoning treated with ethanol or fomepizole (compete with the toxic alcohol for alcohol dehydrogenase). Methotrexate competes with dihydrofolate for DHFR. Statins compete with HMG-CoA for HMG-CoA reductase.
Irreversible competitive: aspirin irreversibly acetylates COX (platelet effect lasts 7-10 days). Organophosphate nerve agents irreversibly bind acetylcholinesterase.
Noncompetitive: heavy metals (lead, mercury) inhibit many enzymes at allosteric sites.

Enzyme activity units: 1 International Unit (U) = the amount of enzyme that catalyzes 1 μmol of substrate per minute under defined conditions. Results in U/L. The SI unit is the katal (1 kat = 1 mol/s = 6 × 10⁷ U), but it’s impractically large and rarely used. Because activity is method-dependent, reference ranges are method-specific.

Enzyme activity depends on pH and temperature. Each enzyme has an optimum pH (pepsin ~2, most serum enzymes 7-8, ALP ~10). Activity rises ~5-10% per 1C of temperature until denaturation (~>60C). Clinical assays are now standardized at 37C by IFCC reference procedures (older methods used 25C or 30C - results are NOT interchangeable; a 30C result is ~60-70% of the 37C result).

Enzyme-substrate (ES) complex forms through non-covalent interactions at the active site (lock-and-key or induced-fit). Cycle: E + S → ES → EP → E + P.

Measurement modes:

Fixed-time (endpoint): measure product at a single time after a set incubation. Simple but assumes linear kinetics across the interval.
Continuous-monitoring (kinetic): measure product formation continuously, selecting the linear (zero-order) portion for rate calculation. More accurate; detects non-linearity immediately. Preferred method in modern analyzers.

Self-indicating reactions directly produce a measurable signal - classic example is NADH/NAD+ at 340 nm (NADH absorbs strongly, NAD+ does not; molar absorptivity of NADH at 340 nm = 6,220 L/(mol·cm)). Dehydrogenases are monitored this way.

Indicator (coupled) reactions add auxiliary enzymes to link the primary reaction to a measurable signal. The primary reaction must be rate-limiting - if the indicator reaction is slower, it becomes rate-limiting and the measured rate no longer reflects the enzyme of interest. A lag phase occurs initially while intermediates build to steady state. Example: CK assay couples through hexokinase and G6PD to monitor NADPH production at 340 nm. AST couples through malate dehydrogenase.

Enzyme activity vs enzyme mass: activity is linear with concentration under optimized conditions. Immunoassays measure enzyme mass (antigenic protein) and can detect inactive forms (zymogens, denatured enzyme, inhibitor-bound enzyme, macroenzyme). This is why immunoassay-based CK-MB mass measurements and activity-based measurements don’t always agree.

Electrochemistry: ISE, Blood Gases, and Biosensors

Electrochemical methods measure current, voltage, charge, or conductivity to quantify analytes. Key techniques:

Potentiometry: measures voltage at zero current (electromotive force, EMF) between two electrodes. EMF is proportional to log of ion activity by the Nernst equation: E = E₀ + (RT/nF)·ln[ion activity]. Theoretical slope at 25C: 59.2 mV per decade for monovalent ions, 29.6 mV for divalent (slope rises to ~61.5 mV/decade at 37C; many sources still quote 59 mV as the canonical board value). ISE performance QC: measure slope - if it deviates >5% from theoretical, the electrode needs service. Temperature affects the slope, so ISE analyzers have temperature compensation.
Amperometry: measures current at fixed voltage. Current is proportional to the electroactive species. Classic example: Clark pO2 electrode.
Coulometry: measures total charge (Faraday’s law: charge is proportional to moles of substance oxidized/reduced). Chloride measurement in sweat (CF diagnosis, pilocarpine iontophoresis).
Conductometry: measures electrical conductivity, which depends on ion number and mobility. Used in Coulter cell counters, blood gas analyzer hematocrit, and sweat chloride.
Voltammetry: applies a varying potential and measures current. Anodic stripping voltammetry (ASV) is used for trace metal analysis (lead).

Galvanic vs electrolytic cells: a galvanic cell produces spontaneous current from a chemical reaction (battery). An electrolytic cell uses external voltage to drive a non-spontaneous reaction. Clark pO2 electrode is technically electrolytic (polarizing voltage applied).

Inert electrodes (platinum, gold, glassy carbon) provide surfaces for electron transfer without participating in the redox reaction. Used in Clark electrodes and amperometric biosensors. Reference electrodes (Ag/AgCl, calomel) maintain a constant potential for comparison.

Ion-selective electrodes (ISE) use ion-specific membranes to generate a potential proportional to the target ion’s activity. Three membrane categories:

Glass: lithium-aluminum silicate (H+/pH), borosilicate (Na+)
Crystalline (solid-state): LaF3 (F-), AgCl/Ag2S (Cl-)
Polymer membrane: PVC with embedded ionophore - valinomycin for K+ (extreme selectivity ~10,000:1 K+/Na+), ETH 1001 for Ca2+, tridodecylamine for H+, methylmonensin for Na+. Polymer membrane ISEs dominate modern multi-analyte cartridges.

Direct vs indirect ISE - critical board distinction:

Direct ISE: undiluted sample. Measures ion activity directly in the water phase of plasma. NOT affected by lipemia or hyperproteinemia. Used in blood gas analyzers and POC devices.
Indirect ISE: sample is diluted 1:20 to 1:35. Also measures activity but assumes normal plasma water content (~93%). In lipemia or hyperproteinemia (multiple myeloma), water fraction falls to as low as 80%, dilution overestimates volume, and Na+ reads falsely low (pseudohyponatremia).

Board pearl: Na+ low on the main chemistry analyzer (indirect ISE) but normal on the blood gas analyzer (direct ISE) → pseudohyponatremia → check lipids and total protein.

Activity vs concentration: activity = γ × [concentration], where γ is the activity coefficient (<1 in biological fluids due to ionic interactions). Direct ISE measures activity, which is the biologically active fraction - matters most for calcium (ionized Ca²⁺ is the physiologically active form; total Ca includes protein-bound fraction).

Clark amperometric pO2 electrode: platinum cathode and Ag/AgCl anode behind a gas-permeable membrane (polypropylene, silicone, Teflon). Polarizing voltage (-0.65V) applied. O2 diffuses through membrane and is reduced at Pt: O2 + 2H2O + 4e⁻ → 4OH⁻. Current is proportional to pO2. Anesthesia gases (N2O, halothane, isoflurane) can be reduced at the Pt and falsely elevate measured pO2 - modern membranes minimize this. The gas-permeable membrane prevents protein fouling; membrane replacement is routine maintenance.

Stow-Severinghaus pCO2 electrode: a modified pH electrode surrounded by a thin (~20 μm) gas-permeable membrane. CO2 diffuses into a bicarbonate electrolyte layer, forms carbonic acid, drops pH; the internal pH electrode measures the shift. Relationship to sample pCO2 is logarithmic (Henderson-Hasselbalch). Response time 1-2 minutes (slower than ISE; diffusion is rate-limiting). Modern differential planar pCO2 sensors use two identical polymer pH electrodes with different internal electrolytes (one CO2-responsive, one reference), canceling out drift and noise.

Optodes (optical electrodes) use a fluorescent/absorbent indicator dye at the tip of a fiber optic. The dye changes optical properties with the analyte. Advantages: no reference electrode, easy to miniaturize, no electrical noise. Used in some POC blood gas analyzers (i-STAT).

Biosensors combine a biological recognition element (enzyme, antibody, nucleic acid, cell, receptor) with a physicochemical transducer (electrochemical, optical, piezoelectric). The classic example is the glucose biosensor (glucose oxidase + amperometric electrode measures H2O2). POC glucometers and i-STAT-type cartridges are biosensors.

Flow-through electrochemical detectors for HPLC: low dead-volume electrodes (glassy carbon, mercury film) oxidize/reduce eluting analytes. Very sensitive and selective for electroactive compounds - catecholamines (dopamine, norepinephrine, epinephrine), serotonin, their metabolites (HVA, VMA, 5-HIAA).

Immunoassays

Immunoassays use antibody-antigen binding to detect and quantify analytes. Critical for hormones, drugs, tumor markers, and proteins at low concentrations.

Immunogen vs hapten: an immunogen (>10 kDa) independently stimulates an immune response. A hapten is small (<5 kDa) and cannot alone - it must be coupled to a carrier protein to provoke antibody formation. Many drug assays use haptens conjugated to carriers during antibody production.

Affinity vs avidity: affinity is the strength of a single epitope-paratope interaction (thermodynamic). Avidity is the total strength including all binding sites on the antibody. IgM has low affinity per site but high avidity (10 sites - pentamer). IgG has higher affinity per site but only 2. This is why IgM fixes complement so well despite weak individual site binding.

Three non-covalent forces drive antigen-antibody binding:

Van der Waals / London dipole-dipole
Hydrophobic interactions
Ionic (coulombic) bonding (COO⁻ on antigen with NH3+ on antibody)

No covalent bonds - interactions are reversible and governed by the law of mass action.

Phases of immune complex formation: Phase 1 (rapid, seconds-minutes): initial Ag-Ab binding, single antibodies binding epitopes. Phase 2 (slow, minutes-hours): cross-linking and lattice formation with multivalent antigens and bivalent antibodies. Phase 2 is rate-limiting and produces the signal in nephelometry/turbidimetry. PEG (polyethylene glycol) accelerates phase 2 by reducing solvent interactions and promoting aggregation. High ionic strength (NaCl) inhibits complex formation.

Precipitin curve: three zones based on Ag:Ab ratio:

Zone A (antibody excess / prozone): too much Ab, small soluble complexes, low precipitate
Zone B (equivalence): optimal ratio, maximum cross-linking
Zone C (antigen excess / postzone): too much Ag, saturated binding sites, low precipitate

Prozone effect causes false negatives in agglutination/precipitation at very high antibody titers (classic with very high-titer syphilis RPR). Solution: test serial dilutions.

Competitive immunoassay (limited-reagent):

Labeled analyte analog competes with patient analyte for a limited amount of antibody
More patient analyte → less labeled analyte binds → lower signal
Signal is inversely proportional to concentration
Used for small molecules (<1000-5000 Da) - drugs, T4, T3, cortisol, estradiol, digoxin, theophylline - because they have only one epitope and can’t accommodate two antibodies for a sandwich
Sensitivity is limited by antibody affinity

Noncompetitive / sandwich / immunometric assay (reagent excess):

Capture antibody on solid phase (microparticle, plate well) extracts analyte from sample
After washing, detection (tracer) antibody binds a different, non-overlapping epitope
More analyte → more sandwiches → higher signal
Signal directly proportional to concentration
Used for larger molecules with multiple epitopes (hCG, TSH, troponin, tumor markers)
More sensitive and wider dynamic range than competitive

The Hook Effect (high-dose hook): in sandwich assays, extremely high analyte concentrations independently saturate both capture and detection antibodies, preventing sandwich formation. Result: falsely normal or low values. Clinical relevance: molar pregnancy (very high hCG), large prolactinoma (very high prolactin), high-burden tumor markers. Solution: serial dilution and reassay.

Heterogeneous vs homogeneous:

Heterogeneous immunoassays require a wash/separation step to remove unbound label before measurement. Most ELISA, chemiluminescent, and radioimmunoassays. Better specificity. Most automated platforms use heterogeneous formats with magnetic microparticle separation.
Homogeneous immunoassays do not require separation - the label’s signal changes with binding. Examples: EMIT (enzyme label is active when free, inhibited when antibody-bound), CEDIA (cloned enzyme donor fragment complementation), FPIA (polarization changes with molecular tumbling). Faster and simpler; primarily for drug-of-abuse screening and therapeutic drug monitoring. Generally less sensitive than heterogeneous.

Homogeneous competitive assays in practice:

EMIT (Enzyme Multiplied Immunoassay Technique): drug-enzyme conjugate competes with free drug; antibody binding inhibits the enzyme. Drug in sample → less antibody bound to conjugate → more enzyme activity.
CEDIA (Cloned Enzyme Donor Immunoassay): drug-labeled β-galactosidase fragment competes; antibody binding prevents fragment reassembly into active enzyme.
FPIA (Fluorescence Polarization Immunoassay): fluorescein-labeled drug (tracer) excited with polarized light. Free small tracer tumbles fast, emits depolarized light. Antibody-bound tracer tumbles slowly, emits polarized light. Patient drug competes with tracer → more drug = less tracer bound = more depolarization = lower polarization signal. Used for vancomycin, aminoglycosides, immunosuppressants, anticonvulsants, drugs of abuse.

Label-free immunoassays detect antigen-antibody binding without a tag. Methods: nephelometry/turbidimetry (light scattering from immune complexes), surface plasmon resonance. Nephelometry/turbidimetry are the most common label-free assays clinically, used for quantifying IgG, IgA, IgM, complement, CRP, RF, ASO titers. Advantage: simpler chemistry, no label interference.

PETINIA (Particle-Enhanced Turbidimetric Inhibition Immunoassay): homogeneous competitive assay for drug monitoring. Drug-coated latex particles + anti-drug antibody → aggregation → turbidity. Patient drug competes with latex-bound drug → less aggregation → less turbidity. Signal is inversely proportional. Runs on standard chemistry analyzers - used for mycophenolic acid, sirolimus, tacrolimus on some platforms.

LOCI (Luminescent Oxygen Channeling Immunoassay): rare homogeneous sandwich assay. Two antibody-coated beads - a sensitizer bead generates singlet oxygen under 680 nm illumination; a chemiluminescer bead emits at 612 nm when activated by singlet oxygen. Only when analyte bridges the two beads (<200 nm apart) does singlet O2 channel and produce light. Used on Siemens Dimension.

Latex agglutination: antibody (or antigen) adsorbed/covalently linked to latex microbeads. Corresponding antigen/antibody cross-links beads → visible agglutination. Qualitative or semi-quantitative (serial dilution titer). Used for RPR, RF, CRP, cryptococcal antigen, Strep group typing.

Polyclonal vs monoclonal antibodies: polyclonals recognize multiple epitopes and have more cross-reactivity (e.g., anti-digoxin polyclonals cross-react with digoxin-like immunoreactive substances [DLIS] in neonates and renal failure). Monoclonals recognize a single epitope - less cross-reactivity but can have their own issues (epitope masking, batch consistency).

Common immunoassay interferences:

Heterophile antibodies (especially HAMA, human anti-mouse antibodies) bridge the capture and tracer antibodies independently of analyte → falsely elevated results in sandwich assays (sometimes falsely low by blocking sandwich formation). Suspect when results are clinically discordant. Confirm by: testing on a different platform, observing non-linearity on serial dilution, or adding heterophile blocking reagent.
Biotin interference (streptavidin-biotin platforms, especially Roche). Exogenous biotin (high-dose for MS therapy at 100-300 mg/day, hair/nail/skin supplements) competes with biotinylated assay components for streptavidin. In sandwich assays → falsely LOW (less capture antibody immobilized). In competitive assays → falsely HIGH (less labeled analog captured). Dangerous example: falsely low TSH + falsely high free T4 in a biotin-supplemented patient can look like hyperthyroidism. Falsely low troponin can miss MI. Solution: stop biotin 2-3 days before testing. FDA issued a safety alert in 2017.

Labels used in immunoassays: enzymes (ELISA, EMIT - HRP or AP), fluorophores (FPIA, DELFIA, time-resolved fluorescence), chemiluminescent molecules (CLIA, ECL), radioisotopes (RIA - largely historical).

Mass Spectrometry

Mass spectrometry has revolutionized clinical chemistry by providing high specificity and the ability to measure multiple analytes simultaneously with robust structural information.

Basic principle:

Ionization: sample molecules are given a charge
Mass analysis: ions separated by mass-to-charge ratio (m/z)
Detection: ion abundance measured

All mass spectrometers have: (1) ion source, (2) vacuum system (ions must travel without collisions at ~10⁻⁵ to 10⁻⁸ torr), (3) mass analyzer, (4) detector, (5) computer.

A mass spectrum plots relative abundance vs m/z. Base peak (tallest) = 100%. For singly charged ions, m/z = molecular weight. In ESI, ions often carry multiple charges; deconvolution determines true MW. Spectra function as molecular fingerprints and support library matching (e.g., NIST EI library).

Ionization methods:

Electrospray ionization (ESI): HPLC effluent through a high-voltage capillary at atmospheric pressure → fine charged droplets → solvent evaporates → multiply charged ions [M+nH]n+. Soft ionization (minimal fragmentation). Good for polar, large molecules. The workhorse for clinical LC-MS/MS (immunosuppressants, steroids, vitamin D, drugs).
APCI (Atmospheric Pressure Chemical Ionization): HPLC effluent nebulized and vaporized, corona discharge ionizes solvent → charge transfer to analyte. Singly charged. Good for less polar, thermally stable compounds (steroids, lipids, fat-soluble vitamins). Second most common in clinical LC-MS/MS.
APPI (Atmospheric Pressure Photoionization): UV lamp (krypton, 10 eV) ionizes analytes, often with dopant (toluene). Good for nonpolar aromatics.
ICP (Inductively Coupled Plasma): argon plasma at ~6,000-10,000 K, hard ionization, complete atomization. Elemental analysis - lead, mercury, arsenic, cadmium, chromium, manganese, thallium, selenium, zinc, copper. Replaced AA for most clinical metal testing. Multi-element, wider dynamic range, lower detection limits. Interferences from isobaric overlaps (ArCl+ at m/z 75 interferes with arsenic).
EI (Electron Ionization) and CI (Chemical Ionization) for GC-MS. EI: 70 eV electrons cause extensive, reproducible fragmentation - perfect for NIST library searching, standard for toxicology confirmation. CI: reagent gas (methane, ammonia) transfers charge via ion-molecule reactions, soft ionization preserves molecular ion for MW determination.
MALDI (Matrix-Assisted Laser Desorption/Ionization): sample mixed with UV-absorbing matrix (sinapinic acid, CHCA), crystallized on target, ionized by pulsed UV laser. Soft, singly charged [M+H]+, large molecules. Classically paired with TOF (MALDI-TOF). Revolutionary clinical application: microbial identification (Bruker Biotyper, bioMerieux VITEK MS) identifies bacteria and fungi by protein profiles in minutes.
SELDI (Surface-Enhanced Laser Desorption/Ionization): variant of MALDI with proteins captured on a chemically modified surface (chip) before ionization. Was investigated for cancer biomarker discovery; largely abandoned in clinical use due to poor reproducibility.

Mass analyzers (beam-type vs trapping-type):

Quadrupole Mass Filter (QMF): four parallel rods with oscillating RF+DC fields create a stability window - only ions of a specific m/z pass through. Workhorse of clinical MS. Unit resolution (~1 Da). Can scan or operate in SIM (selected ion monitoring) for higher sensitivity on known targets.
Triple Quadrupole (QqQ): Q1 selects precursor → q2 is a collision cell (collision-induced dissociation with N2 or Ar, fragments the precursor) → Q3 selects a product ion. This enables MRM / SRM (Multiple/Selected Reaction Monitoring) - the gold standard for quantitative clinical MS. Two-stage mass selection nearly eliminates chemical noise. Each analyte has characteristic precursor→product transitions.
Time-of-Flight (TOF): all ions accelerated to the same kinetic energy, then flight tube - lighter ions arrive first. High m/z range (essentially unlimited), high mass accuracy (~1-5 ppm), fast. Works well with pulsed sources (MALDI). Reflectron TOF improves resolution.
Quadrupole Ion Trap (QIT): RF field traps ions in a 3D or linear trap. Ions can be stored, selectively ejected, or fragmented in place. Unique capability: MSⁿ (MS/MS/MS) for structural elucidation. Trade-off: limited dynamic range and ion capacity.
Orbitrap and FT-ICR (Fourier Transform Ion Cyclotron Resonance): trapping analyzers with ultra-high resolution (>100,000) and mass accuracy (<5 ppm). Used in research and advanced clinical/forensic applications.

Mass resolution: R = m/Δm. Higher R = better separation of close peaks. QMF ~500-2,000; TOF ~10,000-40,000; Orbitrap ~100,000-500,000. For routine MRM quantitation, unit resolution is sufficient because the MRM transition provides additional selectivity.

Mass accuracy: (measured - true) / true, in ppm. High mass accuracy enables identification by molecular formula matching.

Detectors: electron multipliers are the most common. Ion strikes first dynode → cascade of secondary electrons through 10-20 dynodes → ~10⁶ amplification. Microchannel plates (MCPs) are used in TOF for fast time resolution.

Chromatogram types: TIC (Total Ion Chromatogram) sums all m/z at each time point. XIC/EIC (Extracted Ion Chromatogram) shows only selected m/z - more specific. MRM trace is the gold standard for quantitation in QqQ instruments.

Ion suppression is the main matrix effect in clinical LC-MS/MS. Co-eluting matrix components (phospholipids, salts, proteins) reduce ionization efficiency of the target → falsely decreased signal. Mitigation: better chromatographic separation, cleaner sample preparation (SPE or LLE), and critically, stable isotope-labeled internal standards that co-elute with and experience the same suppression as the analyte, compensating for the effect. Must be evaluated during method validation.

Clinical applications of MS:

Therapeutic drug monitoring (immunosuppressants - tacrolimus, sirolimus, cyclosporine, everolimus, MPA; antiretrovirals)
Toxicology confirmation (drugs of abuse)
Endocrinology (steroid panels, vitamin D metabolites, aldosterone, testosterone, estradiol at low levels)
Newborn screening (acylcarnitines, amino acids, succinylacetone - metabolic disorders)
Microbial identification (MALDI-TOF)
Trace metals (ICP-MS)

Proteomics uses MS to study entire protein sets. Techniques: 2D-PAGE + MS, shotgun proteomics (LC-MS/MS), MALDI-TOF, protein microarrays. Challenge: serum protein dynamic range >10 orders of magnitude - abundant proteins (albumin) must be depleted before analyzing low-abundance biomarkers. Clinical impact has been limited; most discovered biomarkers haven’t reached routine use due to reproducibility and specificity issues.

Electrophoresis

Electrophoretic mobility is directly proportional to net charge and inversely proportional to molecular size and medium viscosity. At alkaline pH (~8.6, standard for serum protein electrophoresis), most serum proteins are negative and migrate toward the anode (+). Albumin has the greatest net negative charge → migrates fastest. Gamma globulins migrate slowest.

Ampholytes (amphoteric electrolytes) carry positive or negative charge depending on pH. Proteins are the most important biological ampholytes (amino + carboxyl groups). At pH < pI the protein is positively charged; at pH > pI negatively charged; at pH = pI net charge = 0 (does not migrate). Synthetic carrier ampholytes (mixtures of polyaminopolycarboxylic acids with a range of pI values) are used to create pH gradients for isoelectric focusing.

Slab gel electrophoresis: flat rectangular gel (agarose or polyacrylamide). Multiple samples run simultaneously - direct comparison under identical conditions. Agarose for SPEP and hemoglobin electrophoresis. Polyacrylamide (PAGE) for smaller proteins and isoenzymes. SDS-PAGE adds sodium dodecyl sulfate to denature proteins and separate by molecular weight only.

Densitometry scans stained gels/membranes with a light beam, producing an electropherogram (tracing). Peak area is proportional to protein quantity. Combined with total protein, this gives absolute concentrations of each fraction (albumin, α1, α2, β, γ). Used for SPEP, UPEP, and immunofixation interpretation.

Isoelectric focusing (IEF) separates amphoteric compounds by pI in a stable pH gradient. Proteins migrate until net charge = 0 at their pI, then stop. Extremely high resolution (can distinguish proteins differing by <0.01 pI unit). Clinical uses: hemoglobin variants, CSF oligoclonal bands (MS diagnosis), transferrin isoforms (carbohydrate-deficient transferrin for alcohol use), and the first dimension of 2D electrophoresis.

Capillary electrophoresis: in a small-bore (10-100 μm) fused silica capillary, 20-200 cm long. Very high voltages (up to 30,000 V) for rapid high-efficiency separation. Automated, quantitative, small sample volume.

Capillary zone electrophoresis (CZE): free-solution capillary, separation by charge-to-mass ratio. Electroosmotic flow drives all analytes to the detector. Replacing gel-based SPEP in many labs; also used for hemoglobin variants and HbA1c.
Capillary gel electrophoresis (CGE): polymer matrix inside the capillary for size-based sieving. DNA/RNA analysis, SDS-protein analysis.

2D electrophoresis: IEF (by pI, first dimension) followed by SDS-PAGE (by MW, second dimension). Can resolve >1,000 protein spots. Protein spots are excised, trypsin-digested, identified by MS (peptide mass fingerprinting or LC-MS/MS). Primarily a research tool; not routine clinically due to complexity and reproducibility limits.

Western blot: SDS-PAGE → transfer to nitrocellulose/PVDF membrane → primary antibody → labeled secondary antibody → visualize specific protein bands. Clinical use historically for HIV confirmation (now replaced by HIV-1/2 differentiation immunoassay), Lyme disease confirmation, HTLV.

Dot blot: skip the electrophoresis - spot sample directly on membrane, probe with labeled antibody. Faster and simpler than western but no MW information. Semi-quantitative. Largely a research tool.

Chromatography

Chromatography separates components by differential partitioning between a stationary phase and a mobile phase.

Types of liquid chromatography (LC):

Ion-exchange (IEC): charged stationary phase retains oppositely charged analytes. Cation exchangers have negative groups (sulfonate) and retain cations. Anion exchangers have positive groups (quaternary amine) and retain anions. Elution by salt gradient or pH shift. Clinical use: HbA1c by cation-exchange HPLC (separates Hb variants by charge - HbS, HbC, HbE elute differently and can interfere with A1c).
Adsorption chromatography (LSC, liquid-solid): analytes adsorb to a solid surface (silica, alumina) by surface interactions. TLC (thin-layer) is the most common form in clinical labs (drug screening, toxicology).
Partition chromatography: solutes partition between mobile phase and a liquid stationary phase coated/bonded to a support. The basis of most HPLC. Partition coefficient K = [stationary] / [mobile].
- Normal phase: polar stationary phase (silica, cyano, amino) + nonpolar mobile phase (hexane, chloroform). Retains polar compounds.
- Reversed phase (RP): nonpolar stationary phase (C18/ODS, C8) + polar mobile phase (water, methanol, acetonitrile). Retains nonpolar compounds. Most common clinical LC mode (~80% of applications) because biological samples are aqueous.
Size-exclusion (SEC, gel filtration): porous support excludes large molecules from pores → large molecules elute first; small molecules enter pores and elute later. Ideally no surface interaction. Clinical uses: desalting, estimating MW, macroprolactin screening (large prolactin-IgG complexes elute before monomeric prolactin).
Affinity chromatography: a biologically specific ligand captures the target. Extremely selective. Boronate affinity captures glycated hemoglobin (cis-diol groups) for HbA1c. Protein A/G for IgG purification. Lectin affinity for glycoproteins. Immunoaffinity for targeted capture before MS.

Column vs planar formats: column chromatography (HPLC, GC) packs stationary phase in a tube. Modern UHPLC uses sub-2-μm particles for faster and higher-resolution separations. Planar chromatography (TLC, paper) has a flat medium.

Quantitation: analytes are identified by retention time, retention volume, or Rf (retention factor) in TLC (Rf = distance traveled by analyte / distance traveled by solvent front, 0-1). Quantitation uses peak area (preferred - less affected by peak broadening) or peak height (simpler, useful for unresolved neighbors). Two calibration modes:

External calibration: standards run separately from samples. Assumes identical injection and response.
Internal standard calibration: a known amount of a distinct compound (the IS) is added to both calibrators and samples. Analyte/IS ratio compensates for extraction losses, injection variations, and ion suppression. Stable isotope-labeled internal standards (e.g., deuterated analog) are essential in quantitative clinical LC-MS/MS and GC-MS - they co-elute and ionize with the analyte.

Resolution Rs = 2(t2-t1) / (w1+w2). Rs ≥ 1.5 = baseline separation. Improved by selectivity (different column/mobile phase), efficiency (longer column, smaller particles - more theoretical plates N), or retention (adjust k’).

Gas chromatography (GC): analytes must be volatile or derivatizable. The mobile phase is the carrier gas (inert: helium most common; hydrogen fastest but flammable; nitrogen cheapest). Three GC types by stationary phase:

Gas-solid chromatography (GSC): solid stationary phase (alumina, molecular sieves). Permanent gases.
Gas-liquid chromatography (GLC): liquid stationary phase coated on a support. Classic form.
Bonded-phase GC: stationary phase chemically bonded to capillary wall. Most common modern form (fused silica capillary columns). More stable, lower bleed, longer life.

GC detectors: Flame ionization detector (FID) is the most common clinical GC detector. Column effluent enters H2-air flame, organics combust and produce ions, electrode collects current. Responds to C-H bonds (universal for organics), very wide linear range (10⁷), robust. Does NOT detect H2O, CO, CO2, N2, O2, noble gases, fully halogenated compounds. Clinical use: blood alcohol (ethanol), volatile panel. Other detectors: ECD (electron capture - halogenated), NPD (nitrogen-phosphorus - amines and organophosphates), MS (universal, specific).

12.3 Quality Control: Ensuring Reliable Results

Laboratory QC ensures analytical systems perform consistently within acceptable limits and detects errors before they reach patient reports.

Internal QC: control materials with known values run alongside patient samples. Results plotted on Levey-Jennings charts.
External QC (proficiency testing): unknown samples from an external organization (CAP surveys, API); lab results compared to peer laboratories and target values.

Levey-Jennings Charts

A Levey-Jennings chart plots QC results over time with the mean and ±1/2/3 SD limits:

68% within ±1 SD, 95% within ±2 SD, 99.7% within ±3 SD (for Gaussian distribution)
Results should be randomly distributed around the mean
Patterns (shifts, trends) suggest systematic problems

Westgard Rules

Westgard rules are a systematic way to evaluate QC data for errors:

Rule	Description	Type of Error
1:2s	One control >2 SD from mean	Warning rule (not rejection)
1:3s	One control >3 SD	Random error; reject run
2:2s	Two consecutive controls >2 SD same side	Systematic error (shift)
R:4s	One >+2 SD and one >-2 SD	Random error; reject run
4:1s	Four consecutive controls >1 SD same side	Systematic error (shift)
10x	Ten consecutive controls on same side of mean	Systematic error (trend/shift)

Random errors: unpredictable, affect precision. Causes: bubbles, electrical fluctuations, random pipetting errors.

Systematic errors: consistent direction, affect accuracy. Causes: calibration drift, reagent deterioration, wrong standard, miscalibrated pipette.

Error Detection and Troubleshooting

When QC fails:

Do not report patient results
Repeat the QC
If still fails, troubleshoot:
- Fresh reagents
- Fresh controls
- Recalibrate
- Maintenance
Document corrective action
Repeat QC; if passes, resume testing

12.4 Method Validation

Before implementing a new method or instrument, laboratories must validate that it performs acceptably.

Precision (reproducibility):

Within-run (repeatability): same sample multiple times in one run
Between-run: same sample across different days/runs
Expressed as SD or CV = SD/mean × 100%

Accuracy:

Comparison to reference method or certified reference materials
Recovery studies (spike known amount, measure recovery)
Proficiency testing

Linearity (reportable range):

Test serial dilutions of a high sample
Determine the concentration range where results are accurate and proportional
Analytical measurement range (AMR) = the range over which results can be reported without dilution or modification

Analytical sensitivity:

Limit of blank (LoB): highest result from a blank sample
Limit of detection (LoD): lowest concentration reliably distinguished from blank
Limit of quantitation (LoQ): lowest concentration reliably measured with acceptable precision (often defined as CV ≤20%)

Analytical specificity (interferences):

Test for interference from hemolysis, lipemia, icterus
Test for cross-reactivity with similar compounds

12.5 Reference Ranges

Establishing Reference Ranges

Reference ranges define the expected results in a healthy population.

Define the reference population: healthy individuals; consider age, sex, ethnicity
Sample size: minimum 120 individuals (to calculate 90% confidence intervals for the 2.5th and 97.5th percentiles)
Statistical analysis:
- Parametric (if data are Gaussian): mean ± 2 SD includes 95%
- Non-parametric (most common): 2.5th to 97.5th percentile

Reference Range Considerations

Age and sex differences (creatinine, hemoglobin, alkaline phosphatase)
Pregnancy changes (normal “abnormal” values; e.g., hCG, alkaline phosphatase)
Fasting requirements (glucose, lipids)
Diurnal variation (cortisol peaks AM, iron peaks AM)
Method-specific ranges (different assays may have different reference intervals - particularly important for enzyme assays)

Transgender patients pose a particular GFR challenge because eGFR equations (MDRD, CKD-EPI) use sex as a variable (reflecting expected muscle mass). Testosterone increases muscle mass/creatinine; estrogen decreases both. Sex-independent methods (iothalamate clearance, cystatin C-based eGFR) avoid this confound entirely. Cystatin C is produced by all nucleated cells regardless of muscle mass.

Critical Values

Critical (panic) values are results so far from normal that they indicate life-threatening conditions requiring immediate clinical action.

Examples:

Glucose <40 or >500 mg/dL
Potassium <2.5 or >6.5 mEq/L
Sodium <120 or >160 mEq/L

Requirements:

Immediate notification of caregiver
Read-back verification (the recipient repeats back the value)
Documentation of notification
Time limits (usually within 30-60 minutes)

12.6 Select Clinical Chemistry Applications Tied to Analytical Principles

A few assays are worth highlighting because board questions map analytical principle → clinical result.

B12 deficiency confirmation: if serum B12 is borderline-low, the next step is methylmalonic acid (MMA). MMA is elevated in B12 deficiency but normal in folate deficiency. Homocysteine rises in both. So borderline B12 + elevated MMA = confirmed B12 deficiency; borderline B12 + normal MMA = look elsewhere. Only then test intrinsic factor antibodies (pernicious anemia).

Celiac disease testing algorithm: first-line is anti-tissue transglutaminase (tTG) IgA + total IgA simultaneously. If tTG IgA positive, confirm with duodenal biopsy (villous atrophy, crypt hyperplasia, intraepithelial lymphocytes). If tTG IgA negative but suspicion high, check total IgA. Celiac disease has a 10-15× higher prevalence of selective IgA deficiency (~2-3% vs 0.2%) than the general population. In IgA-deficient patients, use tTG IgG or anti-deamidated gliadin peptide IgG. Classic board trap: negative celiac serology in an IgA-deficient patient does NOT rule out celiac.

hCG kinetics: detectable 6-8 days post-conception, doubles every 48 hours for the first 8-10 weeks, peaks at the end of the first trimester (~100,000 mIU/mL), declines in the second trimester to ~10,000-50,000. After delivery, clears in ~2 weeks (half-life 24-36 hours). Persistent post-partum hCG suggests gestational trophoblastic disease. Alpha subunit is shared with TSH/LH/FSH; beta subunit is unique. In early pregnancy, hyperglycosylated hCG predominates in serum and the beta-core fragment predominates in urine. In ectopic pregnancy, hCG fails to double every 48h; above the discriminatory zone (~1,500-3,500 mIU/mL, some institutions use 6,000) without a visible intrauterine sac on TVUS is diagnostic.

Cardiac biomarkers in cocaine-induced MI: cocaine causes coronary vasospasm (MI without atherosclerosis) AND rhabdomyolysis. Myoglobin and CK-MB lose specificity (both in skeletal muscle). Troponin retains normal specificity (cardiac isoforms are structurally distinct from skeletal muscle). Board pearl: whenever skeletal muscle injury coexists with possible MI (rhabdo, seizures, extreme exercise), troponin is the preferred marker.

Cyanide toxicity labs: cyanide inhibits cytochrome c oxidase (Complex IV), blocking aerobic metabolism. Labs: elevated thiocyanate (metabolite), elevated lactate, anion gap metabolic acidosis, elevated glucose (can’t utilize aerobically), and elevated venous O2 (cells can’t extract oxygen - classic cherry-red venous blood). Treatment: hydroxocobalamin (binds CN⁻) or sodium thiosulfate (converts CN⁻ to thiocyanate).

Urine drug screen adulterant detection: check temperature (90-100F within 4 minutes of collection - cool = substitution), specific gravity (<1.005 suggests dilution, >1.030 suggests additive), pH (abnormal if <4.5 or >8.0), creatinine (<20 mg/dL suggests dilution), and nitrite (>500 μg/mL suggests adulterant). Oxidants (bleach, nitrites, PCC) can destroy drug metabolites.

Chapter 13: Electrolytes, Acid-Base, and Blood Gases

This chapter covers the core chemistries that drive decisions at the bedside: sodium, potassium, calcium, acid-base, and blood gases. A lot of the board yield here lives in pre-analytics - pseudohyponatremia, pseudohyperkalemia, ionized calcium specimen handling, air exposure artifacts on ABGs. The other half is pattern recognition: matching lab values to the right diagnostic framework (volume status for Na, anion gap for acidosis, chloride responsiveness for alkalosis, PTH/Ca/PO4 pattern for parathyroid disease).

13.1 Sodium: The Master Electrolyte

Sodium is the primary extracellular cation and the main determinant of plasma osmolality. Disorders of sodium primarily reflect disorders of water balance.

Normal Physiology

Plasma sodium: 135-145 mEq/L
Normal serum osmolality: 275-295 mOsm/kg
Total body sodium is regulated by aldosterone (controls renal sodium reabsorption)
Plasma sodium concentration is regulated by ADH/vasopressin (controls water excretion)

Sodium contributes ~90% of serum osmolality. Osmolality is tightly regulated by ADH and thirst mechanisms, which is why deviations in sodium are almost always water-balance problems.

The key concept: Hyponatremia is usually a problem of too much water, not too little sodium.

Measured vs. Calculated Osmolality

Two numbers, two different things:

Measured osmolality - freezing point depression on a lab instrument. Measures every osmotically active particle in the sample.
Calculated osmolality = 2(Na) + Glucose/18 + BUN/2.8

Osmolal gap = |Measured - Calculated|. Normal osmolal gap: <10 mOsm/kg.

An elevated osmolal gap means there are osmotically active substances in the blood that the formula doesn’t account for. This is one of the most useful calculated values in toxicology.

Hyponatremia (Sodium <135 mEq/L)

Before calling it true hyponatremia, work through the tonicity categories. True hyponatremia is hypotonic hyponatremia. Everything else is an artifact or a translocation problem.

Hypertonic hyponatremia - serum is hypertonic despite the low sodium number. Caused by hyperglycemia or mannitol. These osmotically active solutes pull water from cells into the blood, diluting sodium. Use the corrected sodium formula: for every 100 mg/dL glucose above 100, add 1.6 mEq/L to measured sodium (some labs use 2.4 when glucose exceeds 400). In DKA, the “hyponatremia” often resolves as glucose is corrected - water shifts back into cells.

Isotonic hyponatremia - measured osmolality is normal. Two main causes:

Pseudohyponatremia (see below)
TURP syndrome - absorption of non-conductive irrigant solutions (glycine, sorbitol) during transurethral resection of the prostate or other urologic/gynecologic procedures dilutes sodium

Hypotonic hyponatremia (true hyponatremia) - classified by volume status:

Hypovolemic hyponatremia (low total body sodium AND water, but relatively more sodium lost):

GI losses (vomiting, diarrhea)
Renal losses (diuretics, salt-wasting nephropathy, mineralocorticoid deficiency)
Third-spacing (pancreatitis, burns, surgery)

Use urine sodium to distinguish the source:

UNa >30 mEq/L = renal losses (diuretics, Addison disease, renal medullary disease, RTA type I, cerebral salt wasting). In a volume-depleted patient the kidneys should be retaining sodium; if they’re wasting it, something is wrong with the kidney or the hormones driving it.
UNa <30 mEq/L = extrarenal losses (GI losses, third-spacing). The kidneys are doing their job.

Euvolemic hyponatremia (total body sodium normal, but excess water):

SIADH (most common cause): Inappropriate ADH secretion causes water retention
Hypothyroidism (severe)
Glucocorticoid deficiency
Psychogenic polydipsia (water intake overwhelms renal dilution capacity)
Beer potomania - mechanistically similar to polydipsia. Beer is mostly water with minimal solute, so the kidneys can’t excrete enough free water despite maximally dilute urine

Hypervolemic hyponatremia (excess total body sodium AND water, but relatively more water):

Congestive heart failure
Cirrhosis
Nephrotic syndrome
All involve “effective hypovolemia” (low effective arterial volume) triggering ADH release and water retention. These same three conditions are also the top three causes of transudative pleural effusions.

SIADH in detail: SIADH occurs when ADH is secreted independent of normal osmotic/volume stimuli. Causes:

CNS disorders (stroke, trauma, infection)
Pulmonary disease (pneumonia, especially small cell lung cancer)
Medications (SSRIs, carbamazepine, cyclophosphamide)
Tumors (ectopic ADH production - small cell lung cancer classic)

SIADH is a diagnosis of exclusion. You can only make the call when you’ve excluded renal insufficiency, adrenal insufficiency, and recent diuretic use - all of which confound free water handling. The hallmarks:

Hyponatremia
Low serum osmolality (<280 mOsm/kg)
Inappropriately concentrated urine (>100 mOsm/kg) when serum is dilute
Elevated urine sodium (>40 mEq/L) - kidneys are excreting sodium appropriately
Euvolemic
Normal thyroid and adrenal function
No diuretic use

Pseudohyponatremia: Artifactually low sodium in the setting of hyperlipidemia (hypertriglyceridemia or hypercholesterolemia) or hyperproteinemia (classically myeloma). The patient’s actual sodium is normal.

Mechanism: the electrolyte exclusion effect. The indirect ion-selective electrode (ISE) method pre-dilutes the specimen, assuming ~93% of plasma volume is water. When lipids or proteins occupy more volume, the water fraction shrinks. The dilution step then overestimates the dilution and reports a falsely low sodium. Direct ISE (no dilution step) and blood gas analyzers avoid this entirely.

The osmolality pattern nails the diagnosis:

Measured osmolality: NORMAL (freezing point depression measures all particles correctly)
Calculated osmolality: LOW (formula uses the falsely low Na)
Osmolal gap: INCREASED (measured > calculated)

Correction methods for pseudohyponatremia:

Direct ISE (no dilution, no exclusion effect)
Ultracentrifugation before indirect ISE (removes the lipid/protein layer)
Blood gas analysis (direct ISE)

Spurious hyponatremia (distinct from pseudohyponatremia): measured sodium is low because the blood was drawn proximal to an IV infusion, diluting the specimen. Classic example - giving D5W via a left radial line and drawing labs from the left antecubital vein. Pre-analytical error. Draw from the opposite arm or distal to the IV site.

Symptoms of hyponatremia: Related to cerebral edema (water shifts into cells)

Mild (Na 130-135): Often asymptomatic
Moderate (Na 120-129): Nausea, headache, confusion
Severe (Na <120): Seizures, coma, death

Treatment principles:

Treat the underlying cause
Acute symptomatic: Hypertonic saline (3% NaCl)
Chronic: Gradual correction; avoid correcting faster than 8-10 mEq/L/day to prevent osmotic demyelination syndrome (central pontine myelinolysis)

Hypernatremia (Sodium >145 mEq/L)

Hypernatremia always indicates a relative deficit of water compared to sodium. It is most commonly seen in dehydrated patients - they’ve lost more water than sodium. Particularly common in the elderly (impaired thirst), infants (dependent on caregivers for water), ICU patients, and anyone with limited water access.

Like hyponatremia, hypernatremia is classified by volume status.

Hypovolemic hypernatremia - patient is volume-depleted AND hypernatremic. Subdivide by source:

Renal water losses: osmotic diuresis (hyperglycemia), post-obstructive diuresis
Extrarenal water losses: sweating, diarrhea, burns, insensible losses (fever, tachypnea)

Both lose more free water than sodium, concentrating the remaining sodium.

Euvolemic hypernatremia - classically diabetes insipidus. Pure free water loss without significant sodium loss. Patients can’t concentrate urine to conserve water, so they produce massive dilute urine output.

Central DI - damage to hypothalamus or posterior pituitary (neurohypophysis) means the body doesn’t make/release ADH. Causes include trauma (especially neurosurgery), space-occupying lesions (craniopharyngioma), Langerhans cell histiocytosis (eosinophilic granuloma), neurosarcoidosis, and idiopathic. DI is the most common endocrine manifestation of LCH. Treatment: desmopressin.
Nephrogenic DI - collecting tubules of the kidney are resistant to ADH. Does NOT respond to exogenous desmopressin.
- Sickle cell disease - sickling in the vasa recta of the renal medulla causes microinfarction, destroys the medullary concentration gradient. Also called hyposthenuria.
- Lithium - the most common drug cause; interferes with aquaporin-2 channels
- Demeclocycline - tetracycline antibiotic; its DI side effect was historically leveraged to treat SIADH (too much ADH plus a drug that blunts ADH response). Now largely replaced by tolvaptan (V2 receptor antagonist).
- Others: colchicine, furosemide, gentamicin, amphotericin B

Hypervolemic hypernatremia (less common):

Hypertonic saline administration
Sodium bicarbonate administration
Salt tablets

Symptoms: Related to cerebral shrinkage

Lethargy, weakness
Seizures, coma
More severe when acute

Treatment:

Free water replacement (oral or D5W IV)
Correct underlying cause
Correct gradually to avoid cerebral edema

13.2 Potassium: The Intracellular Cation

Potassium is the primary intracellular cation (~98% intracellular, intracellular K+ ≈ 140-150 mEq/L vs. plasma K+ ≈ 4 mEq/L). This steep gradient is maintained by Na-K-ATPase and is critical for cell membrane potential, especially in cardiac and neuromuscular cells. Small changes in extracellular K+ have outsized effects on cardiac conduction.

Normal serum potassium: 3.5-5.0 mEq/L

Hypokalemia (Potassium <3.5 mEq/L)

Three broad categories: GI loss, renal loss, transcellular shift.

Inadequate intake: Rare as sole cause (kidneys can conserve K+ well).

GI losses - vomiting, diarrhea, nasogastric suction. Diarrhea loses K+ directly in stool. Vomiting loses K+ both directly (gastric fluid) and indirectly: HCl loss drives metabolic alkalosis, which activates RAAS, which causes renal K+ wasting as a secondary insult.

Renal losses:

Diuretics (loop, thiazide) - most common cause
Hyperaldosteronism (primary or secondary), Cushing syndrome
Renal tubular acidosis types I and II
Hypomagnesemia (impairs K+ conservation)

Transcellular shift (K+ moves into cells):

Metabolic alkalosis - H+ leaves cells to buffer, K+ enters cells to maintain electroneutrality. For every 0.1 pH increase, K+ decreases ~0.4 mEq/L.
Insulin administration - drives K+ into cells via Na-K-ATPase stimulation. Critical in DKA treatment: as insulin is given, K+ plummets and must be supplemented aggressively.
β-agonists (albuterol)
Refeeding syndrome

Urine potassium distinguishes GI vs. renal loss:

UK <30 mEq/day = GI loss (kidneys are appropriately conserving)
UK >30 mEq/day = renal loss (kidneys are inappropriately wasting)

RTA exception to the acidosis rule: Nearly every acidosis causes hyperkalemia (H+ enters cells, K+ exits). The exceptions are RTA type I (distal - can’t secrete H+, wastes K+) and RTA type II (proximal - wastes bicarb AND K+). Both cause hypokalemic metabolic acidosis. Note: RTA type IV causes hyperkalemia (hypoaldosteronism).

Clinical effects: Related to altered membrane potential

ECG changes: U waves, flattened T waves, ST depression, prolonged QT
Muscle weakness, cramps
Ileus
Rhabdomyolysis (severe cases)
Predisposition to digoxin toxicity

Treatment:

Oral KCl preferred (safer)
IV KCl for severe cases (rate limited to 10-20 mEq/hr via peripheral line)
Replace magnesium (hypomagnesemia impairs K+ repletion)

Hyperkalemia (Potassium >5.0 mEq/L)

First, rule out pseudohyperkalemia. Three categories:

Degenerating cells (in vitro K+ leak):

Leukocytosis/leukemia and thrombocytosis - massive numbers of WBCs or platelets lyse in the tube and release K+. Prevention: immediate centrifugation to separate cells from plasma before K+ leaks out. In leukemia patients, the best workaround is to use whole blood potassium via point-of-care testing or ABG analysis - these measure K+ without a centrifugation/sitting step, so cell lysis in the tube doesn’t contaminate the result.

Improper collection:

Hemolysis (most common overall) - K+ leaks from lysed RBCs. Always check the hemolysis index before reporting a critical K+. RBCs contain ~150 mEq/L K+; even mild hemolysis significantly elevates measured K+.
Prolonged tourniquet time, fist clenching, traumatic draw, small-gauge needles (shear stress)
Wrong order of tube draw (if K+ tube sits too long before processing)

Familial pseudohyperkalemia: rare hereditary condition with faulty transmembrane channels on erythrocytes that constantly leak K+. In vivo the body compensates; in the collection tube, K+ accumulates in serum. Think of this in a patient with persistently elevated K+ but no clinical symptoms or ECG findings.

Contaminated draw: a dramatic K+ jump (e.g., 3.6 → 9.4 over 2 hours) during DKA treatment usually means the specimen was drawn proximal to an IV potassium infusion. Same pattern as spurious hyponatremia from IV contamination.

True hyperkalemia causes:

Increased intake (rare unless renal excretion impaired):

Oral/IV potassium supplements
Potassium-containing medications (potassium penicillin)

Decreased excretion:

Acute or chronic kidney disease
Hypoaldosteronism (Addison’s disease, hyporeninemic hypoaldosteronism)
Medications: ACE inhibitors, ARBs, spironolactone, NSAIDs, trimethoprim

Transcellular shift (K+ moves out of cells):

Acidosis (H+ enters cells, K+ exits)
Insulin deficiency (insulin drives K+ into cells)
β-blockers
Tissue damage (rhabdomyolysis, tumor lysis, hemolysis)
Succinylcholine
Digitalis toxicity

Clinical effects: Cardiac toxicity is the major concern

ECG changes (progressive):
1. Peaked T waves (earliest)
2. Prolonged PR interval
3. Loss of P waves
4. Widened QRS
5. Sine wave pattern
6. Ventricular fibrillation/asystole

Treatment (for symptomatic or K+ >6.5 mEq/L):

Stabilize cardiac membrane: Calcium gluconate (immediate effect, doesn’t lower K+)
Shift K+ into cells:

Insulin + glucose
β-agonists (albuterol)
Sodium bicarbonate (if acidotic)

Remove K+ from body:

Loop diuretics (if adequate renal function)
Potassium binders (sodium polystyrene sulfonate, patiromer, sodium zirconium cyclosilicate)
Hemodialysis (definitive; for severe cases or renal failure)

13.3 Calcium, Phosphate, and Parathyroid Physiology

Calcium circulates in two functional forms: ionized (free) and bound. Only ionized calcium is biologically active.

Total vs. Ionized Calcium

Total calcium = free (ionized) + bound
~50% is free (the physiologically active form)
~40% is bound to albumin
~10% is bound to anions (citrate, phosphate)

Normal ranges:

Total calcium: 8.5-10.5 mg/dL
Ionized calcium: 4.5-5.2 mg/dL

Because ~40% of total calcium is bound to albumin, the total number is unreliable when albumin is abnormal. Two ways to handle this:

Corrected calcium formula: Corrected Ca = Measured Ca + 0.8 × (4 − albumin). For every 1 g/dL albumin below 4, add 0.8 mg/dL. Hypoalbuminemic patients look falsely hypocalcemic on total calcium.
Measure ionized calcium directly - the best indicator of calcium status, not affected by albumin. Essential in critical care, neonatal, and transfusion settings.

Ionized calcium specimen requirements:

No air exposure - CO2 loss causes alkalosis, which shifts calcium onto albumin, giving a falsely low iCa
No calcium-chelating anticoagulants - EDTA or citrate tubes chelate calcium
No fist clenching or prolonged tourniquet - venous stasis causes local acidosis and falsely elevates iCa
Keep cool, deliver rapidly
Usually measured on a blood gas analyzer via ion-selective electrode (same instrument as pH, pO2, pCO2)

Hypercalcemia

Primary hyperparathyroidism is the #1 cause of hypercalcemia in outpatients. In hospitalized patients, malignancy takes #1. Together, primary HPT and malignancy account for ~90% of all hypercalcemia.

Primary hyperparathyroidism:

Lab pattern: ↑ Ca, ↓ PO4, ↑ urinary cAMP (PTH acts on bone to release Ca, on kidney to reabsorb Ca, excrete PO4, and activate vitamin D)
Causes, most to least common: parathyroid adenoma (~85%), parathyroid hyperplasia (~10-15%, all four glands), parathyroid carcinoma (rare)

Secondary hyperparathyroidism - PTH is appropriately elevated in response to chronic hypocalcemia, most commonly chronic kidney disease. The glands are doing their job.

CKD lab pattern: Ca LOW, PO4 HIGH, PTH HIGH. Mechanism: failing kidneys can’t excrete phosphate (↑PO4 complexes with Ca → ↓Ca), can’t activate vitamin D (↓1,25-OH-D → ↓Ca absorption), and the hypocalcemia drives PTH.

Complications of chronic secondary HPT:

Renal osteodystrophy - chronic osteoclast activation → bone resorption → osteoporosis. Treated with phosphate binders, calcitriol, cinacalcet.
Calciphylaxis - calcium-phosphate deposition in small blood vessels → ischemic skin necrosis. Driven by high phosphate, not calcium. When Ca × PO4 product exceeds ~55-70, precipitation occurs. Painful purplish skin lesions become necrotic. High mortality. Treatment: lower phosphate aggressively.

Tertiary hyperparathyroidism - follows prolonged secondary HPT. The glands have been chronically overstimulated for so long they become autonomous, continuing to overproduce PTH even when the original hypocalcemia is corrected. Now the patient is hypercalcemic. Classic post-renal-transplant scenario.

Quick comparison:

Type	Ca	PO4	PTH	Mechanism
Primary HPT	↑	↓	↑	Autonomous gland (adenoma)
Secondary HPT (CKD)	↓	↑	↑	Appropriate response to low Ca
Tertiary HPT	↑	variable	↑	Was appropriate, became autonomous
Vitamin D excess	↑	↑	↓	Increased absorption of Ca AND PO4

Humoral hypercalcemia of malignancy:

Mediated by PTH-related peptide (PTHrP)
Produced by tumors: squamous cell carcinoma of lung, renal cell carcinoma, breast cancer
Activates PTH receptors but is a different molecule
Standard PTH immunoassays do NOT detect PTHrP - must order a specific PTHrP assay
PTH itself is suppressed (negative feedback from high Ca) while PTHrP is elevated
PTHrP does NOT activate vitamin D (no effect on 1,25-OH-D). This distinguishes it from primary HPT, which elevates 1,25-OH-D.

Hypervitaminosis D:

Overingestion of vitamin D → increased intestinal absorption of both Ca AND PO4
Lab pattern: ↑ Ca, ↑ PO4, ↓ PTH (PTH suppressed by high Ca)
Vitamin D is fat-soluble and accumulates in adipose - toxicity persists for weeks

Granulomatous disease (sarcoidosis and others):

Macrophages within granulomas express 1α-hydroxylase, activating 25-OH vitamin D to 1,25-OH vitamin D outside the kidney
Normally this step is kidney-only
Results in hypercalcemia from increased Ca absorption

Milk-alkali syndrome:

Triad: hypercalcemia + metabolic alkalosis + acute kidney injury
Excessive calcium supplementation with absorbable alkali (think TUMS/calcium carbonate)
Re-emerging as calcium supplements gain popularity
The alkalosis enhances renal Ca reabsorption, creating a vicious cycle

Hypocalcemia

Top causes of hypoparathyroidism:

Iatrogenic/surgical: most common - inadvertent removal or damage during thyroid or parathyroid surgery
Autoimmune: can be part of autoimmune polyendocrine syndrome

Congenital hypoparathyroidism:

DiGeorge syndrome (22q11.2 deletion) - failure of 3rd and 4th pharyngeal pouches. Absent/hypoplastic thymus (T-cell deficiency) AND parathyroids (hypocalcemia). Classic triad: neonatal hypocalcemia + cardiac defects + T-cell immunodeficiency.

Hypomagnesemia → functional hypoparathyroidism:

PTH secretion requires magnesium
Persistent hypomagnesemia suppresses PTH release → hypocalcemia
The hypocalcemia won’t correct until Mg is replaced
Always check magnesium in a hypocalcemic patient

Massive transfusion → hypocalcemia:

Stored blood products contain citrate (anticoagulant) that chelates calcium
In massive transfusion (>10 units in 24 hours), citrate load overwhelms hepatic metabolism
Citrate binds ionized calcium → symptomatic hypocalcemia (tetany, cardiac effects)
Monitor ionized calcium and supplement

PTH Fragment Assays

PTH circulates as multiple fragments:

Intact PTH (1-84) and N-terminal PTH: biologically active, short half-life
Midregion and C-terminal PTH: biologically inactive, long half-life, cleared by kidneys

Modern intact PTH assays avoid interference from inactive fragments. In renal failure, C-terminal fragments accumulate and can falsely elevate older PTH assays.

13.4 Acid-Base Physiology

Understanding acid-base balance requires grasping the relationship between pH, pCO2, and bicarbonate.

Terminology

Acidemia: pH <7.35
Alkalemia: pH >7.45
Normal pH: 7.35-7.45

Acidemia and alkalemia describe the blood pH. Acidosis and alkalosis describe the underlying process. A patient can have acidosis without acidemia if compensation is complete. Not every acidosis/alkalosis causes acidemia/alkalemia - with compensation, pH can return to near-normal. Compensation is never perfect though; pH will not fully normalize with a single disorder.

Metabolic vs. respiratory:

Metabolic disorders = primary change in HCO3-
Respiratory disorders = primary change in CO2 elimination

Metabolic disorders are compensated by respiratory rate (CO2 change). Respiratory disorders are compensated by renal HCO3- handling.

Normal values:

Normal HCO3-: 22-28 mEq/L
Normal PaCO2: 35-45 mmHg

Compensation timing:

Metabolic compensation (via respiratory rate) is fast - minutes to hours. Full compensation within ~1 hour.
Respiratory compensation (via renal HCO3-) is slow - days. This is why we distinguish acute (<3 days) from chronic (≥3 days) respiratory disorders.

Quick Reference: Primary Acid-Base Disorders

Disorder	pH	Primary Change	Compensation
Metabolic acidosis	↓	↓ HCO3-	↓ pCO2 (hyperventilation)
Metabolic alkalosis	↑	↑ HCO3-	↑ pCO2 (hypoventilation)
Respiratory acidosis	↓	↑ pCO2	↑ HCO3- (renal retention)
Respiratory alkalosis	↑	↓ pCO2	↓ HCO3- (renal excretion)

Systematic Approach to Acid-Base Interpretation

Look at pH: Acidemia (<7.35) or alkalemia (>7.45)?
Identify the primary process: Does pCO2 or HCO3- explain the pH change?
Is compensation appropriate? Use compensation formulas
If metabolic acidosis, calculate anion gap
If elevated anion gap, calculate delta-delta ratio

The Henderson-Hasselbalch Equation

The general form:

pH = pKa + log([base]/[acid])

Applied to the bicarbonate-carbonic acid buffer system (the dominant physiologic buffer):

pH = 6.1 + log([HCO3-] / (0.03 × PaCO2))

The pKa of the bicarbonate system is 6.1. Why is this the most important buffer despite being far from physiologic pH? Because both components are physiologically regulated - kidneys control HCO3-, lungs control CO2 - and bicarbonate is abundant. The 0.03 converts PaCO2 (mmHg) to carbonic acid concentration.

Plugging in normal values: 6.1 + log(24/[0.03×40]) = 6.1 + log(24/1.2) = 6.1 + log(20) = 6.1 + 1.3 = 7.4. A 20:1 ratio of HCO3- to carbonic acid gives normal pH.

pH depends on the ratio of bicarbonate to pCO2:

Metabolic processes change [HCO3-]
Respiratory processes change pCO2
The body compensates to normalize the ratio and pH

Compensation Rules

Respiratory compensation for metabolic disorders (fast, minutes to hours):

Metabolic acidosis → Hyperventilation → ↓ pCO2
- Winter’s formula: Expected PaCO2 = 1.5 × [HCO3-] + 8 ± 2
- If actual PCO2 < expected → concurrent respiratory alkalosis
- If actual PCO2 > expected → concurrent respiratory acidosis (mixed disorder)
Metabolic alkalosis → Hypoventilation → ↑ pCO2
- Expected PaCO2 = 0.7 × [HCO3-] + 20 ± 5
- Limited by hypoxemia (can’t hypoventilate to the point of suffocation)

Metabolic compensation for respiratory disorders (slow, days):

Simple rule of thumb used on these boards:

Acute (<3 days): each 1 mmHg change in PaCO2 changes [HCO3-] by 0.1 (acidosis) or 0.2 (alkalosis) in the same direction (minimal renal compensation; only chemical buffering has occurred)
Chronic (≥3 days): each 1 mmHg change in PaCO2 changes [HCO3-] by 0.4 in the same direction (kidneys have had time to adjust HCO3- reabsorption/excretion)

Equivalently per 10 mmHg:

Respiratory acidosis, acute: HCO3- increases ~1 mEq/L per 10 mmHg rise
Respiratory acidosis, chronic: HCO3- increases ~3.5-4 mEq/L per 10 mmHg rise
Respiratory alkalosis, acute: HCO3- decreases ~2 mEq/L per 10 mmHg fall
Respiratory alkalosis, chronic: HCO3- decreases ~4-5 mEq/L per 10 mmHg fall

The Anion Gap

Anion gap = [Na+] - [Cl-] - [HCO3-]

Normal: 8-12 mEq/L (varies by lab).

The anion gap represents unmeasured anions (primarily albumin, phosphate, sulfate, organic acids) minus unmeasured cations (K+, Ca2+, Mg2+). In the normal state, the unmeasured anions exceed unmeasured cations by 8-12 mEq/L. Metabolic acidosis is subclassified by the gap: an elevated gap means unmeasured anions are accumulating; a normal gap means HCO3- is being replaced by Cl- (hyperchloremic).

Elevated anion gap metabolic acidosis (MUDPILES):

Methanol (metabolized to formic acid)
Uremia (accumulated organic acids)
DKA (ketoacids: β-hydroxybutyrate, acetoacetate)
Propylene glycol / Paraldehyde (in IV medications)
Isoniazid, Iron (lactic acidosis)
Lactic acidosis (hypoperfusion, metformin)
Ethylene glycol (metabolized to glycolic and oxalic acid)
Salicylates (aspirin overdose)

Lactic acidosis and DKA are by far the most common causes clinically.

Normal anion gap (hyperchloremic) metabolic acidosis - mnemonic HARDASS:

Hyperalimentation
Addison disease (primary adrenal insufficiency)
Renal tubular acidosis
Diarrhea (most common - loss of HCO3-)
Acetazolamide
Spironolactone
Saline infusion (dilutional acidosis)

All involve either HCO3- loss or impaired acid excretion, with Cl- stepping in to replace the lost HCO3-. Note: the recovery phase of DKA can present as non-anion gap metabolic acidosis.

The Osmolal Gap

Osmolal gap = Measured osmolality - Calculated osmolality

Calculated osmolality = 2(Na+) + Glucose/18 + BUN/2.8

Normal osmolal gap: <10 mOsm/kg

An elevated osmolal gap indicates osmotically active substances not in the formula. Two clinical patterns matter:

Metabolic acidosis + elevated osmolal gap - think toxic alcohols. Memory aid: M PEEP (the best mnemonic anyone has offered):

Methanol
Propylene glycol
Ethylene glycol
Ethanol (if excessive)
Paraldehyde

These raise the osmolal gap AND their metabolites (formic acid, oxalic acid, glycolic acid) raise the anion gap. Important time course: early in methanol or ethylene glycol poisoning, the parent compound is present but not yet metabolized → osmolal gap high, anion gap normal. Later, as metabolism proceeds → osmolal gap normalizes, anion gap rises. Don’t dismiss a low osmolal gap if you’ve caught the patient late.

Elevated osmolal gap WITHOUT metabolic acidosis - IMAGES mnemonic:

Isopropanol (isopropyl alcohol) - metabolized to acetone (a ketone, not an acid)
Mannitol
Acetone
Glycerol
Ethanol (sometimes)
Sorbitol

These raise the osmolal gap but are not metabolized to acids.

Ethanol’s contribution to the osmolal gap can be calculated: ethanol (mg/dL) / 4.6.

Metabolic Alkalosis Classification

Metabolic alkalosis is classified by chloride responsiveness - whether saline infusion would reverse the abnormality. Use urine chloride to distinguish:

Chloride-responsive metabolic alkalosis (UCl <10 mEq/L):

Kidneys are conserving Cl- because the patient is volume/Cl depleted
Most common causes: diuretics (post-diuretic phase), vomiting, NG suction
Other: villous adenoma, contraction alkalosis
Treatment: normal saline to replace volume and chloride → alkalosis corrects

Mechanism of vomiting-induced alkalosis:

Hypochloremia: HCl lost in vomit
Hypokalemia: Renal K+ wasting from RAAS activation + direct loss
↑ Bicarbonate: Loss of H+ → metabolic alkalosis
↑ BUN/Creatinine ratio (>20:1): Pre-renal azotemia from dehydration

Chloride-resistant metabolic alkalosis (UCl >10 mEq/L):

Kidneys are wasting Cl- (patient is not volume-depleted)
Saline won’t fix this - the problem is hormonal
Causes: hyperaldosteronism, Cushing syndrome, Bartter syndrome, licorice consumption, milk-alkali syndrome
All involve mineralocorticoid excess → Na retention + K/H excretion

Mechanism pearls:

Bartter syndrome = mutation in the Na-K-2Cl channel (mimics loop diuretic)
Licorice = glycyrrhizin inhibits 11β-HSD → apparent mineralocorticoid excess (cortisol can’t be inactivated and binds mineralocorticoid receptors)

Respiratory Acidosis and Alkalosis Causes

Respiratory acidosis occurs from any impairment to ventilation:

Airway obstruction (COPD, asthma)
Alveolar disease, perfusion defects
Neuromuscular weakness (Guillain-Barré, myasthenia gravis)
CNS depression (opioids, sedatives)
Chest wall deformity

Respiratory alkalosis results from hyperventilation:

Hypoxemia (most common - triggers respiratory drive)
Anxiety/panic
Pregnancy - progesterone stimulates the respiratory center; this is physiologic
CNS insults
Sepsis (early), pulmonary embolism

Refeeding Syndrome

Distinct pattern worth memorizing. In a severely malnourished patient who starts getting fed, insulin surges → glucose and K+ rush into cells. Three electrolytes drop simultaneously:

Hypokalemia
Hypophosphatemia (phosphorus consumed for ATP synthesis)
Hypomagnesemia (Mg complexes with ATP for stability; the three negative charges on ATP need Mg for neutralization)

Can cause cardiac arrhythmias, respiratory failure, death. Prevent by starting feeds slowly and supplementing electrolytes proactively.

Worked ABG Examples

These are the kinds of cases boards like to show. A systematic approach:

Look at pH - acidemic or alkalemic?
Examine PCO2 and HCO3- to identify primary disorder(s)
Check compensation using the formulas above
For metabolic acidosis, calculate anion gap

Example 1: pH 7.52, PCO2 31, HCO3- 35, AG 10, symptoms 3 days.

pH is alkalemic
Low PCO2 → respiratory alkalosis. High HCO3- → metabolic alkalosis. Both drive pH up.
If this were compensated metabolic alkalosis, PCO2 should be elevated (retaining CO2), not low.
Answer: mixed respiratory alkalosis and metabolic alkalosis

Example 2: pH 7.31, PCO2 40, HCO3- 18, AG 10.

pH acidemic
Normal PCO2 + low HCO3- + normal AG → non-anion gap metabolic acidosis
Winter’s: expected PCO2 = 1.5×18+8 = 35±2. Actual PCO2=40 (too high) → superimposed respiratory acidosis (lungs should be hyperventilating, aren’t).
Answer: mixed non-anion gap metabolic acidosis and respiratory acidosis

Example 3: pH 7.52, PCO2 40, HCO3- 35.

pH alkalemic, normal PCO2, high HCO3- → metabolic alkalosis
Expected PCO2 = 0.7×35+20 = 44.5±5. PCO2=40 is within range, so respiratory compensation is appropriate; pH hasn’t normalized yet.
Answer: partially compensated metabolic alkalosis

Example 4: pH 7.36, PCO2 33, HCO3- 18, AG 10.

pH near-normal (slightly acid), low PCO2, low HCO3-, normal AG
Winter’s: expected PCO2 = 1.5×18+8 = 35±2 (range 33-37). Actual=33, perfect fit.
Answer: compensated non-anion gap metabolic acidosis

Example 5: pH 7.46, PCO2 46, HCO3- 32.

Barely alkalemic, high PCO2, high HCO3-
Expected PCO2 = 0.7×32+20 = 42.4±5 (range ~37-47). Actual=46, fits.
Answer: compensated metabolic alkalosis

Example 6: pH 7.52, PCO2 30, HCO3- 24, symptoms 3 days.

Alkalemic, low PCO2, normal HCO3- → respiratory alkalosis
3 days = chronic. Expected HCO3- drop = 0.4×(40-30) = 4 mEq/L. Expected HCO3- = 24-4 = 20. Actual=24 → insufficient renal compensation.
Answer: uncompensated respiratory alkalosis

Example 7: pH 7.31, PCO2 50, HCO3- 24, symptoms 3 days.

Acidemic, high PCO2, normal HCO3- → respiratory acidosis
3 days = chronic. Expected HCO3- rise = 0.4×(50-40) = 4. Expected HCO3- = 24+4 = 28. Actual=24 → kidneys haven’t compensated yet.
Answer: uncompensated respiratory acidosis

Example 8: pH 7.47, PCO2 30, HCO3- 20.

Barely alkalemic, low PCO2, low HCO3-
Chronic: expected HCO3- = 24 - 0.4×(40-30) = 24 - 4 = 20. Actual=20, matches.
Answer: compensated respiratory alkalosis

Example 9: pH 7.33, PCO2 50, HCO3- 30.

Mildly acidemic, high PCO2, high HCO3-
Chronic: expected HCO3- = 24 + 0.4×(50-40) = 24 + 4 = 28 (acceptable up to ~30). Actual=30, appropriate.
Answer: compensated respiratory acidosis

13.5 Blood Gas Analysis

Sample Requirements

Arterial blood preferred (for pO2 assessment)
Heparin anticoagulated (lithium heparin or dry balanced heparin)
Anaerobic collection (air bubbles raise pO2, lower pCO2)
Analyzed immediately or kept on ice (metabolism continues)

Pre-analytical artifacts are heavily tested:

Air exposure (sample mixed with room air before analysis):

pO2 increases (air has higher pO2 ~150 mmHg)
pCO2 decreases (air has pCO2 ~0.3 mmHg)
pH increases (from CO2 loss)

Testing delay (cells continue to metabolize = glycolysis by RBCs/WBCs):

pO2 decreases (consumed)
pCO2 increases (produced)
pH decreases (acid generated)

Keep specimens on ice and analyze within ~30 minutes. These are the opposite direction of air-exposure artifacts.

Parameters

Measured (three electrodes on the analyzer):

pH - glass electrode
pCO2 - Severinghaus electrode (modified pH electrode behind a CO2-permeable membrane)
pO2 - Clark electrode (amperometric)

Calculated:

HCO3- is calculated, not measured (from pH and pCO2 via Henderson-Hasselbalch)
Base excess/deficit
Oxygen saturation is calculated from pO2 assuming normal hemoglobin-oxygen affinity (normal O2-Hb dissociation curve, normal 2,3-BPG, no abnormal hemoglobins). If those assumptions break, the calculated SaO2 is wrong.

Oxygen Assessment

pO2: Partial pressure of oxygen in blood

Normal arterial: 80-100 mmHg (depends on age and altitude)
Hypoxemia: pO2 <60 mmHg

Oxygen saturation (SaO2): Percentage of hemoglobin saturated with oxygen

Normal: >95%
Reflects the oxygen-hemoglobin dissociation curve

The oxygen-hemoglobin dissociation curve: Sigmoidal relationship between pO2 and saturation

Factors that shift the curve:

Right shift (decreased O2 affinity, more O2 released to tissues):
Acidosis
Hyperthermia
Increased 2,3-DPG
Hypercapnia
Left shift (increased O2 affinity, less O2 released):
Alkalosis
Hypothermia
Decreased 2,3-DPG
Carbon monoxide
Fetal hemoglobin

Pulse Oximetry vs. Co-oximetry

Pulse oximetry uses 2 wavelengths of light to measure oxyhemoglobin and deoxyhemoglobin (660 nm red for deoxy, 940 nm infrared for oxy). Calculates SpO2 = oxy / (oxy + deoxy). Cannot detect methemoglobin, carboxyhemoglobin, or sulfhemoglobin.

Co-oximetry uses 4+ wavelengths and measures four hemoglobin species:

Oxyhemoglobin - functional, carrying O2
Deoxyhemoglobin - functional, not carrying O2
Carboxyhemoglobin - bound to CO; cannot carry O2
Methemoglobin - iron oxidized to Fe3+; cannot carry O2

Co-oximetry cannot detect sulfhemoglobin - it has a unique absorption spectrum the instrument isn’t calibrated for. Sulfhemoglobin can interfere with methemoglobin readings on some instruments, falsely elevating metHb. Caused by dapsone, sulfonamides.

The key clinical pearl: in carbon monoxide poisoning, pulse oximetry interprets carboxyhemoglobin as oxyhemoglobin (similar absorbance at 660 nm) → falsely normal or elevated SpO2 despite severe hypoxia. Classic presentation: a “cherry red” CO poisoning patient with SpO2 ~100% who is critically hypoxic. You need co-oximetry to detect it.

Chapter 14: Renal Function Assessment

This chapter covers the lab side of renal function: how we estimate GFR, how we interpret BUN and creatinine, how urinalysis works, and how we classify acute kidney injury. Most of the nitrogen we measure in blood comes from catabolism of protein (amino acids deaminated to urea) and nucleic acids (purines to uric acid). These non-protein nitrogen (NPN) compounds are all cleared by the kidney, which is why almost every one of them rises in renal failure. The fractions to remember: urea ~45% of NPN, amino acids ~20%, uric acid ~20%, creatinine ~5%, ammonia ~2%. Urea is the dominant NPN species and the reason BUN exists as a test.

One small conversion worth keeping in your back pocket: BUN measures only the nitrogen fraction of urea, not the whole molecule. To convert BUN to urea, multiply by 2.14 (urea MW 60 divided by 2 nitrogen atoms at MW 28 = 2.14). Most US labs report BUN directly, but some international labs report urea, so you’ll see this conversion come up on boards.

14.1 Creatinine: The Standard Marker

Creatinine is produced from creatine in muscle at a relatively constant rate and is freely filtered by the glomerulus with minimal tubular reabsorption or secretion. This makes it a practical marker of glomerular filtration rate (GFR).

Creatinine Metabolism

Source: Non-enzymatic conversion of creatine and creatine phosphate in muscle
Production rate: Proportional to muscle mass (higher in young men, lower in elderly, women, malnourished)
Elimination: Almost entirely renal (freely filtered, 10-15% tubular secretion)

Because creatinine is both filtered and actively secreted by proximal tubular organic cation transporters, creatinine clearance overestimates true GFR by roughly 10-20%. The tubular secretion adds creatinine to urine beyond what was filtered. This matters clinically: drugs that block tubular creatinine secretion (trimethoprim, cimetidine) cause serum creatinine to rise without actually changing GFR - a pseudorise, not real renal injury. That is a classic board trap.

Contrast the three markers conceptually:

Creatinine - freely filtered + partially secreted → overestimates GFR
BUN/urea - freely filtered + partially reabsorbed (40-50%) → underestimates GFR
Inulin - freely filtered, neither secreted nor reabsorbed → equals true GFR (gold standard)

Inulin clearance is the true reference standard for GFR but is impractical - it requires IV infusion of a fructose polymer, precisely timed urine collections, and repeated blood sampling. Reserved for research. In clinical practice, true GFR lies somewhere between BUN-based and creatinine-based estimates.

Creatinine Measurement

Jaffe method (colorimetric, in use since 1886):

Creatinine reacts with picric acid in an alkaline medium to form an orange-red Janovsky complex, read at ~510 nm
This is a kinetic colorimetric assay, with modern versions using rate-blanking (measuring color change within a defined window) to reduce interference from non-creatinine chromogens
Advantages: Inexpensive, widely available
Positive interferences (falsely elevate creatinine): ascorbic acid, cephalosporins (especially cefoxitin and cefazolin), ketones/acetoacetate, glucose, fructose, protein, urea
Negative interference: bilirubin
Classic board trap: DKA patient with elevated creatinine on Jaffe that normalizes on enzymatic - the ketones are interfering
Despite rate-blanking, Jaffe still overestimates creatinine by ~0.1-0.2 mg/dL vs. enzymatic or HPLC reference methods

Enzymatic methods:

Use creatinine-specific enzymes (creatinine amidohydrolase)
More specific than Jaffe
Less interference
More expensive
IDMS-traceable (isotope dilution mass spectrometry) methods are the current reference standard used for eGFR calculations

Interpreting Creatinine

Critical concept: Because creatinine is related to muscle mass, a “normal” creatinine doesn’t always mean normal kidney function.

Example: An elderly woman with creatinine 1.0 mg/dL may have significantly reduced GFR because her low muscle mass produces little creatinine. A young muscular man with creatinine 1.0 mg/dL has normal GFR.

This is why we calculate or estimate GFR rather than relying on creatinine alone.

Azotemia vs. Uremia

Worth keeping these two terms straight.

Azotemia - elevated BUN as a lab finding, asymptomatic
Uremia - azotemia with clinical symptoms

The uremic syndrome on boards: nausea/vomiting, encephalopathy, peripheral neuropathy, uremic pericarditis (fibrinous pericarditis - an indication for emergent dialysis), uremic platelet dysfunction (prolonged bleeding time with normal platelet count and normal PT/PTT), and uremic frost (urea crystals on skin).

14.2 Glomerular Filtration Rate (GFR)

GFR is the volume of plasma filtered by the glomeruli per unit time (normally ~125 mL/min or ~180 L/day).

Measured Creatinine Clearance

If you have a 24-hour urine collection, you can directly measure creatinine clearance:

$CrCl = \frac{U_{Cr} \times V_{urine}}{P_{Cr} \times T}$ (in mL/min; T = 1440 min for a 24-hour collection)

Normal CrCl: 80-120 mL/min. This is slightly higher than true GFR (~90-120 mL/min) because of the tubular creatinine secretion discussed above. CrCl declines roughly 1 mL/min/year after age 40.

The main pitfall is incomplete collection, which falsely lowers CrCl. Verify completeness by checking total daily creatinine excretion: 15-25 mg/kg/day for women, 20-25 mg/kg/day for men. In advanced CKD, tubular secretion of creatinine becomes proportionally larger, so CrCl increasingly overestimates true GFR.

CKD stages by eGFR (mL/min/1.73 m²): G1 ≥90, G2 60-89, G3a 45-59, G3b 30-44, G4 15-29, G5 <15.

GFR Estimation Equations

Cockcroft-Gault (estimates creatinine clearance, not GFR): CrCl = [(140 - age) × weight (kg)] / [72 × creatinine] × (0.85 if female)

Used for drug dosing (historically)
Not standardized to body surface area
Being replaced by CKD-EPI

MDRD (Modification of Diet in Renal Disease):

Estimates GFR standardized to 1.73 m² BSA
Uses creatinine, age, sex (historically also race)
Validated in cohorts that included White and Black/African American adults ages 18-70 with eGFR < 60 mL/min/1.73 m²
Not validated in pregnant women, children, or acutely ill hospitalized patients
Older equation - more accurate at low GFR (<60), but underestimates and often caps reporting at “>60” for higher values
Historical sex/race modifiers (race coefficient now obsolete): multiply by 0.742 if female; older equations applied a 1.210 modifier for patients categorized as Black/African American; 1.0 otherwise

In pregnancy, GFR rises ~50% so MDRD underestimates badly. In children, use the Schwartz equation. In acutely ill patients with rapidly changing creatinine, no eGFR equation is valid - they all assume steady state.

CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration):

Current preferred equation for all patients (since 2020-2021)
More accurate than MDRD, especially at GFR > 60
Uses a two-slope spline that handles the creatinine-GFR curve better at higher values
Multiple versions: creatinine-based, cystatin C-based, combined creatinine-cystatin C (most accurate)

Recent change: race is no longer used in GFR calculations (2021 CKD-EPI race-free equation, endorsed by NKF-ASN). Older race-adjusted equations used a coefficient for patients categorized as Black/African American in the original datasets, based on average serum creatinine differences for a given measured GFR. Assigning biological significance to a social category was inappropriate, and the race-free equation uses only creatinine, age, and sex.

Important caveat for all eGFR equations: they only work at steady-state creatinine. If creatinine is actively rising or falling (AKI, recent fluid shifts, recent dialysis), the equations are meaningless.

KDIGO recommends confirmatory cystatin C-based eGFR when creatinine-based eGFR is 45-59 mL/min/1.73 m² without other evidence of kidney damage.

Cystatin C: An Alternative Marker

Cystatin C is a 13 kDa cysteine protease inhibitor produced at a constant rate by virtually all nucleated cells. It’s freely filtered by the glomerulus and completely reabsorbed and degraded by the proximal tubule - not returned to circulation. This is different from creatinine (filtered + secreted, not reabsorbed) and inulin (filtered only).

Advantages over creatinine:

Not affected by muscle mass, diet, or sex
More accurate GFR estimation in certain populations (elderly, extremes of body size, amputees, vegetarians)

Disadvantages:

Affected by thyroid dysfunction (hyperthyroidism increases it), high-dose corticosteroids, inflammation
More expensive
Less standardized historically (improving)

Combined creatinine-cystatin C equations provide the most accurate GFR estimates but cost more because they require two analytes.

14.3 Blood Urea Nitrogen (BUN)

Urea is the end product of protein metabolism, synthesized in the liver’s urea cycle.

BUN Physiology

Production: Liver converts ammonia (from amino acid deamination) to urea
Elimination: Freely filtered by glomerulus; 40-50% reabsorbed in the tubules
Unlike creatinine, BUN levels are affected by many non-renal factors

The reabsorption is the key mechanistic fact. Urea is passively reabsorbed in the proximal tubule and in the inner medullary collecting duct (where it participates in the countercurrent multiplier for urine concentration). With hypovolemia, slower tubular flow and ADH-driven water reabsorption cause MORE urea to be reabsorbed, so BUN rises out of proportion to creatinine. This is the mechanistic basis for pre-renal azotemia: the kidneys aren’t actually failing, they’re doing their volume-conservation job. Creatinine is not significantly reabsorbed regardless of volume status, so it doesn’t rise in parallel.

BUN/Creatinine Ratio

Normal ratio: 10-20:1. A useful shorthand is 10:1.

Elevated ratio (>20:1) - “pre-renal pattern”:

Prerenal azotemia (dehydration, heart failure, hemorrhage): Decreased renal perfusion → increased tubular urea reabsorption
GI bleeding: Blood in gut is digested like any protein meal → massive urea load → ratios can hit 30-40:1. Classic board trap in an elderly patient with unexplained high BUN:Cr
High protein intake
Catabolic states (trauma, infection, burns, steroids)

Decreased ratio (<10:1) - “intrinsic renal or low-production pattern”:

Liver disease (decreased urea production - the liver can’t make urea from ammonia)
Low protein diet
Rhabdomyolysis (disproportionate creatinine increase from muscle breakdown)
SIADH (dilutional effect on BUN)
Intrinsic renal disease: damaged tubules can’t preferentially reabsorb urea, so BUN and creatinine rise proportionally

14.4 Urinalysis: The Window to the Kidney

A complete urinalysis includes physical, chemical, and microscopic examination.

Physical Examination

Color:

Yellow (normal): Urochrome pigment
Red/brown: Blood, hemoglobin, myoglobin, beets, rifampin
Orange: Pyridium, carrots
Brown/black: Melanin, metronidazole, melanoma
Green: Pseudomonas UTI, propofol, methylene blue

Clarity/turbidity:

Clear: Normal
Cloudy: WBCs, bacteria, crystals, phosphates (normal in alkaline urine)

Specific gravity:

Normal: 1.005-1.030
Measures urine concentration (kidney’s ability to concentrate or dilute)
Low (<1.005): Dilute urine, diabetes insipidus, water intoxication
High (>1.030): Dehydration, SIADH, contrast dye
Fixed at 1.010: Isosthenuria (loss of concentrating ability - renal tubular damage)

Chemical Examination (Dipstick)

pH: 4.5-8.0

Acidic: Protein diet, acidosis, UTI with E. coli
Alkaline: Vegetarian diet, UTI with urease-positive organisms, stale sample (bacteria convert urea to ammonia)

Protein:

Detects primarily albumin (other proteins poorly detected)
Positive: Glomerular disease, overflow proteinuria, UTI, transient (fever, exercise)
False positive: Very alkaline urine, contamination with antiseptics
False negative: Dilute urine, non-albumin proteinuria (Bence Jones)

Glucose:

Normally not present (renal threshold ~180 mg/dL)
Positive: Diabetes mellitus, pregnancy (lowered threshold), renal glycosuria

Ketones:

Detects acetoacetate (not β-hydroxybutyrate well)
Positive: DKA, starvation, alcoholic ketoacidosis
Note: In DKA, β-hydroxybutyrate predominates - ketones may be only mildly positive initially and increase during treatment as β-hydroxybutyrate converts to acetoacetate

Blood:

Detects hemoglobin (from intact RBCs or free hemoglobin) and myoglobin
Positive blood with no RBCs on microscopy: Hemoglobinuria or myoglobinuria
False positive: Oxidizing agents, peroxidase-producing bacteria
False negative: Ascorbic acid (high doses)

Leukocyte esterase:

Enzyme from neutrophils
Positive: Pyuria (suggests UTI)
Can be positive with sterile pyuria (interstitial nephritis, TB)

Nitrite:

Produced by bacteria that reduce nitrate (most Enterobacteriaceae)
Positive: Suggests UTI with gram-negative bacteria
False negative: Gram-positive organisms, Pseudomonas (don’t reduce nitrate), dilute urine, short bladder incubation time

Bilirubin:

Conjugated (direct) bilirubin is water-soluble and can be excreted in urine
Positive: Hepatocellular disease, biliary obstruction
Unconjugated bilirubin is not water-soluble and will not appear in urine

Urobilinogen:

Product of bilirubin metabolism by gut bacteria
Normal: Small amount present (0.2-1.0 mg/dL)
Increased: Hemolysis (more bilirubin → more urobilinogen), hepatocellular disease
Absent: Complete biliary obstruction (no bilirubin reaches gut)

Microscopic Examination

Red blood cells:

Normal: 0-2/HPF
Elevated: Glomerulonephritis, stones, infection, malignancy, trauma
Dysmorphic RBCs (acanthocytes, irregular shapes): Suggest glomerular origin - they get deformed passing through a damaged GBM. Isomorphic (normal-shaped) RBCs point to lower urinary tract sources (stones, bladder, urothelial cancer)

White blood cells:

Normal: 0-5/HPF
Elevated: UTI, interstitial nephritis, contamination
Eosinophiluria (detected by Hansel stain, NOT Wright stain) is the classic finding in acute interstitial nephritis (AIN). Offenders: NSAIDs, penicillins, cephalosporins, sulfonamides, PPIs. Classic triad for AIN: fever + rash + eosinophilia (but only 10-30% of cases have the full triad). Eosinophiluria also occurs in atheroembolic disease and some glomerulonephritides, so it’s not perfectly specific

Epithelial cells:

Squamous (vaginal/urethral): Suggest contamination
Transitional (bladder): May be normal in small numbers
Renal tubular: Suggest acute tubular necrosis

Casts (cylindrical structures formed in tubules, taking the shape of the tubule):

Cast Type	Composition	Clinical Significance
Hyaline	Tamm-Horsfall protein	Normal, concentrated urine, exercise
RBC	RBCs	Glomerulonephritis, vasculitis
WBC	WBCs	Pyelonephritis, interstitial nephritis
Granular	Degenerating cells/protein	Many causes; “muddy brown” = ATN
Waxy	Degenerated granular	Chronic kidney disease, stasis
Broad	Any type, wide	Chronic kidney disease (dilated tubules)
Fatty	Lipid droplets	Nephrotic syndrome

The presence of RBC casts is highly specific for glomerulonephritis - RBCs originating from the damaged glomerulus get trapped in Tamm-Horsfall protein matrix in the tubules and take on the cast shape. Combined with dysmorphic RBCs and proteinuria, RBC casts nail the diagnosis of glomerular origin.

WBC casts most commonly indicate pyelonephritis. They distinguish upper-tract infection from lower-tract infection (cystitis): both produce pyuria, but only pyelonephritis produces WBC casts because the casts form in the renal tubules. WBC casts also occur in AIN and lupus nephritis.

Quick sediment-to-diagnosis map you should have memorized:

Bland sediment (few cells, maybe hyaline casts) = pre-renal or post-renal AKI
Muddy brown granular casts = acute tubular necrosis (degenerating tubular cells in Tamm-Horsfall matrix)
RBC casts + dysmorphic RBCs + proteinuria = glomerulonephritis
WBC casts = pyelonephritis or AIN
Eosinophiluria (Hansel stain) = AIN
Hyaline casts = normal or dehydration
Waxy / broad casts = chronic renal failure (dilated tubules from nephron loss)
Fatty casts / oval fat bodies = nephrotic syndrome

RBC cast: Red-tinged cast containing RBCs. Pathognomonic for glomerulonephritis.

Hyaline cast: Clear, colorless, and nonspecific. May be seen in concentrated urine or mild dehydration.

Crystals:

Crystal	Appearance	Urine pH	Significance
Calcium oxalate	Envelope, dumbbell	Any	Common; ethylene glycol poisoning
Uric acid	Various (rhomboid, rosette)	Acidic	Gout, tumor lysis
Triple phosphate	Coffin lid	Alkaline	UTI with urease producer
Cystine	Hexagonal	Acidic	Cystinuria (always pathologic)
Tyrosine	Needles	Acidic	Liver disease
Leucine	Spheres with radial striations	Acidic	Liver disease
Ampicillin/Sulfa	Needle clusters	Any	Drug precipitation

Cystine crystals: Pathognomonic hexagonal plates. Always indicates cystinuria.

14.5 Proteinuria and Albumin Testing

Proteinuria is one of the most sensitive and informative lab findings in nephrology. What you’re measuring depends heavily on which test you use, and this is a high-yield board area because it’s easy to miss clinically significant proteinuria with the wrong test.

Normal vs. Abnormal

Normal proteinuria: < 150 mg/day
Microalbuminuria: 30-300 mg/day (earliest sign of diabetic nephropathy, independent CV risk factor)
Nephrotic range: > 3.5 g/day (with edema + hypoalbuminemia + hyperlipidemia = nephrotic syndrome)

Most normal urinary protein is Tamm-Horsfall protein (uromodulin), a glycoprotein secreted by the thick ascending limb of Henle. It forms the matrix of hyaline casts. Crucially, uromodulin is NOT detected by urine dipstick, which only detects albumin.

What the Dipstick Does and Doesn’t See

The urine dipstick uses the protein error of indicators principle: bromophenol blue changes color at different pH when albumin is present. The consequence: the dipstick is sensitive to albumin but insensitive to globulins, Bence Jones light chains, and low-MW proteins.

Dipstick threshold: ~30 mg/dL (roughly equivalent to ~300 mg/day proteinuria). That means it misses microalbuminuria entirely - positive dipstick (1+) already represents overt proteinuria.

Classic board trap: a patient with multiple myeloma has a completely negative dipstick but massive proteinuria on a 24-hour collection. The Bence Jones light chains aren’t detected by the dipstick. To catch non-albumin proteins, use the sulfosalicylic acid (SSA) test (detects all proteins) or a 24-hour urine collection.

Dipstick false positives: very alkaline urine (pH > 8), quaternary ammonium antiseptic contamination, highly concentrated urine, prolonged dipping time.

Microalbumin Assay

Dedicated microalbumin immunoassays (immunoturbidimetric or immunonephelometric) detect albumin at 0.3 mg/dL - roughly 100x more sensitive than the dipstick. This is what you need for diabetic nephropathy screening.

The preferred specimen is a spot urine albumin:creatinine ratio (ACR) from the first morning void. First morning void is the most concentrated and minimizes orthostatic proteinuria (a benign condition, especially in adolescents, where protein is elevated during daytime activity but normal at night). The ACR corrects for urine dilution without requiring a timed collection.

ACR < 30 mg/g = normal (A1)
ACR 30-300 mg/g = microalbuminuria (A2)
ACR > 300 mg/g = macroalbuminuria (A3)

Two of three positive samples over 3-6 months confirms the diagnosis. ACE inhibitors and ARBs can slow progression and regress albuminuria in diabetic nephropathy.

Glomerular vs. Tubular vs. Overflow Proteinuria

The location of the defect determines what proteins leak:

Glomerular proteinuria: damaged GBM leaks large proteins (albumin, transferrin). Detected by dipstick and microalbumin assay. Classic glomerular diseases: diabetic nephropathy, glomerulonephritis, nephrotic syndrome
Tubular proteinuria: proximal tubule fails to reabsorb small proteins that normally get reabsorbed after glomerular filtration. Detect by β2-microglobulin (12 kDa) and lysozyme (14 kDa) assays. Causes: heavy metals, aminoglycosides, Fanconi syndrome
Overflow proteinuria: massive overproduction of a protein overwhelms tubular reabsorption. Examples: Bence Jones light chains in myeloma, myoglobin in rhabdomyolysis, hemoglobin in intravascular hemolysis

Albumin is 68 kDa - too large to cross the normal GBM in any meaningful quantity. Its presence in urine means the glomerular barrier is damaged. Beta-2 microglobulin and lysozyme are small enough to be freely filtered normally; seeing them in urine means the proximal tubule isn’t reabsorbing them.

14.6 Chronic Kidney Disease

Definition

CKD requires either of these for ≥ 3 consecutive months:

GFR < 60 mL/min/1.73 m², OR
Albuminuria (or another marker of kidney damage)

The 3-month duration is what distinguishes CKD from AKI. A single low GFR is NOT sufficient - you have to confirm on repeat testing. Markers of kidney damage beyond albuminuria include urine sediment abnormalities, tubular electrolyte abnormalities, structural abnormalities on imaging, and kidney transplant status.

CKD is staged using a two-dimensional KDIGO grid combining GFR category (G1-G5) and albuminuria category (A1-A3). Both axes matter because CKD can present in multiple patterns: decreased GFR with normal albuminuria (tubulointerstitial disease) or normal GFR with albuminuria (early diabetic nephropathy).

GFR categories (repeated from earlier for convenience): G1 ≥ 90, G2 60-89, G3a 45-59, G3b 30-44, G4 15-29, G5 < 15.

Screening

Individuals at high risk of CKD should be screened yearly with eGFR and urine albumin-creatinine ratio. You need both because each axis can be normal while the other is abnormal.

High-risk categories:

Diabetes
Hypertension
Family history of kidney disease
Age ≥ 60 (GFR declines ~1 mL/min/year after age 40)
Higher CKD burden in some populations (including African American, Hispanic/Latino, Asian/Pacific Islander, and American Indian/Alaska Native groups), reflecting genetic risk, environmental exposures, socioeconomic factors, and access to care
Frequent NSAID use
History of AKI

14.7 Acute Kidney Injury (AKI)

AKI is classified by where the problem is relative to the kidney: pre-renal, intrinsic (intra-renal), or post-renal.

Classification and Frequencies

Pre-renal (~55-60%): hypoperfusion. Kidneys are structurally normal but not getting enough blood. Dehydration, CHF, hemorrhage, sepsis, hepatorenal
Intrinsic (~35-40%): parenchymal damage. ATN, glomerulonephritis, acute interstitial nephritis
Post-renal (~5%): bilateral outflow obstruction

Pre-renal is most common and is reversible with volume repletion if caught in time.

Pre-Renal AKI: The Volume-Conserving Kidney

Pre-renal AKI is a functioning kidney trying to hold onto water and sodium. All the lab findings reflect that:

Parameter	Pre-renal AKI	Intrinsic AKI
BUN:Cr ratio	> 20:1	< 20:1
Urine specific gravity	> 1.020 (concentrated)	~1.010 (isosthenuric)
Urine osmolality	> 500 mOsm/kg	< 300 mOsm/kg
Fractional excretion of Na (FeNa)	< 1%	> 2%
Fractional excretion of urea (FeUrea)	< 35%	> 35%
Urine sediment	Bland, maybe hyaline casts	Muddy brown casts, RBC casts, WBC casts depending on cause

FeUrea is useful when the patient is on diuretics, because diuretics force Na excretion and falsely elevate FeNa even in a pre-renal state, but they don’t affect urea handling. FeNa can also be < 1% in contrast nephropathy, rhabdomyolysis, and early ATN - so FeNa < 1% does not always equal pre-renal.

Isosthenuria (SG ~1.010) = specific gravity of plasma ultrafiltrate, meaning the tubules aren’t modifying the filtrate at all. That’s the hallmark of tubular damage.

Fluid Challenge

Pre-renal AKI is reversible with volume. Give 500 mL-1 L isotonic saline, recheck creatinine in 24-48 hours. If creatinine improves - pre-renal confirmed. If not - the pre-renal state has tipped over into ATN. This is the pre-renal to ATN continuum: prolonged ischemia causes tubular injury, and once ATN develops, recovery takes 1-3 weeks and may require dialysis.

Intrinsic AKI

The three major causes:

Acute tubular necrosis (ATN) - most common intrinsic cause. Two mechanisms:

Ischemic ATN: prolonged pre-renal state. The proximal tubule and thick ascending limb (medullary segments) are most vulnerable because they have the highest O2 demand and lowest O2 supply
Nephrotoxic ATN: aminoglycosides (proximal tubular injury), IV contrast (vasoconstriction + direct toxicity), cisplatin, myoglobin and hemoglobin (pigment nephropathy)

Muddy brown granular casts are the hallmark of ATN. Recovery in 1-3 weeks as tubular epithelium regenerates.

Glomerulonephritis - RBC casts, dysmorphic RBCs, proteinuria. Workup heads into the autoimmune serology / complement territory covered in Chapter 24.

Acute interstitial nephritis (AIN) - drug hypersensitivity. NSAIDs, penicillins, cephalosporins, sulfonamides, PPIs. WBC casts, eosinophiluria. Classic triad of fever + rash + eosinophilia is only present in 10-30% of cases. May need immunosuppression if drug withdrawal doesn’t reverse it.

Post-Renal AKI

Post-renal AKI requires bilateral obstruction (or unilateral obstruction in a solitary functioning kidney). With two functioning kidneys, unilateral obstruction does NOT cause renal failure because the contralateral kidney compensates via hyperfiltration - the same principle that allows living kidney donors to function normally on one kidney.

Common causes: BPH (most common overall, blocks at the urethra), bladder cancer, cervical cancer, retroperitoneal fibrosis, bilateral ureteral stones or tumors.

Diagnosis: renal ultrasound showing hydronephrosis. Treatment: relieve the obstruction (Foley for BPH, ureteral stents for ureteral obstruction). Watch for post-obstructive diuresis after relief - massive sodium and water loss that may require IV replacement.

Board trap: kidney stone in one ureter with rising creatinine - think about why the other kidney isn’t compensating (may have pre-existing disease, or the stone is the only ureter if it’s a solitary kidney).

14.8 Renal Artery Stenosis

Renal artery stenosis (RAS) is a cause of secondary hypertension that’s worth understanding because it connects renal physiology, endocrine feedback (RAAS), and imaging workup.

Mechanism

Stenosis of the renal artery reduces perfusion pressure at the juxtaglomerular apparatus (JGA), which erroneously senses hypotension (less sodium reaches the macula densa). The JGA responds by releasing renin, which drives the renin-angiotensin-aldosterone cascade:

Renal artery stenosis → ↑renin → ↑angiotensin I → ↑angiotensin II (via ACE) → ↑aldosterone → Na+ retention, K+ excretion, vasoconstriction → hypertension

The patient isn’t actually volume-depleted - the kidney just thinks they are. This is the classic example of secondary hyperaldosteronism: elevated aldosterone driven by elevated renin (contrast with primary hyperaldosteronism = Conn syndrome, where renin is suppressed).

Two Etiologies, Two Patient Profiles

Atherosclerosis and fibromuscular dysplasia cause RAS in very different patients and affect different segments of the artery:

Feature	Atherosclerotic RAS	Fibromuscular Dysplasia
Frequency	~80-90% of RAS	~10-20%
Location on artery	Proximal / ostial (origin)	Mid-to-distal segment
Typical patient	Older, atherosclerotic risk factors (DM, HTN, smoking, dyslipidemia)	Young women (15-50 y)
Classic imaging	Smooth, focal proximal stenosis	String of beads on angiography
Treatment response	Angioplasty ± stent; medical management often preferred	More amenable to angioplasty

Diagnosis

Historically: renal vein renin sampling - compare renin from each renal vein. The stenotic side has disproportionately higher renin. Invasive, technically demanding.

Currently: imaging is preferred - Doppler ultrasound (first-line, noninvasive, cheap, operator-dependent), MR angiography, CT angiography. Definitive imaging guides intervention.

14.9 Urine Validity Testing (Adulteration Checks)

Urine drug testing is mostly covered in Chapter 21 (Toxicology), but a few urinalysis-specific validity checks are worth knowing here because they use the same chemical principles as routine urinalysis.

Every observed urine drug screen includes validity checks before interpreting results. Adulteration is suspected when the specimen falls outside physiologic norms:

Temperature: fresh urine should be 90-100°F (32-38°C) within 4 minutes of collection. A cool specimen suggests substitution with room-temperature fluid
Specific gravity < 1.005 = too dilute. Either excessive water intake to flush drugs, or deliberate dilution with water. SG > 1.030 or < 1.001 suggests a non-urine substance was substituted
pH outside 4.5-8.0 = chemical adulteration. Bleach raises pH, vinegar lowers pH. Normal physiology can’t produce pH outside this range
Creatinine < 20 mg/dL = dilute; < 2 mg/dL = not physiologic human urine. Creatinine is the key internal validity marker - tells you whether you’re even looking at real urine
Nitrite > 500 µg/mL = chemical adulteration. Normal urine has trivial nitrite even with UTI. Elevated nitrite means addition of an oxidizing agent (pyridinium chlorochromate is a common commercial adulterant) that destroys drug metabolites

Dilute specimens can cause false negatives because drug metabolites fall below assay cutoffs. SAMHSA/DOT guidelines require reporting specimens as “dilute” alongside the drug result, and some programs require re-collection under direct observation when adulteration is flagged.

Chapter 15: Liver Function and Hepatobiliary Assessment

The liver performs hundreds of functions - synthesis, metabolism, detoxification, and excretion. “Liver function tests” (LFTs) is actually a misnomer for most of these tests; they’re better called “liver chemistries” because they primarily detect liver injury rather than measure function.

A critical conceptual distinction: AST and ALT are markers of hepatocyte INJURY, not hepatocyte FUNCTION. The enzymes that reflect actual synthetic function are albumin and PT/INR (the liver makes both). Keep that split in your head when you read a panel - injury markers tell you damage is happening, synthetic markers tell you how much working liver is left. Ammonia and bilirubin sit somewhere in between (they reflect the liver’s ability to process things).

This chapter also covers the pancreas and porphyrias, because both sit adjacent to liver chemistry on boards and share lab-based diagnostic logic.

Quick Reference: Liver Test Patterns

Pattern	AST/ALT	Alk Phos	Bilirubin	Interpretation
Hepatocellular	↑↑↑	Normal or ↑	Variable	Hepatitis (viral, drug, autoimmune), ischemia, toxins
Cholestatic	Normal or ↑	↑↑↑	↑	Biliary obstruction (stone, stricture, tumor), PBC, PSC, drug-induced
Infiltrative	Normal or ↑	↑↑	Normal or ↑	Granulomas, malignancy, amyloid

AST:ALT Ratio Interpretation (the De Ritis ratio):

Ratio	Suggests
<1	Most liver diseases, viral hepatitis, NAFLD
>1 (reversal)	Alcoholic hepatitis, Wilson disease, cirrhosis of any cause
>2	Classic teaching: alcoholic liver disease
>3	Strongly suggests alcoholic liver disease

Key interpretation principle: The pattern of liver test abnormalities points toward the mechanism of injury. Marked transaminase elevation with modest ALP elevation suggests hepatocellular injury (hepatitis). Marked ALP elevation with modest transaminase elevation suggests cholestasis (biliary obstruction or intrahepatic cholestasis). Mixed patterns require clinical correlation.

15.1 The Aminotransferases: Markers of Hepatocellular Injury

Aspartate Aminotransferase (AST, SGOT)

AST catalyzes the transfer of an amino group from aspartate to α-ketoglutarate, producing oxaloacetate and glutamate. It exists in two isoenzymes: mitochondrial (~80% of hepatic AST) and cytosolic (~20%).

This subcellular distribution matters. Mild hepatocyte injury that spares mitochondria releases cytosolic contents first (both ALT and cytosolic AST), so ALT predominates in most liver injury. When injury is severe enough to damage mitochondria - specifically alcohol and copper - mitochondrial AST floods out and flips the ratio.

Tissue distribution (highest to lowest concentration):

Cardiac muscle > Liver > Skeletal muscle > Kidney > Brain > Lung > Pancreas

This widespread distribution makes AST less specific for liver disease than ALT. Historically AST was a cardiac biomarker; troponin replaced it. Today, when AST is elevated, always consider cardiac (MI, myocarditis), skeletal muscle (rhabdomyolysis, statin myopathy, post-exercise), and hemolytic sources before assuming liver origin.

Strenuous exercise can raise AST (along with CK, LDH, aldolase) from skeletal muscle injury. A marathon runner with elevated AST and a normal ALT has a muscle problem, not a liver problem. Check CK. Ideally have patients abstain from strenuous exercise for 24-48 hours before a liver panel if feasible.

Alanine Aminotransferase (ALT, SGPT)

ALT catalyzes the transfer of an amino group from alanine to α-ketoglutarate, producing pyruvate and glutamate. It is exclusively cytoplasmic.

Tissue distribution: ALT is produced primarily in liver, kidney, and muscle. It is more liver-specific than AST but not perfectly so. CKD patients can have elevated ALT from renal sources, and rhabdomyolysis elevates both AST and ALT because skeletal muscle contains both (AST > ALT in muscle, so rhabdo elevates AST more, but ALT rises too).

ALT half-life (~47 hours) is longer than AST (~17 hours), which is another reason ALT typically predominates in most forms of liver injury.

Sex differences: ALT is higher in males than females, probably due to greater liver mass relative to body weight and hormonal effects. Some guidelines recommend sex-specific upper limits (30 U/L males, 19 U/L females) lower than traditional cutoffs, which is why a “normal” ALT doesn’t always exclude liver disease. ALT also rises with BMI in NAFLD.

Interpreting Aminotransferase Elevations

Degree of elevation:

Mild (<3× ULN): NAFLD, chronic hepatitis B or C, hemochromatosis, medications
Moderate (3-10× ULN): Acute viral hepatitis, alcoholic hepatitis, autoimmune hepatitis
Marked (>10× ULN): Acute viral hepatitis, drug/toxin-induced (acetaminophen), ischemic hepatitis (“shock liver”), acute bile duct obstruction

Rhabdomyolysis as a mimic: Marked muscle injury elevates both AST and ALT because both enzymes are present in skeletal muscle. CK will be dramatically elevated (often >10,000 U/L) in rhabdomyolysis but normal in hepatitis. AST rises more than ALT because muscle has more AST. Classic board trap: elevated AST/ALT in a post-marathon runner or crush injury patient - check CK before assuming liver disease.

The AST:ALT (De Ritis) ratio:

Ratio	Condition	Explanation
<1	Viral hepatitis, NAFLD, drug-induced	ALT predominates in cytosol; cytosolic enzymes released first with membrane injury
>1 (reversal)	Alcoholic hepatitis, Wilson disease, cirrhosis	Mitochondrial damage releases mAST; other mechanisms below
>2	Alcoholic liver disease (classic teaching)	See mechanisms below

Reversal of the De Ritis ratio (AST > ALT) is seen in alcoholic steatohepatitis, Wilson disease, and cirrhosis of any cause.

Why alcoholic hepatitis gives AST:ALT >2:1:

Acetaldehyde (from alcohol) directly damages hepatocyte mitochondria, releasing mitochondrial AST
Alcohol depletes pyridoxal-5’-phosphate (vitamin B6), which is the cofactor for ALT synthesis. Less B6 = less ALT = inflated ratio
In alcoholic hepatitis, transaminases are usually only modestly elevated (typically <500 U/L). If AST or ALT is markedly elevated (>500), consider additional or alternative diagnoses (acetaminophen co-ingestion, viral coinfection, ischemic injury)

Why Wilson disease gives AST > ALT: Copper accumulates in mitochondria causing oxidative damage, releasing mitochondrial AST. Wilson disease can present as fulminant hepatic failure with very high AST, relatively lower ALT, and Coombs-negative hemolytic anemia (from copper-induced RBC injury).

Why cirrhosis (any cause) gives AST > ALT: The cirrhotic liver has fewer hepatocytes left to make ALT, while AST continues to be released from remaining damaged mitochondria and non-hepatic sources.

Isolated AST elevation (normal ALT):

Consider non-hepatic sources: Myocardial injury, skeletal muscle injury (rhabdomyolysis, strenuous exercise), hemolysis (RBCs contain AST)
Check CK if muscle source suspected

Lactate Dehydrogenase (LDH)

LDH is a nonspecific marker of liver injury (and everything else). It’s elevated in hemolysis, megaloblastic anemia, muscle injury, liver disease, kidney disease, lung disease, and malignancies (leukemia, lymphoma, germ cell tumors, carcinomas). As a standalone liver marker it adds almost nothing.

Useful LDH signals in liver workup:

LDH:AST ratio >5 suggests ischemic hepatitis (shock liver) rather than viral hepatitis. Shock liver has disproportionately high LDH because hypoxic cell necrosis releases LDH from many tissues simultaneously
Markedly elevated LDH with only mild transaminase elevation points toward hemolysis or lymphoma rather than primary liver disease

LDH isoenzymes (5 total, tetramers of H and M subunits):

Isoenzyme	Subunit	Main tissue sources
LD1	HHHH	Heart, RBC, kidney
LD2	HHHM	Heart, RBC, kidney
LD3	HHMM	Lung, spleen, lymphocytes, pancreas
LD4	HMMM	Liver, skeletal muscle
LD5	MMMM	Liver, skeletal muscle

Normal serum pattern: LD2 > LD1 > LD3 > LD4 > LD5. The “flipped” pattern (LD1 > LD2) was the historic MI signal, now obsolete because of troponin. In current practice LD1/LD2 elevation is most useful to confirm hemolysis (elevated total LDH + elevated LD1/LD2 + low haptoglobin + elevated indirect bilirubin).

LD4 and LD5 are the liver/skeletal muscle isoenzymes. Their elevation helps localize damage to liver or muscle, but in practice LDH fractionation is rarely ordered - checking AST/ALT + CK is cheaper and gives the same information.

LD6 is an atypical isoenzyme seen only in severely ill patients (multi-organ failure). It migrates cathodal to LD5 on electrophoresis and indicates high mortality. The exact molecular origin is disputed (some sources attribute it to alcohol dehydrogenase, others to a mitochondrial form) [TODO: verify origin]. Board trap: an unexpected extra band beyond LD5 on electrophoresis in an ICU patient.

15.2 Alkaline Phosphatase (ALP): Marker of Cholestasis

ALP is a group of isoenzymes that hydrolyze phosphate esters at alkaline pH (optimum pH ~9). Its sister enzyme class, acid phosphatase, has pH optimum of 5 and is produced by prostate, RBCs, and bone. Prostatic acid phosphatase was replaced by PSA as a prostate cancer marker. The one acid phosphatase that still matters in hemepath: tartrate-resistant acid phosphatase (TRAP), the classic cytochemical marker for hairy cell leukemia. Normal acid phosphatase is inhibited by L-tartrate; the HCL isoform (5b) resists. TRAP is also produced by osteoclasts and Gaucher cells. It has largely been replaced for HCL diagnosis by flow cytometry (CD103+, CD25+, CD11c+, CD123+) and BRAF V600E testing.

ALP tissue sources:

Liver (canalicular membrane of hepatocytes)
Bone (osteoblasts, during bone formation; NOT osteoclasts - those make TRAP)
Intestine
Placenta
Kidney

ALP Isoenzymes

There are four ALP isoenzymes based on tissue of origin. On electrophoresis they migrate in this order from cathode to anode:

Intestinal (cathode) → Placental → Bone → Biliary (anode)

Bone and biliary migrate close together and are hard to separate on standard electrophoresis, which is why GGT and 5’-nucleotidase are used instead (see below).

Isoenzyme differentiation by heat and chemical inactivation:

Isoenzyme	Heat/urea sensitivity	L-phenylalanine inhibition
Intestinal	+ (mild)	+++ (strong)
Placental	- (resistant)	+++ (strong)
Bone	+++ (very sensitive)	- (none)
Biliary	+ (mild)	- (none)

Mnemonic: “bone burns.” Bone ALP is the most heat-sensitive. Placental ALP is heat-resistant (makes sense - it has to survive elevated body temperature during labor). L-phenylalanine inhibits the intestinal and placental isoforms (the two that sit near the cathode on electrophoresis).

The Regan isoenzyme: A placental-like ALP produced by some malignancies (especially lung, ovarian, testicular, GI cancers). Resistant to heat (like placental ALP), inhibited by L-phenylalanine. Named after the patient in whom it was first discovered. Functions as a tumor marker but has low sensitivity. The Nagao isoenzyme is another tumor-associated ALP variant. Both are ectopic oncofetal antigens.

Elevated ALP: Hepatic vs. Bone

The two dominant sources of elevated ALP are liver (biliary) and bone. When ALP is elevated, you must determine which.

Method 1: Check GGT or 5’-nucleotidase (this has largely replaced ALP isoenzyme electrophoresis because the latter is complex and expensive):

GGT is produced by biliary epithelial cells AND hepatocytes
5’-nucleotidase is produced only by biliary epithelial cells (more specific for biliary disease)
Both are elevated in hepatobiliary disease but NOT in bone disease
Elevated ALP + elevated GGT/5’-NT = hepatobiliary source
Elevated ALP + normal GGT and 5’-NT = bone source

GGT is the more commonly available test. It’s more sensitive but less specific because it’s elevated by alcohol, enzyme-inducing drugs (phenytoin, barbiturates, rifampin), obesity, and diabetes even without biliary disease.

Method 2: Heat inactivation (bone burns). If ALP activity drops dramatically on heating, the source is bone. If it resists heat, think placental.

Hepatobiliary causes of elevated ALP:

Biliary obstruction (most marked elevations)
Primary biliary cholangitis (PBC)
Primary sclerosing cholangitis (PSC)
Infiltrative diseases (granulomatous hepatitis, metastases)
Hepatic congestion
Drug-induced cholestasis (oral contraceptives via estrogen-induced cholestasis, phenytoin, carbamazepine, antibiotics)

ALP is the most sensitive single liver chemistry test for detecting hepatic metastases, because tumor deposits obstruct small intrahepatic bile ducts, stimulating ALP production from biliary epithelial cells. It can be elevated even with normal bilirubin and transaminases. Rising ALP + imaging showing liver lesions in a cancer patient = metastatic disease until proven otherwise.

Bone causes:

Paget’s disease (causes the highest ALP elevations of any condition - often >10x ULN; GGT and 5’-NT are normal, confirming bone source)
Bone metastases (osteoblastic > osteolytic, because ALP comes from osteoblast activity; prostate cancer mets elevate bone ALP, breast cancer and myeloma osteolytic mets may not)
Osteomalacia, rickets
Healing fractures
Normal growth in children and adolescents (growth plate activity)
Hyperparathyroidism

Physiologic and Non-Pathologic ALP Elevation

Not every elevated ALP is disease:

Childhood growth (2-3x adult levels; use pediatric reference ranges; peaks during adolescent growth spurt)
Pregnancy (placental ALP in third trimester, 2-3x normal)
Benign transient hyperphosphatasemia of infancy (ALP can exceed 1000 U/L in a young child, resolves spontaneously)
Postprandial intestinal ALP rise (~30% increase for up to 12 hours after a meal, specifically in Lewis-positive blood group B or O secretors - because intestinal ALP is secreted into bile and enters the bloodstream postprandially, and secretors (FUT2+) express ABO antigens in secretions). Minor but board-relevant cause of elevated ALP.

Sex and age: Men have higher ALP than premenopausal women. After menopause, women’s ALP rises to equal men’s because loss of estrogen increases bone turnover. Men > premenopausal women; men = postmenopausal women. Use age- and sex-specific reference ranges.

Medications: Oral contraceptives elevate ALP through an estrogen-mediated cholestatic mechanism (decreased bile flow via altered bile acid composition and canalicular transporter expression - the same mechanism behind intrahepatic cholestasis of pregnancy). Phenytoin, carbamazepine, and various antibiotics also elevate ALP.

Decreased ALP

Less commonly tested but worth knowing:

Hypophosphatasia (ALPL gene mutation, autosomal recessive, tissue-nonspecific ALP deficiency causing defective bone mineralization - rickets/osteomalacia phenotype). The only condition where low ALP is the primary diagnostic finding
Malnutrition (zinc and magnesium are ALP cofactors)
Wilson disease (copper inhibits ALP)
Theophylline therapy, estrogen therapy, hypothyroidism, pernicious anemia, celiac disease

GGT as an Alcohol Marker

GGT is a sensitive marker for recent drinking, induced via microsomal enzyme induction. Heavy drinkers can have GGT elevated 2-3x normal even without liver damage. Sensitivity ~75% for heavy alcohol use. Limitations: also elevated by obesity, diabetes, medications (phenytoin, warfarin), and pancreatic/biliary disease. More specific alcohol markers are CDT (carbohydrate-deficient transferrin) and PEth (phosphatidylethanol). Combining GGT + CDT improves detection. GGT normalizes within 2-3 weeks of abstinence.

15.3 Ammonia and the Urea Cycle

The liver keeps serum ammonia low by converting it to urea (urea cycle - 5 enzymatic steps across mitochondria and cytoplasm of hepatocytes). When the liver can’t do this, ammonia accumulates and crosses the blood-brain barrier, causing hepatic encephalopathy.

Main sources of ammonia:

Skeletal muscle (branched-chain amino acid catabolism and AMP deamination, especially during exercise; normally muscle is a net consumer via glutamine synthetase, but becomes a net producer when activity overwhelms local GS capacity)
Gut (bacterial protein metabolism, glutamine metabolism by enterocytes)

The kidney also produces ammonia (from glutamine) for acid-base regulation. In patients with portosystemic shunts this can add to hyperammonemia.

Ammonia to urea: Ammonia is discarded by the liver as urea, which is excreted in urine. A paradox worth knowing: in severe hepatic dysfunction, BUN may be LOW (liver can’t make urea from ammonia) while ammonia is HIGH. The combination of low BUN + high ammonia should make you suspect severe hepatic failure.

Triggers for Hyperammonemia in Liver Disease

When a patient with known liver dysfunction suddenly gets encephalopathic, look for a trigger:

GI bleeding (especially variceal hemorrhage). Hemoglobin is protein; gut bacteria metabolize it to ammonia. 500 mL of blood in the GI tract = massive protein load
TIPS or other portosystemic shunts bypass the liver, so portal blood (ammonia-rich from gut) enters systemic circulation directly
Infection (increased catabolism)
Constipation (prolonged gut contact)
High-protein diet
Hypokalemia (stimulates renal ammonia production)
Trauma

Treatment of hepatic encephalopathy: lactulose (traps ammonia in gut as NH4+ and accelerates transit) and rifaximin (kills ammonia-producing gut bacteria).

Important caveat: Ammonia levels do NOT correlate well with the grade of hepatic encephalopathy. Some patients with mildly elevated ammonia have severe encephalopathy, and vice versa. Reasons: ammonia is only one of several neurotoxins, blood and brain ammonia don’t equilibrate perfectly, individual sensitivity varies. HE is a clinical diagnosis (asterixis, confusion, altered consciousness), not a lab diagnosis.

Other Causes of Hyperammonemia

Beyond liver failure:

Inborn errors of metabolism (most important in children). Mnemonic: UFO - Urea cycle disorders, Fatty acid oxidation disorders, Organic acidemias
- Urea cycle disorders: OTC deficiency is X-linked and the most common; presents in neonatal males with hyperammonemia. Heterozygous females can have late-onset presentations
- FAO disorders: MCAD deficiency is the most common; presents with hypoketotic hypoglycemia and hyperammonemia during fasting
- Organic acidemias: methylmalonic, propionic; present with metabolic acidosis + hyperammonemia + ketonuria
- Key lab distinction: organic acidemias have anion gap metabolic acidosis; urea cycle defects do NOT (they show respiratory alkalosis from central hyperventilation)
- Acute treatment: stop protein intake, IV dextrose (anabolic state reduces catabolism), ammonia scavengers (sodium benzoate, sodium phenylacetate), dialysis if severe
Total parenteral nutrition (high amino acid load exceeding hepatic urea cycle capacity)
Urease-positive UTI with atonic bladder (urease from Proteus, Klebsiella splits urea to ammonia; stagnant urine allows reabsorption into systemic circulation)
Ureterosigmoidostomy (urine diverted to colon after bladder removal; ammonia reabsorbed via colon)
Valproate therapy (inhibits urea cycle enzymes)
Cigarette smoking (cigarette smoke contains ammonia directly; see below)

In a child with hyperammonemia, think inborn error of metabolism.

Ammonia Pre-Analytical Error

Ammonia is one of the most pre-analytically labile analytes in clinical chemistry. Elevated ammonia is frequently a pre-examination artifact, not a real elevation.

Why ammonia rises in the tube over time:

Proteins and amino acids undergo spontaneous and enzymatic deamination, releasing ammonia
Rate accelerated at room temperature and with hemolysis
Ammonia in a blood sample increases over time - even with proper collection on ice, levels drift after 30 minutes

Common pre-analytical errors causing falsely elevated ammonia:

Hemolysis (RBCs contain 2-3x more ammonia than plasma)
Delay to analysis
Failure to chill the specimen during transport
Patient smoking (cigarette smoke contains ammonia, which is actually added to cigarettes to convert nicotine to its freebase form and increase absorption. Patients should abstain for several hours before an ammonia draw)

Proper protocol: specimen on ice, no hemolysis, immediate transport, analyze within 15-30 minutes, fasting non-smoking patient. If an ammonia result seems inconsistent with the clinical picture, suspect pre-analytical error and recollect. Many labs reject specimens that arrive >15-30 minutes after collection.

15.4 Bilirubin: The Pigment of Hemoglobin Catabolism

Bilirubin Metabolism

Heme catabolism: Senescent RBCs are phagocytosed by macrophages. Heme is converted by heme oxygenase to biliverdin, then by biliverdin reductase to unconjugated bilirubin. ~80% of daily bilirubin comes from hemoglobin degradation; the rest from myoglobin, cytochromes, and ineffective erythropoiesis
Transport: Unconjugated bilirubin is lipophilic and circulates bound to albumin (cannot dissolve in plasma independently). It is neurotoxic - can cross the blood-brain barrier in neonates and cause kernicterus
Hepatic uptake: Hepatocytes take up unconjugated bilirubin via specific transporters
Conjugation: UDP-glucuronosyltransferase 1A1 (UGT1A1) conjugates bilirubin with glucuronic acid (glucuronidation), forming water-soluble bilirubin diglucuronide (conjugated bilirubin)
Excretion: Conjugated bilirubin is actively secreted into bile canaliculi by MRP2 (multidrug resistance-associated protein 2). Defect in MRP2 = Dubin-Johnson syndrome
Intestinal fate: Gut bacteria (beta-glucuronidases) deconjugate bilirubin and reduce it to urobilinogen. Most urobilinogen is further reduced to stercobilin (gives stool its brown color) and excreted in feces. A small portion is reabsorbed (enterohepatic circulation) and either re-excreted by the liver into bile or filtered by the kidneys and excreted as urobilin (gives urine its yellow color)

Solubility and Lab Measurement

Unconjugated bilirubin: NOT water-soluble, bound to albumin in blood, cannot be filtered by glomerulus.

Conjugated bilirubin: water-soluble, dissolved in plasma (not protein-bound), can appear in urine.

This is why bilirubinuria = conjugated hyperbilirubinemia. Only water-soluble conjugated bilirubin passes into urine. Quick bedside test: jaundice + dark tea-colored urine (bilirubinuria) = conjugated; jaundice without bilirubinuria = unconjugated (hemolysis, Gilbert).

Measurement methods (van den Bergh diazo reaction):

Conjugated (direct) bilirubin: diazo colorimetric method WITHOUT accelerator. Only conjugated bilirubin reacts (it’s water-soluble and reacts directly with the diazo reagent). “Direct” comes from “reacts directly without accelerator”
Total bilirubin: diazo method WITH accelerator (methanol, caffeine, or surfactant breaks hydrogen bonds of unconjugated bilirubin so it also reacts), OR direct spectrophotometry at 454 nm (bilirubin’s peak absorbance, used in neonates where sample volumes are small)
Unconjugated (indirect) bilirubin: calculated as total minus direct. Not directly measured, hence “indirect”

The diazo method slightly overestimates direct bilirubin (some unconjugated reacts slowly even without accelerator), so calculated indirect may be slightly underestimated. Hemolysis interferes with both methods (hemoglobin absorbs near bilirubin).

Delta-Bilirubin

When conjugated bilirubin is persistently elevated, some becomes covalently bound to albumin, forming delta-bilirubin. Albumin has a half-life of ~21 days, so delta-bilirubin persists even after the underlying liver disease resolves. Clinically: a patient recovering from hepatitis may remain jaundiced for weeks after LFTs normalize because of residual delta-bilirubin. Delta-bilirubin is measured as part of “direct bilirubin” by most analyzers but is NOT excreted in urine (too large, albumin-bound).

Urine Urobilinogen Patterns

Condition	Urine urobilinogen	Urine bilirubin	Stool color
Normal	Small amount	Negative	Brown
Hemolysis	Increased	Negative	Dark (excess stercobilin)
Hepatocellular	Increased or normal	Positive	Normal or pale
Complete biliary obstruction	Decreased/absent	Positive	Pale/clay-colored (acholic)

Complete biliary obstruction = acholic stools + bilirubinuria + decreased urine urobilinogen + dark urine.

Unconjugated (Indirect) Hyperbilirubinemia

Two mechanisms:

1. Increased production:

Hemolysis (most common - more heme breakdown)
Ineffective erythropoiesis
Hematoma resorption

2. Impaired hepatic uptake/conjugation - via mutations in UGT1A1:

Gilbert syndrome (autosomal dominant, benign, 5-10% of population)
Crigler-Najjar type 2 (autosomal recessive, moderate, responds to phenobarbital)
Crigler-Najjar type 1 (autosomal recessive, severe, no enzyme activity)
Rifampin inhibits bilirubin uptake

Gilbert syndrome:

Mechanism: TA repeat polymorphism in the UGT1A1 promoter (7 repeats instead of 6) causing reduced transcription
Mild, intermittent unconjugated hyperbilirubinemia (usually <3 mg/dL)
Increases with fasting, stress, illness, dehydration
No liver disease, no hemolysis
Benign; requires no treatment

Crigler-Najjar type 1:

Autosomal recessive, complete absence of UGT1A1
Severe unconjugated hyperbilirubinemia (>20 mg/dL) from first days of life
Does NOT respond to phenobarbital (no enzyme to induce)
Requires aggressive phototherapy (12-16 hr/day) to prevent kernicterus
Definitive treatment: liver transplantation
Gene therapy under investigation

Crigler-Najjar type 2:

Autosomal recessive, partial UGT1A1 deficiency (residual activity present)
Less severe (bilirubin typically 6-20 mg/dL)
Responds to phenobarbital (induces residual enzyme, reducing bilirubin >25%)
Kernicterus rare but possible during intercurrent illness
No transplant needed
Phenobarbital response is the classic board differentiator: type 1 no response, type 2 responds

Conjugated (Direct) Hyperbilirubinemia

Causes:

Hepatocellular disease: Hepatitis, cirrhosis
Biliary obstruction: Stones, strictures, tumors
Inherited:
- Dubin-Johnson syndrome: Defective canalicular transport (MRP2 defect); benign; liver grossly BLACK from melanin-like pigment
- Rotor syndrome: Defective hepatic storage; benign; liver is NOT pigmented (distinguishes from Dubin-Johnson)

Neonatal Hyperbilirubinemia

Common causes of neonatal unconjugated hyperbilirubinemia:

Physiologic jaundice (most common; peaks day 3-5, resolves by 1-2 weeks; from immature UGT1A1, shorter neonatal RBC lifespan, and increased enterohepatic circulation)
Breastfeeding jaundice (early, day 2-4, from inadequate intake leading to fewer stools and more enterohepatic recirculation of bilirubin)
Breast milk jaundice (later, appears after day 4, peaks week 2-3, can persist weeks; breast milk contains beta-glucuronidase that deconjugates bilirubin in the gut, increasing reabsorption)
Polycythemia (increased RBC mass = more heme breakdown)

All cause unconjugated hyperbilirubinemia.

Most common causes of SEVERE neonatal hyperbilirubinemia (requiring phototherapy or exchange transfusion):

Hemolytic disease of the fetus and newborn (HDFN) - ABO or Rh incompatibility, maternal antibodies destroy fetal RBCs
Neonatal sepsis - hemolysis from DIC, impaired hepatic conjugation
Also: G6PD deficiency (triggered by oxidant stress), hereditary spherocytosis, Crigler-Najjar

Kernicterus

Unconjugated bilirubin is lipophilic and can cross the blood-brain barrier when levels exceed albumin binding capacity, or when albumin is low (prematurity). It deposits in the basal ganglia, hippocampus, and brainstem nuclei, causing irreversible neuronal damage.

Clinical phases:

Acute: lethargy, hypotonia, poor feeding, seizures
Chronic: cerebral palsy, hearing loss, upward gaze palsy

Treatment:

Phototherapy (converts bilirubin to water-soluble photoisomers excreted without conjugation)
Exchange transfusion for severe cases or rapid rise

Drugs that displace bilirubin from albumin (sulfonamides, aspirin) increase kernicterus risk in neonates because more free unconjugated bilirubin is available to cross the BBB.

15.5 Tests of Hepatic Synthetic Function

The synthetic markers tell you how well the remaining liver is actually working. These are the real “liver function” tests (as opposed to injury markers like AST/ALT).

Albumin

Albumin is synthesized exclusively by the liver. Half-life ~20 days, so it reflects chronic, not acute, liver function.

Decreased albumin:

Chronic liver disease (decreased synthesis)
Nephrotic syndrome (urinary loss)
Protein-losing enteropathy
Malnutrition
Acute phase response (negative acute-phase reactant)

Prothrombin Time (PT) and INR

The liver synthesizes most coagulation factors (I, II, V, VII, IX, X, XI, XII, XIII). Factor VII has the shortest half-life (~6 hours), so PT (which measures the extrinsic/common pathway including factor VII) is the first coagulation test to become abnormal in liver disease.

PT/INR is a marker of hepatic synthetic function, not injury. Together with albumin, it tells you what the liver can still make.

Sensitivity caveat: PT only detects liver injury when it’s severe. The liver has enormous synthetic reserve - factor levels must drop below ~30% of normal before PT prolongs. A patient can lose >70% of synthetic capacity before PT changes. By the time PT is elevated, liver disease is advanced. AST/ALT are far more sensitive for detecting mild injury.

End-stage liver disease paradox: A patient with markedly elevated PT but nearly normal AST/ALT has end-stage (“burned-out”) cirrhosis - so few viable hepatocytes remain that minimal transaminase can be released AND the liver can’t make clotting factors. Counterintuitive but classic: the worse the liver, the lower the transaminases can be.

Distinguishing liver disease from vitamin K deficiency:

Give vitamin K (10 mg IV):
- Vitamin K deficiency: PT corrects within 24-48 hours
- Liver disease: PT does not correct

Why raw PT beats INR for liver disease: INR was developed for warfarin monitoring using the International Sensitivity Index calibrated in warfarin patients, not liver disease patients. In liver disease, all factors are decreased (not just vitamin K-dependent ones), and INR may not accurately reflect bleeding risk. The MELD score uses INR despite this limitation; some hepatologists prefer raw PT for true assessment of synthetic function.

Immunoglobulin Patterns

Liver disease patterns on SPEP:

Autoimmune hepatitis: polyclonal IgG elevation (chronic B-cell stimulation from autoimmune process)
Primary biliary cholangitis (PBC): polyclonal IgM elevation (anti-mitochondrial antibody is the diagnostic marker)
Primary sclerosing cholangitis (PSC): no specific Ig pattern; p-ANCA may be positive; associated with IBD (especially UC)

These appear on SPEP as broad-based polyclonal hypergammaglobulinemia.

Albumin:Globulin (A:G) Ratio

Impaired hepatic synthesis and/or enhanced immunoglobulin synthesis decreases the albumin:globulin ratio.

An A:G ratio <1.0 is usually the result of liver disease (cirrhosis with decreased albumin synthesis + increased immunoglobulin production from portosystemic shunting exposing the immune system to gut antigens). On SPEP this shows as a decreased albumin peak plus elevated gamma region with polyclonal bridging. Other causes: chronic infection, autoimmune disease, nephrotic syndrome (loses albumin but not globulins). The A:G ratio is a quick, free screening tool on every CMP.

15.6 Patterns of Liver Disease

Hepatocellular Pattern

ALT, AST markedly elevated
ALP mildly elevated or normal
Causes: Viral hepatitis, drug-induced, autoimmune hepatitis, ischemic hepatitis

Cholestatic Pattern

ALP markedly elevated (often >3× ULN)
GGT elevated
ALT, AST mildly elevated or normal
Causes: Biliary obstruction, PBC, PSC, drug-induced cholestasis

Wilson Disease

Autosomal recessive copper metabolism disorder (ATP7B gene mutation, encodes a hepatic copper-transporting ATPase).

Diagnostic workup:

Low ceruloplasmin (<20 mg/dL)
Elevated 24-hour urine copper (>40 μg/day; often >100 μg/day)
Kayser-Fleischer rings (copper deposits in Descemet’s membrane) - seen on slit lamp examination
Free (non-ceruloplasmin-bound) copper: Calculated as Total copper − (3 × ceruloplasmin); elevated in Wilson disease
Gold standard: Liver biopsy with hepatic copper quantification (>250 μg/g dry weight)

Paradoxically, serum copper is often LOW because ceruloplasmin-bound copper (which constitutes most serum copper) is decreased.

Wilson disease on liver chemistry often shows AST > ALT (mitochondrial copper damage releases mAST) and can present as fulminant hepatic failure with Coombs-negative hemolytic anemia. Low ALP is another clue (copper inhibits ALP activity).

15.7 Pancreatic Function Testing

The pancreas deserves its own section adjacent to liver chemistry because amylase/lipase testing and cyst fluid analysis come up together on boards, and both share logic with biliary testing.

Amylase

Amylase hydrolyzes starch. Two major serum isoforms: P-type (pancreatic, ~40%) and S-type (salivary, ~60%).

Kinetics in acute pancreatitis:

Rises within 6-12 hours of symptom onset
Peaks at ~48 hours
Returns to normal in 3-5 days in uncomplicated cases
Persistently elevated amylase beyond 5 days suggests a complication (pseudocyst, pancreatic necrosis, abscess, duct disruption)

Short half-life (~2 hours) because it’s a small protein (55 kDa) readily cleared by the kidneys.

Diagnostic thresholds: Amylase >3x ULN has ~95% sensitivity for acute pancreatitis. The higher the amylase, the greater the specificity for pancreatitis - mild elevations (1-3x) can be many things, but >5-10x is much more specific.

Normal amylase in pancreatitis: Up to 10% of acute pancreatitis cases have normal amylase, especially alcoholic pancreatitis. Mechanism: chronic alcohol causes progressive acinar cell loss, so less amylase is available to release. Hypertriglyceridemia (another cause of pancreatitis) can also falsely lower measured amylase due to assay interference. This is why lipase is now preferred as the single best marker.

Non-pancreatic hyperamylasemia (up to 30% of elevations):

Salivary: parotitis/mumps, bulimia, sialadenitis
GI: bowel obstruction, mesenteric ischemia, perforated ulcer, cholecystitis
GYN: ectopic pregnancy, ruptured ovarian cyst, salpingitis
Metabolic: DKA, macroamylasemia, renal failure

If needed, pancreatic-specific isoenzyme (P-amylase) or lipase can distinguish pancreatic from non-pancreatic sources.

Renal clearance:

Amylase is primarily cleared renally, so renal insufficiency causes spurious hyperamylasemia (accumulation, not pancreatitis)
Amylase-to-creatinine clearance ratio (ACCR) >5% suggests true pancreatitis; <1% suggests macroamylasemia

Macroamylasemia

Macroamylasemia: amylase complexed with immunoglobulin (usually IgA or IgG), forming a large molecule that cannot be filtered by the kidney. Result: persistently elevated serum amylase with low urine amylase and low ACCR (<1%). Completely benign. Recognize it to avoid an unnecessary pancreatitis workup.

Lipase

Lipase is more specific than amylase because its tissue distribution is narrower (pancreas >> tongue, stomach, liver). It is also less dependent on renal clearance than amylase.

Kinetics:

Rises 4-8 hours after symptom onset
Peaks at 24 hours
Remains elevated up to 14 days (vs 3-5 days for amylase), making it better for late presentations

Current ACG guidelines prefer lipase over amylase for diagnosing acute pancreatitis. Diagnosis (revised Atlanta criteria) requires 2 of 3: (1) characteristic abdominal pain, (2) lipase or amylase >3x ULN, (3) characteristic imaging findings. Imaging is not required if the first two are met.

Renal disease: In CKD, amylase is chronically elevated (up to 3x normal) making it useless for diagnosing pancreatitis. Lipase is also mildly elevated in CKD but remains a better discriminator.

Trypsinogen/Trypsin

Trypsinogen is the inactive zymogen of trypsin, produced exclusively by pancreatic acinar cells and secreted into the duodenum via the pancreatic duct.

Activation in duodenum by two mechanisms:

Enterokinase (enteropeptidase) - a serine protease on the duodenal brush border that cleaves the trypsinogen activation peptide. The physiologic initiator
Autocatalysis - once some trypsin is formed, it converts more trypsinogen to trypsin (positive feedback)

Trypsin then activates all other pancreatic zymogens (chymotrypsinogen, proelastase, procarboxypeptidase, prophospholipase).

Safety mechanism: Pancreatic secretory trypsin inhibitor (PSTI / SPINK1) inactivates prematurely activated trypsin within the pancreas. Mutations in SPINK1 or PRSS1 (cationic trypsinogen gene) cause hereditary pancreatitis - the intrapancreatic safety net fails, trypsin activates other zymogens inside the pancreas, and autodigestion ensues.

Pancreatic Exocrine Function Tests

For chronic pancreatitis and pancreatic insufficiency diagnosis.

Invasive gold standard: Secretin-cholecystokinin (secretin-CCK) test

Place duodenal tube fluoroscopically
Administer IV secretin (stimulates ductal bicarbonate secretion) and CCK (stimulates acinar enzyme secretion)
Collect serial duodenal aspirates via endoscopy over 60-80 minutes
Measure volume, bicarbonate concentration, and enzyme activity (amylase, lipase, trypsin)

In chronic pancreatitis all three are decreased. Invasive and rarely performed in practice.

Non-invasive stool tests:

Fecal fat (quantitative 72-hr collection or qualitative Sudan III stain) - detects steatorrhea from any cause, not specific to pancreas
Fecal chymotrypsin - low levels suggest exocrine insufficiency, but can be degraded in transit
Fecal elastase-1 - the best non-invasive test. Not degraded during intestinal transit (unlike chymotrypsin) and specific to pancreatic tissue. Values <200 μg/g = insufficiency

Gold standard for steatorrhea: 72-hour fecal fat quantitation after 6 days on a high-fat (100g/day) diet. Collect stool during the last 72 hours. Normal <7 g/day; >7 g/day = steatorrhea; >20 g/day strongly suggests pancreatic insufficiency (vs small bowel malabsorption). Not specific to pancreas - detects fat malabsorption from any cause (exocrine pancreatic insufficiency, bile salt deficiency, celiac disease, short bowel). Qualitative Sudan III screening is faster but less sensitive (75-90%).

Pancreatic Cyst Fluid Analysis

Cyst fluid analysis uses three markers in a simple logic: amylase tracks ductal communication; CEA and CA19-9 track epithelial lining (specifically mucin-producing epithelium).

Cyst type	Amylase	CEA	CA19-9	Key features
Pseudocyst	↑ (high)	↓	↓	No epithelial lining (walled off by fibrosis/inflammation); direct communication with duct
Serous cystadenoma / solid pseudopapillary tumor	↓	↓	↓	No ductal connection; cuboidal (not ductal) epithelium
Mucinous cystic neoplasm (MCN)	↓	↑	↑	Mucinous epithelium but NO ductal connection; ovarian-type stroma
Intraductal papillary mucinous neoplasm (IPMN)	↑	↑	↑	Mucinous epithelium AND ductal connection

Mnemonic: amylase = ductal connection; CEA/CA19-9 = epithelial lining (mucinous type).

Additional clinical context:

Pseudocysts lack epithelial lining (“pseudo”) - walled off by inflammatory/fibrotic tissue; complication of pancreatitis
Serous cystadenomas: benign, no malignant potential, characteristic “central scar with stellate calcification” on imaging
Solid pseudopapillary tumors: classic in young women, low malignant potential
MCNs: occur almost exclusively in women (95%+), body/tail of pancreas, have malignant potential (can progress to invasive carcinoma), should be resected. Ovarian-type stroma is the pathognomonic histologic finding
IPMNs: main-duct type (higher malignant risk, often resected) vs branch-duct type (lower risk, often surveilled); occur in both sexes
CEA >192 ng/mL in cyst fluid has ~75% accuracy for distinguishing mucinous from non-mucinous cysts
Viscous/mucoid aspirate favors a mucinous lesion (MCN or IPMN) over serous or pseudocyst

15.8 Porphyrias

The porphyrias are inherited defects in heme biosynthesis. They split into two clinical phenotypes based on which intermediate accumulates:

Neurovisceral attacks (no photosensitivity): caused by accumulation of early, non-porphyrin intermediates (ALA, PBG). Classic example: AIP
Photosensitivity (skin findings, no acute attacks): caused by accumulation of porphyrins themselves (photoactive). Classic example: PCT

Acute Intermittent Porphyria (AIP)

AIP is caused by deficiency of porphobilinogen deaminase (also called hydroxymethylbilane synthase) - the 3rd enzyme in heme synthesis. Autosomal dominant with variable penetrance (most carriers never have attacks).

Triggers for attacks (anything that induces ALA synthase):

Drugs: barbiturates, sulfonamides, oral contraceptives
Fasting
Alcohol
Stress, infection

Clinical: neurovisceral crises - severe abdominal pain, neuropsychiatric symptoms, autonomic dysfunction, hyponatremia (SIADH). NO photosensitivity (because the porphyrins that cause photosensitivity are not elevated - only ALA and PBG accumulate).

Screening test: urinary porphobilinogen (PBG) during an acute attack. Markedly elevated during attacks; can remain elevated between attacks. Urine may turn port wine / dark red on standing (PBG polymerizes to uroporphyrin under light).

Acute treatment: IV hemin (provides negative feedback on ALA synthase, shutting down the pathway). IV glucose also helps.

Porphyria Cutanea Tarda (PCT)

PCT is caused by deficiency of uroporphyrinogen III decarboxylase (5th enzyme in heme synthesis). Most common porphyria overall.

Clinical: photosensitivity with blistering skin lesions on sun-exposed areas, skin fragility, hypertrichosis, hyperpigmentation. NO acute neurovisceral attacks.

Strong associations: hepatitis C, iron overload (hemochromatosis), alcohol use, estrogen therapy. These conditions precipitate the phenotype in susceptible individuals.

Screening test: urinary and/or plasma porphyrins. Uroporphyrin >> coproporphyrin is the characteristic pattern. Plasma porphyrin fluorescence peak in PCT is ~615-620 nm; the 626 nm peak is the classic finding in variegate porphyria and is the more board-tested wavelength. PBG is NOT elevated in PCT (this is the key distinguisher from AIP).

Treatment: phlebotomy (reduces iron stores) plus treat underlying cause (HCV eradication). Low-dose hydroxychloroquine can also mobilize hepatic porphyrins.

AIP vs PCT at a Glance

Feature	AIP	PCT
Enzyme deficiency	PBG deaminase	Uroporphyrinogen III decarboxylase
Inheritance	AD (low penetrance)	Mostly sporadic (type 1, ~75%); type 2 is AD
Phenotype	Neurovisceral attacks	Skin photosensitivity
Photosensitivity	NO	YES
Acute attacks	YES	NO
Screening test	Urine PBG	Urine/plasma porphyrins
Classic associations	Drug triggers, fasting	HCV, iron overload, alcohol
Urine color	Port wine on standing	May be dark/tea-colored

Chapter 16: Cardiac Biomarkers

The diagnosis of acute myocardial infarction has been transformed by cardiac biomarkers. Understanding these markers requires appreciating both their molecular biology and their clinical kinetics - when they rise, when they peak, and why they behave as they do.

This chapter walks through the markers roughly in the order they rose (and in some cases fell) in clinical importance: troponin first because it is the current gold standard, then natriuretic peptides for heart failure, then the older markers (CK-MB, myoglobin) which still show up on boards, and finally the framework that pulls biomarker results together - the Universal Definition of MI, the ACS spectrum, and the non-ACS causes of myocardial injury you have to know cold.

16.1 Cardiac Troponins: The Gold Standard

The Biology of Troponin

The troponin complex is the master regulator of muscle contraction. It consists of three subunits (T, I, C) that work together to control when actin and myosin can interact. The easy way to keep them straight: T binds Tropomyosin, I is Inhibitory, C binds Calcium. One mnemonic floating around: “a heart attack gives me a TIC.”

Troponin C (TnC): The calcium-binding subunit. When calcium floods into the cytoplasm during depolarization, TnC binds it and undergoes a conformational change. TnC is nearly identical in cardiac and skeletal muscle, making it useless as a cardiac-specific marker.

Troponin I (TnI): The inhibitory subunit. TnI normally holds tropomyosin in a position that blocks actin-myosin interaction. When TnC binds calcium, TnI releases its inhibition, allowing contraction. Cardiac TnI (cTnI) has a unique 31-amino-acid N-terminal extension not found in skeletal TnI - this is what makes it cardiac-specific.

Troponin T (TnT): The tropomyosin-binding subunit. TnT anchors the entire troponin complex to tropomyosin. Cardiac TnT (cTnT) also has unique sequences that distinguish it from skeletal isoforms.

The key concept is that only cTnI and cTnT are cardiac-specific. TnC is not, because its cardiac and skeletal sequences are identical. When you detect cTnI or cTnT in the blood, you know cardiac myocytes have been damaged. This specificity is why troponin has become the cornerstone of MI diagnosis.

Assay vendor note (board-testable): cTnT assays are patented and made by a single manufacturer (Roche). cTnI assays are made by multiple vendors, but the results are not interchangeable between platforms because different monoclonal antibodies target different epitopes. This is why you should not trend troponin across hospitals with different assay manufacturers.

Why Troponin Is Released in Myocardial Injury

About 3-5% of troponin in the myocyte exists free in the cytoplasm (the “cytosolic pool”). The remaining 95%+ is bound to the contractile apparatus (the “structural pool”). This distribution explains troponin kinetics:

Early release (rises 4-8 hours): Membrane damage allows the small cytosolic pool to leak out first. This is why troponin can be detected relatively early after MI onset.

Sustained elevation (remains elevated up to 7-14 days): As myocytes undergo necrosis, the structural pool is gradually released. The contractile apparatus must be degraded before this bound troponin can escape. This proteolytic degradation takes time, explaining why troponin remains elevated for up to two weeks after a large MI.

Peak levels (peaks at 12-24 hours): The peak represents the maximum rate of release from dying myocytes, which occurs during active infarct evolution.

Why this makes troponin bad for reinfarction detection: The two-week tail is the downside. If a patient infarcts again on day 3, the troponin from event 1 is still riding high. Historically this is where CK-MB earned its keep (see 16.3). With high-sensitivity assays, reinfarction can now be called on a >20% rise from a declining baseline, which salvages troponin for this use case.

Serial measurement protocols: With high-sensitivity troponin, the standard rule-in/rule-out strategy uses serial draws at 0 and 3 hours (or 0/1-hour algorithms on the faster platforms). A dynamic rise or fall is what distinguishes acute injury from chronic elevation.

High-Sensitivity Troponin Assays

Traditional troponin assays could detect troponin only when levels were significantly elevated - they had detection limits around 0.01-0.04 ng/mL. High-sensitivity assays (hs-cTnI, hs-cTnT) detect concentrations 10-100 times lower than conventional assays, but their kinetics (when they rise, peak, and fall) are the same as regular troponin. What changes is the floor, not the shape of the curve.

Why this matters clinically:

The 99th percentile upper reference limit (URL) of a healthy reference population is the threshold that defines “elevated” troponin - and the cutoff that defines the upper limit of normal. High-sensitivity assays can actually measure levels at this threshold with acceptable precision, whereas older assays often couldn’t detect anything below several times the 99th percentile.

What makes an assay “high-sensitivity” (formal definition): two criteria. (1) The assay must measure the 99th percentile URL with a coefficient of variation (CV) <10%. (2) The assay must detect troponin in >50% of a healthy reference population. If either criterion fails, the assay is not high-sensitivity, just “sensitive.”

Sex-specific cutoffs: Healthy men have higher baseline troponin than healthy women, so modern high-sensitivity assays now use sex-specific 99th percentile cutoffs. Using a single pooled cutoff under-diagnoses MI in women.

The trade-off: With greater analytical sensitivity comes reduced clinical specificity. High-sensitivity assays detect troponin release from any cause of myocardial injury, not just acute coronary syndrome. More patients test “positive” because of chronic HF, CKD, or sepsis rather than plaque rupture. This has forced clinicians to think more carefully about why troponin is elevated rather than just whether it is elevated, and it is why the Universal Definition of MI (16.5) leans heavily on the rise-and-fall pattern plus clinical context.

Troponin Elevation Without Acute Coronary Syndrome

The rule: troponin detects myocardial injury, not MI specifically. A single elevated troponin does not equal MI. You need a rise-and/or-fall pattern plus clinical criteria. Understanding non-ACS causes of troponin elevation is essential for interpretation:

Type 2 MI (supply-demand mismatch): Conditions that increase oxygen demand (tachycardia, sepsis, anemia) or decrease supply (hypotension, respiratory failure) can cause myocardial injury without coronary plaque rupture. The troponin elevation is real - myocytes are dying - but the treatment is addressing the underlying cause, not revascularization. See 16.5 for the full Type 1 vs Type 2 breakdown.

Chronic elevation in structural heart disease: Heart failure patients often have chronically elevated troponin because the stressed, remodeled myocardium has ongoing low-level myocyte death. These patients need serial measurements to detect change from baseline.

Renal failure: Both cTnI and cTnT can be elevated in ESRD patients. For cTnT, this may partly reflect cross-reactivity with re-expressed skeletal TnT isoforms in uremic myopathy. For clinical purposes, higher thresholds or serial changes are more meaningful in dialysis patients.

Other important causes of troponin elevation (non-ischemic): myocarditis (can cause massive elevation), heart failure, pulmonary embolism (right heart strain), sepsis, cardiac contusion, takotsubo cardiomyopathy, ablation procedures, cardioversion, renal failure (impaired clearance), and even extreme exercise in trained athletes. For boards: if you see elevated troponin with normal coronaries, think myocarditis (see 16.6).

16.2 Natriuretic Peptides: Markers of Myocardial Wall Stress

ANP vs BNP: Where They Come From

The natriuretic peptide family has two cardiac members that matter for boards:

ANP (atrial natriuretic peptide): made by atrial myocytes
BNP (B-type natriuretic peptide): made by ventricular myocytes

The naming is a historical accident worth flagging: BNP stands for Brain natriuretic peptide because it was first isolated from porcine brain tissue. It is not made in the brain in any clinically meaningful amount - the dominant source is the ventricle. You will see both “brain” and “B-type” in the literature. They refer to the same peptide.

Both ANP and BNP are released in response to myocardial wall stretch from volume or pressure overload. Their physiologic actions overlap: vasodilation, natriuresis, diuresis, suppression of RAAS, and suppression of sympathetic tone. All of it aimed at unloading the heart.

The Biology of BNP

BNP is synthesized primarily by ventricular myocytes in response to wall stretch. When the ventricle is volume or pressure overloaded, the mechanical stress triggers transcription of the BNP gene.

Synthesis pathway: The myocyte first produces proBNP (108 amino acids), which is cleaved by the enzyme furin into two fragments (note: corin is the analogous enzyme that cleaves proANP, not proBNP):

BNP (32 amino acids): The biologically active hormone
NT-proBNP (76 amino acids): An inactive fragment

Both are released into circulation and both can be measured clinically.

Physiologic effects of BNP: BNP is the body’s defense against volume overload. It causes:

Vasodilation (reducing afterload)
Natriuresis and diuresis (reducing preload)
Inhibition of the renin-angiotensin-aldosterone system
Inhibition of sympathetic nervous system

In heart failure, BNP levels are elevated because the heart is trying to compensate for the overloaded state - but the compensation is inadequate to overcome the disease.

BNP vs. NT-proBNP: Clinically Important Differences

Half-life (memorize these three):

ANP half-life: ~2 minutes
BNP half-life: ~20 minutes
NT-proBNP half-life: ~120 minutes (2 hours)

The 2-minute half-life of ANP is why ANP is useless as a clinical lab test - it is gone before you can reliably measure it. BNP and NT-proBNP are the ones you actually order. NT-proBNP is the most stable analyte and accumulates to the highest levels, which is part of why it is popular for lab measurement.

Renal clearance: NT-proBNP is cleared primarily by the kidneys. In renal failure, NT-proBNP levels rise disproportionately. BNP is cleared by multiple mechanisms (neprilysin / neutral endopeptidases, natriuretic peptide receptor C, some renal clearance), so it is less affected by renal function.

Drug effects: Sacubitril (in sacubitril/valsartan, Entresto) inhibits neprilysin, the enzyme that degrades BNP. Patients on this drug will have elevated BNP levels that don’t reflect their clinical status. NT-proBNP is not cleaved by neprilysin and is not affected - use NT-proBNP to monitor HF in patients on sacubitril.

Clinical Use: Ruling Out Heart Failure

The greatest utility of natriuretic peptides is their negative predictive value. In a patient presenting with dyspnea:

BNP <100 pg/mL: Heart failure is very unlikely as the cause of symptoms. The negative predictive value exceeds 95%.

NT-proBNP: Age-stratified cutoffs are used because NT-proBNP rises with age even in healthy individuals:

Age <50: <450 pg/mL rules out HF
Age 50-75: <900 pg/mL rules out HF
Age >75: <1800 pg/mL rules out HF

Important caveat: Obese patients may have lower natriuretic peptide levels than expected for their degree of heart failure. Adipose tissue expresses natriuretic peptide clearance receptors, pulling BNP out of circulation.

16.3 Creatine Kinase and Its Isoenzymes

CK Isoenzymes and Electrophoresis

Creatine kinase is a dimer built from two subunits, M (muscle) and B (brain), which pair to form three isoenzymes. On agarose gel electrophoresis, migration order from fastest (most anodal) to slowest (most cathodal) is:

CK-BB: fastest migrating, tissue source is brain and smooth muscle
CK-MB: intermediate, tissue source is cardiac muscle (~25-46% of cardiac CK)
CK-MM: slowest, tissue source is skeletal muscle

A useful way to remember it: alphabetical order on the gel, with BB closest to the anode.

Normal serum composition (worth memorizing cold):

>99% CK-MM
<1% CK-MB
0% CK-BB

CK-MM dominates because total skeletal muscle mass vastly exceeds cardiac muscle mass, so even a tiny percentage of skeletal CK swamps the cardiac contribution.

The baseline CK-MB trap: this is a classic board question. At baseline, the small amount of CK-MB in serum is derived from skeletal muscle, not heart. Skeletal muscle contains roughly 1-3% CK-MB by fraction, but because skeletal muscle mass is enormous, it contributes most of the circulating CK-MB in a healthy person. This is exactly why the CK index (below) matters - an absolute CK-MB elevation with a low index points to skeletal muscle, not cardiac injury.

The CK Index

CK index = (CK-MB / total CK) x 100. The ratio is used to figure out whether an elevation in CK-MB is cardiac or skeletal in origin.

CK index >5%: suggests a cardiac source
CK index <5% with elevated total CK: suggests skeletal muscle

Worked example: a marathon runner may have total CK of 10,000 U/L with CK-MB of 200 - that is an index of 2%, which is skeletal. An MI patient may have total CK of 1,000 with CK-MB of 80 - index of 8%, which is cardiac. The ratio corrects for the fact that massive skeletal muscle injury can drive absolute CK-MB values up, even though the cardiac fraction is not elevated.

CK index that is too high: if the index is >25-30%, stop and think. That ratio is not physiologically plausible for a cardiac event - CK-MB cannot make up that much of total CK by pure cardiac release. A very high CK index, especially in an elderly woman, should prompt a workup for macro CK (see below), not a call to cardiology.

Total CK is Nonspecific

Total CK alone tells you nothing about source. Elevated total CK has a broad differential:

Skeletal muscle: rhabdomyolysis, trauma, strenuous exercise, muscular dystrophy, statin myopathy, seizures, IM injections
Cardiac: MI, myocarditis, cardiac surgery, cardioversion
Brain: stroke (though CK-BB rarely reaches serum in meaningful quantity)
Other: hypothyroidism (myopathy), malignant hyperthermia, malignancy

Bottom line: total CK alone is useless for diagnosing MI. You have to fractionate into isoenzymes, or skip CK entirely and use troponin.

Macro CK: Two Types, Two Very Different Meanings

Macro CK is a high-molecular-weight CK species that interferes with CK-MB immunoassays and produces weird electrophoresis patterns. There are two types, and the board-testable distinction is that one is benign and one is a bad-news finding.

Macro CK type 1: a CK-immunoglobulin complex (usually CK-BB bound to IgG or IgA). On electrophoresis it migrates between CK-MM and CK-MB. It is classically seen in healthy elderly women and is benign and clinically insignificant. It can cause a falsely elevated total CK and CK-MB on immunoassays - which, as noted above, is why you see absurdly high CK indices in these patients.

Macro CK type 2: mitochondrial CK released from severely damaged cells. It migrates cathodal to CK-MM (slower than MM). It is seen in advanced malignancy, severe liver disease, and critically ill patients, and it carries a poor prognosis. Classic board setup: unexplained persistent CK elevation plus known malignancy - think macro CK type 2.

Both types can cause falsely elevated CK-MB on immunoassays. Electrophoresis is how you sort them out.

CK-MB for MI: Kinetics and Remaining Niche

CK-MB from the heart rises during or after myocardial injury, and the historical kinetics (worth memorizing alongside troponin and myoglobin) are:

Rises within 4 hours
Peaks at 12-24 hours
Normalizes by 48 hours

Why it used to matter: the 48-hour return to baseline was CK-MB’s killer app for detecting reinfarction. Troponin stays elevated for 7-14 days, so historically you could not use it to tell whether a patient had a new MI on day 3 post-initial event - the troponin from event 1 was still high. CK-MB would have normalized by then, so a new rise in CK-MB unmasked the second event.

Why it matters less now: high-sensitivity troponin assays can detect reinfarction by demonstrating a >20% rise from a declining baseline. CK-MB is largely redundant in hospitals with modern troponin platforms, though it still shows up on board questions.

16.4 Myoglobin: Fast but Nonspecific

Myoglobin is the earliest-rising cardiac marker, but it is so nonspecific that it has essentially been abandoned in favor of high-sensitivity troponin.

Biology: myoglobin is a small (17 kDa) heme protein found in both cardiac and skeletal muscle. Its small size is why it leaks out of damaged myocytes so quickly - it slips through membrane defects that would still retain the larger troponin and CK-MB molecules.

Kinetics (fastest of the three classical markers):

Rises within 1-2 hours of MI
Peaks at 6-8 hours
Normalizes by 24 hours

Why it was used: its speed gave it a niche for ruling out MI early - high sensitivity, very low specificity. A negative myoglobin shortly after chest pain onset was reassuring.

Why it was abandoned: rhabdomyolysis, trauma, IM injections, renal failure, and even strenuous exercise all elevate myoglobin. And high-sensitivity troponin now rises nearly as fast with far better specificity.

Haptoglobin aside (board trivia worth knowing): myoglobin does not bind haptoglobin the way free hemoglobin does. So in rhabdomyolysis, haptoglobin is normal. In intravascular hemolysis, haptoglobin is consumed and low. If you see red-brown urine, a low haptoglobin points toward hemolysis and a normal haptoglobin points toward rhabdomyolysis. This is the classic lab distinction.

16.5 Acute Coronary Syndrome and the Universal Definition of MI

The ACS Spectrum

Acute coronary syndrome is not a single diagnosis - it is a continuum of unstable angina, NSTEMI, and STEMI. The underlying pathophysiology is the same for all three: coronary plaque rupture or erosion with thrombus formation. What changes is the degree of coronary occlusion and whether myocytes have died.

Entity	Troponin	ECG	Pathophys
Unstable angina	Normal	ST depression or no changes	Partial occlusion, no necrosis
NSTEMI	Elevated	ST depression or T-wave inversions	Partial occlusion with necrosis
STEMI	Elevated	ST elevation or new LBBB	Complete occlusion

With high-sensitivity troponin, the line between unstable angina and NSTEMI has shifted - what used to be called UA is now often reclassified as NSTEMI because the more sensitive assay picks up micro-infarction.

hs-CRP and Cardiovascular Risk

C-reactive protein is an independent predictor of acute coronary syndrome in otherwise healthy individuals. The version used for risk stratification is high-sensitivity CRP (hs-CRP), which reflects the inflammatory component of atherosclerosis.

Risk categories:

hs-CRP <1 mg/L: low cardiovascular risk
hs-CRP 1-3 mg/L: moderate risk
hs-CRP >3 mg/L: high risk
hs-CRP >10 mg/L: likely acute infection or other acute inflammation, not useful for cardiac risk stratification

CRP is a positive acute-phase reactant made by the liver in response to IL-6. The JUPITER trial showed that statin therapy reduced events in patients with elevated hs-CRP even when LDL was normal, which cemented hs-CRP’s role in risk assessment.

The 4th Universal Definition of MI

The 4th Universal Definition of Myocardial Infarction is the framework you have to know cold for boards. It prevents overcalling MI in patients with chronically elevated troponin.

A diagnosis of acute MI requires troponin above the 99th percentile URL with a rise and/or fall pattern, PLUS at least one of the following:

Symptoms of acute myocardial ischemia
New ischemic ECG changes (ST-T depression or elevation, or new LBBB)
Development of pathologic Q waves
Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality
Identification of an intracoronary thrombus by angiography or autopsy

The critical concept: a single elevated troponin is not MI. Patients with CKD, chronic HF, or structural heart disease have stable baseline elevations. Without a dynamic change and clinical criteria, you cannot call it an acute event.

Type 1 vs Type 2 MI

The Universal Definition also classifies MI into types. The two you have to know:

Type 1 MI: caused by coronary plaque rupture or erosion with intracoronary thrombus. Treatment is revascularization (PCI or CABG) plus antiplatelet and antithrombotic therapy.
Type 2 MI: caused by oxygen supply-demand mismatch without plaque rupture. Examples: tachycardia, hypotension or shock, severe anemia, respiratory failure, severe hypertension, aortic stenosis, HOCM, coronary spasm, coronary dissection. Treatment is fixing the underlying cause, not PCI.

Type 2 MI is far more common in hospitalized patients than Type 1. Cardiology is often consulted reflexively for any troponin bump, but many of these are Type 2 and do not get cath.

The other types (mostly procedural, useful for completeness):

Type 3: sudden cardiac death with presumed ischemic features, before biomarkers could be obtained
Type 4a: PCI-related MI
Type 4b: stent thrombosis
Type 5: CABG-related MI

16.6 Myocarditis and Non-Ischemic Causes of Troponin Elevation

When you see elevated troponin with normal coronary angiography, the leading diagnosis is myocarditis. Biopsy shows inflammatory infiltrate and myocyte necrosis. Labs: elevated troponin, often elevated CK, sometimes elevated inflammatory markers.

Causes of Myocarditis

Viral (most common overall): Coxsackie B, adenovirus, parvovirus B19, HHV-6, influenza, SARS-CoV-2. Boards rarely test the exact virus - what they test is that myocarditis is usually viral and that the presentation is elevated troponin with normal coronaries.

Non-viral causes (higher yield for boards because they have specific associations):

Trypanosoma cruzi (Chagas disease): parasite, leading cause of cardiomyopathy in Central and South America. Chronic chagasic cardiomyopathy features dilated cardiomyopathy with apical aneurysms and conduction disease.
Borrelia burgdorferi (Lyme disease): spirochete, classically causes heart block in the acute disseminated phase.
Rheumatic heart disease: post-streptococcal, pancarditis with Aschoff bodies (granulomas with Anitschkow cells) on histology.
Autoimmune: sarcoidosis (noncaseating granulomas), SLE, giant cell myocarditis (see below), eosinophilic myocarditis.
Medications: doxorubicin (dose-dependent dilated cardiomyopathy), immune checkpoint inhibitors (can cause fulminant myocarditis), cocaine, clozapine.

Giant Cell Myocarditis

Giant cell myocarditis is a rare, rapidly progressive, and often fatal autoimmune myocarditis. It is worth flagging on its own because it has specific associations and a different histology.

Associations to memorize: consider giant cell myocarditis in patients with systemic autoimmune disease (IBD, thyroiditis, myasthenia gravis) or thymoma.

Histology: multinucleated giant cells with extensive myocyte necrosis and an eosinophilic infiltrate. No granulomas - this is what distinguishes it from cardiac sarcoidosis, which also shows giant cells but within well-formed noncaseating granulomas.

Course: fulminant, high mortality if untreated. Treatment is aggressive immunosuppression (often cyclosporine plus steroids). Many patients require mechanical circulatory support or heart transplant.

Other Non-Ischemic Causes of Troponin Elevation (Review)

Pulling together what was scattered earlier in the chapter, the full non-ACS troponin differential:

Myocarditis (any cause)
Heart failure (chronic or decompensated)
Pulmonary embolism (right heart strain)
Sepsis
Cardiac contusion (blunt chest trauma)
Takotsubo cardiomyopathy
Ablation procedures, cardioversion, DC shock
Renal failure (impaired clearance plus possible re-expression of skeletal TnT)
Extreme exercise in trained athletes
Aortic dissection, severe hypertensive crisis

For boards, the pattern to recognize: elevated troponin + normal coronaries = myocarditis until proven otherwise.

Chapter 17: Lipids and Cardiovascular Risk

Lipids are hydrophobic molecules that must be transported through the aqueous bloodstream within specialized particles called lipoproteins. Understanding lipoprotein metabolism is essential because derangements in this system cause atherosclerosis - the leading cause of death worldwide.

The relevant lipids are cholesterol, triglycerides, and phospholipids. None of these dissolve in plasma on their own. The body’s solution is to bundle them into particles with a phospholipid monolayer on the outside (hydrophilic heads facing water) and a hydrophobic core. The protein components - apolipoproteins - do most of the functional work: targeting particles to specific receptors, activating enzymes, and holding the whole thing together.

17.1 Lipoprotein Structure and Function

The Architecture of a Lipoprotein

Every lipoprotein has the same basic structure: a hydrophobic core surrounded by an amphipathic shell.

The core contains the truly hydrophobic lipids - triglycerides and cholesteryl esters. These molecules cannot interact with water at all and must be hidden inside the particle.

The shell consists of phospholipids (with their hydrophilic heads facing outward), unesterified (free) cholesterol, and apolipoproteins. The shell makes the particle water-soluble while keeping the hydrophobic cargo sequestered inside.

Apolipoproteins are the functional components that determine what each lipoprotein does. They serve as ligands for receptors, activators for enzymes, and structural scaffolds for the particle.

The Lipoprotein Classes

There are 5 classes, and the name reflects density. Going from chylomicrons to HDL, density increases, size decreases, and the protein:lipid ratio increases. Chylomicrons are roughly 98% lipid / 2% protein. HDL is roughly 50/50. The reason density increases with protein content is mechanical: lipids (especially triglycerides) are lighter than water, proteins are heavier. A particle stuffed with triglyceride floats. A particle that’s half protein sinks.

The order: Chylomicrons > VLDL > IDL > LDL > HDL (largest/lightest to smallest/densest).

Chylomicrons: The dietary lipid transport system

Made by enterocytes from absorbed dietary fat
Largest lipoproteins (80-1000 nm); make serum appear turbid when elevated
Primary apolipoprotein: ApoB-48 (a truncated form of ApoB made only in the intestine; the “48” refers to the fact that it’s 48% of the full-length ApoB-100 gene product, produced via intestinal mRNA editing)
Function: Deliver dietary triglycerides to peripheral tissues via lipoprotein lipase
Chylomicron remnants (TG-depleted) are cleared by the liver via ApoE-mediated uptake through LRP (LDL receptor-related protein)

VLDL (Very Low-Density Lipoprotein): The hepatic triglyceride export system

Made by hepatocytes to export endogenously synthesized triglycerides
Large particles (30-80 nm)
Primary apolipoprotein: ApoB-100 (full-length ApoB)
Function: Deliver hepatic triglycerides to peripheral tissues
Metabolized to IDL, then LDL as triglycerides are stripped off by lipoprotein lipase
Overproduction is the central defect in insulin resistance and metabolic syndrome

IDL (Intermediate-Density Lipoprotein): The intermediate

Transient particle between VLDL and LDL
Normally short-lived - either further depleted to LDL or taken up by the liver via ApoE
Accumulates pathologically in type III hyperlipidemia (ApoE2/E2, see 17.6)
Electrophoretic mobility overlaps pre-beta and beta bands, producing the characteristic broad beta band in type III

LDL (Low-Density Lipoprotein): The cholesterol delivery system

The end-product of VLDL metabolism after triglycerides have been removed
Medium-sized particles (20-25 nm)
Primary apolipoprotein: ApoB-100 - exactly one per particle. This is crucial for ApoB-based risk assessment.
Rich in cholesterol because triglycerides have been depleted
Function: Deliver cholesterol to peripheral tissues via the LDL receptor
The primary atherogenic lipoprotein
Small dense LDL is more atherogenic than large buoyant LDL - it penetrates the arterial wall more easily and oxidizes faster. Small dense LDL is associated with metabolic syndrome and diabetes.

HDL (High-Density Lipoprotein): The reverse cholesterol transport system

Made by liver and intestine; also formed from surface remnants of other lipoproteins
Smallest and densest particles (5-12 nm)
Primary apolipoprotein: ApoA-I
Also rich in lecithin-cholesterol acyltransferase (LCAT), the enzyme that esterifies free cholesterol so it can be packed into the HDL core
Function: Remove cholesterol from peripheral tissues (via the ABCA1 transporter) and deliver it to the liver for biliary excretion
Considered atheroprotective

Apolipoproteins Worth Knowing Cold

A handful of apolipoproteins come up repeatedly in both basic physiology questions and in the familial dyslipidemias:

ApoB-48: chylomicrons only (intestinal)
ApoB-100: VLDL, IDL, LDL, Lp(a). Ligand for the LDL receptor.
ApoA-I: HDL. Structural. Activates LCAT.
ApoC-II: cofactor that activates lipoprotein lipase (LPL) on chylomicrons and VLDL. Deficiency phenocopies LPL deficiency.
ApoE: ligand for hepatic uptake of chylomicron remnants and IDL. Three isoforms: E2, E3, E4.
- E2 binds poorly to hepatic receptors. Homozygous E2/E2 causes familial dysbetalipoproteinemia (type III).
- E3 is the most common “normal” allele.
- E4 is associated with higher LDL and with Alzheimer’s disease risk.

The atherogenic lipoproteins are the ones containing ApoB: VLDL, IDL, LDL (all ApoB-100), plus chylomicrons (ApoB-48) and Lp(a). HDL does not contain ApoB. This is why total ApoB is a useful summary measurement of atherogenic particle number - one ApoB per particle, regardless of class.

Lipoprotein Electrophoresis

Lipoproteins can be separated electrophoretically, and the mobility pattern is occasionally board-testable:

Chylomicrons: remain at the origin (too large to migrate)
VLDL: pre-beta band
IDL: between pre-beta and beta
LDL: beta band (ß1-ß2 interface) (this is why LDL cholesterol is sometimes called “beta-lipoprotein cholesterol”)
HDL: alpha band (albumin-α interface), fastest migration

Lipoprotein electrophoresis is a qualitative technique. It identifies patterns but doesn’t give concentrations. The classic finding it’s used for is the broad beta band in type III hyperlipidemia, where IDL and chylomicron remnants merge the pre-beta and beta regions into a single wide band.

The quantitative reference method is ultracentrifugation, which separates lipoproteins by density using a salt gradient. It’s slow (~18 hours), expensive, and mostly used for research or for definitive diagnosis of rare hyperlipoproteinemias. Routine clinical practice uses the calculated lipid panel.

Immunoassays (immunoturbidimetric or immunonephelometric) are available for specific apolipoproteins: ApoB, ApoA-I, and Lp(a). These are the basis of advanced lipid testing.

Measured vs Calculated, and the Refrigerator Test

The routine lipid panel directly measures three things: total cholesterol, HDL-C, and triglycerides. LDL-C and VLDL-C are calculated using the Friedewald equation (detailed in 17.2). This is why fasting has traditionally been required - non-fasting chylomicrons break the equation’s assumptions.

A quick bedside observation before any of this machinery: lipemia. Only chylomicrons and VLDL cause visible plasma turbidity (they’re large enough to scatter light). LDL and HDL particles are too small to alter plasma appearance. So lipemic plasma means hypertriglyceridemia, not hypercholesterolemia.

The refrigerator test: leave a lipemic sample at 4°C overnight.

Creamy top layer + clear infranatant = chylomicrons (they float; TG-rich, huge particles)
Diffuse turbidity with no discrete layer = VLDL (stays dispersed)
Both = combined chylomicronemia + VLDL (type V)

Lipemia interferes with spectrophotometric and nephelometric assays, which is why hypertriglyceridemia causes spurious results on many chemistry analytes (sodium, bilirubin, etc.).

17.2 LDL: The Central Player in Atherosclerosis

The VLDL-to-LDL Cascade

LDL doesn’t appear out of nowhere. It’s the downstream product of VLDL metabolism. The liver releases triglyceride-rich VLDL. As VLDL circulates, lipoprotein lipase (anchored on capillary endothelium of adipose and muscle, activated by ApoC-II) strips off triglycerides. The particle gets smaller and denser. It passes through IDL (intermediate density) and finally becomes LDL: TG-depleted, cholesterol-enriched, with ApoB-100 still embedded in the shell.

LDL then has two possible fates: uptake by peripheral cells or clearance by the liver. Both happen through the LDL receptor (LDLR), which binds ApoB-100 on the LDL surface. Receptor-bound LDL is internalized by endocytosis. Intracellular cholesterol then feeds back on two pathways: it suppresses HMG-CoA reductase (reducing de novo cholesterol synthesis) and downregulates further LDLR expression. This negative feedback is what statins exploit.

The liver is the primary site of LDL clearance. Hepatic LDLRs binding ApoB-100 is how most circulating LDL leaves the bloodstream.

Why LDL Causes Atherosclerosis

LDL particles are small enough to penetrate the arterial endothelium and enter the intima. Once there, they become trapped in the extracellular matrix. Trapped LDL undergoes oxidation and other modifications that make it immunogenic. Modified LDL is engulfed by macrophages, which become lipid-laden foam cells - the hallmark of early atherosclerosis.

The key insight is that LDL particle number matters more than LDL cholesterol content. Each LDL particle has exactly one ApoB-100 molecule, so ApoB concentration directly reflects particle number. Two people with identical LDL-C may have very different cardiovascular risk if one has many small, cholesterol-depleted particles (high particle number) and the other has fewer large, cholesterol-rich particles (low particle number).

LDL Receptor Pathway and Therapeutic Targets

Everything that lowers LDL pharmacologically works by increasing LDL receptor activity or reducing ApoB production:

Statins inhibit HMG-CoA reductase. Less intracellular cholesterol -> upregulated LDLR -> more hepatic LDL clearance -> lower serum LDL-C. Statins are the backbone of LDL management.
PCSK9 inhibitors (evolocumab, alirocumab): PCSK9 is a protein that targets the LDL receptor for lysosomal degradation after internalization. Inhibiting PCSK9 preserves the LDLR, allowing it to recycle to the cell surface and clear more LDL. Very effective; used in familial hypercholesterolemia and in patients who don’t reach goal on maximum statin.
Bile acid sequestrants: deplete the hepatic bile acid pool, forcing the liver to synthesize more bile acids from cholesterol, reducing intracellular cholesterol and upregulating LDLR.
Ezetimibe: blocks NPC1L1-mediated cholesterol absorption in the intestine.

Familial Hypercholesterolemia

Mutations that break this pathway cause familial hypercholesterolemia (FH), a relatively common (heterozygous ~1:250) autosomal dominant dyslipidemia. Three gene defects account for nearly all cases:

LDLR mutations (most common)
ApoB-100 mutations (LDL can’t bind its own receptor)
PCSK9 gain-of-function mutations (excess LDLR degradation)

Clinical picture: markedly elevated LDL-C with normal triglycerides. Heterozygous FH runs LDL 190-400 mg/dL with MI risk by age 50-60. Homozygous FH (roughly 1:1,000,000) runs LDL >500 mg/dL with MI in childhood or adolescence. Physical findings include tendinous xanthomas, xanthelasma, and corneal arcus. This is Fredrickson type II (IIa = pure hypercholesterolemia; IIb = elevated LDL plus elevated TG).

Measuring LDL Cholesterol

Direct measurement: Homogeneous assays that selectively solubilize LDL and measure its cholesterol content. Used when triglycerides are elevated.

Calculated (Friedewald equation):

LDL-C = Total Cholesterol - HDL-C - (Triglycerides ÷ 5)

The “divide by 5” estimates VLDL cholesterol, assuming VLDL is 20% cholesterol by mass (so TG/5 = VLDL-C).

Worked example: TC 250, HDL 50, TG 200. LDL = 250 - 50 - (200/5) = 250 - 50 - 40 = 160 mg/dL. That’s the “high” LDL category. Board questions are often this mechanical.

When Friedewald fails:

Triglycerides >400 mg/dL: The TG/VLDL-C ratio is no longer 5:1; VLDL becomes enriched with triglycerides
Non-fasting specimens: Chylomicrons are present, and the equation doesn’t account for them
Type III hyperlipoproteinemia: Abnormal VLDL remnants with different composition

In these cases, direct LDL measurement (homogeneous assay or ultracentrifugation) is needed.

The Martin-Hopkins equation is a newer calculation that uses adjustable factors based on triglyceride and non-HDL-C levels, providing more accurate LDL-C estimates across a wider range of triglyceride levels, including very low LDL and high TG.

17.3 HDL: More Complex Than “Good Cholesterol”

The Paradox of HDL

For decades, HDL-C was considered protective based on strong epidemiological associations between high HDL-C and reduced cardiovascular events. However, drugs that raise HDL-C (niacin, CETP inhibitors) have failed to reduce cardiovascular events in clinical trials.

The current understanding is that HDL function matters more than HDL quantity. HDL performs reverse cholesterol transport, but a high HDL-C level doesn’t guarantee that these particles are functional. Dysfunctional HDL (seen in inflammation, diabetes, and chronic kidney disease) may even be pro-atherogenic.

Modifiable Causes of Low HDL

Lifestyle and common conditions drop HDL:

Smoking (most potent modifiable factor, lowers HDL 5-10%)
Obesity
Sedentary lifestyle
Anabolic steroids
Hypertriglyceridemia (CETP swaps TG into HDL; the TG-enriched HDL gets hydrolyzed and cleared faster)
Type 2 diabetes and metabolic syndrome
Non-selective beta-blockers, progestins

The single best lifestyle intervention to raise HDL is aerobic exercise (5-10% bump is realistic). Moderate alcohol also raises HDL, though this is not a recommendation.

Tangier Disease: Absent HDL

Tangier disease is an autosomal recessive disorder caused by ABCA1 transporter deficiency. ABCA1 pumps cholesterol and phospholipid out of cells onto lipid-poor apoA-I to form nascent HDL. Without it, cholesterol accumulates inside macrophages and nascent HDL can’t mature.

Lab picture:

Total cholesterol: low
Triglycerides: normal
HDL: absent or near-absent
ApoA-I: absent or near-absent

The pathognomonic physical finding is orange-yellow enlarged tonsils, packed with cholesterol-laden macrophages (foam cells). Cholesterol also deposits in lymph nodes, spleen, liver, and peripheral nerves (producing peripheral neuropathy) and corneas. Tangier is extremely rare (fewer than 100 cases worldwide).

The surprise finding: despite essentially zero HDL, Tangier patients have only moderately increased cardiovascular risk, not the catastrophic risk you’d predict if HDL alone drove atheroprotection. This is one of the strongest clinical arguments that the “raise HDL, reduce events” model is incomplete.

17.4 Advanced Lipid Testing

Apolipoprotein B (ApoB)

Since every LDL, VLDL, IDL, and Lp(a) particle contains exactly one ApoB molecule, the ApoB concentration equals the total number of atherogenic particles. This makes ApoB a better predictor of cardiovascular risk than LDL-C, especially in patients with discordance between LDL-C and particle number (common in metabolic syndrome, diabetes, and hypertriglyceridemia).

Clinical use: When LDL-C is at goal but ApoB remains elevated, the patient has many small, dense LDL particles and may benefit from more aggressive therapy.

Lipoprotein(a) [Lp(a)]

Lp(a) is an LDL particle with an additional protein called apolipoprotein(a) attached to its ApoB. This apolipoprotein(a) is structurally similar to plasminogen, which may explain Lp(a)’s prothrombotic effects.

Key features:

Levels are >90% genetically determined
Not significantly affected by diet, exercise, or statins
Elevated Lp(a) is an independent cardiovascular risk factor
Associated with aortic stenosis
Can cause falsely elevated LDL-C measurements (Lp(a) cholesterol is included in LDL-C)

Who to test: Family history of premature cardiovascular disease, personal history despite controlled LDL-C, to refine risk assessment in intermediate-risk patients.

17.5 Lipid Panel Interpretation

Non-HDL Cholesterol

Non-HDL-C = Total Cholesterol - HDL-C

This simple calculation captures all atherogenic lipoproteins (LDL + VLDL + IDL + Lp(a)). It’s particularly useful when triglycerides are elevated because it doesn’t require the assumptions of the Friedewald equation. Many guidelines now use non-HDL-C as a secondary target after LDL-C.

Triglycerides: Marker vs. Cause

Elevated triglycerides are clearly associated with cardiovascular disease, but the causal relationship is debated. Very high triglycerides (>500 mg/dL) are more important for pancreatitis risk than cardiovascular risk, and >1000 mg/dL is a strong risk factor. Mechanism: pancreatic lipase hydrolyzes excess TGs in pancreatic capillaries, releasing toxic free fatty acids that damage acinar cells. Hypertriglyceridemic pancreatitis is the 3rd most common cause of acute pancreatitis (after gallstones and alcohol). Treatment: insulin infusion (activates LPL and rapidly lowers TGs), plasmapheresis for severe or refractory cases, and strict fat restriction.

Triglyceride-rich lipoproteins (VLDL, chylomicron remnants) also appear to contribute to atherosclerosis through mechanisms similar to LDL.

How Triglycerides Are Measured (and Why Glycerol Kinase Deficiency Fools the Assay)

The enzymatic triglyceride assay works in two broad steps:

Lipase hydrolyzes triglycerides -> glycerol + 3 fatty acids
Glycerol gets phosphorylated by glycerol kinase to glycerol-3-phosphate, then oxidized to dihydroxyacetone phosphate + H2O2. The H2O2 drives a Trinder reaction that produces a colored product, measured colorimetrically

The critical point: the assay measures glycerol, not triglycerides directly. Any source of free glycerol in the sample gets counted as triglyceride. This produces pseudohypertriglyceridemia.

Sources of free glycerol:

Glycerol kinase deficiency (X-linked recessive). Free glycerol accumulates because it can’t be phosphorylated. Classic board clue: markedly elevated TG that doesn’t respond to diet, exercise, or drugs AND non-lipemic plasma (no actual TG-rich lipoproteins). Confirm with a glycerol-blanked TG method that subtracts free glycerol.
Exogenous glycerol (IV nitroglycerin, some IV medications, detergents on glassware, certain alcoholic beverages)

17.6 Familial Dyslipidemias (Fredrickson Classification)

The Fredrickson classification groups the primary hyperlipoproteinemias by which lipoprotein is elevated on electrophoresis. It’s dated but still testable, and the pathophysiology maps cleanly onto the metabolism covered above.

Type	Elevated particle(s)	Defect	Lipid pattern	Key physical findings
I	Chylomicrons	LPL or ApoC-II deficiency	TG ↑↑↑, chol normal	Eruptive xanthomas, lipemia retinalis, pancreatitis
IIa	LDL	LDLR, ApoB-100, or PCSK9 (FH)	Chol ↑↑, TG normal	Tendinous xanthomas, xanthelasma, corneal arcus
IIb	LDL + VLDL	Familial combined hyperlipidemia	Chol ↑ and TG ↑	Premature atherosclerosis
III	IDL / remnants	ApoE2/E2 (dysbetalipoproteinemia)	Chol ↑ and TG ↑ (similar)	Palmar crease xanthomas, tuberoeruptive xanthomas, broad beta band
IV	VLDL	Familial hypertriglyceridemia (polygenic, secondary)	TG ↑, chol normal	Eruptive xanthomas if severe, pancreatitis
V	Chylomicrons + VLDL	Mixed (often type IV with stress)	TG ↑↑↑, chol ↑	Eruptive xanthomas, pancreatitis

Type I: Familial Chylomicronemia Syndrome (LPL or ApoC-II Deficiency)

Chylomicrons can’t be cleared because lipoprotein lipase is missing or its ApoC-II cofactor is missing. Chylomicrons accumulate to extraordinary levels.

Massive hypertriglyceridemia (TG often 1,000 - 10,000 mg/dL)
Presents in childhood with eruptive xanthomas, lipemia retinalis (creamy retinal vessels visible on fundoscopy), hepatosplenomegaly, and recurrent pancreatitis
Milky / creamy blood (literally - the serum looks like cream)
Fasting chylomicrons are present, which is abnormal (chylomicrons normally clear within 12 hours of fasting)
Treatment: very low-fat diet (<20 g/day), MCT oils (bypass chylomicron formation)

Type II: Familial Hypercholesterolemia

Already covered in 17.2. Three defects in the LDL clearance pathway: LDLR (most common), ApoB-100, PCSK9 gain-of-function. Pure hypercholesterolemia with tendinous xanthomas, xanthelasma, corneal arcus, and premature MI.

IIa = pure hypercholesterolemia (elevated LDL only)
IIb = elevated LDL + elevated VLDL/TG (overlaps with familial combined hyperlipidemia)

Type III: Familial Dysbetalipoproteinemia (ApoE2/E2)

Homozygous ApoE2/E2 binds the hepatic receptor poorly, so IDL and chylomicron remnants accumulate. Because both remnants (cholesterol-rich, from chylomicrons) and IDL (cholesterol + TG) build up, you see BOTH hypercholesterolemia AND hypertriglyceridemia - usually in roughly similar magnitude (both 300 - 600 mg/dL).

Key features:

Broad beta band on electrophoresis (pathognomonic; IDL merges pre-beta and beta into a single wide band)
Palmar crease xanthomas (xanthoma striatum palmare) - pathognomonic for type III
Tuberoeruptive xanthomas (elbows, knees)
Premature peripheral vascular disease
Requires a second hit to express clinically: obesity, diabetes, hypothyroidism, or menopause. ApoE2/E2 alone is common (~1% of population) but clinical type III is rare.
Very responsive to fibrates and weight loss

Familial Combined Hyperlipidemia (FCH)

The most common inherited hyperlipidemia (1 - 2% of the population). Polygenic; no single gene identified. Hepatic overproduction of ApoB-100 / VLDL is the core defect.

Variable phenotype even within the same patient over time: can look like type IIa one year, IIb the next, type IV the year after
Different family members can have different patterns (one with isolated high LDL, another with isolated high TG)
Strong association with metabolic syndrome and premature atherosclerosis
Diagnosis is clinical: mixed hyperlipidemia + family history of premature CAD + phenotypic variability across relatives

Type IV: Familial Hypertriglyceridemia

Elevated VLDL from hepatic overproduction. TG usually 200 - 500 mg/dL but can spike above 1000 during decompensation (uncontrolled diabetes, alcohol binge, steroids).

Strongly associated with insulin resistance, obesity, metabolic syndrome, type 2 diabetes
Complications: eruptive xanthomas, pancreatitis when TG crosses ~500 - 1000
Electrophoresis: increased pre-beta band (VLDL)
Treatment: lifestyle (weight loss, exercise, less refined carb and alcohol), fibrates, omega-3 fatty acids. Statins are less effective for isolated hypertriglyceridemia (statins lower cholesterol more than TG)

Secondary Causes of Mixed Hyperlipidemia

Always exclude secondary causes before committing to a primary diagnosis. The classic triad to memorize:

Diabetes mellitus (insulin resistance drives hepatic VLDL overproduction)
Hypothyroidism (decreased LDLR expression + decreased LPL activity; treat the thyroid before the lipids)
Nephrotic syndrome (the liver ramps up VLDL/LDL synthesis to compensate for urinary albumin loss)

Other common contributors: obesity, excessive alcohol, corticosteroids, thiazides, oral contraceptives and estrogens, protease inhibitors (HIV therapy), atypical antipsychotics.

17.7 Physical Findings in Dyslipidemia

The cutaneous and ocular findings in hyperlipidemia are board-favorite pattern recognition. The key trick: the type of lipid that’s elevated determines the type of xanthoma.

Finding	Pathology	Lipid elevated	Lipoprotein(s)	Classic disease
Eruptive xanthomas	TG-laden macrophages in skin	Triglycerides	Chylomicrons, VLDL	Type I, IV, V; uncontrolled DM
Tendinous xanthomas	Cholesterol in tendons (Achilles, patellar)	Cholesterol	LDL, IDL	Familial hypercholesterolemia (type II)
Xanthelasma	Cholesterol on eyelids	Cholesterol (often)	LDL	FH; ~50% have normal lipids
Palmar crease xanthoma	Lipid in palmar creases	Cholesterol + TG	IDL / remnants	Type III (dysbetalipoproteinemia) - pathognomonic
Tuberoeruptive xanthomas	Mixed lipid on elbows/knees	Cholesterol + TG	Remnants	Type III
Corneal arcus	Cholesterol in corneal periphery	Cholesterol	LDL	Aging (benign); if <45 yrs, suggests FH
Lipemia retinalis	Creamy retinal vessels	Triglycerides	Chylomicrons	Type I; TG usually >2500 mg/dL

Eruptive xanthomas are small yellow papules that appear suddenly on extensor surfaces, buttocks, and trunk in severe hypertriglyceridemia (usually TG >1000 mg/dL). They represent TG-laden macrophages in the dermis. They resolve with TG lowering.

Tendinous xanthomas are firm, painless nodules in tendons (Achilles is most classic, plus patellar and hand extensors). They take years to develop - their presence tells you the patient has had chronic, severe LDL elevation. Essentially pathognomonic for familial hypercholesterolemia.

Xanthelasma are flat yellow cholesterol deposits on the eyelids (usually medial upper). About half of patients with xanthelasma have normal lipids, so the finding alone isn’t diagnostic. But combined with FH features, it supports the diagnosis.

Corneal arcus = a lipid ring around the corneal periphery. Common and benign in elderly patients. The board-relevant point: corneal arcus before age 45 (especially before 40) suggests familial hypercholesterolemia and warrants a lipid panel and family history.

17.8 Reference Ranges and Screening Cutoffs

Memorize these NCEP/ATP III categories - they come up directly.

Total cholesterol (mg/dL):

Desirable: <200
Borderline: 200 - 239
High: ≥240

LDL-C (mg/dL):

Optimal: <100
Near-optimal: 100 - 129
Borderline: 130 - 159
High: 160 - 189
Very high: ≥190 (triggers high-intensity statin regardless of other risk factors; suggestive of FH)

HDL-C (mg/dL):

Low: <40 (men), <50 (women)
High (protective): >60

Triglycerides (mg/dL):

Normal: <150
Borderline: 150 - 199
High: 200 - 499
Very high: ≥500 (pancreatitis risk)

Total cholesterol alone is a blunt tool - it lumps protective HDL in with atherogenic LDL. A patient with TC 250 could have LDL 130 + HDL 80 + VLDL 40 (moderate risk) or LDL 190 + HDL 30 + VLDL 30 (very high risk). Modern guidelines emphasize LDL-C (or non-HDL-C) plus a calculated 10-year ASCVD risk rather than treating off total cholesterol.

Chapter 18: Endocrine Function Testing

Endocrine disorders often present with nonspecific symptoms - fatigue, weight change, mood disturbance. Laboratory testing is essential for diagnosis, but interpreting endocrine tests requires understanding the feedback loops and physiological rhythms that govern hormone secretion. This chapter also folds in pregnancy chemistry, because the analytes that matter in pregnancy (hCG, AFP, amniotic bilirubin, fetal lung maturity markers) are fundamentally hormone / feto-maternal biochemistry problems and share the same interpretive logic as the rest of endocrinology.

18.1 Thyroid Function Testing

The Hypothalamic-Pituitary-Thyroid Axis

The thyroid axis operates through negative feedback:

The hypothalamus releases TRH (thyrotropin-releasing hormone)
TRH stimulates the anterior pituitary to release TSH (thyroid-stimulating hormone)
TSH stimulates the thyroid gland to produce T4 and T3
T4 and T3 feed back to suppress TRH and TSH release

This feedback is the key to interpreting thyroid tests. When the thyroid gland fails (primary hypothyroidism), T4 falls and the pituitary responds by dramatically increasing TSH - trying to whip a failing gland into action. When the pituitary fails (secondary hypothyroidism), TSH is low or inappropriately normal despite low T4.

TRH itself is not monogamous to the thyroid axis. It also stimulates release of prolactin and, to a lesser extent, growth hormone from the anterior pituitary. The prolactin effect is strong enough that one endocrinologist quipped TRH “might as well be called prolactin-releasing hormone.” This is why primary hypothyroidism (high TRH from low feedback) can cause hyperprolactinemia, and why any workup of elevated prolactin should include a TSH.

Why TSH Is the Best Screening Test

TSH has an inverse log-linear relationship with free T4. This means that small changes in thyroid hormone produce large changes in TSH. If free T4 drops by 50%, TSH might increase 100-fold. This amplification makes TSH exquisitely sensitive to early thyroid dysfunction.

Standard testing sequence: TSH first, then free T4 if TSH is abnormal, then T3 in select cases (suppressed TSH + normal T4 = check T3 for T3 toxicosis). TSH alone picks up >95% of thyroid dysfunction.

When NOT to start with TSH alone:

Critically ill patients (euthyroid sick syndrome distorts TSH)
Early or unstable phases of thyroid therapy (TSH takes ~2 months to normalize after treatment changes)
Suspected central (pituitary or hypothalamic) disease (TSH can be inappropriately normal despite low T4)

In these cases, check TSH and free T4 together.

Primary hypothyroidism: TSH markedly elevated (often >10 mU/L), free T4 low

TSH rises before T4 falls below the reference range
“Subclinical hypothyroidism” = elevated TSH with normal free T4 and no symptoms. This is the earliest detectable phase of thyroid failure. Treatment is usually offered when TSH >10 mU/L or symptoms develop.

Primary hyperthyroidism: TSH suppressed (<0.1 mU/L), free T4 and/or T3 elevated

In Graves’ disease, stimulating antibodies activate the TSH receptor, so high thyroid hormone feeds back but can’t suppress the autonomous stimulation.
T3 toxicosis (~5% of hyperthyroid patients): elevated T3 with normal T4 and suppressed TSH. Always check T3 if TSH is suppressed and T4 is normal. Seen in early Graves’ and toxic nodular goiter.
Subclinical hyperthyroidism = suppressed TSH + normal free T4 + no symptoms. Distinguish from central hypothyroidism (both have low TSH, but free T4 is low in central hypo, normal in subclinical hyper).

Secondary (central) hypothyroidism: TSH low or inappropriately normal, free T4 low

The pituitary is damaged and can’t make TSH.
TSH may even be mildly elevated (but the TSH is biologically inactive due to abnormal glycosylation).
Causes: Pituitary adenoma, surgery, radiation, Sheehan syndrome, infiltrative disease.
A TRH stimulation test can confirm: give TRH, measure TSH. Low / absent TSH response = pituitary disease. Appropriate TSH response = hypothalamic (tertiary) disease.
Once central hypothyroidism is diagnosed, do a full pituitary workup (prolactin, ACTH / cortisol, FSH, LH, GH / IGF-1). Isolated TSH deficiency is rare.

Tertiary (hypothalamic) hypothyroidism: T4 low, TSH low or inappropriately normal, TRH low or inappropriately normal. Hypothalamic failure means TRH is not driving the pituitary.

Lab Patterns at a Glance

Condition	TSH	Total T4	Free T4	Total T3	rT3
Hyperthyroidism	↓	↑	↑	↑	no change
Hypothyroidism (primary)	↑	↓	↓	↓	no change or ↓
Euthyroid sick syndrome	no change	no change	no change	↓	↑
Excess TBG (pregnancy, estrogen)	no change	↑	no change	↑	no change
Subclinical hypothyroidism	↑	normal	normal	normal	normal
Subclinical hyperthyroidism	↓	normal	normal	normal	normal

Euthyroid sick syndrome (nonthyroidal illness) is the classic trap: sick patients shunt T4 away from T3 conversion and toward rT3 conversion. T3 drops, rT3 rises, TSH and T4 are usually normal. Mechanism: illness inhibits type 1 deiodinase (T4 to T3) and upregulates type 3 deiodinase (T4 to rT3). Don’t treat, and ideally don’t test thyroid function in critically ill patients unless hypothyroidism is strongly suspected.

Total vs. Free Thyroid Hormones

More than 99% of circulating T4 and T3 is bound to carrier proteins - primarily thyroxine-binding globulin (TBG), but also transthyretin (prealbumin) and albumin. Only the free (unbound) hormone is biologically active. Specifically, only 0.02% of T4 and 0.2% of T3 circulate free.

Total T4 measures both bound and free hormone. Changes in binding protein levels alter total T4 without changing the patient’s thyroid status:

Increased TBG (pregnancy, estrogen, hepatitis): Total T4 increases, but free T4 and TSH remain normal
Decreased TBG (nephrotic syndrome, androgens, severe illness): Total T4 decreases, but the patient is euthyroid

Causes of increased TBG (think “estrogen-driven + some others”):

Pregnancy
Oral contraceptives / estrogen therapy
Liver disease (hepatitis)
Hypothyroidism itself

Causes of decreased TBG:

Hypoproteinemic states (nephrotic syndrome, malnutrition)
Androgen therapy
Corticosteroids / cortisol

Androgens and estrogens have opposite effects on TBG. Same with prealbumin - it drops in malnutrition and in inflammation (it’s a negative acute phase reactant, half-life ~2 days, the best short-term nutritional marker).

Free T4 is the metabolically active fraction and is preferred for clinical assessment. Modern free T4 and free T3 assays are fully automated chemiluminescent immunoassays. True “free” measurements by equilibrium dialysis are the gold standard but rarely needed clinically.

Free T3 is rarely needed, but useful for:

Investigating T4-to-T3 conversion
Monitoring patients on oral T3 (liothyronine)
Diagnosing T3 toxicosis

T3 resin uptake (historical): before automated free hormone assays, free T3 was approximated with a T3 resin uptake study. Radiolabeled 125I-T3 competes for open TBG sites; excess binds to a resin. High resin uptake = few open TBG sites = hyperthyroidism or low TBG. Low resin uptake = many open TBG sites = hypothyroidism or high TBG. Now replaced by direct free T4 assays.

Thyroid Antibodies

Anti-thyroid peroxidase (anti-TPO, also called anti-microsomal): Present in ~95% of Hashimoto thyroiditis and ~75% of Graves’ disease. Indicates autoimmune thyroid disease.

Anti-thyroglobulin (anti-Tg): Often measured alongside anti-TPO but less specific. Important because anti-Tg interferes with the thyroglobulin tumor marker assay used for thyroid cancer monitoring - always measure anti-Tg whenever thyroglobulin is being used as a tumor marker.

TSH receptor antibodies (TRAb, also called TSI or LATS): Can be stimulating (Graves’) or blocking (some hypothyroidism). Useful for confirming Graves’ disease and predicting neonatal thyrotoxicosis in pregnant women.

Radioactive Iodine Uptake (RAIU) Scan

RAIU uses 123I (diagnostic imaging) after patient ingests the isotope, to assess gland activity. Know the three iodine isotopes:

123I - RAIU diagnostic scan
125I - T3 resin uptake test (historical)
131I - thyroid ablation therapy

RAIU sorts causes of hyperthyroidism by whether the gland is actually making hormone:

Increased RAIU (active gland producing hormone):

Graves’ disease - diffusely increased uptake
Functioning (“toxic”) adenoma - localized hot nodule with suppressed surrounding tissue
Toxic multinodular goiter - may be mildly decreased, normal, or mildly elevated

Decreased RAIU (gland not making hormone, excess is from elsewhere):

Thyroiditis - preformed hormone leaking from damaged gland
Struma ovarii - thyroid tissue in an ovarian teratoma
Exogenous thyroid hormone

Causes of Hyperthyroidism (Expanded)

Clinical picture: tachycardia, weight loss, diarrhea, heat intolerance, anxiety, tremor. Everything is sped up. Cardiovascular effects (AF, tachycardia) are the most dangerous; thyroid storm can be fatal. 5:1 female-to-male ratio.

Causes:

Graves’ disease (most common) - TSAb / TRAb autoantibodies stimulate TSH receptor. Uniquely presents with exophthalmos and thyroid dermopathy (pretibial myxedema) because TRAb cross-reacts with TSH receptors on orbital fibroblasts and skin.
Toxic (functioning) adenoma
Toxic multinodular goiter
TSH-producing pituitary adenoma (central)
Thyroiditis (early / thyrotoxic phase)
Amiodarone (can cause BOTH hyper- and hypothyroidism)
Exogenous thyroxine

Causes of Hypothyroidism (Expanded)

Clinical picture: fatigue, cold intolerance, slowed mentation, slowed reflexes, periorbital edema, weight gain, constipation, bradycardia. Everything slowed down. Labs often show dyslipidemia (↑LDL, ↑total cholesterol) and elevated CK. Always check thyroid function in unexplained hypercholesterolemia or elevated CK.

Causes:

Hashimoto thyroiditis (most common) - anti-TPO + anti-Tg
Thyroidectomy
131I therapy
Medications: iodine, amiodarone, lithium, IL-2, interferon-alpha

Amiodarone has iodine in its structure. In a normal euthyroid patient, the excess iodine load inhibits the gland (hypothyroidism). In a patient with iodine deficiency or a multinodular goiter, the same iodine load is used as substrate and drives hyperthyroidism. Outcome depends on underlying thyroid status. Classic board question.

Lithium causes hypothyroidism by inhibiting release of thyroxine (blocks proteolysis of thyroglobulin). 20-40% of lithium patients develop subclinical or overt hypothyroidism. Management: continue lithium, add levothyroxine. Don’t stop lithium just for hypothyroidism - it’s too common and the psychiatric stakes are too high.

Neonatal Hypothyroidism

All neonates are screened because undetected disease causes irreversible mental and growth retardation. Screening window: between 48 and 72 hours of age. Before 48 hours, the physiologic neonatal TSH surge causes false positives. After 72 hours, the delay risks developmental damage.

Three screening approaches (all have some false negatives, all miss mild cases):

TSH only (most common in US) - misses central hypothyroidism and delayed TSH rise
T4 first, TSH if abnormal
TSH + T4 simultaneously

Clinical signs of neonatal hypothyroidism: dry skin, hoarse cry, macroglossia, prolonged jaundice, umbilical hernia, hypothermia. Treatment with levothyroxine must begin within weeks of birth.

Etiology: most commonly thyroid dysgenesis (agenesis, ectopia, hypoplasia), usually sporadic. Can be caused by mutations in PAX8 or TTF (thyroid transcription factor). Less common: familial dyshormonogenesis, Refetoff syndrome (resistance to thyroid hormone, THRB mutation - inappropriately normal or elevated TSH despite high T4/T3), hypopituitarism, transient maternal factors (autoantibodies, medications).

18.2 Adrenal Function Testing

Cortisol Physiology

Cortisol is the body’s primary glucocorticoid, essential for stress response, glucose homeostasis, and immune modulation. Its secretion follows a circadian rhythm - highest in the early morning (to prepare for waking) and lowest at midnight.

The hypothalamic-pituitary-adrenal (HPA) axis controls cortisol:

CRH from the hypothalamus stimulates ACTH release from the pituitary
ACTH stimulates cortisol synthesis in the adrenal cortex
Cortisol feeds back to suppress CRH and ACTH

Testing for Cortisol Excess (Cushing Syndrome)

The challenge in diagnosing Cushing syndrome is that cortisol varies throughout the day and increases with stress. You need tests that either exploit the circadian rhythm or test the integrity of feedback suppression.

Late-night salivary cortisol: Cortisol should be at its nadir at midnight. In Cushing syndrome, the normal circadian rhythm is lost, and midnight cortisol remains elevated. Salivary cortisol reflects free cortisol and is easily collected at home. Two elevated late-night samples are strongly suggestive.

24-hour urine free cortisol: Integrates cortisol production over the entire day, avoiding the problem of spot variability. Elevated in most cases of Cushing syndrome, but can be normal in mild or cyclic disease. False elevations occur with high fluid intake (dilutes urine, increasing collection volume).

1 mg overnight dexamethasone suppression test: Dexamethasone is a potent synthetic glucocorticoid that should suppress ACTH and therefore cortisol. Give 1 mg at 11 PM, measure serum cortisol at 8 AM the next morning. Normal response: cortisol <1.8 μg/dL. Lack of suppression suggests autonomous cortisol production. False positives occur with estrogen use (increases CBG), depression, alcoholism, and drugs that accelerate dexamethasone metabolism.

Testing for Cortisol Deficiency (Adrenal Insufficiency)

Morning cortisol: A random cortisol is useful only at extremes. Morning cortisol <3 μg/dL strongly suggests insufficiency; >18 μg/dL essentially rules it out. Values in between require dynamic testing.

Cosyntropin (ACTH) stimulation test: The gold standard for diagnosing primary adrenal insufficiency. Cosyntropin is synthetic ACTH. In a normal person, ACTH stimulates cortisol production. In primary adrenal insufficiency, the adrenals are destroyed and cannot respond.

Protocol: Administer 250 μg cosyntropin IV or IM. Measure cortisol at 0, 30, and 60 minutes. Normal response: Cortisol ≥18-20 μg/dL at 30 or 60 minutes. Abnormal response: Indicates primary adrenal insufficiency.

Limitation: The cosyntropin test may be normal in recent-onset secondary adrenal insufficiency because the adrenals haven’t yet atrophied. ACTH can still stimulate them. In suspected secondary insufficiency, measure ACTH level (should be low) and consider insulin tolerance test or metyrapone test.

18.3 Diabetes Mellitus Laboratory Diagnosis

The Pathophysiology Behind the Tests

Fasting glucose reflects hepatic glucose output. Normally, insulin suppresses gluconeogenesis overnight. In diabetes, insulin resistance or deficiency allows the liver to produce glucose unchecked.

Postprandial glucose reflects the ability to dispose of a glucose load. After eating, insulin promotes glucose uptake into muscle and fat. In diabetes, this disposal is impaired.

HbA1c reflects average glucose over 2-3 months. Glucose binds non-enzymatically to hemoglobin in proportion to its concentration. Because red blood cells live ~120 days, HbA1c integrates glucose exposure over the RBC lifespan.

Diagnostic Criteria

Any of the following, confirmed on a separate day (unless unequivocal hyperglycemia with symptoms):

Test	Diabetes	Prediabetes
Fasting glucose	≥126 mg/dL	100-125 mg/dL (IFG)
2-hour OGTT	≥200 mg/dL	140-199 mg/dL (IGT)
HbA1c	≥6.5%	5.7-6.4%
Random glucose	≥200 mg/dL + symptoms	-

Why fasting glucose requires confirmation: A single elevated fasting glucose could be stress hyperglycemia. Requiring confirmation on a separate day increases specificity.

HbA1c caveats: Conditions that alter RBC lifespan affect HbA1c accuracy:

Shortened RBC survival (hemolysis, blood loss, hemoglobinopathies): Falsely low HbA1c
Prolonged RBC survival (splenectomy, aplastic anemia): Falsely high HbA1c
Hemoglobin variants (HbS, HbC, HbE): May interfere depending on the assay method

18.4 Hypoglycemia Evaluation

Hypoglycemia in a non-diabetic adult is rare and demands investigation. The diagnostic approach centers on what’s happening to insulin when glucose is low.

The critical sample: When the patient is symptomatic and glucose is <50-55 mg/dL, simultaneously measure:

Glucose (confirm hypoglycemia)
Insulin (should be suppressed if hypoglycemia is appropriate)
C-peptide (marker of endogenous insulin secretion)
Proinsulin (elevated in insulinoma)
β-hydroxybutyrate (should rise when insulin is low)
Sulfonylurea/meglitinide screen

Interpretation Patterns

Insulinoma (endogenous hyperinsulinism):

Insulin: Inappropriately high (≥3 μU/mL when glucose <55)
C-peptide: Elevated (≥0.6 ng/mL) - proves insulin is endogenous
Proinsulin: Elevated (≥5 pmol/L) - insulinomas secrete excess proinsulin
β-hydroxybutyrate: Suppressed (<2.7 mmol/L) - insulin inhibits ketogenesis
Sulfonylurea screen: Negative

Exogenous insulin (factitious hypoglycemia from injected insulin):

Insulin: High
C-peptide: Suppressed (injected insulin has no C-peptide and suppresses endogenous secretion)
The mismatch between high insulin and low C-peptide is diagnostic

Sulfonylurea ingestion (accidental or factitious):

Insulin: High
C-peptide: High (sulfonylureas stimulate endogenous insulin release)
Sulfonylurea screen: Positive
This pattern mimics insulinoma; the drug screen distinguishes them

18.5 Pituitary Function Testing

Pituitary Anatomy and Cell Types

The anterior pituitary (adenohypophysis) is glandular tissue derived from Rathke’s pouch (oral ectoderm). The posterior pituitary (neurohypophysis) is neural tissue derived from the hypothalamus. Craniopharyngiomas arise from Rathke’s pouch remnants.

Anterior pituitary cells stain as either acidophils or basophils:

Acidophils (stain with acidic dyes): Growth hormone and Prolactin
Basophils (stain with basic dyes): FSH, LH, ACTH, TSH (mnemonic “B-FLAT”)

The posterior pituitary is not really glandular. It is made up of axon termini of neurons whose cell bodies sit in the hypothalamus. The hormones are synthesized in the hypothalamus and shipped down the axons for storage and release:

Supraoptic nucleus → ADH (vasopressin)
Paraventricular nucleus → Oxytocin

Pituitary dysfunction almost always presents as panhypopituitarism, not isolated hormone deficiency. The most common cause is a non-functioning pituitary adenoma that compresses and atrophies the surrounding pituitary tissue. Also: Sheehan syndrome, sarcoidosis, Langerhans cell histiocytosis.

Anterior pituitary hormone release is controlled on two levels:

Hypothalamic input via releasing / inhibiting hormones through the hypophyseal portal system
Direct feedback from peripheral target-gland hormones (e.g., T4 directly inhibits TSH at the pituitary)

The Stalk Effect (Classic Board Concept)

If the hypophyseal portal system (pituitary stalk) is severed or compressed, all anterior pituitary hormones fall EXCEPT prolactin, which rises. Reasoning: GHRH, GnRH, TRH, CRH all reach the pituitary through the portal veins and stimulate their targets - interrupt the stalk, those hormones drop. Dopamine also reaches the pituitary through the stalk, and dopamine is the primary tonic inhibitor of prolactin. Interrupt the stalk, dopamine can no longer suppress prolactin, so prolactin rises. Cause: compression from pituitary adenoma, craniopharyngioma.

Growth Hormone

GH secretion is pulsatile and undetectable most of the day, making random GH levels useless.

GH is stimulated by:

First 2 hours of slow-wave sleep (the largest pulse of the day)
Stress
Hypoglycemia (GH is counter-regulatory to insulin)
Arginine (stimulation test alternative to insulin tolerance)
Exercise
Clonidine, glucagon (used pharmacologically)

GH is inhibited by:

Somatostatin from the hypothalamus AND the pancreas
Hyperglycemia (basis of oral glucose suppression test)

Somatostatin inhibits broadly - TSH, insulin, glucagon, GI hormones. This is why octreotide (somatostatin analog) is used for GH-secreting adenomas.

Clinical syndromes:

GH deficiency: children - dwarfism; adults - largely asymptomatic (some increased body fat, decreased muscle mass, fatigue, but no dramatic syndrome because growth plates are fused).
GH excess: children - gigantism (growth plates still open, proportionate tall stature); adults - acromegaly (growth plates fused, so only acral / soft tissue growth - enlarged hands, feet, jaw, coarsened features, organomegaly).

Testing:

Deficiency: provocative testing (insulin tolerance test is gold standard; also arginine, clonidine, glucagon). Normal response = GH >5-10 ng/mL.
Excess (screening): IGF-1 is the ideal test because it’s stable throughout the day (no diurnal variation like GH). Elevated IGF-1 = screen positive for acromegaly.
Excess (confirmation): oral glucose suppression test - give 75g glucose, measure GH. Normal response: GH suppresses to <1 ng/mL. In acromegaly, GH fails to suppress.
A single random GH level cannot exclude GH excess (pulsatile release means you can catch a trough).

IGF-1 serves as an integrated marker of GH status because GH stimulates hepatic IGF-1 production. Low IGF-1 also supports GH deficiency.

GnRH, FSH, LH

GnRH from the hypothalamus stimulates both FSH and LH release from the anterior pituitary. One releasing hormone, two gonadotropins.

Pulsatile vs continuous GnRH matters:

Pulsatile GnRH = stimulates FSH / LH (physiologic pattern)
Continuous GnRH = inhibits FSH / LH (receptor desensitization)

This is therapeutically exploited with leuprolide (GnRH agonist given continuously) - after an initial 1-2 week flare, it suppresses gonadotropins and induces chemical castration. Used for prostate cancer, endometriosis, precocious puberty, IVF protocols.

FSH and menopause: As the ovary fails, estrogen and inhibin B drop, negative feedback is lost, and FSH rises. In young women suspected of premature ovarian insufficiency, FSH >40 IU/L on two occasions at least 4 weeks apart suggests ovarian failure. Premature ovarian failure is defined as menopause before age 40. In older women, menopause is clinical; lab confirmation is rarely needed.

Prolactin

Prolactin is the oddball - it has no dedicated hypothalamic releasing hormone. Release is primarily controlled by tonic inhibition from dopamine. High dopamine = low prolactin. Low dopamine = high prolactin. TRH can stimulate prolactin release but is not its dedicated releasing factor; this is why primary hypothyroidism (high TRH) causes hyperprolactinemia.

Prolactinoma is the most common pituitary adenoma (~40-50%). Treatment is primarily medical with dopamine agonists (cabergoline, bromocriptine), because prolactin is normally tonically inhibited by dopamine. Surgery is reserved for drug-intolerant or resistant cases.

Hyperprolactinemia clinical effects:

Women: amenorrhea + galactorrhea (prolactin inhibits GnRH → suppresses FSH / LH)
Men: testicular atrophy, impotence, gynecomastia (low testosterone from FSH / LH suppression)

Women are typically diagnosed earlier because amenorrhea is noticed; men often present late with large tumors and mass effect.

Drug-induced hyperprolactinemia: dopamine antagonists. Most important: antipsychotics (D2 blockade is their mechanism) and metoclopramide (antiemetic, D2 antagonist). Always review medications before imaging the pituitary in an elevated prolactin.

ADH (Vasopressin) and Diabetes Insipidus

ADH release is stimulated by:

Increased plasma osmolarity (primary stimulus, sensed by hypothalamic osmoreceptors)
Decreased plasma volume (severe, sensed by baroreceptors)

Osmolarity is the fine-tuning mechanism. Volume depletion is the emergency override.

Diagnosing DI: the water deprivation test followed by ADH administration. Restrict water, measure urine osmolarity. Then give desmopressin, remeasure.

Condition	Water deprivation	ADH administration
Central DI	Urine stays dilute	Urine concentrates
Nephrogenic DI	Urine stays dilute	Urine stays dilute
Psychogenic polydipsia	Urine concentrates	(not needed - test stops here)

Central DI = hypothalamus / posterior pituitary can’t make ADH. Kidneys work fine - ADH fixes it. Treatment: desmopressin.
Nephrogenic DI = kidneys don’t respond to ADH. Nothing fixes it acutely. Causes: lithium (most common drug cause), hypercalcemia, hypokalemia, congenital. Treatment: thiazide diuretic (paradoxical effect) + low-sodium diet.
Psychogenic polydipsia = the patient drinks too much water. Everything is intact, so water deprivation alone concentrates the urine and you stop there.

SIADH: inappropriate ADH secretion despite low plasma osmolarity. Kidney retains free water → dilutional hyponatremia with inappropriately concentrated urine. Urine osmolarity >100 mOsm/kg in a hyponatremic patient is the red flag - urine should be maximally dilute in hyponatremia.

Vitamin D Metabolism

The hydroxylation pathway:

Skin (UV light) → Cholecalciferol (Vitamin D3)
Liver (25-hydroxylase) → 25-OH Vitamin D (calcidiol) - this is the best measure of vitamin D status (half-life 2-3 weeks, reflects total body stores)
Kidney (1-alpha hydroxylase) → 1,25-dihydroxyvitamin D (calcitriol) - the active form

Clinical ranges for 25-OH Vitamin D: Deficiency <20 ng/mL, Insufficiency 20-29 ng/mL, Sufficiency ≥30 ng/mL.

Key: In CKD, decreased 1-alpha hydroxylase → low calcitriol → hypocalcemia → secondary hyperparathyroidism → renal osteodystrophy. Measure 1,25-(OH)₂D only for CKD or sarcoidosis workup.

Pre-albumin (Transthyretin) as Nutritional Marker

Pre-albumin has a half-life of ~2 days (vs. albumin ~20 days), making it a much more responsive marker of acute nutritional status and response to nutritional support.

Normal: 20-40 mg/dL. Limitation: Pre-albumin is a negative acute-phase reactant - it decreases in inflammation, liver disease, and overhydration. Always check CRP to interpret; if CRP is elevated, low pre-albumin may reflect inflammation rather than malnutrition.

Board Pearl: Despite “transthyretin” literally meaning “transports thyroxine and retinol,” pre-albumin is NOT used for thyroid diagnosis - use TSH and free T4 instead.

Anti-Müllerian Hormone (AMH)

Produced by granulosa cells of small antral follicles. Reflects ovarian reserve.

Clinical use: Low AMH = diminished ovarian reserve; used for fertility assessment and predicting response to ovarian stimulation in IVF. Also elevated in PCOS.

Key advantage: AMH is relatively stable throughout the menstrual cycle (can be drawn any day), unlike Day-3 FSH which varies cycle-to-cycle.

Pheochromocytoma and Catecholamine Testing

Catecholamine-secreting tumor of the adrenal medulla (or extra-adrenal paraganglioma).

Screening tests:

Plasma free metanephrines: Highest sensitivity (~99%); best initial test for high-risk patients (MEN2, VHL, NF1, SDH mutations)
24-hour urine fractionated metanephrines: More specific (fewer false positives); best for low-risk/sporadic presentation
VMA (vanillylmandelic acid): Low sensitivity for pheochromocytoma; used primarily for neuroblastoma screening in children

The “Rule of 10s”: ~10% bilateral, ~10% extra-adrenal, ~10% malignant, ~10% familial, ~10% pediatric.

18.6 Pregnancy Chemistry and Fetal Testing

Pregnancy introduces a second patient and a new set of analytes. The physiology is distorted so broadly that “normal” reference ranges in pregnancy differ from non-pregnant reference ranges for most chemistries and hematology parameters. This section covers hCG, maternal physiologic lab changes, fetal aneuploidy screening, fetal lung maturity, amniotic bilirubin for HDFN, liver disease of pregnancy, and pregnancy-specific endocrine effects.

hCG: Source, Structure, and Interpretation

Sources of hCG:

Placenta (syncytiotrophoblast - main source in pregnancy)
Pituitary - small amounts; increases in menopause when gonadotropins rise
Certain tumors - germ cell tumors (choriocarcinoma, testicular seminoma / nonseminoma), gestational trophoblastic disease (hydatidiform mole)

hCG structure: two chains. The α-chain is shared with TSH, FSH, and LH. Only the β-chain is unique. This is why pregnancy tests target β-hCG specifically, and it’s why very high hCG can cross-react with the TSH receptor and cause transient biochemical hyperthyroidism.

“False positive” hCG (patient not pregnant but hCG elevated):

Heterophile antibody interference (HAMA - human anti-mouse antibodies that bridge the capture and detection antibodies in the sandwich immunoassay). Confirm by serial dilution non-linearity or testing on a different platform.
Neoplasm (real hCG from germ cell tumors or GTD)
Physiologic pituitary hCG in menopause

Molar pregnancies: complete mole (46,XX, all paternal, diploid, no fetal tissue, diffuse trophoblastic proliferation) has very high hCG (often >100,000 IU/L) and ~15-20% risk of gestational trophoblastic neoplasia. Partial mole (69,XXX or 69,XXY, triploid, with some fetal tissue and focal trophoblastic proliferation) has modestly elevated hCG. Average hCG is higher in complete than partial moles.

hCG kinetics in normal pregnancy:

Detectable in serum ~8-10 days post-conception
Doubles every 48-72 hours in early pregnancy (up to ~10 weeks)
Peaks by week 10, can exceed 100,000 IU/L
Plateaus by week 16 at lower levels (~10,000-20,000 IU/L) for remainder of pregnancy
Urine tests detect hCG at higher thresholds (~20-50 mIU/mL) than serum (~1-2 mIU/mL), so urine can be negative in very early or ectopic pregnancy

After delivery or abortion: hCG may remain detectable up to 2 months. Half-life 24-36 hours. Failure to decline to zero raises concern for retained products of conception, gestational trophoblastic neoplasia, or choriocarcinoma. After molar evacuation, hCG is monitored weekly until negative, then monthly for 6-12 months.

Ectopic pregnancy:

Suspect when hCG fails to double every 48-72 hours in early pregnancy.
Absence of an intrauterine gestational sac when hCG >6,500 IU/L on transabdominal ultrasound strongly suggests ectopic (discriminatory zone for transabdominal). For transvaginal, the discriminatory zone is ~1,500-2,000 IU/L.
Declining hCG suggests miscarriage; a plateau is more concerning for ectopic. Note ~15% of normal pregnancies have slower-than-expected rises and ~17% of ectopics have normal doubling, so this is never absolute.
Treatment: methotrexate (if stable, unruptured, low hCG) or surgery.

Fetal Aneuploidy Screening

Overall trisomy risk: ~1:700 for any trisomy in a neonate. At maternal age ≥35, this rises to ~1:270. Trisomy 21 is the most common viable trisomy; trisomy 18 (Edward) and trisomy 13 (Patau) have very high mortality. Trisomy 16 is the most common trisomy overall but is uniformly lethal, causing first-trimester miscarriage. About 50% of first-trimester miscarriages are chromosomal.

First-trimester screen (10-13 weeks):

hCG (elevated in trisomy 21)
PAPP-A (pregnancy-associated plasma protein A; decreased in trisomy 21 and 18). PAPP-A is a metalloproteinase that cleaves IGF-binding proteins; low PAPP-A in the first trimester also predicts preeclampsia, IUGR, and preterm delivery independent of aneuploidy risk.
Nuchal translucency thickness by ultrasound (increased thickness suggests aneuploidy, cardiac defects, or Turner syndrome)

Detection rate for trisomy 21: ~85%.

Quad screen (second trimester, 15-22 weeks):

α-fetoprotein (AFP)
Unconjugated estriol
hCG
Dimeric inhibin A

Mnemonic: “All Equines Have Incontinence” (AFP, Estriol, hCG, Inhibin A). Detection rate for trisomy 21 ~80% with ~5% false-positive rate. This is a screening test - positive results require diagnostic amniocentesis or CVS.

Integrated screen combines first-trimester (PAPP-A + NT) with second-trimester (quad) for a single risk estimate:

Full integrated = serum markers + NT (~95% detection of trisomy 21)
Serum integrated = serum markers only (no NT), ~88% detection
Downside: results delayed until second trimester
Cell-free DNA (cfDNA) screening has >99% detection for trisomy 21 with very low false-positive rate, valid from week 10 onward

Quad screen patterns (ordered alphabetically - AFP, Estriol, hCG, Inhibin):

Syndrome	AFP	Estriol	hCG	Inhibin A
Down (T21)	↓	↓	↑	↑
Edward (T18)	↓	↓	↓	no change
Patau (T13)	not reliably detected by quad screen	-	-	-

Memory trick (teaching device, not a literal description of every result): Down (T21) affects all 4 analytes - hCG and Inhibin A go up, AFP and Estriol go down. Edward (T18) drops AFP, Estriol, and hCG together, with Inhibin A unchanged. The quad screen does not reliably detect Patau (T13); when AFP is elevated in a T13 pregnancy, that reflects associated structural defects (holoprosencephaly, omphalocele, NTDs) rather than the trisomy itself.

AFP Interpretation

Elevated maternal serum AFP is seen in many fetal malformations:

Neural tube defects (anencephaly, spina bifida, encephalocele)
Abdominal wall defects (omphalocele, gastroschisis; gastroschisis higher AFP because exposed bowel leaks protein directly into amniotic fluid - no membrane)
Renal anomalies, sacrococcygeal teratoma, cystic hygroma, hydrops fetalis, Turner syndrome, bowel obstruction, fetal demise, fetal-maternal hemorrhage

Non-malformation causes of abnormal AFP (must correct for these; results reported as multiples of the median / MoM):

Incorrect fetal dating (most common cause - AFP is gestational-age dependent)
Multiple gestation (more fetuses, more AFP)
Maternal weight (heavier women have lower AFP from dilution)
Maternal diabetes (AFP ~20% lower)

When MSAFP is elevated, the workup is: ultrasound first (verify dating, check for twins, look for structural defects). If ultrasound normal, amniocentesis for amniotic AFP and acetylcholinesterase. Elevated amniotic AChE + elevated AFP has >98% sensitivity and specificity for open neural tube defects. AChE is specific to neural tissue, detected by polyacrylamide gel electrophoresis.

Amniotic Fluid Bilirubin (HDFN)

In hemolytic disease of the fetus and newborn (HDFN), fetal hemolysis produces unconjugated bilirubin that crosses into amniotic fluid. Concentration of amniotic unconjugated bilirubin reflects the degree of fetal hemolysis.

How it’s measured:

Bilirubin absorbs maximally at 450 nm (blue region - this is also why jaundiced patients look yellow: they absorb blue, transmit yellow)
Measured spectrophotometrically as ΔOD 450 - the change in absorbance at 450 nm compared to a theoretical baseline (tangent line drawn from the spectral scan 350-550 nm). Larger deviation = more bilirubin = more hemolysis.
Only meaningful when plotted against gestational age on a Liley chart (27-42 weeks) or Queenan chart (14-40 weeks, extends earlier). Amniotic bilirubin normally decreases across gestation (3rd trimester levels lower than 2nd).

Liley chart zones:

Zone I = mild / no hemolysis, serial monitoring
Zone II = moderate, close surveillance
Zone III = severe, fetal hydrops imminent, intrauterine transfusion or delivery

Now largely replaced by middle cerebral artery peak systolic velocity (MCA-PSV) Doppler, which is non-invasive.

Specimen handling:

Protect from light - bilirubin photodegrades (same principle as neonatal phototherapy)
Minimize blood contamination - oxyhemoglobin absorbs near 450 nm (Soret band at 415 nm) and creates spectral interference

Fetal Lung Maturity

Pulmonary surfactant is produced by type II pneumocytes, reduces alveolar surface tension, and prevents atelectasis. Production begins ~24-28 weeks but doesn’t reach maturity until ~35-36 weeks. Surfactant deficiency in prematures = neonatal respiratory distress syndrome.

Composition: predominantly phospholipid. Lecithin (phosphatidylcholine) is the dominant component, specifically dipalmitoyl phosphatidylcholine (DPPC), ~70-80% of mature surfactant.

Three lung maturity tests:

Test	Mature cutoff	Blood effect	Meconium effect	Maternal DM effect
Lamellar body count (gold standard)	≥20,000/µL	↓	↑	minimal
Lecithin:sphingomyelin ratio	≥2:1 (immature <1.5:1)	normalizes to 1.5:1	↓	delayed maturity
Phosphatidylglycerol	Present = mature	none	none	none

Lamellar body count (LBC) is the gold standard. Lamellar bodies are intracytoplasmic organelles in type II pneumocytes that store surfactant, ~1-5 μm in size - similar to platelets. Cleverly, you run amniotic fluid through the platelet channel of a hematology analyzer; amniotic fluid has no platelets, so whatever the counter identifies as “platelets” is actually lamellar bodies. No special reagents, results in minutes, requires small volume. Do NOT centrifuge the sample - it removes lamellar bodies.

Meconium increases LBC (small particles counted as platelets)
Blood decreases LBC

L:S ratio (historical standard, thin-layer chromatography). Sphingomyelin stays relatively constant through gestation; lecithin rises as lungs mature. Immature <1.5:1, mature ≥2:1 (some sources cite 2.5:1). Transition occurs ~35-36 weeks.

Meconium decreases L:S
Blood normalizes L:S to 1.5:1 - this is bidirectional. Blood contamination can falsely elevate an immature sample or falsely lower a mature one, driving any result toward 1.5:1. Classic board question.
Diabetes delays lung maturation (insulin inhibits surfactant production), so diabetic mothers may need higher L:S cutoffs.

Phosphatidylglycerol (PG) is a minor, late-appearing surfactant phospholipid. Not affected by blood, meconium, or maternal diabetes. Its weakness is that it’s a late marker - PG appears ~35-36 weeks, later than lecithin which begins rising ~32-34 weeks. A fetus can have mature lungs without detectable PG. Qualitative result (present = mature; absent is not proof of immaturity). Best used as a confirmatory test when LBC or L:S results are borderline or contamination is suspected.

In current practice, fetal lung maturity testing is done less often - the decision to deliver is driven more by obstetric indications and gestational age than by lab testing of maturity.

Fetal Fibronectin (fFN)

Fetal fibronectin is a glycoprotein that acts as “glue” between fetal membranes and uterine wall. Normally present before 22 weeks and after 35 weeks. Its presence between 22-35 weeks suggests membrane disruption.

Negative fFN has >99% NPV for delivery within 7-14 days
PPV is low (only ~20-30%)
Clinical use: rule OUT preterm birth and avoid unnecessary tocolytics, steroids, hospital transfer
Collect >24 hours after intercourse or cervical exam (both disrupt the interface, cause false positives)
Specimen: posterior vaginal fornix, Dacron swab
Valid window: 22-34 weeks, intact membranes, minimal cervical dilation

Normal Physiologic Lab Changes in Pregnancy

Pregnancy is a state of volume expansion, hormone-driven liver effects, and altered coagulation. Reference ranges shift.

Parameter	Direction	Magnitude	Mechanism
Plasma volume	↑	~50%	estrogen-driven RAAS activation
RBC mass	↑	~30%	mild erythropoiesis boost
Hematocrit	↓	4-7%	dilutional (plasma expands more than RBCs)
Hemoglobin	↓	1-2 g/dL	dilutional; normal pregnancy Hb ~11-12 g/dL
Albumin	↓	0.5-1 g/dL	dilutional
Total calcium	↓	~10%	tracks albumin drop
Ionized (free) calcium	no change	-	maintained by PTH
Creatinine	↓	0.3 mg/dL	↑GFR ~50%
BUN	↓	~50%	↑GFR + dilution
Urine protein	↑	roughly doubles	↑GFR + decreased tubular reabsorption
Fibrinogen	↑	1-2 g/L	pregnancy is prothrombotic
TBG	↑	-	estrogen-driven hepatic production
Total T4, T3	↑	-	tracks TBG
Free T4, TSH	no change	-	-
Triglycerides	↑	2-4x	estrogen-driven VLDL, decreased LPL
Total cholesterol	↑	25-50%	supports fetal development
Na, K	no change	-	RAAS-maintained

Important traps and board points:

Pregnant women are NOT truly hypocalcemic. Total calcium drops because albumin drops; ionized calcium is normal. Always measure ionized calcium or correct for albumin before diagnosing hypocalcemia. Contributes to peripheral edema via decreased oncotic pressure.
Standard eGFR equations (MDRD, CKD-EPI) are NOT validated in pregnancy. A “normal” creatinine of 1.0 may actually indicate renal impairment in a pregnant patient. Upper limit of normal creatinine in pregnancy ~0.8 mg/dL.
Hb / Hct drop is “physiologic anemia of pregnancy” - the hemodilution improves placental blood flow by reducing viscosity. Inadequate plasma expansion is associated with preeclampsia and IUGR. Iron needs rise ~1000 mg over pregnancy (fetal needs + expanded RBC mass + delivery blood loss); most women need supplementation.
Total T4 is falsely high in pregnancy - use free T4. Pregnant patient with high total T4 and normal free T4 is euthyroid. In the first trimester, hCG can also mildly suppress TSH through TSH-receptor cross-reactivity.
Glycosuria of pregnancy - the increased GFR filters more glucose than tubules can reabsorb, so glucose appears in urine without hyperglycemia. Don’t diagnose gestational diabetes from glycosuria alone.
Insulin resistance of pregnancy is driven by human placental lactogen (hPL), which peaks in the third trimester - hence gestational diabetes screening at 24-28 weeks. Other diabetogenic hormones: cortisol, progesterone, prolactin.
Pregnancy is a hypercoagulable state - fibrinogen up, factors VII / VIII / X / XII / vWF up, protein S down, PAI-1 up. VTE risk 5-10x baseline, worst in third trimester and postpartum. DIC from placental abruption, HELLP, or amniotic fluid embolism is especially dangerous on this baseline.

Pregnancy and Thyroid

Two TSH-related wrinkles in pregnancy:

Estrogen increases TBG, so total T4 and total T3 rise but free T4 and TSH stay normal. Always use free T4.
hCG has mild TSH-like activity (shared α-subunit, structural homology with TSH). In the first trimester, hCG can mildly suppress TSH.

Pregnancy can cause both hypo- and hyperthyroidism:

Transient hyperthyroidism of hyperemesis gravidarum (THHG): severe first-trimester nausea / vomiting + mild biochemical hyperthyroidism (low TSH, mildly elevated free T4). Driven by very high hCG (>200,000 IU/L) cross-reacting with TSH receptors. Self-limited - resolves as hCG drops after 14-18 weeks. Treat supportively (IV fluids, antiemetics). No antithyroid drugs needed. Distinguish from Graves’ by absence of TSI, ophthalmopathy, and goiter.
Postpartum thyroiditis follows a classic triphasic pattern: hyperthyroid phase (1-4 months) → hypothyroid phase (4-8 months) → recovery. ~20% become permanently hypothyroid.
Graves’ disease can present or flare in pregnancy. TSI crosses the placenta and can cause fetal / neonatal hyperthyroidism. PTU is preferred in first trimester (methimazole is teratogenic - aplasia cutis); methimazole can be used after the first trimester.

Pregnancy and Autoimmune Disease

Pregnancy generally alleviates autoimmune disease because estrogen, progesterone, and regulatory T cells produce immune tolerance of the fetus. Diseases that typically improve: RA, MS, Graves’ disease.

Major exception: SLE, which often flares in pregnancy because estrogen enhances B-cell activity and autoantibody production. Lupus flares increase miscarriage, preterm delivery, preeclampsia, and neonatal lupus. SLE in pregnancy → recurrent miscarriage and preterm labor, driven by:

Antiphospholipid antibodies (lupus anticoagulant, anticardiolipin, anti-β2-GPI) causing placental thrombosis
Complement activation damaging trophoblast
Lupus nephritis increasing preeclampsia risk

Treatment: hydroxychloroquine (safe, reduces flares), low-dose aspirin (prevents preeclampsia), heparin (if APS). Avoid mycophenolate and cyclophosphamide (teratogenic).

Neonatal lupus: anti-SSA (Ro) antibodies are IgG and cross the placenta. Most serious manifestation: congenital complete heart block (irreversible fibrosis of fetal cardiac conduction system at 18-24 weeks - requires permanent pacemaker). Other features (transient rash, cytopenias, hepatitis) resolve within 6 months as maternal antibodies clear. Screen anti-SSA positive mothers with fetal echo. Often associated with anti-SSB (La).

Postpartum autoimmune flares: the sudden drop in estrogen / progesterone after delivery removes immunosuppression, and autoimmune activity rebounds. Common: Hashimoto’s, postpartum thyroiditis, RA, MS, SLE.

ITP in pregnancy: ITP antibodies (IgG anti-GPIIb/IIIa) cross the placenta via FcRn (same mechanism as passive immunity). Maternal antibodies destroy fetal platelets → neonatal thrombocytopenia. Usually self-limited (resolves 1-2 months as antibodies clear). Maternal platelet count does NOT predict neonatal platelet count.

Always rule out neonatal alloimmune thrombocytopenia (NAIT) in neonatal thrombocytopenia. NAIT is analogous to HDFN but for platelets: mother lacks a platelet antigen (usually HPA-1a / PlA1) that the fetus inherits from father; maternal alloantibodies cross the placenta and destroy fetal platelets. NAIT is more severe than ITP-related neonatal thrombocytopenia and carries ~10-20% risk of intracranial hemorrhage. NAIT can occur in first pregnancies (unlike HDFN). A maternal transfusion history raises concern for alloimmunization.

Pregnancy-Specific Conditions

UTI and pyelonephritis: Pregnancy increases UTI risk through progesterone-mediated ureteral smooth muscle relaxation (hydroureter, hydronephrosis), decreased peristalsis, mechanical compression by the gravid uterus, and increased urinary glucose. Asymptomatic bacteriuria in pregnancy MUST be treated (unlike in non-pregnant patients) because it progresses to pyelonephritis in 20-40% if untreated (vs 1-2% non-pregnant). Group B Strep bacteriuria also mandates intrapartum prophylaxis.

Sheehan syndrome (postpartum pituitary apoplexy / necrosis): The pituitary enlarges 2-3x during pregnancy (lactotroph hyperplasia from estrogen) but blood supply doesn’t keep up. Severe postpartum hemorrhage causes hypotension and ischemic necrosis of the enlarged gland. 90% have history of severe postpartum hemorrhage. Presents with:

Failure to lactate (first sign - loss of prolactin)
Amenorrhea
Hypothyroidism
Adrenal insufficiency

Labs: panhypopituitarism.

Liver diseases of pregnancy - all three typically present in the third trimester:

Disease	Defining feature	Emergency
Intrahepatic cholestasis of pregnancy (ICP)	pruritis + jaundice, ↑bile acids	Fetal (bile acids cardiotoxic)
Acute fatty liver of pregnancy (AFLP)	hepatic failure + DIC	Yes - immediate delivery
HELLP syndrome	Hemolysis, Elevated Liver enzymes, Low Platelets	Yes - preeclampsia spectrum

Intrahepatic cholestasis of pregnancy presents with intense pruritis (worse at night, palms and soles) and jaundice in the third trimester. Labs: elevated serum bile acids (most sensitive and specific, >10 μmol/L), mild transaminase elevation, elevated bilirubin. Fetal risk: bile acids are toxic to fetal cardiomyocytes → fetal arrhythmias and sudden fetal demise. Treatment: ursodeoxycholic acid and early delivery at 37 weeks. Resolves after delivery.

Acute fatty liver of pregnancy is one of the most dangerous obstetric emergencies. Often complicated by DIC. Histology: microvesicular steatosis worst in zone 3 (centrilobular / paracentral), often with hepatocyte ballooning and some macrovesicular steatosis. Microvesicular = small fat droplets, nucleus central (distinct from macrovesicular in NAFLD / alcoholic liver disease where one large droplet displaces the nucleus). Suggests mitochondrial dysfunction (impaired fatty acid β-oxidation). Labs: elevated transaminases and bilirubin, low glucose (liver can’t gluconeogenize), elevated ammonia, prolonged PT. Associated with fetal LCHAD deficiency (long-chain 3-hydroxyacyl-CoA dehydrogenase). Treatment: immediate delivery regardless of gestational age. Maternal mortality 1-12%, fetal 7-20% if untreated. Aggressive DIC management with FFP, cryoprecipitate, platelets. Recurrence risk is low unless the mother carries LCHAD mutations (~25%).

HELLP is on the preeclampsia spectrum: Hemolysis (schistocytes, elevated LDH, low haptoglobin), Elevated Liver enzymes, Low Platelets. Can overlap clinically with AFLP; both resolved by delivery.

Chapter 19: Tumor Markers

Tumor markers are substances produced by cancer cells or by the body in response to cancer. While the concept is straightforward, their clinical application is nuanced. Very few tumor markers are useful for screening; most are valuable for monitoring treatment response and detecting recurrence.

This chapter covers the core serum and urine tumor markers you’ll see on boards: PSA, AFP, hCG, CA-125, CA 19-9, CEA, thyroid markers (thyroglobulin and calcitonin), β2-microglobulin, carcinoid markers, pheochromocytoma/paraganglioma workup, neuroblastoma markers, and urothelial carcinoma markers. Interference patterns (hook effect, heterophile antibodies, anti-thyroglobulin antibodies) matter as much as the analytes themselves.

19.1 General Principles of Tumor Marker Use

Why Most Tumor Markers Aren’t Good Screening Tests

An ideal screening test has high sensitivity (doesn’t miss cancers) and high specificity (doesn’t produce false positives). Most tumor markers fail on specificity - they’re elevated in many benign conditions.

The problem of false positives: If a marker has 95% specificity and you screen 1,000 healthy people, 50 will have false-positive results. If the cancer prevalence is 1%, only 10 of those 1,000 actually have cancer. Your positive predictive value is dismal - most people with elevated markers don’t have cancer.

The consequence: Inappropriate anxiety, unnecessary imaging, biopsies, and even surgery for benign conditions.

Appropriate Uses of Tumor Markers

The three canonical uses: monitoring recurrence (most important), screening (limited utility), and monitoring treatment response. Of these, recurrence detection is where tumor markers earn their keep.

Monitoring treatment response: If a marker is elevated at diagnosis and falls with treatment, rising levels suggest recurrence or progression. This is the most validated use of tumor markers.

Prognosis at diagnosis: Initial marker levels often correlate with tumor burden and prognosis.

Detecting recurrence: In patients with known cancer history, rising markers may detect recurrence before imaging. A key teaching point: rising markers can precede imaging changes by 3-6 months, so trending over time beats single values, and a single negative reading doesn’t exclude disease.

Diagnosis in specific contexts: When a patient presents with metastatic disease of unknown primary, tumor markers can suggest the tissue of origin.

Assay Interferences Common to Tumor Markers

Two interferences show up repeatedly on boards and in practice:

Hook (prozone) effect: In sandwich immunoassays, extremely high analyte concentrations saturate both the capture and detection antibodies separately, so fewer sandwich complexes form. The result: extremely high analyte gives a falsely low reading. The clue is a result that seems too modest for the clinical picture (massive molar pregnancy, bulky germ cell tumor). Confirm by serial dilution - if the reported concentration increases with dilution, that’s the hook effect. The fix is to dilute and re-run.

Heterophile antibodies: Human antibodies (e.g., HAMA - human anti-mouse antibodies, developed after exposure to mouse monoclonals) can bridge capture and detection antibodies without any analyte present. These usually cause falsely high results. Suspect heterophile interference when a marker is elevated but the clinical picture doesn’t fit. Blocking reagents or a different assay platform can help.

The mnemonic: hook effect goes low, heterophiles go high.

19.2 Prostate-Specific Antigen (PSA)

The Biology of PSA

PSA is a serine protease produced almost exclusively by prostatic epithelial cells. Its physiologic function is to liquefy semen. PSA is prostate-specific, not cancer-specific - any process that disrupts prostatic architecture releases PSA into the bloodstream.

Causes of elevated PSA:

Prostate cancer
Benign prostatic hyperplasia (BPH) - larger prostate = more PSA
Prostatitis (inflammation disrupts tissue)
Prostatic infarction
Recent ejaculation (transient elevation)
Prostate manipulation (biopsy, digital rectal exam, surgery, catheterization)
Urinary retention

Mildly elevated PSA deserves clinical correlation before jumping to cancer. The most common cause of PSA elevation overall is BPH, not malignancy.

Demographic and Age Effects on PSA

PSA distributions and prostate cancer risk vary across populations and by age:

Black men in the United States have higher prostate cancer incidence and mortality, and PSA distributions differ across populations. Population-specific reference ranges have been proposed but remain controversial; age-specific ranges are used more commonly in practice.
In younger patients, PSA has higher sensitivity (small prostates, so any elevation is suspicious). In older patients, PSA has higher specificity (BPH is so common that mild elevations are expected, but very high levels become more specific for malignancy).

PSA Forms and Ratios

PSA circulates in multiple forms:

Complexed (bound) PSA: Bound to protease inhibitors. 75-90% of serum PSA is bound, mostly to α1-antichymotrypsin. A smaller fraction binds α2-macroglobulin (not detected by most assays).
Free PSA: Unbound to any protein, 10-25% of total.

Total PSA is the standard measurement (detects complexed + free PSA; α2-macroglobulin-bound PSA is hidden from the assay because both epitopes are occluded).

Free PSA ratio (% free PSA = free PSA / total PSA × 100): Cancer cells release more complexed PSA and proportionally less free PSA, while BPH releases more free PSA. A low free PSA ratio (<10%) suggests cancer; a high ratio (>25%) suggests BPH. This is most useful when total PSA is in the “gray zone” (4-10 ng/mL) and helps decide whether to biopsy.

PSA density (PSA / prostate volume from transrectal ultrasound): Normalizes for gland size. A small gland producing a lot of PSA is more suspicious than a large gland producing the same PSA. PSA density >0.15 ng/mL/cc is considered suspicious.

PSA velocity: The rate of PSA change over time (ng/mL/year). Rapidly rising PSA is more concerning than stable elevation. Velocity >0.75 ng/mL/year raises suspicion for malignancy, and a reliable velocity requires at least 3 measurements over ≥18 months.

The PSA Screening Controversy

PSA screening detects prostate cancer early, but many detected cancers are indolent and would never cause symptoms or death. Overdiagnosis leads to overtreatment, with attendant morbidity (incontinence, impotence) from surgery or radiation.

Current guidelines recommend shared decision-making: discuss the risks and benefits with patients aged 55-69, and offer screening to those who value early detection after understanding the tradeoffs.

19.3 Colorectal Cancer Screening

CRC screening deserves its own section because the modalities and intervals are board-testable, and because CEA (covered later) is not part of screening.

When to Start

Average-risk CRC screening begins at age 45 (updated from 50, because of rising incidence in younger adults). Earlier screening is recommended for high-risk groups: family history, inflammatory bowel disease, Lynch syndrome.

Screening Modalities and Intervals

Know the intervals cold:

Modality	Interval
Colonoscopy	every 10 years
Flexible sigmoidoscopy	every 5 years
CT colonography	every 5 years
Capsule endoscopy	every 5 years
FIT + stool DNA (Cologuard)	every 3 years
FOBT alone (guaiac or FIT)	every year

Colonoscopy is the gold standard because it allows biopsy and polypectomy in the same session. FIT + DNA (Cologuard) has higher sensitivity than FOBT alone, at the cost of more false positives.

FOBT Methods: Guaiac vs. FIT

Two ways to detect occult blood in stool, and boards like to test the difference:

Guaiac-based FOBT: Hemoglobin’s peroxidase activity oxidizes guaiac in the presence of hydrogen peroxide, producing a blue color. Detects ANY hemoglobin - human, animal, or plant peroxidase.
Fecal immunochemical test (FIT): Uses monoclonal antibodies specific for human hemoglobin. Far more specific for colorectal bleeding sources. No dietary restrictions needed. FIT also doesn’t detect upper GI bleeding well because hemoglobin degrades during transit.

FIT is preferred for most screening programs.

Guaiac FOBT false positives: NSAIDs (cause GI bleeding), eating red meat (animal heme), peroxidase-rich foods (turnips, horseradish, broccoli).

Guaiac FOBT false negatives: Vitamin C (ascorbate blocks the oxidation reaction).

Patients must follow dietary restrictions before guaiac testing - this is the major practical drawback that FIT eliminates.

19.4 Alpha-Fetoprotein (AFP)

Biology and Normal Physiology

AFP is the major serum protein of the fetus, produced by the yolk sac and fetal liver. It functions as the fetal equivalent of albumin (maintains oncotic pressure, transports small molecules). After birth, AFP levels fall rapidly, reaching adult levels by age 1-2 years.

Normal adult AFP is <6 ng/mL. Re-elevation in adulthood suggests malignancy or significant liver disease.

Interpreting Elevation Magnitude

The degree of AFP elevation matters:

AFP ≥500 ng/mL almost always indicates malignancy (HCC or germ cell tumor in the right clinical setting)
Mild elevations (<100 ng/mL) are often non-malignant: pregnancy (fetal AFP crosses placenta), cirrhosis (hepatocyte regeneration), hepatitis (liver inflammation/repair)
The “gray zone” (100-500 ng/mL) requires imaging and clinical correlation

Clinical Applications

Hepatocellular carcinoma (HCC): AFP is elevated in 60-80% of HCC cases. Uses:

Surveillance in high-risk patients (cirrhosis, chronic hepatitis B): AFP every 6 months with ultrasound
Diagnosis: AFP >400-500 ng/mL in a patient with cirrhosis and a liver mass is highly suggestive of HCC
Prognosis: Higher AFP correlates with worse outcomes
Monitoring treatment response

Germ cell tumors: AFP is produced by yolk sac tumor elements. In testicular cancer:

Elevated AFP indicates nonseminomatous germ cell tumor (NSGCT) - pure seminomas do NOT make AFP
AFP elevation with a “seminoma” on histology means there are yolk sac elements that were missed (treat as NSGCT)
AFP is part of the testicular cancer staging triad (AFP + hCG + LDH)

Hepatoid variant tumors: AFP can be elevated in hepatoid variants of non-hepatic malignancies - most famously hepatoid gastric carcinoma. These rare tumors morphologically and immunophenotypically mimic hepatocytes. Elevated AFP in a patient without liver disease should prompt consideration of a hepatoid variant of another primary.

Neural tube defects: Maternal serum AFP screening (part of the quad screen) detects open neural tube defects (spina bifida, anencephaly), where AFP leaks from fetal CSF into amniotic fluid and maternal blood. Low AFP in pregnancy is associated with Down syndrome.

19.5 Human Chorionic Gonadotropin (hCG)

Biology

hCG is a glycoprotein hormone produced by trophoblastic tissue. It maintains the corpus luteum during early pregnancy. The molecule consists of two subunits: α (shared with LH, FSH, TSH) and β (unique to hCG).

β-hCG is what we measure - the β subunit provides specificity for hCG.

hCG Forms in Pregnancy

hCG exists in multiple forms, and different forms dominate different compartments:

In blood during pregnancy, the predominant form is hyperglycosylated hCG. This variant promotes implantation and placental invasion.
In urine during pregnancy, the predominant form is the hCG β-core fragment (a degradation product of intact hCG/free β subunit).

This matters for assay design: point-of-care urine pregnancy tests are engineered to detect intact hCG and β-core fragment. Assays with different specificities can give discordant results between serum and urine.

Clinical Applications

Pregnancy testing: hCG is detectable in serum ~8-11 days after conception and doubles every 48-72 hours in early normal pregnancy.

Gestational trophoblastic disease (GTD): Molar pregnancies and choriocarcinoma produce massive amounts of hCG (often >100,000 mIU/mL). hCG is used for diagnosis, staging, and monitoring. After mole evacuation, hCG is followed weekly until undetectable, then monthly to ensure no persistent disease.

Germ cell tumors: Choriocarcinoma elements in testicular or ovarian tumors produce hCG. Used similarly to AFP for staging and monitoring.

hCG in seminoma: ~15% of pure seminomas produce hCG (from syncytiotrophoblast giant cells scattered in the tumor). But pure seminomas NEVER produce AFP. So: hCG alone can still be a pure seminoma, but elevated AFP means the tumor is not a pure seminoma (mixed germ cell tumor or NSGCT).

Prognosis in hCG-producing tumors: hCG ≥50,000 mIU/mL is associated with poor prognosis in gestational trophoblastic disease and germ cell tumors. hCG is part of the IGCCCG risk stratification for testicular cancer.

The hook effect: In sandwich immunoassays, extremely high hCG concentrations can paradoxically produce falsely low results. Excess hCG saturates both capture and detection antibodies independently, preventing sandwich formation. When molar pregnancy or choriocarcinoma is suspected clinically but hCG seems only modestly elevated, request serial dilution to unmask the true level. Rising reported concentrations with each dilution confirm the hook effect.

19.6 CA-125

Biology

CA-125 (cancer antigen 125) is a high-molecular-weight glycoprotein expressed by coelomic epithelium derivatives, including Müllerian duct-derived tissues (ovary, fallopian tube, endometrium) and mesothelial linings (pleura, pericardium, peritoneum). This explains why so many non-ovarian conditions involving serosal irritation elevate CA-125.

Clinical Applications

Epithelial ovarian cancer: CA-125 is elevated in nonmucinous epithelial ovarian neoplasms (serous, endometrioid, clear cell). Mucinous ovarian tumors do NOT express CA-125 - they often express CA 19-9 instead.

CA-125 is elevated in ~80% of advanced ovarian cancer but only ~50% of stage I disease, which is exactly why it’s a poor screening test.

Monitoring known ovarian cancer: CA-125 is most valuable after diagnosis. Falling levels indicate treatment response; rising levels suggest recurrence, often before clinical or imaging evidence.

Prognostic use:

Pre-operative CA-125 >65 U/mL portends an unfavorable prognosis. Higher levels generally track with advanced disease and greater tumor burden. This cutoff is part of the Risk of Malignancy Index (RMI) along with ultrasound findings and menopausal status.
A shorter CA-125 half-life post-chemotherapy (<20 days) predicts improved prognosis, reflecting better tumor response.

Why CA-125 Is a Poor Screening Test

CA-125 has poor positive predictive value for ovarian cancer screening in the general population. Too many benign conditions elevate it, and only ~50% of stage I cancers have elevated levels.

Benign causes of CA-125 elevation:

Endometriosis
Pelvic inflammatory disease
Menstruation (CA-125 rises in the follicular phase)
Pregnancy (first trimester especially)
Uterine fibroids
Benign ovarian cysts
Ascites from any cause (cirrhosis, heart failure)
Any cause of peritoneal/pleural/pericardial inflammation

All of these share the common mechanism of mesothelial or Müllerian-derived tissue irritation.

Physiologic Variation

CA-125 levels also vary within normal individuals:

Lower in postmenopausal women
Lower reported distributions in African American and Asian women
Increases during the follicular phase of the menstrual cycle

Account for these factors before labeling a level “abnormal.”

Clinical Utility in Defined Scenarios

CA-125 is actually useful in two specific settings:

Differentiating benign vs. malignant adnexal masses, especially in postmenopausal women: CA-125 >65 U/mL in a postmenopausal woman with a palpable adnexal mass has >95% PPV for ovarian malignancy.
Monitoring known ovarian cancer for recurrence.

Risk algorithms: Combining CA-125 with ultrasound and applying algorithms (ROCA, ROMA) improves specificity. These may have a role in high-risk women (BRCA carriers), but CA-125 alone is not recommended for screening average-risk women.

19.7 CA 19-9

Biology

CA 19-9 is a carbohydrate antigen - specifically a sialylated Lewis A (Le^a) antigen expressed on mucin glycoproteins. Its synthesis requires a functional FUT3 gene (which produces the Lewis fucosyltransferase).

The Lewis-negative caveat: Approximately 5-10% of the population is Lewis-negative [Le(a-b-)] (higher prevalence in some ancestry groups, including people of African ancestry). These individuals cannot produce CA 19-9 regardless of disease state, because they lack FUT3. A low or undetectable CA 19-9 in a Lewis-negative patient means nothing - always check Lewis status if CA 19-9 is unexpectedly low in a patient who should have a high tumor burden.

Clinical Applications

Pancreatic adenocarcinoma: CA 19-9 is elevated in ~80% of pancreatic adenocarcinoma at presentation. Uses:

Assessing response to treatment (the most validated use) - declining levels during chemotherapy = tumor responding; rising levels = progression
Prognosis: higher preoperative levels predict worse outcomes
Monitoring for recurrence
NOT recommended for screening (too many benign causes of elevation)

CA 19-9 + Clinical Context for Near-100% PPV: When all three of these features are present in a patient with a pancreatic mass, PPV nears 100% for pancreatic adenocarcinoma:

Weight loss >20 lbs
Elevated serum bilirubin >3 mg/dL
Elevated CA 19-9 >37 U/L

With only 2 of 3 features, PPV drops to ~90%. Still powerful diagnostic information.

Biliary tract cancer: Frequently elevated in cholangiocarcinoma.

How High Is High Enough?

The magnitude of CA 19-9 elevation carries information:

CA 19-9 >100 U/mL: only ~3% have benign disease
CA 19-9 >1000 U/mL: virtually 0% have benign disease

Very high CA 19-9 also correlates with unresectable disease and poorer prognosis.

Benign and Other Malignant Elevations

Benign causes: pancreatitis, cholestasis, cholangitis, cirrhosis, biliary obstruction from any cause. Cholestasis in particular can drive impressive CA 19-9 elevation.

Other malignancies with elevated CA 19-9: hepatobiliary, gastric, colorectal, and breast cancers. CA 19-9 is not specific for pancreatic adenocarcinoma.

19.8 Carcinoembryonic Antigen (CEA)

Biology

CEA is an oncofetal glycoprotein involved in cell adhesion. High levels during fetal development, low levels in adult life (produced in small amounts by normal adult GI mucosa), re-expressed in malignancy.

Smoker vs. Non-smoker Reference Ranges

Smoking stimulates CEA production by bronchial epithelium and causes chronic GI mucosal irritation. Median CEA is higher in smokers than non-smokers, and reference ranges differ:

Non-smokers: up to ~2.5 ng/mL
Smokers: up to ~5 ng/mL

Always account for smoking status when interpreting CEA.

Clinical Applications

Colorectal cancer (the classic association): CEA is the primary tumor marker for CRC. Used for:

Preoperative level - prognostic (higher = worse outcome)
Post-resection monitoring - baseline after surgery; rising CEA may detect recurrence 3-6 months before imaging changes
NOT useful for screening - too many benign causes

Other cancers: Also elevated in lung, breast, gastric, and pancreatic cancers, but not used as the primary marker for these.

CEA as a dedifferentiation marker: CEA is elevated in medullary thyroid carcinoma and carcinoid tumors when they become poorly differentiated. In both contexts, rising CEA indicates loss of differentiation and worsening prognosis, even when the “specific” marker (calcitonin for MTC, serotonin for carcinoid) trends differently.

Benign Causes of CEA Elevation

Mild CEA elevation occurs in many non-cancerous conditions:

Peptic ulcer disease
Inflammatory bowel disease
Pancreatitis
Hypothyroidism
Biliary obstruction
Cirrhosis
Smoking

These are typically mild elevations (often <20 ng/mL). Markedly elevated CEA (>20 ng/mL) is more concerning for malignancy.

19.9 Thyroid Cancer Markers

Thyroid cancer has two very different tumor markers depending on the histologic subtype. Know which marker applies to which.

Thyroglobulin (Tg) for Differentiated Thyroid Cancer

What it marks: Tg is the tumor marker for differentiated thyroid carcinomas - papillary and follicular. These cancers retain enough thyroid differentiation to produce Tg. Tg is NOT useful for medullary thyroid carcinoma (from C cells, not follicular) or anaplastic thyroid carcinoma (too dedifferentiated to make Tg).

When it works: Tg monitoring for recurrence only works after total thyroidectomy plus radioactive iodine ablation. Residual normal thyroid tissue produces Tg, which would cause false-positive results and obscure any signal from recurrent cancer. The goal of total ablation is to eliminate all physiologic Tg production so that any detectable Tg must come from tumor.

Pharmacokinetics: Tg half-life is ~65 hours. After total thyroidectomy, Tg takes ~1 month (roughly 11 half-lives, reducing to 0.05% of baseline) to become undetectable. Don’t draw Tg too early post-op - wait at least 6 weeks. Tg is normally cleared by the liver, so hepatic dysfunction can theoretically elevate Tg independent of thyroid disease.

Serial measurement principles:

Serial Tg is checked every 6-12 months during surveillance
Serial measurements must be performed on identical assays - different assays have different calibrations and antibody specificities, so values are not comparable across platforms. Switching assays mid-surveillance can create artificial trends.
Trending is key: a rising Tg across visits is concerning even if individual values fall within the reference range.

Stimulated Tg testing increases sensitivity by driving any residual cells to produce Tg:

Administer recombinant TSH (Thyrogen) to stimulate residual thyroid tissue
OR withhold thyroxine therapy so endogenous TSH rises and drives Tg production

Both methods unmask low-level Tg production from occult recurrence. Thyroxine normally suppresses Tg production through negative feedback; withdrawing it reverses that suppression.

Anti-Thyroglobulin Antibody Interference

The big caveat: anti-thyroglobulin antibodies interfere with Tg immunoassays, and they’re common in this patient population.

Prevalence: ~10% of the general population, ~20% of thyroid cancer patients have anti-Tg antibodies
Mechanism: In immunometric (sandwich) assays, anti-Tg antibodies bind Tg and sequester it from the capture/detection antibodies, so Tg is falsely underestimated
Heterophile antibodies also interfere (~3% of cases)

Workaround: Always measure anti-Tg antibodies alongside Tg. In patients who have anti-Tg antibodies, serial measurement of the antibodies themselves serves as a surrogate marker for Tg. Declining anti-Tg antibody levels over time suggest the antibodies are being consumed by binding to newly produced Tg from recurrent disease. Rising or stable antibodies with no detectable Tg should raise concern.

Calcitonin for Medullary Thyroid Carcinoma

What it marks: Calcitonin is the primary marker for medullary thyroid carcinoma (MTC), which arises from parafollicular C cells. Levels correlate with tumor mass, and calcitonin doubling time has prognostic significance.

Calcitonin synthesis pathway: preprocalcitonin (141 aa) → procalcitonin (116 aa) → calcitonin (32 aa). Each step involves proteolytic cleavage. Calcitonin assays are specific for the mature 32-aa form; normal adults have values <5 ng/L. Assays do NOT detect preprocalcitonin or procalcitonin.

Procalcitonin is a completely different test: Elevated procalcitonin is a marker of bacterial infection/sepsis, NOT thyroid cancer. In bacterial infection, extra-thyroidal tissues (liver, lungs, kidney) release procalcitonin. Procalcitonin is more specific for bacterial infection than CRP and is used to guide antibiotic stewardship. Same protein family, different assay, different clinical meaning - don’t mix them up.

CEA in Medullary Thyroid Carcinoma

MTC has a second marker: CEA. The two-marker approach for MTC is calcitonin + CEA.

CEA in MTC behaves as a marker of dedifferentiation. As the tumor loses its C-cell differentiation, it produces less calcitonin but more CEA. The most ominous pattern is rising CEA with falling calcitonin - the tumor is becoming more aggressive and less differentiated simultaneously. CEA doubling time also has prognostic value.

Causes of Calcitonin Elevation

Primary (direct production):

Medullary thyroid carcinoma
C-cell hyperplasia (precursor to MTC)
Hashimoto thyroiditis
Small cell lung carcinoma
Breast cancer

Secondary (physiologic, indirect):

Chronic renal failure (impaired clearance)
Zollinger-Ellison syndrome (gastrinoma)
Proton pump inhibitor therapy

The ZE/PPI connection: both cause elevated gastrin, and gastrin stimulates thyroid C cells to produce calcitonin. Important to recognize because you don’t want to misdiagnose MTC in a patient on chronic PPIs with mildly elevated calcitonin.

Normal Calcitonin in MEN2 / Familial MTC

Not all MTCs present with elevated calcitonin. Patients with MEN2 or familial MTC can harbor subclinical MTC with normal calcitonin levels. This matters because missing early disease in these high-risk families has lethal consequences.

Two ways to detect subclinical disease:

Calcium provocation testing - measure calcitonin before and after calcium infusion. In C-cell disease, calcitonin rises abnormally high after calcium. Historical workhorse.
RET gene testing - mutations in RET underlie MEN2a, MEN2b, and familial MTC. Genetic testing has largely replaced calcium provocation for screening at-risk family members.

A RET mutation in an at-risk family member drives the decision for prophylactic thyroidectomy to prevent progression to invasive MTC. Different RET mutations carry different timing recommendations for prophylactic surgery.

19.10 β2-Microglobulin

β2-microglobulin (β2M) is a non-specific but clinically useful marker worth understanding.

Biology

β2M is a component of MHC class I molecules. Every nucleated cell and platelet expresses MHC class I, so every nucleated cell contains β2M. When cells die and MHC I is broken down, β2M is released into the extracellular fluid. This makes β2M an indirect readout of cell turnover.

β2M is cleared by the kidneys. Renal insufficiency elevates β2M independent of cell turnover, and dialysis patients can accumulate enough β2M to deposit as dialysis-related amyloidosis.

Clinical Applications

β2M is a non-specific marker elevated with increased cell turnover. It is most notably elevated in hematologic malignancies - lymphoma and multiple myeloma.

Multiple myeloma: β2M is an independent adverse prognostic factor and is part of the International Staging System (ISS) for myeloma. Higher β2M = higher stage = worse prognosis.

Inflammatory states: β2M is also elevated in rheumatoid arthritis, lupus, and inflammatory bowel disease, reflecting increased immune cell turnover. This non-specificity limits diagnostic utility but makes β2M useful for monitoring disease activity in these conditions.

19.11 Alkaline Phosphatase as a Bone Tumor Marker

Alkaline phosphatase (specifically the bone isoform) serves as a tumor marker for bone lesions:

Osteogenic sarcomas (any sarcoma with increased bone formation - osteoblastoma, osteoblastic osteosarcoma)
Bone metastases

Osteoblastic activity produces ALP. Also elevated in Paget’s disease, healing fractures, and growing children. Bone-specific ALP can be distinguished from liver ALP by isoenzyme analysis or heat stability (bone is heat-labile; liver is heat-stable - “bone burns, liver lasts”).

19.12 Carcinoid Tumors and Carcinoid Syndrome

Carcinoid tumors are neuroendocrine neoplasms that often secrete bioactive substances. The biochemistry of their secretion drives both the clinical syndrome and the lab workup.

Serotonin Metabolism

Carcinoid cells produce serotonin from tryptophan (tryptophan → 5-hydroxytryptophan → serotonin). Once released into the circulation, serotonin is handled two ways:

Platelets take up serotonin from plasma and store it in dense granules. Platelet serotonin (whole blood serotonin) reflects cumulative serotonin production, unaffected by dietary fluctuation.
Free serotonin is metabolized in the renal tubules to 5-HIAA (5-hydroxyindoleacetic acid) by monoamine oxidase (MAO) followed by aldehyde dehydrogenase, acting sequentially. 5-HIAA is the final urinary metabolite.

Carcinoid Syndrome

Carcinoid syndrome develops when tumor products bypass hepatic metabolism. Normally, serotonin from GI carcinoids drains into the portal vein and is inactivated in the liver. Carcinoid syndrome therefore typically requires liver metastases (serotonin enters the systemic circulation directly) or a carcinoid originating outside the portal drainage (bronchial).

Clinical tetrad - “Be FDR”:

Bronchospasm
Flushing
Diarrhea
Right-sided heart failure (endocardial fibrosis from chronic serotonin exposure of right heart valves)

Lab Workup for Carcinoid

Match the test to the question:

Question	Best Test
Carcinoid TUMOR (burden, treatment response)	Serum chromogranin A
Carcinoid SYNDROME, serum	Platelet serotonin
Carcinoid SYNDROME, urine	24-hr urine 5-HIAA or serotonin
Dedifferentiation in carcinoid	CEA

Urine 5-HIAA and serotonin: Classic 24-hour urine test. Good specificity, variable sensitivity by tumor location. Falsely increased by serotonin-rich foods (bananas, pineapples, tomatoes, walnuts, avocados) or 5-hydroxytryptophan supplements - ingested serotonin and 5-HTP feed directly into the 5-HIAA excretion pathway. Patients must follow dietary restrictions 2-3 days before collection.

Chromogranin A (CgA): Released from neuroendocrine secretory granules. The best serum tumor marker for carcinoid and other neuroendocrine tumors because it’s elevated regardless of secretory status (so it works for non-secretory carcinoids too). Used for tumor burden and treatment response. False positives from PPI therapy and renal failure.

Platelet serotonin: The best serum marker for carcinoid syndrome. Because platelets take up serotonin at a constant rate, platelet serotonin reflects cumulative production and is unaffected by diet, unlike urine 5-HIAA or serotonin.

Other neuroendocrine peptides produced by carcinoids: synaptophysin, pancreatic polypeptide, neuropeptide K. These confirm neuroendocrine differentiation but are less commonly used clinically than CgA.

Anatomic Location Matters

Foregut and hindgut carcinoids often have normal urinary 5-HIAA and serotonin (~20-30% of cases), which can mislead the unwary. Product profile by location:

Foregut (stomach, duodenum, lung): produce serotonin, histamine, and catecholamines. Histamine can cause atypical carcinoid syndrome with dry flushing rather than wet flushing. 5-HIAA may be normal because serotonin metabolism differs.
Midgut (small bowel, appendix, proximal colon): produce only serotonin. These are the classic carcinoids causing typical carcinoid syndrome with elevated 5-HIAA. Most common site for carcinoid syndrome.
Hindgut (distal colon, rectum): usually non-secretory. Produce nothing measurable. Found incidentally on colonoscopy. Urinary 5-HIAA and serotonin are normal; CgA may still be elevated.

In foregut/hindgut tumors with negative urine studies, use chromogranin A as the serum marker instead.

19.13 Pheochromocytoma and Paraganglioma

Anatomy and Biochemistry

Tumors of chromaffin cells. Location defines the name:

Pheochromocytoma = chromaffin cell tumor within the adrenal medulla
Paraganglioma = chromaffin cell tumor outside the adrenal medulla

The anatomic distinction matters biochemically because only the adrenal medulla has phenylethanolamine N-methyltransferase (PNMT), the enzyme that converts norepinephrine to epinephrine.

Pheochromocytoma: produces norepinephrine AND epinephrine
Paraganglioma: produces norepinephrine only (lacks PNMT)

This biochemical split helps localize the tumor: if metanephrine (the epi metabolite) is elevated, the tumor is adrenal. If only normetanephrine is elevated, the tumor is extra-adrenal.

Catecholamine Metabolism

Each catecholamine has characteristic metabolites:

NE → normetanephrine and vanillylmandelic acid (VMA)
Epi → metanephrine and VMA
Dopamine → homovanillic acid (HVA)

Metanephrines (normetanephrine + metanephrine) are the most sensitive markers for pheochromocytoma/paraganglioma. VMA is less sensitive. HVA is used for neuroblastoma screening, not pheo.

Screening Tests

The best screening tests are:

Plasma free metanephrines (highest sensitivity, ~96-99%), drawn with the patient supine for 20+ minutes to reduce false positives
24-hour urine catecholamines and metanephrines

Plasma catecholamines alone are NOT recommended because catecholamine release is episodic - levels may be normal between paroxysms. Metanephrines are continuously produced by COMT within the tumor itself, so they stay elevated regardless of the patient’s symptoms at the moment of draw.

Medication and Other Interferences

Several medications can cause false-positive pheo screening:

Tricyclic antidepressants (imipramine)
MAO inhibitors
Nitroglycerin
Reserpine
Acetaminophen (interferes with HPLC methods)
Caffeine, stress, sleep apnea

Review the medication list before interpreting results. Many of these can be stopped safely for 1-2 weeks before testing.

Clonidine Suppression Test

For patients with equivocal screening results: clonidine suppression test. Clonidine is an α2-agonist that normally suppresses sympathetic NE release from the brainstem. In pheochromocytoma/paraganglioma, catecholamine production is autonomous and does not suppress.

Normal response: plasma NE falls after clonidine = no pheo
Positive test: plasma NE fails to suppress = pheo

19.14 Neuroblastoma Markers

Neuroblastoma is a pediatric catecholamine-producing tumor derived from sympathetic neuroblasts. It has its own workup distinct from pheo.

Urine Screening

Neuroblastoma screening uses urinary VMA and HVA, NOT metanephrines or catecholamines. Why: neuroblastoma produces immature catecholamine metabolites, and VMA/HVA are more sensitive than metanephrines for this specific tumor.

VMA = metabolite of NE/epinephrine
HVA = metabolite of dopamine

VMA:HVA Ratio as Prognostic Marker

Poorly differentiated neuroblastomas produce more HVA (dopamine metabolite) than VMA. Well-differentiated tumors produce more mature catecholamines and therefore more VMA. So a low VMA:HVA ratio = worse prognosis, reflecting loss of maturation along the catecholamine synthesis pathway.

Non-specific Markers

Other markers that track neuroblastoma activity:

Neuron-specific enolase (NSE) - reflects neuronal origin
Lactate dehydrogenase (LDH) - tumor burden, adverse prognostic factor
Ferritin - adverse prognostic factor

MYCN amplification (genetic, not serologic) is the single most important molecular prognostic factor in neuroblastoma.

19.15 Urothelial Carcinoma Markers

Urothelial (transitional cell) carcinoma can arise in the bladder, urethra, ureters, or renal pelvis. Two urine markers are used for surveillance of bladder cancer recurrence:

Nuclear Matrix Protein 22 (NMP22)

NMP22 detects the nuclear mitotic apparatus protein, released from dying tumor cell nuclei. Present in urine of patients with active urothelial carcinoma.

Mainly used for surveillance in patients with a history of bladder cancer between cystoscopy appointments
Can be run as a point-of-care test
Sensitive but not very specific

Bladder Tumor Antigen (BTA)

BTA detects complement factor H (and related proteins) in urine. Urothelial carcinoma cells produce complement factor H to evade complement-mediated immune destruction, so its presence in urine suggests active tumor.

False Positives

Both NMP22 and BTA produce false positives in conditions of increased urinary tract cell turnover:

UTI (dying inflammatory cells)
Instrumentation (catheterization, cystoscopy)
Kidney stones
Hematuria from any cause

Neither marker replaces cystoscopy as the gold standard for bladder cancer surveillance. They’re adjuncts, not substitutes.

19.16 Goodpasture Syndrome and Anti-GBM Antibody

Not a cancer marker, but it shows up in the tumor marker tags and is worth a quick inclusion: the young patient with hemoptysis + hematuria + progressive renal failure.

Diagnosis: anti-glomerular basement membrane (anti-GBM) antibody targets the α3 chain of type IV collagen, which is expressed in both glomerular and alveolar basement membranes
Renal biopsy: linear IgG on immunofluorescence
Treatment: plasmapheresis + immunosuppression

Chapter 20: Protein Electrophoresis and Immunofixation

Serum protein electrophoresis is one of the most interpretively rich tests in laboratory medicine. The pattern of protein bands tells a story about inflammation, liver function, renal losses, and clonal B-cell proliferation. Understanding what each band represents physiologically is the key to interpretation.

20.1 The Principles of Protein Electrophoresis

How It Works

Proteins are amphoteric molecules with both acidic and basic groups. At a given pH, each protein has a net charge determined by its amino acid composition. In an electric field, proteins migrate toward the electrode opposite to their charge.

Serum protein electrophoresis (SPEP) separates proteins in serum on an agarose gel at pH 8.6. At this pH, most serum proteins are negatively charged and migrate toward the anode (positive electrode). Proteins separate into bands based on their charge-to-mass ratio.

After separation, the gel is stained with a protein dye and scanned densitometrically to produce a tracing showing protein concentration at each position.

Two Opposing Forces: Electrophoretic Pull and Endosmosis

The final position of a band is set by a tug-of-war between two forces:

Electrical force: pulls negatively charged proteins toward the anode. The more negative the protein, the stronger the pull. Prealbumin and albumin are the most negatively charged, so they migrate the fastest toward the anode.
Endosmosis (electroendosmotic flow): the gel matrix itself carries a slight negative charge. That attracts buffer cations, and bulk solvent flow drags weakly charged proteins back toward the cathode.

γ-globulins have only a weak net negative charge. The endosmotic force overwhelms their feeble electrophoretic pull, and they end up drifting toward the cathode. That’s why the γ region ends up on the opposite end of the gel from albumin despite technically still being acidic proteins.

Capillary Zone Electrophoresis

Most older labs run SPEP on agarose gel. Newer platforms use capillary zone electrophoresis (CZE), which runs the separation inside a narrow capillary instead of a flat gel. CZE gives higher resolution, better detection of small M-proteins, and is increasingly the standard. The interpretive patterns are the same; the hardware is different.

The Five Major Zones

Moving from anode to cathode (most to least negatively charged), the full migration order is: prealbumin → albumin → α1 → α2 → β → γ.

Prealbumin (transthyretin) is the fastest-migrating band, just ahead of albumin. It’s usually faint or absent on routine serum SPEP but becomes prominent on CSF electrophoresis because transthyretin is actively secreted into CSF by the choroid plexus and also crosses the blood-brain barrier. Prealbumin binds ~10% of circulating T4/T3 and carries the retinol-binding protein/vitamin A complex. It’s also the protein that misfolds in transthyretin amyloidosis (ATTR). Half-life is ~48 hours, which makes it the best laboratory marker of recent nutritional status.

Albumin: The largest and most prominent peak, representing ~60% of total serum protein (normal 3.5-5.5 g/dL). Albumin migrates fastest of the major bands because it is small and highly negatively charged at pH 8.6. It’s made exclusively by the liver, has a half-life of ~17 days, and serves as an oncotic buffer plus a carrier for drugs, hormones, and bilirubin. Albumin is a negative acute-phase reactant: during inflammation, the liver reprioritizes synthesis toward positive APRs and albumin drops. The slow half-life makes it a late, confounded marker of nutritional status; prealbumin is better.

α1-globulins: The first small peak after albumin. Major components:

α1-antitrypsin (AAT): The dominant protein in this zone; a serine protease inhibitor (hence the SERPIN family) that protects lung tissue from neutrophil elastase. Positive acute-phase reactant. Encoded by SERPINA1.
α1-acid glycoprotein (orosomucoid): Positive acute-phase reactant (rises 2-5× in inflammation). Binds basic and neutral drugs. Lives at the α1-α2 interface rather than strictly in α1.
HDL (α1-lipoprotein) migrates at the albumin-α1 interface.

α1-α2 interface: Not a “zone” on reports, but worth knowing. Contains:

Gc globulin (vitamin D binding protein): transports 25-OH-D and 1,25-(OH)₂-D, and scavenges actin from damaged cells
α1-antichymotrypsin: positive APR; binds 75-90% of serum PSA
α1-acid glycoprotein

α2-globulins: The second small peak. Major components:

α2-macroglobulin: A very large protein (720 kDa) that cannot pass through the glomerulus. Relatively preserved while smaller proteins are lost in nephrotic syndrome, and the liver actually ups its synthesis to compensate for low oncotic pressure - it can look ~10-fold increased on a nephrotic SPEP.
Haptoglobin: Binds free hemoglobin only, not myoglobin. The complex is cleared by the liver/spleen. Consumed in hemolysis, so low haptoglobin is an early hemolysis marker. Rhabdomyolysis does not consume haptoglobin - use CK instead. Positive APR (which can mask hemolytic consumption when both are happening).
Ceruloplasmin: Copper transport; carries ~95% of serum copper. Positive APR. Decreased in Wilson disease (ATP7B mutation), Menkes syndrome (ATP7A mutation), hepatic failure, and malnutrition. Because it’s a positive APR, ceruloplasmin can be falsely normal during concurrent inflammation, masking Wilson disease.

α2-β interface: Normally empty. If a band appears here, think free hemoglobin from hemolysis (pre-analytical or in vivo) - this can masquerade as a pseudo-M-spike.

β-globulins: Often appears as two peaks (β1 and β2). Major components:

β1: Transferrin - iron transport protein; the dominant β1 protein. Binds ferric (Fe³⁺) iron (two ions per molecule). Normally ~30% saturated; total iron-binding capacity ≈ transferrin concentration. Low saturation = iron deficiency; high saturation = iron overload. Transferrin is a negative acute-phase reactant. It increases in iron deficiency, pregnancy, and estrogen therapy. In iron deficiency, the β1 band can become prominent enough to mimic an M-spike.
β1-β2 interface: β-lipoprotein (LDL).
β2: IgA, C3, and fibrinogen (the last only if the specimen was incompletely clotted).

γ-globulins: The most cathodal zone. Split into γ1 and γ2:

γ1: contains the bulk of immunoglobulins (IgG, IgA, IgM, IgD, IgE). Because immunoglobulins are produced by many different plasma cell clones with slightly different sequences, normal γ-globulin appears as a broad, diffuse band (polyclonal).
γ2: CRP lives here. CRP is the prototypical positive APR and can rise 100-1000×, so a prominent γ2 band in heavy inflammation can look like a spike. CRP has mild seasonal variation with the highest levels in winter, likely reflecting increased respiratory infections and cold-weather inflammation.

Acute Phase Reactants: Quick Reference

Knowing which proteins go up and which go down in inflammation is the single most useful thing for reading SPEP. The table below should be memorizable cold.

Protein	APR direction	SPEP zone
Albumin	Negative	Albumin
Prealbumin (transthyretin)	Negative	Prealbumin
Transferrin	Negative	β1
α1-antitrypsin	Positive	α1
α1-acid glycoprotein	Positive	α1-α2 interface
α1-antichymotrypsin	Positive	α1-α2 interface
Haptoglobin	Positive	α2
Ceruloplasmin	Positive	α2
α2-macroglobulin	Positive (relative)	α2
C3	Positive	β2
Fibrinogen	Positive	β-γ (plasma only)
CRP	Positive	γ2

Two pitfalls fall out of this:

Haptoglobin in inflamed patients who also hemolyze: haptoglobin is consumed by hemolysis but also elevated by inflammation. The two can cancel, producing a deceptively “normal” haptoglobin. If hemolysis is clinically likely but haptoglobin is normal, you may be looking at concurrent inflammation masking consumption.
Iron studies in inflammation: transferrin drops (negative APR) while ferritin rises (positive APR). Serum iron also drops. The overall picture can look like iron deficiency despite adequate iron stores - this is anemia of chronic disease, and ferritin is the distinguishing clue.

Congenital Albumin Variants

Two named conditions can confuse SPEP readers:

Analbuminemia: congenital absence of albumin. Presents with surprisingly mild edema and compensatory hyperlipidemia (the liver ramps up lipoprotein synthesis to maintain oncotic pressure). Symptoms are mild because other serum proteins partly compensate.
Bisalbuminemia (alloalbuminemia): two distinct albumin peaks on SPEP from heterozygosity for different albumin alleles. Most people are homozygous for albumin A, the common allele. Rare albumin variants carry amino acid substitutions that don’t change function but shift mobility. Usually clinically insignificant. The two peaks are in the albumin region, not γ, which helps distinguish from an M-spike. Bisalbuminemia can also be acquired (pancreatitis, high-dose penicillin).

α1-Antitrypsin Deficiency on SPEP

A flat or absent α1 band on SPEP is a clue to AAT deficiency. The classic clinical story is early-onset panacinar emphysema (especially in smokers) plus liver disease from intrahepatic accumulation of misfolded AAT.

Genotypes (PI = protease inhibitor system, classified by mobility on isoelectric focusing):

PiMM: normal, 100% activity
PiMZ: heterozygous, ~60% activity, usually asymptomatic
PiZZ: severe deficiency, ~15% activity. Emphysema plus PAS-positive, diastase-resistant hepatocyte globules.

M = medium mobility (normal), S = slow, Z = very slow. The Z variant polymerizes in hepatocytes - that’s what causes the liver disease.

SPEP is a rapid screen. Confirm with direct AAT level measurement, PI typing by isoelectric focusing, or SERPINA1 genotyping.

20.2 Interpreting SPEP Patterns

The power of SPEP lies in pattern recognition. Each disease state produces a characteristic pattern.

SPEP normal bands: From anode (+) to cathode (-): Albumin, α1, α2, β, γ. Each zone contains specific proteins with characteristic clinical significance.

Acute Inflammation Pattern

What you see: Decreased albumin, increased α1 and α2 globulins

Why it happens: The liver reprioritizes protein synthesis during acute inflammation. Albumin production decreases (albumin is a “negative” acute-phase reactant) while acute-phase proteins in the α1 and α2 zones increase. The most visibly affected bands on routine SPEP are the α1 (AAT, α1-acid glycoprotein) and α2 (haptoglobin, ceruloplasmin, α2-macroglobulin). CRP and other γ2-region APRs also rise but contribute less to the overall visual pattern. Prealbumin also drops but is usually too faint on serum SPEP to appreciate. This shift happens within hours of an inflammatory stimulus.

Common causes: Infection, surgery, trauma, malignancy, autoimmune flares

Chronic Inflammation Pattern

What you see: Polyclonal increase in γ-globulins (broad-based elevation), often with decreased albumin

Why it happens: Chronic antigenic stimulation activates many different plasma cell clones, each producing slightly different immunoglobulins. The result is a diffuse, broad-based increase across the entire γ region, in contrast to the sharp narrow spike of a monoclonal process.

Common causes: Chronic infections (HIV, hepatitis, tuberculosis), autoimmune diseases (SLE, rheumatoid arthritis), cirrhosis

Nephrotic Syndrome Pattern

What you see: Markedly decreased albumin, relatively increased α2-globulins, decreased γ-globulins

Why it happens: The damaged glomerulus leaks proteins based on size. Albumin (66 kDa) and immunoglobulins (150-900 kDa) are lost in urine. α2-macroglobulin (720 kDa) is too large to pass through even a damaged glomerulus, so it’s retained - and the liver actually increases its synthesis to compensate for the low oncotic pressure. The absolute α2-macroglobulin concentration may be unchanged or even slightly decreased, but it looks 10-fold elevated relative to other bands on the densitometric trace. Don’t mistake this tall α2 peak for a pseudo-M-spike. Classic nephrotic syndrome also produces compensatory hyperlipidemia (increased β-lipoprotein).

Cirrhosis Pattern

What you see: Decreased albumin, β-γ bridging (fusion of β and γ zones), blunted α1 and α2 bands

Why it happens: The cirrhotic liver produces less albumin and reduces its output of the α-region APRs (AAT, haptoglobin, ceruloplasmin) - hence blunted α1 and α2 bands. Meanwhile, impaired hepatic clearance of antigens from the gut allows chronic antigenic stimulation, producing polyclonal immunoglobulin elevation. Kupffer cell dysfunction specifically impairs IgA catabolism, so serum IgA rises. IgA migrates at the β-γ junction, filling in the normal valley between these zones and producing the classic β-γ bridging. This pattern is a board-favorite association for cirrhosis.

Monoclonal Gammopathy Pattern

What you see: A sharp, discrete spike (M-spike or M-protein), also called a paraprotein. Location: most commonly γ (IgG), sometimes β or α2 (IgA).

Why it happens: A single clone of plasma cells has expanded and produces a single, homogeneous immunoglobulin. Because all molecules are identical, they migrate to exactly the same position, creating a sharp spike rather than a diffuse band.

M-spike on SPEP: A sharp monoclonal peak in the gamma region indicates clonal plasma cell proliferation. Requires immunofixation to characterize.

M-protein composition, most to least common:

Intact antibody (2 heavy + 2 light chains, i.e. full IgG/IgA/IgM): most common
Light chain only (~16% of myeloma cases; called Bence Jones when in urine): may produce no visible SPEP spike because free light chains are small and rapidly filtered into urine. UPEP and serum free light chains catch these.
Heavy chain only: rare. Heavy chain diseases are named by the chain (α, γ, μ).

Immunoglobulin subtype frequency in multiple myeloma: IgG > IgA > IgD > IgM (IgM paraprotein usually means Waldenström/LPL, not myeloma). Light chain only myeloma in ~16%.

Critical point: An M-spike on SPEP indicates a monoclonal protein is present but doesn’t tell you what it is. You need immunofixation (IFE) to characterize heavy chain class and light chain type. The paraprotein itself is quantitated by nephelometry or turbidimetry (nephelometry measures scattered light; turbidimetry measures transmitted light through a turbid solution), not by densitometry of the SPEP gel.

Pseudo M-Spikes

Not every sharp band in the β or γ region is a paraprotein. A number of non-monoclonal proteins can form a tight band at a specific migration position and mimic an M-spike. You should know where each one lives and the clinical scenario that produces it:

Position	Protein	Scenario
α2-β interface	Hemoglobin	Hemolysis (in vivo or pre-analytical)
β1	Transferrin	Iron deficiency (transferrin upregulated)
β2	C3	Acute inflammation
β-γ	Fibrinogen	Incompletely clotted specimen (plasma contamination of serum)
γ2	CRP	Severe inflammation

Other causes to keep in the back pocket:

Certain antibiotics (some cephalosporins) and radiocontrast agents can form sharp bands
Very high levels of serum tumor markers
Therapeutic monoclonal antibodies (see daratumumab below)

How to tell the difference: the short answer is always IFE. A true paraprotein produces a discrete band in one heavy chain lane and one light chain lane. A pseudo-M-spike shows either no band on IFE or a broad polyclonal smear. If the sample looks like plasma and the spike is at β-γ, request a properly clotted re-draw before calling it a paraprotein.

20.3 Immunofixation Electrophoresis (IFE)

IFE is the reflex test when SPEP shows an M-spike (or is suspicious for a small M-spike). It identifies the specific heavy chain class (IgG, IgA, IgM) and light chain type (κ or λ) of the monoclonal protein.

IFE is the most commonly used confirmatory method. Two related techniques exist:

Immunofixation (IFE): the standard. Antisera applied to a gel after electrophoresis.
Immunotyping (IT): same principle but integrated with capillary electrophoresis instead of gel.
Immunoelectrophoresis (IEP): the old technique using immunodiffusion - largely obsolete.

The Technique

Serum is applied to 6 parallel lanes on a gel and electrophoresed. Then each lane is overlaid with a different reagent:

Lane 1: fixative alone (total protein reference, equivalent to a mini-SPEP)
Anti-γ (IgG heavy chain)
Anti-α (IgA heavy chain)
Anti-μ (IgM heavy chain)
Anti-κ (kappa light chain)
Anti-λ (lambda light chain)

Each antiserum precipitates its target protein in place. The rest of the proteins are washed away. The retained immune complexes are then stained. A monoclonal protein appears as a discrete band in one heavy chain lane and one light chain lane at the same migration position.

On a normal IFE, each lane shows diffuse polyclonal staining - a broad smear, not a discrete band - reflecting the normal diverse repertoire of immunoglobulins.

Interpreting IFE

IgG kappa example: A band appears in the anti-γ lane and the anti-κ lane at the same position. Interpretation: IgG kappa monoclonal protein.

Light chain disease: A band appears in the κ or λ lane but not in any heavy chain lane. This indicates free monoclonal light chains without attached heavy chains - seen in light chain myeloma and AL amyloidosis.

Biclonal gammopathy: Two separate bands, representing two independent clones. True biclonal gammopathy is rare (~3% of plasma cell dyscrasias). Most apparent biclonality is artifact. The two common culprits are:

IgA pseudo-biclonality: IgA can exist as both monomers and dimers, which migrate at different positions and produce two bands of the same heavy/light chain type. Treat the sample with β-mercaptoethanol (a reducing agent that breaks the J-chain disulfide bonds between dimers) and re-run. If the two bands collapse into one, you have pseudo-biclonality from IgA dimerization, not a true second clone.
Therapeutic monoclonal antibody interference: see below.

True biclonal gammopathy shows two bands with different heavy and/or light chain assignments (e.g., IgG-κ and IgA-λ).

Daratumumab and Other Therapeutic Monoclonal Antibody Interference

Daratumumab is an anti-CD38 IgG-κ monoclonal antibody used to treat multiple myeloma. Because it’s literally a monoclonal IgG-κ, it shows up on SPEP and IFE as an IgG-κ band. In a patient previously diagnosed with IgG-κ myeloma who is now in apparent response (normal IgG, normal κ levels, doing well clinically) but still has an IFE band, the most likely explanation is residual drug, not residual disease. This also produces apparent biclonality in patients whose original clone was a different isotype (e.g., IgA-λ myeloma + daratumumab = apparent IgA-λ + IgG-κ biclonal pattern). Always check treatment history when reading IFE on myeloma patients. The same principle applies to other therapeutic monoclonal antibodies (elotuzumab, isatuximab, rituximab, etc.).

Initial Screening for Monoclonal Gammopathy

The 2021 CAP guideline recommends that the best initial screen for a suspected monoclonal gammopathy is SPEP plus serum free light chains (sFLC). The combination identifies 94.3% of M-proteins, including 100% of multiple myeloma, 100% of Waldenström, and 99.5% of smoldering myeloma cases. SPEP alone misses light chain only myeloma because the light chains are filtered into urine too quickly to produce a visible serum spike. Ordering SPEP + sFLC together catches the ones that SPEP alone would miss. Add UPEP and urine IFE when needed (especially for confirming Bence Jones proteinuria or characterizing light chains in AL amyloidosis).

20.4 Serum Free Light Chains

This assay measures light chains that are circulating free (unbound to heavy chains), not those incorporated into intact immunoglobulins.

Why This Test Exists

Normal plasma cells produce a slight excess of light chains over heavy chains. These free light chains are small (~25 kDa) and rapidly filtered by the glomerulus, then reabsorbed by the proximal tubule. Serum levels are normally low.

In plasma cell neoplasms, the malignant clone may produce:

Intact immunoglobulin + excess free light chains
Free light chains only (light chain myeloma)
Little or no measurable protein (nonsecretory myeloma)

Free light chain testing is more sensitive than SPEP for detecting small amounts of monoclonal light chains, especially in:

Light chain myeloma (no intact immunoglobulin to see on SPEP)
AL amyloidosis (often low levels of monoclonal protein)
Oligosecretory myeloma (minimal protein production)

The κ/λ Ratio

Polyclonal plasma cell expansion produces both κ and λ light chains in proportion, maintaining a normal ratio (~0.26-1.65, laboratory dependent).

Clonal plasma cell expansion produces either κ or λ light chains (whichever the clone happens to make), skewing the ratio:

Elevated κ/λ ratio: κ-producing clone
Decreased κ/λ ratio: λ-producing clone

Renal impairment caveat: Free light chains are cleared by the kidneys. In renal failure, both κ and λ accumulate, but κ accumulates more (it’s cleared faster normally). The reference range for patients with eGFR <60 is wider (~0.37-3.1).

20.5 Urine Protein Electrophoresis (UPEP)

UPEP uses the same electrophoretic principle applied to a concentrated urine sample. Three patterns dominate interpretation, each with a distinct mechanism.

Glomerular Proteinuria

Mechanism: damaged glomerular basement membrane allows large proteins (>65,000 daltons) to leak into urine. Seen in glomerulonephritis and nephrotic syndrome.

Pattern: prominent albumin, β1, and β2 bands. Albumin dominance is the key feature.

Tubular Proteinuria

Mechanism: the glomerulus is intact, but the proximal tubule can’t reabsorb low molecular weight proteins that normally pass through the glomerulus and get recaptured upstream. Causes include acute tubular necrosis, interstitial nephritis, and Fanconi syndrome.

Pattern: small albumin band (glomeruli intact, so little albumin leak) with prominent α2 and β2 bands (low MW proteins like β2-microglobulin and α1-microglobulin that the tubule failed to reabsorb).

The small-albumin-with-prominent-α2/β2 signature is what distinguishes tubular from glomerular proteinuria.

Overflow Proteinuria

Mechanism: plasma concentration of a specific protein is so high that it exceeds the normal tubular reabsorptive capacity, even though the kidney itself is working fine. The problem is upstream.

Most common causes:

Bence Jones proteinuria (monoclonal light chains from myeloma)
Myoglobinuria (rhabdomyolysis)
Hemoglobinuria (intravascular hemolysis)

Clinical clue: a urine dipstick only detects albumin. A patient with Bence Jones proteinuria may have a negative or mild dipstick despite massive light chain excretion on UPEP. If you’re screening for myeloma in a patient with renal failure, dipstick is not enough - order UPEP with urine IFE.

20.6 CSF Electrophoresis

CSF protein electrophoresis has two clinical uses you should know cold.

Confirming CSF Leak

Fluid leaking from the nose (rhinorrhea) or ears (otorrhea) after head trauma or skull-base surgery may or may not be CSF. Electrophoresis of the fluid settles it. Two hallmarks confirm CSF:

Double transferrin peak: transferrin normally migrates as one band in the β1 region. In CSF, a fraction is enzymatically desialylated (sialic acid removed) as it crosses the blood-brain barrier, producing a second band called β2-transferrin (also known as τ protein). β2-transferrin is highly specific for CSF and is the gold standard marker for CSF leaks.
Prominent prealbumin: transthyretin is actively secreted into CSF by the choroid plexus, so the prealbumin band is much more conspicuous in CSF than in serum.

A fluid showing both features is CSF.

Oligoclonal Bands and Multiple Sclerosis

Standard SPEP won’t catch the small monoclonal populations relevant to neurologic disease. High-resolution CSF electrophoresis (typically isoelectric focusing) can resolve oligoclonal bands: two or more discrete bands in the γ region reflecting intrathecal immunoglobulin synthesis.

For multiple sclerosis, the clinically relevant finding is oligoclonal bands in CSF that are not present in paired serum. That mismatch indicates the bands are being made inside the CNS, not spilling over from systemic circulation. Oligoclonal bands are found in ~90% of MS patients but are not specific to MS - they also appear in neurosyphilis, Lyme disease, and SSPE.

20.7 Other Transferrin Variants

Carbohydrate-Deficient Transferrin (CDT)

CDT is a marker of chronic heavy alcohol use. Alcohol inhibits glycosyltransferases, so transferrin produced during chronic intake has fewer carbohydrate side chains than normal. The threshold for detection is roughly ≥60 g/day of ethanol for ≥2 weeks. CDT has better specificity than GGT for chronic alcohol use and returns to baseline after 2-4 weeks of abstinence.

20.8 Cryoglobulinemia

Cryoglobulins are immunoglobulins that precipitate reversibly at cold temperatures and redissolve on warming. The clinically important consequence is that specimens must be collected and transported at 37°C, otherwise cryoglobulins precipitate in the tube, get centrifuged out of the serum, and are lost. A cold-handled specimen gives a false negative.

The Three Types

Type	Composition	Most common association
I	Monoclonal Ig alone	Plasma cell / lymphoid neoplasm
II	Monoclonal IgM with RF activity + polyclonal IgG	Hepatitis C (most common cryo overall)
III	Two polyclonal Igs (typically IgG + IgM)	Chronic infection, autoimmune disease

Types II and III are called mixed cryoglobulinemias because they contain more than one immunoglobulin class. Type I is simple (single monoclonal Ig).

In Type II, the monoclonal IgM has rheumatoid factor activity - it binds the Fc region of polyclonal IgG. That IgM-IgG complex is what precipitates in the cold and deposits in small-vessel walls.

Clinical Features

Meltzer’s triad of cryoglobulinemia:

Palpable purpura (leukocytoclastic vasculitis from immune complex deposition in dermal vessels)
Arthralgias
Weakness

Other features: sensorimotor neuropathy, hepatosplenomegaly, lymphadenopathy, anemia. Renal biopsy shows membranoproliferative glomerulonephritis (MPGN) with the classic “tram-tracking” appearance on silver stain from duplication of the GBM.

Labs

Mixed cryoglobulinemia consumes complement: low C3 and C4, because the IgM-IgG immune complexes activate the classical pathway. The combination of palpable purpura + low complement + positive hepatitis C serology is the classic board presentation.

Treatment of the underlying cause matters: direct-acting antivirals for HCV usually resolve the cryoglobulinemia.

20.9 Clinical Applications: Plasma Cell Dyscrasias

Multiple Myeloma

Multiple myeloma is a malignant proliferation of plasma cells producing monoclonal immunoglobulin. The diagnosis requires demonstrating clonal plasma cells AND evidence of myeloma-related organ damage or biomarkers of malignancy.

A key point that confuses trainees: despite a high total immunoglobulin from the M-protein, patients with myeloma are immunocompromised. The malignant clone suppresses normal B-cell function, so polyclonal (normal) immunoglobulin production is decreased (immunoparesis). Infections are the leading cause of death in myeloma.

Beyond plasma cell dyscrasia, other B-cell neoplasms can also produce monoclonal immunoglobulin and show an M-spike: lymphoplasmacytic lymphoma (Waldenström), marginal zone lymphoma, and CLL/SLL.

Diagnostic Criteria (IMWG):

≥10% clonal bone marrow plasma cells OR biopsy-proven plasmacytoma PLUS one or more of:

CRAB criteria (myeloma-defining organ damage):

Calcium: >11 mg/dL (or >1 mg/dL above normal)
Renal: Creatinine >2 mg/dL (or CrCl <40 mL/min)
Anemia: Hemoglobin <10 g/dL (or >2 g/dL below normal)
Bone: ≥1 lytic lesion on skeletal survey, CT, or PET-CT

SLiM criteria (biomarkers predicting progression):

Sixty: ≥60% clonal plasma cells in bone marrow
Light chain ratio: Involved/uninvolved serum free light chain ratio ≥100
MRI: >1 focal lesion on MRI (≥5 mm)

The SLiM criteria allow treatment of “smoldering” myeloma that has high-risk features, before CRAB damage occurs.

MGUS (Monoclonal Gammopathy of Undetermined Significance)

MGUS is a premalignant condition found in ~3% of people over 50. Definition:

M-protein <3 g/dL
Clonal plasma cells <10% of bone marrow
No CRAB or SLiM criteria
No other B-cell lymphoproliferative disorder

Risk of progression: ~1% per year transform to myeloma or related disorder. Risk stratification uses M-protein size, type (IgG lower risk than non-IgG), and free light chain ratio.

Waldenström Macroglobulinemia

A lymphoplasmacytic lymphoma producing monoclonal IgM. The high molecular weight of IgM pentamers (~900 kDa) causes hyperviscosity syndrome at high concentrations: blurred vision, headache, epistaxis and mucosal bleeding, neurological symptoms, retinal hemorrhages on fundoscopy. SPEP shows an M-spike; IFE confirms IgM type.

Hyperviscosity risk ranking: IgM > IgA > IgG. IgM is the clear leader because of its pentameric structure. IgA can cause hyperviscosity when it forms dimers at high concentrations. IgG rarely causes hyperviscosity outside of extreme paraprotein levels. Waldenström is the classic hyperviscosity-associated disease.

Chapter 21: Toxicology

Clinical toxicology is fundamentally about understanding the relationship between drug concentration and effect. Whether assessing overdose, therapeutic drug monitoring, or workplace drug testing, the laboratory provides critical information - but only if you understand both the analytical methods and their limitations.

21.1 Pharmacokinetic Foundations

Before you can interpret a drug level, you need to know when it was drawn relative to dosing, whether the patient is at steady state, and how the drug distributes. These four numbers - half-life, steady-state, peak/trough, and volume of distribution - show up behind every therapeutic drug monitoring decision.

Half-Life

Half-life (t½) is the time it takes a substance to reach half of its starting concentration. After 1 t½ you have 50% remaining, 2 t½ = 25%, 3 t½ = 12.5%, 4 t½ = 6.25%, 5 t½ = 3.125% (effectively zero). This is why board questions keep asking about “5 half-lives” - that’s the practical definition of “gone.”

Steady State

Steady state is the plateau where drug input equals drug elimination. Before steady state, levels are still climbing with each dose. Steady state is achieved after approximately 5 half-lives - i.e. 5 doses given at intervals of 1 half-life each, at which point you’re at >96% of the final level.

This rule works both directions: it takes 5 half-lives to reach a new steady-state after starting or changing a dose, and 5 half-lives to fully clear a drug after stopping. Therapeutic drug monitoring should ideally happen at steady state - levels drawn earlier are still climbing and underestimate the eventual plateau.

Peak and Trough

Trough: lowest concentration, drawn just before the next dose
Peak: highest concentration, drawn shortly after a dose (exact timing varies by drug and route)

Most drugs are monitored at trough because it’s the most reproducible time point. Aminoglycosides were traditionally monitored at both (peak for efficacy, trough for toxicity), though extended-interval dosing has simplified this.

Protein Binding

Most drugs are partially bound to plasma proteins (mostly albumin). Only the free (unbound) fraction is pharmacologically active and available to cross membranes.

If binding protein decreases (cirrhosis, nephrotic syndrome, malnutrition), less binding → more free drug → increased effect/toxicity at the same total level. Parallel rule: if binding protein increases, total drug may look fine but free drug is low and the patient may be subtherapeutic. This is the same logic as TBG changes affecting total T4 but not free T4 (see Chapter 18).

When two drugs compete for the same binding sites, both get displaced - free concentrations of both rise. Classic example: NSAIDs displacing warfarin from albumin, increasing bleeding risk.

When albumin is abnormal, order a free drug level (phenytoin, valproic acid) or correct the total level for albumin (Winter-Tozer for phenytoin - see section 21.4).

Volume of Distribution (Vd)

Vd is the theoretical volume needed to uniformly distribute a drug at its observed plasma concentration. Formula: Vd = Dose / Plasma concentration.

It’s not a real anatomic compartment - it’s a math construct. A large Vd means the drug distributed extensively into tissues (low plasma levels); a small Vd means it stayed in blood.

Hydrophilic drugs have low Vd - they stay in vascular and interstitial (extracellular aqueous) spaces. These are amenable to hemodialysis because they’re actually in the blood.
Hydrophobic (lipophilic) drugs have high Vd - they dissolve into adipose tissue and cell membranes. These are hard to dialyze because the drug isn’t in the blood to filter out.

Clinically: lithium is hydrophilic with a small Vd → dialyzable. Digoxin is lipophilic with a huge Vd (~7 L/kg) → not effectively removed by dialysis; treatment of severe toxicity requires digoxin-specific Fab fragments instead.

21.2 Principles of Drug Screening

Specimens for Drug Testing

Urine is the usual specimen of choice for drug-of-abuse (DOA) screening. Drugs and metabolites concentrate in urine, giving a longer detection window than blood.

Urine advantages: more stable specimen, longer window of detection (days to weeks vs. hours for blood).

Urine disadvantages: susceptible to tampering (dilution with water, substitution with clean urine, additives like bleach or vinegar). This is why specimen integrity tests - temperature, creatinine, pH, specific gravity - are performed routinely.

Other specimens:

Serum/plasma: for tox testing, draw in a red-top tube with no additives. Some clinical tox tests specifically require serum (acetaminophen, salicylate, ethanol for clinical management). Gel-separator tubes may absorb some drugs - check your lab’s requirements.
Saliva: detects recent use only (hours to ~1-2 days). Upside: observed collection makes tampering hard, it’s noninvasive, and it correlates better with blood levels (and therefore impairment) than urine does. Increasingly used in roadside testing. Downside: short window.
Hair: historic use over weeks to months. Mostly in forensic contexts.

Detection Windows in Urine

Approximate detection periods by class (memorize these - commonly tested):

Cannabinoids: 3 days single use; up to 30 days chronic user (THC is fat-soluble and slowly releases from adipose)
Benzodiazepines: 2-10 days (long-acting benzos last longer)
Amphetamines: 2-3 days
Cocaine: 2-3 days
Opiates: 2-3 days
Alcohol: ~1 day

Chain of Custody

Some tox tests require chain-of-custody precautions - a documented record of specimen handling from collection to reporting. Every person who touches the specimen must sign. Required for forensic, workplace, and any medicolegal testing. Breaks in chain of custody can invalidate results in court. Practically this means: no leaving the specimen unattended outside a locked area, and seal/signature tape on containers.

The Two-Tier Testing System

Drug testing uses a two-tier approach: immunoassay screening followed by confirmatory testing when needed.

Immunoassay screening is designed for speed and sensitivity. Antibodies are raised against drug molecules or their metabolites. When the target drug is present in the sample, it competes with labeled drug for antibody binding, generating a measurable signal. Common platforms: EMIT, CEDIA, KIMS.

The trade-off: To achieve high sensitivity, immunoassays use antibodies with some degree of cross-reactivity. They detect drug classes rather than specific drugs. A positive screen means “something structurally similar to the target was detected” - not necessarily the target drug itself. Immunoassay results are considered presumptive and must be confirmed before being used for clinical or legal decisions.

Confirmatory testing by mass spectrometry (GC-MS or LC-MS/MS) identifies the exact molecular species present. It is highly specific and legally defensible. However, it is more expensive and slower than immunoassay, so it’s reserved for confirming positive screens. Confirmatory methods for toxicology are usually gas or liquid chromatography/mass spectrometry (GC-MS or LC-MS/MS).

Understanding Immunoassay Cross-Reactivity

Cross-reactivity is both a feature and a bug. Some cross-reactants are clinically relevant (other drugs of abuse), while others are innocent (medications, foods). Classic board false positives to know:

Dextromethorphan → false-positive PCP
Diphenhydramine → false-positive TCA
Pseudoephedrine → false-positive amphetamines
Poppy seeds → false-positive opiates (poppy seeds genuinely contain morphine and codeine)

Amphetamine immunoassays cross-react with:

Methamphetamine and MDMA (true positives)
Pseudoephedrine, phenylephrine (decongestants)
Bupropion and its metabolites
Phentermine (diet pills)
Selegiline (MAO inhibitor)

The clinical implication: A positive amphetamine screen in a patient taking Wellbutrin (bupropion) should prompt confirmatory testing before concluding they used methamphetamine.

Opiate immunoassays vary in what they detect:

Traditional “opiate” assays detect morphine and codeine well
They poorly detect synthetic opioids: fentanyl, methadone, oxycodone, buprenorphine
Many labs now use separate assays for these synthetics
False positives: Poppy seeds (contain morphine and codeine), quinolone antibiotics (some assays), rifampin

Know the terminology: Opiate = natural opioids from the opium poppy (morphine, codeine, heroin). Opioid = the broader term for anything that binds opioid receptors, including natural opiates, semi-synthetics (oxycodone, hydrocodone), and fully synthetic opioids (fentanyl, methadone). Standard “opiate” immunoassays may miss fentanyl and methadone - specific assays are needed.

Benzodiazepine immunoassays are optimized for older benzodiazepines (diazepam, chlordiazepoxide) that metabolize to oxazepam-like compounds. They often miss:

Lorazepam (direct glucuronidation, no oxazepam metabolite)
Clonazepam (different metabolic pathway)
Alprazolam (metabolite may not cross-react well)

Cannabinoid immunoassays detect THC-COOH, the major urinary metabolite of marijuana. THC is lipophilic and accumulates in fat, then slowly releases. Heavy chronic users may test positive for weeks after last use.

Cut-off Concentrations

Immunoassays use cut-off concentrations to define “positive” and “negative.” These cut-offs are not toxicological thresholds - they’re administrative decisions balancing sensitivity against false-positive rates.

Federal workplace testing cut-offs (SAMHSA) are intentionally high to minimize false positives:

Marijuana metabolites: 50 ng/mL (screening), 15 ng/mL (confirmation)
Cocaine metabolites: 150 ng/mL (screening), 100 ng/mL (confirmation)
Opiates: 2000 ng/mL (screening), varies by compound (confirmation)
Amphetamines: 500 ng/mL (screening), 250 ng/mL (confirmation)

Clinical testing may use lower cut-offs to maximize detection. Always know your laboratory’s cut-offs when interpreting results.

21.3 Confirmation by Mass Spectrometry

Mass spectrometry identifies compounds by their mass-to-charge ratio and fragmentation patterns. It is the definitive method for drug identification.

Gas Chromatography-Mass Spectrometry (GC-MS): The historical gold standard. Sample is vaporized, separated by gas chromatography, then ionized (electron impact ionization) and analyzed by mass spectrometry. Excellent for volatile compounds. Requires derivatization for many drugs.

Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS): Increasingly dominant. Sample stays in liquid phase, separated by LC, then analyzed by two mass spectrometers in sequence (MS/MS). More versatile than GC-MS; handles polar and high-molecular-weight compounds without derivatization.

The key advantage of mass spectrometry is specificity: each compound has a unique mass spectrum. A positive result means that specific molecule was present, not merely something that cross-reacted with an antibody.

21.4 Therapeutic Drug Monitoring

Therapeutic drug monitoring (TDM) optimizes dosing for drugs with narrow therapeutic windows - where the difference between effective and toxic concentrations is small. Understanding the pharmacokinetics, proper sampling times, and interpretation of levels is essential.

Principles of TDM

Pharmacokinetics determines how concentration changes over time:

Absorption: Rate and extent of drug reaching systemic circulation
Distribution: How drug spreads to tissues; affects volume of distribution
Metabolism: Hepatic (mainly CYP450), renal, or other biotransformation
Elimination: Clearance from the body; determines half-life

Key concepts:

Half-life: Time for concentration to decrease by 50%
Steady-state: Achieved after 4-5 half-lives; input equals output
Trough level: Lowest concentration, just before next dose
Peak level: Highest concentration, after distribution phase complete

Why some drugs need monitoring:

Narrow therapeutic index (small margin between effective and toxic)
Large interindividual variability in pharmacokinetics
Serious consequences of under- or overdosing
Concentrations correlate better with effect than doses do
Need to verify compliance or absorption

The TDM Quick Reference Table

Drug	Therapeutic Range	Sample Timing	Key Toxicities	Notes
Gentamicin/Tobramycin	Peak 5-10 μg/mL; Trough <2 μg/mL	Peak: 30 min post-infusion; Trough: just before dose	Nephrotoxicity, ototoxicity	Extended-interval dosing uses random level
Amikacin	Peak 20-35 μg/mL; Trough <10 μg/mL	Same as above	Same as above	Higher target peaks
Vancomycin	Trough 10-20 μg/mL; AUC 400-600	Trough: just before dose	Nephrotoxicity (with troughs >20)	AUC/MIC monitoring now preferred
Digoxin	0.8-2.0 ng/mL (0.5-1.0 for HF)	≥6 hours post-dose	Arrhythmias, GI, visual	Hypokalemia increases toxicity
Lithium	0.6-1.2 mEq/L	12 hours post-dose	Tremor, renal, thyroid, cardiac	Dehydration increases levels
Phenytoin	Total: 10-20 μg/mL; Free: 1-2 μg/mL	Trough	Ataxia, nystagmus, gingival hyperplasia	Use free level if low albumin
Valproic acid	50-100 μg/mL	Trough	Hepatotoxicity, tremor, weight gain	Highly protein-bound
Carbamazepine	4-12 μg/mL	Trough	Aplastic anemia, SIADH, rash	Autoinduction over 2-4 weeks
Phenobarbital	15-40 μg/mL	Trough (long half-life)	Sedation, respiratory depression	Very long half-life (~100 hours)
Tacrolimus	5-15 ng/mL (varies)	Trough (12 hours post-dose)	Nephrotoxicity, neurotoxicity, DM	Whole blood; CYP3A4 interactions
Cyclosporine	100-400 ng/mL (varies)	Trough or C2 (2-hour)	Nephrotoxicity, HTN, tremor	Whole blood; CYP3A4 interactions
Sirolimus	5-15 ng/mL (varies)	Trough	Hyperlipidemia, myelosuppression	Long half-life (~60 hours)
Theophylline	10-20 μg/mL	Trough	Seizures, arrhythmias, GI	Rarely used now

Aminoglycosides: Detailed Considerations

Mechanism of action: Bind 30S ribosomal subunit; concentration-dependent killing (higher peaks = better bacterial kill)

Aminoglycosides are eliminated by the kidneys (hydrophilic, low Vd, not protein-bound → can be removed by dialysis). Dose must be adjusted in renal impairment.

Toxicity mechanisms: primary toxicities are nephrotoxicity and ototoxicity.

Nephrotoxicity: Accumulates in proximal tubule cells; causes acute tubular necrosis
Risk correlates with duration of exposure (trough levels)
Usually reversible
Ototoxicity: Damages hair cells in cochlea and vestibular apparatus
May be irreversible
Risk correlates with cumulative dose and peak levels

Traditional dosing (multiple daily doses):

Peak (30 min after infusion): Target 5-10 μg/mL (gentamicin/tobramycin)
Trough (just before next dose): Target <2 μg/mL

Extended-interval dosing (once daily):

Exploits concentration-dependent killing and post-antibiotic effect
Single high dose (5-7 mg/kg) followed by drug-free interval
Monitoring: Single random level with nomogram (Hartford nomogram)
Lower nephrotoxicity than traditional dosing (longer drug-free interval allows tubule recovery)
NOT appropriate for: endocarditis, pregnancy, severe renal impairment, burns >20% BSA

Vancomycin: The AUC/MIC Era

Traditional approach: Trough-based monitoring

Trough target 10-20 μg/mL (15-20 for serious MRSA infections)
Problem: Trough correlates with nephrotoxicity but imperfectly with efficacy

Current approach: AUC/MIC-guided dosing

AUC (area under the concentration-time curve) better predicts efficacy
Target AUC/MIC 400-600 for MRSA infections
Can be estimated from two levels (peak and trough) or Bayesian software with one level
Reduces nephrotoxicity compared to targeting high troughs

Nephrotoxicity risk factors:

Troughs >20 μg/mL
Concurrent nephrotoxins (aminoglycosides, piperacillin-tazobactam, NSAIDs, contrast)
Prolonged therapy
ICU patients, those with underlying renal disease

Digoxin: The Challenging Drug

Why monitoring is complex:

Very long half-life (36-48 hours; longer in renal impairment)
Extensive tissue binding; slow distribution phase
Symptoms of toxicity overlap with indications (arrhythmias)

Sampling time is critical: Draw level at least 6 hours (preferably 8-12 hours) after dose. Earlier sampling overestimates true steady-state concentration because the drug is still distributing.

Factors increasing toxicity at any given level:

Hypokalemia: Digoxin competes with K+ for Na-K-ATPase; low K+ increases binding and toxicity
Hypomagnesemia: Often coexists with hypokalemia; increases toxicity risk
Hypercalcemia: Sensitizes heart to digoxin
Hypothyroidism: Decreases clearance
Renal impairment: Decreases clearance (digoxin is ~70% renally eliminated)

Drug interactions:

Amiodarone, verapamil, quinidine: Increase digoxin levels (reduce digoxin dose by 50%)
Diuretics: Cause hypokalemia, increasing toxicity risk

Toxicity manifestations:

GI: Nausea, vomiting, anorexia
Visual: Yellow-green halos, photophobia
Cardiac: Almost any arrhythmia, but classically:
Increased automaticity (PVCs, atrial tachycardia)
Decreased conduction (AV block)
“Regularization” of atrial fibrillation (suggests AV nodal toxicity)
Bidirectional VT (classic but rare)

Digoxin immune Fab (Digibind): Binds free digoxin; used for life-threatening toxicity. After administration, total digoxin level becomes uninterpretable (measures bound + unbound). Note that although digoxin is renally eliminated, its huge Vd means it’s not effectively removed by dialysis - Fab fragments are the mainstay for severe overdose.

When to check levels: after dose changes or when renal function changes (digoxin is ~70% renally eliminated, so declining GFR raises levels even on a stable dose).

Digoxin-like immunoreactive substances (DLIS): Endogenous compounds that cross-react with digoxin immunoassays, producing false-positive levels in patients who are not even on digoxin (or falsely elevating true digoxin levels). DLIS are seen in neonates, pregnant women (third trimester), liver failure, renal failure, and volume-overloaded states. They’re thought to be ouabain-like endogenous substances and are not pharmacologically active like digoxin. If DLIS interference is suspected, reflex to LC-MS/MS.

Procainamide and NAPA: Why You Must Monitor Both

Procainamide is metabolized in the liver to N-acetylprocainamide (NAPA) by acetylation, and NAPA is eliminated by the kidneys. NAPA is pharmacologically active with its own antiarrhythmic effect.

Acetylation rate is genetically determined by NAT2 genotype - fast vs. slow acetylators. Same polymorphism matters for isoniazid and hydralazine.

Fast acetylators: low procainamide, high NAPA. If you measure only procainamide, they’ll look subtherapeutic even though total active drug is fine.
Slow acetylators: high procainamide, low NAPA. Measuring only procainamide can make them look toxic when total active drug isn’t.

Always monitor both procainamide AND NAPA together - the sum is what matters. This is a classic TDM board question.

Phenytoin: Nonlinear Kinetics

Unique pharmacokinetics: Phenytoin exhibits Michaelis-Menten (saturable) kinetics - as dose increases, metabolism becomes saturated, and small dose increases cause disproportionately large concentration increases. This makes dosing adjustments tricky.

Protein binding considerations: Phenytoin is ~90% bound to albumin. Only free (unbound) phenytoin is pharmacologically active.

When to measure free phenytoin:

Hypoalbuminemia (cirrhosis, nephrotic syndrome, malnutrition, critical illness)
Renal failure (uremia displaces phenytoin from albumin)
Pregnancy (altered protein binding)
Concurrent drugs that displace phenytoin (valproic acid, aspirin)

The Winter-Tozer equation: Estimates total phenytoin level that would be expected if albumin were normal:

Adjusted phenytoin = Measured phenytoin / [(0.2 × albumin) + 0.1]

For renal failure, use: Adjusted = Measured / [(0.1 × albumin) + 0.1]

Or simply measure free phenytoin level (therapeutic: 1-2 μg/mL)

Immunosuppressants: Transplant Survival Depends on TDM

Why whole blood? Tacrolimus, cyclosporine, and sirolimus concentrate in red blood cells. Measuring in whole blood (not serum/plasma) captures the true body burden.

Tacrolimus:

Mechanism: Binds FKBP12, inhibits calcineurin, blocks T-cell activation
Trough target: Varies by organ (kidney typically 5-15 ng/mL), time post-transplant (higher early), and rejection risk
Major toxicities: Nephrotoxicity (vasoconstriction), neurotoxicity (tremor, headache, seizures), new-onset diabetes
Metabolized by CYP3A4 and affected by P-glycoprotein
Levels INCREASE with: azoles, diltiazem, macrolides, grapefruit juice
Levels DECREASE with: rifampin, phenytoin, St. John’s wort

Cyclosporine:

Mechanism: Same as tacrolimus (calcineurin inhibitor)
Monitoring: Trough (C0) or 2-hour post-dose (C2, better correlates with exposure)
Major toxicities: Nephrotoxicity, hypertension, tremor, gingival hyperplasia, hirsutism
Same CYP3A4 interactions as tacrolimus

Sirolimus (rapamycin):

Mechanism: Binds FKBP12 but inhibits mTOR (not calcineurin) - antiproliferative
Very long half-life (~60 hours); takes 1-2 weeks to reach steady-state
Trough monitoring; target varies by use
Major toxicities: Hyperlipidemia, myelosuppression, impaired wound healing, interstitial pneumonitis
Often used to minimize calcineurin inhibitor exposure (to spare kidneys)

Lithium: Narrow Window and Dangerous Interactions

Therapeutic range: 0.6-1.2 mEq/L for maintenance; 1.0-1.5 mEq/L for acute mania

Sampling: Always 12 hours post-dose (standardized because of variability in absorption)

Toxicity spectrum:

Mild (1.5-2.0 mEq/L): Tremor, nausea, diarrhea, lethargy
Moderate (2.0-2.5 mEq/L): Confusion, ataxia, dysarthria, coarse tremor
Severe (>2.5 mEq/L): Seizures, coma, cardiac arrhythmias, death

Factors that increase lithium levels (all decrease renal clearance):

Dehydration/volume depletion: Any cause (vomiting, diarrhea, sweating, poor intake)
NSAIDs: Decrease renal blood flow
ACE inhibitors/ARBs: Decrease GFR
Thiazide diuretics: Increase proximal tubule lithium reabsorption (paradoxically; loop diuretics are safer)
Renal impairment: Lithium is 100% renally eliminated

Chronic lithium effects to monitor:

Nephrogenic diabetes insipidus: Lithium impairs collecting duct response to ADH; polyuria/polydipsia
Hypothyroidism: Lithium inhibits thyroid hormone release (check TSH regularly)
Chronic kidney disease: Long-term lithium can cause interstitial nephritis

Ethanol Metabolism

Ethanol is metabolized in two steps:

Ethanol → acetaldehyde (three enzymes can do this; alcohol dehydrogenase does the majority)
Acetaldehyde → acetate via aldehyde dehydrogenase 2 (ALDH2), in the mitochondria

Three enzymes that convert ethanol → acetaldehyde (memorize the cellular compartments - frequently tested):

Cytosol: alcohol dehydrogenase (majority)
Endoplasmic reticulum: CYP2E1 (induced by chronic alcohol use)
Peroxisomes: catalase (minor role)

CYP2E1 induction in chronic drinkers explains faster ethanol metabolism and is the same system that produces NAPQI from acetaminophen - that’s why chronic alcohol use increases APAP hepatotoxicity risk.

Acetaldehyde is the toxic intermediate responsible for hangover symptoms. Disulfiram (Antabuse) blocks ALDH2 → acetaldehyde accumulates → severe nausea and flushing to deter drinking. Genetic ALDH2 deficiency (common in people with East Asian ancestry) produces the same effect at lower ethanol doses, causing the ALDH2-associated flushing phenotype.

Blood Alcohol: Specimen, Method, and Legal Limits

BAL can be measured in serum, plasma, or whole blood. These are not interchangeable: serum/plasma ethanol is ~15-20% higher than whole blood (~1.15-1.18×) because ethanol distributes in water, and serum/plasma is ~93% water while whole blood is only ~80% water (RBC mass dilutes).

Clinical enzymatic assay: uses alcohol dehydrogenase to convert ethanol → acetaldehyde. It is specific for ethanol and does NOT measure methanol, ethylene glycol, or isopropanol. A negative clinical ethanol level does NOT exclude toxic alcohol ingestion - for that you need the osmolal gap or specific assays.

Breath alcohol: blood alcohol diffuses across alveolar septa and is excreted in expired air. The blood-to-breath ratio is ~2100:1 (2100 mL expired air contains the same alcohol as 1 mL blood). Variation in body temperature, breathing pattern, and lung physiology can introduce error.

Legal limits (based on whole blood):

0.08% (80 mg/dL) in all US states
Utah: 0.05% (50 mg/dL) (the exception)

Don’t directly compare clinical serum/plasma values to legal whole-blood limits without converting.

BAL severity progression:

0.08% = legal impairment
0.1-0.2% = impaired
0.2-0.3% = confusion, nausea
0.3-0.4% = stupor
>0.4% (400 mg/dL) = coma and death (though chronic heavy drinkers can tolerate much higher levels due to CYP2E1 induction and pharmacodynamic tolerance)

Alcohol Biomarkers

Marker	Detection Window	Sensitivity	Specificity	Use
BAC (Blood Alcohol)	Hours	High	High	Acute intoxication only
GGT	Weeks	High	Low (many conditions elevate GGT)	Screening; not specific to alcohol
CDT (Carbohydrate-Deficient Transferrin)	2-4 weeks	Moderate	HIGH	Chronic heavy use (≥4-7 drinks/day × ≥2 weeks); forensic sobriety monitoring
EtG (Ethyl Glucuronide)	Up to 72-80 hours	High	High	Extended “recent use” window beyond BAC

CDT Mechanism: Alcohol metabolite (acetaldehyde) interferes with enzymes that add sialic acid chains to transferrin → desialylated (carbohydrate-deficient) transferrin accumulates. Transferrin half-life ~7-14 days → CDT reflects chronic intake over weeks.

Note: when ethanol and cocaine are co-ingested, transesterification produces cocaethylene, a pharmacologically active metabolite with a much higher risk of sudden cardiac death (see Cocaine section below).

21.5 Toxicology of Specific Agents

Acetaminophen (Paracetamol) Toxicity

Acetaminophen is safe at therapeutic doses but highly hepatotoxic in overdose. Understanding the mechanism is essential.

Normal metabolism: Preferred pathway is conjugation with glucuronide or sulfate → nontoxic metabolites (~90%). A small fraction is oxidized by CYP2E1 (the P450 system) to NAPQI (N-acetyl-p-benzoquinone imine), a highly reactive metabolite. NAPQI is immediately conjugated with glutathione and excreted.

Overdose metabolism: Glucuronidation and sulfation saturate → more drug shunted through CYP2E1 → more NAPQI → glutathione stores deplete → unconjugated NAPQI covalently binds hepatocyte proteins → centrilobular hepatic necrosis (zone 3, where CYP2E1 activity is highest). Chronic alcohol use upregulates CYP2E1, which is why alcoholics are more susceptible to APAP hepatotoxicity at lower doses.

Three clinical phases:

Phase I (0-24 hrs): mild nausea and abdominal discomfort - deceptively well. This is the dangerous part because patients feel fine and delay treatment.
Phase II (24-48 hrs): liver injury begins (rising AST/ALT, RUQ pain)
Phase III (>48 hrs): fulminant hepatic failure, coagulopathy, encephalopathy, multi-organ failure

Acetaminophen toxicity is potentially reversible during Phase I (<24 hrs) if treated with NAC. After 24 hours, liver damage has begun and may be irreversible.

The Rumack-Matthew nomogram: Plots serum acetaminophen concentration against time since ingestion to predict hepatotoxicity risk and guide NAC treatment.

Draw serum level after 4 hours post-ingestion (before 4 hours, absorption may be incomplete and levels are still rising, so an early level can underestimate risk)
Three risk zones: Probable hepatic toxicity, Possible hepatic toxicity, No hepatic toxicity. Treat the first two with NAC.
Only valid for single acute ingestions with known time
Not valid for chronic/repeated overdoses or extended-release formulations
The original “150 line” starts at 150 μg/mL at 4 hours

Treatment: N-acetylcysteine (NAC) replenishes glutathione, allowing it to neutralize NAPQI. Most effective within 8 hours of ingestion but still provides benefit up to 24+ hours. Oral or IV. Very safe (anaphylactoid reaction is rare). When in doubt, treat - the cost of missing toxicity is catastrophic.

Salicylate (Aspirin) Toxicity

Salicylate toxicity produces a characteristic mixed acid-base disturbance - respiratory alkalosis + anion gap metabolic acidosis simultaneously.

Mechanisms:

Direct respiratory center stimulation → Hyperventilation → Respiratory alkalosis (early)
Uncoupling of oxidative phosphorylation → Impaired ATP production → Increased lactate → Anion gap metabolic acidosis
Interference with carbohydrate/fat metabolism → Ketoacidosis

Early in poisoning, respiratory alkalosis dominates; later, metabolic acidosis takes over. Both occurring together is classic.

Clinical features: Tachypnea, tinnitus, altered mental status, hyperthermia, diaphoresis

Laboratory findings:

Salicylate level (>30 mg/dL significant; >100 mg/dL severe)
ABG: Mixed respiratory alkalosis/metabolic acidosis
Elevated anion gap
Hypokalemia (from renal compensation)

Treatment: Alkalinization of urine (sodium bicarbonate) enhances salicylate excretion by “ion trapping” - salicylic acid becomes ionized in alkaline urine and cannot be reabsorbed. Hemodialysis for severe cases.

Toxic Alcohols

Methanol, ethylene glycol, and isopropanol are the three classic toxic alcohols. The parent alcohols themselves aren’t very toxic - it’s the metabolites that cause the damage, which is exactly why treatment works by blocking alcohol dehydrogenase.

Sources (know these for clinical scenarios):

Ethylene glycol: antifreeze (sweet taste; often ingested by children/pets)
Methanol: windshield wiper fluid, poorly made homemade alcohol (moonshine)
Isopropanol: rubbing alcohol (the most commonly ingested toxic alcohol in the US)

Methanol → formaldehyde → formic acid (via the same two enzymes as ethanol: alcohol dehydrogenase, then aldehyde dehydrogenase 2).

Terminal metabolites: formic acid, formaldehyde
Formic acid inhibits cytochrome oxidase and is directly toxic to the optic nerve and retina
Classic presentation: visual disturbances (“snowstorm” vision), blindness
Lab: anion gap metabolic acidosis, elevated osmolal gap (early)

Ethylene glycol → glycoaldehyde (via alcohol dehydrogenase) → glycolic acid (via aldehyde dehydrogenase 2) → oxalate.

Terminal metabolites: glycolic acid (causes the AG acidosis) and oxalate (causes the kidney damage)
Oxalate deposits in kidney → renal failure + flank pain, and appears in urine as calcium oxalate crystals (envelope-shaped)
Classic presentation: CNS depression → cardiopulmonary → renal failure
Lab: AG metabolic acidosis, elevated osmolal gap (early), Ca oxalate crystals in urine

Isopropanol → acetone (via alcohol dehydrogenase).

Terminal metabolite: acetone (a ketone, NOT an acid)
Result: ketonemia/ketonuria WITHOUT acidosis
Elevated osmolal gap, but NORMAL anion gap - this is what distinguishes isopropanol from methanol/ethylene glycol
Fruity breath odor from acetone
Isopropanol is the least dangerous of the three because its metabolite is relatively nontoxic. Fomepizole is typically NOT needed

Key concept - the osmolal gap: Early after ingestion, the parent alcohol is present but not yet metabolized. These alcohols are osmotically active, creating an elevated osmolal gap. As metabolism proceeds, the alcohols are converted to organic acids (which contribute to anion gap but not osmolal gap). So:

Early: High osmolal gap, normal anion gap
Late: Normal osmolal gap, high anion gap
In between: Both elevated

Treatment for methanol and ethylene glycol: block alcohol dehydrogenase with either:

Fomepizole: direct ADH inhibitor (preferred, fewer side effects)
Ethanol: competitive substrate that occupies ADH preferentially

Hemodialysis removes both the parent alcohol and its metabolites. Isopropanol doesn’t need fomepizole because acetone is relatively benign.

Toxidromes

Toxidromes are constellations of signs and symptoms that point to a class of toxin. Pattern recognition of vitals + pupils + skin + mental status lets you identify the class before labs return. The major ones are sympathomimetic, anticholinergic, cholinergic, sedative, narcotic (opioid), and hallucinogenic.

Toxidrome	HR / BP	Pupils	Skin	Mental Status	Classic Examples
Sympathomimetic	↑ / ↑	Mydriasis	Diaphoretic (WET)	Agitated, hyperthermic	Cocaine, amphetamines
Anticholinergic	↑ / ↑	Mydriasis	Dry, flushed	Delirious, hyperthermic	Diphenhydramine, TCAs, atropine
Cholinergic	↓ / ↓	Miosis	Diaphoretic	Sedated ± seizures	Organophosphates, carbamates
Sedative	↓ / ↓	Small/normal	Variable	CNS depression	Benzos, barbiturates, alcohol
Narcotic/opioid	↓ / ↓	Pinpoint miosis	Cool	CNS depression, ↓RR	Heroin, morphine, fentanyl
Hallucinogenic	↑ / ↑	Mydriasis	Variable	Hallucinations	LSD, psilocybin

Sympathomimetic: tachycardia, hypertension, mydriasis, diaphoresis, hyperthermia, agitation. Everything is “revved up.”

Anticholinergic: blind as a bat (mydriasis), dry as a bone (anhidrosis), red as a beet (flushed), hot as a hare (hyperthermia), mad as a hatter (delirium). The key feature distinguishing it from sympathomimetic is dry skin - anticholinergics block sweating.

Why do sympathomimetics sweat but anticholinergics don’t? Sweat glands are innervated by sympathetic fibers but use acetylcholine as the neurotransmitter (an unusual setup). So sympathomimetics activate those cholinergic sweat glands, while antimuscarinics shut them down.

Cholinergic: DUMBBBELS - Diarrhea, Urination, Miosis, Bradycardia, Bronchospasm/Bronchorrhea, Emesis, Lacrimation, Salivation. Everything is “wet” - the opposite of anticholinergic. Pinpoint pupils are key.

Sedative: CNS depression, hypotension, bradypnea, hypothermia, decreased reflexes, small-to-normal pupils. Unlike opioids, pupils are not pinpoint.

Narcotic/opioid: the triad = pinpoint miosis + respiratory depression + altered mental status. Also: bradycardia, hypotension, constipation. If naloxone reverses symptoms, that confirms opioid etiology.

Hallucinogenic: visual/auditory hallucinations, altered time perception, mydriasis, tachycardia, nystagmus. Usually not life-threatening - PCP is the exception because of its NMDA antagonism plus aggression.

Pupils, Skin, and Odors: Pattern Recognition

Know these bedside findings - they’re high-yield board pattern questions.

Pinpoint pupils (miosis): opioids, cholinergics, benzodiazepines.

Dilated pupils (mydriasis): anticholinergics, sympathomimetics, carbon monoxide.

Diaphoresis: sympathomimetics, cholinergics, organophosphates. (Because sweat glands use ACh even though they’re sympathetic.)

Cherry red skin: carbon monoxide, cyanide, anticholinergics. Different mechanisms:

CO: carboxyhemoglobin is intrinsically cherry red
Cyanide: venous blood stays oxygenated because cells can’t use O2 (“arterialized venous blood”)
Anticholinergics: vasodilation (“red as a beet”)

Breath odors:

Bitter almond = cyanide (only ~50% of the population has the genetic ability to detect it - don’t rely on odor alone)
Mothball = camphor
Garlic = arsenic or organophosphates
Fruity = DKA or isopropanol/acetone

Dystonia: sustained muscle contractions, abnormal posturing → think neuroleptics/antipsychotics (D2 blockers). Most common in young males after a first dose. Treated with anticholinergics (benztropine, diphenhydramine). Related reactions to know: akathisia, tardive dyskinesia, neuroleptic malignant syndrome.

Cocaine

Cocaine is a sympathomimetic that blocks catecholamine reuptake → sympathetic overdrive: tachycardia, hypertension, diaphoresis, mydriasis, agitation, hyperthermia.

Cardiac complications: coronary vasoconstriction (vasospasm) + increased cardiac demand (tachycardia, HTN, ventricular hypertrophy) → acute MI even in young patients with normal coronaries. Classic board scenario: young patient with chest pain + cocaine use.

Biomarker specificity in cocaine MI: myoglobin and CK-MB have low specificity in cocaine users because cocaine also damages skeletal muscle (rhabdomyolysis), and both are released from skeletal muscle. Troponin retains normal specificity and is the preferred biomarker.

Cocaethylene: when cocaine and ethanol are co-ingested, hepatic transesterification by carboxylesterase produces cocaethylene (ethylbenzoylecgonine).

Cocaethylene has a longer half-life (3-4 hours vs. ~1 hour for cocaine)
More potent vasoconstrictor than cocaine alone
Risk of sudden cardiac death 18-25× higher than cocaine alone
Only known example of a pharmacologically active metabolite formed by transesterification of two recreational substances
Detected on GC-MS/LC-MS/MS

Opioids

Acute opioid intoxication: sedation, pinpoint pupils (miosis), constipation, bradycardia, hypotension, respiratory depression. The triad of miosis + respiratory depression + altered mental status is opioid overdose until proven otherwise.

Treatment: naloxone (Narcan), a competitive opioid receptor antagonist that rapidly reverses opioid effects. Also nalmefene.

Naloxone half-life is short (~30-90 min) - patients can re-sedate after initial reversal, especially with long-acting opioids like methadone. Repeat dosing or a continuous infusion may be needed.

Historical note: propoxyphene was an opioid that uniquely caused cardiac conduction abnormalities and seizures (atypical for opioids). Withdrawn from the US market in 2010. May still appear on boards as historical knowledge.

Stimulants and PCP

Amphetamines and methylphenidate: stimulant ADHD medications that induce release of dopamine and norepinephrine in the CNS. Amphetamines (Adderall) release stored catecholamines; methylphenidate (Ritalin) blocks reuptake. Both produce the sympathomimetic toxidrome at high doses.

Phencyclidine (PCP): unique dual mechanism - NMDA receptor antagonist + catecholamine reuptake inhibitor. This produces both dissociative/hallucinogenic effects AND sympathomimetic effects.

Classic PCP intoxication = horizontal (or rotary) nystagmus + violent aggression. Patients may exhibit superhuman strength due to pain insensitivity. Board scenario: violent patient with nystagmus → PCP.

Sedatives: Barbiturates vs. Benzodiazepines

Both enhance GABA (inhibitory neurotransmitter) at the GABA-A receptor, but they do it differently:

Barbiturates: increase duration of Cl⁻ channel opening
Benzodiazepines: increase frequency of Cl⁻ channel opening

Mnemonic: “Barb likes it longer, Ben likes it more often.”

Barbiturates are more dangerous in overdose because they can directly open the Cl⁻ channel without GABA. Benzos require GABA to act - they just amplify what’s already there - which gives them a wider safety margin.

Immunoassay note: standard benzodiazepine immunoassays miss lorazepam, clonazepam, and often alprazolam (see 21.2). Know this for clinical interpretation.

Tricyclic Antidepressants (TCAs)

TCAs produce a mixed toxidrome. Anticholinergic effects dominate the symptoms (dry mouth, urinary retention, constipation, mydriasis, altered mental status), but the dangerous part is cardiac.

TCAs block cardiac sodium channels → QRS prolongation:

QRS >100 ms = seizure risk
QRS >160 ms = ventricular arrhythmia risk

Treatment: sodium bicarbonate - provides Na⁺ to overcome channel blockade and alkalinizes blood to reduce TCA binding.

Carbon Monoxide

CO binds hemoglobin with ~200-250× the affinity of O2, forming carboxyhemoglobin (COHb). COHb cannot carry oxygen AND it shifts the O2-Hb dissociation curve left, so the remaining oxyhemoglobin holds onto O2 more tightly - worse tissue delivery. Double harm: less O2 carried + less O2 released.

Endogenous CO: produced normally from heme breakdown via heme oxygenase → biliverdin + CO. Normal baseline COHb <2% in nonsmokers. Hemolytic conditions (sickle cell, G6PD deficiency) increase endogenous CO production.

COHb levels and symptoms (memorize):

0.4-2% = normal nonsmoker
2-6% = normal smoker
10-20% = mild symptoms (headache, dyspnea on exertion)
20-50% = severe (confusion, lethargy, loss of consciousness)
≥50% = coma and death

CO half-life depends on oxygen:

~6 hours on room air
~1 hour on 100% O2
~20 min on hyperbaric O2

Treatment is high-flow 100% O2. Hyperbaric O2 is considered for severe cases (loss of consciousness, cardiac ischemia, pregnancy, COHb >25%).

Pulse oximetry pitfall: pulse oximeters cannot distinguish COHb from oxyhemoglobin - they read SpO2 as falsely normal while the patient is actually hypoxic. See oxygen saturation gap below.

Cyanide

Sources:

Smoke inhalation from fires (burning plastics/synthetic materials releases HCN)
Industrial exposure (mining, metallurgy, photography)
Intentional ingestion
Prolonged nitroprusside infusion (CN⁻ byproduct)

Fire victims with altered mental status should trigger consideration of dual CO + cyanide exposure.

Mechanism: cyanide binds cytochrome c oxidase (complex IV) in the mitochondrial electron transport chain - specifically the cytochrome a3 subunit. This halts oxidative phosphorylation. Cells can’t use O2 for ATP, so they switch to anaerobic glycolysis → pyruvate shunted to lactate.

Clinical/lab findings:

Anion gap metabolic acidosis from lactic acidosis (often profound lactate)
Increased venous oxygen and increased serum glucose (tissues can’t use either - “cellular starvation in a sea of O2”)
Arterialized venous blood - arterial-venous O2 difference is LOW because mitochondria can’t extract oxygen
Cherry red skin from oxygenated venous blood (different mechanism from CO)
Profound lactic acidosis + high venous O2 = think cyanide
Bitter almond breath odor (only ~50% can detect)
Serum thiocyanate (the principal metabolite) can be measured to confirm exposure

Treatment triad:

Sodium nitrite / amyl nitrite: induce methemoglobinemia → MetHb binds cyanide more avidly than cytochrome oxidase does
Sodium thiosulfate: substrate for rhodanese, which converts CN⁻ to thiocyanate for renal excretion
Hydroxocobalamin (vitamin B12a): directly binds cyanide → forms cyanocobalamin (B12); now the preferred agent because there’s no MetHb risk

Oxygen Saturation Gap and Dyshemoglobins

The oxygen saturation gap is the difference between pulse-oximeter SpO2 and the co-oximeter (or calculated) SpO2. A gap ≥5% is significant and should prompt co-oximetry.

The gap exists because pulse oximetry can’t distinguish abnormal hemoglobin species from normal oxy/deoxyhemoglobin. Four classic causes (mnemonic SCMC - Saturation Chasm Mediates Cyanosis): Sulfhemoglobin, Carboxyhemoglobin, Methemoglobin, Cyanide.

Related concept: in cyanide and hydrogen sulfide poisoning, the arterial-venous O2 difference is LOW (arterialized venous blood) because mitochondria can’t extract oxygen. The patient has plenty of O2 in the blood but can’t use it.

Low anion gap sidebar: hypoalbuminemia causes a low anion gap because albumin is the primary unmeasured anion contributing to the normal AG. For every 1 g/dL decrease in albumin, the AG drops by ~2.5 mEq/L. Practical implication: a “normal” AG in a hypoalbuminemic patient may hide an AG acidosis - always correct the AG for albumin.

Lead

Mechanism: lead inhibits multiple enzymes in the heme synthesis pathway. The two to know:

δ-ALA dehydratase (early step)
Ferrochelatase (final step: inserting iron into protoporphyrin)

Result: accumulated ALA and protoporphyrin, decreased heme production → microcytic sideroblastic anemia with ring sideroblasts. Zinc substitutes for iron in the protoporphyrin ring → free erythrocyte protoporphyrin (FEP) and zinc protoporphyrin (ZPP) are elevated.

Elevated ZPP/FEP is seen in three conditions where iron isn’t available for heme synthesis:

Iron deficiency (no iron to use)
Lead toxicity (ferrochelatase can’t insert iron)
Anemia of chronic inflammation (iron sequestered)

Lead effects on RBCs:

Inhibits Na-K-ATPase → increased osmotic fragility → shortened RBC survival→ hemolysis
Inhibits 5’-nucleotidase → ribosomal RNA degradation is impaired → ribosomal aggregates persist → basophilic stippling on peripheral smear

Basophilic stippling is the classic board finding for lead poisoning.

Blood lead level: CDC historical action level was ≥10 μg/dL; the reference value for children is now 3.5 μg/dL. No safe level of lead exposure exists. Venous blood is the definitive test - capillary specimens are prone to skin contamination.

Chelation agents:

Dimercaprol (BAL) - IM, for severe/encephalopathic lead poisoning
Succimer (DMSA) - oral, for moderate lead poisoning in children
EDTA (CaNa2EDTA) - IV, combined with BAL for severe cases

Arsenic

Mechanism: arsenic inhibits oxidative phosphorylation. Arsenate substitutes for phosphate in ATP synthesis; arsenite binds sulfhydryl groups in enzymes. Different mechanism from cyanide but similar end result - disrupted cellular energy.

Distribution and excretion: mainly excreted in urine; the rest distributes into keratinized tissues (skin, nails, hair). 24-hour urine arsenic is the diagnostic test. Hair and nail arsenic levels reflect chronic exposure over weeks to months.

Acute arsenic toxicity (within hours of ingestion):

Rice-water diarrhea (profuse, watery - similar to cholera)
Severe abdominal pain
Encephalopathy
Can progress to multi-organ failure

Rice-water diarrhea is the board buzzword.

Chronic arsenic toxicity:

Peripheral neuropathy
Hyperkeratosis / hyperpigmentation of palms and soles
Transverse Mees lines on nails (white bands; also seen in thallium poisoning)
Increased risk of skin, lung, and bladder cancer

Bone marrow effects: arsenic causes cytopenias (pancytopenia in severe toxicity). Paradoxically, arsenic trioxide (ATO) is used therapeutically in acute promyelocytic leukemia - at therapeutic doses it induces differentiation and apoptosis of leukemic cells; at toxic doses it suppresses normal hematopoiesis.

Mercury

Exposure: mercury toxicity producing symptoms usually occurs via inhalation of mercury vapor (industrial/occupational). Elemental mercury is poorly absorbed from the GI tract but readily absorbed through the lungs. Organic mercury (methylmercury from fish) causes chronic low-level toxicity.

“Mad Hatter” disease = chronic mercury exposure, historically from hat-making industry (mercury was used in felt).

Chronic mercury: two classic presentations:

Acrodynia (Feer syndrome):

Autonomic manifestations (sweating, hemodynamic instability)
Desquamative, erythematous rash on palms and soles
Increased urinary catecholamines - can mimic pheochromocytoma

If a “pheo” patient also has rash on palms/soles → consider mercury poisoning. The rash is the giveaway.

Erethism:

Personality changes
Irritability, insomnia, memory loss
Fine motor disturbances (tremor)

These are CNS effects from mercury crossing the blood-brain barrier.

Organophosphates and Carbamates

Organophosphates and carbamates are insecticides used in agriculture. Both inhibit acetylcholinesterase, so ACh accumulates at synapses → overstimulation of muscarinic and nicotinic receptors.

Key difference: organophosphates cause irreversible AChE inhibition (through “aging” of the bond); carbamates are reversible (the bond spontaneously dissociates).

Clinical picture: cholinergic toxidrome - DUMBBBELS: Diarrhea, Urination, Miosis, Bradycardia, Bronchospasm/Bronchorrhea, Emesis, Lacrimation, Salivation. Nicotinic effects include muscle fasciculations and weakness. Garlic breath odor is a clue.

Laboratory confirmation: decreased erythrocyte or plasma cholinesterase activity.

RBC AChE (true cholinesterase) - more specific for OP poisoning
Plasma cholinesterase (pseudocholinesterase, butyrylcholinesterase) - more sensitive; recovers faster so useful for monitoring

Treatment:

Atropine: blocks muscarinic effects (dose to dry secretions)
Pralidoxime (2-PAM): reactivates AChE - but only if given before the organophosphate-enzyme bond “ages” (time-sensitive). Not needed for carbamates (which reverse on their own).

21.6 Postmortem Chemistry

Postmortem chemistry is forensic pathology’s overlap with clinical chemistry. The rules that govern living-patient specimens break down after death, and knowing which analytes stay stable versus which change is essential for interpreting autopsy biochemistry.

Specimen Selection: Location Matters

Significant differences exist between left heart and right heart blood, and between central and peripheral blood, in postmortem specimens. Central blood (heart, great vessels) is contaminated by diffusion from adjacent organs - hepatic enzymes leak from the liver, gastric contents diffuse through the diaphragm.

Peripheral blood - femoral vein is preferred - is more reliable for drug levels and most chemistries.

The gold standard specimen for postmortem chemistry is vitreous humor. It is anatomically sequestered behind the blood-retinal barrier, resistant to bacterial contamination and autolysis, and stable for electrolytes (Na, Cl) and urea/creatinine.

Vitreous Humor

Up to ~3 mL can be obtained from each eye (total ~6 mL from both). Always collect from both eyes and pool the specimen to minimize the concentration gradient between central vitreous and retinal-adjacent vitreous. Solute concentrations differ between the two because retinal vasculature creates a diffusion gradient; glucose, for example, diffuses out from retinal vessels, so periretinal vitreous can have higher glucose than central vitreous early after death (and lower later, as glycolysis proceeds inward from the retinal surface).

Technique: needle and syringe directed at the center of the globe, aspirating slowly to avoid retinal contamination. Replace the volume with saline for cosmetic restoration.

Postmortem Glucose: Opposite Directions

This is a frequently tested concept:

Serum glucose INCREASES after death due to hepatic glycogenolysis (liver dumps glycogen stores as glucose into portal blood within the first few hours)
Vitreous glucose DECREASES after death due to glycolysis (anaerobic breakdown by residual cells and bacteria)

Vitreous glucose is useful for evaluating hyperglycemia, not hypoglycemia. Why? A vitreous glucose >200 mg/dL strongly supports antemortem hyperglycemia - postmortem glycolysis can only decrease it, so if it’s still high, it must have been very high before death. A low vitreous glucose is uninterpretable (could be true hypoglycemia OR just postmortem glycolysis).

Hypoglycemia cannot be reliably diagnosed postmortem. Both serum and vitreous glucose move for artifactual reasons (serum up, vitreous down), so a low glucose in any postmortem specimen could be an artifact. If insulin overdose is suspected as cause of death, check insulin and C-peptide at the injection site and in blood (exogenous insulin = high insulin, low C-peptide).

Postmortem DKA Diagnosis

Vitreous fluid is the preferred specimen for postmortem DKA diagnosis. Analyze for glucose and ketones (beta-hydroxybutyrate is most reliable).

Vitreous glucose >200 mg/dL supports antemortem hyperglycemia
Vitreous beta-hydroxybutyrate >250 mg/dL strongly supports DKA as cause of death

Adjunctive tests:

HbA1c is the most reliable postmortem indicator of diabetes. It reflects a 3-month average and is stable postmortem for weeks. Unaffected by the postmortem glucose chaos.
Glycosuria and ketonuria (urine)
Serum acetone (headspace gas chromatography)

Postmortem Urea, Creatinine, and Electrolytes

BUN (or vitreous urea nitrogen, VUN) and creatinine are stable in both blood and vitreous after death. They don’t undergo significant postmortem production or metabolism, making them reliable for diagnosing antemortem renal failure and dehydration.

Sodium and chloride in serum decrease after death at ~0.9 mEq/L per hour (dilution from intracellular water release as cells lyse). Serum electrolytes are unreliable postmortem. Vitreous sodium and chloride are remarkably stable (the blood-retinal barrier protects the vitreous from rapid equilibration with decomposing blood), which is why vitreous is the specimen of choice for postmortem electrolyte assessment.

Vitreous potassium is a special case - see below.

Four Classic Vitreous Patterns

Memorize these patterns - they’re the postmortem chemistry version of ABG interpretation.

Pattern	Na	Cl	K	BUN	Creatinine	Diagnosis
Dehydration	↑	↑	normal (<15)	↑	↑	Hypovolemia / neglect
Uremia	normal	normal	normal (<15)	↑	↑	Renal insufficiency
Decomposition	↓	↓	↑ (>15)	normal	normal	Prolonged PMI
Polydipsia / SIADH	↓	↓	normal (<15)	↓	↓	Water intoxication

How to read the table:

Dehydration concentrates ALL solutes (high Na, Cl, urea, creatinine); K stays normal.
Uremia selectively impairs nitrogen waste clearance (high urea, creatinine; normal Na and Cl).
Decomposition releases intracellular K into vitreous (K rises) and dilutes Na and Cl as cells lyse (they fall); urea/creatinine remain stable.
Polydipsia is the opposite of dehydration - excess water dilutes everything.

Water intoxication pattern is seen in psychiatric patients, MDMA (ecstasy) users, and marathon runners.

Vitreous Potassium and Postmortem Interval

Vitreous potassium rises linearly after death (~0.14-0.55 mEq/L/hr depending on temperature). Blood and CSF potassium rise abruptly and unpredictably (cell lysis, bacterial contamination), so they’re useless for timing.

Vitreous potassium is the single most reliable chemical test for estimating postmortem interval (PMI). Multiple regression formulas incorporate vitreous K⁺ and ambient temperature to estimate time since death, usually accurate to ±6-12 hours.

Interpretation cutoff: initial vitreous K⁺ at time of death is ~5-8 mEq/L. K⁺ <15 mEq/L indicates a reasonable PMI; K⁺ >15 mEq/L indicates a prolonged PMI or significant decomposition, at which point other vitreous chemistries become less reliable. At typical room temperature the K⁺ reaches 15 mEq/L roughly 14-20 hours postmortem.

Limitations:

Temperature fluctuations affect the rate of rise
Eye trauma can contaminate vitreous
Results less reliable as decomposition advances

Postmortem Digoxin

Digoxin levels RISE progressively after death - sometimes dramatically. The huge Vd (tissue-bound) means that as cells break down, sequestered digoxin is released into blood, pushing postmortem levels far above what the patient had antemortem.

Board point: a postmortem digoxin level of 5 ng/mL does NOT necessarily mean the patient died of digoxin toxicity. Use peripheral (femoral) blood rather than central, and use antemortem levels if available.

Second pitfall: digoxin-like immunoreactive substances (DLIS) can also produce false-positive digoxin levels in patients who were never on the drug. DLIS are seen in:

Neonates
Pregnant women
Liver failure
Renal failure
Volume-overloaded states

So the combination of postmortem redistribution + DLIS means postmortem digoxin interpretation requires extreme caution. Use LC-MS/MS to distinguish true digoxin from DLIS if digoxin toxicity is suspected.

Postmortem Tryptase for Anaphylaxis

Postmortem serum tryptase is the biochemical marker for anaphylaxis. Tryptase is a mast cell protease that remains stable for days postmortem (unlike histamine, which degrades rapidly).

Levels >10-11 ng/mL (some use >44 ng/mL for postmortem specifically) support anaphylaxis
Collect from femoral blood, not central - cardiac blood can be contaminated by postmortem mast cell degranulation
Differential for elevated tryptase: systemic mastocytosis, some hematologic malignancies

Pulmonary edema and laryngeal edema at autopsy are nonspecific - they occur in many causes of death. Only tryptase provides biochemical evidence for anaphylaxis.

Chapter 22: Newborn Screening

Newborn screening is one of public health’s greatest success stories - detecting treatable disorders before clinical symptoms cause irreversible damage. Understanding the methodology, the disorders screened, and the follow-up process is essential for clinical pathology practice.

The Philosophy of Newborn Screening

The principle is simple: some devastating diseases are treatable if detected early but cause permanent damage if diagnosis is delayed until symptoms appear. The classic example is phenylketonuria (PKU) - untreated, it causes severe intellectual disability; treated from birth with dietary phenylalanine restriction, children develop normally.

Wilson and Jungner criteria (1968) guide decisions about which conditions to screen:

The condition is an important health problem
There is an accepted treatment
Facilities for diagnosis and treatment are available
There is a recognizable latent or early symptomatic stage
A suitable test exists
The test is acceptable to the population
The natural history is adequately understood
There is agreement on whom to treat
The cost of finding cases is economically balanced with expenditure on treatment
Case-finding should be a continuous process

Specimen Collection

The dried blood spot (DBS): Blood is collected by heel stick (avoiding the central heel to prevent bone damage) onto special filter paper (Guthrie card). The blood must saturate the paper completely, creating a uniform spot. Proper collection technique is critical - inadequate saturation or layered spots cause invalid results.

Timing: Collection at 24-48 hours of age balances two concerns:

Too early: Some metabolites haven’t accumulated to detectable levels; early discharge may miss abnormalities
Too late: Delays treatment initiation and risks damage

Most programs recommend collection after 24 hours of life but before hospital discharge. Premature and sick infants may need repeat screening.

Tandem Mass Spectrometry: The Technology Revolution

Tandem mass spectrometry (MS/MS) transformed newborn screening. Previously, each disorder required a separate test. MS/MS can detect dozens of metabolites simultaneously from a single blood spot, enabling screening for 40+ disorders at once.

How it works:

Metabolites are extracted from the dried blood spot
Electrospray ionization creates charged molecules
First mass analyzer (Q1) selects parent ions by mass-to-charge ratio
Collision cell fragments the ions
Second mass analyzer (Q3) measures specific fragment ions
The combination of parent mass and fragment mass uniquely identifies each metabolite

Why MS/MS is ideal for newborn screening:

High specificity (identifies specific metabolites, not just classes)
High sensitivity
Multiplex capability (many analytes from one specimen)
Rapid throughput
Small sample volume

Disorders Screened

The Recommended Uniform Screening Panel (RUSP) standardizes newborn screening across the US. The core panel includes:

Amino Acid Disorders

Disorder	Analyte	Consequence if Untreated	Treatment
Phenylketonuria (PKU)	↑ Phenylalanine	Intellectual disability, seizures	Phe-restricted diet
Maple Syrup Urine Disease (MSUD)	↑ Leucine, isoleucine, valine	Encephalopathy, death	BCAA-restricted diet
Homocystinuria	↑ Methionine	Intellectual disability, lens dislocation, thrombosis	B6, methionine-restricted diet
Tyrosinemia Type I	↑ Tyrosine, succinylacetone	Liver failure, HCC	Nitisinone + diet
Citrullinemia	↑ Citrulline	Hyperammonemia, encephalopathy	Protein restriction, nitrogen scavengers
Argininosuccinic aciduria	↑ Argininosuccinic acid	Hyperammonemia	Protein restriction, arginine supplementation

PKU in detail: Phenylalanine hydroxylase deficiency prevents conversion of phenylalanine to tyrosine. Phenylalanine accumulates and is neurotoxic. Screening measures phenylalanine; elevated levels trigger confirmatory testing. Treatment is lifelong dietary phenylalanine restriction with special medical formulas. Maternal PKU is critical - women with PKU must maintain strict dietary control during pregnancy or their babies (regardless of genotype) suffer teratogenic effects from in utero phenylalanine exposure.

Fatty Acid Oxidation Disorders

Disorder	Analyte	Consequence if Untreated	Treatment
MCAD deficiency	↑ C8 acylcarnitine (octanoylcarnitine)	Hypoketotic hypoglycemia, sudden death	Avoid fasting, carnitine supplementation
VLCAD deficiency	↑ C14:1 acylcarnitine	Cardiomyopathy, hypoglycemia	Avoid fasting, MCT supplementation
LCHAD deficiency	↑ 3-OH C16, C18 acylcarnitines	Cardiomyopathy, retinopathy, neuropathy	Avoid fasting, MCT diet
Carnitine uptake defect	↓ Free carnitine	Cardiomyopathy, hypoglycemia	Carnitine supplementation

MCAD deficiency in detail: Medium-chain acyl-CoA dehydrogenase deficiency is the most common fatty acid oxidation disorder (~1:15,000). During fasting, the body cannot oxidize medium-chain fatty acids for energy. Patients present with hypoketotic hypoglycemia (low glucose, inappropriately low ketones) during illness or fasting, which can cause sudden death. A common mutation (c.985A>G, p.K304E) accounts for ~80% of mutant alleles. Treatment is remarkably simple: avoid prolonged fasting, ensure adequate carbohydrate intake during illness. With early diagnosis and simple precautions, outcomes are excellent.

Organic Acid Disorders

Disorder	Analyte	Consequence if Untreated	Treatment
Propionic acidemia	↑ C3 acylcarnitine	Metabolic acidosis, hyperammonemia, developmental delay	Protein restriction, carnitine
Methylmalonic acidemia	↑ C3 acylcarnitine	Similar to propionic acidemia	Protein restriction, B12 (some forms), carnitine
Isovaleric acidemia	↑ C5 acylcarnitine	“Sweaty feet” odor, metabolic crisis	Leucine restriction, glycine supplementation
Glutaric acidemia type I	↑ C5DC (glutarylcarnitine)	Macrocephaly, dystonia after metabolic crisis	Lysine restriction, carnitine, emergency protocol
3-MCC deficiency	↑ C5OH acylcarnitine	Often benign; some have symptoms	Variable; often no treatment needed

Endocrine Disorders

Congenital Hypothyroidism:

Screening: TSH (primary) or T4 (with TSH reflex)
Incidence: ~1:2,000-4,000
Consequences: Intellectual disability, growth failure (cretinism)
Treatment: Levothyroxine - early treatment leads to normal development

Congenital Adrenal Hyperplasia (CAH):

Most commonly 21-hydroxylase deficiency
Screening: 17-hydroxyprogesterone (17-OHP)
Consequences: Salt-wasting crisis (life-threatening), virilization
Treatment: Glucocorticoid and mineralocorticoid replacement
Challenge: High false-positive rate in premature and stressed infants (17-OHP is normally elevated)

Hemoglobinopathies

Screening method: Isoelectric focusing (IEF), high-performance liquid chromatography (HPLC), or capillary electrophoresis

Sickle cell disease (HbSS, HbSC, HbS/β-thalassemia):

Pattern: FS (fetal hemoglobin + sickle hemoglobin, no HbA)
Consequences: Vasoocclusive crises, splenic sequestration, infection susceptibility
Treatment: Penicillin prophylaxis, vaccinations, hydroxyurea, transfusion support
The screened infant may be affected (SS) or carrier (AS)

Hemoglobin interpretation patterns:

Pattern	Interpretation
FA	Normal (F = fetal, A = adult HbA)
FAS	Sickle cell trait (carrier)
FS	Sickle cell disease (or S/β0-thal) - no HbA
FSC	HbSC disease
FSA	Sickle/β+-thalassemia (some HbA present)
FAC	HbC trait
FC	HbC disease
F only	May indicate β-thalassemia major (follow up needed)

Cystic Fibrosis

Screening approach: Immunoreactive trypsinogen (IRT) is elevated in CF due to pancreatic duct obstruction. High IRT triggers either:

DNA analysis for CFTR mutations, OR
Repeat IRT at 2 weeks

Confirmation: Sweat chloride testing (>60 mmol/L diagnostic)

Treatment impact: Early identification allows nutritional optimization, pulmonary therapy initiation, and family counseling before symptoms develop.

Other Conditions

Galactosemia: Deficiency of galactose-1-phosphate uridyltransferase (GALT). Galactose and galactose-1-phosphate accumulate. Presents with liver disease, cataracts, sepsis (especially E. coli). Treatment: Galactose-free diet (no breast milk or standard formula).

Biotinidase deficiency: Cannot recycle biotin. Presents with seizures, hypotonia, skin rash, alopecia. Treatment: Biotin supplementation - simple, cheap, effective.

Severe Combined Immunodeficiency (SCID): Screened by measuring T-cell receptor excision circles (TRECs) - low or absent TRECs indicate T-cell lymphopenia. Early diagnosis allows hematopoietic stem cell transplant before fatal infections occur.

Spinal Muscular Atrophy (SMA): Screened by detecting homozygous SMN1 deletion. New treatments (nusinersen, gene therapy) are most effective when given presymptomatically.

Second-Tier Testing

Some conditions have high false-positive rates on primary screening. Second-tier tests improve specificity:

Condition	Primary Screen	Second-Tier Test
CAH	17-OHP	Steroid profile by LC-MS/MS
Tyrosinemia type I	Tyrosine	Succinylacetone
MSUD	Leucine/Isoleucine	Allo-isoleucine
Homocystinuria	Methionine	Total homocysteine

Follow-Up and Confirmation

An abnormal newborn screen is NOT a diagnosis - it indicates increased risk requiring follow-up testing.

Follow-up process:

Contact family urgently (some conditions cause rapid deterioration)
Repeat screen and/or confirmatory testing
Confirmatory tests may include: plasma amino acids, urine organic acids, acylcarnitine profile, enzyme assays, molecular genetic testing
Refer to metabolic specialist

False positives and negatives:

False positives cause parental anxiety and additional testing costs
False negatives occur with some metabolite disorders if specimen collected too early, with blood transfusion (dilutes abnormal hemoglobin), or with some variants
No screening test is perfect; clinical suspicion should prompt testing even with normal newborn screen

Chapter 23: Body Fluid Analysis

Body fluids provide a window into disease processes occurring in specific anatomic compartments. Unlike blood, which reflects systemic conditions, body fluids reveal local pathology - infection, inflammation, malignancy, or abnormal fluid dynamics. Proper collection, handling, and interpretation are essential because many body fluid tests are time-sensitive and specimen-dependent.

A recurring theme across this chapter: when cells or organisms sit in a closed fluid space, they consume glucose and produce acid. That’s why bacterial meningitis, empyema, septic arthritis, and SBP all share the same biochemical fingerprint - low glucose, low pH, high lactate, and high LDH. Learn that pattern once and it applies everywhere.

Quick Reference: Body Fluid Analysis

Pleural Fluid: Light’s Criteria (Exudate if ANY ONE met)

Criterion	Exudate Threshold
Protein (fluid/serum ratio)	>0.5
LDH (fluid/serum ratio)	>0.6
LDH (fluid value)	>2/3 upper limit of normal serum LDH

Interpretation: Transudates result from systemic factors (CHF, cirrhosis, nephrotic syndrome). Exudates result from pleural disease (infection, malignancy, PE, autoimmune).

Ascitic Fluid: SAAG (Serum-Ascites Albumin Gradient)

SAAG	Interpretation
≥1.1 g/dL	Portal hypertension (cirrhosis, CHF, Budd-Chiari)
<1.1 g/dL	Non-portal hypertension (malignancy, TB, nephrotic syndrome, pancreatic)

Synovial Fluid Classification

Type	WBC Count	%PMN	Appearance	Causes
Normal	<200	<25%	Clear	-
Non-inflammatory	<2,000	<25%	Clear/yellow	OA, trauma
Inflammatory	2,000-50,000	>50%	Cloudy	RA, crystals, reactive
Septic	>50,000	>75%	Purulent	Bacterial infection

Crystal identification:

Gout (MSU): Needle-shaped, negative birefringence (yellow parallel to axis)
Pseudogout (CPPD): Rhomboid, weak positive birefringence (blue parallel to axis)

Urine Dipstick: The Most-Missed Pitfalls

Test	Detects	Classic pitfall
Glucose (oxidase)	Glucose only	Ascorbate → false negative
Copper sulfate (Clinitest)	Any reducing sugar	Galactosemia screen in neonates
Protein (dipstick)	Albumin only	Misses Bence Jones (light chains)
Protein (SSA)	Albumin + globulins	SSA+/dipstick− = myeloma
Ketones	Acetoacetate only	Misses β-hydroxybutyrate in DKA; captopril/methyldopa false+
Blood	Hb + myoglobin + RBCs	Can’t distinguish - need microscopy
Nitrite	Nitrate-reducing bacteria	Negative with S. saprophyticus, Enterococcus, Chlamydia, GC
Leukocyte esterase	Neutrophil enzyme	False+ with Trichomonas, eosinophils (AIN)

23.1 Cerebrospinal Fluid (CSF)

Cerebrospinal fluid surrounds the brain and spinal cord, providing mechanical protection, nutrient delivery, and waste removal. CSF is produced by the choroid plexus (about 500 mL/day), circulates through the ventricles and subarachnoid space, and is absorbed by arachnoid granulations. Total CSF volume is only about 150 mL, so it turns over several times daily.

CSF Collection and Handling

CSF is obtained by lumbar puncture, typically at L3-L4 or L4-L5 (below the spinal cord termination). The fluid is collected into sequential tubes:

Tube 1: Chemistry, serology (least likely to have blood from traumatic tap) Tube 2: Microbiology (Gram stain, culture) Tube 3: Cell count (most representative if traumatic tap clears) Tube 4: Special studies (cytology, flow cytometry if indicated)

Critical handling: CSF must be analyzed promptly. WBCs begin to lyse within 1-2 hours at room temperature. Glucose decreases over time. Cell count tubes should be analyzed immediately or refrigerated. Delay also causes in vitro RBC lysis, which releases hemoglobin and can simulate xanthochromia - a false positive for subarachnoid hemorrhage. Process CSF ASAP.

Opening Pressure

Measured with the patient in the lateral decubitus position, legs relaxed. Normal is 6-20 cm H2O (varies by position and patient factors).

Elevated opening pressure indicates:

Increased CSF production (rare)
Decreased CSF absorption (meningitis, subarachnoid hemorrhage)
Obstruction to flow (mass lesion)
Increased venous pressure (venous sinus thrombosis, heart failure)
Idiopathic intracranial hypertension (pseudotumor cerebri)

Quick Reference: CSF Findings in Meningitis

Parameter	Bacterial	Viral	Fungal/TB
Opening pressure	↑↑	Normal or ↑	↑
WBC count	↑↑↑ (>1000)	↑ (10-500)	↑ (10-500)
Cell type	Neutrophils (>80%)	Lymphocytes	Lymphocytes
Protein	↑↑ (>100 mg/dL)	Normal or ↑	↑↑
Glucose	↓↓ (<40 mg/dL)	Normal	↓
CSF/serum glucose	<0.4	>0.6	<0.4

CSF Cell Count

Normal: 0-5 WBCs/μL in adults (all mononuclear cells, predominantly lymphocytes and monocytes); 0 RBCs. Neonates have a higher normal range: 0-20 WBCs/μL due to immature blood-brain barrier. Any neutrophils or elevated counts should raise concern for infection.

Correcting for traumatic tap: When a bloody tap introduces peripheral blood, you can estimate the “true” CSF WBC count by subtracting 1 WBC for every 700 RBCs in the CSF. This matters when evaluating for meningitis on a bloody specimen.

The cell count distinguishes types of meningitis and other CNS processes:

Bacterial meningitis produces a robust neutrophilic response. Expect hundreds to thousands of WBCs, predominantly neutrophils (>80%). The meninges are acutely inflamed, and bacteria in the subarachnoid space trigger massive PMN recruitment.

Viral (aseptic) meningitis produces a lymphocytic response, typically 10-500 WBCs with lymphocyte predominance. Early in viral infection (first 24-48 hours), neutrophils may predominate before shifting to lymphocytes.

Tuberculous and fungal meningitis cause a lymphocytic pleocytosis, often with mixed cells including monocytes. The cell count is typically lower than bacterial meningitis (50-500/μL). The chronicity of these infections produces a more indolent inflammatory response. The profile looks almost identical to viral meningitis on WBC, differential, and protein - the giveaway is low glucose (<50 mg/dL) in fungal/TB vs. normal glucose in viral. Low glucose + lymphocytes = think TB or fungal, pursue India ink, AFB stain, and fungal cultures.

HSV encephalitis is the viral exception to “normal glucose in viral infection.” HSV can cause hypoglycorrhachia along with RBCs in the CSF (hemorrhagic encephalitis, predilection for temporal lobes). Diagnosed by CSF HSV PCR; treat empirically with acyclovir while awaiting results.

Traumatic Tap vs. Subarachnoid Hemorrhage

When RBCs are present, distinguishing traumatic tap from true hemorrhage is critical.

Traumatic tap features:

RBC count decreases from tube 1 to tube 4 (blood clears as CSF flows)
No xanthochromia (fresh blood hasn’t broken down)
Clot may form in tube

Subarachnoid hemorrhage features:

RBC count similar in all tubes
Xanthochromia: Yellow discoloration from bilirubin (takes 2-4 hours to develop as RBCs lyse)
Elevated opening pressure
On microscopy, macrophages containing RBCs (erythrophagocytosis) and/or hemosiderin (golden-brown iron pigment) are specific for true hemorrhage. These take hours to develop and are NOT seen in acute traumatic taps.

Xanthochromia is best detected by spectrophotometry (measures oxyhemoglobin and bilirubin) rather than visual inspection, which is insensitive.

Xanthochromia color by timing:

Pink xanthochromia = free hemoglobin from acute SAH (<12 hours)
Yellow xanthochromia = bilirubin from heme breakdown, old SAH (12-72 hours)

Artifactual (false) xanthochromia - things that turn CSF yellow without hemorrhage:

Elevated serum bilirubin (bilirubin crosses the BBB when very high)
Elevated serum protein
Rifampin (turns body fluids orange-red, including CSF, urine, tears)
Delay to analysis (in vitro RBC lysis)
Traumatic tap specimen that sat too long

Always ask about medications and clinical context before calling xanthochromia a SAH. You do not want to send someone to angiography because their rifampin turned the CSF orange.

CSF Protein

Normal: 15-45 mg/dL (lower than serum due to blood-brain barrier)

Elevated protein indicates disruption of the blood-brain barrier or intrathecal production:

Bacterial meningitis: Often >100 mg/dL, sometimes >500 mg/dL
Viral meningitis: Mild elevation (50-100 mg/dL)
TB/fungal meningitis: Moderate elevation
Guillain-Barré: Elevated protein with normal cell count (albuminocytologic dissociation)
Multiple sclerosis: Mildly elevated or normal

CSF albumin as a BBB marker: Because albumin is made in the liver and not in the CNS, its appearance in CSF reflects blood-brain barrier leak. Normal CSF:serum albumin ratio is <1:230. A higher ratio means the BBB is leaky. This number becomes important when you try to separate “IgG leaked in from blood” from “IgG synthesized within the CNS” - see the IgG index below.

CSF glutamine is elevated in hepatic encephalopathy. Mechanism: liver failure raises blood ammonia, ammonia crosses the BBB, astrocytes detoxify it by converting it to glutamine via glutamine synthetase, and glutamine accumulates in CSF. The CSF glutamine level correlates with severity of encephalopathy.

Detecting a CSF Leak

When clear fluid drips out of someone’s nose or ear after head trauma or surgery, the question is “is this CSF?” Three tests help:

Glucose: CSF has glucose (40-80% of serum); nasal/ear secretions generally don’t. Quick but low specificity - a small amount of blood can throw this off.
Prealbumin (transthyretin): Prominent on CSF electrophoresis.
β2-transferrin (double transferrin peak): A desialylated form of transferrin made in the CNS. Produces a “double transferrin peak” on electrophoresis. The most specific test for CSF.

CSF Glucose

Normal: 50-80 mg/dL, approximately 40-80% of serum glucose (60% rule of thumb).

Always interpret CSF glucose relative to serum glucose (draw serum glucose simultaneously). Hypoglycorrhachia is the term for low CSF glucose.

Low CSF glucose (hypoglycorrhachia) occurs when glucose is consumed by cells or organisms:

Bacterial meningitis: Bacteria metabolize glucose; often <40 mg/dL or <40% of serum
TB/fungal meningitis: Also low due to organism metabolism and inflammation
Viral meningitis: Usually normal (viruses don’t metabolize glucose)
Carcinomatous meningitis: Malignant cells consume glucose

Special CSF Studies

Oligoclonal bands (OCB): Immunoglobulins produced intrathecally appear as discrete bands on CSF electrophoresis that aren’t present in matched serum. Finding ≥2 unique CSF bands (not in serum) supports MS diagnosis. Present in 85-95% of MS patients. Also seen in other inflammatory CNS conditions (neurosarcoidosis, CNS vasculitis, neurosyphilis).

Critical technique point: OCB testing requires simultaneous CSF and serum electrophoresis. The bands must be present in CSF but ABSENT in serum to count as positive. If bands appear in BOTH CSF and serum, the IgG is coming from the systemic circulation (e.g., systemic inflammation) - the test is NEGATIVE for intrathecal synthesis. Bands must be restricted to the CSF.

IgG index: Compares CSF IgG to serum IgG, corrected for albumin (which reflects blood-brain barrier integrity): \[\text{IgG index} = \frac{\text{CSF IgG} / \text{Serum IgG}}{\text{CSF albumin} / \text{Serum albumin}}\] Normal <0.7. Elevated values indicate intrathecal IgG synthesis.

Why the albumin correction matters: albumin is made in the liver, not the CNS, so the CSF:serum albumin ratio is a pure measure of BBB leak. Dividing IgG ratios by albumin ratios removes the “how leaky is the BBB” component and isolates the “how much IgG is being made within the CNS” signal. If the IgG index is elevated, the CNS is making IgG locally.

Cryptococcal antigen: Highly sensitive and specific for cryptococcal meningitis. Latex agglutination or lateral flow assay.

VDRL: Specific (not sensitive) for neurosyphilis. Positive CSF VDRL with appropriate clinical picture confirms neurosyphilis. Negative doesn’t rule it out.

23.2 Pleural Fluid

The pleural space normally contains only a few milliliters of fluid, which lubricates the lung surfaces during respiration. Pleural effusions accumulate when fluid production exceeds absorption - either from increased hydrostatic pressure, decreased oncotic pressure, increased capillary permeability, or impaired lymphatic drainage.

The Fundamental Question: Transudate or Exudate?

This distinction guides the entire diagnostic approach.

Transudates result from systemic factors affecting hydrostatic or oncotic pressure. The pleura itself is not diseased. Major causes:

Congestive heart failure (most common cause of pleural effusion overall)
Cirrhosis with ascites (hepatic hydrothorax)
Nephrotic syndrome
Hypoalbuminemia

Exudates result from local pleural disease - inflammation, infection, or malignancy. The pleura is actively involved. Major causes:

Parapneumonic effusion (adjacent pneumonia)
Empyema (pus in pleural space)
Malignancy (lung cancer, breast cancer, lymphoma, mesothelioma)
Pulmonary embolism
Autoimmune disease (lupus, rheumatoid arthritis)
Tuberculosis

Light’s Criteria

The pleural fluid is an exudate if it meets ANY of the following:

Pleural protein / serum protein ratio > 0.5
Pleural LDH / serum LDH ratio > 0.6
Pleural LDH > 2/3 the upper limit of normal serum LDH

These criteria are extremely sensitive for exudates (~98%) but less specific (~83%). Some transudates are misclassified as exudates, particularly in patients on diuretics (concentration of the fluid).

If Light’s criteria indicate exudate but clinical picture suggests transudate (e.g., CHF patient on diuretics), check the serum-pleural albumin gradient:

Gradient > 1.2 g/dL suggests transudate despite meeting Light’s criteria

Additional features correlating with exudate (supplementary to Light’s criteria):

Specific gravity >1.016
Pleural fluid protein >3 g/dL
Pleural fluid cholesterol >45 mg/dL
Pleural fluid bilirubin:serum bilirubin ratio >0.6

Note the overlap with transudative causes of ascites and hypervolemic hypotonic hyponatremia - CHF, cirrhosis, and nephrotic syndrome are the big three. Same physiology (hydrostatic push or oncotic loss), same answer across multiple testable scenarios.

Additional Pleural Fluid Tests

pH: Normal pleural fluid pH is 7.60 (higher than blood due to bicarbonate transport). Low pH (<7.30) indicates acid production by bacteria or metabolism of glucose by cells.

pH <7.20 in parapneumonic effusion suggests empyema; drainage usually needed
pH <7.30 in malignant effusion predicts poor prognosis

Glucose: Low glucose (<60 mg/dL or <50% of serum) suggests:

Complicated parapneumonic effusion/empyema
Rheumatoid effusion (can be very low)
Malignancy
TB

Adenosine deaminase (ADA): Elevated (>40 U/L) in tuberculous pleuritis (sensitivity ~90-100%, specificity ~85-95%). ADA is produced by activated T lymphocytes, which dominate the TB inflammatory response. Useful in areas of high TB prevalence.

Cytology: Positive in about 60% of malignant effusions on first sample; increases with repeated sampling.

Triglycerides: >110 mg/dL indicates chylothorax (thoracic duct injury). Milky appearance.

Amylase: Elevated pleural amylase has three testable causes - pancreatitis, esophageal perforation, and malignancy. Pancreatitis characteristically produces a left-sided effusion (the left hemidiaphragm is closer to the pancreas) with pancreatic-type amylase. Esophageal perforation produces salivary-type amylase (which leaks from the ruptured esophagus). Amylase isoenzyme analysis distinguishes them.

Pleural Fluid Patterns by Cause

Parapneumonic effusion: Exudate from adjacent pneumonia, no bacteria in the fluid. If bacteria are present, it’s no longer a parapneumonic effusion - it’s an empyema. Parapneumonic effusions may resolve with antibiotics alone.

Empyema: Bacteria present in the pleural space. Criteria include pH <7.2, glucose <40, neutrophils >100,000/μL, and bacteria on Gram stain. Requires drainage (chest tube) plus antibiotics. The extreme values reflect active bacterial metabolism of glucose and lactic acid production.

Tuberculous effusion: Lymphocyte predominant with a paucity of mesothelial cells (granulomatous inflammation traps them). Elevated ADA is diagnostic.

Pulmonary embolism effusion: Usually exudative, often bloody, and cytologically shows mesothelial hyperplasia often with atypia. This is the polar opposite of TB effusion: PE has MANY mesothelial cells; TB has FEW. Don’t over-call malignancy on a reactive mesothelial proliferation - correlate with clinical.

Rheumatoid effusion: Exudate with characteristic findings pH <7.2, glucose <30 mg/dL, LDH >700. The extremely low glucose and very high LDH are among the most extreme values seen in any effusion. Rheumatoid factor may be detected in the fluid.

Uremic effusion: Exudative effusion from uremic pleuritis. Same pathophys also causes uremic pericarditis. Uremia = symptomatic elevation of BUN/creatinine (encephalopathy, pericarditis, nausea); azotemia = elevated numbers without symptoms.

Meigs syndrome: Ovarian fibroma + pleural effusion + ascites. The effusion is typically right-sided and transudative, resolves after tumor resection. Classic board triad - think ovarian fibroma in a woman with pleural effusion + ascites.

Chylothorax vs. pseudochylothorax: Both are milky. The distinction is important.

Feature	Chylous	Pseudochylous
Cause	Lymphatic (thoracic duct) obstruction	Cellular breakdown in long-standing effusion
Microscopy	Lymphocyte-predominant	Mixed inflammatory cells + cholesterol crystals
Triglycerides	>110 mg/dL	<50 mg/dL
Chylomicrons (on electrophoresis)	Present	Absent

Chylous causes include lymphoma (most common), trauma/surgery, lymphangioleiomyomatosis, sarcoidosis, and infection. Pseudochylous fluid gets its milky look from lipids released by degenerating inflammatory cell membranes in a long-standing effusion.

Eosinophils in pleural fluid: Suggests prior instrumentation (prior thoracentesis) or air in the pleural space (pneumothorax). Can also reflect drug reactions, parasitic infection, and Churg-Strauss. Usually benign and most commonly a footprint of previous procedures.

23.3 Synovial Fluid

Synovial fluid is a viscous ultrafiltrate of plasma combined with hyaluronic acid secreted by synoviocytes. It lubricates joints and provides nutrition to cartilage (which is avascular). Normal joints contain only 1-4 mL of fluid.

Joint aspiration (arthrocentesis) is indicated for any acute monoarticular arthritis to rule out septic arthritis, which is a medical emergency.

Gross Examination

Normal: Clear to straw-colored, highly viscous

Non-inflammatory: Clear/straw colored, similar to normal but increased in volume

Inflammatory: Yellow, cloudy/turbid (from cells), decreased viscosity (hyaluronic acid degraded)

Septic, gout, or RA (very high WBC states): Can all produce yellow or purulent fluid. Gross appearance alone does not distinguish them.

Hemorrhagic: Bloody - suggests trauma, coagulopathy, or (if recurrent) pigmented villonodular synovitis

Viscosity test: Normal synovial fluid forms a long string (4-6 cm) when dripped from a syringe due to hyaluronic acid. Inflammation activates enzymes that degrade hyaluronic acid, so inflammatory fluid has low viscosity and drips like water. The “string test” is a quick bedside check: if the fluid breaks easily, it’s inflammatory.

Cell Count Classification

The WBC count is the most useful parameter for classification:

Non-inflammatory (<2,000 WBC/μL, <25% PMNs):

Osteoarthritis
Trauma
Early rheumatoid arthritis (may be higher)

Inflammatory (2,000-50,000 WBC/μL, often >50% PMNs):

Rheumatoid arthritis
Crystal arthropathies (gout, pseudogout)
Reactive arthritis
Psoriatic arthritis

Septic (typically >50,000 WBC/μL, >75% PMNs):

Bacterial infection
Prosthetic joint infection (lower counts may still indicate infection)

Important caveat: These ranges overlap. Septic arthritis, gout, and rheumatoid arthritis can ALL produce WBC counts >100,000/mm³ with >90% neutrophils. These three can produce the most extreme synovial fluid WBC counts. Septic arthritis is the most dangerous and must always be ruled out. Gout and RA can mimic septic arthritis biochemically. Partially treated or indolent infections may have lower counts. When in doubt, treat as potentially infected.

Septic Arthritis Biochemistry

Septic arthritis produces a predictable biochemical fingerprint (same logic as bacterial meningitis or empyema - bacteria and WBCs consume glucose and produce acid):

Low pH
Low glucose
High lactate
Very high WBC (usually >50,000-100,000)

Gram stain and culture are essential. Considerable overlap with RA and severe gout biochemically.

Glucose differential (Δ glucose) = serum glucose − synovial fluid glucose. Higher Δ glucose means more inflammation (WBCs and bacteria eating glucose inside the joint). Δ glucose >40 mg/dL suggests septic arthritis, gout, or rheumatic fever. Non-inflammatory effusions have a small Δ glucose.

Crystal Analysis

This is one of the most important and underutilized laboratory tests. Compensated polarized microscopy can definitively diagnose gout and pseudogout at the bedside or in the lab.

Monosodium urate crystals (GOUT):

Shape: Needle-shaped, pointed ends
Birefringence: Strongly negative (yellow when parallel to slow axis of compensator)
Size: 5-25 μm, often intracellular during acute attacks
Mnemonic: “Yellow when parallel” = urate

Gout: Needle-shaped MSU crystals with strong negative birefringence (yellow parallel to compensator axis).

Calcium pyrophosphate dihydrate crystals (PSEUDOGOUT/CPPD):

Shape: Rhomboid or rod-shaped, blunt ends
Birefringence: Weakly positive (blue when parallel to slow axis)
Size: Smaller, 2-10 μm
Often associated with chondrocalcinosis on X-ray

Pseudogout: CPPD crystals are rhomboid with weak positive birefringence (blue parallel to compensator axis).

Why this matters: Crystal arthritis and septic arthritis can coexist. Finding crystals doesn’t rule out infection - always send culture if there’s clinical suspicion.

23.4 Ascitic Fluid (Peritoneal Fluid)

Ascites is the pathological accumulation of fluid in the peritoneal cavity. The approach to ascites parallels pleural fluid - first determine if it’s due to portal hypertension or peritoneal disease.

Serum-Ascites Albumin Gradient (SAAG)

The SAAG has replaced the transudate/exudate classification for ascites because it better reflects the underlying pathophysiology.

SAAG = Serum albumin - Ascites albumin

High SAAG (≥1.1 g/dL) indicates portal hypertension:

Cirrhosis (most common)
Alcoholic hepatitis
Cardiac failure (cardiac cirrhosis)
Budd-Chiari syndrome (hepatic vein thrombosis)
Portal vein thrombosis
Hepatic veno-occlusive disease
Myxedema

Low SAAG (<1.1 g/dL) indicates peritoneal disease (no portal hypertension):

Peritoneal carcinomatosis
Tuberculous peritonitis
Pancreatic ascites
Nephrotic syndrome
Serositis (lupus)

Why SAAG works: Portal hypertension creates a hydrostatic pressure gradient that forces fluid (low in albumin) into the peritoneal cavity. The serum albumin is higher than ascitic albumin by at least 1.1 g/dL due to this pressure gradient. In peritoneal disease, the capillaries leak protein, so the gradient is smaller.

Spontaneous Bacterial Peritonitis (SBP)

SBP is infection of ascitic fluid without an obvious intra-abdominal source (e.g., perforation). It occurs almost exclusively in cirrhotic patients due to bacterial translocation from the gut and impaired immune defenses.

Diagnosis: Ascitic fluid PMN count ≥250 cells/μL. Culture may be negative in up to 40% of cases (culture-negative neutrocytic ascites) - the PMN threshold is what drives treatment, not the culture.

Clinical presentation: Fever, abdominal pain, altered mental status, worsening liver function. But many patients have subtle or no symptoms - paracentesis should be performed liberally in cirrhotic patients with new ascites, clinical deterioration, or GI bleeding.

Culture: Often negative even in true SBP. Use blood culture bottles inoculated at the bedside to maximize yield. Monomicrobial in SBP; polymicrobial suggests secondary peritonitis (perforation).

Primary vs. secondary peritonitis on Gram stain:

Primary (SBP): Gram stain usually negative (low bacterial load, monomicrobial)
Secondary (perforated viscus): Gram stain usually positive (high bacterial load, polymicrobial)

Secondary peritonitis is a surgical emergency and cannot be treated with antibiotics alone - source control (surgery) is needed.

Treatment: Third-generation cephalosporin (cefotaxime) empirically. Albumin infusion reduces mortality by preventing hepatorenal syndrome.

23.5 Urinalysis

Urine is the most accessible body fluid and the most information-dense for the effort. A properly-read urinalysis gives information on renal function, metabolic status, infection, hemolysis, liver disease, and more. The trick is knowing what each dipstick pad actually measures, what can fool it, and what the microscopic findings mean.

Urine analysis is also where pre-analytical errors bite hardest. Urine left sitting at room temperature becomes alkaline (bacteria convert urea to ammonia, CO2 escapes), loses glucose (bacterial glycolysis), and has RBCs that lyse. Process urine promptly or refrigerate.

Urine Color: Beyond Yellow

Yellow foam after shaking: Evidence of bilirubin (specifically conjugated bilirubin, which is water-soluble). Normal urine foam is white (urobilinogen gives urine its yellow color, but not the foam). Yellow foam is a quick bedside signal of conjugated hyperbilirubinemia - obstructive jaundice or hepatocellular disease.

Red urine - the three common causes: Hematuria (intact RBCs), hemoglobinuria (free Hb from intravascular hemolysis), myoglobinuria (rhabdomyolysis). All three turn the urine dipstick positive for blood (discussed below). To distinguish: centrifuge the urine. RBCs pellet (hematuria); the supernatant stays red if it’s hemoglobin or myoglobin.

Red urine with negative hemoglobin dipstick: Not one of the three. Consider:

Porphyria (porphyrins)
Beets (“beeturia”)
Medications: rifampin, L-dopa

These substances don’t have peroxidase activity, so the dipstick is negative.

Purple urine bag syndrome: Purple discoloration of the catheter bag or container. Caused by bacteria producing indigo and indirubin pigments from tryptophan metabolism; these pigments react with plastic in catheters and bags. Associated with UTIs in catheterized patients. Usually benign, treat the UTI.

Urine Dipstick: What Each Pad Actually Measures

The dipstick is a stack of enzyme reactions on paper. Knowing the chemistry tells you the pitfalls.

Glucose (glucose oxidase method):

Step 1: Glucose oxidase converts glucose → gluconate + H2O2
Step 2: H2O2 oxidizes a chromogen (dye) → color change
Specific for glucose (no cross-reactivity with other sugars)
Inhibited by ascorbic acid (vitamin C), which causes false negatives

Glucose (copper sulfate method, Clinitest):

Glucose (and other reducing substances) reduce Cu(II) sulfate → Cu(I) sulfate, changing color from blue to orange
Not specific for glucose - detects any reducing sugar (fructose, lactose, galactose, pentoses) and drugs (ascorbic acid, cephalosporins)
Clinical use: screening neonates for galactosemia (dipstick negative for glucose, copper sulfate positive → non-glucose reducing sugar → pursue galactosemia workup)

Renal glucose threshold: Urine typically lacks glucose until serum glucose exceeds ~180 mg/dL. Below this, the proximal tubule reabsorbs all filtered glucose; above it, the transporters saturate and glucose spills.

Glucosuria at normal serum glucose: Pregnancy (increased GFR lowers the renal threshold) and renal tubular dysfunction (Fanconi syndrome, SGLT2 defects). Don’t assume diabetes if glucose is in the urine - check serum glucose.

Standing specimen effect on glucose: If urine sits at room temperature, glucose decreases because environmental organisms metabolize it (glycolysis). Pre-analytical false-negative.

Protein (dipstick, protein error of indicators):

Detects albumin only
Based on the protein error of indicators (albumin shifts the pKa of the indicator dye)

Protein (precipitation/sulfosalicylic acid/SSA):

Detects albumin AND globulins
The discrepancy matters: if SSA is positive but dipstick is negative → non-albumin protein is present → think Bence Jones proteins (light chains) in myeloma. Classic board scenario.

Benign proteinuria - not always kidney disease:

Intermittent proteinuria: exercise, fever, dehydration
Postural (orthostatic) proteinuria: protein appears only when standing, absent when supine. Common in adolescents (up to 5%), resolves by age 30. Confirmed by a split urine collection.

Ketones (nitroprusside reaction):

Dipstick detects acetoacetate only
Does NOT detect β-hydroxybutyrate, the predominant ketone body in DKA
During DKA treatment, β-hydroxybutyrate converts to acetoacetate as the patient improves - so the dipstick can paradoxically become MORE positive even as the patient gets better
False-positive causes: sulfhydryl-containing drugs like captopril and methyldopa (free -SH groups react with nitroprusside)

Blood (peroxidase reaction):

Detects peroxidase activity, so it’s positive for hemoglobin, myoglobin, AND intact RBCs
Cannot distinguish among hematuria, hemoglobinuria, and myoglobinuria - microscopy is required
But RBCs lyse if the sample stands too long, so microscopy may miss them in a delayed specimen

Distinguishing hematuria / hemoglobinuria / myoglobinuria after a positive dipstick:

Microscopy: RBCs present = hematuria
Hemoglobinuria = intravascular hemolysis (free Hb overwhelms haptoglobin). Plasma is also pink/red. Check haptoglobin, LDH, smear.
Myoglobinuria = rhabdomyolysis. Accompanied by very high CK, often >10,000 U/L. Causes: crush injury, statins, seizures, cocaine, extreme exercise, heat stroke. Myoglobin is nephrotoxic and precipitates in renal tubules (especially acidic urine) - AKI risk. Treatment: aggressive IV fluids, urine alkalinization.

Bilirubin vs. urobilinogen on dipstick:

Urine bilirubin reflects conjugated bilirubin specifically (water-soluble, crosses glomerulus). Unconjugated bilirubin is bound to albumin and never makes it into urine.
Urine urobilinogen reflects either conjugated or unconjugated bilirubin excess. Urobilinogen is made in the gut by bacteria from conjugated bilirubin, reabsorbed, and excreted in urine.

Biliary obstruction pattern (complete picture to memorize):

Parameter	Finding
Stool color	Pale (no stercobilin)
Urine color	Dark/yellow with yellow froth
Urine dipstick bilirubin	↑ Increased
Urine dipstick urobilinogen	↓ Decreased

Why: bile can’t reach the gut → less conjugated bilirubin for bacteria to convert to urobilinogen → ↓ urobilinogen in urine. Conjugated bilirubin backs up into blood → spills into urine. Contrast with hemolytic jaundice: ↑ urobilinogen but NO bilirubin in urine (because unconjugated bilirubin isn’t water-soluble and doesn’t cross the glomerulus).

Nitrite: Detects bacteria that convert nitrate to nitrite (mainly gram-negative Enterobacteriaceae, e.g., E. coli, Klebsiella, Proteus). Nitrite-negative UTI organisms that don’t reduce nitrate:

Staphylococcus saprophyticus (2nd most common UTI in young women)
Enterococci
Chlamydia
Neisseria gonorrhoeae

A negative nitrite does NOT exclude UTI. Gram-positives and atypicals generally don’t reduce nitrate.

Leukocyte esterase (LE): Enzyme from neutrophils (primary UTI WBC). Also produced by eosinophils (allergic interstitial nephritis - positive LE without UTI) and Trichomonas (STI producing LE - false positive for UTI). LE detects the enzyme, not intact WBCs, so it can be positive after WBCs have lysed.

Ascorbic acid interferes with multiple dipstick tests: glucose, hemoglobin, bilirubin, nitrite, leukocyte esterase - all oxidase-based reactions → false negatives in patients taking vitamin C supplements. Classic board pearl.

Specific Gravity

Measures urine concentration (density relative to water). Normal: 1.005-1.030. Reflects the kidney’s ability to concentrate or dilute urine.

Decreased SG (dilute urine):

Diabetes insipidus (can’t concentrate without ADH action)
Polydipsia (excess water intake)
Diuretics
Isosthenuria

Increased SG (concentrated urine):

SIADH
Dehydration
Diabetes mellitus (glucose as osmotic solute)
Proteinuria
CHF
Addison disease

Isosthenuria: Specific gravity fixed at ~1.010 (same as plasma ultrafiltrate). The damaged kidney can neither concentrate nor dilute urine. Causes: CKD, acute tubular injury, sickle cell trait/disease. In sickle cell, RBCs sickle in the hyperosmolar, hypoxic renal medulla, causing microinfarction of the vasa recta and loss of the medullary concentration gradient - often one of the earliest renal findings in sickle cell.

Dipstick SG method limitation: The dipstick uses a pH-indicator-based reaction most sensitive to small ionic solutes (Na, Cl, K) and less sensitive to large molecules (glucose, protein, contrast dye). Can give falsely low SG when only large solutes are elevated. Refractometry is more accurate for overall specific gravity.

Urine pH

Normal: 5.5-6.5 (mildly acidic). Kidneys excrete metabolic acids.

Alkaline urine causes:

UTIs with urea-splitting organisms (Proteus, Klebsiella) - urease converts urea → ammonia → alkalinizes urine → precipitates magnesium ammonium phosphate (struvite, “coffin lid” crystals). Classic scenario: recurrent Proteus UTI with staghorn calculus.
Renal tubular acidosis type I (distal) - can’t secrete H+
Vegetarian diet
Standing specimen (bacterial overgrowth, urea → ammonia; CO2 escapes)

Urine pH in RTA:

Type I (distal): urine pH >5.5 (can’t secrete H+)
Type II (proximal): variable (early = high due to bicarbonate wasting, late = low once serum bicarbonate depleted)
Type IV: urine pH <5.5

Kidney Stones

Stones are most commonly analyzed by crystallography or infrared spectroscopy for chemical composition. Knowing the composition guides prevention.

Stone frequency (most to least common):

Calcium oxalate (~70%)
Calcium phosphate
Magnesium ammonium phosphate (struvite)
Uric acid (urate)
Cystine (rarest)

Calcium oxalate risk factors: low urine volume, low urine citrate (citrate chelates calcium, preventing stone formation), high urine calcium, high urine oxalate. Prevention: increase fluids, potassium citrate supplementation.

Hyperoxaluria causes:

Status post small bowel resection or bypass (enteric hyperoxaluria: fat malabsorption → calcium binds fat instead of oxalate → free oxalate is absorbed and excreted in urine). Increasingly common post-bariatric surgery.
High-oxalate foods: spinach, rhubarb, nuts
Ethylene glycol poisoning (metabolized to oxalate - classic scenario: alcoholic with AG metabolic acidosis, increased osmolal gap, and envelope-shaped oxalate crystals in urine)

Calcium phosphate risk factors: low urine volume, high urine pH (alkaline), high urinary calcium/phosphate. Associated with RTA type I (persistently alkaline urine). Calcium phosphate and struvite both form in alkaline urine.

Uric acid (urate) risk factors: low urine volume, low urine pH (acidic), high urine uric acid. Uric acid stones are radiolucent (not visible on plain X-ray). Prevention: alkalinize the urine (urine pH >6.0).

Cystine stones: Occur exclusively in cystinuria - an autosomal recessive deficiency of the dibasic amino acid transporter in the proximal tubule. The transporter normally reabsorbs cystine, ornithine, lysine, arginine (mnemonic: COLA). Cystine is poorly soluble and precipitates. Hexagonal crystals (benzene ring shape) are pathognomonic. Confirmed by cyanide-nitroprusside test (turns magenta). Note: cystinuria, NOT cystinosis (separate disease not associated with cystine stones).

Struvite (MAP): Infection stones in alkaline urine from urea-splitting organisms. Treat the UTI to prevent recurrence.

Crystal Identification

Knowing the classic crystal descriptions is pure memorization. The board-testable patterns:

Crystal	Shape / Description	Clinical context
Calcium oxalate	Envelope-shaped (dihydrate) or dumbbell (monohydrate)	Most common; ethylene glycol poisoning
Uric acid (urate)	Diamond/rhomboid, also rods, elongated hexagons	Acidic urine, gout, tumor lysis; “brick dust” in newborn diapers
Struvite (MAP)	Coffin lid	Alkaline urine, urease-producing bacteria, staghorn calculi
Cystine	Hexagonal (flat, colorless plates)	Pathognomonic for cystinuria
Ammonium biurate	Thorn apple (round with spiky projections)	Clinically insignificant; alkaline/old urine
Tyrosine	Sheaves of wheat / silky needles	Tyrosinemia, liver disease, hyperbilirubinemia
Cholesterol	Broken window glass (rectangular plates with notched corner)	Nephrotic syndrome, hypercholesterolemia
Leucine	Yellow concentric circles (oil drop)	Severe liver disease, maple syrup urine disease
Bilirubin	Yellow-brown needles or granules	Bilirubinuria (obstructive jaundice)

Note: tyrosine crystals also appear histologically in pleomorphic adenoma of the salivary gland - same crystal, different context.

Urine Microscopy: Glomerular vs. Non-Glomerular Hematuria

Glomerular hematuria signs: Dysmorphic RBCs (distorted by passing through GBM), RBC casts, erythrophagocytosis. RBC casts are pathognomonic for glomerulonephritis.

Non-glomerular bleeding (ureter, bladder): isomorphic RBCs, no casts.

Urine Casts

Casts are protein cylinders formed in the distal tubules, embedded with whatever was there at the time. The matrix is Tamm-Horsfall protein.

Hyaline casts: Clear, transparent, composed of Tamm-Horsfall protein alone. Nonspecific - seen in dehydration, exercise, concentrated urine, and many conditions. Finding them alone is not significant.

Granular casts: Rough granular surface, from degenerating cellular material. Can be fine or coarse. Muddy brown granular casts are the classic finding in acute tubular necrosis (ATN). Also appear after vigorous exercise, dehydration, and heat-related injury.

Waxy casts: Smooth, waxy, sharp edges. Represent end-stage cast degeneration (granular → waxy). Indicate advanced/chronic renal disease with prolonged tubular stasis. Distinguished from hyaline casts by noticeable contour and opacity.

RBC casts: Pathognomonic for glomerulonephritis. RBCs embedded in tubular protein matrix, red-orange color, visible RBC shapes. Always significant.

Fatty casts: Hallmark of nephrotic syndrome. Massive proteinuria → compensatory hepatic lipid overproduction → hyperlipidemia → lipiduria → lipid droplets in casts. Under polarized light, the cholesterol-containing lipid droplets form Maltese cross patterns - pathognomonic for nephrotic syndrome. “Oval fat bodies” are tubular cells containing lipid droplets.

Chapter 24: Autoimmune Serology

Autoimmune diseases occur when the immune system attacks self-antigens. Laboratory testing detects autoantibodies, which serve as markers of disease (and sometimes participate in pathogenesis). Understanding these tests requires knowing which antibodies associate with which diseases - and equally important, understanding their limitations.

This chapter also covers the broader clinical immunology needed to reason about autoimmune testing: how B cells, T cells, NK cells, APCs, and granulocytes actually do their jobs, how complement is organized (beyond the consumption patterns in 24.7), what cytokines drive which responses, how to work up suspected primary immunodeficiency, and how transplant rejection and GVHD fit into the same conceptual framework. Sections 24.1-24.8 are the serology-first content. Sections 24.9+ are the underlying immunology that the serology sits on top of.

General principle of autoimmunity: Autoimmunity = breakdown of self-tolerance. Central tolerance (negative selection in thymus/bone marrow) and peripheral tolerance (anergy, Tregs, AICD) normally delete or silence self-reactive clones. When those fail, self-antigens become targets.

Who gets autoimmune disease: Mostly females of childbearing age. Estrogen enhances B cell activity and Th2 skewing. Key exceptions worth remembering - primary sclerosing cholangitis (men) and Sjögren (tends to be postmenopausal women).

HLA associations (high yield):

HLA-B27: seronegative spondyloarthropathies (PAIR - Psoriatic, Ankylosing spondylitis, IBD-associated, Reactive)
HLA-B51: Behcet
HLA-DR2: narcolepsy (DQB1*06:02, >98%), Goodpasture, MS
HLA-DR3: SLE, type 1 DM, Sjögren, myasthenia gravis
HLA-DR4: rheumatoid arthritis (shared epitope), type 1 DM
HLA-DR5: Hashimoto
HLA-DQ2 / HLA-DQ8: celiac disease (near-100% NPV)
HLA-A3: hereditary hemochromatosis (linkage disequilibrium with HFE)

Drug-induced autoimmunity worth memorizing:

Drug-induced lupus - SHIPP mnemonic: Sulfa drugs, Hydralazine, Isoniazid, Procainamide, Phenytoin. Also TNF-alpha inhibitors and minocycline.
Autoimmune hemolytic anemia - methyldopa (true autoantibody, anti-RhD) and penicillins (hapten mechanism).
Penicillamine - triggers vasculitis, drug-induced lupus, myasthenia gravis, pemphigus, Goodpasture.
Hepatitis B - triggers polyarteritis nodosa via immune complex deposition.

Lab patterns across autoimmune disease:

Elevated inflammatory markers (CRP, ESR, ferritin) and anemia of chronic disease via hepcidin.
SLE specifically causes cytopenias (anti-cell antibodies, type II hypersensitivity).
Lupus anticoagulant - prolonged aPTT but hypercoagulable in vivo. The aPTT is prolonged because LA binds phospholipid in the reagent, but in vivo it activates platelets and endothelium.

Immunofluorescence methods:

Direct IF: patient tissue + fluorescein-labeled anti-human globulin. Detects bound (in situ) autoantibodies. Used for pemphigus/pemphigoid skin biopsies, lupus band test, renal biopsies.
Indirect IF: patient serum + substrate (HEp-2 for ANA, rat liver for smooth muscle, monkey esophagus for pemphigus, Crithidia for anti-dsDNA) + fluorescein-labeled anti-human globulin. Detects circulating antibodies.

24.1 Antinuclear Antibodies (ANA)

The ANA test is the cornerstone of autoimmune serology, serving as a screening test for systemic autoimmune diseases, particularly systemic lupus erythematosus (SLE).

The ANA Test: Method and Interpretation

Indirect immunofluorescence (IIF) on HEp-2 cells remains the gold standard. Patient serum is incubated with HEp-2 cells (a human epithelial cell line derived from laryngeal carcinoma), then fluorescent anti-human IgG is added. If the patient has antibodies against nuclear antigens, the nuclei fluoresce. Both resting and mitotically active cells are evaluated, because some antigens (e.g., centromere) only reveal their pattern during mitosis.

Other ANA detection methods (mnemonic: Evaluate Immunoglobulin Location):

ELISA (enzyme-linked immunosorbent assay) - fast, automated, good for screening
IIF on HEp-2 - gold standard, gives titer + pattern
Luminex bead-based flow cytometry - multiplex screen for common ENA antibodies

The IIF pattern matters because it narrows the target antigen before you order the more specific follow-up tests.

Titer: Serum is serially diluted. The titer is the highest dilution that still shows fluorescence (e.g., 1:160 means positive at 1:160 dilution). Higher titers are more clinically significant.

<1:40: Negative
1:40-1:80: Borderline (often seen in healthy individuals)
≥1:160: More likely to be clinically significant

Pattern: The pattern of nuclear staining suggests which antigens are targeted, guiding further testing.

ANA Patterns and Their Meaning

Homogeneous (diffuse): Uniform staining of the entire nucleus

Antigens: dsDNA, histones, chromatin
Associations: SLE, drug-induced lupus
Most common pattern, but least specific

Speckled: Discrete dots throughout the nucleus (nucleoplasm), sparing the nucleoli

Antigens: Extractable nuclear antigens (ENAs) - Sm, RNP, SSA (Ro), SSB (La)
Associations: SLE, mixed connective tissue disease (MCTD), Sjögren syndrome
Most common pattern overall; often seen in healthy individuals at low titers

Nucleolar: Staining limited to the nucleoli (3-5 bright spots per nucleus)

Antigens: RNA polymerase I/III, fibrillarin, PM-Scl
Associations: Systemic sclerosis (especially diffuse cutaneous)

Centromere (discrete speckled): 46 discrete dots per cell (corresponding to centromeres of chromosomes)

Antigen: Centromere proteins (CENP-A, B, C)
Association: Limited cutaneous systemic sclerosis (CREST syndrome)
Very specific pattern - strongly suggests this diagnosis

Nuclear membrane (rim): Staining at the nuclear periphery

Antigens: dsDNA, nuclear envelope proteins (lamins)
Associations: SLE (especially with nephritis); primary biliary cholangitis (anti-gp210, anti-lamin B receptor)

ANA Limitations

Low specificity: ANA is positive in many conditions besides autoimmune disease:

5% of healthy young adults
10-25% of elderly (increases with age)
Infections (EBV, hepatitis C)
Medications (hydralazine, procainamide, isoniazid, many others)
Malignancy

The appropriate response to a positive ANA is to pursue specific antibody testing based on the pattern and clinical context - not to diagnose lupus based on ANA alone.

ANA has high NPV for SLE: sensitivity for SLE is 95-100%, so a negative ANA essentially rules out SLE. That makes it a good screening test but a bad confirmatory test. ANA is positive in SLE (~99%), drug-induced lupus (>95%), scleroderma (90%), Sjögren (70%), MCTD (~100%), dermatomyositis/polymyositis (60%), and many non-autoimmune states.

Mnemonic for ENA speckled pattern - “Speckled Stuff Reacts Spectacularly”: SSA (Ro), SSB (La), U1-RNP, Smith. These four are the classic extractable nuclear antigens that produce a speckled pattern on IIF.

24.2 SLE-Associated Antibodies

SLE Demographics and Clinical Features

SLE is predominantly a disease of women aged 15-45 (F:M ~9:1), more common and often more severe among Black and Hispanic/Latina patients, influenced by ancestry, social determinants, and access to care. The peak during reproductive years reflects estrogen-driven immune activation.

Classic SLE diagnostic criteria (need 4 of 11): malar rash, discoid rash, photosensitivity, oral ulcers, arthritis (≥2 joints), serositis (pleuritis or pericarditis), renal dysfunction (proteinuria or cellular casts), neurologic dysfunction, cytopenias, immunologic criteria (anti-dsDNA, anti-Sm, antiphospholipid), and positive ANA.

SLE cytopenia mechanisms: anti-RBC antibodies (AIHA in ~10%), anti-lymphocyte antibodies (lymphopenia is highly characteristic), anti-platelet antibodies (ITP-like thrombocytopenia). All type II hypersensitivity.

Anti-dsDNA (Anti-double-stranded DNA)

Clinical utility: Highly specific for SLE (~95% specificity). Present in 50-70% of SLE patients.

Why it matters:

Correlates with disease activity - levels rise during flares
Strongly associated with lupus nephritis (forms immune complexes that deposit in glomeruli)
Useful for monitoring: Rising titers may predict flares

Testing methods:

Farr assay (radioimmunoassay): Most specific, detects high-avidity antibodies
Crithidia luciliae IIF: Uses a flagellate with a kinetoplast containing pure dsDNA and no human nuclear antigens - very specific substrate
ELISA: Most common; may detect low-avidity antibodies (less specific)

Anti-Smith (Anti-Sm)

Clinical utility: Most specific antibody for SLE (~99% specificity), but low sensitivity (~25-30%)

What it targets: Sm proteins are components of small nuclear ribonucleoproteins (snRNPs) involved in mRNA splicing.

Clinical meaning: If positive, it essentially confirms SLE. However, it doesn’t correlate with disease activity and levels remain stable over time.

Anti-Histone

Clinical utility: Present in >95% of drug-induced lupus; also in 50-70% of idiopathic SLE. Target is the H2A-H2B dimer of the nucleosome.

Drug-induced lupus: Classically caused by the SHIPP drugs - Sulfa-containing drugs, Hydralazine, Isoniazid, Procainamide, Phenytoin. Also TNF-alpha inhibitors and minocycline. Presents with arthritis, serositis, constitutional symptoms. Notably spares kidneys and CNS. Anti-histone positive, anti-dsDNA usually negative. Resolves when drug is stopped.

Anti-SSA (Ro) and Anti-SSB (La)

Clinical associations:

Sjögren syndrome (primary and secondary)
SLE (especially subacute cutaneous lupus, which causes photosensitive rashes)
Neonatal lupus

Neonatal lupus: Maternal anti-SSA (and sometimes anti-SSB) cross the placenta. In the fetus, they can cause:

Congenital heart block: Permanent; requires pacing in severe cases. Risk ~2% in anti-SSA positive mothers
Neonatal lupus rash: Transient; resolves as maternal antibodies clear
Cytopenias: Transient

All pregnant women with known anti-SSA should have fetal heart monitoring.

Laboratory note: Anti-SSA antibodies may cause a negative ANA with a positive anti-SSA because Ro antigens may be lost during HEp-2 cell fixation. Always test specifically for anti-SSA if Sjögren syndrome is suspected, regardless of ANA result.

Anti-U1-RNP (Mixed Connective Tissue Disease)

Very high titers of anti-U1-RNP define mixed connective tissue disease (MCTD), which has overlapping features of SLE, systemic sclerosis, and polymyositis. Anti-RNP is also seen in SLE, but typically at modest titers and usually alongside anti-Sm. MCTD tends to respond better to treatment than “pure” lupus and has a lower rate of severe nephritis.

24.3 Systemic Sclerosis (Scleroderma) Antibodies

Anti-Centromere

Association: Limited cutaneous systemic sclerosis (lcSSc), formerly called CREST syndrome

Calcinosis
Raynaud phenomenon
Esophageal dysmotility
Sclerodactyly
Telangiectasia

Pattern on ANA: Distinctive discrete speckled pattern (46 dots per nucleus)

Prognosis: Generally better prognosis than diffuse SSc. Pulmonary hypertension is the major concern (not interstitial lung disease).

Anti-Scl-70 (Anti-topoisomerase I)

Association: Diffuse cutaneous systemic sclerosis (dcSSc)

Clinical implications: Increased risk of interstitial lung disease (ILD), which is a major cause of mortality. Requires monitoring with pulmonary function tests and high-resolution CT.

Anti-RNA Polymerase III

Association: Diffuse SSc with increased risk of scleroderma renal crisis (acute hypertensive emergency with renal failure)

Also associated with increased risk of concurrent or developing malignancy (SSc as paraneoplastic phenomenon).

24.4 Inflammatory Myopathy Antibodies

Anti-Jo-1 and Other Antisynthetases

Antisynthetase syndrome: A subset of inflammatory myopathy (dermatomyositis/polymyositis) characterized by:

Inflammatory myopathy
Interstitial lung disease (major morbidity)
Arthritis
Mechanic’s hands (rough, cracked skin)
Raynaud phenomenon
Fever

Anti-Jo-1 (anti-histidyl-tRNA synthetase) is the most common antisynthetase antibody (~20% of myositis patients). Other antisynthetases (anti-PL-7, anti-PL-12, etc.) have similar clinical associations.

Anti-Mi-2

Associated with dermatomyositis with classic skin findings (heliotrope rash, Gottron papules). Generally better prognosis - good response to treatment, lower risk of ILD and malignancy.

Anti-MDA5 (Anti-CADM-140)

Associated with clinically amyopathic dermatomyositis (skin findings without muscle weakness) and rapidly progressive interstitial lung disease - a life-threatening complication requiring aggressive immunosuppression.

24.5 ANCA (Anti-Neutrophil Cytoplasmic Antibodies)

ANCA-associated vasculitides are systemic vasculitides affecting small vessels. ANCA testing is essential for diagnosis.

ANCA Method

ANCA refers to a pattern of reactivity when patient serum is incubated with alcohol (ethanol)-fixed neutrophils. Ethanol fixation is key: it permeabilizes the cells and causes MPO (positively charged) to migrate toward the negatively charged nucleus, producing the perinuclear artifact. On formalin-fixed cells, MPO stays cytoplasmic. This is why specific PR3 and MPO antibody confirmation by ELISA matters.

Patterns and Targets

c-ANCA (cytoplasmic pattern): Diffuse granular staining of the cytoplasm

Target: Proteinase 3 (PR3), a 29-kDa serine protease in azurophilic granules
Association: Granulomatosis with polyangiitis (GPA, formerly Wegener’s), positive in >90%
c-ANCA = PR3-ANCA

p-ANCA (perinuclear pattern): Staining around the nucleus (artifact of fixation - the antigen redistributes)

Target: Myeloperoxidase (MPO), 140-kDa enzyme in azurophilic granules
Association: MPA, EGPA, and (with atypical patterns) several non-vasculitis conditions
p-ANCA = MPO-ANCA

p-ANCA associations mnemonic - pMEUP: Primary sclerosing cholangitis, Microscopic polyangiitis, Eosinophilic GPA, Ulcerative colitis, Polyarteritis nodosa. Atypical p-ANCA in IBD and autoimmune hepatitis targets antigens other than MPO (lactoferrin, elastase, BPI) - always confirm by specific MPO ELISA.

Important: Always confirm IIF pattern with specific PR3 and MPO antibody testing by ELISA or other immunoassay. Atypical p-ANCA patterns can be seen in IBD and autoimmune hepatitis (directed against other antigens, not MPO).

Clinical Correlations

GPA (Wegener’s): Upper respiratory tract (sinusitis, saddle nose), lower respiratory tract (pulmonary nodules, hemorrhage), and kidney (rapidly progressive glomerulonephritis). PR3-ANCA positive in ~90%.

MPA: Similar to GPA but without granulomatous inflammation. Pulmonary-renal syndrome common. MPO-ANCA positive in ~70%.

EGPA (Churg-Strauss): Asthma, eosinophilia, systemic vasculitis. MPO-ANCA positive in ~40% (more often in those with glomerulonephritis).

24.6 Other Important Autoantibodies

Anti-GBM (Anti-Glomerular Basement Membrane)

Target: Type IV collagen in glomerular and alveolar basement membranes

Disease: Goodpasture syndrome - rapidly progressive glomerulonephritis with pulmonary hemorrhage

Mechanism: The antibody directly attacks the basement membrane (type II hypersensitivity). Linear IgG deposits on kidney biopsy immunofluorescence.

Clinical urgency: Medical emergency. Untreated, leads to irreversible renal failure. Treatment: Plasmapheresis (remove the antibody) + immunosuppression.

Anti-CCP (Anti-Cyclic Citrullinated Peptide)

Disease: Rheumatoid arthritis

Utility: More specific than rheumatoid factor (>95% specific vs. ~80%). Positive in 60-70% of RA patients. Can be positive years before clinical RA develops.

Anti-CCP positivity predicts more aggressive, erosive disease.

Rheumatoid Factor (RF)

What it is: IgM antibody directed against the Fc portion of IgG

Sensitivity: Present in 70-80% of RA patients

Specificity: Poor - RF is elevated in many conditions:

Sjögren syndrome (60-90%)
SLE (30%)
Chronic infections (endocarditis, hepatitis C)
Elderly without disease (up to 5-10%)

Clinical use: Combined with anti-CCP for RA diagnosis. RF-positive/anti-CCP-positive RA has the worst prognosis. RF is also elevated in Sjögren (60-90%), cryoglobulinemia (100% of type II/III), endocarditis, hepatitis C, sarcoidosis, and up to 5-10% of healthy elderly.

Anti-Mitochondrial Antibody (AMA)

Disease: Primary biliary cholangitis (PBC) - present in >95%, highly specific.

Target: The M2 mitochondrial antigen, specifically the E2 subunit of the pyruvate dehydrogenase complex on the inner mitochondrial membrane. M2 is expressed in gastric parietal cells, renal tubular cells, and hepatocytes, so IIF on rat kidney/stomach/liver composite substrate shows cytoplasmic staining of all three. AMA is also used as an IHC stain for oncocytic neoplasms (thyroid, salivary).

PBC: progressive destruction of intrahepatic bile ducts, cholestatic liver disease, middle-aged women, elevated alkaline phosphatase.

Anti-Smooth Muscle Antibody (ASMA)

Disease: Autoimmune hepatitis type 1, the most common form. Primarily young women, elevated transaminases, elevated IgG, interface hepatitis on biopsy.

Target: F-actin (filamentous actin). On IIF, reactivity with smooth muscle fibers in gastric mucosa and renal arterioles.

Anti-LKM (Liver Kidney Microsomal)

Disease: Autoimmune hepatitis type 2. Less common than type 1, affects children and young women, tends to present more severely. Target is cytochrome P450 2D6 (CYP2D6).

Anti-Parietal Cell and Anti-Intrinsic Factor

Disease: Autoimmune gastritis (atrophic body gastritis). Anti-H+/K+ ATPase antibodies destroy parietal cells; anti-IF antibodies block B12 absorption. Result: achlorhydria, iron deficiency (no acid to reduce Fe3+), and pernicious anemia (B12 deficiency). Increased risk of gastric adenocarcinoma and carcinoid (from unopposed gastrin driving ECL cell hyperplasia).

Anti-Thyroid Antibodies

Anti-TPO (thyroid peroxidase, formerly anti-microsomal) and anti-thyroglobulin: Hashimoto thyroiditis (anti-TPO ~90-95% sensitive, more specific than anti-Tg).

Thyroid-stimulating antibody (TSAb / TRAb): Graves disease. Mimics TSH at the TSH receptor, driving hyperthyroidism, diffuse goiter, and Graves ophthalmopathy (though the orbital disease also involves TSHR on orbital fibroblasts).

Anti-Acetylcholine Receptor and Anti-MuSK

Disease: Myasthenia gravis. Anti-AChR antibodies against the muscle nicotinic AChR at the neuromuscular junction (~85% of generalized MG). Seronegative MG often has anti-MuSK (muscle-specific kinase) or anti-LRP4 antibodies - MuSK/LRP4/agrin are needed to cluster AChRs at the junction.

~10-15% of MG patients have thymoma; conversely, ~30-50% of thymoma patients develop MG. Anti-titin antibodies in an MG patient strongly suggest thymoma as the driving cause - titin antibodies themselves are “passenger” antibodies, not pathogenic.

Serum ACE in Sarcoidosis

Serum angiotensin-converting enzyme (ACE) is produced by epithelioid cells in sarcoid granulomas and is elevated when sarcoidosis is active. It’s not specific - also elevated in primary biliary cholangitis, Gaucher disease, and leprosy - but supports the diagnosis alongside noncaseating granulomas and appropriate clinical/radiographic features.

24.7 Complement Testing

The complement system is a cascade of proteins that enhances (“complements”) the immune response. In autoimmune diseases, complement activation leads to consumption, causing low levels.

C3 and C4 Levels

Both low (C3 and C4 decreased): Classical pathway activation with consumption

Active SLE (especially nephritis) - immune complexes activate classical pathway
Cryoglobulinemia
Severe bacterial infection (consumption)

Low C4, normal C3: Early classical pathway activation or hereditary deficiency

C4 is consumed first in the classical pathway
C4 deficiency (genetic) - associated with SLE susceptibility
Hereditary angioedema (C1 inhibitor deficiency causes C4 consumption)

Low C3, normal C4: Alternative pathway activation

C3 nephritic factor (autoantibody stabilizes C3 convertase, causing C3 consumption)
Membranoproliferative glomerulonephritis

CH50 (Total Hemolytic Complement)

The CH50 is a functional assay measuring the entire classical pathway (C1 through C9). Patient serum is serially diluted and mixed with sheep RBCs coated with antibody. CH50 is reported as the reciprocal of the maximum dilution that lyses 50% of the RBCs (e.g., if dilution is 1:32, result is 32). The higher the dilution that still lyses, the more robust the complement activity.

Utility: Screens for complete deficiency of any classical pathway component. A very low or absent CH50 suggests homozygous deficiency of one component. To distinguish which pathway is activated - low C3 alone points to alternative pathway activation, whereas low C4 or C1q points to classical pathway activation (recall C4 and C1q are upstream of classical-pathway C3 convertase).

Hereditary complement deficiencies:

Early classical components (C1q, C2, C4): lupus-like illness. Without the classical pathway, immune complexes cannot be properly cleared, so they accumulate and drive autoimmunity. (Note: C1q deficiency is different from C1 esterase inhibitor deficiency.)
C3: severe recurrent encapsulated bacterial infections - without C3b opsonization, encapsulated organisms (which hide under a polysaccharide capsule) cannot be phagocytosed efficiently.
Late components (C5-C9): Neisseria infections - MAC is uniquely required to kill Neisseria. Any patient with ≥2 meningococcal infections needs CH50 and individual component testing.

C1 esterase inhibitor deficiency - hereditary angioedema (HAE): C1-INH normally blocks C1r/C1s (and MASP-1/2, kallikrein, Factor XIIa). Without it, the classical pathway runs unchecked and - more importantly - kinin pathway activation produces excess bradykinin, causing episodic non-pruritic, non-urticarial swelling of face, extremities, GI mucosa (surgical abdomen mimic), and larynx (airway emergency). ACE inhibitors worsen HAE because ACE normally degrades bradykinin.

HAE flare labs: increased urinary histamine, decreased C4 and C2 (substrates consumed by overactive classical pathway), decreased CH50. C1 activity is paradoxically increased because its inhibitor is absent.

24.8 Celiac Disease Serology

First-line screening: tTG IgA (tissue transglutaminase IgA) - >95% sensitivity and specificity.

Confirmatory: EMA IgA (endomysial antibody IgA) - ~99% specificity; labor-intensive immunofluorescence on monkey esophagus or human umbilical cord.

The IgA Deficiency Trap: ~2-3% of celiac patients have selective IgA deficiency → tTG IgA and EMA IgA will be FALSE NEGATIVE. Always check total serum IgA; if deficient, order tTG IgG or DGP IgG (deamidated gliadin peptide).

HLA: HLA-DQ2 (>90%) and/or HLA-DQ8 - near-100% NPV: if negative for both, celiac is essentially excluded.

Gold standard: Small bowel biopsy - Marsh classification:

Marsh 0: Normal
Marsh 1: Increased intraepithelial lymphocytes (>25/100 enterocytes)
Marsh 2: + Crypt hyperplasia
Marsh 3a/3b/3c: Progressive villous atrophy (partial → subtotal → total)

24.9 B Cells and Immunoglobulins

Everything in autoimmune serology depends on how B cells make antibodies. If you understand V(D)J recombination, class switching, and affinity maturation, the ANA/anti-dsDNA/RF/anti-CCP stories all stop feeling like random facts to memorize.

B Cell Maturation

B cells develop entirely in the bone marrow (memory aid: B = Bone marrow, T = Thymus). The pathway:

Lymphoid stem cell → Pro-B cell (heavy chain VDJ rearrangement begins) → Pre-B cell (cytoplasmic mu heavy chain + surrogate light chain; light chain VJ rearrangement begins) → Immature B cell (surface IgM only; undergoes negative selection against self-antigens) → Mature/naive B cell (co-expresses surface IgM and IgD; exits bone marrow) → Plasma cell (terminally differentiated; secretes large quantities of Ig; loses surface Ig).

Each stage has characteristic flow markers - this scaffolding is how we classify B-cell neoplasms in hematopathology.

Immunoglobulin Structure

Immunoglobulins have 2 heavy chains and 2 light chains (4 total) linked by disulfide bonds.

Heavy chain determines isotype: γ (IgG), α (IgA), µ (IgM), δ (IgD), ε (IgE) - mnemonic “GAMED”.
Light chain is either kappa or lambda (never mixed on the same antibody). Allelic exclusion means each B cell expresses only one. Normal serum ratio is ~2:1 kappa:lambda; deviation suggests clonality.
Variable domains (different between clones) contain the antigen-binding site (paratope). Constant domains are shared across all antibodies of an isotype and mediate effector function.
Fab = antigen-binding fragment (tips of Y). Fc = Fragment crystallizable, the constant region that binds complement and Fc receptors.

Gene locations (high yield):

Heavy chain: chromosome 14. Translocations involving 14q32 (t(14;18) follicular, t(8;14) Burkitt, t(11;14) mantle cell) drive B-cell lymphomas.
Kappa light chain: chromosome 2
Lambda light chain: chromosome 22

Memory aid: “kappa = 2, lambda = 22”.

V(D)J Recombination

Immunoglobulin rearrangement only occurs in the variable regions. Constant regions are spliced in later.

Heavy chain: VDJ rearrangement (includes D segment for extra diversity)
Light chain: VJ rearrangement (no D segment)
TCR analogy: TCR β and γ chains undergo VDJ (like heavy chain); TCR α and δ chains undergo VJ (like light chain)

V(D)J recombination is mediated by RAG1/RAG2 recombinases. Hypomorphic RAG mutations cause Omenn syndrome (SCID variant with oligoclonal autoreactive T cells producing a GVHD-like phenotype). Total diversity after V(D)J + junctional diversity + somatic hypermutation: ~10^11 antibody specificities.

CDRs (complementarity-determining regions): 6 per binding site (3 from VH, 3 from VL). CDR3 is the most variable and contributes most to antigen specificity.

Idiotope: a paratope is itself an epitope. Antibodies directed against another antibody’s variable region are anti-idiotypic antibodies - this is the basis of regulatory antibody networks and of some monoclonal assays.

Initial Isotype and Class Switching

All immunoglobulins are initially assembled with the µ (IgM) heavy chain - µ is the first constant gene downstream of J segments, so it is the default after VDJ. Mature naive B cells co-express surface IgM and IgD by alternative RNA splicing (same variable region, different constant).

After antigen binding + T cell help (CD40L on T cells, CD40 on B cells, plus cytokines), B cells undergo:

Affinity maturation: somatic hypermutation of the variable region in germinal centers, selecting for higher-affinity binders.
Isotype (class) switching: the constant region changes from µ to γ/α/ε, preserving specificity but changing effector function. Cytokines direct the switch (IL-4 → IgE; TGF-β → IgA; IFN-γ → IgG1/IgG3).

Immunoglobulin Isotypes and Valency

Isotype	Form in serum	Binding sites	Serum conc.	Key roles
IgG	Monomer	2	~1200 mg/dL	Most abundant; only Ig to cross placenta (FcRn); opsonization; long half-life ~21 days; 4 subclasses (IgG1/3 fix complement; IgG2 responds to polysaccharides; IgG4 in IgG4-related disease)
IgA	Dimer in secretions (monomer in serum)	2 (serum monomer) / 4 (secretory dimer)	~300 mg/dL	Mucosal immunity; secretory component from epithelial cells; 2 subclasses (IgA2 at mucosa, protease-resistant); neutralizes Giardia and enteroviruses
IgM	Pentamer (J chain)	10	~150 mg/dL	First Ig in primary response; most potent classical complement activator (one pentamer = enough C1q binding); naive B cell receptor
IgD	Monomer	2	~0.3 mg/dL	B-cell receptor co-receptor; minimal secreted role
IgE	Monomer	2	~0.05 mg/dL	Binds FcεRI on mast cells/basophils with very high affinity (Ka ~10^10); type I hypersensitivity; parasites

Complement fixation rank: IgM > IgG3 > IgG1. IgG2 and IgG4 are poor complement activators. Classical pathway requires two IgG Fc regions in close proximity (hence IgG needs to be clustered on a target) or one IgM pentamer.

Fc receptors:

CD16 (FcγRIII): IgG receptor on NK cells, macrophages, neutrophils - mediates ADCC.
FcεRI: high-affinity IgE receptor on mast cells/basophils - mediates type I hypersensitivity.
FcRn (neonatal Fc receptor): recycles IgG to extend its half-life and transports IgG across the placenta.

Clinical pearls:

Elevated IgG: chronic infection, autoimmune disease, liver disease, IgG myeloma.
Elevated IgM: Waldenstrom macroglobulinemia, primary biliary cholangitis, acute infection, hyper-IgM syndrome.
Elevated IgE: allergic disease, Job syndrome, parasites, some lymphomas.
IgA deficiency and anti-IgA antibodies → anaphylaxis from IgA-containing blood products; use washed RBCs and IgA-depleted products.
Bence Jones protein = urinary free light chains, pathognomonic for multiple myeloma.

24.10 T Cells

T cells are the coordinators (CD4 helpers) and the assassins (CD8 cytotoxics) of adaptive immunity. T cell defects present as viral and fungal infections; B cell defects as bacterial infections.

T Cell Maturation

T cells mature in the thymus. Precursors migrate from bone marrow as CD4-/CD8- (double-negative) thymocytes, rearrange TCR genes, become CD4+CD8+ (double-positive), then undergo:

Positive selection: must recognize self-MHC (or die).
Negative selection: must NOT strongly recognize self-antigen (or die). AIRE drives expression of tissue-specific self-antigens in thymic medullary epithelial cells - AIRE mutations cause APS-1 (autoimmune polyendocrine syndrome 1).

The survivors become single-positive CD4+ or CD8+ and exit as mature naive T cells.

TCR Structure

T-cell receptors are heterodimers of either:

αβ chains (95% of mature T cells) - conventional CD4/CD8 T cells
γδ chains (~5% circulating, enriched at skin and mucosal surfaces) - bridge innate/adaptive immunity, respond without classical MHC restriction

Chain recombination mirrors Ig genetics: β and γ undergo VDJ; α and δ undergo VJ. TCR has 1 constant region per chain (unlike Ig, no class switching, no somatic hypermutation, no affinity maturation).

TCRs associate with CD3, a multi-subunit signaling complex (γ, δ, ε, ζ chains) carrying ITAMs on the ζ chain that transduce the signal - the TCR alone has a short cytoplasmic tail and can’t signal.

TRECs and SCID Screening

V(D)J recombination excises intervening DNA as circular T-cell receptor excision circles (TRECs). TRECs aren’t replicated during cell division, so they dilute out - recent thymic emigrants have many, older peripheral T cells have few.

Newborn SCID screening uses TRECs from dried blood spots. Absent or very low TRECs suggest SCID (no T cell output from thymus). CHARGE syndrome can also show low TRECs from thymic hypoplasia.

CD4 vs CD8 and MHC Restriction

T cells broadly split into:

CD4+ helper T cells - recognize antigen on MHC Class II. MHC II is on APCs (dendritic cells, macrophages, B cells) and presents extracellular/exogenous antigens. Memory aid: “CD4 × MHC II = 8”; “CD8 × MHC I = 8” - they multiply to 8.
CD8+ cytotoxic T cells - recognize antigen on MHC Class I. MHC I is on all nucleated cells (not mature RBCs) and presents endogenous/intracellular antigens (viral, tumor). CD8 T cells kill via perforin/granzyme or Fas/FasL.

Normal CD4:CD8 ratio is ~2:1 (range 1-4:1). Inversion (CD8 predominant) in HIV, CMV, EBV. Highly elevated CD4:CD8 (>3.5:1 in BAL) characteristic of sarcoidosis.

T cells cannot recognize free/soluble antigen - TCR is MHC-restricted. This is the fundamental difference from B cells/antibodies.

Th Subsets

Th1 (IL-12 → IFN-γ): intracellular pathogens, macrophage activation
Th2 (IL-4, IL-5, IL-13): helminths, allergy, IgE class switching
Th17 (IL-23 → IL-17, IL-22): extracellular bacteria and fungi at mucosa (anti-Candida)
Treg (FOXP3, IL-10, TGF-β): peripheral tolerance

24.11 NK Cells

NK cells are the innate immune system’s lymphocytes - no prior sensitization, no MHC restriction required.

~10% of circulating lymphocytes (5-15% range). Large granular lymphocytes (LGLs) - visibly granular on smear.
Flow phenotype: CD3-, CD16+, CD56+, CD57+. CD56-bright are immunoregulatory (cytokine producers); CD56-dim are cytotoxic. Also express CD2, CD7 (shared with T cells).
Kill via two triggers:
- Missing self: absent or reduced MHC I on target (viral infection, tumor). MHC I normally inhibits NK killing via KIRs; its loss unleashes the NK cell.
- ADCC: CD16 (FcγRIII) binds IgG Fc on antibody-coated targets → degranulation → perforin/granzyme. This is how rituximab (anti-CD20) kills B cells.
Secrete IFN-γ, activating macrophages during the early innate phase before adaptive T cells are up.

24.12 Antigen Presenting Cells

APCs express MHC Class II and present extracellular antigens to CD4+ T cells. The pathway: engulf by endocytosis/phagocytosis → degrade in endosome/lysosome → load peptide onto MHC II in MIIC compartment → display on surface → T cell recognition via TCR + CD4.

Three APC types:

Monocyte-derived APCs - dendritic cells (most potent, activate naive T cells) and macrophages (activate memory T cells)
B cells (present antigen specifically to T cells that will help them)

Tissue-specific macrophages / dendritic cells (worth memorizing):

Kupffer cells - liver sinusoids. Largest fixed macrophage population.
Langerhans cells - epidermis. Contain Birbeck granules (tennis racket-shaped on EM), express CD1a and langerin (CD207). Migrate to lymph nodes. Central to contact dermatitis.
Hofbauer cells - placenta (chorionic villi). Maintain maternal-fetal tolerance, protect against vertical transmission.
Interdigitating reticulum cells - interfollicular (paracortical) T-cell zones of lymph nodes. Migrated dendritic cells presenting antigen. Express S100, CD1a.
Follicular dendritic cells (dendritic reticulum cells) - germinal centers. Do NOT express MHC II, NOT bone marrow derived. Trap immune complexes on CD21 (also the EBV receptor) and present intact antigen to B cells during affinity maturation.

A small subset of intracellular antigens can be “cross-presented” on MHC II - relevant for viral immunity.

24.13 Granulocytes and Mast Cells

Neutrophils

Short-lived (6-8 hours in blood, 1-2 days in tissue) phagocytes. Recruited by IL-8 (CXCL8) - memory aid: “IL-8 → neutrophils eight the bacteria”. Produced by macrophages, endothelium, epithelium in response to IL-1, TNF-α, and bacterial products.

Self-destructive: kill pathogens and die. Two main mechanisms:

Degranulation - release MPO, elastase, defensins, lysozyme into phagosome or extracellularly
NETosis - expel chromatin coated in antimicrobial proteins to trap bacteria

Neutrophils have limited phagocytic activity compared to macrophages, and are NOT antigen-presenting cells (no MHC II).

Basophils

Rarest circulating granulocyte. Express FcεRI like mast cells → bind IgE → allergen crosslinks → degranulate (histamine, leukotrienes). Functionally a circulating mast cell equivalent.

Eosinophils

Driven by Th2 cytokines - IL-4, IL-5, IL-13 (also IL-6, IL-9). IL-5 is the master eosinophil cytokine - development, recruitment, survival, activation. Anti-IL-5 (mepolizumab, reslizumab) shrinks eosinophil counts in eosinophilic asthma.

Th2 cytokine functions:

IL-4 → IgE class switching, Th2 polarization, M2 macrophages
IL-5 → eosinophil recruitment
IL-13 → mucus production, smooth muscle contraction (asthma pathophysiology), IgE

Degranulated eosinophil contents coalesce to form Charcot-Leyden crystals - needle-shaped/hexagonal crystals of galectin-10/lysophospholipase, found in sputum (asthma, ABPA), stool (parasites), and nasal polyps. Marker of intense eosinophilic inflammation.

Mast Cells

Tissue-resident, express FcεRI, bind IgE with extremely high affinity (Ka ~10^10). IgE sits on the mast cell surface for weeks-months waiting for allergen. When multivalent allergen crosslinks two adjacent IgE, the receptor clusters, tyrosine kinase cascade fires, and the cell degranulates - histamine, tryptase, heparin, leukotrienes, prostaglandins. This is the fundamental mechanism of type I hypersensitivity.

24.14 Complement System (detailed)

Section 24.7 covered the consumption patterns clinically. This section covers the cascade itself.

Three Pathways

All three converge at C3 convertase, which cleaves C3 → C3a (anaphylatoxin) + C3b (opsonin). C3b then feeds into the C5 convertase, which cleaves C5 → C5a (anaphylatoxin) + C5b (MAC initiator).

Pathway	Trigger	Initiating protease	C3 convertase	C5 convertase
Classical	Antibody-antigen complex (IgG or IgM) binding C1q	C1s (cleaves C4 and C2)	C4b2b	C4b2b3b
Lectin	Mannose-binding lectin binds bacterial sugars	MASP-2 (MBL-associated serine protease 2; functionally equivalent to C1s)	C4b2b	C4b2b3b
Alternative	Spontaneous C3 hydrolysis (“tick-over”), accelerated by LPS, venom, aggregated IgA	Factor D cleaves Factor B	C3bBb	C3bBb3b

C3 hydrolysis happens continuously at low levels in blood. Regulatory proteins on host cells keep it in check; pathogen surfaces lack those regulators, so C3b builds up and amplifies.

Products and Their Jobs

C3b: opsonin - coats pathogens, binds complement receptors on phagocytes (CR1/CR3) → phagocytosis. Along with IgG, the two most important opsonins.
C3a, C5a: anaphylatoxins - trigger histamine release from basophils/mast cells and chemotaxis. C5a is the stronger.
C5b-C9 (MAC): membrane attack complex. C5b binds C6, C7 (inserts in membrane), C8 (pore formation begins), C9 (polymerizes into full pore) → osmotic lysis. Essential for killing Neisseria.

Regulators

C1 esterase inhibitor (C1-INH): blocks C1r/C1s (classical), MASP-1/2 (lectin), kallikrein/Factor XIIa (kinin). Deficiency → hereditary angioedema.
Factor I: serine protease that inactivates C3b → iC3b (and C4b). Cofactors: Factor H (fluid phase), MCP/CD46 (membrane), C4BP.
Factor H: cofactor for Factor I + decay-accelerating activity for alternative pathway C3bBb. Factor H deficiency → atypical HUS (uncontrolled alternative pathway → renal endothelial thrombotic microangiopathy). Treatment: eculizumab (anti-C5).
DAF (CD55): binds C4b and C3b, prevents formation of both C3 convertases. GPI-anchored.
CD59 (protectin/MIRL): binds C5b678, blocks C9 polymerization, prevents MAC. GPI-anchored.

GPI anchor loss (somatic PIG-A mutation in a hematopoietic stem cell) = paroxysmal nocturnal hemoglobinuria (PNH). Lose DAF and CD59 → unchecked complement-mediated RBC lysis. Classic triad: hemolytic anemia, pancytopenia, thrombosis (especially intra-abdominal veins). Treatment: eculizumab.

24.15 Cytokines (Board-Relevant Roll Call)

IL-1, IL-6, TNF-α: acute inflammation. Drive acute phase reactants (CRP, fibrinogen, ferritin, hepcidin, C3, C4, SAA, ceruloplasmin, α1-antitrypsin). Negative APRs drop: albumin, transferrin, transthyretin. CRP is the most useful clinically.
IL-2: T cell proliferation. Target of basiliximab, cyclosporine, tacrolimus in transplant immunosuppression.
IL-3, G-CSF, GM-CSF: hematopoietic growth factors. Filgrastim (G-CSF) for neutropenia; sargramostim (GM-CSF) for granulocytes/monocytes.
IL-4: IgE class switching, Th2 polarization.
IL-5: eosinophil recruitment.
IL-6: hepcidin → anemia of chronic disease. Target of tocilizumab (RA, CRS).
IL-8 (CXCL8): neutrophil chemotaxis.
IL-10, TGF-β: immunosuppressive (Treg).
IL-12: from macrophages/dendritic cells → Th1 differentiation and IFN-γ production.
IL-13: mucus, bronchoconstriction, IgE.
IL-17 (Th17): anti-fungal at mucosa, neutrophil recruitment.
IL-1β (via pyrin inflammasome): elevated in familial Mediterranean fever.
TNF-α: macrophage-derived; endothelial activation, fever, cachexia, septic shock. Target of infliximab, adalimumab, etanercept.
IFN-α, IFN-β (Type I): dendritic cells and fibroblasts. Antiviral - induce antiviral state, activate NK and CD8.
IFN-γ (Type II): from Th1, CD8 T cells, NK cells. Most potent macrophage activator - upregulates MHC I/II, enhances phagocyte killing. Essential for granuloma formation.

High-yield deficiency: IL-12 deficiency or IFN-γ receptor deficiency → mycobacterial susceptibility (disseminated BCG, atypical NTM, severe TB). The IL-12/IFN-γ axis is how we control mycobacteria.

24.16 Immune Function and Hypersensitivity

Pattern Recognition for Immunodeficiency Workup

Which cell type is broken often determines what organism is invading:

B cell / Ig defects→ bacterial respiratory and GI infections, especially encapsulated organisms (S. pneumoniae, H. influenzae, Moraxella). Also Giardia (loss of secretory IgA) and enteroviruses. Infections classically start after 6 months (maternal IgG wanes).
T cell defects → viral and fungal infections. CMV, EBV, HSV, VZV, measles, Candida, Pneumocystis, Aspergillus, mycobacteria. Also recurrent mucocutaneous candidiasis signals T cell defect (Th17/IL-17 axis) - contrast with systemic candidiasis, which signals neutropenia.
Phagocyte defects → catalase-positive organisms (S. aureus, Serratia, Nocardia, Aspergillus, Burkholderia). Catalase-positive organisms destroy their own H2O2, depriving the failing phagocyte of substrate.
Terminal complement deficiency → Neisseria.

Other clues:

Absent lymph nodes/tonsils on physical exam → B cell defect (classically Bruton). Germinal centers and follicles fail to develop without B cells.
Absolute lymphopenia on CBC → T cell defect (T cells are 70% of lymphocytes).
Failure of antibody response to protein antigens → B OR T cell defect (T-dependent response).
Failure of antibody response to polysaccharide antigens → B cell defect only (T-independent).
Most common isolated Ig deficiency: IgA deficiency (~1:500 people of European ancestry). Most are asymptomatic; symptomatic patients have sinopulmonary and GI infections. Also high celiac and other autoimmune risk. Anaphylaxis to IgA-containing blood products is the transfusion medicine pearl.

Primary Immunodeficiency Workup Tests

Flow cytometry: T cell subsets (CD3, CD4, CD8, CD4:CD8). SCID = absent CD3+; DiGeorge = decreased CD4+; HIV = inverted ratio.
Delayed-type hypersensitivity (tuberculin skin test / PPD): screens T cell function. Positive induration at 48-72 hours = functional Th1 immunity. Anergy (no response to any DTH antigen) suggests T cell dysfunction.
Lymphocyte proliferation assays: T cells exposed to mitogens like phytohemagglutinin (PHA) or concanavalin A (ConA), measure DNA synthesis (tritiated thymidine historically; CFSE dilution or BrdU now). Failure to proliferate = functional T cell defect.
NK/CTL function: chromium-51 release assay (target cells labeled with Cr-51, incubated with effectors, measure released radioactivity = lytic activity) or cytokine release. Also intracellular granzyme B by flow cytometry.
Neutrophil oxidative burst: DHR (dihydrorhodamine 123) flow cytometry - DHR is oxidized to fluorescent rhodamine by ROS. Normal = fluorescence shift after PMA. CGD = no shift. MPO deficiency = partial shift. Nitroblue tetrazolium (NBT) is the older version (NBT reduced to blue formazan by superoxide).
Complement activation: CH50 for total classical pathway. Low C3 = alternative pathway activation. Low C4 or C1q = classical pathway activation.

Hypersensitivity Reactions

The four types, each with a distinct mechanism:

Type	Mechanism	Examples
Type I (immediate)	IgE bound to mast cells/basophils, crosslinked by allergen → degranulation → histamine	Anaphylaxis, atopy (asthma, rhinitis), food allergy, insect venom
Type II (cytotoxic / antibody-mediated)	IgG or IgM antibody binds cell-surface antigen → complement, ADCC, phagocytosis	Goodpasture, myasthenia gravis, AIHA, HDFN, Graves, pemphigus
Type III (immune complex)	Circulating immune complexes deposit in tissue → complement → neutrophil recruitment	SLE, HSP / IgA vasculitis, serum sickness, post-strep GN, Arthus reaction
Type IV (delayed / cell-mediated)	T cells and macrophages, no antibody	TB skin test, contact dermatitis, type 1 DM, MS, GVHD

Anaphylaxis labs: serum tryptase is the best marker - released from mast cell granules, peaks at 1-2 hours, stays elevated 6-12 hours. Also elevated in systemic mastocytosis. Serum histamine is a poor marker (very short half-life); urinary histamine is more durable (up to 24 hours) but not routinely used.

Important non-hypersensitivity mimics:

Hereditary angioedema and anaphylactoid reactions are NOT type I hypersensitivity - no IgE involved. HAE is bradykinin-mediated (C1-INH deficiency). Anaphylactoid reactions (IV contrast, vancomycin red man) are direct mast cell activation without IgE.

Selected Autoimmune Diseases Not Covered in 24.1-24.8

Sjögren syndrome: dry eyes (keratoconjunctivitis sicca) and dry mouth (xerostomia), older women. Anti-SSA/SSB positive. Diagnosis: minor salivary gland (labial) biopsy showing focal lymphocytic sialadenitis, focus score ≥1 (focus = aggregate of ≥50 lymphocytes per 4 mm²). Increased risk of MALT lymphoma.

Rheumatoid arthritis: most common chronic inflammatory arthritis, ~1% prevalence, 2-3x more common in women. Symmetric small-joint polyarthritis, morning stiffness >1 hour. Joint histology shows papillary synovial hyperplasia (pannus) with dense lymphoplasmacytic infiltrates. Anemia of chronic disease via IL-6 → hepcidin.

IgG4-related sclerosing disease: fibroinflammatory lesions with sclerosis and IgG4+ plasma cell infiltrates. Protean manifestations: sclerosing mediastinitis, retroperitoneal fibrosis, Riedel thyroiditis, sclerosing cholangitis, orbital pseudotumor, autoimmune (type 1) sclerosing pancreatitis.

Primary sclerosing cholangitis: biliary disease, most common in men, beading of biliary tree on MRCP/ERCP, strongly associated with ulcerative colitis (~70% of PSC have UC). UC may be mild while PSC is severe. Increased risk of cholangiocarcinoma and colon cancer.

Celiac disease: villous atrophy, intraepithelial lymphocytes, crypt hyperplasia on small bowel biopsy (see 24.8 for serology). HLA-DQ2 (>90%) or HLA-DQ8.

Inclusion body myositis: most common inflammatory myopathy in older adults (>50), refractory to steroids. No specific serologic marker, so biopsy is required.

Familial Mediterranean Fever: autosomal recessive MEFV mutations → mutant pyrin → uncontrolled inflammasome → excess IL-1β → episodic fevers and serositis (1-3 days). Most feared complication: AA amyloidosis with renal failure (from sustained serum amyloid A elevation). Treatment: colchicine reduces symptoms and amyloid deposition by inhibiting microtubule-dependent neutrophil function.

Vasculitides - quick reference:

Vasculitis	Vessel	Key features
Giant cell arteritis	Large	>50 yo, temporal headache, jaw claudication, vision loss, polymyalgia rheumatica overlap, temporal artery biopsy gold standard (skip lesions → need a long segment)
Takayasu arteritis	Large	Women 10-40, aortic arch and branches, pulseless disease
Polyarteritis nodosa	Medium	Associated with HBV, spares lungs, segmental fibrinoid necrosis of muscular arteries
Kawasaki	Medium	Children <5, post-viral, coronary artery aneurysms, CRASH and Burn (Conjunctivitis, Rash, Adenopathy, Strawberry tongue, Hand/foot erythema, fever)
Thromboangiitis obliterans (Buerger)	Small/medium	Young male smokers, digital ischemia
GPA / MPA / EGPA	Small (ANCA)	See 24.5. EGPA = asthma + eosinophilia + vasculitis
HSP / IgA vasculitis	Small	Children, palpable purpura + abdominal pain + arthritis + IgA nephritis. Abnormally galactose-deficient IgA1.
Behcet	Any size	Oral + genital ulcers + uveitis, Middle East/Silk Road, HLA-B51, positive pathergy test (papule >2 mm at 24-48 hours after 20-gauge needle stick)

Small-vessel ANCA-associated vasculitides cause pauci-immune crescentic glomerulonephritis - crescents without immune complex deposits on IF.

24.17 Primary Immunodeficiencies

Primary immunodeficiencies (inborn errors of immunity) disproportionately affect males because many are X-linked: Bruton (BTK), X-linked SCID (IL-2Rγ), Wiskott-Aldrich (WAS), X-linked hyper-IgM (CD40L), CGD (gp91phox most common form), Duncan/XLP (SH2D1A).

Severe Combined Immunodeficiency (SCID)

Absent/severely reduced T cell function, low/absent Igs, thymic dysplasia. Fatal within 1-2 years without bone marrow transplant or gene therapy. Detected by low TRECs on newborn screening.

ADA (adenosine deaminase) deficiency: autosomal recessive. Toxic dATP accumulates in lymphocytes → apoptosis of T and B cells. Unique associations: pulmonary alveolar proteinosis and cognitive defects. Amenable to enzyme replacement or gene therapy.
IL-2 receptor γ chain (common γ chain) deficiency: X-linked, the most common SCID (~50%). The γc chain is shared by IL-2R, IL-4R, IL-7R, IL-9R, IL-15R, IL-21R. T cell SCID with preserved B cell numbers (but nonfunctional because of no T cell help).
Omenn syndrome: hypomorphic RAG1/RAG2 mutations. A few oligoclonal, autoreactive T cells escape and cause a GVHD-like phenotype (erythroderma, lymphadenopathy, hepatosplenomegaly, eosinophilia) layered on top of SCID.

SCID patients need irradiated blood products - they lack T cells to reject donor lymphocytes, so TA-GVHD is almost uniformly fatal.

DiGeorge Syndrome (22q11.2 deletion)

Failure of development of the 3rd and 4th pharyngeal pouches → absent thymus (→ T cell defect) and parathyroids (→ hypocalcemia).

CATCH-22 mnemonic: Cardiac anomalies (conotruncal: tetralogy of Fallot, truncus arteriosus, TGA), Abnormal facies (hypertelorism, low-set ears, mandibular hypoplasia, bifid uvula), Thymic aplasia, Cleft palate, Hypocalcemia, 22q11 deletion.

Neonatal hypocalcemic tetany is a classic presentation.
Partial DiGeorge is more common than complete - hypoplastic thymus, reduced but not absent T cells, milder course.
Susceptible to fungi and viruses (Candida, PCP, CMV, EBV).
Histology: depleted paracortical (T cell) areas in lymph nodes, poorly developed periarteriolar lymphatic sheaths (PALS) in spleen.
Need irradiated blood products (TA-GVHD risk).

CHARGE syndrome: related thymic hypoplasia → may show low TRECs on screening. CHD7 mutation. Coloboma, Heart defects, choanal Atresia, Retardation of growth/development, Genital hypoplasia, Ear anomalies.

Bruton (X-linked) Agammaglobulinemia

X-linked recessive BTK mutation. BTK is required for B cell development at the pre-B to immature B cell transition.

Severely reduced serum Igs of all isotypes, absent B cells and plasma cells.
Recurrent bacterial infections starting at 6 months (maternal IgG wanes).
Susceptibility to enteroviruses (polio, coxsackie, echovirus) - normally cleared by intestinal IgA.
Absent lymph nodes and tonsils on physical exam - lymphoid architecture needs B cells.
Increased risk of lymphoid neoplasms and autoimmune disease.

Common Variable Immunodeficiency (CVID)

Heterogeneous group, defined clinically.

Presents in 2nd-3rd decade (teens to young adults) - key distinguisher from Bruton.
Low IgG, usually low IgA and/or IgM, but B cells ARE present (they just fail to differentiate into plasma cells).
Recurrent respiratory and GI infections, Giardia, autoimmune disease, increased malignancy risk.
Hyperplastic germinal centers in lymph nodes (B cells try and fail to become plasma cells).
Absent plasma cells on GI biopsy - striking finding in duodenal/colonic lamina propria.

Selective IgA Deficiency

Most common primary immunodeficiency (~1:500 people of European ancestry). Serum IgA <7 mg/dL with normal IgG and IgM. Most asymptomatic. Symptomatic: sinopulmonary and GI infections, Giardia, autoimmunity (celiac, SLE).

Anaphylaxis risk with IgA-containing blood products - patients may develop anti-IgA antibodies that trigger IgE-mediated anaphylaxis on subsequent exposure. Use washed RBCs and IgA-depleted products.

Ataxia-Telangiectasia

Autosomal recessive ATM (11q22.3) mutation. ATM detects and repairs double-strand DNA breaks. Impaired DSB repair disrupts V(D)J recombination → combined T and B cell defects.

ATM mnemonic: Ataxia (cerebellar), Telangiectasia (oculocutaneous), Malignancy (lymphoma, leukemia).

Labs: decreased IgA (~70%), decreased IgG2, very high serum AFP (>95% of patients), elevated CEA, lymphopenia. Radiosensitive - avoid unnecessary CT scans.

Wiskott-Aldrich Syndrome

X-linked recessive WAS gene mutation → defective WASP (actin cytoskeleton signaling in hematopoietic cells).

WATER mnemonic: Wiskott-Aldrich = microThrombocytopenia, Eczema, Recurrent infections.

Characteristic Ig pattern: low IgM, high IgA and IgE, normal IgG. Low IgM reflects impaired T-independent (polysaccharide) antibody response.

Microthrombocytopenia is distinctive - platelets are few AND small (low MPV), distinguishing WAS from ITP (normal/large platelets).

Duncan Disease (X-Linked Lymphoproliferative, XLP)

X-linked recessive SH2D1A mutation → defective SAP (SLAM-associated protein). Essential for T, NK, NKT cell signaling.

Fulminant, often fatal response to primary EBV - massive lymphoproliferation, hemophagocytic lymphohistiocytosis, hepatitis. Also inverted CD4:CD8 ratio, hypogammaglobulinemia. Named after the Duncan family in which multiple boys died of mono.

XLP2 (XIAP/BIRC4 mutations) is a related syndrome.

Job Syndrome (Hyper-IgE)

Usually autosomal dominant STAT3 mutations (~70% AD-HIES; less commonly autosomal recessive DOCK8). STAT3 impairs Th17 → anti-Candida and anti-Staph defense.

Markedly elevated IgE (often >2000 IU/mL) without classical allergic symptoms.

FATED mnemonic: coarse Facies, cold Abscesses (S. aureus) without warmth/erythema, retained primary Teeth, Eosinophilia + elevated IgE, allergic Dermatitis. Also scoliosis, pathologic fractures.

Chronic Mucocutaneous Candidiasis

T cell defect affecting Th17/IL-17 anti-Candida axis. Can be isolated or part of APS-1 (autoimmune polyendocrine syndrome 1), which is caused by AIRE mutations (defective thymic expression of self-antigens → autoreactive T cells). APS-1 triad: chronic mucocutaneous candidiasis, hypoparathyroidism, Addison disease.

Chronic Granulomatous Disease (CGD)

Defective NADPH oxidase, mostly X-linked recessive (CYBB / gp91phox, Xp21, ~65% of cases); AR forms involve p47/p67/p22/p40phox. No oxidative burst → failure to kill catalase-positive organisms (S. aureus, Serratia, Burkholderia, Nocardia, Aspergillus).

Extensive granuloma formation - macrophages wall off organisms that neutrophils can’t kill.

Diagnosis: DHR (preferred) or NBT. On DHR, CGD neutrophils show no fluorescence shift after PMA.

McLeod phenotype association: X-linked CGD patients with large deletions that include XK (near CYBB on Xp21) have weak Kell antigen expression, acanthocytes, and neuromuscular features. Transfusion issue in the blood bank.

Chediak-Higashi Syndrome

Autosomal recessive LYST (lysosomal trafficking regulator) mutation → giant granules in all granulated cells.

Neutrophils: giant granules can’t degranulate properly → recurrent pyogenic infections (S. aureus, Streptococcus)
Platelets: delta storage pool deficiency → absent dense granules → absent/reduced second wave of platelet aggregation
Melanocytes: oculocutaneous albinism (defective melanosome transfer)
Nerves: peripheral neuropathy
Accelerated phase = hemophagocytic lymphohistiocytosis (HLH) - ~85% of patients, usually EBV-triggered

May-Hegglin Anomaly

Autosomal dominant MYH9 (non-muscle myosin heavy chain IIA) mutation.

Neutrophils with Döhle body-like inclusions (actually aggregates of mutant myosin, not true rough ER Döhle bodies) + macrothrombocytopenia (large platelets, low count). On smear: neutrophil has pale blue cytoplasmic inclusion, giant platelets scattered about.

Most commonly presents with bleeding (easy bruising, menorrhagia, post-surgical bleeding), usually mild because giant platelets partially compensate functionally. Immunodeficiency is actually not common.

MYH9 is also the gene most commonly fused with USP6 in nodular fasciitis - trivia.

Alder-Reilly Anomaly

Large azurophilic granules resembling toxic granulation in all WBC types (neutrophils, lymphocytes, monocytes, eosinophils) - distinguishes from toxic granulation, which is neutrophils only and reactive.

Associated with mucopolysaccharidoses (Hunter, Hurler, I-cell disease) - accumulated mucopolysaccharides deposit in lysosomes.

Pelger-Huet Anomaly

Autosomal dominant lamin B receptor (LBR) mutation → hypolobulated (bilobed) neutrophils with mature, coarsely clumped chromatin and a thin filament between lobes (“pince-nez” / “spectacle” appearance). Functionally normal.

Heterozygous: bilobed
Homozygous: monolobated “Stodtmeister cells”, rare

Pseudo-Pelger-Huet: acquired form in myelodysplastic syndrome. Key differentiators - hereditary affects all neutrophils, lifelong, no other dysplasia; pseudo-PHA is patchy and accompanied by other dysplastic features.

Jordan Anomaly

Fat (lipid) vacuoles in leukocytes (neutrophils, monocytes, sometimes lymphocytes) on peripheral smear. Well-demarcated, uniform, clear round inclusions in multiple cell types.

Associated with neutral lipid storage disorders (Chanarin-Dorfman). Distinguish from toxic vacuolation (neutrophils only, reactive, less uniform).

Complement Deficiencies

Recapping 24.14, the classic associations:

C1q, C2, C4 (early classical) → lupus-like illness (immune complex clearance failure)
C3 → encapsulated bacterial infections
C5-C9 (terminal, MAC) → Neisseria infections
C1 esterase inhibitor → hereditary angioedema
Factor H → atypical HUS
GPI anchor (PIG-A somatic) → PNH

24.18 Transplant Rejection

Rejection is the immune system doing its job on the wrong target. Three main categories by timing and mechanism.

Type	Timing	Mediator	Mechanism
Hyperacute	Within minutes to 24 hours	Preformed anti-ABO or anti-HLA antibodies	Bind donor endothelium → complement activation → fibrin/platelet thrombi → ischemic necrosis
Acute cellular	Days to weeks/months	Recipient T cells (mostly CD8, with CD4 help)	Recognize donor HLA (direct: intact donor MHC on donor APCs; indirect: processed donor MHC on recipient APCs) → cytotoxicity. Most common type clinically.
Humoral / antibody-mediated (AMR)	Days to weeks (can be acute or chronic)	De novo donor-specific antibodies (DSAs)	Deposit in vessel walls → complement → endothelial damage. Tracked by solid-phase assays (Luminex single-antigen beads).
Chronic	Months to years	Cumulative cellular + humoral	Progressive dysfunction, fibrosis is the predominant histologic feature regardless of organ

Hyperacute Rejection

Preformed antibodies (anti-ABO or anti-HLA) bind donor endothelium immediately. Microscopic: fibrin and platelet thrombi, neutrophilic infiltration of vessel walls, endothelial necrosis, hemorrhagic necrosis.

Prevention: ABO compatibility + negative HLA crossmatch before transplant. If it happens, the graft is lost.

Acute Cellular Rejection

Most common type encountered clinically. Days to weeks/months post-transplant, most common in first 3-6 months. Histology: lymphocytic infiltration of parenchyma with tissue damage (tubulitis in kidney, myocyte necrosis in heart, endothelialitis in liver).

Treatment: pulse corticosteroids first-line, anti-thymocyte globulin for steroid-resistant cases. IL-2 pathway inhibitors (basiliximab, cyclosporine, tacrolimus) prevent it.

Humoral (Antibody-Mediated) Rejection

De novo DSAs developing post-transplant. Differs from hyperacute because antibodies are not preformed.

Diagnostic triad:

DSAs detected by Luminex
C4d immunohistochemical staining in peritubular capillaries (kidney) or small vessels - C4d is a degradation product of C4b that covalently binds tissue at sites of classical complement activation, so its presence means antibody-mediated complement activation happened here.
Histologic microvascular injury

Chronic Rejection

Cumulative damage over months to years. Leading cause of late graft loss. Pathology is organ-specific but fibrosis is universal:

Kidney: interstitial fibrosis and tubular atrophy (IF/TA) + transplant glomerulopathy
Heart: accelerated coronary artery disease (transplant vasculopathy) with concentric intimal fibroproliferation
Lung: bronchiolitis obliterans (fibrotic obliteration of small airways)
Liver: vanishing bile duct syndrome (ductopenia)

24.19 Graft-Versus-Host Disease

GVHD is rejection in reverse: donor T cells attack recipient tissues. Requires three conditions:

Immunocompetent donor T cells in the graft
Immunosuppressed recipient (can’t reject the donor T cells)
Antigenic differences between donor and recipient (HLA mismatch)

Most commonly occurs after bone marrow (hematopoietic stem cell) transplant - the graft carries a full donor immune system. Also can occur with solid organs rich in passenger lymphocytes (liver, intestine, lung) and with transfusion in susceptible recipients.

Timing

Acute GVHD: within 100 days post-transplant (most <30 days)
Chronic GVHD: after 100 days - classic definition. Modern definitions increasingly based on clinical features rather than timing.

Organ Targets

Acute GVHD hits three organs classically:

Skin - earliest and most common. Maculopapular rash starting on palms, soles, and ears. Microscopic: lymphocytic infiltration at dermal-epidermal junction with satellite cell necrosis (apoptotic keratinocytes surrounded by lymphocytes), resembling erythema multiforme.
GI tract - profuse watery diarrhea (can be >2 L/day, sometimes bloody), abdominal pain. Microscopic: crypt apoptotic bodies. Severe cases: crypt dropout, mucosal sloughing. (DDx: mycophenolate can mimic with apoptotic bodies.)
Liver - jaundice, cholestatic pattern (elevated direct bilirubin and alk phos). Microscopic: portal inflammation with endothelialitis, bile duct damage (ductitis, ductopenia).

Chronic GVHD

More widespread, resembles autoimmune/connective tissue disease with sclerosis (fibrosis) as the hallmark. Features:

Sclerodermoid skin changes (hide-bound skin, loss of sweat glands)
Sicca syndrome (dry eyes/mouth)
Bronchiolitis obliterans
Chronic cholestatic liver disease with ductopenia
Oral/esophageal strictures
Cytopenias

Leading cause of late non-relapse mortality after allogeneic transplant. Has a beneficial graft-vs-tumor effect - some chronic GVHD correlates with lower relapse risk in leukemia.

Transfusion-Associated GVHD (TA-GVHD)

Different from BMT-GVHD in one key way: TA-GVHD targets the bone marrow in addition to skin/GI/liver, causing fatal pancytopenia. Almost always fatal.

High-risk recipients (need irradiated blood products, typically 25 Gy gamma):

SCID, DiGeorge, other T cell deficiencies
Hodgkin lymphoma, other heavy-chemo patients
Post-hematopoietic transplant
Directed donations from blood relatives (shared HLA haplotypes → donor T cells see only one foreign haplotype → easier to engraft)
Intrauterine transfusions

Irradiation inactivates donor T cells while preserving RBC and platelet function. Psoralen pathogen reduction is an alternative for platelets.

Chapter 25: Infectious Disease Serology

Serological testing for infectious diseases relies on detecting antibodies (indicating exposure or immunity) or antigens (indicating active infection). Understanding the patterns of antibody response - which antibodies appear when, and what they mean - is essential for interpreting these tests.

Hepatitis B Serology: The Master Table

Hepatitis B serology is notoriously confusing because there are multiple markers, each with different meanings. The key is understanding what each marker represents and then using the pattern to determine the clinical state.

The Markers and What They Mean

HBsAg (Hepatitis B Surface Antigen): The viral coat protein. Its presence indicates the virus is actively replicating - the patient has active infection (acute or chronic). This is the first marker to appear (1-2 weeks before symptoms) and the first marker of infection.

Anti-HBs (Antibody to Surface Antigen): Neutralizing antibody that provides immunity. Its presence indicates either:

Recovery from past infection (with immunity), OR
Successful vaccination

Anti-HBc (Antibody to Core Antigen): Indicates exposure to the virus at some point. The core antigen is internal and not released into blood, so anti-HBc forms only from actual infection - never from vaccination.

Anti-HBc IgM: Indicates recent/acute infection (present for ~6 months)
Anti-HBc IgG (total anti-HBc): Indicates any past or current infection (persists for life)

HBeAg (Hepatitis B e Antigen): A secreted viral protein indicating high viral replication and high infectivity. Its presence means the patient is highly contagious.

Anti-HBe (Antibody to e Antigen): Indicates lower viral replication (seroconversion). The transition from HBeAg-positive to anti-HBe-positive is generally favorable.

HBV DNA: Direct measure of viral load. Most sensitive marker of active replication.

The Interpretation Table

Clinical State	HBsAg	Anti-HBs	Anti-HBc (total)	Anti-HBc IgM	HBeAg	Interpretation
Susceptible (never infected, not immune)	-	-	-	-	-	No exposure, no vaccination
Immune (vaccination)	-	+	-	-	-	Anti-HBs from vaccine; no core antibody because never infected
Immune (past infection)	-	+	+	-	-	Cleared infection; anti-HBs provides immunity
Acute infection (early)	+	-	+	+	+	Active replication; IgM indicates recent infection
Acute infection (window)	-	-	+	+	-	HBsAg cleared but anti-HBs not yet formed; IgM confirms acute
Chronic infection (active)	+	-	+	-	+/-	HBsAg >6 months; IgM negative; high infectivity if HBeAg+
Chronic infection (inactive carrier)	+	-	+	-	-	Low replication; anti-HBe positive; lower (but not zero) infectivity

The Window Period

The “window period” is a critical concept. After acute infection, HBsAg may become undetectable before anti-HBs appears. During this window (which can last weeks), the only positive marker is anti-HBc IgM. This is why blood banks test for anti-HBc - it catches donors in the window period who would be missed by HBsAg testing alone.

Special Scenarios

Isolated anti-HBc positive (anti-HBc positive, everything else negative):

Window period (acute infection) - check IgM
Resolved infection with waned anti-HBs (most common)
Occult HBV infection (low-level chronic infection; check HBV DNA)
False positive

HBsAg positive >6 months = Chronic hepatitis B (by definition)

“e antigen seroconversion”: Transition from HBeAg-positive to HBeAg-negative with anti-HBe appearance. Generally indicates reduced viral replication and improved prognosis - but watch for “e-negative” mutants that replicate despite anti-HBe.

Hepatitis C Serology

Hepatitis C testing is simpler than hepatitis B because there are fewer markers.

The Markers

Anti-HCV: Antibody to hepatitis C virus. Indicates exposure at some point but does NOT distinguish active from resolved infection. Takes 4-10 weeks to appear after infection.

HCV RNA: Direct detection of viral genetic material. Indicates active, current infection.

Interpretation

Anti-HCV	HCV RNA	Interpretation
-	-	No HCV infection (or very early acute, before antibody)
+	+	Active HCV infection (acute or chronic)
+	-	Past infection (cleared spontaneously or treated) OR false positive antibody
-	+	Very early acute infection (RNA positive before antibody develops)

Key point: Unlike hepatitis B, there is no protective antibody for HCV. Anti-HCV does NOT provide immunity - reinfection is possible. About 20-25% of people clear HCV spontaneously; the rest develop chronic infection.

Testing algorithm:

Screen with anti-HCV
If positive, confirm active infection with HCV RNA
If RNA positive, genotype to guide treatment

Hepatitis A Serology

Anti-HAV IgM: Acute hepatitis A infection. Appears at symptom onset, persists ~3-6 months.

Anti-HAV IgG (or total anti-HAV): Past infection or vaccination; indicates immunity.

Key point: Hepatitis A does NOT cause chronic infection. IgM = acute; IgG = immune.

EBV (Epstein-Barr Virus) Serology

EBV serology is used to diagnose infectious mononucleosis and determine immune status. The pattern of antibodies tells you where the patient is in the infection timeline.

The Markers

VCA IgM (Viral Capsid Antigen IgM): Appears early in acute infection; disappears within 4-8 weeks. Indicates acute/recent infection.

VCA IgG (Viral Capsid Antigen IgG): Appears during acute infection; persists for life. Indicates current or past infection.

EA (Early Antigen): Appears during acute infection in ~70-80% of patients; disappears after recovery in most. Persistence may indicate chronic active infection or reactivation. Less commonly used.

EBNA (EBV Nuclear Antigen): Appears late (3-6 months after infection); persists for life. Its presence excludes acute primary infection.

The Interpretation Table

VCA IgM	VCA IgG	EBNA	Interpretation
-	-	-	Susceptible (never infected)
+	+	-	Acute primary infection
-	+	-	Early convalescence (recent infection, EBNA not yet positive)
-	+	+	Past infection (immune)
+	+	+	Probable false positive IgM, OR reactivation (rare)

Key pattern for boards: In acute infectious mononucleosis, expect VCA IgM positive, VCA IgG positive, EBNA negative. If EBNA is positive, the infection is not acute.

EBV serology interpretation: VCA IgM indicates acute infection; EBNA appears late and excludes acute infection if positive.

Heterophile antibody (Monospot): A rapid test that detects heterophile antibodies produced during acute EBV infection. Positive in ~85% of adults with infectious mono but may be negative in children and early in illness. A positive Monospot confirms acute EBV in the right clinical context; a negative test should prompt specific EBV serology.

CMV (Cytomegalovirus) Serology

CMV serology is used for determining immune status (important for transfusion and transplant) and diagnosing acute infection.

The Markers

CMV IgM: Indicates recent or active infection. May persist for months, so presence doesn’t definitively indicate acute infection.

CMV IgG: Indicates past infection. Persists for life.

CMV IgG Avidity: Measures how tightly antibodies bind to antigen. Low avidity = recent infection (antibodies mature over time). High avidity = past infection (>3-4 months ago). Useful when IgM is positive and you need to distinguish recent from past infection.

Interpretation

CMV IgM	CMV IgG	Interpretation
-	-	Susceptible (seronegative)
+	-	Very early acute infection (IgG not yet formed)
+	+	Recent infection OR reactivation (check avidity)
-	+	Past infection (seropositive, immune)

Clinical applications:

Pregnancy: Primary CMV during pregnancy carries highest risk to fetus. Low avidity IgG with positive IgM suggests recent infection.
Transplant: CMV-seronegative recipients receiving organs from seropositive donors are at highest risk for CMV disease.
Blood transfusion: CMV-seronegative or leukoreduced blood for at-risk recipients (premature infants, seronegative transplant recipients).

HIV Serology and Testing

The Testing Algorithm

Fourth-generation (Ag/Ab combination) assay: Detects both HIV-1/2 antibodies AND p24 antigen. This is the recommended initial screening test. The p24 antigen component shortens the window period by detecting acute infection before antibodies develop.

4th Gen Screen	HIV-1/2 Differentiation	HIV-1 NAT	Interpretation
Negative	-	-	HIV negative (or very early acute)
Positive	HIV-1 positive	-	HIV-1 infection confirmed
Positive	HIV-2 positive	-	HIV-2 infection confirmed
Positive	Negative or indeterminate	Positive	Acute HIV-1 (antibody not yet formed, but RNA present)
Positive	Negative or indeterminate	Negative	False positive screen (or rare late seroconversion)

Window periods (approximate):

RNA (NAT): Detectable ~10-14 days after infection
p24 antigen: Detectable ~14-18 days after infection
Antibody: Detectable ~21-28 days after infection

Syphilis Serology

Covered in the microbiology section, but the key serological points:

Non-treponemal tests (RPR, VDRL):

Detect antibodies to cardiolipin (not specific to syphilis)
Quantitative titers correlate with disease activity
Decrease with treatment (useful for monitoring)
False positives: pregnancy, autoimmune disease, other infections

Treponemal tests (FTA-ABS, TP-PA, EIA/CIA):

Detect antibodies specific to T. pallidum
More specific than non-treponemal tests
Remain positive for life (can’t use to monitor treatment)

Reverse sequence algorithm (increasingly common):

Screen with treponemal EIA/CIA
If positive, reflex to non-treponemal (RPR)
If discordant (EIA+, RPR-), confirm with different treponemal test (TP-PA)

PART III: HEMATOPATHOLOGY

Hematopathology is the study of diseases affecting blood cells and their precursors. It encompasses benign conditions (anemias, coagulopathies) and malignancies (leukemias, lymphomas). Understanding hematopathology requires integrating morphology, laboratory findings, immunophenotype, and molecular genetics.

Taxonomic Overview of Hematologic Diseases

The following classification organizes hematologic diseases by cell lineage. All blood cells derive from a common hematopoietic stem cell (HSC) that differentiates into myeloid and lymphoid progenitors. Understanding where each disease fits in this hierarchy aids in predicting morphology, immunophenotype, clinical behavior, and treatment approach.

Comprehensive Lineage of Hematopoietic Stem Cell Differentiation

Structural Hierarchy of Blood Cell Development

I. MYELOID LINEAGE DISORDERS

A. RED BLOOD CELL DISORDERS

Anemias - Decreased Production (Hypoproliferative)

Microcytic (MCV <80)
Iron deficiency anemia - most common worldwide; low ferritin, high TIBC
Anemia of chronic disease/inflammation - low iron, low TIBC, high ferritin, high hepcidin
Thalassemias - alpha (gene deletions) and beta (point mutations); target cells, microcytosis out of proportion to anemia
Sideroblastic anemia - ringed sideroblasts, defective heme synthesis (lead, B6 deficiency, MDS-RS)
Normocytic (MCV 80-100)
Anemia of chronic disease (early)
Chronic kidney disease - decreased erythropoietin
Aplastic anemia - pancytopenia, hypocellular marrow, fatty replacement
Pure red cell aplasia - parvovirus B19, thymoma, autoimmune
Myelophthisic anemia - marrow infiltration (tumor, fibrosis)
Macrocytic (MCV >100)
Megaloblastic: B12 deficiency (neurologic symptoms, methylmalonic acid elevated), Folate deficiency (no neuro, homocysteine elevated) - hypersegmented neutrophils, oval macrocytes
Non-megaloblastic: liver disease, hypothyroidism, MDS, alcohol, reticulocytosis

Anemias - Increased Destruction (Hemolytic)

Intrinsic (RBC defect)
Membrane: Hereditary spherocytosis (spectrin/ankyrin, osmotic fragility), Hereditary elliptocytosis, PNH (CD55/CD59 loss, complement-mediated)
Enzyme: G6PD deficiency (oxidant stress, bite cells, Heinz bodies), Pyruvate kinase deficiency (rigid cells)
Hemoglobin: Sickle cell disease (HbSS, vaso-occlusion), HbSC disease, HbC disease (target cells, crystals)
Extrinsic (external to RBC)
Immune: Warm AIHA (IgG, spherocytes, spleen), Cold AIHA (IgM, complement, agglutination), Drug-induced
Mechanical: MAHA (TTP, HUS, DIC - schistocytes), Mechanical heart valves, March hemoglobinuria
Infectious: Malaria, Babesia, Clostridial sepsis

Polycythemia (Increased RBCs)

Primary: Polycythemia vera - JAK2 V617F (95%), panmyelosis, low EPO, risk of thrombosis and transformation to myelofibrosis/AML
Secondary: Appropriate (hypoxia, high altitude, lung disease, cyanotic heart disease) vs Inappropriate (EPO-secreting tumors - RCC, HCC, hemangioblastoma)
Relative: Dehydration, stress polycythemia (Gaisbock syndrome)

B. WHITE BLOOD CELL DISORDERS - MYELOID NEOPLASMS

Acute Myeloid Leukemia (AML) - ≥20% blasts, aggressive

AML with recurrent genetic abnormalities (generally better prognosis)
APL: t(15;17) PML-RARA - Auer rods, DIC, ATRA therapy, HLA-DR negative
t(8;21) RUNX1-RUNX1T1 - large blasts, Auer rods, good prognosis
inv(16)/t(16;16) CBFB-MYH11 - abnormal eosinophils, good prognosis
t(9;11) KMT2A (MLL) - monocytic features, intermediate prognosis
NPM1 mutation - cup-like nuclear invaginations, favorable (if no FLT3-ITD)
Biallelic CEBPA mutation - favorable prognosis
FLT3-ITD - poor prognosis, targetable with midostaurin/gilteritinib
AML with myelodysplasia-related changes - multilineage dysplasia, poor prognosis
Therapy-related AML - prior alkylating agents (del 5/7) or topoisomerase II inhibitors (11q23)
AML, NOS - classified by differentiation (minimally differentiated through monocytic, erythroid, megakaryocytic)

Myelodysplastic Syndromes (MDS) - cytopenias with dysplasia, <20% blasts

Refractory anemia, refractory cytopenia with multilineage dysplasia
MDS with ring sideroblasts - SF3B1 mutation, favorable
MDS with excess blasts (5-19%) - higher risk of AML transformation
MDS with isolated del(5q) - thrombocytosis, hypolobated megakaryocytes, lenalidomide responsive
Key mutations: SF3B1, TET2, ASXL1, TP53 (poor prognosis), RUNX1

Myeloproliferative Neoplasms (MPN) - clonal proliferation, hypercellular marrow, increased mature cells

Polycythemia vera (PV) - JAK2 V617F (95%), erythrocytosis, pruritus after bathing, thrombosis
Essential thrombocythemia (ET) - JAK2, CALR, or MPL mutations; thrombocytosis, bleeding or thrombosis
Primary myelofibrosis (PMF) - JAK2, CALR, or MPL; marrow fibrosis, teardrop cells, leukoerythroblastic smear, splenomegaly
Chronic myeloid leukemia (CML) - t(9;22) BCR-ABL1 Philadelphia chromosome; basophilia, three phases (chronic → accelerated → blast crisis); imatinib

MDS/MPN Overlap

Chronic myelomonocytic leukemia (CMML) - persistent monocytosis >1000/μL, dysplasia, ±splenomegaly
Atypical CML - neutrophilia with dysplasia, BCR-ABL1 negative
Juvenile myelomonocytic leukemia (JMML) - children, RAS pathway mutations

Mastocytosis

Systemic mastocytosis - KIT D816V mutation, tryptase elevated, urticaria pigmentosa
Mast cell leukemia - ≥20% mast cells in marrow, aggressive

C. PLATELET AND MEGAKARYOCYTE DISORDERS

Thrombocytopenia - Decreased Production

Marrow failure (aplastic anemia, MDS, infiltration)
Congenital: Wiskott-Aldrich (small platelets, eczema, immunodeficiency), TAR syndrome, Bernard-Soulier (giant platelets, GPIb deficiency)

Thrombocytopenia - Increased Destruction

Immune: ITP (antiplatelet antibodies, diagnosis of exclusion), HIT (PF4 antibodies, thrombosis), Drug-induced, Post-transfusion purpura, Neonatal alloimmune
Non-immune: TTP (ADAMTS13 deficiency, pentad), HUS (Shiga toxin, renal failure), DIC (consumption)

Thrombocytopenia - Sequestration

Hypersplenism, massive splenomegaly

Thrombocytosis

Reactive: Iron deficiency, infection, inflammation, malignancy, post-splenectomy
Clonal: Essential thrombocythemia, other MPNs, MDS with del(5q)

Qualitative Platelet Disorders

Glanzmann thrombasthenia - GPIIb/IIIa deficiency, no aggregation to any agonist (except ristocetin)
Bernard-Soulier - GPIb deficiency, no aggregation to ristocetin, giant platelets
Storage pool disorders - dense granule or alpha granule deficiency
Acquired: Uremia, aspirin/NSAIDs, myeloproliferative neoplasms

II. LYMPHOID LINEAGE DISORDERS

A. PRECURSOR LYMPHOID NEOPLASMS

B-Lymphoblastic Leukemia/Lymphoma (B-ALL)

TdT+, CD19+, CD10+ (most), surface Ig negative
Children: most common childhood cancer, excellent prognosis
Favorable: Hyperdiploidy (>50 chromosomes), ETV6-RUNX1 t(12;21)
Unfavorable: Hypodiploidy, KMT2A rearrangements (infants), BCR-ABL1 (Philadelphia chromosome ALL)
Ph-like ALL - poor prognosis, ABL-class or JAK-STAT alterations

T-Lymphoblastic Leukemia/Lymphoma (T-ALL)

TdT+, CD3 (cytoplasmic)+, CD7+, variable CD4/CD8
Often presents as mediastinal mass (thymic), adolescent males
NOTCH1 mutations common

B. MATURE B-CELL NEOPLASMS

Small Lymphocytic Lymphoma / Chronic Lymphocytic Leukemia (CLL/SLL)

Same disease: SLL (nodal), CLL (blood/marrow)
CD5+, CD19+, CD23+, CD200+, dim CD20, dim surface Ig
Smudge cells on smear, proliferation centers in lymph node
Good prognosis: del(13q), mutated IGHV
Poor prognosis: del(17p)/TP53, del(11q), unmutated IGHV
Richter transformation to DLBCL (aggressive)

Mantle Cell Lymphoma

CD5+, CD19+, CD23 negative, Cyclin D1+, SOX11+
t(11;14) CCND1-IGH
Aggressive, often presents advanced stage
Blastoid variant - very aggressive

Follicular Lymphoma

CD10+, CD19+, CD5 negative, BCL2+, BCL6+
t(14;18) BCL2-IGH - antiapoptotic, constitutive BCL2 expression
Indolent, nodular/follicular pattern, centrocytes and centroblasts
Grading: 1-2 (low grade, indolent), 3A (intermediate), 3B (treat as DLBCL)
Transformation to DLBCL

Marginal Zone Lymphoma

CD19+, CD20+, CD5 negative, CD10 negative, CD23 negative
Extranodal (MALT): stomach (H. pylori - may regress with antibiotics), lung, thyroid (Hashimoto’s), salivary (Sjogren’s)
Nodal: generalized lymphadenopathy
Splenic: splenomegaly, villous lymphocytes

Lymphoplasmacytic Lymphoma / Waldenstrom Macroglobulinemia

IgM monoclonal protein (paraprotein)
MYD88 L265P mutation (>90%)
Hyperviscosity syndrome, neuropathy, cryoglobulinemia, cold agglutinins

Hairy Cell Leukemia

CD19+, CD11c+, CD25+, CD103+, CD123+, TRAP (tartrate-resistant acid phosphatase)+
BRAF V600E mutation (almost 100%)
Pancytopenia, splenomegaly, “dry tap” on marrow aspirate (fibrosis)
“Hairy” cytoplasmic projections, “fried egg” appearance in marrow

Diffuse Large B-Cell Lymphoma (DLBCL)

Most common NHL in adults
CD19+, CD20+, variable CD10, BCL6, BCL2
Aggressive but curable with R-CHOP
Cell of origin: GCB (germinal center B-cell, better prognosis) vs ABC (activated B-cell, worse)
MYC rearrangement + BCL2 or BCL6 = “double-hit” (very aggressive)

Burkitt Lymphoma

CD10+, CD19+, BCL6+, BCL2 negative, Ki-67 ~100%
t(8;14) MYC-IGH (or variants t(2;8), t(8;22))
“Starry sky” pattern (tingible body macrophages)
Endemic (Africa, EBV+, jaw), Sporadic (abdomen, children), Immunodeficiency-associated (HIV)
Highly aggressive but curable, tumor lysis syndrome risk

Plasma Cell Neoplasms

MGUS - <3g/dL M-protein, <10% plasma cells, no end-organ damage; 1%/year progression to myeloma
Smoldering myeloma - higher M-protein/plasma cells but no symptoms
Multiple myeloma - CRAB criteria (Calcium, Renal, Anemia, Bone lesions), CD138+, CD38+, often CD56+ (aberrant)
Rouleaux formation, lytic bone lesions, serum/urine protein electrophoresis
High-risk: t(4;14), t(14;16), del(17p), 1q gain
Primary amyloidosis (AL) - light chain deposition, Congo red birefringence
Waldenstrom - see lymphoplasmacytic lymphoma above (IgM)

C. MATURE T-CELL AND NK-CELL NEOPLASMS

T-Cell Prolymphocytic Leukemia

Aggressive, splenomegaly, skin involvement
inv(14) or t(14;14) involving TCL1

T-Cell Large Granular Lymphocytic Leukemia (T-LGL)

CD3+, CD8+, CD57+
Indolent, neutropenia, rheumatoid arthritis association, STAT3 mutations
Large lymphocytes with azurophilic granules

Adult T-Cell Leukemia/Lymphoma (ATLL)

HTLV-1 associated, endemic in Japan, Caribbean
“Flower cells” (multilobated nuclei)
Hypercalcemia, lytic bone lesions, aggressive

Mycosis Fungoides / Sezary Syndrome

Cutaneous T-cell lymphoma
CD4+ T-cells with cerebriform nuclei
MF: patches → plaques → tumors, Pautrier microabscesses (epidermotropism)
Sezary: erythroderma + circulating Sezary cells (leukemic phase)

Peripheral T-Cell Lymphoma, NOS

Heterogeneous group, aggressive, poor prognosis

Angioimmunoblastic T-Cell Lymphoma (AITL)

CD4+ T-follicular helper phenotype (PD-1+, CXCL13+, CD10+)
Systemic symptoms, polyclonal hypergammaglobulinemia, EBV+ B-cells in background
TET2, DNMT3A, RHOA mutations

Anaplastic Large Cell Lymphoma (ALCL)

CD30+ (hallmark), ALK+ or ALK-
ALK+ (younger, better prognosis) - t(2;5) NPM1-ALK
“Hallmark cells” with horseshoe/kidney-shaped nuclei
Breast implant-associated ALCL - ALK negative, localized, good prognosis if capsulectomy

Extranodal NK/T-Cell Lymphoma, Nasal Type

EBV-associated, more frequent in East Asian and Central/South American populations
Midline destructive lesions (nose, palate), angiocentric/angiodestructive
CD56+, cytoplasmic CD3+, surface CD3 negative

Hepatosplenic T-Cell Lymphoma

Young males, immunosuppression (IBD on thiopurines + anti-TNF)
Gamma-delta T-cell origin, sinusoidal infiltration
Very aggressive

D. HODGKIN LYMPHOMA

Classical Hodgkin Lymphoma (95%)

Reed-Sternberg cells: CD30+, CD15+, PAX5 weak+, CD45 negative, CD20 usually negative
“Owl eye” binucleated RS cells in inflammatory background
Nodular sclerosis - most common, mediastinal mass, lacunar cells, young adults
Mixed cellularity - EBV-associated, older/HIV patients
Lymphocyte-rich - best prognosis
Lymphocyte-depleted - worst prognosis, HIV-associated, older patients

Nodular Lymphocyte-Predominant Hodgkin Lymphoma (5%)

LP cells (“popcorn cells”): CD20+, CD45+, CD15 negative, CD30 negative
Indolent, excellent prognosis
Can transform to DLBCL

III. HEMOSTASIS AND COAGULATION DISORDERS

A. BLEEDING DISORDERS

Platelet/Primary Hemostasis Disorders - mucocutaneous bleeding

Quantitative: Thrombocytopenia (see above)
Qualitative: von Willebrand disease, Glanzmann, Bernard-Soulier, acquired

Coagulation Factor/Secondary Hemostasis Disorders - deep tissue bleeding, hemarthrosis

Inherited
Hemophilia A (Factor VIII) - X-linked, most common severe deficiency
Hemophilia B (Factor IX) - X-linked, Christmas disease
Hemophilia C (Factor XI) - autosomal, people of Ashkenazi Jewish ancestry
von Willebrand Disease - most common inherited bleeding disorder; Type 1 (quantitative), Type 2 (qualitative), Type 3 (severe)
Acquired
Vitamin K deficiency - Factors II, VII, IX, X, Protein C, S
Liver disease - decreased synthesis of all factors except VIII
DIC - consumption of factors and platelets, microthrombi
Acquired inhibitors - Factor VIII inhibitor (autoantibody)

B. THROMBOTIC DISORDERS (Thrombophilias)

Inherited

Factor V Leiden - most common; activated protein C resistance
Prothrombin G20210A mutation - elevated prothrombin levels
Antithrombin deficiency - heparin resistance
Protein C deficiency - warfarin-induced skin necrosis
Protein S deficiency

Acquired

Antiphospholipid syndrome - lupus anticoagulant, anticardiolipin, anti-beta2-glycoprotein I; arterial and venous thrombosis, pregnancy loss
Malignancy (Trousseau syndrome)
HIT - heparin-induced thrombocytopenia with thrombosis
PNH - complement-mediated hemolysis and thrombosis

Key Classification Principles:

Lineage	Key Markers	Disease Examples
Myeloid	CD13, CD33, CD117, MPO	AML, MDS, MPN, CML
B-lymphoid (precursor)	TdT+, CD19+, CD10+	B-ALL
B-lymphoid (mature)	CD19, CD20, surface Ig, CD5 (some)	CLL, FL, DLBCL, MCL
T-lymphoid (precursor)	TdT+, cytoplasmic CD3, CD7	T-ALL
T-lymphoid (mature)	Surface CD3, CD4 or CD8, CD7	PTCL, ALCL, MF/SS
NK-cell	CD56+, surface CD3 negative	NK/T-cell lymphoma
Plasma cell	CD138+, CD38+, CD19 negative	Myeloma
Hodgkin RS cell	CD30+, CD15+, CD45 negative	Classical HL

Quick Reference: Key Immunophenotypes in Hematologic Malignancies

Disease	Key Markers
B-ALL	TdT+, CD19+, CD10+ (usually), surface Ig-
T-ALL	TdT+, CD3+ (cytoplasmic), CD7+, variable CD4/CD8
CLL/SLL	CD5+, CD19+, CD23+, dim CD20, dim surface Ig, CD200+
Mantle cell lymphoma	CD5+, CD19+, CD23-, Cyclin D1+, SOX11+
Follicular lymphoma	CD10+, CD19+, CD5-, BCL2+, BCL6+
Hairy cell leukemia	CD19+, CD11c+, CD25+, CD103+, TRAP+
Marginal zone lymphoma	CD19+, CD20+, CD5-, CD10-, CD23-
Burkitt lymphoma	CD10+, CD19+, BCL6+, BCL2-, Ki-67 ~100%
AML	CD13+, CD33+, CD117+, MPO+
APL	CD13+, CD33+, CD117+, HLA-DR-
Plasma cell myeloma	CD138+, CD38+, often CD56+ (aberrant), CD19-, CD20-
Hodgkin lymphoma (RS cells)	CD30+, CD15+, CD45-, PAX5 weak+, CD20-

Practical use: When you see a lymphocyte malignancy with CD5+, the main differential is CLL vs. mantle cell. The distinguishing feature is CD23 - positive in CLL, negative in mantle cell. Cyclin D1 confirms mantle cell.

Chapter 26: Normal Hematopoiesis and the Complete Blood Count

This chapter is the foundation for everything downstream - anemias, leukemias, lymphomas, all of it. If you can’t read a CBC and a smear fluently, you can’t diagnose the downstream stuff. The goal here: understand what each number means, know what can spuriously move it, and know what cells look like at each stage so you can recognize when something is off.

26.1 Hematopoiesis: The Production of Blood Cells

All blood cells derive from a common hematopoietic stem cell (HSC) in the bone marrow. This stem cell has two defining properties:

Self-renewal: Ability to replicate without differentiation
Multipotency: Ability to differentiate into all blood cell lineages

The Hematopoietic Hierarchy

The HSC differentiates through increasingly lineage-restricted progenitors:

Hematopoietic Stem Cell ↓ Multipotent Progenitor ↓ (splits into) Common Myeloid Progenitor → Common Lymphoid Progenitor

The Common Myeloid Progenitor gives rise to:

Megakaryocyte-Erythroid Progenitor → Megakaryocytes (platelets) and Erythrocytes
Granulocyte-Monocyte Progenitor → Neutrophils, Eosinophils, Basophils, and Monocytes

The Common Lymphoid Progenitor gives rise to:

B lymphocytes
T lymphocytes
NK cells

Sites of Hematopoiesis Through Life

Yolk sac (weeks 2-8 gestation): Primitive hematopoiesis
AGM region (aorta-gonad-mesonephros): generates the first definitive HSCs
Liver and spleen (weeks 6-birth): Fetal hematopoiesis, liver peaks at 3-4 months
Bone marrow (week 20-throughout life): Definitive adult hematopoiesis, becomes primary site by 7th month

By the end of the first trimester, fetal hematopoiesis occurs in liver, spleen, thymus, lymph nodes, and marrow. Nucleated RBCs can be found within the chorionic villus vessels by early second trimester.

At birth, essentially all bones are hematopoietic (including long bones). Hepatic and splenic hematopoiesis cease shortly after birth - any persistence of extramedullary hematopoiesis (EMH) beyond this is pathologic. Gradually through the first two decades, long bone marrow is replaced by fat (yellow marrow), and by age 18-20 hematopoiesis retreats to the axial skeleton (vertebrae, sternum, ribs, pelvis, skull) and proximal long bones (proximal femur/humerus, epiphyses).

Extramedullary hematopoiesis (EMH) in adults occurs when bone marrow cannot meet demand or is replaced by disease. Common sites are liver and spleen, which retain embryonic hematopoietic capability. Stress (e.g., severe hemolysis, marrow infiltration, myelofibrosis) can reactivate EMH.

Key Growth Factors

Growth Factor	Primary Action
Erythropoietin (EPO)	RBC production (produced by kidney)
Thrombopoietin (TPO)	Platelet production (produced by liver)
G-CSF	Neutrophil production
GM-CSF	Granulocyte and monocyte production
IL-3	Multi-lineage stimulation
IL-7	Lymphocyte development
SCF (Stem Cell Factor)	HSC maintenance and early differentiation

TPO signals through the c-Mpl receptor (MPL gene). Loss-of-function MPL mutations cause congenital amegakaryocytic thrombocytopenia (CAMT), which presents in the neonatal period with severe thrombocytopenia and absent marrow megakaryocytes, progressing to pancytopenia by early childhood because c-Mpl also supports HSC self-renewal. HSCT is curative. Contrast with TAR syndrome (thrombocytopenia with absent radii): bilateral absent radii with present thumbs (distinguishes from Fanconi anemia, where thumbs are absent), RBM8A mutations, thrombocytopenia improves with age.

Clinically used cytokines: GM-CSF causes leukocytosis with prominent toxic granulation, nucleated RBCs, and abnormal nuclear segmentation. G-CSF causes similar neutrophilia but with fewer toxic changes. Both cause a left shift. Always know the medication history when interpreting a smear.

26.2 The Complete Blood Count (CBC)

The CBC is the most commonly ordered laboratory test. Understanding it requires knowing what each parameter measures and what affects it - including the spurious movements that are high-yield board material.

Red Blood Cell Parameters

RBC count: Number of red blood cells per unit volume (millions/μL)

Hemoglobin (Hgb): Concentration of hemoglobin (g/dL) - the oxygen-carrying protein. Most automated analyzers measure Hgb by the cyanmethemoglobin (hemiglobin cyanide, HiCN) method: RBCs are lysed, hemoglobin is converted to cyanmethemoglobin by potassium ferricyanide and potassium cyanide, and absorbance is read at 540 nm. All hemoglobin forms are converted EXCEPT sulfhemoglobin. Newer instruments use sodium lauryl sulfate (SLS) to avoid cyanide.

Anything that raises plasma turbidity can falsely elevate Hgb (the spectrophotometric reading is affected by light scattering). Classic interferences: lipemia, paraproteinemia, hyperleukocytosis (WBC >100k), cryoglobulins. Correction: replace plasma with saline and remeasure.

Hematocrit (Hct): Percentage of blood volume occupied by RBCs

Calculated on most analyzers as: RBC count × MCV
Direct measurement by centrifugation (microhematocrit) may differ due to trapped plasma

WHO defines anemia as Hgb <13 g/dL in men, <12 in women, <11 in pregnant women. Other authors use variable thresholds.

Red Cell Indices:

Mean Corpuscular Volume (MCV): Average RBC volume in femtoliters (fL)

Directly measured by impedance or light scatter on modern analyzers
Manual formula: MCV = (Hct / RBC count) × 10, where Hct is %
Classification: Microcytic (<80 fL), Normocytic (80-100 fL), Macrocytic (>100 fL)

Microcytosis (<80 fL): iron deficiency, thalassemia, anemia of inflammation (late), sideroblastic anemia, lead poisoning. Macrocytosis (>100 fL): B12/folate deficiency, MDS, liver disease, alcohol, reticulocytosis, drugs (hydroxyurea, AZT, methotrexate). MCV >120 is almost always megaloblastic. Spurious macrocytosis: cold agglutinins (doublets counted as large cells), hyperglycemia (RBC swelling), pronounced reticulocytosis (reticulocytes are ~120-150 fL and drag up the mean).

Mean Corpuscular Hemoglobin (MCH): Average hemoglobin per RBC in picograms

Calculated: Hgb / RBC count
Parallels MCV (low in microcytic anemias, high in macrocytic)

Mean Corpuscular Hemoglobin Concentration (MCHC): Average hemoglobin concentration within RBCs (g/dL)

Manual formula: MCHC = (Hgb / Hct) × 100
Represents hemoglobin density; the most tightly regulated RBC index
Elevated (>36 g/dL): Spherocytosis (true), cold agglutinins (falsely - doublets counted as single large cells with double hemoglobin), lipemic specimens (false Hgb elevation)
Decreased: Hypochromic anemias (iron deficiency, thalassemia)

An MCHC >36 should always prompt a smear check: true spherocytes (warm AIHA or HS) vs. agglutinates (cold agglutinins) vs. lipemia. HS MCHC elevation reflects membrane loss without proportional hemoglobin loss - the cell shrinks but keeps its hemoglobin, concentrating it. MCHC >36 g/dL combined with increased RDW is highly suggestive of HS.

Red Cell Distribution Width (RDW): Coefficient of variation of RBC volume - a measure of anisocytosis

Normal RDW: ~11.5-14.5%
Anisocytosis = size variation; poikilocytosis = shape variation. RDW quantifies anisocytosis.
Elevated RDW: iron deficiency (classic, mixed microcytic + normocytic populations), mixed deficiencies, sideroblastic, early treatment response, post-transfusion
Normal RDW with microcytosis: thalassemia trait (uniform small cells)

This is one of the most useful parameters for distinguishing iron deficiency (high RDW) from thalassemia trait (normal RDW). Thalassemia has decreased hematocrit and normal to slightly increased RDW, with increased RBC count (distinctive “thalassemic indices”).

Reticulocyte Count

Reticulocytes are immature RBCs that still contain ribosomal RNA. Differentiated from mature RBCs based on abundance of ribosomal RNA. Detected by:

Supravital stains (new methylene blue, brilliant cresyl blue) precipitate the RNA into a visible reticular network
Automated analyzers use fluorescent RNA-binding dyes (thiazole orange, polymethine)

Normal: 0.5-1.5% (or 25,000-75,000/μL absolute)

Interpretation:

Elevated: Appropriate bone marrow response to anemia (hemolysis, blood loss, treatment response)
Low or normal in setting of anemia: Hyporegenerative anemia - inadequate marrow response (marrow failure, nutritional deficiency, anemia of chronic disease)

Caveats: partially treated iron/folate/B12 deficiency may show reticulocytosis. Immune hemolysis can uncommonly target mature AND immature erythrocytes, resulting in reticulocytopenia. Chronic hemolysis or blood loss may deplete iron/folate/B12 and eventually present with reticulocytopenia.

Absolute reticulocyte count = % reticulocytes × RBC count. More informative than percentage alone because the percentage can be misleadingly elevated due to a low RBC count denominator.

Corrected Reticulocyte Count (CRC) = % reticulocytes × (patient Hct / 45). Corrects for the dilutional effect of anemia on the reticulocyte percentage.

Reticulocyte Production Index (RPI) = CRC × (1 / maturation factor)

Maturation factor corrects for the longer life span of prematurely released reticulocytes in the blood (more anemia = earlier release = longer circulation time). Maturation factors: Hct 45% = 1.0; 35% = 1.5; 25% = 2.0; 15% = 2.5.

RPI >2-3 suggests appropriate marrow response (hemolysis or blood loss)
RPI <2 suggests inadequate response (hyporegenerative)

Immature Reticulocyte Fraction (IRF): proportion of reticulocytes with the highest RNA content (most immature). Rises BEFORE the reticulocyte count increases, making it the earliest indicator of marrow recovery (e.g., post-chemo recovery, early response to EPO).

Reticulocyte Hemoglobin (CHr or Ret-He): hemoglobin content of reticulocytes - a real-time snapshot of iron available for hemoglobin synthesis in the marrow right now. Low CHr indicates functional iron deficiency. Useful in differentiating functional iron deficiency from ferritin-stored iron sequestration in anemia of chronic disease.

Hemolysis Labs

When evaluating anemia with reticulocytosis or suspected hemolysis:

Classic hemolysis labs: elevated LDH, decreased haptoglobin, elevated indirect bilirubin, elevated reticulocyte count. Haptoglobin is the most sensitive marker - it drops before LDH rises. Low haptoglobin alone is not specific (decreased in liver disease, congenital ahaptoglobinemia). The combination of low haptoglobin + elevated LDH has >90% specificity for hemolysis.

Intravascular vs. extravascular hemolysis: important distinction.

Intravascular: RBCs lyse within vessels, releasing free hemoglobin into plasma. Hemoglobinemia, hemoglobinuria, and hemosiderinuria (specific for intravascular hemolysis; iron in urine sediment, Prussian blue positive, persists for weeks after the event). Causes: microangiopathic (TTP, HUS, DIC, HELLP), complement-mediated (PNH, PCH, ABO incompatibility), mechanical heart valves, Clostridium perfringens, severe G6PD crises, snake envenomation, malaria/babesiosis
Extravascular: RBCs destroyed by macrophages in spleen/liver. Increased urine/fecal urobilinogen. Causes: HS, AIHA, thalassemia

Acute exacerbation of chronic hemolysis: aplastic crisis (Parvovirus B19 infects erythroid precursors via P antigen, halts erythropoiesis for 7-10 days, reticulocytes drop to zero), splenic sequestration crisis (young SCD patients, hypovolemic shock), hyperhemolytic crisis (infection-driven), megaloblastic crisis (folate depletion from chronic high turnover).

White Blood Cell Parameters

WBC count: Total white blood cells per unit volume (thousands/μL). On automated impedance counters, particles larger than ~36 fL are counted as leukocytes (after RBC lysis). Nucleated RBCs can escape lysis and be counted as WBCs, falsely elevating the count. Modern analyzers have NRBC correction algorithms.

Differential: Percentage or absolute count of each WBC type

Cell Type	Normal %	Normal Absolute
Neutrophils	40-70%	1.8-7.7 × 10³/μL
Lymphocytes	20-40%	1.0-4.8 × 10³/μL
Monocytes	2-8%	0.1-0.8 × 10³/μL
Eosinophils	1-4%	0.0-0.4 × 10³/μL
Basophils	0-1%	0.0-0.1 × 10³/μL

Absolute neutrophil count (ANC): WBC × (% neutrophils + % bands)

ANC <1500/μL: Neutropenia
ANC <500/μL: Severe neutropenia (high infection risk)
ANC <100/μL: Profound neutropenia

Immature Granulocyte (IG) fraction: sum of metamyelocytes, myelocytes, and promyelocytes in peripheral blood. Detected by automated analyzers using differences in light scatter, fluorescence, or peroxidase activity. More reproducible than manual band counts. Used in sepsis screening.

Causes of Neutrophilia, Lymphocytosis, and Beyond

Reactive neutrophilia: usually does not exceed 30 × 10³/μL. Above this, consider leukemoid reaction (severe infection, G-CSF therapy, solid tumors producing G-CSF) or CML. Reactive neutrophilia has:

Toxic changes: toxic granulation (prominent dark azurophilic granules - retained primary granules from accelerated maturation), Döhle bodies (pale blue cytoplasmic inclusions of rough ER), cytoplasmic vacuolization (phagocytic activity)
Left shift with mainly bands and occasional metamyelocytes (blasts absent, few myelocytes). Contrast with CML (smooth maturation pyramid, all stages proportional)
Increased LAP score. CML has decreased LAP

Causes of reactive neutrophilia: infection, medications (epinephrine, corticosteroids), trauma/burns, systemic inflammation, seizures, exercise, post-splenectomy, pregnancy, juvenile rheumatoid arthritis in children.

Reactive lymphocytosis:

Infectious mononucleosis syndromes: proliferation of reactive T cells (the “atypical lymphocytes” are activated CD8+ cytotoxic T cells responding to EBV-infected B cells). Moderate to abundant cytoplasm, possibly cytoplasmic granules, and characteristic “burnt” (scalloped) cytoplasmic borders that are identified by adjacent erythrocytes. Most common causes: EBV, CMV, acute HIV, toxoplasmosis. EBV infects B cells via CD21 (complement receptor 2). Heterophile antibody (Monospot) detects IgM against horse/sheep RBCs.
Transient stress lymphocytosis: T, B, and NK cells demarginate from endothelium due to catecholamine release during acute physiologic stress (trauma, surgery, MI, seizures, sickle cell crisis). Resolves within hours. Morphologically non-reactive.
Persistent polyclonal B lymphocytosis (PPBL): young adult female smokers, HLA-DR7+, mild absolute lymphocytosis with bilobed/binucleated lymphocytes (morphologic hallmark), polyclonal IgM hypergammaglobulinemia. Polyclonal B cells (both kappa and lambda). Not a neoplasm but small increased lymphoma risk. CD5-negative (distinguishes from CLL).
Pertussis in children: marked lymphocytosis (can be 30-100k) with small mature lymphocytes having cleaved/clefted nuclei (Rieder cells). Mechanism: pertussis toxin inhibits chemokine receptors, preventing lymphocyte egress from blood into lymph nodes.
Adults >40 with absolute lymphocytosis: always suspicious for CLL. Flow cytometry is essential.

Monocytosis (>1.0 × 10³/μL): frequently reactive. Persistent unexplained monocytosis for >3 months raises concern for CMML (requires monocytes ≥0.5 × 10⁹/L AND ≥10% of WBC differential, with no BCR-ABL1, no MPN, with dysplasia or clonal cytogenetic/molecular abnormality - key mutations TET2, SRSF2, ASXL1). Reactive causes: collagen vascular diseases, chronic infection (classically Listeria or tuberculosis, also brucellosis, endocarditis, syphilis), malignancy, neutropenia (compensatory), post-splenectomy, recovery from chemo. Serum lysozyme is elevated in monocytic proliferations but does not distinguish reactive from neoplastic. Neoplasia suggested by promonocytes on smear and/or splenomegaly.

Automated counters may misclassify hairy cells, blasts, abnormal T cells (Sezary), or hypogranular immature neutrophils as monocytes - always review the smear.

Eosinophilia (>0.5 × 10³/μL): almost always reactive - allergies and helminthic infections. Protozoa (malaria, Giardia) do NOT cause eosinophilia, only helminths do (Ascaris, Strongyloides, hookworm, Toxocara, Trichinella, filariae). Other causes: drug hypersensitivity (DRESS), adrenal insufficiency, Churg-Strauss/eGPA, Hodgkin, T-cell neoplasms, colonic carcinoma, IBD, GM-CSF therapy, eosinophilic cellulitis (Well), eosinophilic pneumonia (Löffler), eosinophilic fasciitis (Shulman).

After reactive causes are excluded, persistent eosinophilia raises concern for: chronic eosinophilic leukemia (CEL) (check PDGFRA, PDGFRB, FGFR1 rearrangements, PCM1-JAK2), MPN with eosinophilia, or hypereosinophilic syndrome (HES - idiopathic). FIP1L1-PDGFRA rearrangement is the most important to identify because it responds dramatically to imatinib (FISH for CHIC2 deletion is the standard surrogate test). Morphologically abnormal eosinophils (trilobate, monolobate) and/or splenomegaly increase the likelihood of neoplasia.

Neutropenia: medications are the most common cause. Mechanisms: dose-dependent marrow suppression (chemotherapy) or idiosyncratic immune-mediated. Specific drugs (must know):

Antithyroidals (methimazole, propylthiouracil, carbimazole) - classic board answer
Antibiotics (chloramphenicol, penicillin, sulfasalazine, trimethoprim-sulfa)
Anticonvulsants (valproate, carbamazepine)
Antiarrhythmics (procainamide)
Clozapine (mandatory monitoring - weekly to monthly CBC for first 6 months)
Ticlopidine, phenothiazines, dapsone

Mnemonic: “Toxic Big Chemical Agents” - anti-Thyroidals, anti-Biotics, anti-Convulsants, anti-Arrhythmics.

Other neutropenia causes: LGL leukemia (LGLs suppress granulopoiesis via Fas/FasL-mediated killing of myeloid precursors; triad: neutropenia + splenomegaly + rheumatoid arthritis, Felty-like), autoimmune (HLA-DR4, SLE, RA/Felty syndrome), consumption in overwhelming sepsis (especially neonates and elderly; bandemia or left shift with low total WBC is ominous), splenomegaly, infection (typhoid, brucellosis, tularemia, rickettsial).

Lymphopenia: isolated lymphopenia is rare. Causes - “Some Viruses Reduce Cells Constantly”:

SLE
Viral infections (HIV, SARS)
Rituximab (anti-CD20) therapy
Corticosteroids
Congenital immunodeficiency (Bruton, SCID, DiGeorge, CVID)

Monocytopenia: hairy cell leukemia (hallmark finding - hairy cells in marrow/spleen suppress monocyte production) or steroid therapy (transient, via redistribution). In chemotherapy, monocytopenia heralds the onset of neutropenia. MonoMAC syndrome (GATA2 deficiency): monocytopenia + mycobacterial infections + risk of MDS/AML.

Platelet Parameters

Platelet count: Number of platelets per unit volume (thousands/μL)

Normal: 150-400 × 10³/μL
<150: Thrombocytopenia
<50: Increased bleeding risk with surgery
<10-20: Risk of spontaneous bleeding

On impedance-based analyzers, particles measuring 2-20 fL are counted as platelets. Giant platelets (>20 fL) may be counted as RBCs or WBCs (falsely low platelet count) - classic in MYH9 disorders, Bernard-Soulier. Schistocytes, microcytic RBCs, cryoglobulins, fibrin strands, Pappenheimer bodies can all be counted as platelets and falsely elevate the count. Optical/fluorescent platelet counting (CD61-based) is more accurate for giant platelets.

Mean Platelet Volume (MPV): Average platelet volume

Increased: Young platelets (bone marrow compensating), ITP, MYH9 disorders, Bernard-Soulier, Mediterranean macrothrombocytopenia, gray platelet syndrome, MPN
Decreased: Marrow failure, Wiskott-Aldrich syndrome

Thrombocytopenia and Thrombocytosis

Children: ITP is the most common cause of thrombocytopenia (diagnosis of exclusion). Rapid response to steroids is consistent with ITP. Poor steroid response but good response to transfused platelets suggests an inherited syndrome (in ITP, transfused platelets are rapidly destroyed by the same antibodies).

Neonates: infection-induced marrow suppression (TORCH, sepsis), neonatal alloimmune thrombocytopenia (NAIT) (maternal anti-HPA-1a antibodies cross placenta, analogous to Rh HDN for platelets - most common cause of severe thrombocytopenia in an otherwise well neonate), maternal ITP (passive IgG transfer), DIC, congenital syndromes (CAMT, TAR, Wiskott-Aldrich), chromosomal anomalies (trisomy 13/18/21, Turner). In bleeding NAIT before antigen-negative or maternal platelets are available, transfuse the most readily available platelet unit + consider IVIG.

Adults: ITP, TTP, medications (heparin -> HIT, quinine, vancomycin, linezolid, abciximab, quinidine), DIC, liver disease/hypersplenism, bone marrow failure (MDS, aplastic anemia), gestational thrombocytopenia (benign, >70k, 3rd trimester), HELLP. Drug timing is the most important clue for drug-induced thrombocytopenia.

Critical distinction by clotting times:

TTP and HUS: NORMAL PT/aPTT (microvascular platelet aggregation only, no coag cascade activation)
DIC and HELLP: PROLONGED PT/aPTT (consumption of clotting factors, low fibrinogen, elevated D-dimer)

ITP mechanism: splenic destruction and sequestration of platelets coated with antiplatelet autoantibodies (IgG against GPIIb, GPIIIa, GPIb, or GPV). Predominantly anti-GPIIb/IIIa or anti-GPIb/IX. Splenic macrophages phagocytose via Fc receptors. Thrombopoiesis may also be impaired (antimegakaryocyte antibodies). Diagnosis is one of exclusion.

ITP treatment: >80% respond to methylprednisolone with/without IVIG. Refractory: platelet count >30k cannot be achieved despite initial therapy; may need splenectomy, rituximab, TPO-RA (eltrombopag, romiplostim), fostamatinib. IVIG blocks Fc receptors on macrophages (rapid but temporary). Anti-D Ig works similarly in Rh+ non-splenectomized patients. Substantial response to immunomodulatory treatment is the best diagnostic test.

Drug-induced immune thrombocytopenia: quinidine is associated with antibodies against GPIX component of GPIb-IX-V. Drug-dependent antibodies require the drug to be present for binding.

Large and variably sized platelets on smear:

Bernard-Soulier syndrome: giant platelets, absent ristocetin aggregation (GPIb-IX-V deficiency)
May-Hegglin anomaly: autosomal dominant, triad of thrombocytopenia + giant platelets + Döhle-like bodies in leukocytes. Caused by MYH9 mutations (non-muscle myosin heavy chain IIA). The “inclusions” are aggregated myosin heavy chain, NOT true Döhle bodies (which are RER). Spectrum includes Fechtner (+ sensorineural deafness, nephritis), Sebastian, Epstein syndromes. Most patients have mild bleeding despite thrombocytopenia (giant platelets are functionally effective).
ITP: large young platelets from accelerated turnover
Gray platelet syndrome (NBEAL2): large pale agranular platelets, alpha-granule deficiency, mild thrombocytopenia, splenomegaly, myelofibrosis
Mediterranean macrothrombocytopenia: southern European ancestry, mild thrombocytopenia, decreased expression of GPIb-IX-V (VWF receptor, chromosome 17), clinically benign

Glanzmann thrombasthenia (GT): autosomal recessive, deficiency of GPIIb/IIIa (integrin αIIb-β3). Normal platelet size (unlike Bernard-Soulier). Platelet aggregation absent to ALL agonists EXCEPT ristocetin (which works via VWF-GPIb, not GPIIb/IIIa). Mnemonic: GT = normal size, no aggregation; BSS = giant, no ristocetin.

Pseudothrombocytopenia: spuriously low platelet count from in vitro clumping/satellitosis, most commonly due to EDTA-dependent anti-platelet antibodies (IgG against GPIIb/IIIa exposed by EDTA-induced conformational change). Always check the smear and feathered edge when an unexpectedly low platelet count is reported. Recollection in citrate (light blue top) or acid-citrate-dextrose gives an accurate count (citrate dilution factor: multiply by 1.1 because 9:1 blood:citrate ratio). Platelet satellitism: platelets form a rosette around neutrophils (EDTA-dependent IgG bridges platelet GPIIb/IIIa to neutrophil CD16). Thrombophagocytosis: WBCs ingest platelets - can be EDTA artifact or in vivo (severe sepsis, DIC, HLH).

Reactive thrombocytosis: causes include iron deficiency (most common cause of very high counts, can exceed 1 million), systemic inflammation, malignancy, post-splenectomy, post-surgery/trauma. IL-6 and thrombopoietin drive production. At very high platelet counts, reactive thrombocytosis is NOT associated with thrombosis/hemorrhage (critical distinction from essential thrombocythemia, where extreme thrombocytosis >1.5 million can paradoxically cause acquired von Willebrand disease).

Alloimmune platelet refractoriness: in repeatedly transfused patients, HLA class I antibodies (especially anti-HLA-A and anti-HLA-B) cause rapid destruction of transfused platelets. Diagnosed by poor corrected count increment (CCI). Managed with HLA-matched or crossmatch-compatible platelets.

26.3 RBC Morphology on Peripheral Smear

This is the highest-yield smear content. Know each morphology, what it looks like, and what to think when you see it.

Size and Shape Abnormalities

Microcytosis: small RBCs (MCV <80 fL). Iron deficiency, thalassemia, immune hemolysis, anemia of inflammation (usually normocytic), lead poisoning, sideroblastic anemia.

Macrocytes: RBCs larger than normal (MCV >100 fL). Two types:

Oval macrocytes (macro-ovalocytes): megaloblastic anemia (B12/folate). With hypersegmented neutrophils = megaloblastic until proven otherwise
Round macrocytes: liver disease, alcohol, reticulocytosis, hypothyroidism, MDS

Spherocytes: small, round, hyperchromic RBCs lacking central pallor. Decreased surface area:volume ratio. Elevated MCHC. Two main causes:

DAT-positive spherocytes = AIHA
DAT-negative spherocytes = HS (and other causes)
Also: delayed hemolytic transfusion reactions, ABO HDN, Clostridium perfringens sepsis, burns, Wilson disease

Target cells (codocytes): bullseye appearance, excess membrane relative to hemoglobin. Thalassemia, HbC disease (up to 90% of RBCs in HbC homozygotes), liver disease (lecithin loading), iron deficiency, post-splenectomy, obstructive jaundice. Target cells + microcytosis = thalassemia or HbC; target cells + macrocytosis = liver disease.

Sickle cells (drepanocytes): crescent-shaped from HbS polymerization. Pathognomonic for sickle cell disease (HbSS, HbSC, HbS-β-thal). Irreversibly sickled cells (ISCs) keep their shape even when re-oxygenated. Not seen in sickle trait under normal conditions.

Teardrop cells (dacrocytes): one pointed end, one rounded end. RBCs squeezed through fibrotic marrow or enlarged spleen. Myelofibrosis, myelophthisis, thalassemia major, megaloblastic anemia. Leukoerythroblastic picture (teardrops + nucleated RBCs + immature granulocytes) demands a bone marrow biopsy.

Schistocytes: fragmented RBCs, helmet-shaped cells. Mechanical shearing by fibrin strands or abnormal vessels. 58% of normal adults have schistocytes (mean 0.05%, range 0-0.27%). >1% schistocytes is significant. Classic associations: TTP (>3/HPF), HUS, DIC, HELLP, mechanical heart valves, malignant hypertension, MAHA. Schistocytes + thrombocytopenia = MAHA until proven otherwise - urgent workup for TTP (ADAMTS13), HUS, DIC.

Elliptocytes (ovalocytes): twice as long as wide. A few are normal. >25% elliptocytes is characteristic of hereditary elliptocytosis. Also iron deficiency, B12/folate deficiency, MDS. Southeast Asian ovalocytosis (SAO) produces rigid ovalocytes that resist malaria invasion (band 3 deletion).

Acanthocytes (spur cells): irregular, unevenly spaced thorny projections of varying length and thickness. Altered membrane lipid composition (increased cholesterol:phospholipid ratio). Associations: liver disease (spur cell hemolytic anemia in cirrhosis - poor prognosis), abetalipoproteinemia, McLeod phenotype (Kell-null), post-splenectomy, neuroacanthocytosis (chorea-acanthocytosis). Irreversible.

Echinocytes (burr cells): evenly spaced, uniform, short projections around the entire cell. Most commonly artifact (slow drying, EDTA effect, glass slides). True causes: uremia, pyruvate kinase deficiency, post-transfusion (storage lesion), burns, post-splenectomy, gastric cancer. Reversible. Key distinction from acanthocytes: echinocytes are regular/uniform; acanthocytes are irregular/varied.

Stomatocytes: mouth-shaped (slit-like) central pallor instead of the normal round pallor. Most commonly artifact (slow drying). True causes: hereditary stomatocytosis (overhydrated or dehydrated types), liver disease/alcohol, phenytoin, Rh-null phenotype. Hereditary stomatocytosis patients should NOT undergo splenectomy (paradoxically increases thrombotic risk).

Bite cells (degmacytes): semicircular “bite” from spleen pitting out Heinz bodies. Classic in G6PD deficiency during oxidative crisis, also unstable hemoglobins, drug-induced oxidative hemolysis. Blister cells: variant with eccentric vacuole-like space beneath the membrane. Blisters + bite cells = oxidative hemolytic anemia.

Rouleaux: RBCs stacked like coins. Caused by increased plasma proteins (fibrinogen, immunoglobulins) that neutralize the negative surface charge (zeta potential). Classic in multiple myeloma, Waldenström macroglobulinemia, chronic inflammation, pregnancy. Rouleaux disperses with saline dilution; true agglutination does not. Interferes with blood bank testing (mimics agglutination).

Agglutination (true): irregular clumps of RBCs, distinct from uniform rouleaux stacks. Cold agglutinins (IgM anti-I or anti-i) cause agglutination that worsens at room temperature and disappears at 37°C. Causes spurious results: falsely elevated MCV (doublets), falsely decreased RBC count, falsely elevated MCHC. Warming the specimen to 37°C corrects these.

Ghost cells: pale, hemoglobin-depleted RBC membranes after intravascular hemolysis. Severe intravascular hemolysis (complement-mediated, mechanical), Clostridium perfringens sepsis (alpha-toxin destroys membranes), severe thermal burns.

Dimorphism: two distinguishable RBC populations. Post-transfusion, sideroblastic anemia, post-iron therapy in iron deficiency, MDS with ring sideroblasts, post-B12 treatment. Bimodal RBC histogram on the analyzer.

Hypochromasia: increased central pallor (>1/3 cell diameter). Iron deficiency (most common), thalassemia, anemia of chronic disease (advanced), sideroblastic, lead poisoning. Low MCHC.

Polychromasia: slightly larger bluish-gray RBCs = reticulocytes on Wright stain. Correlates with reticulocytosis.

RBC Inclusions

Basophilic stippling: small blue dots from aggregated ribosomal RNA/ribosomes. Do NOT stain for iron.

Fine: reticulocytes, thalassemia trait
Coarse: lead poisoning (inhibits pyrimidine 5’ nucleotidase, preventing RNA degradation), pyrimidine 5’ nucleotidase deficiency, thalassemia, MDS, severe megaloblastic, alcohol

Coarse basophilic stippling + microcytic anemia in a child = lead poisoning until proven otherwise.

Howell-Jolly bodies: small, round, dark purple nuclear DNA remnants. Normally removed by spleen pitting function. Present in: functional asplenia (SCD autosplenectomy), post-splenectomy, hyposplenic states (celiac, IBD, amyloidosis). Usually one per cell. Multiple HJ bodies per cell suggest megaloblastic anemia.

Pappenheimer bodies: iron-containing granules (ferritin aggregates in siderosomes). Visible on Wright stain, confirmed as iron by Prussian blue. Larger, more irregular, and grayer than basophilic stippling. Post-splenectomy, sideroblastic anemia, thalassemia, hemolytic anemias, MDS. Can cause falsely elevated platelet counts on automated analyzers.

Composition summary: basophilic stippling = RNA/ribosomes; Howell-Jolly = DNA/nuclear remnants; Pappenheimer = iron.

Heinz bodies: gray-black round inclusions of denatured hemoglobin adhered to inner RBC membrane. NOT visible on Wright stain - require supravital stain (crystal violet or brilliant cresyl blue). G6PD deficiency, unstable hemoglobins, alpha-thalassemia (HbH inclusions ARE Heinz bodies), drug-induced oxidative hemolysis (dapsone, primaquine). Spleen pits them out to create bite cells. Post-splenectomy: Heinz bodies persist.

Cabot rings: thin, figure-eight or ring-shaped inclusions, remnants of mitotic spindle apparatus (microtubules). Rare, nonspecific. Megaloblastic anemia, MDS, lead poisoning, severe anemias. Visible on Wright stain. Not to be confused with Howell-Jolly bodies (DNA) or Pappenheimer bodies (iron).

Hemozoin (“malaria pigment”): dark brown-black pigment in malaria parasites and phagocytes that have ingested parasitized RBCs. Crystallized heme produced by the parasite. Hemozoin-laden phagocytes indicate malaria even when parasites are not seen.

Neonatal anemia causes: significant fetomaternal hemorrhage (>30 mL, quantified by Kleihauer-Betke), twin-twin transfusion, intrapartum cord injuries (velamentous cord insertion, vasa previa), or rare inherited causes (Fanconi anemia, Diamond-Blackfan, congenital parvovirus B19). Hemolysis (ABO/Rh HDN, G6PD, HS, alpha-thal/Barts hydrops fetalis). Fetomaternal hemorrhage is the most common cause of significant neonatal anemia.

Transient neonatal polycythemia: infants of diabetic mothers (chronic fetal hypoxia from placental insufficiency stimulates EPO), Down syndrome, twin-twin transfusion (recipient), delayed cord clamping, SGA infants.

Erythrocytosis Differential

EPO measurement is pivotal:

Low/suppressed EPO: primary (polycythemia vera - check JAK2 V617F, then JAK2 exon 12; primary familial and congenital polycythemia - EPOR mutations)
Elevated EPO: secondary
- Appropriate (hypoxia-driven): COPD, high altitude, cyanotic heart disease, sleep apnea, carbon monoxide, high-affinity hemoglobins
- Inappropriate (EPO-secreting tumors): renal cell carcinoma (most common), cerebellar hemangioblastoma, hepatocellular carcinoma, uterine leiomyoma, pheochromocytoma, meningioma
Relative: dehydration, Gaisböck syndrome (stress polycythemia: hypertension, erythrocytosis, leukocytosis, thrombocytosis, mild obesity)

Erythrocytosis + elevated EPO + renal mass = RCC until proven otherwise.

26.4 WBC Morphology on Peripheral Smear

Normal WBC Stages

Myeloid maturation sequence: myeloblast → promyelocyte → myelocyte → metamyelocyte → band → segmented neutrophil.

Myeloblast: large, high N:C, fine chromatin, 1-3 prominent nucleoli, scant basophilic cytoplasm. Auer rods are pathognomonic for myeloid lineage.
Promyelocyte: large, eccentric nucleus, dispersed chromatin, prominent nucleoli, abundant primary (azurophilic) granules, prominent perinuclear hof (Golgi). Most granular normal marrow cell. In APL, faggot cells (bundles of Auer rods) are virtually pathognomonic.
Myelocyte: round/oval nucleus, condensing chromatin, both primary and secondary (specific, pink/lilac) granules. Dawn of secondary granules = dawn of the myelocyte stage. Last dividing stage.
Metamyelocyte: kidney-bean indented nucleus (indentation <50% of nuclear width), condensing chromatin, predominantly secondary granules. First non-dividing stage.
Band: horseshoe or C-shaped nucleus, uniform width throughout (no thin filaments). Bandemia (>10% bands or >700/μL) suggests bacterial infection.
Segmented neutrophil: 2-5 lobes connected by thin chromatin filaments, pink-lilac granules.

Eosinophil maturation: eosinophil precursors mirror neutrophil stages, but granules are characteristically large orange-red. Mature eosinophil: bilobed nucleus, abundant refractile orange-red granules containing major basic protein, eosinophil peroxidase, eosinophil cationic protein. Normal circulating eosinophils are bilobed; trilobed or monolobed = abnormal.

Basophils: bilobed nucleus (often obscured) with large dark purple-blue metachromatic granules. Granules contain histamine, heparin, leukotrienes. Rarest WBC (<1%). Hypogranulation seen in CML blast crisis, MDS, MPN. Automated basophil counts are often inaccurate; manual counting on a 200-cell differential may be required, especially when considering MDS or MPN.

Monocyte maturation: monoblast → promonocyte → monocyte.

Monoblast: large, abundant basophilic cytoplasm (often vacuolated), round to folded nucleus, fine chromatin, prominent nucleoli. NSE+ (inhibited by NaF).
Promonocyte: folded/grooved nucleus (more convoluted than monoblast), delicate chromatin, 1+ nucleoli, gray-blue cytoplasm with fine azurophilic granules. In CMML and AML M5, promonocytes count as blast equivalents.
Monocyte: largest normal circulating WBC (15-20 μm), kidney-shaped or folded/lobulated nucleus, lacy delicate chromatin, “ground glass” gray-blue cytoplasm, fine azurophilic granules with occasional vacuoles.

Lymphocyte variants:

Normal mature lymphocyte: small (7-10 μm), round densely staining nucleus (clumped chromatin), scant pale blue cytoplasm, no visible nucleoli
Pediatric lymphocytes can appear morphologically atypical (sparse cytoplasm, nuclear irregularity, larger) in health
Large granular lymphocyte (LGL): 12-15 μm, abundant pale cytoplasm with prominent azurophilic granules (perforin/granzyme). Most are NK cells or cytotoxic T cells. Persistent LGL >2 × 10³/μL for >6 months with clonal TCR = LGL leukemia.
Reactive (atypical) lymphocyte: large, abundant basophilic cytoplasm that indents around adjacent RBCs (“scalloped” or “burnt” borders). Activated CD8+ T cells in viral infection.
Plasmacytoid lymphocyte: intermediate between lymphocyte and plasma cell. Classic in lymphoplasmacytic lymphoma/Waldenström (MYD88 L265P in >90%).
Prolymphocyte: medium-large with single prominent central nucleolus, condensed chromatin, moderate cytoplasm. >55% defines PLL.
Sézary cell: medium-large lymphocyte with cerebriform (convoluted, brain-like) nucleus. CD3+ CD4+ CD7- CD26-. Sézary syndrome requires ≥1000/μL or CD4:CD8 ≥10.
Bilobed/binucleated lymphocyte = PPBL
“Soccer ball” / “cracked earth” chromatin + smudge cells = CLL
Hairy cell: medium lymphocyte with abundant pale blue cytoplasm and hair-like circumferential villi, “spongy/fried-egg” chromatin. Classic phenotype CD19+, CD20 bright, CD11c bright, CD25+, CD103+, CD123+, annexin A1+, BRAF V600E (characteristic of classic HCL; absent in HCL-variant). Monocytopenia is a CBC hallmark.

Plasma cells: oval with eccentric nucleus, cartwheel (clock-face) chromatin, deep basophilic cytoplasm, prominent perinuclear hof (Golgi). Normal marrow ≤3%; myeloma requires ≥10% clonal plasma cells. Variants: flame cells (bright pink cytoplasm, often IgA), Mott cells (packed with Russell bodies - intracytoplasmic Ig-filled grape-like globules), Dutcher bodies (intranuclear pseudoinclusions, actually cytoplasmic invaginations).

Erythroid maturation: proerythroblast (pronormoblast) → basophilic erythroblast → polychromatic erythroblast → orthochromatic erythroblast → polychromatic erythrocyte (reticulocyte) → normochromic erythrocyte.

Proerythroblast: large (20-25 μm), round nucleus, fine chromatin, 1-2 nucleoli, deep blue cytoplasm with perinuclear hof. Glycophorin A+, CD71+ bright, CD36+, weak CD45. Parvovirus B19 targets these cells via P antigen (globoside), causing pure red cell aplasia.
Basophilic erythroblast: round nucleus, coarsening chromatin, no nucleoli, deeply basophilic cytoplasm (ribosome-rich).
Polychromatic erythroblast: small round nucleus, condensing chromatin, gray-blue cytoplasm (mixture of blue ribosomes and pink hemoglobin). Last mitotic stage.
Orthochromatic erythroblast: small pyknotic nucleus (ready for extrusion), nearly pink cytoplasm. Cannot divide. Nucleus is extruded with a rim of cytoplasm, forming a reticulocyte.
Polychromatic erythrocyte (reticulocyte): slightly larger, bluer cells on Wright stain due to residual ribosomal RNA. Mature in marrow for ~1 day, circulate for 1-2 days.
Normochromic erythrocyte: biconcave disc, 7-8 μm, central pallor ~1/3 diameter. No nucleus, no organelles. Lifespan ~120 days. Relies entirely on glycolysis for energy (no mitochondria) and the hexose monophosphate shunt (G6PD) for oxidative defense.

Platelets: small (2-4 μm), anucleate, disc-shaped, fine purple-red granules (alpha and dense granules). Form from megakaryocyte cytoplasmic fragmentation via proplatelets. Normal count 150-400k, lifespan 8-10 days.

Abnormalities in WBCs

Toxic changes in neutrophils: reactive/infectious triad.

Toxic granulation: prominent dark azurophilic granules (retained primary granules from accelerated emergency granulopoiesis)
Döhle bodies: pale blue-gray oval cytoplasmic inclusions of stacked rough ER
Cytoplasmic vacuolization: active phagocytosis

Auer rods: needle-shaped crystallized azurophilic granule contents in myeloid blasts. Pathognomonic for AML (never seen in ALL or reactive). Most common in APL/AML M3 (faggot cells = bundles), AML with t(8;21), AML M2. Composed of MPO-containing crystallized granule proteins.

Pseudo-Chediak-Higashi granules: large fused azurophilic granules in leukemic blasts that resemble CHS giant granules. AML with t(8;21), APL. Unlike true CHS, limited to the neoplastic population.

True Chédiak-Higashi syndrome: pathognomonic giant azurophilic granules in neutrophils, lymphocytes, monocytes, eosinophils from fused lysosomes (LYST/CHS1 gene). Impairs neutrophil chemotaxis and bactericidal activity. Partial oculocutaneous albinism. Risk of accelerated phase (HLH-like).

Alder-Reilly anomaly: large azurophilic granules in ALL WBCs including lymphocytes, resembling toxic granulation. Associated with mucopolysaccharidoses (Hurler, Hunter, Maroteaux-Lamy). Granules contain partially degraded GAGs in lysosomes. Distinguished from toxic granulation by lifelong presence, lymphocyte involvement, and MPS clinical features.

Jordan anomaly: fat vacuoles in leukocytes (neutrophils, monocytes, eosinophils). Associated with Chanarin-Dorfman syndrome (neutral lipid storage with ichthyosis, ABHD5/CGI-58 mutations). Triglyceride droplets not metabolized due to lipase cofactor deficiency.

Pelger-Huët anomaly: autosomal dominant, LBR gene. Bilobed (“pince-nez”) neutrophils with coarsely clumped chromatin, connected by thin filament. Function is NORMAL (contrast with MDS pseudo-Pelger-Huët). Homozygotes: round unilobed neutrophils (Stodtmeister cells). Important to recognize to avoid unnecessary MDS workup.

Pseudo-Pelger-Huët: acquired bilobed/hyposegmented neutrophils in MDS, AML, infections, drugs - accompanied by other dysplastic features.

Hypersegmented neutrophils: ≥5 lobes in a single cell or ≥5% of neutrophils with 5+ lobes. Classic earliest peripheral finding of megaloblastic anemia (appears before macrocytosis). Also hydroxyurea, renal failure, iron deficiency (rare), MDS. Macropolycytes: abnormally large tetraploid (4n) hypersegmented neutrophils with 6+ lobes from endomitosis without division in severe megaloblastic anemia.

Hypogranular neutrophils: pale cytoplasm lacking secondary granules. MDS (most common), AML, CMML. Dysplastic feature. Decreased side scatter on flow cytometry.

Botryoid nucleus: grape-like nuclear morphology in neutrophils, associated with severe hyperthermia (>40°C). Heat-induced nuclear protein denaturation.

Blue-green/“critical green” inclusions: blue-green inclusions in neutrophils associated with critical illness with high short-term mortality (severe sepsis, hepatic failure, multi-organ dysfunction). Should be communicated as a critical finding.

Drumstick (Barr body appendage): small round nuclear protrusion on neutrophils representing the inactivated X chromosome. ~3% of female neutrophils. Absent in XY males. Present in Klinefelter (XXY). Historical finding.

Neutrophil Howell-Jolly-like inclusions: rare, suggest dysmyelopoiesis (MDS), cytotoxic chemotherapy, or severe sepsis. Not to be confused with RBC Howell-Jolly bodies.

Hypersegmented eosinophil (>2 lobes): suggests neoplastic process - CEL, MPN with eosinophilia, AML with inv(16).

Abnormal monocytes: nuclear irregularities or cytoplasmic abnormalities beyond reactive changes. Suggest CMML, AML with monocytic differentiation. Promonocytes with nucleoli count as blast equivalents.

Erythrophagocytosis: macrophage engulfing RBCs. Morphologic hallmark of HLH but not diagnostic alone (seen in transfusion reactions, autoimmune, sepsis). HLH diagnosis requires clinical criteria (fever, splenomegaly, cytopenias, hyperferritinemia, elevated sCD25, hypertriglyceridemia, low fibrinogen, decreased NK activity).

Mitotic figures in peripheral blood: abnormal - suggests circulating neoplastic cells (acute leukemia, aggressive lymphoma with leukemic involvement).

Apoptotic cells: condensed fragmented chromatin with shrinkage and blebbing. Increased in MDS, chemotherapy effect, viral infections.

Platelet Morphology Findings

Giant platelets (macrothrombocytes) >RBC diameter: MYH9 disorders, Bernard-Soulier, ITP, gray platelet syndrome, MPN, MDS.

Hypogranular platelets: MPNs (especially ET), MDS, gray platelet syndrome (NBEAL2, alpha-granule deficiency - pale gray agranular platelets, mild thrombocytopenia, splenomegaly, myelofibrosis).

Micromegakaryocytes: abnormally small megakaryocytes (lymphocyte-sized) with round hypolobulated nucleus. Dysplastic feature of MDS. Can be confused for blasts on aspirate; CD41 or CD61 IHC confirms megakaryocytic lineage.

Platelet aggregates/fibrin strands/cryoglobulins:

Platelet aggregates: EDTA, heparin, citrate artifact
Fibrin strands: specimen partially clotted before smear preparation
Cryoglobulins: amorphous blue-purple precipitate, precipitate <37°C, interfere with multiple parameters. Type I (monoclonal - myeloma, Waldenström), Type II (mixed - HCV-associated, most common), Type III (polyclonal - autoimmune, infections)

26.5 Automated Hematology Analyzers

Counting Principles

Electrical impedance (Coulter principle): Cells pass through a small aperture between electrodes. Each cell causes a momentary increase in electrical resistance proportional to cell volume. This measures cell counts and sizes.

Light scatter: Laser light is scattered by cells.

Forward scatter correlates with size
Side scatter correlates with internal complexity (granularity, nuclear lobulation)

Fluorescent staining: Nucleic acid dyes identify cell types (reticulocytes, nucleated RBCs, immature granulocytes).

Coulter VCS technology creates a 5-part leukocyte differential using three measurements: impedance (Volume), conductivity (internal complexity/nuclear characteristics), and Side angle light scatter (granularity). Each WBC type occupies a distinct region in the 3D plot.

Other instruments use multiangle light scatter + fluorescence for the 5-part differential. Neither instruments nor humans can precisely count bands.

Sizing Thresholds

WBC channel: particles >36 fL (after RBC lysis)
Platelet channel: particles 2-20 fL
RBC channel: typically >36 fL

Schistocytes can be enumerated using the platelet channel on some instruments (Sysmex has a specific FRC parameter). Manual smear review remains the gold standard.

Flags

Analyzers flag abnormalities requiring manual review:

Blasts
Atypical lymphocytes
Nucleated RBCs
Left shift
Cell clumps
Platelet clumps

26.6 Hemoglobin Analysis

Sickle Screen Tests

All diseases with HbS have positive metabisulfite AND positive dithionite screening tests.

Metabisulfite (sickling) test: sodium metabisulfite reduces oxygen tension, inducing sickling of HbS-containing cells. Microscopic examination of sickle-shaped cells. Requires ≥10% HbS to be positive. Can be negative in neonates or aggressively transfused patients. Also positive in HbC-Harlem (double mutant).
Dithionite tube test (solubility, Sickledex): RBCs lysed, mixed with sodium dithionite → HbS polymerizes when deoxygenated → insoluble → increased turbidity (positive). HbA remains soluble (clear).

Both detect HbS but CANNOT distinguish trait from disease. Electrophoresis or HPLC is needed for definitive diagnosis. Positive in SS, SA, SC, SD, and C-Harlem. Non-S sickling hemoglobins can also be positive.

Alkaline Hemoglobin Electrophoresis (Cellulose Acetate, pH 8.4)

Hemoglobins migrate from cathode (-) to anode (+). Order from anode to cathode: A > F > S > C. Mnemonic: “A Fat Santa Claus” or “A Fast Sickle Cell.”

HbA migrates fastest, followed by HbF (both separate cleanly from other hemoglobins)
HbS position comigrates with HbD, HbG, Hb-Lepore
HbC position comigrates with HbA2, HbE, HbO-Arab (mnemonic: “CEO” and HbA2)
Fast hemoglobins (beyond HbA): HbH (β4), Hb Barts (γ4), HbI. Mimicked by hyperbilirubinemia

Controls must include HbA, HbF, HbS, and HbC to span the entire migration range.

Acid Hemoglobin Electrophoresis (Citrate Agar, pH 6.0-6.4)

Separates HbS from HbD and HbG (which comigrate with HbA on acid gel). Also separates HbC from HbO-Arab, HbE, and HbA.

HbC and HbS migrate toward anode (Hb-agaropectin complex)
HbF and HbA migrate toward cathode with sharp separation (excellent for HbF quantification)
Limitations: does NOT separate HbD from HbG or Hb-Lepore, or HbE from HbO-Arab

Typical workflow: alkaline gel for screening, acid gel for confirmation, HPLC for quantitation.

Distinguishing HbD/HbG from HbS:

Sickle screen (solubility or metabisulfite): HbS positive; HbD and HbG negative (only HbS polymerizes)
Citrate agar (acid electrophoresis): HbS migrates differently from HbD/G at acid pH

Board trap: a band in the HbS position on alkaline gel + negative sickle test = HbD or HbG, not HbS.

HbF Detection Methods

Acid elution (Kleihauer-Betke test): blood smear in acid buffer; HbF-containing cells resist elution and stain pink while HbA cells appear as ghosts. Used to quantify fetomaternal hemorrhage.
HPLC: precise quantitation of HbF%.
Flow cytometry: anti-HbF antibodies detect F cells.
Alkali denaturation (Singer test, Apt test): HbF resists alkali denaturation. Historical.

Hereditary persistence of fetal hemoglobin (HPFH) has HbF in a pancellular distribution on Kleihauer-Betke (every RBC has HbF equally). Contrast with heterocellular distribution in β-thal major, δ-β thal, SCD, and fetomaternal hemorrhage.

High-Performance Liquid Chromatography (HPLC)

Separates hemoglobin variants by charge using cation-exchange columns. Each variant has a characteristic retention time and elution window. Allows precise quantitation of HbA, A2, F, S, C, and others. Standard confirmatory method.

Limitations: HbE and HbA2 have similar retention times, HbC and HbO-Arab have similar retention times. Bilirubin elutes with Hb Barts - elevated bilirubin (neonatal jaundice, hemolysis) can create a false Hb Barts peak.

About 1-2% of variants cannot be definitively identified by HPLC or gel. Mass spectrometry or globin gene sequencing is required.

Thalassemia Electrophoresis Findings

β-thalassemia minor: increased HbA2 (>3.5%, typically 4-8%) is the diagnostic hallmark. When β chains are underproduced, δ chains are relatively overproduced (HbA2 = α2δ2). Must quantify by HPLC or capillary electrophoresis (not gel estimation).
α-thalassemia trait: NORMAL hemoglobin electrophoresis with thalassemic indices (low Hct, increased RBC count, low MCV, normal/low RDW). HbA2 and HbF are normal. Diagnosis of exclusion; definitive diagnosis requires molecular testing.

26.7 Cytochemistry

Romanowsky Stains

Wright stain: Romanowsky-type stain with methylene blue (basic dye, stains acidic structures blue-purple: DNA, RNA) + eosin (acidic dye, stains basic structures red-orange: hemoglobin, eosinophil granules) + alcohol fixation. Standard peripheral blood and bone marrow smear stain. Wright-Giemsa is a modification adding Giemsa for better nuclear detail. Buffer pH matters: too acidic = everything red; too alkaline = everything blue.

Leukocyte Alkaline Phosphatase (LAP) Score

100 bands/neutrophils (not earlier precursors) are scored 0-4 for cytoplasmic staining intensity. Score = sum of 100 cells. Range 0-400.

Normal LAP: 40-120
Low LAP (0-15): CML, PNH (loss of GPI-anchored surface alkaline phosphatase), MDS, hereditary hypophosphatasia, neonatal septicemia (paradoxical decrease)
Elevated LAP: leukemoid reaction (reactive neutrophilia), polycythemia vera, primary myelofibrosis, Hodgkin lymphoma, Down syndrome, glucocorticoid therapy, 3rd-trimester pregnancy

Classic use: distinguishing a leukemoid reaction (high LAP) from CML (low LAP). Now largely replaced by flow cytometry and molecular testing (BCR-ABL1) for CML diagnosis, but still a testable concept.

Cytochemical Stains for Blast Typing

The classic blast cytochemistry table:

Cell Type	PAS	SBB	MPO	CAE	NSE	Auer rods
Undifferentiated (M0)	-	-	-	-	-	rare
Myeloblasts (M1, M2, M4)	-	+ (black)	+	+	-	50%
Promyelocytes (APL/M3)	-	+ (strong)	+ (strong)	+	-	95% (faggot cells)
Monoblasts (M4, M5)	-	-	-/+	-	+ (inhibited by NaF)	0%
Erythroblasts (M6)	+ (diffuse granular)	-/+	-/+	-	-/+ (resists NaF)	50%
Megakaryoblasts (M7)	+ (diffuse granular)	-	-	-	-/+ (resists NaF)	rare
Lymphoblasts (ALL)	+ (block, rosary bead)	-/+ (faint)	-	-	-	-

Key points:

MPO positivity in ≥3% of blasts defines myeloid lineage (WHO criterion)
SBB and MPO are the primary myeloid markers (SBB stains phospholipids in primary granules)
NSE (non-specific esterase) is positive in monocytic lineage, inhibited by NaF - specific for monocytic esterase
NSE in erythroblasts and megakaryoblasts is weak and resists NaF
PAS pattern distinguishes lymphoblasts (block/rosary) from erythroblasts (diffuse granular)
APL promyelocytes are strongly MPO-positive (massive primary granule content) with faggot cells virtually pathognomonic
M0 diagnosis requires flow cytometry (CD13, CD33, CD117) or ultrastructural MPO by EM - cytochemically bland
Megakaryoblasts: flow cytometry with CD41, CD61, CD42b confirms lineage; reticulin fibrosis often makes aspiration difficult (dry tap)

TdT (terminal deoxynucleotidyl transferase) is positive in >95% of ALL cases - the most useful marker for lymphoblast identification (positive in both B-ALL and T-ALL).

26.8 Immunophenotyping

This section covers the CD markers you need to know cold. Organized by lineage association.

Gating Markers

CD45 (leukocyte common antigen, LCA): expressed by nearly all leukocytes, NOT by Reed-Sternberg cells. CD45 is the “gating” marker in flow cytometry. CD45 vs side scatter plots separate WBC populations:

Mature lymphocytes and monocytes: bright CD45
Granulocytes: intermediate CD45, high side scatter
Erythrocytes: almost no expression
Dim CD45 + strong CD34 = blasts (the “blast gate”)
Dim CD45 + bright CD38 = plasma cells

CD34: marks immature/stem cells - hematopoietic stem cells, endothelial cells, mesenchymal precursors. Used to enumerate stem cells for transplant. Lost during maturation. APL (AML M3) blasts are characteristically CD34-negative (distinctive). Also expressed by GIST, DFSP, solitary fibrous tumor.

HLA-DR (MHC class II): normally expressed by monocytes, B cells, activated T cells, dendritic cells. Most AML blasts are HLA-DR+ EXCEPT APL (HLA-DR-negative). Most B-ALL is HLA-DR+; T-ALL variable.

Myeloid Markers

CD13: aminopeptidase N. Granulocytes, monocytes, and precursors. One of the earliest myeloid markers. Characteristic of AML M1, M2, M3, M4, M5, M6. CD13 has biphasic expression during neutrophil maturation (bright promyelocyte → dim myelocyte/metamyelocyte → bright mature - the “Nike swoosh”).

CD33: Siglec-3. Myeloid and monocytic cells. Target of gemtuzumab ozogamicin (anti-CD33 ADC). Used with CD13 as primary myeloid markers.

CD117 (c-Kit): receptor tyrosine kinase. HSCs/progenitors, mast cells, melanocytes, interstitial cells of Cajal (GIST), germ cells. Positive on AML blasts, negative on mature granulocytes/monocytes. KIT D816V drives systemic mastocytosis; KIT mutations drive GIST. Imatinib target.

CD14: monocyte-specific. LPS receptor. GPI-anchored (diminished on PNH monocytes). Appears at promonocyte stage, brightest on mature monocytes. AML M4/M5 positivity.

CD11b (MAC-1): monocytes, granulocytes, NK cells. Acquired at metamyelocyte stage - marks transition from proliferative to non-proliferative maturation.

CD11c: monocytes, granulocytes, dendritic cells. Bright CD11c in hairy cell leukemia. Also in marginal zone lymphoma.

CD15 (Leu-M1): mature monocytes and granulocytes. Carbohydrate antigen (Lewis X). Reed-Sternberg cells are CD15+. Absence is distinctive for ALCL. In AML, CD15 indicates maturing phenotype (strongly expressed in APL).

CD64: high-affinity FcγRI. Increases with monocyte maturation.

CD16 (FcγRIII): NK cells and granulocytes. Low-affinity IgG Fc receptor. Mediates ADCC. CD16a (transmembrane) on NK cells/macrophages; CD16b (GPI-anchored) on neutrophils, lost in PNH.

B-Cell Markers

CD19: earliest B-cell surface marker (pro-B stage through plasma cells). Part of BCR coreceptor complex. Target of blinatumomab and CAR-T therapy. Often dimly expressed in FL. May be dim on some AML with t(8;21).

CD20: mature B cells (after CD19, before CD22). Not expressed by plasma cells (rituximab does not target plasma cells). Reed-Sternberg cells CD20+ in ~20% of cases; NLPHL uniformly CD20+. CLL/SLL has characteristically DIM CD20. Target of rituximab.

FMC7: antibody recognizing a conformational CD20 epitope. Positive when CD20 is bright; negative when CD20 is dim (CLL/SLL = FMC7-negative).

CD22: mature B cells and plasma cells.

CD79a: part of the BCR complex (analogous to CD3 for TCR). From early pre-B through plasma cells (broader than CD20). Sensitive B-cell marker in IHC.

CD10 (CALLA): germinal center B cells, mature granulocytes, some T cells. CALLA = common ALL antigen. Also = neprilysin. Positive in B-ALL, FL, Burkitt, GCB-type DLBCL. Part of the Hans algorithm for DLBCL classification (CD10+ = GCB subtype, better prognosis). Also expressed by many non-hematolymphoid tumors (RCC, endometrial stroma, HCC).

CD23: CD23 positivity + CD5 + dim CD20 = CLL/SLL.

CD5: pan-T cell marker (NOT NK cells). CD5+ B cell lymphomas = CLL/SLL (CD5+, CD23+) and mantle cell lymphoma (CD5+, CD23-, cyclin D1+). Small population of normal CD5+ B-1 cells exists. Expressed in most thymic carcinomas.

CD43: pan-T cell marker, also granulocytes/monocytes/NK. Normally absent on B cells. Aberrant CD43 on B cell lymphoma suggests: CLL/SLL, MCL, MZL, Burkitt. Not expressed in FL.

CD25 (IL-2R alpha): activated T and B cells, regulatory T cells. Characteristic of hairy cell leukemia and ATLL. Soluble CD25 (sIL-2R) elevated in lymphomas and is an HLH diagnostic criterion. Mast cell clonality marker when expressed.

CD103: most sensitive and specific marker for hairy cell leukemia. Normally expressed by intraepithelial T lymphocytes (binds E-cadherin). HCL phenotype: CD19+, CD20 bright, CD22+, CD11c bright, CD25+, CD103+, CD123+, annexin A1+ (most specific), BRAF V600E (characteristic of classic HCL; absent in HCL-variant, also seen in melanoma and other tumors).

CD138 (syndecan-1): most specific plasma cell marker. Heparan sulfate proteoglycan. Used for myeloma diagnosis/staging/MRD. May be lost in plasma cell leukemia (circulating cells). Also expressed by some epithelial/mesenchymal tumors.

CD38: activated T and B cells, brightest on plasma cells. In CLL, ≥30% CD38+ correlates with unmutated IGHV and poor prognosis. Target of daratumumab in myeloma.

BCL2: T cells and mantle B cells; negative in germinal center B cells (which downregulate BCL2 to allow apoptosis during affinity maturation). BCL2-positive germinal centers = follicular lymphoma (t(14;18) drives constitutive BCL2). Also in synovial sarcoma, solitary fibrous tumor.

BCL6: zinc finger transcription factor in germinal center B cells. Essential for somatic hypermutation. Expressed in FL, GCB-DLBCL, Burkitt, NLPHL, ALCL.

T-Cell Markers

CD2: T and NK cells from very early stage. Sheep RBC receptor (historic E-rosette). Binds LFA-3.

CD3: definitive T-cell lineage marker. Forms signaling complex with TCR. Cytoplasmic CD3 appears before surface CD3 - earliest specific T-cell marker by ICC. Surface CD3 requires functional TCR.

CD4: T-helper cells (~60%), monocytes, macrophages, dendritic cells. Binds MHC II. HIV coreceptor. CD4+ T-cell lymphomas: PTCL-NOS, ATLL, Sézary.

CD7: T and NK cells from earliest stage. Most commonly aberrantly lost T-cell marker in T-cell neoplasms (hallmark of Sézary/mycosis fungoides). Can be aberrantly expressed in ~10% of AML (especially M4/M5).

CD8: cytotoxic/suppressor T cells (~30%), some NK cells. Binds MHC I. CD8+ T-cell neoplasms: hepatosplenic T-cell lymphoma (γδ), T-LGL, CD8+ EATL, aggressive epidermotropic CD8+ T-cell lymphoma.

CD1a: cortical thymocytes (immature T cells in thymus), Langerhans cells (along with S100 and langerin/CD207), interdigitating dendritic cells. Langerhans cell histiocytosis: CD1a+, S100+, CD207+. Lost at medullary thymic stage.

CD45RO (UCHL1): memory and activated T cells. Alternative splicing isoform of CD45 (excludes exons A, B, C). Naive T cells are CD45RA+; upon activation, switch to CD45RO.

CD30: activated lymphoid cells (immunoblasts), RS cells, some plasma cells, NK cells. Classic Hodgkin lymphoma (CD30+, CD15+), ALCL, primary cutaneous ALCL, embryonal carcinoma. Target of brentuximab vedotin.

ZAP70: tyrosine kinase in normal T and NK cells. In CLL, ≥20% ZAP70+ is a poor prognostic marker, strongly correlating with unmutated IGHV.

NK Cells and Other

NK cells: CD2+, CD7+, CD16+, CD56+, CD57+ (subset); surface CD3 NEGATIVE; TCR germline; no surface Ig. Some express CD8.

Two NK subsets:

CD56 bright / CD16 dim: cytokine-producing, immunoregulatory
CD56 dim / CD16 bright: cytotoxic, mediates ADCC

NK cells use antibody-dependent cellular cytotoxicity (ADCC) to kill cells bearing foreign antigens. Mechanism: IgG coats target → NK CD16 binds Fc → perforin/granzymes released. Major mechanism of rituximab action. Also use natural cytotoxicity (NKG2D activating receptors) via “missing self” when target loses MHC I.

CD56 (NCAM): NK cells, plasma cells, neuroepithelial cells. Aberrant CD56 in myeloma (most myeloma plasma cells are CD56+, unlike normal). NK/T-cell lymphoma (nasal type). Blastic plasmacytoid dendritic cell neoplasm (CD56+, CD4+, CD123+). Small cell carcinoma marker.

CD57 (Leu-7): subset of NK cells (terminally differentiated, CD56 dim/CD57+) and neuroepithelial cells. Rosettes around germinal centers in angioimmunoblastic T-cell lymphoma (AITL).

CD59 (protectin, MIRL): GPI-anchored complement regulatory protein on virtually all cells. Inhibits MAC formation. Decreased CD59 and CD55 in PNH (GPI anchor defect). Flow cytometry for CD59/CD55/FLAER on RBCs, granulocytes, monocytes is the PNH diagnostic test. CD59 > CD55 for clinical hemolysis in PNH.

CD68: macrophages, histiocytes - poor specificity. Lysosomal membrane glycoprotein. PG-M1 clone is more specific for macrophages than KP1. CD163 is a more specific macrophage marker.

CD71 (transferrin receptor): highly metabolic cells. Brightest on erythroid precursors (iron for hemoglobin), activated lymphocytes, tumor cells. Identifies erythroid lineage with glycophorin A. Upregulated in iron-deficient erythroid precursors.

CD41 and CD61: megakaryocyte/platelet markers. GPIIb (CD41) + GPIIIa (CD61) = fibrinogen receptor. Glanzmann thrombasthenia = GPIIb/IIIa deficiency. Confirms megakaryocytic lineage in AML M7.

Fascin: actin-bundling protein. Reed-Sternberg cells and dendritic reticulum cells. Supports RS identity.

Clusterin: megakaryocytes (strong diffuse cytoplasmic). ALCL: distinctive Golgi (perinuclear dot) pattern - highly specific.

S100: neural/neural crest (Schwann, melanocytes, glial, sustentacular cells), Langerhans cells, interdigitating dendritic cells, myoepithelial cells, chondrocytes, adipocytes. Mnemonic: “N CALM” - Neural, Chondrocytes, Adipocytes, Langerhans, Myoepithelial. Melanoma marker (most sensitive), LCH, Rosai-Dorfman (S100+ histiocytes with emperipolesis), nerve sheath tumors.

Normal B-Cell Maturation (Flow Cytometry)

Stage	CD34	TdT	HLA-DR	CD19	CD10	CD20	cyto Ig	surface Ig	IgH gene
Lymphoid stem	+	+	+	-	-	-	-	-	germline
Pro-B	+	+	+	+	+	-	-	-	rearranging
Pre-B	-	-	+	+	+	+	+ (cyt μ)	-	L chain rearranging
Mature B	-	-	+	+	+/-	+	+	+	complete
Plasma	-	-	-/+	+/-	-	- (CD138+)	+	-	complete

Key transitions:

Lymphoid stem → Pro-B: gain CD19 and CD10
Pro-B → Pre-B: lose CD34/TdT, gain CD20, acquire cytoplasmic μ heavy chain
Pre-B → Mature B: light chain rearranges, surface Ig expressed (CD21, CD22 added)
Mature B → Plasma cell: lose CD20, lose surface Ig, gain CD138 bright and CD38 bright, cytoplasmic Ig only

Normal T-Cell Maturation

Stage	CD34	TdT	CD1a	CD2	CD3 (cyt)	CD3 (surf)	CD4	CD8	CD5	CD7	TCR
Prothymocyte	+	+	-	+	-	-	-	-	-	+	germline
Common/immature thymocyte	-	+	+	+	+	-	+	+	+	+	rearranging
Mature thymocyte (SP)	-	-	-	+	+	+	+/-	-/+	+	+	rearranged
Mature T cell	-	-	-	+	+	+	+/-	-/+	+	+	rearranged

CD4/CD8 notation in mature stages: each cell expresses CD4 OR CD8 (single-positive), not both. Read CD4 +/- and CD8 -/+ as “either CD4+ CD8- (helper) or CD4- CD8+ (cytotoxic).”

Key transitions:

Prothymocyte → Common thymocyte: gain CD1a, cytoplasmic CD3, CD4, CD5, CD8 (double-positive)
Common → Mature thymocyte: lose TdT and CD1a, single-positive selection (CD4 OR CD8)
Mature thymocyte → Mature T: surface CD3 acquisition

Normal Monocyte Maturation

Monoblast: CD34+, myeloid markers
Promonocyte: lose CD34, gain CD11b, CD14 begins
Monocyte: gain bright CD14, CD64, CD36, CD11b+, HLA-DR+

Throughout maturation, monocytes are CD15 negative or dim (distinguishes from granulocytes).

Normal Myeloid Maturation

Myeloblast: CD34+, HLA-DR+, CD38+, CD117+, dim CD13, CD33
Promyelocyte: lose CD34 and HLA-DR, retain CD117, continue CD13/CD33, acquire bright CD15, bright MPO (primary granules packed with MPO). APL blasts mirror this phenotype: CD34-, HLA-DR-, CD117+, bright MPO.
Metamyelocyte: acquire CD11b (marks transition to non-proliferative maturation); CD16 begins
Bands/segmented neutrophils: acquire CD10 (marks terminal maturation), CD11b, bright CD13 (second peak of the “Nike swoosh”), CD15, CD16. CD33 dim, HLA-DR negative, CD34 negative.

26.9 Immunoglobulin and TCR Gene Rearrangement

Gene Locations

IgH (heavy chain): chromosome 14
Kappa (κ) light chain: chromosome 2
Lambda (λ) light chain: chromosome 22

IgH Gene Segments

V (variable, ~40 functional), D (diversity, ~25), J (joining, 6), C (constant - μ, δ, γ, ε, α). In germline configuration, segments are separated by considerable distances on chromosome 14.

D-J rearrangement occurs first (pro-B cell), then V-DJ. Successful V-DJ rearrangement produces a functional mu heavy chain. RAG1/RAG2 recombinases mediate V(D)J recombination. TdT adds random nucleotides at junctions for additional diversity. Class switch recombination changes the C region (isotype switching).

Light chains have V and J segments only (no D).

Ig Rearrangement Order (B Cell Development)

IgH first → kappa → lambda (only if kappa is nonproductive)

Allelic exclusion ensures each B cell produces only one specificity. Normal kappa:lambda ratio is ~2:1. An inverted ratio or lambda predominance suggests a monoclonal B-cell neoplasm.

Light chain restriction (all kappa or all lambda) is the hallmark of B-cell clonality on flow cytometry.

Clonality Testing

Clonal IgH rearrangement = B-cell neoplasm (most commonly) Clonal TCR rearrangement = T-cell neoplasm (most commonly)

Exceptions: ~10% of B-cell lymphomas show TCR rearrangement (lineage infidelity), and vice versa. TCR-γ is most commonly tested (fewer V and J segments, simpler to amplify).

PCR-based clonality:

Uses consensus (framework) primers amplifying multiple V and J segments (FR1, FR2, FR3 for IgH)
Monoclonal: single dominant band/peak
Polyclonal: Gaussian smear/ladder of many sizes
Sensitivity: 1 clonal cell in 100-1000 polyclonal cells
False negatives: somatic hypermutation in GCB-derived lymphomas (use multiple primer sets - EuroClonality/BIOMED-2 protocol)

Southern blot (Sequencing by hybridization with RFLP): genomic DNA digested with restriction enzymes, gel-separated, transferred to membrane, hybridized with radiolabeled probes for IgH, Igκ, TCRβ. Polyclonal = smear; monoclonal = one or two bands. Gold standard but requires large DNA amounts. Replaced by PCR in most labs.

FISH: most sensitive for oncogene translocations:

BCL1/CCND1 t(11;14) = mantle cell lymphoma (PCR only 40-50% sensitive - FISH preferred)
BCL2 t(14;18) = follicular lymphoma
BCL6 3q27 = DLBCL
MYC 8q24 = Burkitt, DLBCL

Break-apart or dual-fusion probes. Works on interphase nuclei, paraffin, cytogenetic preparations.

26.10 Bone Marrow Evaluation

Aspirate and Touch Imprint

A 500-cell manual differential count should be performed on aspirate smears and touch imprints, including: blasts, promyelocytes, myelocytes, metamyelocytes, bands, segmented neutrophils, eosinophils, basophils, monocytes, lymphocytes, plasma cells, erythroid precursors. Spicules confirm adequacy.

A cell line is considered dysplastic only if ≥10% of cells show morphologic abnormalities (WHO threshold for MDS):

Erythroid dysplasia: megaloblastoid changes, nuclear budding, internuclear bridges, multinucleation, ring sideroblasts
Granulocytic dysplasia: hypolobulation (pseudo-Pelger-Huët), hypogranularity, abnormal nuclear shapes
Megakaryocytic dysplasia: micromegakaryocytes, hypolobulated nuclei, separated nuclear lobes

Dyserythropoiesis increases with time to fixation (smear preparation artifact).

Iron Stain (Prussian Blue / Perls)

Best performed on spicular aspirate, clot section, or trephine biopsy - the decalcification procedure diminishes stainable iron. The stain reveals:

Storage iron in macrophages (graded 0-6)
Siderotic granules in erythroid precursors (sideroblasts)
Ring sideroblasts: ≥5 granules encircling ≥1/3 of nucleus

Absent stainable iron = iron deficiency.

Bone Marrow Biopsy

Cellularity: fat:cell ratio expressed as percent cellularity. Rule of thumb: normal cellularity (%) = 100 minus age (±10%). A 30-year-old should have ~70% cellularity; a 70-year-old ~30%. Subcortical (immediately paratrabecular) areas are always more cellular and should not be used for overall assessment.

Adults with cellularity >70% = hypercellular (MPN, acute leukemia, MDS, reactive conditions, regenerating marrow)
Total fatty replacement = aplastic anemia at any age (consider sampling error if peripheral counts are normal)

M:E ratio: normal adult myeloid:erythroid ratio is 2:1 to 4:1. Increased M:E: myeloid hyperplasia, CML, decreased erythropoiesis. Decreased M:E: erythroid hyperplasia (hemolysis, hemorrhage, EPO therapy), pure red cell aplasia (if reduced), MDS.

Generative zone: rim of immature cells, paratrabecular, 1-2 cells thick. Contains the most immature precursors because the endosteal niche provides stem cell-maintaining signals (osteoblasts, CXCL12). An increase in generative zone girth may be seen in MPNs and MDS.

Megakaryocyte distribution: normally scattered, only rarely touching each other. Megakaryocyte clusters (≥3 in direct contact) or paratrabecular megakaryocytes are ABNORMAL - key criterion for MPN (especially PMF, ET).

Intrasinusoidal hematopoiesis: feature of fibrotic marrow, seen in MPNs (especially PMF).

Reticulin: normal marrow has minimal reticulin restricted to paratrabecular and perivascular sites. WHO reticulin grades:

MF-0: scattered thin fibers
MF-1: loose network, some intersections
MF-2: dense network, frequent intersections, focal collagen
MF-3: dense diffuse fibrosis with thick collagen bundles

MF-2/3 defines fibrotic-stage myelofibrosis.

Lymphoid aggregates: normal aggregates are interstitial/perivascular (away from trabeculae) and mixed. Paratrabecular lymphoid aggregates are highly characteristic of follicular lymphoma (also TCRBCL). Always prompt IHC for BCL2, CD10, CD20.

26.11 Spleen Normal Histology

Relevant here because the spleen is an obligate part of hemolysis, sequestration, and ITP discussions.

Compartments

Two main compartments:

White pulp (B- and T-cell lymphoid tissue organized around central arterioles)
- Three concentric zones around central arteriole:
  - Germinal center: somatic hypermutation, affinity maturation, class switching (BCL6+, BCL2-, high Ki-67)
  - Mantle zone: naive B cells (IgD+, BCL2+)
  - Marginal zone: post-germinal center memory B cells (CD21+, IgM+, IgD-)
- T-cell compartment: periarteriolar lymphatic sheath (PALS), primarily CD4+ T cells
Red pulp (venous sinuses + splenic cords)

Red Pulp Components

Venous sinuses lined by cuboidal littoral cells
Small capillaries lined by flat endothelial cells
Sheathed capillaries (unique to spleen; macrophage checkpoints)
Splenic cords (cords of Billroth): tissue between sinuses containing macrophages, reticular cells, plasma cells, transiting blood cells

Littoral Cells

Specialized sinus-lining endothelial cells unique to the spleen.

Express BOTH histiocytic markers (CD68, CD163) AND endothelial markers (CD31, factor VIII-related antigen/vWF, ERG)
CD8-positive (distinctive - most endothelium is CD8-negative; lymph node and bone marrow sinuses are CD8-)
CD34-negative (unlike most vascular endothelium)
May also express CD21

Littoral cell angioma arises from these cells.

Blood Flow

Splenic artery → trabecular arteries → central arterioles (surrounded by PALS) → follicular arterioles → penicillar arteries → sheathed capillaries → open into splenic cords (open circulation) → RBCs squeeze through discontinuous reticulin network of sinus walls → venous sinuses → trabecular veins → splenic vein.

The discontinuous reticulin network with 1-3 μm fenestrations is the anatomic basis of splenic filtering. Normal deformable RBCs squeeze through. Rigid cells (spherocytes, sickle cells, malaria-infected, senescent, antibody-coated) cannot pass and are trapped in the cords for macrophage destruction - the anatomic basis of extravascular hemolysis.

Abundant hemosiderin in red pulp macrophages is normal (iron recycling from senescent RBC destruction).

Splenic Congestion

Dilated red pulp sinuses engorged with blood, compressed white pulp. Chronic: congestive splenomegaly, fibrosis, Gamna-Gandy bodies (fibrotic iron-encrusted nodules), secondary hypersplenism. Portal hypertension is the most common cause.

Chapter 27: Red Blood Cell Disorders

Quick Reference: Anemia Classification by MCV

MCV	Category	Common Causes
<80 fL	Microcytic	Iron deficiency, thalassemia, anemia of chronic disease (some), sideroblastic anemia, lead poisoning
80-100 fL	Normocytic	Acute blood loss, anemia of chronic disease, renal disease, hemolysis (if not compensated), bone marrow failure, early iron or B12/folate deficiency
>100 fL	Macrocytic	B12/folate deficiency (megaloblastic), liver disease, hypothyroidism, myelodysplasia, reticulocytosis, drugs (methotrexate, hydroxyurea, AZT)

Iron Studies Interpretation

Condition	Serum Iron	TIBC	Ferritin	Transferrin Saturation
Iron deficiency	↓	↑	↓	↓ (<15%)
Anemia of chronic disease	↓	↓ or N	↑ or N	↓ or N
Thalassemia trait	N	N	N	N
Sideroblastic anemia	↑	N or ↓	↑	↑
Hemochromatosis	↑	↓	↑↑	↑↑ (>45%)

Key distinction: In iron deficiency, the body is hungry for iron (high TIBC = more transferrin being made to carry iron). In anemia of chronic disease, the body is sequestering iron (normal or low TIBC = body isn’t making more transferrin because the problem isn’t iron supply, it’s iron release from macrophages). Ferritin is the best single test to distinguish them - it’s truly low only in iron deficiency.

27.1 Approach to Anemia

Anemia is defined as decreased hemoglobin/hematocrit below normal for age and sex. The approach begins with classifying by RBC size (MCV).

Microcytic Anemia (MCV <80 fL)

The differential for microcytic anemia can be remembered by the mnemonic TAILS:

Thalassemia
Anemia of chronic disease (sometimes microcytic)
Iron deficiency
Lead poisoning
Sideroblastic anemia

Iron Deficiency Anemia - The most common cause of anemia worldwide

Pathophysiology: Iron is essential for heme synthesis. Deficiency leads to impaired hemoglobin production, causing small (microcytic), pale (hypochromic) red cells.

Causes:

Blood loss (GI bleeding, menstruation) - most common in adults
Inadequate intake (infants, restricted diets)
Malabsorption (celiac disease, gastrectomy)
Increased demand (pregnancy, growth)

Laboratory findings:

Test	Finding
Serum iron	↓
TIBC (transferrin)	↑ (liver makes more transferrin)
Transferrin saturation	↓↓ (<15%)
Ferritin	↓ (<12 ng/mL highly specific)
Reticulocytes	↓ or normal

Peripheral smear: Hypochromic, microcytic cells; target cells; elliptocytes (pencil cells); high RDW (variation in cell size)

Iron deficiency anemia: Hypochromic, microcytic RBCs with pencil cells (thin elliptocytes). High RDW reflects variation in cell size.

Progression: Iron stores (ferritin) deplete first → serum iron falls → microcytosis develops → anemia develops

Important caveat: Ferritin is an acute phase reactant - it may be normal or elevated in iron deficiency with concurrent inflammation. In this setting, check:

Soluble transferrin receptor (elevated in iron deficiency, not affected by inflammation)
Ferritin index: sTfR/log ferritin

Iron deficiency vs. thalassemia - the platelet clue: Iron deficiency often has reactive thrombocytosis due to cross-reactivity between erythropoietin and thrombopoietin signaling. Thalassemia trait has a normal platelet count. So if you see microcytic anemia with elevated platelets, lean iron deficiency until proven otherwise.

Thalassemia - Decreased globin chain synthesis

Pathophysiology: Hemoglobin consists of 4 globin chains - 2 alpha (α) and 2 non-alpha. In adults, HbA (α2β2) predominates. Thalassemias result from decreased production of one globin chain type, leading to:

Decreased hemoglobin synthesis (microcytosis)
Imbalanced chains forming inclusions (hemolysis)

Thalassemia is a quantitative defect (not enough of a structurally normal chain), in contrast to a hemoglobinopathy, which is a qualitative defect (structurally abnormal chain from a point mutation). The imbalance between α and non-α chain output is what drives pathology - excess unpaired chains precipitate and damage the red cell.

Thalassemia peripheral smear: Microcytic hypochromic RBCs with target cells, basophilic stippling, and nucleated RBCs in severe cases.

Alpha vs. beta - different types of mutations: α-thalassemia usually results from large structural deletions (the tandemly duplicated α genes are prone to unequal crossover). β-thalassemia usually results from point mutations (>300 described, affecting transcription, splicing, or mRNA stability). Practical consequence: α-thal is diagnosed by gap-PCR or MLPA (detects deletions); β-thal by sequencing.

Timing of onset: α-thalassemia manifests at birth because α chains are needed for every hemoglobin at every developmental stage. β-thalassemia manifests at 4-6 months when the fetal-to-adult switch (HbF → HbA) occurs. Board classic.

Alpha Thalassemia: Decreased α-chain production (genes on chromosome 16)

Normal individuals have 4 α-globin genes (2 on each chromosome 16). Severity depends on how many are deleted/mutated:

# Genes Affected	Syndrome	Clinical Features
1 (-α/αα)	Silent carrier	Normal; slight microcytosis
2 (–/αα or -α/-α)	α-thal trait	Mild microcytic anemia
3 (–/-α)	HbH disease	Moderate hemolytic anemia; HbH (β4) inclusions
4 (–/–)	Hb Bart’s hydrops fetalis	Incompatible with life; Hb Bart’s (γ4)

Note: The genotype (–/αα) is called cis deletion (both deletions on same chromosome, common in people with Southeast/East Asian ancestry) and is more likely to produce severely affected offspring than (-α/-α) trans deletion (one deletion per chromosome, common in people with African ancestry). This is exactly why Hb Bart’s hydrops fetalis is predominantly seen in Southeast Asian populations, not in most African ancestry populations - two α0 (–/αα) carriers can produce a –/– child, but two α+ (-α/αα) carriers can at most produce HbH disease.

α-thalassemia 1 (α0) genotype: (–/αα), most prevalent in people with Southeast/East Asian ancestry. α-thalassemia 2 (α+) genotype: (-α/αα), most prevalent in African American populations (~25-30% carrier rate).

Genetic diagnostic pitfall for α-thal trait: The 2-gene deletion state produces thalassemic indices on CBC (MCV 65-75 fL, elevated RBC count, low Hct) but NORMAL hemoglobin electrophoresis. HbA2 and HbF are normal. You cannot diagnose α-thal trait by electrophoresis. Presumptive diagnosis: microcytic anemia + normal iron studies + normal electrophoresis + ancestry/family origin consistent with the expected variant distribution. Definitive: α gene deletion analysis (gap-PCR/MLPA).

HbH disease (3 α genes deleted): Thalassemic indices on CBC, Heinz body preparation positive (HbH inclusions visible with brilliant cresyl blue supravital stain), fast-migrating HbH band (β4 tetramers) on electrophoresis (~5-30% HbH). HbH is unstable - precipitates as RBCs age, creating inclusions that drive splenic destruction.

Hb Bart’s hydrops fetalis (4 α genes deleted, –/–): Peripheral smear with profound hypochromia and nucleated RBCs. Fast-migrating Hb Bart’s (γ4 tetramers) on electrophoresis. γ4 has extremely high oxygen affinity and cannot release O2 to tissues - severe fetal anemia, hydrops fetalis, death in utero or shortly after birth.

Why the γ4 and β4 tetramers form: In α-thal, α chain deficit leaves excess β (adults) or γ (fetuses) that self-associate into β4 (HbH) or γ4 (Hb Bart’s). Both have very high O2 affinity (useless for delivery) and are unstable.

Beta Thalassemia: Decreased β-chain production (genes on chromosome 11)

There are 2 β-globin genes (one on each chromosome 11). Mutations are usually point mutations (not deletions). β0 alleles (complete absence of β chain production) are usually nonsense or frameshift mutations. β+ alleles (reduced β chain production) are usually promoter or 5’UTR mutations that reduce transcription.

Genotype	Syndrome	Clinical Features
β/β+ or β/β0	β-thal minor (trait)	Mild microcytic anemia; elevated HbA2 (>3.5%)
β+/β+ or β0/β+	β-thal intermedia	Moderate anemia; may need transfusions
β0/β0	β-thal major (Cooley’s)	Severe transfusion-dependent anemia; hepatosplenomegaly

β-thalassemia is most common in Mediterranean populations (Italy, Greece, Turkey), the Middle East, India, and Southeast Asia. Like sickle trait and G6PD, β-thal trait provides some protection against P. falciparum malaria.

Why is HbA2 elevated in β-thal trait? With less β-chain available, more α-chains bind to δ-chains (forming HbA2 = α2δ2). This is the key diagnostic finding - elevated HbA2 (>3.5%) confirms β-thal trait.

β-thal major electrophoresis: HbF 50-95%, HbA2 normal to elevated, little to no HbA. γ chain expression is upregulated when β chains are absent. In β0/β0: NO HbA at all. In β+/β0: trace HbA may be present.

Pathophysiology of β-thal major: The primary mechanism is not peripheral hemolysis but ineffective erythropoiesis. Excess α chains form α4 tetramers that are extremely unstable - they precipitate almost immediately within erythroid precursors, causing oxidative damage and apoptosis in the marrow. Intramedullary destruction drives the anemia.

Pitfall - β-thal trait masked by iron deficiency: Concurrent iron deficiency normalizes HbA2 in β-thal trait (iron deficiency suppresses all hemoglobin synthesis, including HbA2). A patient with microcytic anemia and normal HbA2 could have iron deficiency alone, α-thal trait, or β-thal trait masked by iron deficiency. Always correct iron deficiency first, then repeat electrophoresis.

Laboratory comparison:

Finding	Iron Deficiency	β-Thal Trait
MCV	↓	↓↓ (often very low for degree of anemia)
RDW	↑	Normal
RBC count	↓	Normal or ↑
Ferritin	↓	Normal
HbA2	Normal	↑ (>3.5%)

Thalassemic indices: RBC count >5.5 x 10^12/L in men (>5.0 in women), MCV 55-65 fL for β-thal or 65-75 fL for α-thal, normal platelets. The small cells + high count combination is the clue - unlike iron deficiency (small cells + LOW count), thalassemia has compensatory erythropoiesis.

The Mentzer index (MCV/RBC) can help: <13 suggests thalassemia; >15 suggests iron deficiency. Between 13-15 is indeterminate. Sensitivity ~80%, specificity ~85% - a screen, not a diagnosis.

Peripheral smear in thalassemia: Three key findings - (1) microcytic hypochromic cells, (2) occasional target cells (more prominent in β-thal than α-thal), (3) basophilic stippling from aggregated ribosomes.

δβ-thalassemia: Deletion of both δ and β genes. Heterozygotes show mild thalassemic phenotype with elevated HbF (5-20%) and LOW/normal HbA2 (delta gene is also deleted - the reverse of β-thal trait’s high HbA2 pattern). Homozygotes: 100% HbF.

Hb Lepore: δ-β fusion hemoglobin from unequal crossover. Fusion gene has a weak δ promoter driving β coding sequence → reduced output (thalassemic phenotype). Runs with HbS on alkaline gel but at ~10-15% (not the ~35-40% you’d see in sickle trait). Predominantly seen in Mediterranean populations, especially Italy. Three named variants (Washington, Baltimore, Hollandia) based on the crossover point. Heterozygotes resemble β-thal trait but with a Hb Lepore band in the HbS region.

Hereditary persistence of fetal hemoglobin (HPFH): Delayed γ-to-β/δ chain switch. Heterozygotes have 15-30% HbF with pancellular distribution (every cell has HbF), NO anemia, NO microcytosis. Clinically benign. Deletional HPFH (large deletions removing δ, β, and regulatory elements) vs. non-deletional (γ promoter point mutations).

Heterocellular vs. pancellular HbF: In β-thal major and most conditions with elevated HbF, the distribution is heterocellular - some RBCs contain HbF, some don’t. In HPFH, HbF is pancellular. The Kleihauer-Betke acid elution test distinguishes them: HbF-containing cells resist acid elution and remain pink, HbA cells elute and appear as ghost cells. This matters clinically because pancellular HbF protects every cell (e.g., SS-HPFH is mild) while heterocellular HbF leaves some cells vulnerable (e.g., S-β thal is severe).

HbA2 prime (HbA2’): A clinically insignificant δ chain variant in ~1-2% of African American individuals. Produces an extra band near HbA2 on alkaline electrophoresis. The concern is that this may cause underestimation of total HbA2 on some gel-based platforms, potentially missing concurrent β-thal trait. HPLC is preferred in these patients because it separates and quantifies HbA2 and HbA2’ in distinct windows. Total HbA2 = HbA2 + HbA2’; if total >3.5%, β-thal trait is present.

Sideroblastic Anemia - Defective heme synthesis with iron accumulation

Pathophysiology: Defective protoporphyrin synthesis in mitochondria leads to iron accumulation in mitochondria, visible as ring sideroblasts (erythroblasts with iron-laden mitochondria forming a ring around the nucleus).

Causes:

Hereditary: X-linked (ALAS2 mutation) - responds to pyridoxine (vitamin B6)
Acquired:
Myelodysplastic syndrome (most common; especially MDS with ring sideroblasts, often SF3B1 mutation)
Drugs/toxins: Alcohol, lead, isoniazid, chloramphenicol
Copper deficiency

Laboratory findings:

Microcytic anemia (or dimorphic with normal + microcytic cells)
Elevated serum iron, ferritin, transferrin saturation (iron overload pattern)
Ring sideroblasts on Prussian blue stain of marrow (≥15% defines the category)

Lead Poisoning

Pathophysiology: Lead inhibits several enzymes in heme synthesis:

δ-aminolevulinic acid dehydratase (accumulation of δ-ALA)
Ferrochelatase (decreased iron incorporation)

Laboratory findings:

Microcytic anemia
Basophilic stippling (ribosomal aggregates due to inhibited pyrimidine 5’-nucleotidase)
Elevated blood lead level
Elevated free erythrocyte protoporphyrin (FEP) or zinc protoporphyrin (ZPP)

Clinical features: Abdominal pain, constipation, neurological symptoms (encephalopathy in severe cases), “lead lines” on gingiva and long bone metaphyses

Normocytic Anemia (MCV 80-100 fL)

Anemia of Chronic Disease (ACD)

Also called “anemia of inflammation” - the second most common cause of anemia.

Pathophysiology: The key player is hepcidin, a liver-produced hormone that regulates iron metabolism:

Inflammation (via IL-6) → Increased hepcidin
Hepcidin binds and degrades ferroportin (iron exporter on macrophages and enterocytes)
Iron is trapped in macrophages and cannot be absorbed from gut
Result: Low serum iron despite adequate iron stores

This is a protective response - sequestering iron limits its availability to pathogens.

Laboratory findings:

Test	Finding
Serum iron	↓
TIBC	↓ or normal (unlike iron deficiency)
Transferrin saturation	↓ or normal
Ferritin	↑ or normal (acute phase reactant)

MCV: Usually normocytic, may be mildly microcytic

Distinguishing from iron deficiency: Ferritin is the key - elevated in ACD, low in iron deficiency. If both coexist, soluble transferrin receptor (sTfR) or sTfR/log ferritin index helps.

Anemia of Chronic Kidney Disease

Pathophysiology: The kidneys produce erythropoietin (EPO). In CKD, EPO production decreases, leading to inadequate stimulation of erythropoiesis.

Additional factors:

Uremic toxins suppress marrow
Shortened RBC survival
Blood loss from dialysis
Functional iron deficiency

Treatment: EPO-stimulating agents (ESAs) with adequate iron supplementation

Hemolytic Anemia (covered in detail in next section)

Hemolysis can be normocytic (if compensated) or macrocytic (reticulocytosis - reticulocytes are larger).

Acute Blood Loss

Immediately after acute hemorrhage, hemoglobin and hematocrit are normal because whole blood is lost. As the body compensates with fluid shifts and IV fluids, anemia becomes apparent. Reticulocytosis takes 3-5 days to appear.

Macrocytic Anemia (MCV >100 fL)

Megaloblastic vs. Non-megaloblastic: Critical distinction

Megaloblastic anemia results from impaired DNA synthesis (while RNA and protein synthesis continue normally). This causes nuclear-cytoplasmic asynchrony - cells grow large but nuclei remain immature.

Hallmark finding: Hypersegmented neutrophils (≥5% with 5 lobes, or any with 6+ lobes)

Causes: Vitamin B12 (cobalamin) or folate deficiency

Vitamin B12 (Cobalamin) Deficiency

Biochemistry: B12 is a cofactor for two enzymes:

Methionine synthase: Converts homocysteine → methionine (requires B12 and folate)
Methylmalonyl-CoA mutase: Converts methylmalonyl-CoA → succinyl-CoA (requires B12 only)

Deficiency causes:

Elevated homocysteine (shared with folate deficiency)
Elevated methylmalonic acid (MMA) (specific to B12 deficiency)

Causes of B12 deficiency:

Pernicious anemia: Autoimmune destruction of gastric parietal cells (anti-intrinsic factor, anti-parietal cell antibodies) → loss of intrinsic factor → impaired B12 absorption in terminal ileum
Gastrectomy, gastric bypass
Terminal ileum disease (Crohn’s, resection)
Strict vegan diet (takes years to deplete stores)
Metformin (impairs absorption)

Clinical features:

Anemia symptoms
Neurological symptoms: Subacute combined degeneration (posterior columns + lateral corticospinal tracts), peripheral neuropathy, dementia, mood changes
Glossitis, angular cheilitis

Important: Neurological symptoms can occur without anemia and may be irreversible if not treated promptly.

Laboratory findings:

Macrocytic anemia (MCV often >110 fL)
Low reticulocyte count
Hypersegmented neutrophils
Low serum B12 (<200 pg/mL)
Elevated MMA (>0.4 μmol/L)
Elevated homocysteine
Elevated LDH (ineffective erythropoiesis)
Elevated indirect bilirubin (same reason)

Folate Deficiency

Biochemistry: Folate is essential for DNA synthesis - specifically, for thymidylate synthesis (dTMP).

Causes:

Inadequate intake (alcoholism, poor diet)
Increased demand (pregnancy, hemolysis, malignancy)
Malabsorption (celiac disease, tropical sprue)
Drugs: Methotrexate (DHF reductase inhibitor), phenytoin, trimethoprim

Laboratory findings:

Macrocytic anemia, hypersegmented neutrophils (same as B12)
Low serum or RBC folate
Elevated homocysteine
Normal MMA (this distinguishes from B12 deficiency)

Non-megaloblastic Macrocytosis

Many conditions cause macrocytosis without the nuclear-cytoplasmic asynchrony:

Alcoholism: Direct toxic effect on marrow; also often have folate deficiency
Liver disease: Altered lipid metabolism affects RBC membrane
Hypothyroidism: Mechanism unclear
Myelodysplastic syndrome: Clonal marrow disorder
Reticulocytosis: Reticulocytes are larger; high reticulocyte count raises MCV
Drugs: Hydroxyurea, azathioprine, zidovudine (interfere with DNA synthesis)

Key clue: Absence of hypersegmented neutrophils helps distinguish non-megaloblastic from megaloblastic causes.

27.2 Hemolytic Anemias

Hemolytic anemia results from increased RBC destruction. The bone marrow compensates by increasing production (reticulocytosis).

Laboratory Evidence of Hemolysis

Finding	Mechanism
↑ Reticulocyte count	Compensatory marrow response
↑ Indirect bilirubin	Heme catabolism
↑ LDH	Released from RBCs
↓ Haptoglobin	Binds free hemoglobin; complex cleared by liver
↑ Urobilinogen in urine	Increased bilirubin metabolism

Intravascular hemolysis (hemolysis in the bloodstream) additionally shows:

Hemoglobinemia (pink plasma)
Hemoglobinuria (pink/red urine positive for blood but no RBCs)
Hemosiderinuria (hemosiderin in urine sediment - late finding)
Very low/undetectable haptoglobin

Extravascular hemolysis (hemolysis in spleen/liver by macrophages):

Haptoglobin moderately decreased
No hemoglobinemia/hemoglobinuria
Splenomegaly often present

Classification

Hemolytic anemias are classified as:

Intrinsic (defect within the RBC): Membrane, enzyme, or hemoglobin disorders
Extrinsic (external cause): Immune, mechanical, infectious, toxins

Membrane Disorders

Hereditary Spherocytosis (HS)

Pathophysiology: Defects in proteins linking the RBC cytoskeleton to the lipid bilayer. The membrane becomes unstable, and bits are lost as the cell passes through the spleen. The cell becomes progressively smaller and spherical. Spherocytes have decreased deformability and cannot squeeze through the 1-3 micron fenestrations between splenic sinus endothelial cells - they get trapped in the splenic red pulp cords and undergo further membrane conditioning by macrophages until they lyse. This is extravascular hemolysis.

Genetics and inheritance: Usually autosomal dominant (75%); remainder are autosomal recessive or de novo. Family history of anemia, jaundice, gallstones, splenomegaly, or splenectomy is suggestive. HS is most common in Northern Europeans (~1:5000) but occurs in all ethnic groups. It’s the most common congenital hemolytic anemia in populations of Northern European ancestry.

Which proteins are affected: All the proteins involved link the lipid bilayer (via band 3) to the underlying spectrin cytoskeleton (via ankyrin and protein 4.2).

ANK1 (ankyrin-1): ~60% of cases - the most common. Ankyrin anchors band 3 to spectrin. Without ankyrin, band 3 is unstable and lost.
SPTB (β-spectrin): ~15%
SLC4A1 (band 3): ~20%
EPB42 (protein 4.2): ~5%, more common in Japanese populations, autosomal recessive
SPTA1 (α-spectrin): ~5%

Clinical features:

Variable severity: (1) asymptomatic/well-compensated, (2) moderate chronic hemolytic anemia with jaundice and splenomegaly, (3) severe transfusion-dependent (rare, usually AR)
Jaundice, splenomegaly
Pigment (calcium bilirubinate) gallstones from chronic hemolysis
Aplastic crisis from parvovirus B19 infection
Folate deficiency can develop from increased marrow turnover

Laboratory findings:

Spherocytes on peripheral smear - dense, round, absent central pallor, decreased diameter
Elevated MCHC (>36 g/dL) - same hemoglobin in smaller volume
Elevated RDW
Mild reticulocytosis (3-8%) reflecting compensated hemolysis
Extravascular hemolysis labs: ↑ indirect bilirubin, ↑ LDH, ↓ haptoglobin, NO hemoglobinuria/hemosiderinuria
Negative DAT (the key distinguishing feature from AIHA)
Increased osmotic fragility (spherocytes lyse at higher osmolarity)
Decreased eosin-5-maleimide (EMA) binding on flow cytometry - modern diagnostic test of choice

Spherocyte differential: Artifact (most common cause on smears - thick areas). True spherocytes: (1) AIHA (DAT positive), (2) HS (DAT negative), (3) burns, (4) Clostridium perfringens sepsis. The DAT is the single test that most often settles HS vs. AIHA.

EMA binding test mechanics: Eosin-5-maleimide binds band 3 protein on the RBC surface. In HS, band 3 is decreased (either directly mutated or secondarily lost), so EMA fluorescence is reduced. Sensitivity and specificity >90%. Results in hours. Has largely replaced the osmotic fragility test for routine HS diagnosis.

Osmotic fragility test: Measures RBC susceptibility to lysis in hypotonic saline of decreasing concentration (0.85% to 0%). Spherocytes show increased fragility (lyse at higher NaCl concentrations - they can’t expand, already minimal surface-to-volume ratio). Target cells, thalassemia, sickle cells, and iron deficiency have decreased fragility (extra membrane allows greater expansion before lysis). The incubated version (37°C × 24h) is more sensitive for HS. Not recommended for routine use (long turnaround, normal in ~25% of HS patients, sensitivity >80%, specificity >60%).

SDS-PAGE of membrane proteins: Can identify specific protein deficiencies but is labor-intensive and not widely available. Clinical diagnosis relies on: clinical features + spherocytes + elevated MCHC + negative DAT + decreased EMA binding. Genetic testing is increasingly available for definitive diagnosis.

Treatment: Splenectomy is curative for hemolysis in severe HS (though spherocytes persist on smear because the defect is intrinsic to the RBC). Indications: transfusion dependence, severe anemia, symptomatic gallstones, growth retardation. Defer until age >6 due to risk of overwhelming post-splenectomy sepsis. Partial splenectomy may preserve some immune function. Pre-splenectomy vaccination for pneumococcus, meningococcus, H. influenzae. Folate supplementation in moderate/severe disease.

Hereditary Elliptocytosis (HE)

Pathophysiology: Defects in horizontal (lateral) interactions of cytoskeleton proteins. Spectrin heterodimers must self-associate into tetramers to form the cytoskeletal meshwork; mutations in the self-association site prevent tetramer formation. The cell deforms into an elliptical shape during microcirculation passage and cannot return to a discoid shape.

Genetics: Usually autosomal dominant. More common than HS in populations with African or Southeast Asian ancestry. Most common mutation target: α-spectrin (SPTA1, ~65% of cases). Others: β-spectrin, band 4.1, glycophorin C. The common type of HE is most prevalent in people with African ancestry, including African American and West African populations - likely reflects malaria protection (similar to sickle trait and G6PD).

Clinical features: Most HE patients are asymptomatic carriers. ~10% have clinically significant hemolysis. Heterozygotes are mild; homozygotes/compound heterozygotes can be moderate to severe. Severity depends on the specific mutation.

Smear and diagnosis: >25% elliptocytes is diagnostic (normal has <15%). Most HE patients have 60-90% elliptocytes on smear. The smear alone is diagnostic in most cases - no additional lab test needed. Elliptocytes are defined as RBCs twice as long as they are wide. Osmotic fragility is usually normal (unlike HS). EMA binding is usually normal. Low numbers of elliptocytes (<25%) occur in iron deficiency, B12/folate deficiency, and myelophthisis (marrow infiltration).

Hereditary spherocytic elliptocytosis: Double/compound heterozygosity for both HS and HE mutations - combined phenotype with both spherocytes and elliptocytes. More severe hemolysis than either condition alone. Caused by SPTA1 or SPTB variants producing both spectrin deficiency (HS feature) and abnormal spectrin tetramer formation (HE feature).

Hereditary pyropoikilocytosis (HPP): Severe variant of HE. Autosomal recessive (homozygous or compound heterozygous spectrin mutations), in contrast to dominant HE. RBCs fragment at temperatures as low as 45-46°C (normal RBCs fragment at 49°C). Marked anisopoikilocytosis with microspherocytes, fragments, and elliptocytes. MCV often very low (50-60 fL) from membrane fragmentation. Severe hemolytic anemia presenting in infancy. Smear findings mimic a severe burn patient. Thermal instability test is diagnostic.

Southeast Asian Ovalocytosis (SAO): Stomatocytic type of HE. Caused by a 27 bp deletion in SLC4A1 (band 3). Stomatocytic elliptocytes are extremely rigid. SAO homozygosity is lethal in utero. Heterozygotes are asymptomatic with stomatocytic ovalocytes on smear (longitudinal slits or dense central band). Provides resistance to cerebral malaria (P. falciparum). Common in Malaysia and Papua New Guinea.

Hereditary Stomatocytosis

Autosomal dominant disorder with stomatocytes (RBCs with elongated, mouth-like central pallor; “stoma” = mouth). Caused by mutations in ion channels (PIEZO1, RHAG, SLC4A1) that alter Na/K permeability, leading to net water gain or loss. Rare. Small numbers of stomatocytes are commonly artifact (thick smear area) or associated with alcohol use - true hereditary stomatocytosis is distinct.

Two types based on hydration state:

Overhydrated (hydrocytotic) stomatocytosis: Na+ influx > K+ efflux → water enters → macrocytosis, low MCHC, moderate-severe hemolysis. Increased RBC volume.
Dehydrated stomatocytosis (hereditary xerocytosis): K+ efflux > Na+ influx → water loss → high MCHC, normocytic or slightly microcytic cells with spiculated desiccocytes, mild stomatocytosis, target cells. Mild hemolysis. Most commonly PIEZO1 mutations (mechanosensitive cation channel). May present with perinatal edema/hydrops or iron overload in adults.

Rh null phenotype and stomatocytosis: Complete absence of Rh antigens is associated with stomatocytosis and chronic hemolytic anemia. The Rh complex (RhAG, RhD, RhCE) is essential for membrane integrity. Rh null cells show spherostomatocytosis. These patients can only receive blood from other Rh null donors (extremely rare phenotype, ~1:6 million).

CRITICAL - splenectomy contraindicated: Splenectomy is contraindicated in hereditary stomatocytosis due to markedly increased risk of post-splenectomy thrombosis. Dehydrated, rigid stomatocytes are highly thrombogenic after splenectomy (portal vein thrombosis, chronic thrombosis → pulmonary hypertension). Board favorite: distinguish HS (splenectomy appropriate) from hereditary stomatocytosis (splenectomy dangerous).

Paroxysmal Nocturnal Hemoglobinuria (PNH)

Pathophysiology: Acquired mutation in the PIGA gene in a hematopoietic stem cell. PIGA is required to synthesize the GPI anchor, which attaches many proteins to the cell surface, including:

CD55 (DAF = Decay Accelerating Factor): Accelerates decay of C3/C5 convertases of both classical and alternative complement pathways.
CD59 (MIRL = Membrane Inhibitor of Reactive Lysis): Prevents formation of the membrane attack complex (MAC, C5b-9). CD59 is more critical than CD55 for protecting against complement-mediated hemolysis because it blocks the final lytic step.

Without these regulators, RBCs are susceptible to complement-mediated lysis. PNH also has decreased acetylcholinesterase (AChE), CD16, CD24, CD48, CD58, and leukocyte alkaline phosphatase (LAP - also GPI-anchored).

Clinical features:

Intravascular hemolysis (episodic or chronic)
Classic morning hemoglobinuria (nocturnal acidosis activates complement) - but most patients don’t have this classic presentation
Episodic abdominal pain, back pain, headaches, especially at night
Thrombosis (leading cause of death): Unusual sites - hepatic vein (Budd-Chiari), cerebral, mesenteric
Often arises from or coexists with aplastic anemia; may progress to aplastic anemia or AML (~2-5%). The PNH clone has a selective survival advantage in the setting of immune-mediated marrow destruction (GPI-anchor-deficient cells escape cytotoxic T-cell killing).

Laboratory findings:

Hemolysis labs (↑ LDH, ↓ haptoglobin, ↑ reticulocytes)
Negative DAT (complement-mediated hemolysis via alternative pathway tick-over, not antibody-mediated)
Decreased leukocyte alkaline phosphatase (LAP) score (LAP is GPI-anchored)
Mildly hypercellular bone marrow with erythroid hyperplasia and depleted iron stores (chronic urinary iron loss)
Often have concurrent cytopenias

Flow cytometry diagnosis: Definitive PNH diagnosis requires loss of ≥2 GPI-anchored antigens on ≥2 cell lineages (RBCs, neutrophils, monocytes). This criterion ensures specificity and prevents false positives from recent transfusion (donor RBCs may mask the PNH clone).

RBC analysis: CD55 and CD59. RBCs classified as type I (normal), type II (partial deficiency), type III (complete deficiency).
Granulocyte/monocyte analysis: CD14, CD15, CD16, CD24, CD33, CD58, and FLAER.
FLAER (fluorescein-labeled proaerolysin): Aerolysin is a virulence factor of Aeromonas hydrophila that binds selectively and with high affinity to GPI anchors. FLAER binds the GPI anchor itself, not a specific protein, making it the most sensitive reagent for small PNH clones. FLAER works on neutrophils and monocytes but not on mature RBCs (too few GPI anchors for reliable detection). FLAER + CD24 on neutrophils is the most sensitive combination.

Treatment:

Eculizumab (anti-C5 monoclonal antibody) blocks C5 cleavage, preventing MAC formation and dramatically reducing intravascular hemolysis and thrombosis risk. Patients need meningococcal vaccination before starting (terminal complement blockade increases meningococcal infection risk).
Bone marrow transplant (only cure)

Enzyme Deficiencies

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

Biochemistry: G6PD is the rate-limiting enzyme of the pentose phosphate pathway (hexose monophosphate shunt) and is the ONLY source of NADPH in RBCs. NADPH regenerates reduced glutathione (GSH) via glutathione reductase. GSH detoxifies reactive oxygen species (H2O2). Without G6PD → no NADPH → no GSH → oxidative damage to hemoglobin → hemolysis.

Genetics: X-linked recessive (gene on Xq28). The most common enzyme deficiency worldwide (~400 million people). Males are hemizygous and fully affected. Female heterozygotes are usually asymptomatic due to random X-inactivation (lyonization), though skewed inactivation can occasionally cause clinical disease. Complete absence of G6PD is thought to be incompatible with life - all deficient variants retain some residual activity.

Malaria protection: G6PD deficiency prevalence maps closely to historical P. falciparum malaria distribution (Africa, Mediterranean, Middle East, Southeast Asia). Mechanism: infected G6PD-deficient RBCs are more susceptible to oxidative damage and are cleared more rapidly by the spleen before the parasite can complete its cycle.

WHO variant classification (by enzyme activity):

Class I: Severely deficient with chronic hemolysis
Class II: Severely deficient (<10% residual), acute episodic hemolysis. G6PD Mediterranean is classic.
Class III: Moderately deficient (10-60% residual), acute episodic hemolysis. G6PD A- is classic.
Class IV: Mildly deficient to normal (60-100%), no hemolysis
Class V: Increased activity (>150%)

The two classic variants - why they behave differently:

G6PD A- (African American populations, ~10-15% of males): Young RBCs (including reticulocytes) have adequate G6PD levels, but the enzyme is unstable and short-lived (half-life ~13 days vs. 62 days for wild-type). Only older RBCs lack enough enzyme. During oxidative stress, older cells hemolyze but young cells survive. Result: self-limited episodes - hemolysis stops once the older deficient cells are destroyed, even if the trigger persists. Recovery within days.
G6PD Mediterranean (Italy, Greece, Spain, Middle East): Young AND old RBCs have severely decreased G6PD (half-life on the order of hours). Hemolysis is more severe, NOT self-limited, and can be life-threatening. Favism is most associated with this variant.

Triggers of hemolysis:

Infections (most common trigger overall) - neutrophil respiratory burst generates oxidative stress
Drugs: methylene blue, sulfonamides, nitrofurantoin, dapsone, primaquine, rasburicase, phenazopyridine, toluidine blue, naphthalene (mothballs)
Fava beans (favism) - contain divicine and isouramil, generate H2O2. Mainly Mediterranean variant
Metabolic acidosis (DKA)

Methylene blue is a trap: Methylene blue is used to treat methemoglobinemia but is CONTRAINDICATED in G6PD deficiency (requires NADPH for its action, and it generates oxidative stress when NADPH is absent). Classic board question.

Pathophysiology of hemolysis: Oxidative stress → hemoglobin denatures and precipitates on the inner membrane as Heinz bodies (aggregates of denatured globin chains) → splenic macrophages “pit” the Heinz bodies out of the cell (creating bite cells / degmacytes) or push hemoglobin to one side (blister cells) → eventually the whole cell is removed. Hemolysis is typically intravascular (hemoglobinuria, hemoglobinemia).

Laboratory findings during episode:

Acute intravascular hemolysis
Bite cells (degmacytes) - semicircular defects. Double bite cells resembling apple cores are highly characteristic.
Blister cells - hemoglobin pushed to one side with a clear area beneath the membrane
Heinz bodies - visible with supravital stains (crystal violet, methyl violet), NOT visible on Wright stain. Also seen in unstable hemoglobins.
Elevated reticulocytes (after a few days)

Testing - the fluorescence spot test: Patient blood + NADP+ + G6P on filter paper → NADPH is produced if G6PD is present → NADPH fluoresces under long-wave UV; NADP+ does not. Fluorescence = G6PD present. No fluorescence or dim fluorescence = G6PD deficient. Simple, rapid, inexpensive qualitative screen.

False negatives during reticulocytosis: Reticulocytes and young cells have higher G6PD levels. Testing during acute hemolysis (when old deficient cells have already lysed and reticulocytes flood the circulation) can give a falsely normal result. Always delay G6PD testing until 2-3 months after an acute episode. Also falsely negative in mild deficiencies and samples with elevated leukocyte/platelet counts.

Pyruvate Kinase (PK) Deficiency

Biochemistry: PK catalyzes the final rate-limiting step of glycolysis: phosphoenolpyruvate → pyruvate + ATP. RBCs depend entirely on glycolysis (Embden-Meyerhof pathway) for ATP - they have no mitochondria. Without PK → ATP depletion → Na/K-ATPase fails → cation leak → dehydrated, rigid RBCs → splenic destruction (extravascular hemolysis). The upstream intermediate 2,3-DPG accumulates, shifting the O2 dissociation curve rightward.

Genetics: Autosomal recessive. Most common enzyme deficiency of the glycolytic pathway causing hemolytic anemia. High frequency in Northern Europeans and the Pennsylvania Amish.

Clinical features: Unlike G6PD (episodic, oxidant-triggered), PK deficiency causes chronic, non-spherocytic hemolytic anemia from birth. Extravascular hemolysis. Splenomegaly, jaundice, pigment gallstones. Splenectomy can help (unlike in stomatocytosis, where it’s contraindicated).

Laboratory:

Elevated 2,3-DPG (increased substrate proximal to block)
Echinocytes (burr cells) on smear - evenly spaced, blunt projections due to ATP depletion. Distinguish from acanthocytes (irregular, pointed - liver disease, abetalipoproteinemia)
Decreased PK activity assay

Diagnostic tests:

Quantitative PK enzyme assay is the gold standard. Spectrophotometric measurement in RBC lysate. Pitfalls: (1) reticulocytes have higher PK activity → falsely normal during reticulocytosis (same issue as G6PD); (2) leukocytes have their own PK isoenzyme → must deplete WBCs from the sample.
Fluorescent spot test for PK: Different from the G6PD version. NADH + phosphoenolpyruvate added; PK converts PEP → pyruvate, then LDH uses NADH to convert pyruvate → lactate. NADH fluoresces; NAD+ does not. Normal patients: fluorescence disappears (NADH consumed). PK deficient: fluorescence persists (cannot consume NADH).
Autohemolysis test (now largely historical): RBCs incubated at 37°C for 48 hours. PK deficiency: does NOT correct with glucose (PK is needed to process glucose through glycolysis) but DOES correct with ATP (bypasses the block). In HS: corrects with glucose.

Pyrimidine 5’ Nucleotidase Deficiency

Biochemistry: Pyrimidine 5’ nucleotidase degrades RNA within reticulocytes. Deficiency causes unmetabolized pyrimidines to precipitate within the cell, visible as basophilic stippling (aggregated ribosomes/RNA). Autosomal recessive. Chronic hemolytic anemia with prominent basophilic stippling - more prominent than other causes.

Connection to lead poisoning: Lead inhibits pyrimidine 5’ nucleotidase, which is why basophilic stippling is seen in lead poisoning. Lead also inhibits δ-ALA dehydratase and ferrochelatase, producing a broader picture of impaired heme synthesis.

Hemoglobinopathies

Normal hemoglobin structures - necessary background:

HbA (α2β2): major adult hemoglobin (~97%)
HbA2 (α2δ2): minor adult hemoglobin (1-3.5%). Elevated HbA2 (>3.5%) is the hallmark of β-thal trait.
HbF (α2γ2): major late fetal hemoglobin. Higher O2 affinity than HbA (left-shifted) to facilitate placental transfer.
Hb Gower 1 (ζ2ε2): major early embryonic hemoglobin (yolk sac, first 8 weeks)
Hb Gower 2 (α2ε2): minor embryonic hemoglobin

Developmental switches: embryonic (ζ, ε chains, yolk sac) → fetal (α, γ chains, fetal liver) → adult (α, β/δ chains, bone marrow). At birth HbF is 60-80%, declines to adult levels over 6-12 months.

Gene locations - important for understanding thalassemia and variants:

α-globin genes (HBA1, HBA2) on chromosome 16p13.3. 2 α genes per chromosome (4 total). Gene duplication provides redundancy - losing 1 or 2 α genes may cause minimal clinical effect. α-thalassemia severity depends on how many of 4 genes are lost.
β-globin gene (HBB) on chromosome 11p15.4. 1 β gene per chromosome (2 total). Also on chromosome 11: δ, γ-A, γ-G, and ε genes, arranged 5’-ε-Gγ-Aγ-δ-β-3’. This cluster arrangement drives the developmental switches.

Hemoglobinopathy vs. thalassemia: A hemoglobinopathy is a qualitative defect - point mutation producing a structurally abnormal chain (HbS, HbC, HbE). A thalassemia is a quantitative defect - reduced production of a structurally normal chain. Some variants (like HbE) straddle both categories because the mutation reduces output.

Common β-chain variants (all affect adults, since β chains are adult-predominant):

HbS: Glu → Val at position 6. Creates a hydrophobic patch that promotes polymerization of deoxy-HbS.
HbC: Glu → Lys at position 6. Same position as HbS, different substitution. Second most common hemoglobin variant worldwide.
HbE: Glu → Lys at position 26. Second most common hemoglobinopathy worldwide. Extremely common in Southeast Asia (Thailand, Cambodia, Laos carrier rates up to 30-50%). The mutation also activates a cryptic splice site, producing a thalassemic phenotype from the mutant allele.
HbD (Punjab/Los Angeles): Glu → Gln at position 121. Most common HbD variant. Comigrates with HbS on alkaline gel; separable by acid gel.
HbO Arab: Glu → Lys at position 121 (same position as HbD). Comigrates with HbC/HbA2/HbE on alkaline gel. HbO Arab/HbS compound heterozygotes have sickle cell disease.
HbC Harlem: Double mutation - Glu→Val at 6 (same as HbS) AND Asp→Asn at 73. Can sickle but migrates with HbC on alkaline gel. Classic electrophoresis trap - use acid gel to separate from HbC.

Electrophoresis memory hook: HbC, HbE, and HbO are all Glu → Lys alterations and all migrate together with HbA2 on alkaline gel.

Sickle Cell Disease (HbSS)

Genetics: Autosomal recessive; point mutation in β-globin gene (glutamic acid → valine at position 6, GAG → GTG). In the United States, carrier frequency is highest among people with African ancestry (roughly ~1 in 10), and sickle cell disease occurs in roughly ~1 in 500 Black/African American births. Geographic prevalence mirrors historical P. falciparum distribution.

Pathophysiology: Deoxygenated HbS exposes a hydrophobic patch (Val6) that fits into a complementary pocket on adjacent HbS molecules, forming long rigid polymers. Sickled cells are rigid and sticky, causing:

Vaso-occlusion: Sickled cells obstruct small vessels → ischemia → pain crises
Hemolysis: Sickled cells have shortened lifespan (~17 days vs. 120 days)

What triggers polymerization: Requires (1) deoxygenation and (2) HbS concentration >50% of total hemoglobin. This is why sickle trait (35-45% HbS) is usually asymptomatic under normal conditions. The delay time before polymerization determines whether sickling occurs during capillary transit. HbF disrupts HbS polymerization because γ chains cannot participate in the hydrophobic contacts. This is why SCD manifestations are not apparent until 4-6 months of age as the fetal-to-adult switch completes, and why hydroxyurea (which raises HbF) is therapeutic.

Clinical features:

Acute pain (vaso-occlusive) crises - the hallmark of SCD. Bones (especially long bones, spine), chest, abdomen. Precipitants: cold exposure, dehydration, infection, alcohol, high altitude, stress. Treatment: hydration, opioid analgesia, oxygen if hypoxic. Dactylitis (hand-foot syndrome) in infants is often the first presentation.
Acute chest syndrome - most common cause of death. Defined as new pulmonary infiltrate + at least one of: chest pain, fever, tachypnea, cough, hypoxia. Caused by pulmonary vaso-occlusion, fat embolism from bone marrow infarction, or infection. Treatment: antibiotics (cover atypicals), supplemental O2, simple or exchange transfusion if severe. Rib infarction → splinting → atelectasis → ACS (why incentive spirometry matters during pain crises).
Stroke: Especially in children
Splenic sequestration crisis: Massive pooling in spleen during a viral illness → acute anemia, thrombocytopenia, hypovolemic shock. Most common in children 6 months - 5 years with HbSS (before autosplenectomy) and any age in HbSC (spleen persists longer).
Autosplenectomy: Repeated infarction → fibrosis → functional asplenia by age 5 in HbSS
Aplastic crisis: Parvovirus B19 infects erythroid progenitors via the P antigen receptor → arrest of erythropoiesis for 7-10 days → severe reticulocytopenic anemia. Reticulocyte count drops to near zero (the key distinguishing feature from other crises).
Hyperhemolytic crisis: Sudden severe hemolysis, possibly from concurrent G6PD deficiency or delayed transfusion reaction. Reticulocyte count is elevated (unlike aplastic crisis). Some patients have delayed hemolytic transfusion reactions with hyperhemolysis - destruction of both donor and autologous RBCs (bystander hemolysis). Requires immunosuppression, not more transfusions.
Chronic complications: Renal disease, retinopathy, avascular necrosis, priapism, leg ulcers

Infections:

S. pneumoniae is the most common cause of sepsis, pneumonia, and meningitis in SCD (functional asplenia → encapsulated organism risk). Prevention: pneumococcal vaccination, daily penicillin prophylaxis (age 2 months to at least 5).
Salmonella is the most common cause of osteomyelitis in SCD (not S. aureus, which is #1 in the general population). Bone infarcts create a favorable environment. S. aureus is still common in SCD, but Salmonella is disproportionately so - a board classic.

Pregnancy complications: Preeclampsia (increased risk), intrauterine growth restriction, fetal demise, preterm birth, more frequent vaso-occlusive crises, ACS, infection risk. Hydroxyurea must be discontinued before conception (teratogenic).

Transfusion complications: SCD patients are at high risk for alloimmunization (frequent transfusions + antigenic disparity between donor pools enriched for European-ancestry antigen profiles and many recipients with African-ancestry antigen profiles). Delayed hemolytic transfusion reactions can paradoxically worsen anemia (hemoglobin drops below pre-transfusion level).

Laboratory findings:

Chronic hemolytic anemia (Hgb 6-9 g/dL)
Sickled cells (drepanocytes) on peripheral smear - ONLY in sickle cell disease (HbSS, HbSC, S-β thal), NOT in sickle trait. Can also be seen in S-C, S-D, and C.
Target cells (reduced surface-to-volume ratio after membrane loss)
Howell-Jolly bodies (nuclear remnants - indicates splenic dysfunction)
Elevated reticulocytes
Hemoglobin electrophoresis: HbS >80%, HbF 1-20%, HbA2 1-4%, HbA 0%. The complete absence of HbA is the key finding confirming HbSS (vs. S-β+ thal which has some HbA).

Sickle Solubility Test: HbS polymerizes in reducing agent (sodium dithionite), causing turbidity. Positive in HbS trait and disease. Does not distinguish amount of HbS - use electrophoresis for diagnosis.

Sickle Cell Trait (HbAS)

Carriers of one HbS allele (heterozygotes).

Hemoglobin electrophoresis: HbS 35-45%, HbA 50-55%, HbA2 1-3%, HbF <1%. A > S always in classical sickle trait. If HbS > HbA in what looks like trait, consider: (1) HbS/β+ thal (the normal β allele has reduced output), (2) HbS with concurrent α-thalassemia, or (3) lab error.

Clinical significance: Generally asymptomatic with normal CBC and peripheral smear (no sickled cells). Provides protection against P. falciparum malaria. The kidney is the most commonly affected organ in sickle trait:

Hematuria and isosthenuria (urine osmolality equal to plasma) from renal papillary necrosis - the hypertonic, hypoxic renal medulla promotes sickling
Splenic infarction at high altitude
Exercise-induced rhabdomyolysis and sudden cardiac death (notable in military recruits and athletes)
Renal medullary carcinoma - rare but nearly pathognomonic

Renal medullary carcinoma and sickle trait: Rare, aggressive tumor almost exclusively in young patients with sickle trait or HbSC. Presents <40 years, male predominance 2:1, right kidney predominance 3:1. Presents at advanced stage. Loss of SMARCB1 (INI1) expression by IHC is characteristic. Median survival ~13 months. One of the few malignancies specifically associated with sickle cell trait.

Compound sickle states - coinheritance matters:

S-α thalassemia: Fewer α chains → less total hemoglobin → lower %HbS (below polymerization threshold) → milder disease. HbS is ~30-35% with 1 α gene deletion, ~25-30% with 2 α gene deletions. Lower MCV. Alpha-thal makes sickle milder.
S-β+ thalassemia: Normal β allele has reduced output, while sickle β allele produces full HbS → HbS increases to 60-80% → more severe disease. HbA is present (~20-30%). Distinguished from HbSS by presence of HbA. Usually elevated HbA2 (>3.5%). Also has low MCV. Beta-thal makes sickle worse.
S-β0 thalassemia: No normal β output. >80% HbS. Electrophoresis indistinguishable from HbSS alone (both have 0% HbA). Need family studies or genetic testing. Clinically similar to HbSS. Low MCV.
SS-HPFH (sickle cell disease + hereditary persistence of fetal hemoglobin): ~25-30% HbF in pancellular distribution. High pancellular HbF protects every cell from polymerization → clinically mild sickle syndrome despite >80% HbS. Contrast with β-thal major where HbF is heterocellular (not all cells protected).

Electrophoresis trap: Both SS-HPFH and S-β thalassemia show HbS, HbF, and HbA2 bands. The Kleihauer-Betke test distinguishes them - SS-HPFH is pancellular (every cell stains for HbF), S-β thal is heterocellular (only some cells stain). Pancellular = HPFH (mild). Heterocellular = thal (severe). High-yield for boards.

Hemoglobin C Disease (HbCC)

Genetics: Glutamic acid → lysine at position 6 of β-globin. Most prevalent in West Africa (Ghana, Burkina Faso - carrier rates 15-30%). Carrier rate is ~4% in African American populations. Likely confers malaria protection.

Clinical features: Mild hemolytic anemia, splenomegaly.

Electrophoresis:

HbAC trait: 40-50% HbC, remainder HbA. Asymptomatic. Scattered target cells on smear. HbC comigrates with HbA2/HbE on alkaline gel, so this 40-50% band should not be confused with extremely elevated HbA2.
HbCC disease: ~90% HbC, 7% HbF, 3% HbA2, 0% HbA.

Smear: Target cells abundant (HbC dehydrates the RBC via Gardos channel activation → excess membrane relative to volume). Hexagonal or rod-shaped HbC crystals within RBCs are pathognomonic, especially after splenectomy. The spleen normally removes crystals. HbC increases MCHC to levels that promote crystallization.

Hemoglobin SC Disease

Compound heterozygote for HbS and HbC. Intermediate severity between HbSS and HbCC.

Electrophoresis: ~50% HbS and ~50% HbC. No HbA. On alkaline gel, HbS and HbC are clearly separated.

Smear: Abundant target cells (from HbC) + mild sickling (from HbS) + occasional HbC crystals (rhomboid/hexagonal). The combination of target cells + mild sickling should prompt HbSC consideration. Reticulocytosis present but less than HbSS.

Clinical features: Notable for proliferative retinopathy and avascular necrosis - as often or more often than HbSS. This is thought to be because HbSC patients have higher hematocrits (less anemic) → higher viscosity → more vaso-occlusion in certain microvascular beds (retina, femoral head). HbC dehydrates RBCs, increasing MCHC and promoting HbS polymerization despite the lower %HbS. The spleen persists longer than in HbSS, making splenic sequestration a risk at any age.

Hemoglobin E (HbE)

Second most common abnormal hemoglobin worldwide after HbS. Common in Southeast Asia. Usually benign in homozygotes (HbEE) - asymptomatic or mild microcytic anemia (MCV 55-70 fL). The microcytosis occurs because the HbE mutation at β-26 also activates a cryptic splice site, reducing β-globin mRNA (thalassemic phenotype). Target cells on smear.

The major clinical problem is HbE/β-thalassemia compound heterozygosity, a major cause of transfusion-dependent anemia in Southeast Asia. Severity ranges from mild (with β+ thal) to transfusion-dependent (with β0 thal). On HPLC, HbE and HbA2 coelute, so HbA2 cannot be quantified in HbEE.

Other variants worth knowing:

HbD vs. HbG: Both comigrate with HbS on alkaline gel. HbD is a β-chain defect; HbG is an α-chain defect. Because there are 2 α genes per chromosome (4 total) but only 1 β gene per chromosome (2 total), an α-chain variant produces ~25% abnormal hemoglobin per mutant allele, while a β-chain variant produces ~50%. HbG uniquely produces two HbA2 bands on electrophoresis - the abnormal α-variant pairs with δ chains to create an abnormal HbA2 (in addition to normal HbA2). This is a classic alpha-chain-variant clue.
Hb Constant Spring (HbCS): Mutation in the α-globin stop codon (TAA → CAA) produces an elongated, unstable α chain (+31 amino acids). Unstable mRNA → very low levels (~1-2%). Common in Southeast Asia. HbCS allele + α0 deletion on the opposite chromosome = HbH disease (non-deletional form, more severe than deletional HbH).

High oxygen affinity hemoglobins: Examples include Hb Chesapeake, J-Capetown, Malmö, Yakima, Ypsilanti, Rainier, Denver. Leftward shift → hemoglobin holds O2 more tightly → tissues receive less O2 → compensatory erythrocytosis (EPO-mediated). Patients have elevated Hgb/Hct with low P50. Often discovered incidentally during erythrocytosis workup. Not distinguished on gel/HPLC - the O2 dissociation curve (P50) is diagnostic.

Low oxygen affinity hemoglobins: Examples include Hb Beth Israel, Kansas, Providence. Rightward shift → hemoglobin releases O2 more readily → tissues well-oxygenated at lower Hgb → pseudocyanosis (deoxyhemoglobin is blue-purple) and mild anemia. Elevated P50. Despite the low Hgb and cyanotic appearance, tissue oxygenation is fine. No treatment needed.

Unstable hemoglobins: >100 variants described (Hb Hasharon, Köln, Seattle, Tacoma, Ann Arbor, Zurich). Mutations disrupt globin structure → susceptible to oxidative denaturation → Heinz bodies + bite cells (same findings as G6PD deficiency). Chronic hemolysis with episodes of exacerbation. Many are autosomal dominant.

Isopropanol instability test: Lysed RBCs + 17% isopropanol at 37°C → unstable Hbs precipitate in 5 minutes (normal Hb precipitates at 30-40 minutes)
Heat instability test (50°C) is an alternative
Distinction from G6PD: G6PD is episodic (triggered by oxidants); unstable Hbs cause chronic hemolysis. G6PD assay and hemoglobin electrophoresis differentiate.

Hemoglobin Electrophoresis Interpretation:

Condition	HbA	HbS	HbA2	HbF	MCV	Clinical
Sickle Trait (AS)	~60%	~35%	Normal	Normal	Normal	Usually asymptomatic; A > S always
Sickle Cell (SS)	0%	~90%+	Normal	Elevated	Nl/↑	Severe crises; functional asplenia
Sickle β⁺ Thalassemia	Present (~70%)	~24%	↑ (~4%)	~1%	LOW	Milder; HbA present = β⁺ (leaky promoter)
Sickle β⁰ Thalassemia	0%	~80%+	↑	Elevated	LOW	Clinically similar to SS
HbSC Disease	0%	~50%	Normal	Normal	Normal	HbC peak; moderate severity

Key: HbA2 >3.5% = β-thalassemia trait. In sickle-β⁺ thalassemia, HbA is present but elevated HbA2 distinguishes it from simple sickle trait (where A2 is normal).

Abnormal Hemoglobin Species - Methemoglobin, Sulfhemoglobin, Carboxyhemoglobin

These are not genetic hemoglobinopathies but functional alterations of hemoglobin that impair oxygen delivery. Worth knowing for the toxicology and critical-care slant on the boards.

Methemoglobin

Methemoglobin has iron in the oxidized ferric (Fe3+) state instead of the usual ferrous (Fe2+). Fe3+ cannot bind O2. The remaining functional heme groups in the tetramer have increased O2 affinity (left-shifted curve), so whatever O2 is bound is held too tightly. Result: impaired O2 delivery from multiple directions. Normal methemoglobin level is <1%.

Hereditary methemoglobinemia - two mechanisms:

Cytochrome b5 reductase (CYB5R3) deficiency: Autosomal recessive. NADH + methemoglobin → NAD+ + hemoglobin is the major reduction pathway; without this enzyme, methemoglobin accumulates.
- Type I: enzyme deficiency limited to RBCs (cyanosis only, otherwise well)
- Type II: enzyme deficiency in all cells (progressive neurologic deterioration, usually fatal)
- Methylene blue works for Type I
M hemoglobins (HbM): Autosomal dominant. Amino acid substitutions near the heme pocket that stabilize Fe3+. Examples: HbM-Iwate, HbM-Boston, HbM-Saskatoon, HbM-Hyde Park. Run with HbA on routine electrophoresis - need spectrophotometry. Heterozygotes have ~25-30% methemoglobin, cyanotic but asymptomatic. Do NOT respond to methylene blue.

Cyanosis from hereditary methemoglobinemia appears at 6 months of age (fetal hemoglobin doesn’t form methemoglobin the same way), unless there is M fetal hemoglobin, in which case cyanosis abates at 6 months.

Acquired methemoglobinemia - drug/chemical induced:

Nitrites (amyl nitrite, sodium nitrite - used therapeutically for cyanide poisoning) and nitroglycerin - most common acute cause
Dapsone - most common chronic cause
Sulfonamides, phenacetin, quinones
Local anesthetics (benzocaine, prilocaine) - topical spray before endoscopy is a classic clinical scenario
Methylene chloride (see CO below)

Clinical threshold: Cyanosis appears at 10% methemoglobin (~1.5 g/dL). Important distinction: deoxyhemoglobin causes cyanosis at ~5 g/dL, but methemoglobin causes cyanosis at only ~1.5 g/dL - a much lower threshold. Chocolate-brown blood that does not turn red on O2 exposure is the classic clinical clue. Symptom staircase: headache >20%, dyspnea/fatigue >30%, lethargy >50%, coma/seizures >60%, death >70%.

Detection: Co-oximetry (measures multiple hemoglobin species by spectrophotometry at multiple wavelengths). Pulse oximetry is unreliable for methemoglobinemia (uses only 2 wavelengths) - typically reports a saturation of ~82-86% regardless of actual methemoglobin level. Standard ABG also doesn’t measure it.

Treatment:

Methylene blue - reduces methemoglobin via the NADPH-methemoglobin reductase pathway. Acts as an electron carrier: NADPH (from G6PD) → methylene blue → leucomethylene blue → reduces Fe3+ to Fe2+.
CRITICAL: Methylene blue is contraindicated in G6PD deficiency (no NADPH to reduce it; causes oxidative stress). Also ineffective for M hemoglobins (the Fe3+ is structurally stabilized).
Deliberate induction of methemoglobinemia treats cyanide poisoning. Methemoglobin (Fe3+) binds cyanide more avidly than cytochrome c oxidase. Cyanide antidote kit: amyl nitrite + sodium nitrite (induce methemoglobin) + sodium thiosulfate (converts cyanide to thiocyanate for renal excretion). Hydroxocobalamin directly binds cyanide - newer agent.

Sulfhemoglobin (SHb)

Irreversible oxidation of hemoglobin in the presence of sulfur compounds. Cannot carry O2. Precipitates as Heinz bodies. Has a distinct absorption spectrum - not measured as methemoglobin by co-oximetry. Cyanosis at very low levels (~0.5 g/dL).

Causes: Sulfonamides (sulfasalazine, dapsone) and C. perfringens bacteremia (enterogenous cyanosis - C. perfringens produces hydrogen sulfide in the gut, which is absorbed and reacts with hemoglobin). Sulfhemoglobinemia + Heinz bodies in a sepsis patient = think C. perfringens.

Unlike methemoglobin: Sulfhemoglobin is irreversible (persists for the RBC lifespan). Not reducible by methylene blue.

Carbon Monoxide Poisoning

CO binds hemoglobin with ~240x the affinity of O2, forming carboxyhemoglobin (COHb). Multiple mechanisms of toxicity:

Reduced O2 carrying capacity (occupied hemoglobin)
Left-shifts the O2 dissociation curve (remaining heme groups hold O2 tighter)
Directly inhibits cytochrome oxidase (like cyanide)
==Increases nitric oxide == → vasodilation → hypotension

Normal levels: Nonsmokers 0.4-2% (from endogenous heme breakdown). Smokers 2-6%. Mild elevation in smokers is normal and does not indicate acute poisoning.

Acute poisoning: Symptom staircase - headache/nausea 10-20%, confusion 20-30%, lethargy/syncope 30-40%, seizures/coma >40%, death >60%. Sources: house fires, engine exhaust, indoor heaters and stoves, especially unventilated burning.

Dichloromethane (methylene chloride) trap: Dichloromethane, found in paint strippers and adhesive removers, is metabolized by hepatic CYP2E1 to CO. Causes delayed and prolonged COHb elevation (CO is generated internally over hours). Patient with CO poisoning hours after exposure to paint stripper - think dichloromethane. Treatment: 100% O2 or hyperbaric O2, but response may be slower because CO generation is ongoing.

Immune Hemolytic Anemias

(Covered extensively in Blood Banking section)

Key points for Hematopathology:

Warm AIHA: IgG autoantibodies; spherocytes on smear; DAT positive for IgG
Cold agglutinin disease: IgM autoantibodies; RBC agglutination on smear; DAT positive for C3 only
Drug-induced: Various mechanisms; DAT may show IgG, C3, or both

Warm AIHA - secondary causes:

70% of warm AIHA cases are secondary (primary/idiopathic is ~30%). Causes:

Lymphoma, most commonly CLL/SLL
Autoimmune diseases (SLE, RA) and collagen vascular disease
Thymoma (rare)
Solid organ / stem cell transplantation
Medications: methyldopa (classic), fludarabine, cephalosporins, penicillin

IgG autoantibodies (usually anti-Rh) bind RBCs at 37°C → splenic Fc-receptor-mediated phagocytosis (extravascular hemolysis).

Cold agglutinin disease (cold AIHA):

Cold agglutinins are IgM antibodies, most commonly directed against the I antigen. IgM fixes complement at low temperatures in the extremities → hepatic clearance by C3b receptors on Kupffer cells (extravascular) or direct complement-mediated lysis (intravascular). DAT is positive for C3d only (not IgG).

Cold Agglutinin Syndrome (CAS / idiopathic cold AIHA): Chronic cold AIHA in elderly patients. Monoclonal IgM kappa, often with underlying lymphoplasmacytic lymphoma (Waldenström). Presents with acrocyanosis (bluish discoloration of fingers, toes, ears, nose in cold), Raynaud phenomenon, and chronic hemolytic anemia. Distinguish from secondary cold agglutinins, which are acute, polyclonal IgM, often post-infectious (Mycoplasma pneumoniae, EBV).

CBC pitfalls in cold agglutinin disease: Cold agglutinins cause RBC clumping at room temperature. Automated CBC analyzers (which run at room temp or cooler) count clumps as single large cells → falsely elevated MCV, falsely decreased RBC count, falsely elevated MCHC. Only hemoglobin is reliable (measured after cell lysis). Warming the sample to 37°C before analysis corrects the spurious results.

Evaluating for underlying alloantibodies in cold AIHA: The cold autoantibody interferes with antibody screening and crossmatching. Options:

Prewarmed technique: perform all testing at 37°C to prevent cold reactivity
Cold autoadsorption: patient’s own cells adsorb the cold autoantibody at 4°C, removing it
RESt (Rabbit Erythrocyte Stroma): selectively adsorbs cold autoantibodies with specificity to the “I” or “IH” antigens (IH = co-expression of I and H)

Pr antigen: Rare cold AIHA target. Proteolytic enzyme treatment destroys the Pr antigen - if cold agglutinin activity disappears after enzyme treatment of test cells, the antibody may be anti-Pr. Most cold AIHA targets (anti-I, anti-i) are enzyme-resistant. H and ABO antigens are also enhanced (not destroyed) by enzymes.

Paroxysmal Cold Hemoglobinuria (PCH):

IgG biphasic (Donath-Landsteiner) antibody. Binds and fixes complement at cold temperatures, then causes intravascular hemolysis at 37°C. Most common in post-viral settings in children. Despite the name, causes acute severe hemolysis, not paroxysmal episodes. Peripheral smear can show intraneutrophilic hemophagocytosis (neutrophils engulfing RBCs) - a distinctive finding.

Cryoglobulins vs. cryofibrinogens:

Cryoglobulins are immunoglobulins that precipitate in cold serum (blood collected without anticoagulant, allowed to clot)
Cryofibrinogen is a mixture of fibrin, fibrinogen, factor VIII, and fibronectin that precipitates in cold plasma (blood collected with anticoagulant)
Testing requires proper collection: warm the blood at 37°C, separate, then cool to detect precipitation

Cryoglobulinemia types:

Type I: monoclonal, associated with myeloma/Waldenström
Type II: mixed - monoclonal IgM against polyclonal IgG, strongly associated with HCV. Most common form. Causes vasculitis, glomerulonephritis, neuropathy
Type III: polyclonal, associated with autoimmune disease

Smear finding: Pale purple cloudy aggregates of protein on peripheral smear represent precipitated cryoglobulins.

Cryoglobulinemic glomerulonephritis: Can histologically resemble thrombotic microangiopathy and MPGN type II. Characteristic finding is intraluminal hyaline “thrombi” (actually cryoglobulin deposits) in glomerular capillaries. IF shows IgM, IgG, complement. Wire-loop capillary walls and subendothelial deposits resemble lupus nephritis. EM shows subendothelial immune complex deposits with fibrillary/tubular structure in a fingerprint-like pattern. Always check cryoglobulins and HCV in MPGN.

Hemophagocytic Lymphohistiocytosis (HLH)

Though not strictly an RBC disorder, HLH is a systemic immune dysregulation syndrome that often presents with hemolysis, cytopenias, and marrow hemophagocytosis. The bone marrow findings can be picked up on routine evaluation for anemia.

Mortality: Nearly 100% if untreated. With modern protocols (HLH-94, HLH-2004), reduced to under 10% initially but 5-year survival is ~50%. Familial HLH (genetic defects in perforin, MUNC13-4, syntaxin-11) requires hematopoietic stem cell transplant for cure.

Diagnostic criteria (5 of 8 required):

Fever
Splenomegaly
Bicytopenia
Hypertriglyceridemia and/or hypofibrinogenemia
Hemophagocytosis in marrow, spleen, or lymph node
Low/absent NK activity
Ferritin >500 (often >10,000)
Elevated soluble IL-2R (CD25)

Markers:

Elevated soluble CD25 (soluble IL-2 receptor alpha) - reflects T-cell activation, one of the most sensitive markers
Elevated soluble CD163 - reflects macrophage/histiocyte activation. CD163 is a hemoglobin-haptoglobin scavenger receptor

Hemophagocytosis vs. emperipolesis:

HLH: histiocytes with partially digested hematolymphoid cells (the ingested cells appear degraded/fragmented). Active destruction.
Emperipolesis: lymphocytes passing through histiocytes WITHOUT being destroyed (cells are intact within cytoplasm). Passive process. Seen in Rosai-Dorfman disease.

Hemophagocytosis is not specific for HLH - rare hemophagocytic cells may be seen in normal marrows, and they may be increased in recent transfusion, sepsis, or myelodysplasia. The diagnosis requires the full clinical picture.

Associations: Primary immunodeficiencies - Chédiak-Higashi syndrome (LYST mutation, impaired lysosomal trafficking → giant granules, defective NK/CTL degranulation), Griscelli syndrome (RAB27A), X-linked lymphoproliferative disease (SH2D1A/SAP), familial HLH (PRF1, UNC13D, STX11, STXBP2). Infection triggers: EBV (most common), CMV, HIV. Malignancy-associated (T-cell lymphomas). Macrophage activation syndrome (MAS) in rheumatologic disease (systemic JIA, Still’s disease).

Microangiopathic Hemolytic Anemia (MAHA)

Pathophysiology: Mechanical shearing of RBCs as they pass through abnormal microvasculature (fibrin strands, platelet aggregates, or damaged endothelium).

Hallmark finding: Schistocytes (helmet cells, fragmented RBCs) on peripheral smear

Causes:

TTP: ADAMTS13 deficiency → large vWF multimers → platelet microthrombi
HUS: Typical (STEC: Shiga toxin-producing E. coli O157:H7); Atypical (complement dysregulation)
DIC: Widespread coagulation activation
HELLP syndrome: Hemolysis, Elevated Liver enzymes, Low Platelets in pregnancy
Malignant hypertension
Prosthetic heart valves

Laboratory findings:

Schistocytes (>1% suggests MAHA)
Thrombocytopenia (platelets consumed in microthrombi)
Elevated LDH, indirect bilirubin
Decreased haptoglobin
Negative DAT (not immune-mediated)
Coag studies: Normal in TTP/HUS; abnormal in DIC

27.3 Aplastic Anemia and Bone Marrow Failure

Aplastic Anemia: Pancytopenia with hypocellular bone marrow (fat replacing hematopoietic tissue)

Causes:

Idiopathic (most common) - likely immune-mediated
Drugs: Chloramphenicol, NSAIDs, carbamazepine, benzene
Infections: Parvovirus B19 (if underlying hemolytic anemia), hepatitis (seronegative hepatitis)
Inherited: Fanconi anemia (DNA repair defect, congenital anomalies, cancer predisposition)

Diagnosis:

Pancytopenia
Reticulocytopenia (marrow not responding)
Bone marrow biopsy: <25-30% cellularity with fat spaces

Treatment:

Immunosuppression (ATG + cyclosporine) for older or no matched donor
Bone marrow transplant (curative, best for young patients with matched donor)

27.4 Miscellaneous

Copper Deficiency

Copper deficiency presents with a hematologic picture that mimics B12 deficiency or MDS. Anemia (normocytic > macrocytic > microcytic), severe neutropenia, occasionally pancytopenia, and myeloneuropathies (subacute combined degeneration-like picture).

Mechanism: Copper is a cofactor for ceruloplasmin (ferroxidase), which oxidizes Fe2+ to Fe3+ for transferrin binding. Without ceruloplasmin, iron cannot be mobilized from stores → functional iron-restricted erythropoiesis. Copper is also required for cytochrome c oxidase in mitochondria.

Most common cause: Zinc supplementation (zinc induces metallothionein in enterocytes, which preferentially binds copper, blocking absorption). Sources of excess zinc: denture fixatives, therapeutic formulations, dietary supplements. Other causes: gastric bypass, malabsorption, prolonged parenteral/enteral nutrition without copper.

Peripheral smear: Dimorphic RBCs.

Bone marrow findings (can mimic MDS):

Hypercellular marrow with erythroid hyperplasia (can also be hypocellular or normocellular)
Vacuolated erythroid and myeloid precursors (particularly basophilic normoblasts/pronormoblasts and promyelocytes/myelocytes) - similar to alcohol or chloramphenicol toxicity
Ring sideroblasts
Megakaryocytic hyperplasia and left-shifted myelopoiesis
Sometimes increased lymphocytes with hematogone hyperplasia

Key clue: Concurrent neutropenia + anemia in a patient with zinc supplementation or prior gastric surgery. Check serum copper and ceruloplasmin. Treatment with copper replacement rapidly corrects the cytopenias.

Russell Bodies and Mott Cells

Russell bodies are eosinophilic cytoplasmic inclusions in plasma cells, consisting of condensed, non-secreted immunoglobulin trapped in dilated rough ER. Large, round, refractile globules on Wright stain. Dutcher bodies are similar but intranuclear (pseudo-inclusions from invaginated cytoplasm). Mott cells are plasma cells filled with multiple Russell bodies (grape-like appearance).

Seen in: reactive plasmacytosis, multiple myeloma, lymphoplasmacytic lymphoma. There is also an entity called Russell body gastritis, with extensive Mott cells in the antral mucosa lamina propria.

Hematogones

Normal B-lymphocyte precursors in the bone marrow. Most abundant in children (up to 5-10% of marrow cells), decrease with age. Important for flow cytometric distinction from B-ALL.

Flow cytometry pattern: Hematogones form a continuous maturation spectrum from CD34+/CD10+/CD20- (most immature) through CD10+/CD20 dim to CD10-/CD20+ (most mature), creating a characteristic arc or rainbow pattern. Polyclonal (no light chain restriction).

Maturation sequence:

Earliest hematogones: CD19+, CD34+, CD10 bright, CD9 bright, CD38 bright, CD20-, CD45 dim
As they mature: gain CD20, then lose CD10, then lose CD34, then lose CD9
CD45 intensity increases; CD38 intensity decreases

B-ALL vs. hematogones: B lymphoblasts in ALL show a fixed, aberrant phenotype rather than a continuous maturation spectrum. Aberrancies: over-expression (CD10, CD34, CD58), under-expression (CD38, CD45), non-typical co-expression (CD20 + CD34), aberrant expression of non-B markers (CD13, CD33). The maturation continuum in hematogones is the feature that separates reactive from neoplastic.

Chapter 28: White Blood Cell Disorders

This chapter covers the benign side of white blood cell pathology: too few neutrophils, too many, inherited disorders of neutrophil function or egress, and the reactive lineage shifts (eosinophilia, basophilia, monocytosis, lymphocytosis). The malignant counterparts live in the acute leukemia, myeloproliferative neoplasm, and lymphoma chapters. What lives here is what you need to recognize on a CBC and peripheral smear when the question is “is this reactive, congenital, or something clonal.”

28.1 Neutrophil Disorders

Neutropenia

Definition: ANC <1500/μL

Causes:

Drugs (most common): Chemotherapy, clozapine, carbamazepine, methimazole
Infections: Viral (HIV, EBV), bacterial (typhoid, TB)
Autoimmune: Lupus, Felty syndrome (RA + splenomegaly + neutropenia)
Nutritional: B12, folate, copper deficiency
Primary marrow disorders: MDS, leukemia, aplastic anemia
Cyclic neutropenia: Regular 21-day cycles of neutropenia
Severe congenital neutropenia (Kostmann syndrome)

Risk of infection: Increases as ANC falls below 500/μL. Below 500 is “severe”; below 100 is “profound” and patients are at real risk for sepsis from their own flora.

Neutrophilia

Definition: ANC >7700/μL

Causes:

Infection: Bacterial most common cause
Inflammation: Rheumatic diseases, IBD, vasculitis
Stress/steroids: Demargination of neutrophils
Drugs: G-CSF, lithium
Neoplastic: CML, other myeloproliferative neoplasms

Left shift: Increased band forms (>10%) and/or immature forms (metamyelocytes, myelocytes) indicating accelerated release from marrow

Leukemoid reaction: WBC >50,000/μL with reactive cause (infection, severe hemolysis, marrow recovery). Distinguished from CML by:

High LAP score (leukocyte alkaline phosphatase) - elevated in reactive conditions, low in CML
Absence of BCR-ABL1 fusion
Toxic granulation, Döhle bodies in reactive conditions

Congenital Neutropenia - Overview

Congenital neutropenia is a spectrum. Some causes stay mild and never cause real trouble; others are severe with near-absent neutrophils from birth and lifelong infection risk. Severity of congenital neutropenia is variable (mild to severe) depending on the underlying cause or syndrome, and severity of neutropenia correlates directly with infection risk. So the first question with any inherited neutropenia is always “what is the ANC nadir?”

Severe neutropenia (ANC <1000, often <500) typically presents with oral ulcers, fever, cervical lymphadenopathy, gingivitis, sinusitis, and pharyngitis - the organisms that colonize mucosal surfaces take advantage the moment neutrophil defense drops. Mild congenital neutropenia (constitutional, benign ethnic) does not carry that infection risk.

Neutropenia is a component of several larger syndromes worth keeping straight:

WHIM syndrome / myelokathexis (discussed below)
Chédiak-Higashi syndrome (neutropenia is mild but function is terrible)
Shwachman-Diamond syndrome (marrow failure + pancreatic insufficiency + skeletal findings)
Fanconi anemia (bone marrow failure syndrome, covered in the aplastic anemia chapter)
Dyskeratosis congenita (telomere disorder, marrow failure)
Cyclic neutropenia (same ELANE gene as Kostmann, different phenotype - see below)

Duffy-Null Associated Neutrophil Count (Constitutional Neutropenia)

Mild chronic neutropenia, usually ANC >1000 (sometimes lower), particularly common in patients of Mediterranean, African, Middle Eastern, or Caribbean descent. Caused by the Duffy-null phenotype (ACKR1/DARC gene variant) - the same variant that confers resistance to Plasmodium vivax.

Not associated with increased infection risk despite the “low” ANC. No treatment needed. The point on boards: recognize this so you do not start a fruitless workup or inappropriately withhold chemotherapy from a patient with a Duffy-null phenotype whose baseline ANC is 1200.

Severe Congenital Neutropenia (Kostmann Syndrome)

Pathophysiology: Most commonly ELA2 (ELANE) mutation on 19p encoding neutrophil elastase. Mutant elastase triggers the unfolded protein response in promyelocytes, driving apoptosis before cells can mature. Marrow shows a maturation arrest at the promyelocyte / myelocyte stage.

Clinical features:

Marked chronic neutropenia / agranulocytosis, ANC persistently <500
Presents in the neonatal period or early childhood, classically with omphalitis (infection of the umbilical stump) in newborns
Severe bacterial infections: skin abscesses, oral ulcers, pneumonia, peritonitis, sepsis
Distinguished from cyclic neutropenia (which also involves ELANE mutations but with periodic rather than constant neutropenia)

Complications and surveillance:

~20% cumulative risk of MDS/AML at 10-15 years on chronic G-CSF therapy - severe congenital neutropenia confers increased risk of acute myeloid leukemia
Acquired CSF3R (G-CSF receptor) mutations precede transformation and serve as a molecular warning - these truncate the cytoplasmic domain, impair receptor internalization, and cause constitutive signaling
Uncommonly progresses to aplastic anemia
Annual marrow surveillance with cytogenetics is standard
The leukemia risk is baked into the disease; G-CSF does not cause it (the G-CSF just unmasks the natural history by letting patients live long enough)

Treatment: Chronic G-CSF; hematopoietic stem cell transplant is curative and considered for refractory disease or evidence of clonal evolution.

Cyclic Neutropenia

Same gene family as Kostmann (ELANE mutations), very different clinical course. Neutropenia comes in regular 21-day cycles - ANC drops to near-zero for 3-5 days, then recovers. Patients are symptomatic only during the nadir: aphthous ulcers, fevers, and occasional bacterial infections that track the cycle.

Diagnosis requires serial CBCs (two or three times weekly for 6-8 weeks) to capture the oscillation. Management is G-CSF during nadirs; leukemia risk is much lower than in severe congenital neutropenia, probably because the pool of surviving progenitors never collapses the way it does in Kostmann.

Myelokathexis and WHIM Syndrome

Myelokathexis - Greek for “neutrophil retention (kathexis) in bone marrow (myelo).” The marrow is hypercellular and packed with mature-appearing neutrophils, but the periphery is neutropenic because those cells can’t egress.

Morphology - this is the board buzzword: hypersegmented pyknotic nuclei with fine chromatin filaments connecting lobes, reduced BCL-X expression, and abnormal neutrophil function. The cells are essentially apoptotic in the marrow.

WHIM syndrome packages myelokathexis with three other features:

Warts (extensive, HPV-related, recalcitrant)
Hypogammaglobulinemia
Infections (bacterial, recurrent)
Myelokathexis

Pathophysiology: Gain-of-function mutations in CXCR4 (chemokine receptor). The mutant CXCR4 signals too strongly in response to SDF-1/CXCL12 produced by marrow stroma, so neutrophils and lymphocytes get held in the marrow instead of being released. The hypogammaglobulinemia follows from impaired B cell egress and maturation.

Treatment: G-CSF and IVIG are the traditional approach. Plerixafor (CXCR4 antagonist, also used for stem cell mobilization) is the mechanism-targeted therapy.

Mycophenolate mimic: Mycophenolate mofetil can cause neutrophil morphology that looks like myelokathexis - hypersegmented nuclei with thin chromatin bridges and pyknotic changes. This is a drug effect, not a congenital disorder. Always check the med list before you commit the patient to a WHIM workup.

Shwachman-Diamond Syndrome (SDS)

Genetics: Autosomal recessive, SBDS gene mutations (7q11). The SBDS protein is involved in ribosome maturation (60S subunit joining) - so SDS joins Diamond-Blackfan anemia and dyskeratosis congenita in the “ribosomopathies.”

Classic triad:

Exocrine pancreatic insufficiency with fatty (lipomatous) replacement of pancreatic parenchyma - this is the second most common cause of inherited pancreatic insufficiency after cystic fibrosis
Skeletal abnormalities, classically metaphyseal dysostosis
Bone marrow failure - neutropenia is most common and can progress to pancytopenia or aplastic anemia

Associations:

HLA-DR4 positivity is often seen (significance unclear, possibly related to immune dysregulation). Also a feature of large granular lymphocyte leukemia - a useful mnemonic pairing.
~30% lifetime risk of MDS/AML

Diagnosis: Genetic testing for SBDS mutations, low serum trypsinogen and pancreatic isoamylase (markers of pancreatic insufficiency), and X-ray findings. Pancreatic function can actually improve with age in some patients; the marrow failure does not.

Inherited Neutrophil Function Disorders

Chronic Granulomatous Disease (CGD)

Pathophysiology: Defective NADPH oxidase cannot generate superoxide for respiratory burst. Neutrophils phagocytose organisms but cannot kill them. The catalase story: catalase-positive organisms destroy their own hydrogen peroxide, so they do not supply the neutrophil with exogenous H2O2 that could substitute for absent endogenous superoxide. Catalase-negative organisms effectively help kill themselves, which is why they are not classic CGD pathogens.

Genetics:

X-linked (most common, ~65%) - CYBB gene, gp91phox subunit
Autosomal recessive - p47phox (NCF1), p67phox (NCF2), p22phox (CYBA), p40phox (NCF4)

Infections: Recurrent severe infections with catalase-positive organisms. Classic organisms:

Staphylococcus aureus
Aspergillus (most common cause of death)
Serratia marcescens
Nocardia
Burkholderia cepacia

Granulomas form in response to persistent intracellular organisms and can obstruct hollow viscera - granulomatous colitis mimicking Crohn disease, bladder granulomas, hepatic abscesses.

Diagnosis:

Dihydrorhodamine (DHR) flow cytometry - current standard, most sensitive. DHR123 is non-fluorescent; reactive oxygen species oxidize it to rhodamine 123, which fluoresces. In CGD, no fluorescence shift after stimulation (PMA). X-linked carrier females show a bimodal pattern (lyonization).
Nitroblue tetrazolium (NBT) test - historical. NBT turns blue when reduced by respiratory burst. No color change in CGD.

Treatment: Prophylactic TMP-SMX, itraconazole, interferon-gamma. HSCT is curative.

Leukocyte Adhesion Deficiency (LAD)

Pathophysiology: Neutrophils can’t stick to endothelium and can’t extravasate. Three subtypes, each breaking a different step of the adhesion cascade:

LAD-I - most common. CD18 (β2 integrin, ITGB2) deficiency. CD18 is the shared beta chain for LFA-1, Mac-1, p150/95. Breaks firm adhesion.
LAD-II - defect in fucosyl transferase (SLC35C1), so sialyl Lewis X on neutrophils is absent. Breaks selectin-mediated rolling. Also presents with developmental delay and the Bombay-like blood group phenotype (no H antigen fucosylation).
LAD-III - mutation in kindlin-3 (FERMT3). Integrins are present but cannot be activated (“inside-out signaling” fails). Phenocopies LAD-I plus a Glanzmann-like bleeding disorder (platelet integrin alpha-IIb/beta-3 also fails to activate).

Clinical features:

Delayed umbilical cord separation (>30 days) - classic buzzword (normal is ~7-14 days)
Recurrent bacterial infections without pus (neutrophils can’t get to tissue) - absent pus despite bacterial infection is a board giveaway
Markedly elevated WBC count (neutrophils trapped in circulation, often >25,000-100,000 at baseline)
Poor wound healing, severe periodontitis

Diagnosis: Flow cytometry for CD18 / CD11 expression (absent or severely reduced in LAD-I). Genetic testing for LAD-II and LAD-III.

Chédiak-Higashi Syndrome

Pathophysiology: Autosomal recessive, LYST gene mutation. LYST encodes a lysosomal trafficking regulator; without it, lysosomes and related granules fuse abnormally. The result: giant azurophilic granules in neutrophils, lymphocytes, monocytes, and melanocytes.

The neutropenia is mild but function is the real problem. Patients aren’t dangerously neutropenic - they’re dangerously dysfunctional. The giant granules can’t be released or processed normally, so neutrophils have:

Impaired chemotaxis
Defective degranulation
Reduced bactericidal activity

This combination produces recurrent pyogenic infections despite a near-normal neutrophil count.

Findings:

Giant granules in neutrophils, lymphocytes, and monocytes on peripheral smear (pathognomonic)
Partial oculocutaneous albinism (same trafficking defect in melanocytes)
Recurrent pyogenic infections (especially S. aureus, Streptococcus)
Peripheral neuropathy
Accelerated phase: HLH-like lymphohistiocytic infiltration of organs - the usual cause of death without transplant

Treatment: Hematopoietic stem cell transplant for the immune phenotype; it does not fix the neurologic disease or albinism.

28.2 Morphologic Abnormalities

Abnormality	Appearance	Associations
Toxic granulation	Dark azurophilic granules	Infection, inflammation, G-CSF
Döhle bodies	Blue cytoplasmic inclusions (rough ER)	Infection, inflammation, May-Hegglin
Hypersegmented neutrophils	≥5 lobes	Megaloblastic anemia (B12, folate), MDS, uremia
Pelger-Huët anomaly	Bilobed nuclei (“pince-nez”)	Benign inherited (LBR gene) or pseudo-PH (see below)
Myelokathexis-like	Pyknotic hypersegmented nuclei with thin chromatin bridges	WHIM syndrome, mycophenolate
Chédiak-Higashi granules	Giant azurophilic granules	Chédiak-Higashi syndrome
Auer rods	Pink rod-shaped inclusions	AML (especially APL)

Pelger-Huët vs Pseudo-Pelger-Huët

The congenital form is benign: heterozygous LBR (lamin B receptor) mutation, bilobed (“pince-nez” / “aviator goggles”) neutrophils on smear, no functional defect. Pseudo-Pelger-Huët is the acquired version - hyposegmented neutrophils seen in MDS, post-chemotherapy, and occasionally with drugs (mycophenolate, tacrolimus, valproate). In MDS it’s a dyspoiesis marker, not a benign anomaly.

Toxic Changes - what they actually mean

Toxic granulation, Döhle bodies, and cytoplasmic vacuolization appear together in severe bacterial infection, sepsis, and anytime the marrow is pushing out neutrophils fast. They are not specific to infection - G-CSF therapy produces the same picture by accelerating granulopoiesis. Döhle bodies are aggregates of rough endoplasmic reticulum, light blue, peripherally located in the cytoplasm. They are also a classic feature of May-Hegglin anomaly (MYH9 disorder with macrothrombocytopenia).

28.3 Eosinophilia

Definition: Absolute eosinophil count >500/μL. Mild 500-1500, moderate 1500-5000, severe >5000.

Causes (mnemonic NAACP):

Neoplastic: Hodgkin lymphoma, T-cell lymphomas, mastocytosis, CML (paraneoplastic)
Allergic: Asthma, hay fever, drug reactions (DRESS, eosinophilic pneumonia)
Addison disease (adrenal insufficiency - lost cortisol suppression of eosinophils)
Collagen vascular disease (EGPA / Churg-Strauss classically)
Parasites: Tissue-invasive helminths (not protozoa, not Giardia, not pinworms)

Drug reactions worth remembering: DRESS syndrome (drug rash with eosinophilia and systemic symptoms) - aromatic anticonvulsants (phenytoin, carbamazepine), allopurinol, sulfonamides, minocycline. Look for fever, rash, LAD, liver involvement, and high eosinophils 2-8 weeks after drug start.

Hypereosinophilic Syndrome (HES):

Eosinophils >1500/μL for >6 months
End-organ damage (endomyocardial fibrosis (Löffler endocarditis), neuropathy, skin, GI)
Exclude secondary causes
Myeloid HES: FIP1L1-PDGFRA fusion (cryptic del 4q12) - responds beautifully to imatinib. Other kinase fusions: PDGFRB, FGFR1, JAK2.
Lymphoid HES: clonal T cell population producing IL-5; treat underlying clone.

Mepolizumab (anti-IL-5) is the go-to biologic for idiopathic HES without a targetable fusion.

28.4 Basophilia

Definition: Absolute basophil count >200/μL (rare)

Causes:

CML: Basophilia is characteristic
Allergic reactions
Myeloproliferative neoplasms
Ulcerative colitis, hypothyroidism

28.5 Monocytosis

Definition: Absolute monocyte count >800/μL (some use >1000)

Causes:

Chronic infections (TB, endocarditis, brucellosis, syphilis)
Autoimmune / inflammatory diseases (SLE, IBD, sarcoidosis)
Recovery from neutropenia (monocyte recovery precedes neutrophil recovery, good prognostic sign after chemo)
CMML (Chronic Myelomonocytic Leukemia) - sustained monocytosis >1000 and >10% of WBC, with dysplasia

GATA2 deficiency (MonoMAC syndrome) is worth recognizing on boards: young adult with monocytopenia (not monocytosis), disseminated NTM infections, warts, and high risk of MDS/AML. It’s the counterpoint to monocytosis and came up in the heme-onc testing literature.

28.6 Lymphocytosis

Definition: Absolute lymphocyte count >4000/μL in adults (higher cutoffs in children). Values >20,000 should raise concern for a clonal process.

Reactive lymphocytosis:

Viral infections (EBV, CMV, HIV acute seroconversion, hepatitis, adenovirus)
Pertussis (Bordetella pertussis pertussis toxin ADP-ribosylates Gi, blocking lymphocyte egress from blood back into lymphoid tissue - so lymphocytes pile up in circulation)
Toxoplasmosis
Stress / post-splenectomy / post-trauma (transient)

Reactive (atypical / Downey) lymphocytes: Large activated lymphocytes with abundant basophilic cytoplasm, often indented by surrounding RBCs, and eccentric nucleus with coarse chromatin. These are activated CD8+ T cells responding to virus-infected B cells. Classic in infectious mononucleosis (EBV) but seen in any viral illness. Not clonal, not atypical in the malignant sense.

Clonal lymphocytosis:

CLL/SLL (smudge cells on smear, mature-appearing small lymphocytes)
Monoclonal B cell lymphocytosis (MBL) - clonal B cells <5000/μL without other CLL criteria; precursor to CLL
Other lymphoproliferative disorders (mantle cell, hairy cell, follicular in leukemic phase)
Large granular lymphocyte (LGL) leukemia - see below

The workup when you can’t tell reactive from clonal: peripheral flow cytometry. A monotypic B cell population (kappa or lambda restricted) or a clonal T cell population (TCR rearrangement, aberrant CD4/CD8 ratios, loss of pan-T markers) favors malignancy.

Large Granular Lymphocyte (LGL) Leukemia

Worth a callout because it connects to a couple of earlier threads. LGL leukemia is a clonal proliferation of cytotoxic T cells (CD3+ CD8+ CD57+) or less commonly NK cells. On smear: lymphocytes with abundant cytoplasm containing azurophilic granules.

Associated with rheumatoid arthritis and HLA-DR4 (the same HLA association as Shwachman-Diamond - useful pairing)
Presents with neutropenia (often severe), anemia, splenomegaly - clinically mimics Felty syndrome
STAT3 mutations common
Indolent; treat with methotrexate, cyclosporine, or cyclophosphamide if cytopenias are symptomatic

Chapter 29: Platelet and Coagulation Disorders

Hemostasis happens in three phases, and the way you organize your thinking about bleeding should follow the same structure. Primary hemostasis is the platelet plug: vWF grabs exposed collagen, platelets adhere via GPIb, release ADP and thromboxane, and clump together via GPIIb/IIIa-fibrinogen bridges. Secondary hemostasis is the coagulation cascade that reinforces the plug with crosslinked fibrin. Tertiary hemostasis (fibrinolysis) eventually dissolves the clot once the wound is repaired. Every bleeding disorder in this chapter falls somewhere in this framework, and the clinical presentation usually tells you which phase is broken: mucocutaneous bleeding (petechiae, epistaxis, menorrhagia) means primary hemostasis. Deep tissue bleeding (hemarthrosis, muscle hematomas, delayed bleeding after surgery) means secondary hemostasis.

29.1 The Vessel, the Platelet, and Primary Hemostasis

The Vascular Response

When a vessel is injured, the first thing that happens is vasoconstriction. Damaged endothelial cells release endothelin, the most potent endogenous vasoconstrictor known, which reduces blood flow to the injured area. Healthy endothelium normally produces the opposite - NO and prostacyclin (PGI2) - to keep platelets quiet and blood flowing.

Exposed subendothelial collagen and tissue factor then kick off the rest of hemostasis. Vascular disorders that weaken this interface (Ehlers-Danlos, scurvy, hereditary hemorrhagic telangiectasia, amyloid angiopathy) present like platelet disorders - mucocutaneous bleeding with normal PT, PTT, and platelet count.

von Willebrand Factor and Platelet Adhesion

vWF is the bridge between collagen and platelets. Under high shear (arterioles, capillaries), direct platelet-collagen binding via GPIa/IIa and GPVI is not enough - vWF uncoils from its globular form, binds exposed subendothelial collagen, and captures flowing platelets via GPIb. In low-shear veins, fibrinogen can mediate adhesion directly.

vWF is made in endothelial cells (Weibel-Palade bodies, ~85%) and platelet alpha granules (~15%). It’s synthesized as a monomer, dimerizes and is stored, then multimerizes to the ultra-large form. When released, ADAMTS13 cleaves the ultra-large multimers down to the plasma form. vWF also carries and stabilizes Factor VIII - without vWF, FVIII half-life drops from ~12 hours to ~2 hours. This is why severe vWD patients have low FVIII activity and a prolonged PTT.

DDAVP (desmopressin) works in Type 1 vWD by releasing stored vWF from Weibel-Palade bodies - but note tachyphylaxis occurs with repeated dosing because stores deplete.

Platelet Receptors: Know These

GPIb binds vWF - deficiency = Bernard-Soulier (adhesion defect). GPIb = CD42b
GPIIb/IIIa binds fibrinogen - deficiency = Glanzmann (aggregation defect). GPIIb/IIIa = CD41/CD61
GPIa/IIa binds collagen (“la-lla for co-lla-gen”)
GPIc/IIa binds fibronectin (“fibron-IC-tin”)
P2Y12 binds ADP - target of clopidogrel. ADP-P2Y12 signaling exposes the high-affinity conformation of GPIIb/IIIa
Fc-gamma-RIIa (CD32) - binds immune complexes, key receptor in HIT platelet activation

The PLA-1 (HPA-1a) antigen sits on GPIIIa (CD61). This is the antigen behind most cases of neonatal alloimmune thrombocytopenia in individuals of European ancestry. Glanzmann patients lack GPIIb/IIIa entirely, so they are PLA-1 negative and can develop iso-antibodies after platelet transfusion, making future transfusions ineffective.

Platelet Granules

Dense (delta) granules hold small non-protein molecules: ADP, ATP, Ca2+, serotonin, polyphosphates. These amplify platelet activation. ATP release is measured by lumiaggregometry (luciferin-luciferase bioluminescence). Serotonin release is the readout in the serotonin release assay for HIT.

Alpha granules hold proteins: vWF, fibrinogen, fibronectin, P-selectin (CD62P), PDGF, PF4, thrombospondin, Factor V. (“alFa granules have molecules with F.”) P-selectin expression on the platelet surface after activation is a marker of activation on flow cytometry. All alpha granule disorders show loss of CD62P upregulation after thrombin stimulation and low PF4 by ELISA.

The Activation Cascade and Weak Platelet Plug

After GPIb-mediated adhesion, platelets activate and release Ca2+, ADP, and TXA2. This is a positive feedback loop - each activated platelet recruits more. Aspirin blocks this by irreversibly acetylating COX-1, preventing TXA2 synthesis.

ADP binding to P2Y12 exposes GPIIb/IIIa in its high-affinity conformation, which then binds fibrinogen and bridges adjacent platelets. This is the weak platelet plug, which then requires fibrin reinforcement and Factor XIII crosslinking to become stable.

Platelet Count Basics

As platelet count increases, mean platelet volume decreases. TPO regulates both: when platelets drop from consumption, TPO rises and drives megakaryocytes to make larger, hemostatically more active platelets. High MPV + low platelets = peripheral destruction (ITP, DIC, TTP, HIT). Normal/low MPV + low platelets = marrow failure.

Platelets express HLA class I (not class II). HLA antibodies are the most common immune cause of platelet transfusion refractoriness. Leukoreduction reduces HLA alloimmunization. Refractoriness is assessed by the corrected count increment (CCI) from pre- and 1-hour post-transfusion counts: CCI <7500 on two consecutive transfusions = refractoriness.

29.2 Thrombocytopenia

Definition: Platelet count <150,000/μL

Causes by Mechanism

Decreased Production:

Bone marrow failure (aplastic anemia, MDS, leukemia)
Marrow infiltration (metastatic cancer, fibrosis)
Nutritional deficiency (B12, folate)
Drugs (chemotherapy, alcohol)
Viral infections (HIV, hepatitis C)
Congenital syndromes (TAR, Fanconi, Wiskott-Aldrich)

Increased Destruction:

Immune:

Immune Thrombocytopenic Purpura (ITP): Autoantibodies to platelet glycoproteins (usually GPIIb/IIIa or GPIb/IX). Isolated thrombocytopenia; diagnosis of exclusion.
Drug-induced: Heparin (HIT), quinine, sulfonamides, vancomycin
Post-transfusion purpura: Anti-HPA antibodies
Neonatal alloimmune thrombocytopenia (NAIT): Maternal anti-HPA (usually HPA-1a) crossing placenta

Non-immune:

TTP, HUS (microangiopathic consumption)
DIC (widespread coagulation consuming platelets)
Sepsis

Sequestration:

Splenomegaly (hypersplenism): Spleen pools up to 90% of platelets (normal 30%)

Dilutional:

Massive transfusion

Pseudothrombocytopenia

Spuriously low platelet count caused by EDTA-dependent platelet clumping. EDTA exposes a cryptic epitope on GPIIb/IIIa, causing autoantibodies to crosslink platelets into clumps. The automated analyzer mistakes clumps for large cells → falsely low platelet count.

Diagnosis: Review peripheral smear for platelet clumps at feathered edge. Solution: Redraw in citrate (blue-top) tube; multiply result by 1.1 to correct for citrate dilution. Alternatively, use immediate analysis or EDTA-alternative anticoagulant. Prevalence: ~0.1% of EDTA specimens.

TAR Syndrome (Thrombocytopenia with Absent Radii)

Bilateral absent radii + thrombocytopenia + present thumbs = TAR. Contrast with Fanconi anemia: absent radii + absent thumbs + pancytopenia = Fanconi. TAR is autosomal recessive, thrombocytopenia is worst in infancy and improves with age. Also associated with cow’s milk allergy and cardiac defects.

Neonatal Alloimmune Thrombocytopenia (NAIT)

Platelet equivalent of HDFN. Maternal antibodies (usually anti-HPA-1a in individuals of European ancestry, anti-HPA-4b in East Asian populations) cross the placenta and destroy fetal platelets. Unlike HDFN, NAIT can occur in the first pregnancy because HPA antigens are expressed on other fetal tissues and can sensitize mothers earlier.

Management:

Platelet count >30,000 and stable: IVIG alone (blocks Fc receptors on splenic macrophages)
Platelet count <30,000 or bleeding: platelet transfusion. Ideally HPA-compatible (antigen-negative). If unavailable, use random donor platelets plus IVIG, or washed/irradiated maternal platelets
Subsequent pregnancies: IVIG weekly starting at 20-28 weeks, add corticosteroids if needed, cesarean delivery to avoid ICH from head compression

Recurrence risk in subsequent pregnancies is >90% and severity often worsens.

Heparin-Induced Thrombocytopenia (HIT)

Two types:

Type I: Non-immune, mild decrease (100-150K), within 1-2 days, self-resolves even with continued heparin, clinically insignificant. Caused by direct physicochemical heparin-platelet interaction.
Type II: Immune-mediated, serious. This is what we mean by “HIT.”

Pathophysiology of Type II HIT:

Heparin binds to platelet factor 4 (PF4) released from platelet alpha granules
Conformational change exposes neoepitope
IgG antibodies form against PF4-heparin complex
Immune complexes bind platelet Fc-gamma-RIIa (CD32) receptors
Platelet activation → microparticle release, thrombin generation, endothelial activation → thrombosis + platelet consumption

The paradox: Despite thrombocytopenia, HIT is a hypercoagulable state. Thrombosis (venous > arterial) not bleeding. 30-75% of untreated HIT II patients develop thrombosis.

Timing: Typically days 5-10 after first heparin exposure. Rapid-onset (within hours) if prior heparin within past 100 days. Delayed-onset (1-3 weeks after stopping heparin) is rare.

Clinical features:

Platelet drop >50% from baseline, nadir typically 20,000-80,000
Thrombosis (DVT, PE, arterial thrombosis, skin necrosis at injection sites)
Venous limb gangrene if warfarin started too early

Risk factors:

Unfractionated heparin >> LMWH >> fondaparinux for causing HIT
Surgical patients > medical patients (3-5x risk)
Female > male (2x risk)
Higher doses, longer exposure

Diagnosis - The 4T Score (Warkentin):

Thrombocytopenia severity/% drop
Timing (typical 5-10 days)
Thrombosis (new or skin necrosis)
oTher causes excluded

Score 0-3 = low probability (<1% HIT). 4-5 = intermediate. 6-8 = high. Clinical gestalt still matters.

Testing:

PF4/heparin ELISA (screening): Sensitive (~97%) but low specificity. Detects IgG, IgA, IgM antibodies against PF4-heparin complexes. High OD (>2.0) increases specificity. Negative ELISA effectively rules out HIT.
Serotonin Release Assay (SRA) - gold standard confirmation: Donor platelets loaded with 14C-serotonin + patient serum + heparin. Low-dose heparin (0.1 U/mL) positive AND high-dose heparin (100 U/mL) negative = positive SRA. If both doses positive, think HLA antibodies (non-HIT) rather than HIT.

Management:

Stop ALL heparin (including flushes, heparin-coated catheters)
Switch to a direct thrombin inhibitor (argatroban - hepatically cleared, drug of choice in renal failure; bivalirudin - used in PCI/cardiac surgery) or fondaparinux (off-label, no cross-reactivity)
Do NOT use LMWH (>90% in vitro cross-reactivity with HIT antibodies)
Do NOT give warfarin until platelets recover >150,000 (risk of venous limb gangrene / warfarin-induced skin necrosis from early Protein C depletion)
Do NOT transfuse platelets unless life-threatening bleeding (fuels thrombosis)

Future heparin is contraindicated. If heparin is required (cardiopulmonary bypass), plasma exchange can remove anti-PF4-heparin antibodies first, or bivalirudin can substitute during bypass. Antibodies typically disappear by ~100 days, but brief surgical re-exposure is generally feasible once SRA is negative.

Thrombotic Thrombocytopenic Purpura (TTP)

TTP is a medical emergency with >90% mortality if untreated, reduced to <10-20% with prompt plasma exchange. Understanding the molecular pathophysiology explains why treatment works.

Pathophysiology - The ADAMTS13 Story:

ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motif, member 13) is a plasma enzyme that cleaves von Willebrand factor (vWF) at the Tyr1605-Met1606 bond in the A2 domain when vWF is unfolded under shear stress.

Why this matters:

Both platelets and endothelial cells produce ultra-large vWF (ULvWF) multimers - much larger and more adhesive than the vWF that normally circulates
ULvWF multimers are anchored to endothelial cells and extend into flowing blood like streamers
Under shear stress (in arterioles and capillaries), these multimers bind platelets via GPIb, causing spontaneous platelet adhesion and aggregation
Normally, ADAMTS13 rapidly cleaves these ULvWF multimers, releasing them from endothelium
When ADAMTS13 is absent or inhibited, ULvWF accumulates, causing widespread platelet microthrombi in small vessels
These microthrombi shear red blood cells mechanically, producing schistocytes
End-organ ischemia (brain, kidney, heart, GI tract) results from microvascular occlusion

The shear-dependence is why TTP thrombi occur in microvasculature (high shear), not large vessels.

Three forms:

Idiopathic/acquired (~95%): IgG autoantibodies against ADAMTS13
Secondary: Pregnancy (especially 3rd trimester/postpartum), stem cell transplant, medications (ticlopidine, clopidogrel, quinine, cyclosporine, tacrolimus, mitomycin C), SLE, HIV. ADAMTS13 may not be severely deficient (>10%)
Familial (Upshaw-Schulman syndrome): Homozygous/compound heterozygous ADAMTS13 mutations, autosomal recessive

The Classic Pentad - FAT RN (present in <10% of patients at diagnosis - don’t wait for all five):

Fever: Low-grade; high fever suggests sepsis
Anemia (MAHA): Schistocytes, elevated LDH, low haptoglobin, elevated indirect bilirubin, negative DAT
Thrombocytopenia: Often severe (<20,000/μL), median ~15,000 at presentation
Renal dysfunction: Usually mild (creatinine ~1.5-2.0); severe renal failure suggests HUS instead
Neurologic abnormalities: Fluctuating and varied - headache, confusion, visual disturbances, transient focal deficits, seizures, stroke

Clinical Clue: MAHA + thrombocytopenia + normal PT/aPTT should immediately suggest TTP (or other thrombotic microangiopathy).

Laboratory Findings:

Peripheral smear: Schistocytes - essential finding
Platelet count: Severely decreased
LDH: Markedly elevated (from hemolysis AND tissue ischemia)
Haptoglobin: Low or absent
PT/aPTT: Normal (distinguishes from DIC)
ADAMTS13 activity <10% with inhibitor detected - diagnostic for acquired TTP
Note: ADAMTS13 testing takes days; don’t wait to initiate treatment

PLASMIC Score: Predicts severely deficient ADAMTS13 before testing returns. Scores: Platelet <30, Lysis (reticulocytes >2.5%, undetectable haptoglobin, indirect bilirubin >2), Absence of active cancer, absence of Stem cell/organ transplant, MCV <90, INR <1.5, Creatinine <2. Score 6-7 = high probability, start PLEX empirically.

Differential Diagnosis of Thrombotic Microangiopathies:

Feature	TTP	HUS	DIC	HELLP
ADAMTS13	<10%	Normal	Normal	Normal
Renal failure	Mild	Severe	Variable	Moderate
Diarrhea prodrome	No	Yes (typical HUS)	No	No
PT/aPTT	Normal	Normal	Prolonged	Normal
Pregnancy	Sometimes	Rare	Variable	Yes

Treatment - Why Plasma Exchange Works:

Removes: Autoantibodies against ADAMTS13 and accumulated ULvWF multimers
Provides: Functional ADAMTS13 in replacement plasma

Treatment Protocol:

Plasma exchange (TPE): Begin immediately - don’t wait for ADAMTS13 results. Exchange 1-1.5 plasma volumes daily using FFP or cryopoor plasma. Continue until platelet count >150,000 for 2 consecutive days and LDH normalizes
If PLEX is delayed (waiting on line or apheresis), start slow FFP infusion (15-30 mL/kg/day) to provide exogenous ADAMTS13
Corticosteroids: Prednisone 1 mg/kg/day or methylprednisolone - suppresses autoantibody production
Rituximab: Anti-CD20 monoclonal; used for refractory cases or to prevent relapse
Caplacizumab: Anti-vWF nanobody that blocks vWF-GPIb interaction, accelerates response (FDA approved 2019)
Splenectomy: Last resort for truly refractory cases
Do NOT transfuse platelets unless life-threatening bleeding (may fuel microthrombi)

Familial TTP (Upshaw-Schulman): Periodic plasma infusion every 2-4 weeks (provides ADAMTS13, no antibodies to remove so PLEX not needed). Recombinant ADAMTS13 is in trials.

Relapse: ~30-50% of acquired TTP patients relapse. Higher risk with persistent ADAMTS13 <5% in remission and high-titer inhibitor. Preemptive rituximab can reduce relapse.

29.3 Thrombocytosis

Definition: Platelet count >450,000/μL

Reactive (secondary) thrombocytosis:

Infection, inflammation
Iron deficiency
Post-splenectomy
Malignancy

Reactive thrombocytosis rarely causes thrombosis.

Clonal (primary) thrombocytosis:

Essential thrombocythemia (ET)
Other myeloproliferative neoplasms (PV, PMF, CML)

Clonal thrombocytosis carries risk of both thrombosis and bleeding (acquired von Willebrand syndrome at very high counts).

29.4 The Coagulation Cascade

The Coagulation Cascade: A Logical Framework

**Figure 1. The Coagulation Cascade.** The intrinsic (contact activation) pathway is initiated by contact with damaged surfaces and proceeds through Factors XII → XI → IX → VIIIa, measured by the aPTT. The extrinsic (tissue factor) pathway is initiated by trauma exposing tissue factor, which complexes with Factor VII/VIIa, measured by the PT. Both pathways converge on the common pathway: Factor X → Xa (with Va as cofactor) converts Prothrombin (II) to Thrombin (IIa), which converts Fibrinogen (I) to Fibrin (Ia). Factor XIII crosslinks fibrin into a stable clot. Natural anticoagulants shown in red: Antithrombin inhibits thrombin and Xa; TFPI inhibits the TF-VIIa complex; Activated Protein C (with Protein S as cofactor, activated via thrombomodulin) degrades Factors Va and VIIIa.

The traditional model divides coagulation into three pathways. In vivo the process is more integrated (the tissue factor pathway initiates and the intrinsic pathway amplifies), but this framework is essential for interpreting coagulation tests.

Factor Names, Half-Lives, and Where They’re Made

All coagulation factors are made in the liver EXCEPT vWF (endothelial cells and megakaryocytes). Factor VIII is made in liver sinusoidal endothelial cells, not hepatocytes - which is why FVIII can be normal even in liver disease and why liver transplant cures hemophilia A.

Neonates: only Factor VIII, vWF, and fibrinogen are at adult levels at birth. All other factors (especially VK-dependent II, VII, IX, X and contact factors XII, XI, HMWK, prekallikrein) are physiologically low. Age-adjusted reference ranges needed. Adult levels by ~6 months. This means vWD and hemophilia A can be diagnosed in neonates (FVIII and vWF are made in endothelial cells at adult levels), but most other factor deficiencies cannot be reliably diagnosed until after 6 months.

Factor names (board-testable):

Factor I = Fibrinogen
Factor II = Prothrombin
Factor III = Tissue Factor (a lot in brain and placenta - retained placenta can cause DIC)
Factor IV = Calcium
Factor V = Labile factor (degrades in stored plasma)
Factor VII = Stable factor (ironically has the shortest half-life)
Factor VIII = Antihemophilic factor
Factor IX = Christmas factor (named after Stephen Christmas, first patient described)
Factor X = Stuart-Prower factor
Factor XI = Plasma thromboplastin antecedent (PTA)
Factor XII = Hageman factor (Mr. Hageman died of PE, not bleeding)
Factor XIII = Fibrin stabilizing factor
Prekallikrein = Fletcher factor
HMWK = Fitzgerald factor

Vitamin K-dependent factors: II, VII, IX, X (plus Proteins C and S). Mnemonic “1972.” Vitamin K is required for gamma-carboxylation of glutamic acid residues, which is needed for Ca2+-dependent binding to phospholipid surfaces. Warfarin inhibits VKORC1 (vitamin K epoxide reductase), preventing regeneration of reduced vitamin K.

Half-lives (know these):

Factor VII: ~6 hours (shortest - why PT prolongs first)
Factor VIII: ~12 hours (when bound to vWF; ~2 hours without)
Factors V, IX, vWF: ~24 hours
Protein C: ~8 hours (why it depletes first on warfarin, driving warfarin skin necrosis)
Factor II: ~60-72 hours (longest)
Factor XIII: ~10 days (long, so prophylaxis every 4-6 weeks works)

Factor activity needed for normal hemostasis:

30%: Factors I, II, VIII, IX
20%: Factors V, VII, X, XI
<5%: Factors XII, XIII, HMWK, prekallikrein (XII/HMWK/prekallikrein deficiency does NOT cause bleeding)

The Three Pathways

Extrinsic Pathway (PT): Only Factor VII. Tissue factor exposed by vascular injury binds Factor VII, forming the TF-VIIa complex, which activates Factor X (primary substrate) and Factor IX (cross-activation). TF is constitutively expressed on subendothelial cells (fibroblasts, smooth muscle) but NOT on healthy endothelium. Monocytes express TF during inflammation/sepsis (contributes to DIC).

Intrinsic Pathway (aPTT): Factors XII, XI, IX, VIII (plus HMWK and prekallikrein). Factor XIIa (triggered by contact with negatively charged surfaces) activates XI, which activates IX. Factor IXa + Factor VIIIa = intrinsic tenase complex, which activates Factor X. Note: thrombin also directly activates Factor XI, which is the major physiologic activation pathway - this explains why FXII deficiency doesn’t cause bleeding but FXI deficiency does.

Common Pathway (PT and aPTT): Factors X, V, II, I. Factor Xa + Factor Va = prothrombinase complex, which converts prothrombin (II) to thrombin (IIa). Thrombin converts fibrinogen (I) to fibrin monomers, which polymerize. Factor XIII crosslinks the fibrin polymers (gamma-glutamyl-epsilon-lysine covalent bonds) into a stable clot.

Thrombin is the central enzyme: Converts fibrinogen to fibrin, activates Factors V, VIII, XI, XIII, activates platelets, and (bound to thrombomodulin) activates the anticoagulant Protein C pathway. This dual pro- and anti-coagulant role is key to understanding balance.

Natural Anticoagulants

Antithrombin (AT): Inhibits Factors IIa, VIIa, IXa, Xa, XIa, XIIa. Heparin enhances AT activity ~1000-fold by inducing a conformational change. Without AT, heparin doesn’t work.
Protein C / Protein S: Activated Protein C + Protein S as cofactor inactivate Factors Va and VIIIa. Protein C is activated by the thrombin-thrombomodulin complex on endothelium - once enough thrombin is generated, this redirects thrombin to shut down further coagulation
TFPI: Inhibits the TF-VIIa complex
Tissue thromboplastin inhibitor, heparin cofactor II: Minor players

29.5 Coagulation Laboratory Tests

PT (Prothrombin Time)

Reagent: thromboplastin (tissue factor + phospholipid) + CaCl2. Measures extrinsic and common pathways: Factors VII, X, V, II, I. Normal <15 seconds.

Prolonged by: warfarin (Factor VII depletes first), vitamin K deficiency, liver disease, Factor VII deficiency, common pathway factor deficiencies, DIC.

PT is the most sensitive test for early liver synthetic dysfunction because Factor VII has the shortest half-life.

INR (International Normalized Ratio)

INR = (Patient PT / Mean Normal PT)^ISI

MNPT = Geometric mean of PT from ≥20 healthy donors
ISI (International Sensitivity Index) = reagent sensitivity correction. ISI = 1.0 is reference thromboplastin; closer to 1.0 = more sensitive reagent. Given with each new lot

Therapeutic INR: 2.0-3.0 for most indications (DVT, PE, atrial fibrillation), 2.5-3.5 for mechanical heart valves. INR is used exclusively for warfarin monitoring - it is NOT validated for liver disease coagulopathy (ISI calibration assumes warfarin mechanism).

Chromogenic Factor X assay can monitor warfarin when PT/INR is unreliable (lupus anticoagulant, argatroban, baseline prolonged INR). Target Factor X activity: 20-40%.

aPTT (Activated Partial Thromboplastin Time)

Reagent: phospholipid + contact activator (kaolin, silica, or ellagic acid) + CaCl2. Measures intrinsic and common pathways: Factors XII, XI, IX, VIII, X, V, II, I. Normal <40 seconds. “Partial” refers to phospholipid without tissue factor (contrast PT = “complete” thromboplastin).

Prolonged by: heparin, factor deficiencies (VIII, IX, XI, XII), lupus anticoagulant, factor inhibitors.

Sample handling: If PTT testing delayed >4 hours, sample should be frozen (Factor VIII and V are labile). Centrifuge within 1 hour for platelet-poor plasma.

Artifacts:

Low-volume draw or polycythemia (Hct >55%) prolongs PTT falsely (altered citrate-to-plasma ratio). Adjust citrate volume for Hct >55%: citrate volume = 0.00185 × blood volume × (100 - Hct)
AT deficiency, severe anemia, or elevated Factor VIII (acute-phase reactant) can shorten PTT
Hepzyme (heparinase I): Can neutralize heparin contamination in samples

Mixing Studies

Mix patient plasma 1:1 with normal pooled plasma, then repeat the test. Normal plasma has 100% of all factors; you only need ~50% for normal clotting.

Corrects immediately and stays corrected: Factor deficiency
Does not correct: Inhibitor present
Corrects immediately, prolongs on incubation at 37°C for 1-2 hours: Factor VIII inhibitor (time- and temperature-dependent)

Rosner Index provides an objective correction cutoff:

Rosner = (mixed PTT - control PTT) / patient PTT × 100

<12% = corrected (factor deficiency)
15% = not corrected (inhibitor)
12-15% = indeterminate

Prolonged PT that corrects: Factor VII deficiency, vitamin K deficiency, early warfarin Prolonged PT and PTT that correct: common pathway deficiency (V, X, II, I), DIC, severe liver disease

The lupus anticoagulant caveat: LA is an antiphospholipid antibody that prolongs aPTT by interfering with the phospholipid-dependent assay. Despite prolonging aPTT, LA actually causes thrombosis, not bleeding. LA mixing studies may partially correct because it’s a relatively weak inhibitor; confirm with specific LA testing (dRVVT, hexagonal phase phospholipid neutralization).

Specific Factor Assays

Principle: Patient plasma (at multiple dilutions) + factor-deficient plasma (contains all factors EXCEPT the target). PT or PTT performed; result compared to standard curve.

PTT-based: Factors VIII, IX, XI, XII
PT-based: Factors II, V, VII, X

Must be performed in three dilutions to detect non-parallelism. If dilution corrects the apparent factor activity, an inhibitor is interfering (LA, heparin).

Thrombin Time (TT) and Reptilase Time

Thrombin time: Plasma + exogenous thrombin; measures fibrinogen → fibrin.

Prolonged by:

Hypofibrinogenemia
Dysfibrinogenemia
Heparin (thrombin inhibitor) - TT is very sensitive to heparin contamination
Direct thrombin inhibitors (dabigatran, argatroban, bivalirudin)
Elevated FDPs (interfere with fibrin polymerization)
Paraproteins (interfere with fibrin polymerization)
Amyloidosis

Reptilase time: Similar, but uses reptilase (thrombin-like enzyme from Bothrops venom, cleaves only fibrinopeptide A). Reptilase is NOT inhibited by heparin or direct thrombin inhibitors.

Classic board pattern:

Prolonged TT, normal reptilase → heparin (or DTI)
Prolonged TT, prolonged reptilase → fibrinogen problem or paraproteins

Fibrinogen Assays

Clauss method (functional): Excess thrombin added to diluted plasma; time to clot inversely proportional to fibrinogen. Most commonly used
Immunologic assay: Measures antigen regardless of function
PT-derived fibrinogen: Calculated from PT curve, less accurate

Interpretation:

Low Clauss, low immunologic → hypofibrinogenemia (quantitative)
Low Clauss, normal immunologic → dysfibrinogenemia (qualitative)

D-Dimer and FDPs

D-dimer is a specific fibrin degradation product from plasmin cleavage of Factor XIII-crosslinked fibrin. Indicates that BOTH coagulation (made fibrin) AND fibrinolysis (broke it down) have been active. D-dimers have two D-domains joined by a gamma-gamma crosslink.

D-dimer is NOT generated from fibrinogen cleavage. Fibrinogen degradation products are X, Y, D, and E fragments but NOT D-dimer (fibrinogen was never crosslinked).

D-dimer has >95% sensitivity for VTE - a negative D-dimer with low clinical probability effectively rules out DVT/PE. Low specificity - elevated in DIC, infection, malignancy, pregnancy, post-surgery, liver disease, atrial fibrillation (stagnant blood → microthrombi), CHF. Age-adjusted cutoffs (age × 10 μg/L for age >50) improve specificity in elderly.

Anti-Xa Assay

Measures inhibitory activity of anticoagulants acting through Factor Xa. Used for:

Unfractionated heparin: therapeutic 0.3-0.7 IU/mL
LMWH: 0.5-1.0 IU/mL treatment, 0.2-0.5 prophylaxis (only monitored in obesity, renal failure, pregnancy)
Fondaparinux
Direct Xa inhibitors (rivaroxaban, apixaban) - drug-specific calibrated assays for special situations

Activated Clotting Time (ACT)

Point-of-care whole blood test used to monitor high-dose heparin during CPB, ECMO, catheterization. Normal 70-120 seconds. Target for CPB >400-480 seconds. PTT is too prolonged to be useful at these doses. Activators: kaolin, celite, glass beads. Real-time titration in the OR.

Bethesda Assay

Quantifies coagulation factor inhibitor titer. Patient plasma serially diluted + equal volumes normal pooled plasma, incubate 2 hours at 37°C (Nijmegen modification buffers normal plasma for accuracy). 1 BU = dilution at which factor activity = 50% of control.

<5 BU = low titer, may respond to high-dose factor replacement
5 BU = high titer, requires bypassing agents (rFVIIa, aPCC/FEIBA) or immune tolerance induction

PFA-100 (Platelet Function Analyzer)

Whole blood aspirated through small aperture coated with collagen/epinephrine or collagen/ADP. Platelets adhere and aggregate, eventually occluding the aperture. Closure time reflects platelet function. Sensitive to vWD, severe platelet disorders, aspirin effect. Not sensitive to mild platelet dysfunction or clopidogrel.

Drug-induced (aspirin) pattern: Col/epi prolonged, col/ADP normal Intrinsic platelet defect (vWD, BSS, Glanzmann, SPD) pattern: Both prolonged

Affected by platelet count <100,000 and Hct <30% (false results). Thrombocytosis >600K can also affect. Largely replaced bleeding time as a screening test.

Bleeding Time (Ivy Method)

Largely obsolete. Normal 2-7 minutes. Standardized forearm incision, BP cuff at 40 mmHg, horizontal cut, time to stop bleeding. Prolonged in vWD, platelet function disorders, thrombocytopenia <100K. Replaced by PFA-100 due to poor reproducibility, operator dependence, scarring, and poor correlation with surgical bleeding risk.

TEG and ROTEM (Viscoelastic Global Hemostasis Assays)

Evaluate all three phases of hemostasis on whole blood (preserving platelet-RBC-plasma interactions). Increasingly used in trauma, cardiac surgery, liver transplant.

TEG: rotating cup, stationary pin (more sensitive to mechanical force) ROTEM: stationary cup, rotating pin (more resistant to mechanical force, better in clinical settings)

ROTEM has specific reagent panels: INTEM (intrinsic), EXTEM (extrinsic), FIBTEM (fibrinogen only - cytochalasin D inhibits platelets), APTEM (with antifibrinolytic to confirm fibrinolysis), HEPTEM (with heparinase to confirm heparin effect).

Five phases / parameters:

Phase	TEG	ROTEM	What it measures	Normal	Abnormal → Treatment
1	R	CT	Time to initial fibrin (2 mm amplitude); clotting factors	5-10 min	↑ → FFP
2	K	CFT	Time to 20 mm clot; fibrin polymerization	1-5 min	↑ → cryo/fibrinogen
3	α angle	α angle	Rate of crosslinking; fibrinogen	45-75°	↓ → cryo/fibrinogen
4	MA	MCF	Max clot strength; platelets (~80%) + fibrinogen (~20%)	50-75 mm	↓ → platelets or DDAVP
5	LY-30	CL	Clot lysis at 30 min post-MA; fibrinolysis	0-10%	↑ → TXA/aminocaproic acid

Fulminant hyperfibrinolysis (100% lysis within 30-60 min) = massive hemorrhage, very high mortality.

Assay Types

Chronometric: PT, PTT, TT, one-stage factor assays - measure time to clot
Chromogenic (amidolytic): AT, Protein C, chromogenic factor X - enzyme cleaves p-nitroanilide substrate, color measured spectrophotometrically. Not affected by LA
Immunoturbidometric: vWF antigen, Protein S antigen - latex particles + antibody, turbidity from immune complexes
ELISA: Anti-PF4/heparin, anti-cardiolipin, anti-β2GPI

29.6 Factor Deficiencies

Hemophilia A

Factor VIII deficiency. X-linked recessive. Most common severe inherited bleeding disorder (~1 in 5,000 males). 30% of cases are de novo mutations (no family history).

Severity:

Severe: FVIII <1% - spontaneous joint bleeds
Moderate: 1-5% - bleeds with minor trauma
Mild: 6-40% - bleeds only with major trauma/surgery

Most common mutation: intron 22 inversion (~50% of severe cases). Detected by Southern blot or inverse PCR. Other mutations: point mutations, small deletions.

Female carriers: ~50% FVIII activity. Most asymptomatic. Skewed lyonization (X-inactivation) can cause symptomatic carriers needing treatment for surgery.

Labs: Normal platelet count, normal PT, prolonged PTT. Confirmed with FVIII activity assay.

Clinical: Hemarthrosis (hallmark), muscle hematomas, prolonged bleeding after surgery, intracranial hemorrhage.

Treatment:

Recombinant or plasma-derived FVIII concentrate. Dose every 8-12 hours for surgical prophylaxis (based on 12-hour half-life). Extended half-life (EHL) products (PEGylated, Fc fusion): 14-19 hour half-life
DDAVP for mild cases (releases stored FVIII from endothelial cells, 2-5 fold rise)
Emicizumab: bispecific antibody mimicking FVIII function. Works regardless of inhibitors. Given weekly/biweekly
Gene therapy: emerging

Factor VIII Inhibitors

Anti-FVIII is the most common coagulation factor inhibitor. Two types:

Alloantibody in hemophilia A patients receiving replacement (25-30% of severe hemophilia A). IgG that neutralizes exogenous FVIII
Autoantibody (acquired hemophilia A): Rare (1-4 per million/year) but serious. Bimodal distribution: young postpartum women and elderly (autoimmune, malignancy, idiopathic)

Clinical - acquired hemophilia A: Sudden-onset severe bleeding in patient with no prior bleeding history. Extensive soft tissue hematomas, muscle bleeding, retroperitoneal hemorrhage. Joint bleeding uncommon (unlike congenital hemophilia). Mortality ~20% if unrecognized.

Labs: Isolated prolonged aPTT. Mixing study corrects immediately but prolongs on incubation at 37°C (time-dependent inhibitor). FVIII level very low. Bethesda assay positive.

Treatment:

Control acute bleeding:
- Low titer (<5 BU): high-dose Factor VIII may overcome
- High titer (≥5 BU): bypassing agents - rFVIIa or activated PCC (FEIBA)
Eradicate inhibitor: immunosuppression (steroids ± cyclophosphamide or rituximab). Weeks to months

Hemophilia B (Christmas Disease)

Factor IX deficiency. X-linked recessive. ~5x less common than hemophilia A. Named after Stephen Christmas. Clinically identical to hemophilia A - differentiated only by specific factor assays. Factor IX is vitamin K-dependent.

Treatment: recombinant FIX (Benefix, Rixubis) or plasma-derived FIX concentrate. EHL products (Alprolix, Idelvion) allow less frequent dosing. Gene therapy for hemophilia B (etranacogene dezaparvovec/Hemgenix) is FDA-approved.

Hemophilia C

Factor XI deficiency. Autosomal recessive. Common in people of Ashkenazi Jewish ancestry (~8% carrier rate, ~1 in 450 with disease). Two common mutations: Type II (Glu117stop) and Type III (Phe283Leu). Associated with Noonan syndrome.

Unlike hemophilia A/B, bleeding severity does NOT correlate with factor levels. Bleeding typically provoked (surgery, trauma, especially tissues with high fibrinolytic activity - oral cavity, urinary tract, tonsils). Spontaneous bleeding rare.

Treatment: FFP (no FXI concentrate widely available in US; available in Europe but thrombotic risk). Antifibrinolytics (TXA) useful adjuncts.

Factor XII / HMWK / Prekallikrein Deficiencies

Markedly prolonged PTT (often >150 seconds) but NO bleeding. Critical board concept. These factors matter for in vitro contact activation but not in vivo hemostasis. Prekallikrein deficiency PTT may normalize with prolonged incubation (autoactivation of Factor XII).

Factor VII Deficiency

Most common autosomal recessive coagulation factor deficiency (~1 in 500,000). Isolated prolonged PT with normal PTT (FVII is the only factor exclusively in the extrinsic pathway). Bleeding severity does NOT correlate well with factor levels. Can present with CNS hemorrhage in neonates, menorrhagia, epistaxis, post-surgical bleeding.

Treatment: rFVIIa (NovoSeven) or FFP. Short half-life requires frequent dosing.

Factor V Deficiency

Normal platelet count, normal TT, prolonged PT AND PTT. Factor V is common pathway so both PT and PTT prolonged. TT normal because FV acts upstream of fibrinogen→fibrin. FV is NOT vitamin K-dependent and is NOT in PCC. Treatment: FFP is the only source of Factor V.

Factor X Deficiency

Same lab pattern as FV: normal platelet count, normal TT, prolonged PT and PTT. Acquired FX deficiency classically associated with light-chain (AL) amyloidosis, where amyloid fibrils adsorb FX from circulation. Also prolonged dRVVT (Russell viper venom directly activates FX).

Factor XIII Deficiency

PT and PTT are NORMAL - Factor XIII acts AFTER fibrin formation, so standard coagulation tests miss it.

Pathophys: Without FXIII, fibrin clots form but are unstable - held together only by weak hydrogen bonds, susceptible to premature breakdown by plasmin.

Inheritance: Autosomal recessive. Heterozygotes usually asymptomatic.

Clinical:

Delayed bleeding (clot forms then dissolves hours to days later)
Umbilical stump bleeding in neonates (classic presentation, ~80% of cases)
Bleeding with circumcision
Intracranial hemorrhage (higher risk than other bleeding disorders)
Poor wound healing
Recurrent miscarriage (FXIII important for placental implantation)

Diagnosis:

Urea clot solubility test: patient’s clot placed in 5M urea or 1% monochloroacetic acid. Normal crosslinked clots are insoluble. FXIII-deficient clots dissolve within 24 hours
Factor XIII activity assay (quantitative)

Treatment: Factor XIII concentrate. Prophylaxis every 4-6 weeks maintains hemostasis (FXIII half-life ~10 days).

Familial Combined Factor Deficiencies

Five rare AR disorders where defects in protein transport or post-translational modification affect multiple factors:

Type 1: FV and FVIII deficiency from LMAN1 (ERGIC-53) mutation. LMAN1 is the ER cargo receptor for FV and FVIII. Also caused by MCFD2 (LMAN1 cofactor). Most common combined deficiency
Type 2: FVII and FIX
Type 3: All VK-dependent factors (II, VII, IX, X, protein C/S) from vitamin K carboxylase (GGCX) mutation. Does NOT respond to vitamin K supplementation
Type 4: FVII and FVIII
Type 5: FVIII, FIX, FXI

29.7 von Willebrand Disease

Most common inherited bleeding disorder (~1% of population). Most cases autosomal dominant (Types 1, 2A, 2B, 2M) except Type 3 and Type 2N (AR).

Clinical: Mucocutaneous bleeding - menorrhagia (most common presenting symptom in women), epistaxis, gingival bleeding, easy bruising, post-dental extraction bleeding, post-surgical bleeding. Type 3 can present with deep tissue bleeding (hemophilia-like) due to very low FVIII.

Labs:

Normal PT, normal or prolonged PTT (prolonged only when FVIII is significantly decreased)
Prolonged bleeding time / prolonged PFA-100 closure time

Test panel:

vWF:Ag (antigen - immunoturbidometric)
vWF:RCo (ristocetin cofactor activity - functional)
Factor VIII activity
Ristocetin-induced platelet aggregation (RIPA)
Multimer analysis

Ristocetin assay principle: Ristocetin binds vWF and induces vWF-GPIb interaction, causing platelet aggregation. Tests the vWF-GPIb axis. Abnormal in all types of vWD except Type 2N (Type 2N has defect in FVIII binding site, not GPIb binding site).

vWF activity/antigen ratio (vWF:RCo / vWF:Ag):

0.7 = proportional (quantitative defect) = Type 1, 2N, or 3
<0.7 = disproportionate (qualitative defect) = Types 2A, 2B, 2M

Multimer analysis distinguishes within qualitative group: normal in 2M, absent large multimers in 2A, slightly decreased large multimers in 2B.

Classification:

Type	Defect	vWF:Ag	vWF:RCo	FVIII	Platelets	Multimers
Type 1	Partial quantitative (~80% of vWD)	↓	↓	↓	Normal	Normal pattern, decreased intensity
Type 2A	HMW multimer loss (2nd most common)	↓	↓↓	↓	Normal	Absent HMW
Type 2B	Gain-of-function A1 domain (↑ GPIb binding)	↓	↓↓	↓	↓	Slightly ↓ HMW
Type 2M	Loss-of-function A1 domain (↓ GPIb binding)	↓	↓↓	↓	Normal	Normal
Type 2N (Normandy)	FVIII binding site defect	Normal	Normal	↓↓↓	Normal	Normal
Type 3	Virtually absent vWF	↓↓↓	↓↓↓	↓↓↓	Normal	Absent
Platelet-type pseudo-vWD	GPIb gain-of-function	↓	↓↓	↓	↓	Slightly ↓ HMW

Type 2B vs Platelet-type: Both have enhanced RIPA with low-dose ristocetin (<0.8 mg/mL) and slightly decreased HMW multimers. Adding cryoprecipitate (contains vWF, no platelets) corrects Type 2B but NOT platelet-type (patient platelets are the problem in platelet-type). Definitive: genetic testing of VWF vs GP1BA.

Type 2N mimics hemophilia A: Isolated low FVIII, normal vWF:Ag, normal ristocetin. Differentiated by inheritance (2N = AR, affects both sexes; hemophilia A = X-linked, males) and vWF:FVIII binding assay (decreased in 2N). Genetic testing is definitive.

Factors that elevate vWF (causing false-negative testing):

Vigorous exercise
Inflammation / infection (vWF is acute-phase reactant - check CRP alongside)
Pregnancy (5x increase at delivery - low bleeding risk during pregnancy but common 1 week postpartum)
Estrogen therapy, OCP
Stress, surgery
Liver disease (impaired clearance)

If suspicion is high but initial testing normal, retest in 3-6 months at baseline (not during illness, not during estrogen peak). Test during first 3 days of menstrual cycle if possible.

Blood group O individuals have 25-30% lower vWF levels (Group O vWF is cleared faster due to ABO antigens on vWF). AB has the highest vWF. Some labs use ABO-specific reference ranges. Board pearl: Group O with borderline-low vWF may not truly have vWD.

Newborns: vWF is normal to increased at birth (not made in liver, reaches adult levels early). vWD can be diagnosed in neonates, unlike most other factor deficiencies.

Treatment:

Type 1: DDAVP (releases stored vWF, 3-5 fold rise, peak 30-60 min IV, 60-90 min intranasal). Tachyphylaxis with repeated doses. Caution: hyponatremia from antidiuretic effect, restrict fluids
Type 2A, 2M, 2N (mild): DDAVP trial
Type 2B: DDAVP is CONTRAINDICATED (releases more abnormal vWF → worsening thrombocytopenia, potential microthrombi)
Type 3 and severe Type 1/2: Humate-P or Wilate (vWF/FVIII concentrate). Recombinant vWF (Vonvendi) for adults
Aminocaproic acid or TXA: antifibrinolytic adjunct, especially for mucosal procedures

Acquired vWD: Lymphoproliferative disorders (anti-vWF antibodies), autoimmune disorders (SLE), essential thrombocythemia (platelets absorb vWF), cardiopulmonary bypass (shear destroys HMW multimers), aortic stenosis (Heyde syndrome: AS + GI bleeding from angiodysplasia + acquired vWD; valve replacement corrects the vWD). Type 1 pattern most common.

29.8 Qualitative Platelet Disorders

Present with mucocutaneous bleeding despite normal platelet counts (or mild thrombocytopenia). The aggregometry pattern is the key to diagnosis.

Glanzmann Thrombasthenia

Pathophys: Autosomal recessive deficiency of GPIIb/IIIa (integrin αIIbβ3), the fibrinogen receptor essential for platelet aggregation.

GPIIb/IIIa is the final common pathway of platelet aggregation. Activated platelets expose GPIIb/IIIa, which binds fibrinogen and crosslinks adjacent platelets. Without functional GPIIb/IIIa, platelets can adhere to the vessel wall but can’t aggregate.

Clinical:

Severe mucocutaneous bleeding from birth (epistaxis, gingival, menorrhagia, GI)
Prolonged bleeding time / abnormal PFA-100
Normal platelet count and morphology
Abnormal in vitro clot retraction (platelets can’t pull fibrin strands together)

Aggregometry - pathognomonic pattern:

No aggregation to ADP, collagen, epinephrine, thrombin, arachidonic acid
Normal aggregation to ristocetin (tests vWF-GPIb, doesn’t require GPIIb/IIIa)

Absent response to all agonists except ristocetin.

PLA-1 negative: Glanzmann patients lack GPIIb/IIIa, so they’re PLA-1 (HPA-1a) negative. Platelet transfusion can cause iso-antibodies against GPIIb/IIIa → future transfusions ineffective. This is why rFVIIa (NovoSeven) is important for refractory bleeding.

Treatment: Platelet transfusions for bleeding (HLA-matched if alloimmunized), rFVIIa for refractory bleeding, avoid antiplatelet agents, antifibrinolytics.

Bernard-Soulier Syndrome

Pathophys: Autosomal recessive deficiency of the GPIb/IX/V complex (CD42b = GPIbα, CD42c = GPIbβ, CD42a = GPIX, CD42d = GPV), the receptor for vWF essential for platelet adhesion.

GPIb/IX/V mediates initial platelet adhesion under high shear. vWF binds exposed collagen, undergoes conformational change, captures flowing platelets via GPIb. Without this receptor, platelets can’t adhere at sites of injury.

Clinical:

Moderate to severe mucocutaneous bleeding
Giant platelets on blood smear (approaching size of lymphocytes) - “Big Suckers” mnemonic. GPIb/IX/V is also important for normal platelet production
Mild thrombocytopenia (often 50,000-100,000/μL)
May be misdiagnosed as ITP (thrombocytopenia + bleeding)
Automated counters may misclassify giant platelets as RBCs or WBCs

Aggregometry:

No aggregation to ristocetin
Normal aggregation to ADP, collagen, epinephrine, thrombin

Opposite pattern from Glanzmann - isolated failure to respond to ristocetin.

Distinguishing BSS from vWD: Both show absent ristocetin aggregation.

Adding normal plasma (contains vWF) corrects vWD but NOT BSS (the defect is in the platelet receptor, not the plasma)
vWF:Ag and vWF:RCo are normal in BSS, decreased in vWD
BSS has giant platelets

Heterozygous carriers: Mediterranean macrothrombocytopenia - large platelets, mild thrombocytopenia, minimal symptoms. Often misdiagnosed as ITP. Should NOT receive steroids or splenectomy.

Treatment: Platelet transfusions. DDAVP is NOT effective (vWF is normal). Avoid alloimmunization.

Storage Pool Disorders

Dense granule disorders (people names, all have non-protein contents like ADP):

Hermansky-Pudlak (AR, oculocutaneous albinism, ceroid deposits in macrophages, common in Puerto Rico) - triad: albinism + bleeding + ceroid deposition. Pulmonary fibrosis (HPS1/HPS4 subtypes, leading cause of death) and granulomatous colitis in some. Ceroid deposits PAS-positive and autofluorescent
Chediak-Higashi (AR, LYST/CHS1 gene mutation on 1q42, giant granules in all lineages, partial albinism with silver-gray hair, recurrent pyogenic infections, peripheral neuropathy) - accelerated phase (HLH-like) in ~85%, often fatal without BMT. BMT corrects hematologic/immunologic but NOT albinism or neuropathy
Wiskott-Aldrich (X-linked recessive, WASp gene at Xp11.23, WATER mnemonic: Wiskott-Aldrich Thrombocytopenia Eczema Recurrent infections) - small platelets (unique - low MPV). High risk of lymphoma (especially EBV). Milder mutations → X-linked thrombocytopenia (XLT)

Alpha granule disorders (all have “platelet” in the name):

Gray platelet syndrome (AR, NBEAL2 mutation, pale/gray platelets on Wright stain, mild thrombocytopenia, splenomegaly, progressive myelofibrosis from PDGF/TGF-β release into marrow)
White platelet syndrome
Quebec platelet syndrome (AD, overproduction of uPA in alpha granules, premature clot lysis, delayed bleeding; antifibrinolytics work better than platelet transfusion)

Aggregometry patterns:

Dense granule disorders: blunted secondary aggregation with low-dose ADP and epinephrine (the release reaction depends on granule contents for amplification). Normal primary wave
Alpha granule disorders: no CD62P expression after thrombin stimulation on flow cytometry, low PF4 by ELISA

Diagnostic tests for dense granule disorders:

Diminished ATP secretion on lumiaggregometry (luciferin-luciferase)
Absent dense granules on whole-mount EM (gold standard)
Decreased mepacrine (quinacrine) uptake on flow cytometry

Scott Syndrome

Rare AR disorder. Platelets can’t expose phosphatidylserine (PS) on outer membrane after activation, impairing assembly of tenase/prothrombinase complexes on the platelet surface. PT and PTT are normal (use exogenous phospholipid). Diagnosed by flow cytometry with absent annexin V binding after stimulation.

Acquired Platelet Dysfunction

Uremia: Multiple mechanisms (decreased TXA2, increased NO/PGI2, abnormal vWF-GPIb interaction). Platelet transfusion NOT effective (uremic toxins impair transfused platelets too). Treatment: dialysis, DDAVP, cryoprecipitate, conjugated estrogens
Cardiopulmonary bypass: Mechanical activation and consumption. Platelet transfusion IS effective (once platelets leave the circuit). Also acquired vWD (shear destroys HMW multimers), hypofibrinogenemia, heparin effects
Paraproteinemia (IgA or IgM most commonly): Paraprotein coats platelets, blocks receptor-ligand interactions. Platelet transfusion NOT effective. Treatment: address underlying plasma cell dyscrasia, plasmapheresis
Medications: See below
MPN: Acquired storage pool deficiency, acquired vWD at very high counts (>1,000,000). Poor primary phase aggregation with epinephrine on aggregometry

29.9 Antiplatelet Drugs

Aspirin

Irreversibly acetylates COX-1, blocking TXA2 synthesis. Effect lasts 7-10 days (platelet lifespan) because platelets are anucleate and can’t synthesize new COX-1. Low-dose (81 mg) preferentially inhibits platelet COX-1 over endothelial COX-2.

Aggregometry: flat-line to arachidonic acid, absent collagen response, absent 2nd wave with ADP, normal ristocetin. PFA-100: col/epi prolonged, col/ADP normal.

Platelet transfusion IS effective for aspirin-related bleeding (aspirin is not in active circulating form - it’s rapidly hydrolyzed; transfused platelets have functional COX-1).

NSAIDs

Reversibly inhibit COX-1; effect wears off as drug clears.

P2Y12 Inhibitors (ADP Receptor Inhibitors)

Block P2Y12, preventing GPIIb/IIIa activation. Aggregometry: absent ADP response.

Clopidogrel: Prodrug, activated by CYP2C19. Irreversible
- ==CYP2C19*2 (loss-of-function) → poor metabolizer → clotting (inadequate clopidogrel activation)==
- ==CYP2C19*17 (gain-of-function) → ultra-rapid metabolizer → bleeding==
- ~2-15% of people of European ancestry and ~15-20% of East Asian populations are poor metabolizers
- FDA black box warning regarding CYP2C19 genotyping
- VerifyNow P2Y12 assay for monitoring
Prasugrel: Prodrug, not CYP2C19-dependent. Irreversible
Ticagrelor: Reversible, non-thienopyridine, binds allosterically. Shorter duration (3-5 days vs 5-7 days clopidogrel)
Cangrelor: IV, reversible, very short half-life (~3-6 min)

Platelet transfusion MAYBE effective for clopidogrel (active metabolite may still circulate briefly). Transfused platelets are affected only if drug still in active form.

GPIIb/IIIa Antagonists

Abciximab, eptifibatide, tirofiban. Directly block the fibrinogen receptor. Aggregometry: absent to all agonists except ristocetin - identical pattern to Glanzmann thrombasthenia.

29.10 Fibrinolysis and Tertiary Hemostasis

The Fibrinolytic System

Dissolves clots after wound repair. Imbalance leads to either bleeding (excess) or thrombosis (deficient).

Key components:

Plasminogen: Inactive zymogen; has lysine-binding sites that anchor it to fibrin
tPA (tissue plasminogen activator): Released from endothelium; most active when bound to fibrin (localizes fibrinolysis to clots, efficiency 1000-fold greater on fibrin surface)
Urokinase (uPA): Additional plasminogen activator
Plasmin: Active enzyme; cleaves fibrin AND fibrinogen, also FV and FVIII. Not specific for fibrin

Inhibitors of fibrinolysis (deficiency causes excess bleeding):

PAI-1 (plasminogen activator inhibitor-1): Inhibits tPA and uPA. Elevated in obesity, metabolic syndrome
α2-Antiplasmin: Rapidly inactivates free plasmin in seconds. Fibrin-bound plasmin partially protected
TAFI (thrombin-activatable fibrinolysis inhibitor): Removes C-terminal lysine residues from fibrin, reducing plasminogen binding

D-Dimer vs FDP Revisited

D-dimers come ONLY from plasmin cleavage of Factor XIII-crosslinked fibrin. Fibrinogen cleavage → fragments X, Y, D, E (no D-dimer). D-dimer elevation requires BOTH coagulation AND fibrinolysis.

Disorders of fibrinolysis do NOT elevate D-dimer (because coagulation cascade isn’t activated - plasmin just acts on fibrinogen). But fibrinogen is decreased. This distinguishes primary fibrinolysis from DIC:

DIC: low fibrinogen + elevated D-dimer + schistocytes + thrombocytopenia
Primary fibrinolysis: low fibrinogen + normal D-dimer + no schistocytes + normal platelets

Hyperfibrinolysis

Primary (rare):

α2-antiplasmin deficiency (congenital AR, or acquired: liver disease, amyloidosis, DIC consumption, nephrotic syndrome urinary loss, malignancy)
PAI-1 deficiency - delayed wound healing, post-surgical bleeding, delayed menstrual bleeding. Normal PT/PTT. Shortened euglobulin clot lysis time. Treat with antifibrinolytics
Quebec platelet syndrome - uPA overproduction

Secondary:

DIC (consumption of α2-antiplasmin)
Liver disease (decreased clearance of tPA/uPA, decreased synthesis of inhibitors, decreased α2-antiplasmin)
APL cells release tPA and uPA
Trauma (tPA release)
Prostate surgery

Labs: low fibrinogen, elevated FDPs, shortened euglobulin clot lysis time, normal D-dimer (if truly primary).

Euglobulin clot lysis time: Euglobulin fraction (precipitated from acidified diluted plasma) contains plasminogen, tPA, and fibrinogen but NOT inhibitors (α2-antiplasmin, PAI-1). Clotted and observed - accelerated lysis = excess fibrinolytic activity. Normal >90-120 minutes. Short ECLT + no schistocytes/thrombocytopenia = primary fibrinolysis (virtually rules out DIC, TTP, HUS).

Urea clot solubility test: NORMAL in fibrinolytic disorders (Factor XIII crosslinking is intact). Clot dissolves in urea only if Factor XIII has NOT crosslinked it. Specifically diagnoses Factor XIII deficiency (clot dissolves in 5M urea or 1% monochloroacetic acid within 24 hours if FXIII <1%).

Antifibrinolytic Therapy

Tranexamic acid (TXA) and aminocaproic acid (EACA/Amicar) - lysine analogs that block plasminogen binding to fibrin. TXA is ~10x more potent than EACA.

Uses: dental procedures in hemophilia, menorrhagia, trauma (CRASH-2 trial: TXA reduces mortality if given within 3 hours), cardiac surgery, prostate surgery, Quebec platelet syndrome, α2-antiplasmin/PAI-1 deficiency.

Dosing: TXA 1g IV over 10 min, then 1g over 8 hours. EACA: 5g IV loading, then 1g/hr infusion.

Contraindicated in DIC with dominant thrombotic features.

29.11 Acquired Coagulation Disorders

Vitamin K Deficiency

Vitamin K is required for γ-carboxylation of Factors II, VII, IX, X, and Proteins C/S.

Causes:

Dietary deficiency (rare; vitamin K in green vegetables)
Malabsorption (biliary obstruction, celiac, CF, IBD)
Antibiotic-mediated (gut bacteria produce vitamin K)
Warfarin therapy
Neonatal vitamin K deficiency bleeding (VKDB): born with low stores (poor placental transfer, sterile gut). Classic VKDB days 2-7, late VKDB 2-12 weeks (exclusively breastfed infants, breast milk low in vitamin K). Prenatal exposure to phenytoin or phenobarbital (CYP inducers in fetus) accelerates deficiency. Vitamin K 1 mg IM at birth prevents virtually all cases - bleeding 1 week to several weeks after birth without Vitamin K shot has germinal matrix hemorrhage as a classic presentation

Laboratory:

Prolonged PT first (Factor VII shortest half-life)
Then prolonged aPTT
Corrects with vitamin K administration within 12-24 hours if liver function intact

Warfarin

Inhibits VKORC1 (vitamin K epoxide reductase), preventing regeneration of reduced vitamin K. Takes 5-7 days for full effect. Metabolized by CYP2C9 (primarily S-enantiomer, 3-5x more potent). Drug interactions: CYP2C9 inhibitors (fluconazole, amiodarone) increase effect; inducers (rifampin) decrease.

Pharmacogenomics: CYP2C9 and VKORC1 polymorphisms together explain ~40-50% of dose variability. VKORC1 -1639G>A: AA = high sensitivity (lower doses), GG = resistance (higher doses). CYP2C92 and 3 are reduced-function variants requiring lower doses.

Warfarin Reversal - “The Relay Race”:

Agent	Speed	Duration	Role
4-Factor PCC (Kcentra)	Immediate (15-30 min)	Short (4-8 hrs, limited by FVII t½)	Stop the acute bleed; give for ICH/life-threatening hemorrhage. Small volume, no thawing
IV Vitamin K (10 mg)	Slow (6-12 hrs)	Long-term (days)	Prevent INR rebound; restarts hepatic factor synthesis
FFP	Moderate (30-60 min)	Short	Second-line if PCC unavailable; large volume → TACO risk

Warfarin Skin Necrosis

Occurs in patients with hereditary Protein C or S deficiency. Mechanism: warfarin depletes VK-dependent factors. Protein C has ~8-hour half-life, so it drops faster than procoagulant factors (II = 60-72 hours). This transient hypercoagulable state → microvascular thrombosis in skin and subcutaneous fat → skin necrosis.

Prevention: bridge with heparin when starting warfarin. Continue heparin for ≥5 days AND until INR therapeutic for ≥24 hours.

Liver Disease Coagulopathy

The liver synthesizes all clotting factors except vWF. In liver disease:

Decreased synthesis of procoagulant factors (II, V, VII, IX, X)
Also decreased synthesis of anticoagulants (Protein C, S, antithrombin)
Impaired clearance of tPA/uPA → hyperfibrinolysis
Decreased α2-antiplasmin
Net effect: Complex coagulopathy with both bleeding and thrombosis risk

PT/INR elevated but does NOT accurately reflect thrombotic risk - patients may be hypercoagulable despite elevated INR. PT is part of the MELD score. INR calibration is not valid for liver disease.

Factor V and Factor VII are most sensitive to acute liver injury (shorter half-lives). Factor VIII may be normal or elevated (made in liver sinusoidal endothelial cells, acute-phase reactant) - helps distinguish liver disease from DIC (where FVIII is consumed).

Disseminated Intravascular Coagulation (DIC)

Pathophysiology: Systemic activation of coagulation, leading to:

Widespread microthrombi → organ ischemia
Consumption of clotting factors and platelets → bleeding

DIC is always secondary to an underlying condition. Natural anticoagulants (AT, Protein C) are consumed faster than replaced.

Causes (“STOP Making New Thrombi” mnemonic):

Sepsis (most common - especially gram-negative, meningococcemia)
Snake bite (procoagulant venoms)
Trauma, burns
Obstetric complications (abruptio placentae, amniotic fluid embolism, eclampsia, HELLP)
Pancreatitis (acute)
Malignancy (especially acute promyelocytic leukemia, mucin-secreting adenocarcinomas)
Nephrotic syndrome
Transfusion reactions (acute hemolytic)

Amniotic fluid embolism is catastrophic - amniotic fluid contains tissue factor and thrombogenic substances. Sudden cardiovascular collapse, respiratory distress, seizures, then massive hemorrhage. Mortality 20-60%.

Laboratory findings:

Prolonged PT and aPTT (factor consumption; PT rises first)
Thrombocytopenia (platelet consumption)
Low fibrinogen (consumed; note fibrinogen is acute-phase reactant, so in early/chronic DIC may still be normal - a declining trend is more useful than a single value)
Elevated D-dimer and FDPs (best screening test, >95% sensitivity)
Schistocytes (microangiopathic hemolysis)
Decreased α2-antiplasmin

ISTH DIC Score: Combines platelet count, PT prolongation, fibrinogen, D-dimer/FDP. Score >5 = compatible with overt DIC.

Acute DIC: Bleeding predominates (factors consumed faster than produced)

Chronic (compensated) DIC: Thrombosis may predominate (liver can keep up with production); seen with malignancy

Treatment:

Treat the underlying cause: antibiotics for sepsis, delivery for obstetric complications, ATRA for APL, surgery for trauma. Without treating the trigger, blood products are futile
Bleeding patient: platelets if <50K, cryoprecipitate if fibrinogen <100 mg/dL, FFP for factor replacement
Non-bleeding patient: platelets if <10K, cryo if fibrinogen <50 mg/dL
Do NOT give coagulation factor concentrates (provide procoagulants without natural anticoagulants, could fuel thrombosis). FFP preferred for factor replacement (physiologic ratios of pro- and anti-coagulants)
Heparin: may be used in initial hypercoagulable phase (purpura fulminans in meningococcemia, large vessel thrombosis). Contraindicated in hemorrhagic phase

29.12 Hypercoagulable States (Thrombophilia)

Thrombophilia Testing Timing

Clot-based assays are best done >2 months after acute episode and >2 weeks off anticoagulation. Acute thrombosis consumes natural anticoagulants (falsely low levels). Anticoagulants interfere: warfarin decreases Protein C/S (VK-dependent), heparin decreases AT (consumed). Acute-phase FVIII elevation masks deficiency. Genetic tests (FV Leiden, prothrombin 20210A) can be done anytime since they measure DNA.

Inherited Thrombophilias

Factor V Leiden

Most common inherited thrombophilia in people of European ancestry (~5% prevalence). Rare in East Asian populations (<1%) and African ancestry populations (~1%).

Mechanism: R506Q point mutation (G1691A) near the APC cleavage site in Factor V. Arginine to glutamine. APC normally cleaves FVa at R506, R306, R679 (R506 primary). Mutation eliminates primary cleavage site → ~10-fold more resistant to APC inactivation → prolonged thrombin generation.

Risk: 5-7x increased VTE risk (heterozygous); 50-100x (homozygous). Autosomal dominant with incomplete penetrance. Most carriers never develop thrombosis without additional risk factors.

Testing:

APC resistance assay (screening): PTT with/without APC, calculate ratio. Normal >2.0, FVL <2.0. Modified version dilutes patient plasma in FV-deficient plasma to eliminate interference
Genetic testing for G1691A mutation (confirmation)

Prothrombin G20210A Mutation

Second most common inherited thrombophilia in people of European ancestry (~2% prevalence).

Mechanism: Point mutation in the 3’ untranslated region → increased mRNA stability → elevated prothrombin levels (~30% higher). Protein structure is normal; quantity is increased.

Risk: 3-5x increased VTE risk (heterozygous). Particularly associated with cerebral vein thrombosis (sagittal sinus).

Testing: Genetic testing (PCR) - prothrombin level is nonspecific (overlaps with normal).

Autosomal dominant with incomplete penetrance. Compound heterozygosity with FVL significantly increases risk.

Antithrombin Deficiency

Mechanism: AT inactivates Factors IIa, VIIa, IXa, Xa, XIa, XIIa. Heparin enhances AT ~1000-fold. Deficiency → unregulated coagulation + heparin resistance.

Risk: 10-50 fold increased VTE risk.

Types: Type I (quantitative - both antigen and activity decreased), Type II (qualitative - normal antigen, decreased activity). Prevalence 1 in 500-5000. First VTE often in 20s-40s.

Clinical clue: Heparin resistance - PTT doesn’t prolong with standard heparin doses. Options: give AT concentrate (Thrombate III, ATryn) to enable heparin, or switch to DTI (argatroban, bivalirudin).

Testing: AT activity assay (functional, chromogenic). Test BEFORE heparin or >24 hours after stopping heparin (heparin consumes AT, false lows).

Acquired AT deficiency: Heparin, L-asparaginase, estrogen therapy, nephrotic syndrome, pregnancy, DIC, liver disease, sepsis.

Protein C Deficiency

Mechanism: Protein C is activated by thrombin-thrombomodulin complex; APC inactivates Factors Va and VIIIa. Both Protein C and S are vitamin K-dependent.

Clinical:

VTE risk (5-10x increased)
Warfarin-induced skin necrosis (see above): Protein C half-life ~8 hours, drops before procoagulant factors on warfarin

Protein S Deficiency

Protein S is the cofactor for APC. Similar clinical features. 60% of Protein S is bound to C4b binding protein (an acute-phase protein); only free Protein S (~40%) is functionally active.

Testing pitfall: Estrogen, pregnancy, inflammation increase C4b-BP → more Protein S bound → functional deficiency. Measure during baseline health. Free Protein S is the meaningful measurement.

Homocysteine / MTHFR

Elevated homocysteine → thrombosis via endothelial toxicity, oxidative stress, impaired NO production, platelet activation, inhibition of natural anticoagulants.

Severe hyperhomocysteinemia (>100 μmol/L): Homozygous mutations in cystathionine β-synthase (CBS) or MTHFR.

Homocystinuria (CBS deficiency most common, AR): Marfanoid habitus + downward lens subluxation (contrast with Marfan - upward subluxation). Intellectual disability, osteoporosis, thromboembolism. Treatment: pyridoxine (B6, CBS cofactor), folate, B12, methionine-restricted diet, betaine.

Moderate/mild (15-100): Heterozygous mutations or nutritional deficiencies. MTHFR C677T heterozygous is very common (~10-15% of population), mild elevation with low folate.

B6, B9 (folate), or B12 deficiency elevates homocysteine. B12/folate are cofactors for methionine synthase (remethylation). B6 is cofactor for CBS (transsulfuration). Smoking and renal failure also elevate.

Elevated homocysteine + normal methylmalonic acid = folate deficiency; elevated both = B12 deficiency.

Treatment: folate and B-vitamin supplementation reduces homocysteine but clinical benefit for thrombosis prevention remains debated.

Acquired Thrombophilias

Paroxysmal Nocturnal Hemoglobinuria (PNH)

Somatic mutation in PIG-A gene (phosphatidylinositol glycan class A) in hematopoietic stem cells. PIG-A is required for first step of GPI anchor biosynthesis. PIG-A is X-linked so a single mutation inactivates the only active copy. Acquired, NOT inherited.

Without GPI anchors: absent CD55 (DAF) and CD59 (MIRL) on cell surfaces. CD55 accelerates decay of C3/C5 convertases. CD59 prevents MAC assembly by blocking C9 insertion. Result: unregulated complement activation → intravascular hemolysis + thrombosis.

PNH features (mnemonic PNHt):

Pancytopenia
Nocturnal dark urine (hemoglobinuria - actually continuous, not strictly nocturnal)
Hemolytic anemia (Coombs-negative, intravascular)
Venous Thrombosis (classically in unusual sites: hepatic/Budd-Chiari, mesenteric, cerebral veins)

Testing:

Screening: sucrose lysis test (sugar water) - low ionic strength promotes complement activation. Ham test (historical alternative)
Confirmation: flow cytometry - gold standard. FLAER (fluorescent aerolysin) on WBCs (binds GPI anchors directly, most sensitive), CD59 on RBCs, CD24 on granulocytes. Can detect clones as small as 0.01%. Classifies cells Type I (normal), Type II (partial), Type III (complete deficiency)

Treatment: Eculizumab (anti-C5 monoclonal) blocks terminal complement activation. Ravulizumab (longer-acting). Both require meningococcal vaccination (Neisseria risk).

Budd-Chiari syndrome: Hepatic vein thrombosis. Consider MPN screening (JAK2 V617F), PNH, Factor V Leiden, APS in all Budd-Chiari patients.

Antiphospholipid Syndrome (APS)

Pathophysiology: Autoantibodies against phospholipid-binding proteins - β2-glycoprotein I is the primary target (anti-cardiolipin antibodies actually bind cardiolipin-β2GPI complexes). Cause thrombosis through endothelial activation, platelet activation, and complement activation.

Clinical criteria (Sapporo/Sydney):

Vascular thrombosis (venous or arterial)
Pregnancy morbidity (recurrent miscarriage typically 2nd/3rd trimester, stillbirth, preeclampsia)

Laboratory criteria (must be persistent - positive on 2 occasions ≥12 weeks apart, and at medium-high titer >40 GPL/MPL units or >99th percentile):

Lupus anticoagulant (LA) (functional assay)
Anticardiolipin antibodies (aCL) IgG or IgM (ELISA)
Anti-β2-glycoprotein I antibodies (aβ2GPI) IgG or IgM (ELISA)

IgA is NOT diagnostic. Diagnosis requires ≥1 clinical + ≥1 laboratory criterion.

Lupus anticoagulant testing algorithm (three-step):

Screen: prolonged phospholipid-dependent test (dRVVT or sensitive PTT). Two different assay principles needed for adequate sensitivity
Mix: does not correct (inhibitor present)
Confirm: corrects with excess phospholipid (antibody is phospholipid-dependent). Methods: hexagonal phase phospholipid neutralization, platelet neutralization procedure, high-concentration phospholipid reagents

LA testing cannot be done on Factor Xa inhibitors or direct thrombin inhibitors (interfere with phospholipid-dependent assays → false positives). Anti-Xa assay is performed first to confirm no Xa inhibitor before LA testing.

Antibody ELISAs (anti-cardiolipin, anti-β2GPI) CAN be done on anticoagulants (measure antibody levels, not clotting times).

Anti-cardiolipin antibodies cause false-positive VDRL/RPR for syphilis (assays use cardiolipin as antigen). Specific treponemal tests (FTA-ABS, TP-PA) will be negative. Classic association.

Associated conditions: SLE (most common - 30-40% of SLE patients have aPL antibodies), HIV, other infections (hepatitis C, syphilis, Lyme, malaria), malignancy, medications (phenytoin, hydralazine, procainamide, chlorpromazine). Infection-associated often transient and rarely cause thrombosis. Primary APS (no underlying disease) ~50% of cases.

Catastrophic APS: Simultaneous multi-organ thrombosis, >50% mortality.

Nephrotic Syndrome

Hypercoagulable state from urinary loss of antithrombin III (similar MW to albumin, lost through damaged GBM). Plus increased hepatic synthesis of clotting factors (compensatory), elevated fibrinogen, platelet hyperactivation, decreased Protein C/S. Renal vein thrombosis is classic, especially with membranous nephropathy.

29.13 Anticoagulants and Reversal

Unfractionated Heparin (UFH)

Enhances AT activity ~1000-fold. Binds AT via pentasaccharide sequence. Inhibits Factors IIa and Xa (1:1 ratio). Long chains (≥18 saccharides) needed to simultaneously bind AT and thrombin.

Monitoring: PTT (therapeutic 1.5-2.5x control, or anti-Xa 0.3-0.7 IU/mL). Unpredictable PK due to non-specific protein binding (acute-phase reactants, PF4).

Reversal: Protamine sulfate (positively charged, binds negatively charged heparin). Dose: 1 mg protamine per 100 units heparin given in previous 2-3 hours. Side effects: hypotension (give slowly), anaphylaxis (higher risk with fish allergy, prior protamine, NPH insulin).

Low Molecular Weight Heparin (LMWH)

Enoxaparin (Lovenox), dalteparin (Fragmin). Shorter chains preferentially inhibit Factor Xa (anti-Xa:anti-IIa ~3-4:1) - too short to form AT-heparin-thrombin ternary complex.

Advantages: subcutaneous, predictable PK (no PTT monitoring in most), lower HIT risk (~0.1% vs 1-5% UFH), once/twice daily.

Renally cleared. Dose adjust for CrCl <30. Not effectively removed by dialysis. Obesity/pregnancy may need anti-Xa monitoring.

Reversal: protamine only partially reverses LMWH (~60% of anti-Xa activity).

Fondaparinux

Synthetic pentasaccharide, inhibits only Xa via AT. Very low HIT risk (no cross-reactivity with HIT antibodies). Renally cleared.

Warfarin

See above. Monitored by PT/INR.

Direct Oral Anticoagulants (DOACs)

Direct Xa inhibitors:

Rivaroxaban (Xarelto)
Apixaban (Eliquis)
Edoxaban

Oral, fixed dosing, no routine monitoring. Short half-lives (8-15 hours). Renal and hepatic clearance.

Labs: Neither PT nor PTT reliably monitors these. Drug-specific calibrated anti-Xa assay for special situations (overdose, emergency surgery, renal failure, extreme body weight).

Reversal: Andexanet alfa (Andexxa) - recombinant modified FXa decoy, binds and sequesters the Xa inhibitor. IV bolus + infusion. Very expensive. 4-factor PCC as alternative.

Direct thrombin inhibitors (DTIs):

Argatroban: IV, hepatically cleared - drug of choice for HIT in renal failure
Dabigatran (Pradaxa): oral, renally cleared, for afib and VTE
Bivalirudin: IV, proteolytically cleared, used in PCI and cardiac surgery

All prolong PTT and TT. Argatroban also prolongs PT/INR (complicates warfarin transition). Dabigatran causes very prolonged TT - a normal TT excludes clinically significant dabigatran. Quantitative: dilute thrombin time (dTT) or ecarin clotting time.

Reversal:

Dabigatran: Idarucizumab (Praxbind) - monoclonal antibody fragment, binds dabigatran with 350x higher affinity than thrombin. Complete reversal within minutes, lasts ~24 hours. Hemodialysis is also effective (65% dialyzable, low protein binding)
If idarucizumab unavailable: rFVIIa (bypassing agent) or 4-factor PCC
Argatroban, bivalirudin: no specific antidote, supportive care

29.14 Acquired Factor Inhibitors

Covered in detail above (Factor VIII inhibitors are the prototype). Key points revisited:

Anti-FVIII is the most common coagulation factor inhibitor
Mixing study: may correct immediately but prolongs on incubation (time-dependent)
Bethesda assay quantifies titer in BU
Low titer (<5 BU): high-dose factor replacement. High titer (≥5 BU): bypassing agents (rFVIIa, aPCC/FEIBA)
Acquired hemophilia A: sudden severe bleeding in previously healthy patient. Bimodal (young postpartum, elderly)
Treatment of acquired: control bleeding + eradicate inhibitor (immunosuppression: steroids ± cyclophosphamide or rituximab)

Chapter 30: Myeloid Neoplasms

Myeloid neoplasms arise from hematopoietic stem cells with abnormal differentiation, proliferation, or both. The WHO classification organizes them by molecular, morphologic, and clinical features.

The conceptual spine of this chapter is a single dichotomy that everything else hangs off of:

MDS = ineffective hematopoiesis. The marrow is hypercellular but the peripheral blood is cytopenic. The cells are being made, but they’re dysplastic and die in the marrow before they can exit (intramedullary apoptosis). You get dysplastic morphology and cytopenias.
MPN = effective but uncontrolled proliferation. The cells mature normally, look normal, and function normally. They’re just overproduced. You get non-dysplastic morphology and cytoses. Because the marrow eventually can’t contain the proliferation (or becomes fibrotic), hematopoiesis spills into the spleen - splenomegaly is an MPN feature, not an MDS feature.

Both sit “upstream” of AML. Both can transform to AML (≥20% blasts). Only MPN (specifically CML) can also transform to ALL, because BCR-ABL1 sits in a pluripotent stem cell capable of both myeloid and lymphoid differentiation.

Before diagnosing either, you must rule out reactive causes. Reactive cytoses: infection, hypoxia, inflammation, G-CSF exposure. Reactive cytopenias: infection, B12/folate deficiency, cytotoxic drugs, chronic disease, autoimmune. The longer an unexplained CBC abnormality persists, the more suspicious for a clonal process - in practice, persistent unexplained cytopenia/cytosis for more than 4 to 6 months warrants bone marrow evaluation.

This chapter covers the three families - AML, MDS, MPN - plus the MDS/MPN overlap syndromes (CMML, JMML, aCML), mast cell neoplasms, and the “pre-clonal” states CHIP and CCUS. It connects to Chapter 12 (DIC in APL), Chapter 20 (flow immunophenotyping), and Chapter 23 (mutation detection, MRD).

30.1 Acute Myeloid Leukemia (AML)

Definition: Clonal expansion of myeloid blasts (≥20% blasts in blood or marrow) OR presence of certain genetic abnormalities regardless of blast count.

AML is overwhelmingly a disease of older adults - median age 65. Only about 10% of childhood leukemias are myeloid (the rest are ALL). A few AML subtypes skew younger: APL hits young adults (median ~40s), t(9;11) KMT2A-rearranged hits children, t(1;22) RBM15-MKL1 and Down syndrome GATA1-mutated AML hit infants (<5 years).

The Three Diagnostic Pathways

You can reach an AML diagnosis through any one of three independent routes:

Morphologic: ≥20% myeloid blasts in blood or marrow
Genetic: An AML-defining genetic abnormality regardless of blast count (ICC says >10% blasts, WHO5 says “increased”)
Extramedullary: Myeloid sarcoma (chloroma) - a soft tissue mass of myeloid blasts is sufficient regardless of marrow blast percentage

This is a paradigm shift from purely morphologic classification. The genetic abnormalities carry such strong biology and treatment implications that waiting for the 20% threshold would delay therapy. Myeloid sarcoma gets a similar “shortcut” because the tissue mass proves the disease.

Myeloid sarcoma (formerly chloroma or granulocytic sarcoma) is a tumor of myeloid blasts outside the marrow. The green color that gave “chloroma” its name comes from MPO in the tumor cells. Common sites: skin (leukemia cutis), gingiva, orbit, bone, lymph nodes. It can appear de novo, precede marrow involvement by months, or mark relapse/progression.

Blast Counting

Blast percentage comes from a 500-cell count on the bone marrow aspirate differential and a 200-cell count on the peripheral blood smear. Use whichever compartment gives the higher percentage. The aspirate is the gold standard; CD34 IHC on the biopsy is supportive, useful for dry taps or hemodilute aspirates. If aspirate and biopsy disagree, the higher count generally wins.

Promyelocytes in APL and promonocytes in monocytic leukemias count as blast equivalents. They’ve begun partial differentiation but behave biologically like blasts. Without counting them, you may not hit the 20% threshold in these subtypes.

Myeloid Lineage Markers

Myeloperoxidase (MPO) is the lineage-defining marker for myeloid blasts. It lives in the primary (azurophilic) granules of granulocytes. MPO positivity in ≥3% of blasts by flow or cytochemistry confirms myeloid lineage. This matters when morphology is ambiguous (minimally differentiated AML, acute leukemias of ambiguous lineage). Note that monocytic, erythroid, and megakaryocytic AML subtypes may be MPO-negative - they rely on other lineage markers (CD14/CD64 for monocytic, CD41/CD61 for megakaryocytic, glycophorin A for erythroid).

The classic myeloid blast immunophenotype: CD34+, CD13+, CD33+, HLA-DR+, MPO+. Two important exceptions where CD34 is negative: (1) APL with t(15;17) and (2) NPM1-mutated AML. APL is also HLA-DR negative. So CD34-/HLA-DR- screams APL; CD34-/HLA-DR+ with cup-shaped nuclei suggests NPM1-mutated AML.

Classification (WHO 2022)

AML with recurrent genetic abnormalities (defined by specific translocations/mutations)
AML with myelodysplasia-related changes
Therapy-related myeloid neoplasms
AML, not otherwise specified (NOS)

AML with Recurrent Genetic Abnormalities

Subtype	Genetics	Features	Prognosis
APL	t(15;17) PML-RARA	Auer rods, DIC, promyelocytes	Good (with ATRA)
AML with t(8;21)	RUNX1-RUNX1T1	Auer rods, maturation	Favorable
AML with inv(16)/t(16;16)	CBFB-MYH11	Abnormal eosinophils	Favorable
AML with t(9;11)	KMT2A-MLLT3	Monocytic features	Intermediate
AML with NPM1 mutation	NPM1	Cup-like nuclear indentation	Favorable (if no FLT3-ITD)
AML with CEBPA mutation	Biallelic CEBPA	Often normal karyotype	Favorable
AML with FLT3-ITD	FLT3 internal tandem duplication	High WBC	Adverse
AML with TP53 mutation	TP53	Complex karyotype	Very adverse

Disease-Defining Genetics (≥20% blasts NOT required)

These nine abnormalities define AML at any blast count. Know them cold - the board loves testing which ones are disease-defining vs. which still require the 20% threshold.

RUNX1-RUNX1T1 t(8;21) - favorable (CBF leukemia)
PML-RARA t(15;17) - favorable (APL)
CBFB-MYH11 inv(16)/t(16;16) - favorable (CBF leukemia)
NPM1 mutation - favorable if no FLT3-ITD
KMT2A rearrangement t(9;11) - intermediate
DEK-NUP214 t(6;9) - adverse
MECOM rearrangement inv(3)/t(3;3) - adverse
RBM15-MKL1 t(1;22) - infant megakaryoblastic AML
NUP98 rearrangement - pediatric

Not disease-defining (still need ≥20% blasts):

t(9;22) BCR-ABL1 - must distinguish from CML blast crisis
RUNX1 mutation (point mutation, not translocation - adverse prognosis; contrast with the favorable t(8;21) translocation)
Biallelic CEBPA mutation (favorable if no FLT3-ITD, but still need blast count)
FLT3-ITD, TP53 mutation

Favorable Prognosis AML - The Five

Only five genetic abnormalities confer favorable prognosis in AML:

t(8;21) RUNX1-RUNX1T1
inv(16)/t(16;16) CBFB-MYH11
t(15;17) PML-RARA (APL)
NPM1 mutation - only if FLT3-ITD absent
Biallelic CEBPA mutation - only if FLT3-ITD absent

Think of FLT3-ITD as the “favorable prognosis eraser”. It downgrades NPM1 and biallelic CEBPA from favorable to intermediate/adverse. That’s why molecular panels in AML always test NPM1, CEBPA, AND FLT3 together - you need all three to decide prognosis. The CBF leukemias and APL stay favorable regardless of FLT3 status.

Morphologic and Genetic Associations

The board exam will show you a peripheral smear or marrow morphology and ask which genetic abnormality is present. The high-yield pairings:

inv(16) → abnormal eosinophils (large basophilic granules mixed with normal eosinophilic granules)
t(6;9) → increased basophils
inv(3) → small hypolobated megakaryocytes + paradoxical thrombocytosis
t(8;21) → Auer rods, abundant gray-blue cytoplasm, pseudo-Pelger-Huet granulocytes
NPM1 → cup-shaped (cupped) nuclei, CD34-negative
t(15;17) → hypergranular promyelocytes with faggot cells (multiple Auer rods), DIC

Acute Promyelocytic Leukemia (APL)

The most important AML to recognize because of its unique presentation and treatment.

Genetics: t(15;17)(q24;q21) creating PML-RARA fusion

Pathophysiology: The fusion protein blocks myeloid differentiation at the promyelocyte stage. Normally, RARα (retinoic acid receptor alpha) promotes myeloid differentiation. The PML-RARA fusion acts as a super-repressor - it recruits co-repressors and the histone deacetylase (HDAC) complex to silence differentiation genes. Cells arrest at the promyelocyte stage. These accumulated promyelocytes release procoagulant granule contents, causing DIC.

The DIC is why APL kills early. The mechanism: APL promyelocytes are packed with granules containing tissue factor, cancer procoagulant (a cysteine protease that directly activates factor X), and annexin II (which enhances fibrinolysis). When these cells lyse - or even just during active disease - massive release of these factors triggers simultaneous consumption of coagulation factors and platelets (DIC) and hyperfibrinolysis (bleeding). Patients die of intracranial hemorrhage. Start ATRA immediately on clinical suspicion, don’t wait for molecular confirmation.

Clinical presentation:

Often presents with DIC (bleeding and/or thrombosis)
May have hyperleukocytosis or leukopenia
Medical emergency - high early mortality from hemorrhage
Young adults (median age ~40s), not the usual AML age of 65

Morphology:

Hypergranular variant (classic): Hypergranular promyelocytes with numerous Auer rods. Multiple Auer rods in a single cell = “faggot cells” and this is pathognomonic for APL.
Microgranular variant: Bilobed/folded nuclei, granules so fine they’re barely visible on light microscopy. Can mimic monocytes. Often presents with very high WBC. Both variants are PML-RARA positive - the board loves showing the microgranular variant to test whether you recognize it as APL despite atypical morphology.

Promyelocytes count as blast equivalents in APL - without counting them, you may not reach diagnostic threshold.

APL promyelocytes: Hypergranular cells with abundant Auer rods. Multiple Auer rods in a single cell (“faggot cell”) is pathognomonic.

Immunophenotype:

CD34 negative
HLA-DR negative
CD117 positive (c-kit, confirms myeloid progenitor)
CD13+, CD33+
CD15 dim or negative (normal promyelocytes are CD15 bright - the dim CD15 is useful for MRD monitoring)

The CD34-/HLA-DR- pattern is characteristic. The only other commonly CD34-negative AML is NPM1-mutated AML, but that one is usually HLA-DR positive.

Treatment:

ATRA (all-trans retinoic acid): Vitamin A derivative. Pharmacologic doses overwhelm PML-RARA repression, so promyelocytes differentiate into mature granulocytes
Arsenic trioxide (ATO): Directly degrades PML-RARA protein and promotes apoptosis
Often no traditional chemotherapy needed for low- and intermediate-risk
With modern ATRA + ATO, cure rates >90%

Differentiation syndrome: Complication of ATRA/ATO therapy. As blasts differentiate, the inflammatory response causes fever, pulmonary infiltrates, hypoxia, and effusions. Treat with steroids (dexamethasone) - don’t stop the ATRA.

Core Binding Factor (CBF) Leukemias

t(8;21) and inv(16) are the two “CBF leukemias.” Both disrupt the core binding factor complex that is essential for definitive hematopoiesis. RUNX1 encodes core binding factor alpha (CBFα) - the DNA-binding subunit. CBFB encodes CBFβ, its obligate partner. Both translocations produce dominant-negative fusion proteins that block normal transcription.

t(8;21) disrupts CBFα
inv(16) disrupts CBFβ

Both are favorable prognosis and disease-defining regardless of blast count.

AML with t(8;21) RUNX1-RUNX1T1:

Morphology: Blasts with abundant gray-blue cytoplasm, large granules, Auer rods. Perinuclear clearing (“hofs”), sometimes salmon-colored granules. The blasts look somewhat differentiated - more cytoplasm than typical blasts.
Peripheral blood: Maturing granulocytes can show pseudo-Pelger-Huet nuclei (hypolobation - dysplastic change).
Immunophenotype: Standard myeloid markers (CD34, CD13, CD33, HLA-DR, MPO) PLUS aberrant CD19. CD19 is normally a B-cell marker. CD19 on myeloid blasts should make you think t(8;21). Useful for MRD monitoring.
KIT mutation modifier: If the flow shows loss of CD19 with gain of CD56 (NK marker, aberrant), suspect a concurrent KIT mutation (especially D816V). KIT mutations downgrade CBF-AML from favorable to intermediate/adverse.

AML with inv(16)/t(16;16) CBFB-MYH11:

Morphology: Myelomonocytic differentiation with characteristic abnormal eosinophils - large basophilic (dark purple) granules mixed with normal eosinophilic granules.
Cytochemistry trick: The abnormal eosinophils stain positive with α-naphthyl acetate esterase (ANAE). Normal eosinophils are ANAE-negative; ANAE is the monocyte stain. This is a classic boards cytochemistry question.
Immunophenotype: Standard myeloid markers plus monocytic markers (CD14, CD64) reflecting the myelomonocytic differentiation.

To separate the two myelomonocytic AMLs:

inv(16): adults, abnormal eos, CBF leukemia, favorable prognosis
t(9;11) KMT2A: children, no abnormal eos, intermediate prognosis

AML with KMT2A (MLL) Rearrangement t(9;11)

KMT2A (formerly MLL, “mixed lineage leukemia”) sits at 11q23 and encodes a histone methyltransferase critical for maintaining gene expression during development. Rearrangements appear in AML, ALL, and MPAL. The t(9;11) partner MLLT3 is the most common partner in AML.

More common in children
Myelomonocytic differentiation (CD14, CD64, CD11b)
Intermediate prognosis
Disease-defining regardless of blast count

AML with t(6;9) DEK-NUP214

DEK fuses with the nucleoporin NUP214. Adverse prognosis. Rare (<2% of AML). Characteristic basophilia in the marrow. Often has concurrent FLT3-ITD, which further worsens the outlook.

AML with inv(3)/t(3;3) MECOM

MECOM (EVI1) overexpression + GATA2 haploinsufficiency. Adverse prognosis. The board clue: paradoxical thrombocytosis (elevated platelets in a leukemia - unusual) with small hypolobated dysmorphic megakaryocytes and often multilineage dysplasia.

AML with t(1;22) RBM15-MKL1

Megakaryoblastic AML in infants (usually <1 year). MKL1 stands for megakaryoblastic leukemia 1 - the name tells you the lineage. More common in females. Occurs in phenotypically normal infants (not Down syndrome).

Immunophenotype: CD13+, CD33+, HLA-DR+ plus CD41 (GPIIb), CD61 (GPIIIa), CD42b (GPIb) - megakaryocytic markers.
Often MPO negative (megakaryocytes lack peroxidase).
Disease-defining regardless of blast count.

The three causes of megakaryoblastic AML:

t(1;22) - infants without Down syndrome, females
GATA1-mutated - infants WITH Down syndrome (favorable prognosis)
Acute megakaryoblastic leukemia NOS - adults, poor prognosis

AML with Mutated NPM1

NPM1 (nucleophosmin) is the most common mutation in AML (~30% of cases). NPM1 normally shuttles between nucleus and cytoplasm, involved in ribosome biogenesis and genomic stability. The mutation creates a new nuclear export signal so the protein gets stuck in the cytoplasm.

Disease-defining regardless of blast count
Cup-shaped (cupped) nuclei on morphology - nuclei with invaginations creating a “cup” or “fish mouth” appearance
CD34 negative (along with APL, one of only two commonly CD34-negative AMLs)
Usually HLA-DR positive (distinguishes from APL which is HLA-DR negative)
Often monocytic differentiation
Favorable prognosis only if FLT3-ITD is absent
NPM1 IHC: Detects the mutant protein’s aberrant cytoplasmic localization. Cytoplasmic staining = mutated (positive); nuclear staining = wild-type. Alternative to molecular testing.

AML with Biallelic CEBPA

CEBPA encodes a transcription factor essential for granulocytic differentiation. Biallelic (both alleles mutated) is what confers favorable prognosis; monoallelic doesn’t. Favorable only without FLT3-ITD. Often normal karyotype, younger patients. NOT disease-defining - still requires ≥20% blasts.

AML with BCR-ABL1 t(9;22)

NOT disease-defining in AML - still needs ≥20% blasts. Usually p210 fusion (same as CML). Must distinguish from CML blast crisis - look for prior chronic phase history, basophilia, splenomegaly.

BCR-ABL1 breakpoint cheat sheet:

p210 (major) - classic CML; most common in BCR-ABL1+ AML
p190 (minor) - most common in childhood Ph+ ALL; if in CML, associated with monocytosis (can mimic CMML); p190 protein has higher kinase activity than p210
p230 (micro) - rare; produces a milder CML-like picture with thrombocytosis and neutrophilic leukocytosis (resembles CNL)

Therapy-Related AML (t-AML)

AML that develops after cytotoxic therapy for a prior malignancy. Overall poor prognosis. The three causative exposures:

Agent	Latency	Classic genetics	Pattern
Topoisomerase II inhibitors (etoposide, doxorubicin)	1-5 years	KMT2A (MLL) rearrangement at 11q23	Presents as overt AML, no preceding MDS
Alkylating agents (cyclophosphamide, melphalan)	5-10 years	del(5q), del(7q), complex karyotype, TP53	Usually evolves through an MDS phase first
Radiation therapy	5-10 years	Similar to alkylators	Usually MDS phase first

The logic: topo II inhibitors cause specific double-strand breaks that get mis-repaired into balanced translocations, so you jump straight to an acute leukemia with a defining fusion gene. Alkylators and radiation cause cumulative widespread damage, leading to chromosome losses and a slower evolution through dysplasia.

Down Syndrome-Associated AML

Counterintuitive fact: Down syndrome AML has favorable prognosis. Cure rates approach 80-90%. The blasts are exquisitely sensitive to methotrexate and cytarabine - the triplicated cystathionine-β-synthase gene on chromosome 21 sensitizes them.

Age rule in Down syndrome leukemia:

AML → before age 5 (megakaryoblastic with GATA1 mutations)
ALL → after age 5 (B-ALL, usually favorable)

GATA1 mutations are the defining molecular feature. GATA1 is an X-linked transcription factor essential for erythroid and megakaryocytic differentiation. Mutations produce a truncated protein (GATA1s) that drives abnormal megakaryoblast proliferation. Somatic, not inherited.

Transient Abnormal Myelopoiesis (TAM): Occurs in ~10% of Down syndrome neonates, typically in the first week of life. Elevated WBC, hepatosplenomegaly, circulating megakaryoblasts, GATA1 mutation. Looks exactly like AML but spontaneously resolves within 1-2 months in most cases. But ~30% of TAM cases later develop true AML within 1-3 years, so follow them closely. A small percentage have severe/fatal TAM with liver fibrosis or hydrops. Can occur in phenotypically normal neonates with mosaic trisomy 21.

Acute Monoblastic/Monocytic Leukemia

Diagnosed when >80% of leukemic cells are monocytic lineage (monoblasts + promonocytes + monocytes). If 20-80% monocytic, it’s acute myelomonocytic leukemia.

Monocytic leukemias have classic clinical features driven by tissue tropism - monocytes naturally traffic to tissues (becoming macrophages), so malignant monocytes do the same:

Leukemia cutis (violaceous skin papules/nodules)
Gingival hyperplasia/infiltration
CNS involvement (leptomeningeal disease)
Elevated serum and urine lysozyme (muramidase) - can cause renal tubular damage

Immunophenotype: Predominantly monocytic markers with minimal myeloid markers - CD14, CD64, CD11b, CD11c. CD14 is the most specific monocyte marker (LPS receptor). MPO typically negative or weakly positive. Non-specific esterase (NSE) positive, inhibited by sodium fluoride.

Promonocytes count as blast equivalents in monocytic leukemias (critical for reaching the 20% threshold).

Acute Erythroid Leukemia (DiGuglielmo Syndrome)

Rare and aggressive. Requires >80% of marrow cells are erythroid AND >30% of those are proerythroblasts. The eponym DiGuglielmo syndrome comes from the 1917 description. Morphology: markedly dysplastic erythroid precursors with megaloblastoid changes, multinucleation, nuclear budding. Large proerythroblasts with deep basophilic cytoplasm.

Immunophenotype: CD71 (transferrin receptor), CD36, CD235 (glycophorin A), CD117. Typically negative for MPO, CD34, and standard myeloid markers. PAS stain shows block positivity in erythroblasts.

Cytochemistry and Immunophenotype

Cytochemical stains (historical, still useful):

Myeloperoxidase (MPO): Positive in myeloid cells
Sudan Black B: Positive in myeloid cells (similar to MPO)
Non-specific esterase: Positive in monocytic cells; inhibited by sodium fluoride

Flow cytometry immunophenotype (standard of care):

Lineage	Markers
Myeloid	CD13, CD33, CD117 (c-kit), MPO
Monocytic	CD14, CD64, CD11b, CD11c
Erythroid	CD71, Glycophorin A
Megakaryocytic	CD41 (GPIIb), CD61 (GPIIIa)
Blasts	CD34 (most), CD38

30.2 Myelodysplastic Syndromes (MDS)

Definition: Clonal hematopoietic disorders characterized by:

Ineffective hematopoiesis (cytopenias)
Morphologic dysplasia in ≥1 lineage
<20% blasts (otherwise = AML)
Risk of transformation to AML

MDS is predominantly a disease of older adults (>50, median ~70). MDS-like features in a younger patient should push you to consider: (1) therapy-related MDS (prior chemo/radiation), (2) inherited bone marrow failure syndromes (Fanconi anemia, dyskeratosis congenita), (3) reactive mimics, or (4) germline predisposition syndromes (GATA2 deficiency, RUNX1 familial platelet disorder, DDX41 mutations). Primary MDS under age 40 is genuinely rare.

Rule Out Reactive Mimics First

Before diagnosing MDS, you must exclude reactive/secondary causes of dysplasia - a board-favorite trap:

Drugs: methotrexate, azathioprine, mycophenolate, valproic acid, G-CSF
Alcohol: ring sideroblasts, vacuolated erythroid precursors
B12/folate deficiency: megaloblastic changes
Copper deficiency: ring sideroblasts + cytopenia - closely mimics MDS
Infections: HIV, parvovirus
Autoimmune conditions

The board will present a case that looks like MDS with a treatable reversible cause. Always check nutritional and medication history before committing.

Diagnostic Criteria (Morphologic MDS)

Cytopenia: defined as Hb <10, ANC <1800, or platelets <100K
Bone marrow showing dysplasia in ≥10% of cells in at least one lineage
Genetics are supportive but not required for morphologic MDS

Separately, there are genetically-defined MDS subtypes: del(5q), SF3B1 mutation, biallelic TP53 - these have their own specific criteria and can be diagnosed even without meeting full morphologic criteria.

Morphologic Dysplasia

Dyserythropoiesis:

Nuclear budding, multinucleation, megaloblastic changes
Ring sideroblasts (iron-laden mitochondria around nucleus)
Nuclear-cytoplasmic asynchrony (nucleus matures at different rate than cytoplasm)
Internuclear bridging, karyorrhexis, PAS positivity, vacuolization

Dysgranulopoiesis:

Hypogranulation (pale cytoplasm lacking normal granules)
Hypolobation - pseudo-Pelger-Huet (“pince-nez” or “aviator glasses” bilobed nuclei)
Hypersegmentation is NOT considered dysplasia (it’s a feature of B12/folate deficiency)
Small size, Auer rods (in MDS-IB), pseudo-Chédiak-Higashi granules (large fused granules)

Hypolobation and hypogranulation are the most reliable granulocytic dysplasia features.

Dysmegakaryopoiesis:

Small size, hypolobated “naked” nuclei, clustered in groups - these are micro-megakaryocytes that look like bare nuclei with minimal cytoplasm
Widely separated nuclear lobes

Megakaryocyte morphology is the best morphologic tool for distinguishing MDS from MPN:

MDS megas: small, hypolobated, naked nuclei, clustered
Normal megas: medium-sized, 2-4 nuclear lobes
MPN megas: large, hyperlobated “cloud-like” nuclei, staghorn shapes

Think of it as a spectrum: MDS (small/hypolobated) → Normal → MPN (large/hyperlobated).

MDS dysplastic megakaryocytes: Hypolobated, small megakaryocytes with separated nuclear lobes. Key morphologic finding in MDS.

Peripheral blood dysplasia: Macrocytes, hypogranular neutrophils, giant platelets, circulating blasts.

Intramedullary complement sensitivity (Ham’s test): MDS erythroblasts show increased susceptibility to complement-mediated lysis, similar to PNH, because MDS clones can have GPI-anchor deficiencies. MDS and PNH can coexist (“MDS-PNH overlap”). The Ham’s test (acidified serum lysis) is historical - now replaced by flow cytometry for CD55 and CD59 (GPI-anchored proteins) for PNH diagnosis. Still board-testable.

Ring Sideroblasts

Ring sideroblast definition: ≥5 iron granules (siderosomes) encircling ≥1/3 of the nuclear circumference on Prussian blue (iron) stain of the aspirate. Iron accumulates in mitochondria wrapped around the nucleus - the cell can’t incorporate iron into heme properly, so iron gets “trapped” in mitochondria. Normal sideroblasts have scattered fine granules, not a perinuclear ring.

Ring sideroblast thresholds for MDS-RS diagnosis:

>15% ring sideroblasts = MDS with ring sideroblasts
>5% if SF3B1 mutation is present

SF3B1 essentially IS the molecular explanation for ring sideroblasts. SF3B1 (splicing factor 3b subunit 1) is mutated in ~80% of MDS-RS cases. The mutation causes aberrant mRNA splicing of mitochondrial iron transport genes → mitochondrial iron accumulation → ring sideroblasts. And SF3B1 is the only mutation with favorable prognosis in MDS.

MDS Subtypes

Blast count drives classification:

Subtype	PB blasts	BM blasts	Other features
MDS with low blasts (MDS-LB)	<2%	<5%	Dysplasia ≥10%, RS <15%, monocytes <1000
MDS-LB with ring sideroblasts	<2%	<5%	RS ≥15% (or ≥5% with SF3B1)
MDS-LB with del(5q)	<2%	<5%	del(5q) sole or with one other (NOT del(7q))
MDS-IB1 (increased blasts 1)	2-4%	5-9%	RS criteria drops
MDS-IB2 (increased blasts 2)	5-19%	10-19%	OR Auer rods (even with low blast count)
MDS with biallelic TP53	Any	Any	Biallelic TP53 is disease-defining

Key teaching points:

In MDS-LB, the dysplastic lineage is often different from the cytopenic lineage (e.g., erythroid dysplasia but neutropenia). Board classic.
MDS-IB trumps MDS-LB-RS - if blasts are increased, RS criteria is dropped
Auer rods automatically classify as MDS-IB2 regardless of blast count
At ≥20% blasts, it becomes AML

MDS with isolated del(5q) deserves special attention. Unique features:

Macrocytic anemia, often with thrombocytosis
Characteristic hypolobated/monolobated megakaryocytes (medium-sized with unilobated round nuclei - different from the small/naked megas of other MDS; sometimes called “pawn ball” megakaryocytes)
Responds to lenalidomide (selectively targets del(5q) cells)
One additional cytogenetic abnormality allowed EXCEPT del(7q)
Favorable prognosis

Board favorite: macrocytic anemia + thrombocytosis + hypolobated megas + del(5q) = lenalidomide.

MDS with biallelic TP53 inactivation: Defined by TP53 mutations on BOTH alleles - either two mutations, or one mutation + loss of heterozygosity (del(17p)). Can be diagnosed regardless of blast count. Extremely poor prognosis with rapid AML progression, chemoresistance, complex karyotype, median survival <1 year.

Cytogenetics

Disease-defining cytogenetics in MDS: del(5q), del(7q)/monosomy 7, complex karyotype. These can define MDS even with minimal morphologic dysplasia.

Prognostic groupings:

Good prognosis: del(5q), del(11q), del(20q), -Y
Poor prognosis: complex karyotype (≥3 abnormalities), chromosome 7 abnormalities

Complex karyotype is the most common cytogenetic abnormality in MDS and carries poor prognosis. It often coexists with TP53 mutations (genomic instability breeds more abnormalities). Both complex karyotype and chromosome 7 loss are associated with therapy-related MDS.

Loss of Y is seen in elderly men and may not even be clonal (age-related), which is why it’s “good prognosis” - often effectively benign.

Genetics Summary

Biallelic TP53 is the ONLY disease-defining gene mutation in MDS (WHO 2022)
SF3B1 is the ONLY gene mutation with favorable prognosis in MDS
Commonly associated but not defining: ASXL1, TET2, DNMT3A, RUNX1, EZH2, SRSF2

Always Check the Monocyte Count

Check the peripheral blood monocyte count when working up MDS. If monocytes ≥1000/μL and ≥10% of the WBC differential, consider CMML instead of (or overlapping with) MDS. CMML is an MDS/MPN overlap with both dysplastic and proliferative features - missing it means missing a different prognosis and treatment approach.

IPSS-R Risk Stratification

The International Prognostic Scoring System (IPSS-R) incorporates:

Cytogenetics
Blast percentage
Hemoglobin
Platelet count
ANC

Categories range from Very Low to Very High risk, guiding treatment decisions. The newer IPSS-M (molecular) additionally incorporates gene mutations.

CHIP and CCUS - The Pre-MDS States

Two entities sit upstream of MDS that the board expects you to know. Both involve “MDS-type” mutations (DNMT3A, TET2, ASXL1, TP53, JAK2, SF3B1, and others) detected in blood, but without meeting full MDS criteria.

CHIP (Clonal Hematopoiesis of Indeterminate Potential): An MDS-type mutation detected in blood without cytopenias, morphologic dysplasia, or criteria for any myeloid neoplasm. Extremely common in the elderly (>10% of people over 70). CHIP increases cardiovascular risk (CHIP-associated atherosclerosis) and carries a low risk of progressing to MDS/AML (~0.5-1% per year). Risk factors for progression: higher variant allele frequency (VAF), multiple mutations, specific high-risk mutations (TP53, splicing factors like SRSF2, IDH1/2).

CCUS (Clonal Cytopenia of Undetermined Significance): An MDS-type mutation plus cytopenia but NOT meeting full MDS criteria (no significant dysplasia, no defining cytogenetics). Higher progression risk than CHIP because the cytopenia suggests the clone is already affecting hematopoiesis.

The hierarchy to remember:

Normal → CHIP (mutation, no cytopenia) → CCUS (mutation + cytopenia) → MDS (mutation + cytopenia + dysplasia or defining genetics)

30.3 Myeloproliferative Neoplasms (MPN)

MPNs are clonal stem cell disorders characterized by effective (not ineffective) proliferation of one or more myeloid lineages. Unlike MDS, the cells are morphologically normal and function properly - the problem is overproduction.

The four major MPNs:

CML (BCR-ABL1 positive) - stands apart with a defining translocation and targeted TKI therapy
Polycythemia vera (PV) - erythrocytosis
Essential thrombocythemia (ET) - thrombocytosis
Primary myelofibrosis (PMF) - fibrosis and extramedullary hematopoiesis

PV, ET, and PMF are the “BCR-ABL1-negative MPNs” - all three driven by JAK-STAT pathway activation. Two additional rare MPNs round out the group: chronic neutrophilic leukemia (CNL) and chronic eosinophilic leukemia (CEL).

Chronic Myeloid Leukemia (CML)

Defining feature: BCR-ABL1 fusion gene from t(9;22)(q34;q11) - the Philadelphia chromosome. BCR-ABL1 is required for diagnosis - without it, it’s not CML (consider atypical CML or other MPNs).

Pathophysiology: BCR-ABL1 encodes a constitutively active tyrosine kinase that drives uncontrolled proliferation of myeloid cells (especially granulocytes).

Detection methods:

Cytogenetics (see t(9;22))
FISH (BCR-ABL1 fusion signal)
RT-PCR (most sensitive, used for MRD monitoring - can detect 1 CML cell among 100,000 to 1,000,000 normal cells)

Breakpoint isoforms:

p210 (major) - nearly all CML; also most common in BCR-ABL1+ AML
p190 (minor) - most common in childhood Ph+ ALL; in CML, associated with monocytosis (CMML-like)
p230 (micro) - rare; “gentle” CML with thrombocytosis and neutrophilic leukocytosis (CNL-like)

Phases:

Chronic phase: Leukocytosis with full maturation spectrum (myelocytes, metamyelocytes, bands, neutrophils); basophilia; <10% blasts
Accelerated phase: 10-19% blasts, basophilia >20%, platelets <100K or >1000K unresponsive to therapy, cytogenetic evolution (new chromosomal abnormalities)
Blast phase (blast crisis): ≥20% blasts - still called “blast phase of CML,” NOT AML. The terminology matters - TKI therapy continues alongside acute leukemia treatment. 70% myeloid, 30% B-lymphoid (B-ALL). The lymphoid blast crisis is B-ALL because BCR-ABL1 also drives Ph+ B-ALL.

Laboratory findings:

Neutrophilic leukocytosis is the most common CBC abnormality - often >100,000/μL
All stages of granulocyte maturation (“myelocyte bulge”) - distinguishes from AML which has a hiatus
Basophilia (characteristic) - when you see basophilia, think CML; basophils are rarely reactive
Thrombocytosis (common)
Low LAP score (leukocyte alkaline phosphatase) - distinguishes from leukemoid reaction (which has high LAP). High LAP is also seen in PV and PMF.
Bone marrow: hypercellular, typically 90-100% cellular at diagnosis, packed with granulocytic precursors, often with small “dwarf” megakaryocytes characteristic of CML

CML: t(9;22) BCR-ABL1 creates the Philadelphia chromosome. TKI therapy (imatinib) has transformed prognosis.

Prognosis: Good - transformed by TKIs. Before TKIs, CML was fatal within 3-5 years. Now most patients achieve deep molecular remission and have near-normal life expectancy. Prognosis worsens with progression to accelerated phase or blast crisis.

MRD monitoring: PCR for BCR-ABL1 is the gold standard for CML MRD. Results are reported as BCR-ABL1/ABL1 ratio on the International Scale (IS). Key milestones:

Major molecular response (MMR): BCR-ABL1 ≤0.1% IS
Deep molecular response: ≤0.01% or undetectable
Patients in sustained deep molecular response may attempt TKI discontinuation (treatment-free remission)

Treatment: Tyrosine kinase inhibitors:

1st generation: imatinib
2nd generation: dasatinib, nilotinib, bosutinib
3rd generation: ponatinib (for T315I “gatekeeper” mutation - confers resistance to all other TKIs except ponatinib and asciminib)

Chronic Neutrophilic Leukemia (CNL)

Rare MPN defined by:

WBC >25,000/μL with mature neutrophilia
<10% immature myeloid forms (mature, not blasts)
No dysplasia
BCR-ABL1 negative
Splenomegaly

Looks like CML but with mature (not immature) granulocytes. Defining mutation: CSF3R (colony-stimulating factor 3 receptor = G-CSF receptor). CSF3R mutations cause constitutive activation of G-CSF signaling → excessive mature neutrophil production. Two CSF3R mutation types: membrane-proximal (T618I most common, activates JAK-STAT) and truncation mutations (activate SRC kinase).

CSF3R is to CNL what BCR-ABL1 is to CML.

Chronic Eosinophilic Leukemia (CEL)

Defined as:

Persistent eosinophilia >1500/μL
Bone marrow involvement
Evidence of clonality
Exclusion of ALL other causes

CEL is essentially a diagnosis of exclusion. You must rule out:

Reactive eosinophilia (parasites, allergy, drugs)
Myeloid/lymphoid neoplasms with eosinophilia-defining gene rearrangements

The eosinophilia-defining rearrangements matter because they have targeted therapy:

PDGFRA (responds dramatically to imatinib, even at low doses) - FIP1L1-PDGFRA is most common, a cryptic deletion not visible on karyotype - need FISH
PDGFRB (responds to imatinib)
FGFR1 (aggressive, may present as lymphoma or leukemia)
PCM1-JAK2

These rearrangements are classified as “myeloid/lymphoid neoplasm with (mutation)” and are separate from CEL. Don’t miss them - imatinib works.

The BCR-ABL1-Negative MPNs

Three related disorders, all driven by mutations activating the JAK-STAT pathway. All three converge on JAK-STAT even when the primary mutation isn’t in JAK2:

JAK2 V617F - activating mutation in the JAK2 kinase itself
CALR - calreticulin mutations make the protein bind and activate MPL (thrombopoietin receptor) → JAK2 activation
MPL - direct activating mutations in the thrombopoietin receptor → JAK2 activation

Driver Mutation	PV	ET	PMF
JAK2 V617F	~95%	~60%	~60%
CALR mutation	Rare	~25%	~25%
MPL mutation	Rare	~5%	~5%
Triple-negative	Rare	~10%	~10%

Prognosis ranking of the JAK2-associated MPNs: ET (best) → PV (intermediate) → PMF (worst).

ET: median survival >20 years, low AML transformation risk
PV: median survival ~14 years, can progress to post-PV MF or AML
PMF: median survival 5-7 years, highest AML transformation risk

In ET, mutation status affects prognosis: CALR-mutated has better prognosis than JAK2-mutated; triple-negative ET has the worst.

Polycythemia Vera (PV)

Defining feature: Increased red cell mass (polycythemia) independent of erythropoietin

Pathophysiology: JAK2 V617F causes constitutive activation of erythropoietin receptor signaling → EPO-independent erythropoiesis. Red cell production continues even without EPO. The high Hgb triggers negative feedback suppressing EPO production → serum EPO is LOW in PV. This is the opposite of secondary polycythemia (COPD, sleep apnea, high altitude, EPO-secreting tumors) where EPO is high because it’s driving the erythrocytosis.

So the lab pattern: ↑Hct + ↓EPO = PV. Low EPO = primary (PV); high EPO = secondary. Serum EPO is often the first step in polycythemia workup.

Clinical features:

Ruddy complexion (plethora)
Pruritus, especially after bathing - aquagenic pruritus (mast cell degranulation triggered by water; highly specific for PV - board vignette clue)
Splenomegaly
Headaches (from hyperviscosity)
Thrombosis (stroke, MI, Budd-Chiari syndrome, portal vein thrombosis)
Erythromelalgia (burning pain in hands/feet)

Classic vignette: pruritus after bathing + splenomegaly + headaches + erythrocytosis = PV.

Laboratory findings:

Elevated hemoglobin/hematocrit: Hgb >16.5 (men) or >16 (women); Hct >49% (men) or >48% (women)
Often elevated WBC and platelets
Low serum EPO
JAK2 V617F positive (~95%); if negative, check JAK2 exon 12 (~5%) - exon 12 is unique to PV and tends to cause isolated erythrocytosis without leukocytosis or thrombocytosis

Diagnosis (WHO criteria):

Erythrocytosis (Hgb/Hct above threshold)
Bone marrow: hypercellular with panmyelosis (all three lineages increased), megakaryocytes of varying sizes with bulbous “cloud-like” nuclei
JAK2 V617F or JAK2 exon 12 mutation (essentially required - PV must have a JAK2 mutation)
Minor criterion: low EPO

Must rule out secondary polycythemia (check EPO, oxygen saturation, smoking history).

Typical PV bone marrow morphology: hypercellular with panmyelosis + megakaryocytes with bulbous cloud-like nuclei of varying sizes. Unlike ET (normocellular with only megakaryocyte hyperplasia), PV has everything increased. Unlike ET (uniformly large staghorn megas), PV has varying sizes.

Complications:

Thrombosis (leading cause of death) - elevated Hct → increased viscosity → sluggish flow. Classic events: stroke, MI, DVT/PE, Budd-Chiari, portal vein thrombosis
Progression to post-PV myelofibrosis (“spent phase”) in ~15% - blood counts drop (cytopenias develop), smear shows leukoerythroblastic changes, marrow shows fibrosis. Essentially PV burning itself out and evolving into a PMF picture.
Transformation to AML (<5%)

Treatment: Phlebotomy (target Hct <45%); low-dose aspirin; cytoreduction (hydroxyurea) if high-risk.

Essential Thrombocythemia (ET)

Defining feature: Sustained thrombocytosis (≥450,000/μL) with characteristic megakaryocyte morphology. Megakaryocytes produce excess platelets because of activated JAK-STAT signaling.

Clinical features:

Thrombosis (arterial > venous) - the major risk
Paradoxical bleeding at very high platelet counts (>1,000,000) due to acquired von Willebrand syndrome - large vWF multimers get adsorbed by excess platelets, leaving less functional vWF in circulation
Erythromelalgia (burning pain in hands/feet)

Bone marrow: Normocellular (not hyper!) with increased megakaryocytes having “staghorn” nuclei (deeply hyperlobated, resembling deer antlers). Minimal or no reticulin fibrosis (MF-0 or MF-1). Erythroid and granulocytic lineages are unremarkable.

The normocellularity is key - it distinguishes ET from PV (hypercellular) and PMF (may be hypercellular early, then hypocellular late with fibrosis). If you see hypercellularity + erythroid hyperplasia in a patient with thrombocytosis, think PV (which often has concurrent thrombocytosis).

Diagnosis (WHO criteria) - all four must be met:

Platelet count ≥450,000/μL
Bone marrow showing increased megakaryocytes with “staghorn” nuclei, no significant fibrosis (MF-0 to MF-1)
Does not meet criteria for CML, PV, PMF, MDS, or other myeloid neoplasm
JAK2, CALR, or MPL mutation (or other clonal marker) - supports but not absolutely required; ~10% are triple-negative and diagnosed by exclusion

ET has the best prognosis of the JAK2-associated MPNs.

Primary Myelofibrosis (PMF)

Defining feature: Bone marrow fibrosis with extramedullary hematopoiesis

Pathophysiology: Clonal megakaryocytes release profibrotic cytokines (TGF-β, PDGF), stimulating fibroblast proliferation and collagen deposition. Fibrosis replaces hematopoietic marrow, so hematopoiesis moves to spleen and liver. Note: the fibrosis is reactive - the fibroblasts themselves are not part of the neoplastic clone.

Clinical features:

Massive splenomegaly (extramedullary hematopoiesis)
Constitutional symptoms (fatigue, weight loss, night sweats)
Cytopenias (despite being an MPN)
Portal hypertension

Biphasic CBC course:

Early: thrombocytosis (similar to ET - “prefibrotic” PMF)
Late: progressive cytopenias (anemia, thrombocytopenia, leukopenia) as fibrosis destroys the marrow

Peripheral blood findings:

Leukoerythroblastic picture: Nucleated RBCs + immature granulocytes (cells squeezed out of fibrotic marrow or from extramedullary hematopoiesis)
Teardrop cells (dacrocytes): Distorted by trying to exit fibrotic marrow
Large platelets, thrombocytosis or thrombocytopenia

Bone marrow: Hypercellular early with increased megakaryocytes having angulated, hyperchromatic nuclei and varying fibrosis. The megakaryocytes in PMF look different from ET or PV - more atypical with dense, dark nuclei and angular shapes. Dry tap on aspiration is common because fibrosis prevents marrow extraction - core biopsy is essential. Late stages can show osteosclerosis (new bone formation in the marrow cavity - end-stage fibrosis where even collagen is replaced by bone; marrow becomes rock-hard).

Diagnosis (WHO criteria):

CBC showing anemia or leukocytosis
Bone marrow showing “typical PMF morphology” (atypical megakaryocytes + fibrosis)
JAK2, CALR, or MPL mutation
Splenomegaly and/or increased LDH (reflects extramedullary hematopoiesis and cell turnover)

Myelofibrosis grading (reticulin stain):

MF-0: scattered linear reticulin with no intersections - normal or near-normal
MF-1: loose reticulin network with intersections - mild, can be seen in ET or prefibrotic PMF
MF-2: thin and thick reticulin bands with extensive intersections - overt myelofibrosis
MF-3: dense reticulin bands with focal collagen and/or osteosclerosis - most severe, confirmed with trichrome stain (collagen stains blue)

MF grade ≥2 is required to diagnose overt PMF (vs. prefibrotic PMF with MF-0/1).

Post-PV/ET myelofibrosis: When PV or ET develops MF grade 2-3, it’s reclassified as post-PV or post-ET myelofibrosis. Clinically behaves like PMF - splenomegaly, leukoerythroblastic smear, teardrop cells, cytopenias. Original MPN diagnosis remains important for understanding disease trajectory and prior therapy.

Accelerated phase of PV/ET/PMF: 10-19% blasts (same threshold as CML). At ≥20% blasts, it becomes blast phase/AML transformation. Accelerated phase signals imminent leukemic transformation - consider allogeneic transplant in eligible patients.

Prognosis: Worse than PV or ET; median survival 5-7 years.

Treatment: JAK inhibitors (ruxolitinib) for symptoms and splenomegaly; allogeneic stem cell transplant is the only curative option for eligible patients.

30.4 MDS/MPN Overlap Syndromes

These disorders have features of BOTH: dysplasia (MDS-like) AND proliferation/cytosis (MPN-like). They don’t fit neatly into MDS or MPN alone - they bridge both categories.

The three MDS/MPN overlap syndromes:

Chronic myelomonocytic leukemia (CMML) - adults with monocytosis + dysplasia
Juvenile myelomonocytic leukemia (JMML) - children with monocytosis + RAS pathway mutations
Atypical CML (aCML) - neutrophilic leukocytosis with dysplasia, BCR-ABL1 negative

Chronic Myelomonocytic Leukemia (CMML)

Diagnostic criteria:

Persistent monocytosis ≥1000/μL (and ≥10% of WBC differential)
Bone marrow dysplasia in ≥1 lineage
<20% blasts + promonocytes (promonocytes are blast equivalents)
BCR-ABL1 negative
Excluded reactive monocytosis and eosinophilia-defining gene rearrangements

Most common MDS/MPN overlap syndrome. The BCR-ABL1 exclusion matters because CML with p190 breakpoint can present with monocytosis mimicking CMML.

CMML subtypes by WBC:

Proliferative type (WBC >13K): more MPN-like, splenomegaly common, higher AML transformation risk
Dysplastic type (WBC <13K): more MDS-like, cytopenias predominate

CMML blast-based subtypes:

CMML-0: <2% PB blasts, <5% BM blasts
CMML-1: 2-4% PB or 5-9% BM
CMML-2: 5-19% PB or 10-19% BM or Auer rods

Monocyte morphology: ranges from normal to atypical (immature, dysplastic, irregular nuclei). You can’t rely on morphology alone - you need the absolute count. Flow cytometry with increased “classical” monocytes (CD14+/CD16-) >94% is characteristic of CMML, even when morphology is unremarkable.

Commonly mutated genes: TET2 (~60%), SRSF2 (~50%), ASXL1 (~40%), SETBP1 (~15%). SRSF2 mutations are particularly characteristic of CMML. The RAS pathway (NRAS, KRAS) is also frequently mutated (~30%).

With eosinophilia: If CMML is suspected AND there’s eosinophilia, test for PDGFRA, PDGFRB, FGFR1, PCM1-JAK2 rearrangements. Critical - these have targeted therapy (imatinib for PDGFRA/PDGFRB) and get classified as a different entity.

Atypical CML (aCML)

Differs from CML:

Leukocytosis with spectrum from neutrophils to promyelocytes (showing dysplasia) - left-shifted with dysgranulopoiesis. CML has predominantly mature neutrophils without dysplasia.
BCR-ABL1 negative (required for exclusion)
Granulocytic dysplasia is prominent

More aggressive than CML. Doesn’t respond to TKIs. Characteristic mutations: SETBP1 and ETNK1. SETBP1 stabilizes SET → inhibits PP2A tumor suppressor → promotes proliferation.

Juvenile Myelomonocytic Leukemia (JMML)

The pediatric counterpart of CMML. Most patients <4 years old.

Key features:

Monocytosis ≥1000
Splenomegaly
Increased hemoglobin F (HbF) - fetal hemoglobin reactivated in the neoplastic clone
GM-CSF hypersensitivity - JMML cells proliferate in response to much lower GM-CSF concentrations than normal (the hallmark in vitro test)

Molecular: All JMML mutations converge on the RAS pathway. Usually mutually exclusive - one mutation per case:

PTPN11 (~35%) - encodes SHP-2 phosphatase, activates RAS; germline in Noonan syndrome
NRAS/KRAS (~25%) - direct RAS activation
NF1 (~15%) - loss of neurofibromin (a RAS-GAP that inactivates RAS); germline in neurofibromatosis type 1
CBL (~15%) - E3 ubiquitin ligase that normally targets RAS for degradation

Mutations are usually somatic, except NF1 (germline in NF1 syndrome) and PTPN11 (germline in Noonan syndrome).

Clinical pearl: A small portion of JMML patients have NF1. Loss of neurofibromin → constitutive RAS activation → myeloid proliferation - the same mechanism as sporadic RAS mutations, just reached by a different route.

30.5 Mast Cell Neoplasms

Mast cell neoplasms range from solitary skin lesions to widespread disease with marrow involvement. All driven by the KIT receptor.

Spectrum:

Cutaneous mastocytosis - skin only, most common in children, usually benign
Systemic mastocytosis - extracutaneous organs (bone marrow, others)

Cutaneous Mastocytosis

A dermal-based lesion of increased abnormal mast cells (dermis, not epidermis). Most common form:

Urticaria pigmentosa - reddish-brown macules/papules that urticate when stroked (Darier sign - wheal formation due to mast cell degranulation)
Solitary mastocytoma
Diffuse cutaneous mastocytosis

Most pediatric cases are self-limited and may resolve at puberty. Adult cutaneous mastocytosis is more likely to be associated with or progress to systemic disease.

Systemic Mastocytosis

Diagnostic criteria - need 1 major + 1 minor, OR 3 minor criteria:

Major criterion: Multifocal dense mast cell aggregates (≥15 cells) in BM or other extracutaneous organ.

Minor criteria (need any 3 if no major):

>25% of mast cells are spindle-shaped or atypical morphology
KIT D816V mutation
Aberrant CD2, CD25, or CD30 on CD117+ cells
Persistent serum tryptase >20 ng/mL

Neoplastic mast cell immunophenotype: The key to distinguishing neoplastic from normal/reactive mast cells is aberrant expression of CD25 and CD2 (not expressed on normal mast cells) with diminished CD117 (not lost, just decreased). Normal mast cells express CD117 (strongly), CD45, CD11c, CD33, CD43. Neoplastic = CD25+, CD2+, diminished CD117. CD30 was added as another aberrant marker in the latest WHO.

CD25 expression correlates with KIT D816V mutation - they almost always co-occur. The mechanism: KIT D816V drives aberrant gene expression programs including CD25 (IL-2 receptor alpha chain). CD25 positivity on flow or IHC serves as a surrogate marker for KIT D816V.

KIT D816V:

Found in >90% of adult systemic mastocytosis
KIT is a receptor tyrosine kinase (CD117) - normally activated by stem cell factor
D816V is a point mutation in the kinase domain → constitutive activation without ligand
Confers resistance to imatinib (targets a different KIT conformation) but sensitivity to midostaurin and avapritinib
Same mutation can be found in CBF-AML as a secondary adverse prognostic modifier

Serum tryptase is the blood marker for mast cell burden - persistent elevation >20 ng/mL is a minor criterion.

Chapter 31: Lymphoid Neoplasms

Lymphoid neoplasms are a huge category. They range from indolent conditions that look benign under the scope (CLL) to the fastest-growing human tumors (Burkitt). The classification has gotten progressively molecular over time. For boards you need to know the defining translocation or mutation for most of these entities, the immunophenotype that nails the diagnosis, and the clinical context that makes a particular entity likely.

This chapter is organized by cell of origin and compartment. We start with normal lymph node architecture and reactive patterns (because you can’t diagnose lymphoma without knowing what benign looks like), then work through B-cell neoplasms (ALL, CLL, hairy cell, follicular, mantle cell, marginal zone, DLBCL, Burkitt, plasma cell), T/NK neoplasms, and Hodgkin lymphoma.

31.1 Normal Lymph Node Anatomy and Reactive Conditions

You cannot call something a lymphoma if you don’t know what a reactive lymph node looks like. This section covers the normal architecture and the reactive patterns that get confused with lymphoma on boards.

Lymph Node Architecture

From outside in: capsule → subcapsular sinus → cortex → paracortex → medulla. The cortex contains B-cell follicles. The paracortex contains T-cells. The medulla contains medullary cords (plasma cells) and medullary sinuses.

Compartments and what lives there:

B-cells reside in follicles (cortex)
T-cells reside in the paracortex
Plasma cells reside in medullary cords
NK cells and macrophages in sinuses

Why this matters for lymphoma: B-cell lymphomas often recapitulate follicular architecture (FL is nodular). T-cell lymphomas often show paracortical/interfollicular growth. Metastatic carcinoma deposits first in the subcapsular sinus (the “front door” delivered by afferent lymphatics). Lymphomas efface the node and compress sinuses; metastatic carcinoma fills sinuses while initially preserving architecture.

Primary vs. Secondary Follicles

Primary follicles are resting, composed of naive B-cells, no germinal center. Secondary follicles are active, with germinal centers where affinity maturation happens. The presence of a germinal center is what distinguishes secondary from primary follicles.

Mantle zone: contains naive B-cells passing through the node looking for antigen. Small nuclei, compact chromatin (look like dark ink dots). These are the cells that mantle cell lymphoma arises from. Mantle zone cells are BCL2+, variably CD5+, IgD+.

Germinal Center Biology

Germinal centers have two zones with opposite staining intensity:

Dark zone: centroblasts - large, mitotically active, undergo somatic hypermutation. Multiple nucleoli at the nuclear membrane.
Light zone: centrocytes - small, elongated or “twisted” (cleaved) nuclei. Compete for antigen presented by follicular dendritic cells. Those that fail to bind undergo apoptosis.

Centroblasts → centrocytes is the normal maturation. Centrocytes that survive selection become either memory B-cells (recirculate) or plasma cells (migrate to medullary cords).

Centroblast vs. immunoblast (both large): centroblasts have multiple nucleoli at the nuclear membrane, immunoblasts have ONE large central nucleolus. This matters for FL grading (count centroblasts, not immunoblasts) and for distinguishing reactive immunoblast proliferations from lymphoma.

Tingible body macrophages (TBMs): hallmark of reactive germinal centers. TBMs clear apoptotic B-cells that failed affinity maturation. Their presence indicates active, healthy GC activity. TBMs are absent in follicular lymphoma because BCL2 prevents apoptosis (nothing to clear).

GC polarity: light zone vs. dark zone distinction is a feature of reactive GCs and is lost in follicular lymphoma.

IHC in Normal Lymph Node

Knowing normal IHC patterns is half of hemepath. Key markers:

Marker	Normal Pattern	Notes
CD20	Whole follicle (GC + mantle zone)	Membranous; blocked by rituximab
PAX5	Nuclear B-cell marker	Misses plasma cells (they downregulate PAX5)
CD79a	Pan B-cell including plasma cells	More comprehensive than PAX5
CD10	Germinal center only, membranous	First marker in Hans algorithm
BCL6	Germinal center only, nuclear	GC transcription factor
BCL2	Mantle zone + T-cells, GC NEGATIVE	Membranous. GC positivity = think FL
CD21	FDC meshwork	Expanded/disrupted in AITL, FL
CD23	FDC meshwork + reactive mantle zone	Positive in CLL, negative in MCL
CD138	Plasma cells in medullary cords	Syndecan-1, membranous
Ki-67	Stains centroblasts (dark zone) > centrocytes	Shows GC polarity

Cyclin D1 (BCL1) is nuclear, BCL2 is membranous. Don’t confuse these.

Normal GC B-cells express both CD10 and BCL6. When you see CD10/BCL6 positivity in a B-cell neoplasm, it indicates GC origin (FL, Burkitt, GCB-DLBCL). Normal GC B-cells are BCL2 negative because they need to be able to undergo apoptosis during selection.

Germinal center T-cells (TFH) express pan T-cell markers (CD3, CD2, CD5) plus CD57 and PD1. These are the T-follicular helper cells. Angioimmunoblastic T-cell lymphoma (AITL) arises from TFH cells and shares these markers.

CD79a vs. PAX5 coverage: CD79a covers all B-cells including plasma cells. PAX5 misses plasma cells. So if you need to identify B-lineage cells including plasma cells, use CD79a. If rituximab is blocking CD20, use PAX5 or CD19.

CD4:CD8 Ratios - A Cheat Sheet

Normal CD4:CD8 ratio is 3-4:1 (helpers predominate).

Inverted (CD8 > CD4): viral infections, EBV lymphadenitis, immunoblast hyperplasia. EBV infects immunoblasts, triggering a massive CD8+ response.
Increased: classical Hodgkin lymphoma microenvironment (immunosuppressive).
Markedly decreased CD4: HIV/AIDS.
>10 (CD4 dominant): Sezary syndrome diagnostic criterion.

Reactive T-Cell Patterns

Reactive T-cells show progressive (partial) downregulation of CD7 - not complete loss, just gradual decrease. Complete CD7 loss is a feature of T-cell lymphomas. Reactive = progressive/partial. Neoplastic = aberrant complete loss or loss of other pan-T markers.

Early (immature) T-cells express cytoplasmic CD3 only. They don’t gain surface CD3 until reaching the medullary thymocyte/peripheral T-cell stage. Important for T-ALL: cytoplasmic CD3 is the earliest T-cell marker.

Reactive Patterns - Common Entities

Reactive lymph nodes have two dominant patterns: follicular (B-cell mediated: infections, autoimmune) and interfollicular/paracortical (T-cell mediated: viral infections, drug reactions).

Keys to diagnosing reactive lymphadenopathy: variably-sized follicles and open sinuses. Plus TBMs, polarized GCs, and mixed cell populations.

Reactive vs. neoplastic (FL):

Reactive: variable follicles, open sinuses, TBMs, polarity, BCL2-negative GCs
FL: uniform follicles, back-to-back, compressed sinuses, no TBMs, no polarity, BCL2+ GCs

Specific Reactive Entities

Toxoplasma lymphadenitis: classic triad - reactive follicles, epithelioid histiocytes within follicles (encroaching on GCs), and monocytoid B-cells in sinuses. The intrafollicular histiocytes are the most distinctive feature, giving a “moth-eaten” appearance to the GC.

Syphilis lymphadenitis (leutic lymphadenitis): reactive follicles, inter- and intrafollicular plasmacytosis, and fibrosis. Back-to-back follicles can mimic lymphoma but TBMs, polarized GCs, and open sinuses support reactive. Plasmacytosis in lymph nodes - think syphilis, rheumatoid arthritis, or Castleman disease (plasma cell variant).

Kikuchi-Fujimoto disease (histiocytic necrotizing lymphadenitis): necrosis, absent neutrophils, crescent-shaped histiocytes. The absence of neutrophils is the distinguishing feature from other necrotizing lymphadenopathies. Demographics: young women, with higher reported frequency in East Asian populations; cervical lymph nodes. Self-resolving over 1-4 months. Must clinically rule out lupus - the histology can look identical (lupus may show hematoxylin bodies, neutrophils, plasma cells).

HIV lymphadenopathy:

Early (hyperplastic): irregularly-shaped follicles (“follicular lysis”), minimal mantle zones, Warthin-Finkeldey cells (multinucleated polykaryocytes, also seen in measles).
Late (involutional): atrophic (burnt-out) follicles. Immune exhaustion as CD4 T-cells decline.

Persistent generalized lymphadenopathy (PGL): lymphadenopathy >2 cm in ≥2 regions for >3 months due to HIV alone. First sign of HIV infection. After initial workup excludes other causes, no repeat biopsy needed if nodes are stable.

Castleman disease: benign neoplasm of follicular dendritic cells. Two types:

Hyaline vascular type (~90%, unicentric): burnt-out germinal centers, onion-skinning lymphocytes, hyalinized vessels (lollipop appearance). Curable with excision.
Plasma cell variant (multicentric): associated with HHV-8 and HIV. Plasma cells in the interfollicular region. Classically lambda-restricted plasma cells (most commonly IgM lambda), though overall polytypic. Background PCs express IgA. HHV-8 produces a viral IL-6 homolog driving the plasmacytosis and systemic symptoms.

Progressive transformation of germinal centers (PTGC): ill-defined, markedly enlarged germinal centers with mantle zone lymphocytes infiltrating into the GC. Benign but associated with NLPHL (not a significant risk factor - just associated). If you see PTGC, search for features of NLPHL.

EBV lymphadenitis: inverted CD4:CD8 ratio (EBV infects immunoblasts, triggers massive CD8 response). Differentiate EBV lymphadenitis from EBV-positive lymphoma by size range of EBER+ cells: reactive has EBER+ cells of variable sizes (small AND large); lymphoma has EBER+ cells all the same size (monotonous, reflecting clonality).

Dermatopathic lymphadenitis: melanin-pigmented histiocytes in a reactive node from a patient with skin disease/rash. Histiocytes have phagocytosed melanin from the inflamed skin. Classically drains chronic dermatitis, psoriasis, or eczema. Can also be seen in nodes draining mycosis fungoides.

Cat scratch disease (Bartonella henselae): unilateral cervical/axillary lymphadenopathy with stellate granulomas (star-shaped with central necrosis, palisading histiocytes). Classic scenario: child/young adult with cat scratch, regional lymphadenopathy 1-3 weeks later.

Stellate granulomas DDx: cat scratch disease, lymphogranuloma venereum, tularemia. All infectious, all with suppurative necrosis.

Other granuloma patterns:

Necrotizing (caseating) granulomas → think mycobacterial TB first. AFB stain and culture essential. Also consider other mycobacteria, Histoplasma, Coccidioides.
Non-necrotizing granulomas with giant cells → sarcoidosis (diagnosis of exclusion, rule out infection). Schaumann and asteroid bodies are suggestive but not specific.

HSV lymphadenitis: inguinal lymphadenopathy with necrosis and cells with multinucleation, margination (nuclear chromatin pushed to the edge), and molding - classic herpesvirus cytopathic changes.

Histiocytic Disorders

Langerhans cell histiocytosis (LCH): cells with folded “coffee bean” nuclei (nuclear grooves) and abundant eosinophils. Birbeck granules (racket-shaped) on EM are pathognomonic but rarely needed. IHC: S100+, CD1a+, Langerin+ (CD207). CD68 is variable. Molecular: BRAF V600E in ~50-60% (MAPK pathway activation). MAP2K1 in BRAF wild-type cases. Establishes LCH as a clonal neoplasm. If LCH is found in a lymph node, it’s likely multisystem disease - check bone marrow.

Rosai-Dorfman disease (RDD) (sinus histiocytosis with massive lymphadenopathy): atypical histiocytes with emperipolesis (intact lymphocytes within the cytoplasm of histiocytes). Defining morphologic feature. Presents as massive painless cervical lymphadenopathy. IHC: S100+, CD163+, CD68+. CD1a-negative, Langerin-negative (distinguishes from LCH).

LCH vs. RDD: both are S100+. LCH = CD1a+/Langerin+/CD163-. RDD = CD163+/CD68+/CD1a-/Langerin-.

Macrophage markers: CD68, CD163, lysozyme. CD68 is broad. CD163 is more specific for macrophages (also marks RDD). Lysozyme is a functional marker of monocyte/macrophage lineage.

NK cell markers: CD56, CD16, CD57. CD3-negative (distinguishes from T-cells).

Benign Incidental Findings in Lymph Nodes

These get confused with metastases. Recognize them.

Endosalpingiosis/mullerianosis: benign ciliated epithelium in a pelvic LN of a woman, PAX8+/ER+. Similar benign inclusions can occur in other sites: salivary in upper cervical LN, thyroid in lower cervical LN, breast in axillary LN, mesothelial in thoracic LN.
Capsular nevi: bland melanocytes within the collagen of the LN capsule. S100+, MelanA+, but PRAME-negative and HMB-45-negative. PRAME negativity is key to distinguishing from melanoma metastasis.
Lipomatosis: fatty infiltration of lymph nodes, most common in pelvic nodes. Incidental, benign, mature fat without lipoblasts.
Vascular transformation of sinuses: LN sinuses replaced by anastomosing small vascular channels. Benign. Usually associated with venous obstruction or prior surgery. Lacks cytologic atypia (distinguishes from Kaposi).
Lymphangioleiomyomatosis in LN: spindle cell proliferation with features of both smooth muscle and melanocytes (SMA+/HMB-45+). Pelvic location. Importantly, LN LAM is NOT associated with pulmonary LAM or TSC - it’s incidental.
Angiomyomatous hamartoma: inguinal LN in males with sclerotic fibrosis and thick-walled smooth muscle vessels. Benign incidental.
Silicone: axillary LN in a woman post breast implant. Clear/refractile foreign material with foreign body giant cell reaction. Don’t mistake for metastasis.

Mantle Zone Hyperplasia Pitfall

When primary follicles show mantle zone hyperplasia with small or absent GCs, always order follicular IHC (CD10 and BCL6) to confirm whether GCs are truly absent or just compressed. Also add Cyclin D1 to exclude mantle zone pattern of MCL. Don’t rely on BCL2 alone - primary follicles and mantle zones are normally BCL2+.

31.2 Acute Lymphoblastic Leukemia/Lymphoma (ALL)

ALL is a malignancy of lymphoid precursor cells (lymphoblasts). It’s the most common malignancy in children but can occur at any age.

Classification

B-lymphoblastic leukemia/lymphoma (B-ALL):

Most common (85%)
Derived from B-cell precursors

T-lymphoblastic leukemia/lymphoma (T-ALL):

15% of cases
Often presents with mediastinal mass (thymic involvement)
Derived from T-cell precursors

Immunophenotype

B-ALL markers:

TdT positive (blast marker)
CD19, CD79a positive
CD10 (CALLA) usually positive
Surface Ig negative (immature)
Cytoplasmic mu may be present (pre-B)

T-ALL markers:

TdT positive
CD3 positive (cytoplasmic early, surface later)
CD7 positive (most sensitive T marker)
CD4/CD8: Variable (may be double positive, double negative, or single positive)
CD1a positive in cortical T-ALL

Cytogenetic/Molecular Subtypes (B-ALL)

Abnormality	Features	Prognosis
Hyperdiploidy (>50 chromosomes)	Common in children	Favorable
t(12;21) ETV6-RUNX1	Children	Favorable
t(9;22) BCR-ABL1 (Ph+)	Increases with age	Unfavorable (improved with TKI)
KMT2A rearrangements	Infants	Unfavorable
Hypodiploidy (<44 chromosomes)	Any age	Unfavorable
Ph-like ALL	Kinase-activating alterations	Unfavorable

31.3 Chronic Lymphocytic Leukemia (CLL) / Small Lymphocytic Lymphoma (SLL)

CLL and SLL are the same disease:

CLL: Primarily involves blood and marrow (lymphocytosis >5000/uL)
SLL: Primarily involves lymph nodes without significant lymphocytosis

Epidemiology: Most common leukemia in Western adults; median age ~70. Much more common in people of European ancestry than in East Asian populations. (Most common acute leukemia in adults = AML. Most common leukemia in children = ALL.)

Cell of origin: memory B-cells. This explains CD5 expression (a T-cell-associated marker also found on B-1 cells) and why IGHV mutation status matters for prognosis. Mutated IGHV = cells went through the GC → better prognosis. Unmutated IGHV = cells bypassed the GC → worse prognosis.

Immunophenotype (key for diagnosis):

CD5 positive (unusual for B-cells - helps distinguish CLL)
CD19, CD20 positive (CD20 dim)
CD23 positive
Surface Ig weak/dim (lambda or kappa light chain restriction)
CD200 positive
LEF1 positive
FMC7 negative, CD79b weak (helps distinguish from mantle cell)

Blood smear: Small, mature-appearing lymphocytes; smudge cells (“soccer ball” or “tortoise shell” chromatin, fragile cells disrupted during smear preparation). Adding albumin to blood smear preparation eliminates smudge cell formation if you need an accurate count.

CLL/SLL flow cytometry: CD5+, CD19+, CD23+, dim CD20, dim surface immunoglobulin. The CD5+ CD23+ pattern distinguishes CLL from mantle cell lymphoma.

CLL vs. Mantle Cell (high-yield DDx):

Feature	CLL/SLL	MCL
CD23	Positive	Negative
Cyclin D1	Negative	Positive
CD200	Positive	Negative
sIg intensity	Dim	Bright

The key differentiators are CD23 (positive in CLL) and Cyclin D1 (positive in MCL).

Prognostic Markers in CLL

IGHV mutation status: Mutated = favorable; unmutated = unfavorable
ZAP-70 (flow, must be in ≥30% of cells) - associated with unmutated IGHV
CD38 and CD49d (flow, ≥30% of cells) - worse prognosis
Ki-67 >40% - worse prognosis; may herald Richter transformation
FISH hierarchy (worst → best):
1. del(17p) - worst (TP53 loss)
2. del(11q) - poor (ATM loss)
3. +12 - intermediate
4. Normal - intermediate
5. Isolated del(13q) - best prognosis

CD38, CD49d, ZAP-70, and unmutated IGHV cluster together as the aggressive phenotype.

del(17p) patients should receive targeted therapy (BTK inhibitors, venetoclax) rather than chemoimmunotherapy.

Monoclonal B-Cell Lymphocytosis (MBL)

Pre-CLL state, analogous to MGUS being pre-myeloma.

Monoclonal B-cell population <5000/uL with CLL-like phenotype (CD5+, CD23+)
No lymphadenopathy, organomegaly, or cytopenias
Found in ~5% of adults over 40
Very low risk of progression (~1-2%/year for high-count MBL >500/uL; negligible for low-count)

Spectrum: MBL (<5,000) → CLL Rai 0 (≥5,000, lymphocytosis only) → CLL advanced (cytopenias, organomegaly, lymphadenopathy).

CLL Complications

Autoimmune hemolytic anemia (warm type, IgG mediated; IgM also possible but less common). Any new anemia in a CLL patient needs DAT, retic, haptoglobin, LDH.
Autoimmune thrombocytopenia
Hypogammaglobulinemia → infections
Richter transformation (to DLBCL in ~90%; rarely Hodgkin lymphoma ~10%). Occurs in 5-10% of CLL. Very poor prognosis.

Clinical clues to Richter: rapidly enlarging node(s), rising LDH, new B symptoms, worsening cytopenias, rising Ki-67 (>40%).

B-Cell Prolymphocytic Leukemia (B-PLL)

ICC recognizes B-PLL as a distinct entity; WHO5 no longer does (considers it transformed CLL/MCL). Still worth knowing.

Monoclonal B-cell neoplasm with ≥55% prolymphocytes in blood (large cells >2x RBC, prominent nucleoli)
Aggressive
Must exclude t(11;14) (that would make it MCL with prolymphocytic features)
Check CLL markers (CD23, CD200) to exclude prolymphocytic transformation of CLL
Mutations: MYC (~60%), TP53 (~40%)
Immunophenotype: light chain restricted, IgM+, CD19+, CD20+ bright, CD23-negative

Rituximab Pitfall

Rituximab (anti-CD20) blocks CD20 detection by both IHC and flow. Workarounds: use CD19 on flow, or PAX5 on IHC (nuclear, not affected). CD79a is another alternative.

31.4 Hairy Cell Leukemia

Clinical features: Middle-aged men; splenomegaly; pancytopenia; monocytopenia (nearly pathognomonic, mechanism unclear, thought to involve marrow microenvironment).

Pathophysiology: Clonal B-cell with characteristic “hairy” cytoplasmic projections (due to beta-actin overexpression; best seen on phase contrast microscopy).

Blood smear: Lymphocytes with circumferential, irregular cytoplasmic projections

Hairy cell leukemia: Lymphocytes with circumferential “hairy” cytoplasmic projections. BRAF V600E mutation is pathognomonic.

Immunophenotype:

CD19, CD20, CD22 positive
CD11c positive (bright)
CD25 positive
CD103 positive
CD123 positive
Annexin A1 positive (most specific)
CD5 and CD23 negative (helps distinguish from CLL)

HCL variant lacks CD25, Annexin A1, and BRAF V600E - worse prognosis.

Molecular: BRAF V600E mutation in ~100% of classic HCL (pathognomonic; therapeutic target with vemurafenib). Absence of BRAF V600E should prompt reconsidering the diagnosis. BRAF V600E also seen in LCH (~50-60%) and Erdheim-Chester (~50%).

Special stain: TRAP (tartrate-resistant acid phosphatase) positive - historical marker, largely replaced by flow and BRAF testing.

Spleen: Red pulp disease (contrast with SMZL which is white pulp). Massive splenomegaly from red pulp expansion.

Does HCL cause lymphadenopathy? No. HCL involves spleen (red pulp), bone marrow, and peripheral blood only. Splenomegaly WITHOUT lymphadenopathy + pancytopenia + monocytopenia = HCL until proven otherwise.

Bone marrow: Interstitial infiltration with “fried egg” appearance (cells with abundant pale cytoplasm). NOT paratrabecular (that’s FL).

BM patterns to memorize: Interstitial = HCL. Paratrabecular = FL. Nodular = CLL. Sinusoidal = SMZL or hepatosplenic TCL.

Often dry tap (marrow fibrosis). Classic dry tap DDx: primary myelofibrosis, hairy cell leukemia, aplastic anemia. Can also be operator error, packed marrow, or metastasis.

Treatment: Purine analogs (cladribine, pentostatin) - highly effective; most patients achieve durable remission.

31.5 Follicular Lymphoma

Second most common NHL (most common indolent NHL). Arises from germinal center B-cells.

Pathophysiology and Architecture

Characteristic translocation: t(14;18)(q32;q21) - fuses IGH (14q32) with BCL2 (18q21). The IGH enhancer drives constitutive BCL2 expression → anti-apoptosis. Found in up to 85% of FL grade 1-2. Less common in grade 3B.

Morphology: architectural effacement with back-to-back follicles that are remarkably uniform in size and shape. No tingible-body macrophages (because BCL2 prevents apoptosis - nothing to clear). Loss of GC polarity.

Growth patterns:

Follicular (>75% follicular)
Follicular and diffuse (25-75% follicular)
Focally follicular (<25% follicular)

A predominantly diffuse pattern should raise concern for transformation to DLBCL, especially if large cells are present.

FL vs. reactive follicular hyperplasia:

FL: uniform follicles, back-to-back, no TBMs, no polarity, BCL2+ GCs
Reactive: variable-sized follicles, polarized GCs, TBMs present, open sinuses, BCL2-negative GCs

Immunophenotype

CD19, CD20, CD10 positive
BCL2 positive within germinal centers (the diagnostic IHC finding; normal GCs are BCL2-negative)
BCL6 positive
CD5 negative (distinguishes from CLL, mantle cell)

BCL2 pitfall: primary follicles (no GC) and mantle zones are normally BCL2+. Always correlate BCL2 with GC markers (CD10, BCL6) before calling FL. You must confirm you’re looking at a true germinal center.

Grading

Based on number of centroblasts per 10 HPF. Grading is now optional per WHO5.

Grade 1-2: 0-15 centroblasts/HPF (Grade 1 = 0-5; Grade 2 = 6-15; now combined)
Grade 3A: >15 centroblasts with centrocytes still admixed
Grade 3B: centroblasts only, rare/absent centrocytes

Grade 3B is now termed follicular large B-cell lymphoma in WHO5 because it behaves like DLBCL (aggressive, needs R-CHOP, often BCL2-negative).

Follicular lymphoma grading: Count centroblasts per HPF. Grade 1 (0-5), Grade 2 (6-15), Grade 3A (>15 with centrocytes), Grade 3B (sheets of centroblasts).

Why grade 3B is different:

Often BCL2 negative by IHC and FISH (lacks t(14;18))
Instead has BCL6 rearrangement
Biologically closer to DLBCL
Treated with R-CHOP

Two molecular pathways of FL: BCL2-driven (t(14;18)) → grades 1-3A → indolent; BCL6-driven → grade 3B → aggressive.

Follicular Lymphoma In Situ (FLIS)

Reactive-appearing lymph node with normal architecture
Scattered germinal centers are BCL2 positive
Architecture is NOT effaced
Low risk of progression to overt FL

The cells carry t(14;18) but lack additional hits (KMT2D, CREBBP, EZH2) for full transformation.

In situ B-cell neoplasias (MBL, FLIS, in situ MCN) all share the pattern of clonal population with defining genetic alteration but without malignant transformation.

Bone marrow: paratrabecular infiltrate is classic for FL (though also seen in LPL and MZL).

Clinical: indolent; waxes and wanes; not curable with standard therapy but patients live many years. Risk of transformation to DLBCL.

31.6 Mantle Cell Lymphoma (MCL)

Pathophysiology: Arises from naive B-cells in the mantle zone.

Defining translocation: t(11;14)(q13;q32) - fuses IGH with CCND1 (Cyclin D1). Cyclin D1 overexpression drives G1→S transition. Same translocation is found in ~25% of myeloma (differentiate by plasma cell markers).

Mantle cell lymphoma: t(11;14) results in Cyclin D1 overexpression. Cyclin D1 IHC is the diagnostic hallmark.

Immunophenotype

CD19, CD20 positive
CD5 positive (like CLL - important DDx)
CD23 negative (distinguishes from CLL)
Cyclin D1 positive (diagnostic hallmark; nuclear)
CD200 negative
SOX11 positive (most cases)
sIg bright (unlike CLL which is dim)

Cyclin D1-Negative MCL

Rare. Classic histology and immunophenotype but Cyclin D1 negative. SOX11 positive. May overexpress Cyclin D2 or D3 instead.

Decision tree: CD5+ B-cell → CD23+? CLL. Cyclin D1+? MCL. Cyclin D1-/SOX11+? Cyclin D1-negative MCL.

Morphology and Variants

Cells resemble centrocytes - small to medium with slightly irregular nuclear contours.

Four cytologic variants:

Classic (small cell-like)
Marginal zone-like
Blastoid (bad prognosis; additional TP53 mutations; Ki-67 >50%; mimics ALL)
Pleomorphic (bad prognosis)

Prognostication

Ki-67 is the most important IHC prognostic marker in MCL. Ki-67 >30% = high risk. Blastoid MCL has Ki-67 >50% and additional TP53 mutations.

In Situ Mantle Cell Neoplasia

Reactive-appearing lymph node
Patchy Cyclin D1 positivity in some mantle zones
Architecture preserved
Low progression risk (MCL equivalent of FLIS)

Clinical: Aggressive course despite often indolent-appearing cells. GI involvement common (lymphomatous polyposis).

31.7 Marginal Zone Lymphomas

Three subtypes, all CD19+/CD20+/CD5-/CD10-. Diagnosis of exclusion among small B-cell lymphomas.

Nodal Marginal Zone Lymphoma

Effacement with monotypic expansion of the marginal zone
Infiltrates and compresses germinal centers
Cells are medium-sized with pale/clear cytoplasm (monocytoid B-cells)
IgM positive
Most commonly has KMT2D mutations
Trisomies 3 and 18 are common; t(11;18) is NOT a feature of nodal MZL (that’s MALT)

Splenic Marginal Zone Lymphoma (SMZL)

Splenomegaly, villous lymphocytes in blood
Gross: white nodules in spleen (white pulp disease; HCL is red pulp)
Histology: enlarged white pulp nodules with dark centers (residual GCs), neoplastic cells spreading into red pulp
IgD positive (unusual; naive marginal zone B-cells co-express IgM and IgD, SMZL retains this)
t(11;18) negative
del(7q) characteristic
NOTCH2 mutations most common (~20-25%)
Associated with HCV

Memory aid: NOTCH2 = Splenic, KMT2D = Nodal, MYD88 = LPL.

Extranodal Marginal Zone Lymphoma (MALT Lymphoma)

MALT = mucosa-associated lymphoid tissue = extranodal MZL. Same disease, different names. Arises from chronic antigenic stimulation (infection or autoimmunity). Normal MALT doesn’t exist in most organs - it develops from chronic inflammation (acquired MALT).

Classic pathogen/autoimmune associations:

Infection/Autoimmune	MALT Site
H. pylori	Gastric (most common, ~50%)
Sjogren syndrome	Salivary
Hashimoto thyroiditis	Thyroid
C. psittaci	Ocular adnexa
C. jejuni	Intestinal (IPSID/alpha heavy chain disease)
B. burgdorferi	Cutaneous (European cases)
EBV	Post-transplant

Gastric MALT and H. pylori: H. pylori+/t(11;18)-negative cases may regress with antibiotic eradication (~75%). t(11;18)+ cases do NOT respond to eradication and need chemo/radiation. One of the few cancers treatable with antibiotics.

Sjogren syndrome: 15-20x increased lymphoma risk. New parotid mass in Sjogren → rule out MALT lymphoma or DLBCL transformation.

EBV/PTLD pitfall: low-grade B-cell lymphomas do NOT qualify as PTLD, with the exception of MALT lymphoma. High-yield exception.

31.8 Burkitt Lymphoma

Highly aggressive but potentially curable. The fastest-growing human tumor.

Pathophysiology: All cases have MYC translocation (100% - if MYC FISH is negative, it’s not Burkitt).

t(8;14) - MYC/IGH (most common, ~80%)
t(2;8) - IGK/MYC (light chain variant)
t(8;22) - MYC/IGL (light chain variant)

All deregulate MYC. MYC drives proliferation.

Clinical variants:

Endemic (African): EBV-associated (~100%), jaw involvement, children. Chronic malaria impairs T-cell surveillance → permissive for MYC translocation.
Sporadic (Western): abdominal involvement, usually EBV-negative
Immunodeficiency-associated: HIV/AIDS, variable EBV

Morphology:

Medium-sized cells with deeply basophilic cytoplasm and lipid vacuoles
Starry-sky pattern - tingible body macrophages (stars) clearing apoptotic debris among sheets of tumor cells (sky)
Very high mitotic rate
Ki-67 approaches 100%

Burkitt lymphoma: “Starry sky” pattern from tingible body macrophages clearing apoptotic debris. Ki-67 approaches 100%.

Starry-sky DDx: Burkitt (classic), DLBCL, ALL, extracavitary PEL. Not specific but Burkitt is the archetype.

Immunophenotype:

CD19, CD20, CD10, BCL6 positive
BCL2 negative (unlike FL; MYC drives proliferation without BCL2 protection)
Ki-67 ~100%

31.9 Diffuse Large B-Cell Lymphoma (DLBCL)

Most common NHL (most common aggressive NHL).

Cell of Origin Classification

GCB type (germinal center B-cell origin): better prognosis, responds better to R-CHOP
ABC/non-GCB type (activated B-cell): post-germinal center B-cells arrested during plasma cell differentiation (hence MUM1+, CD10-/BCL6-). Depends on NF-kB signaling → potential target for BTK inhibitors.

Immunophenotype: CD19, CD20 positive; variable CD10, BCL6, MUM1.

Hans Algorithm for Cell of Origin

Start with CD10.

CD10 positive → GCB subtype (regardless of other markers)
CD10 negative → assess BCL6
- BCL6 negative → ABC
- BCL6 positive → assess MUM1
  - MUM1 negative → GCB (still in GC)
  - MUM1 positive → ABC (transitioning out)

CD10 → BCL6 → MUM1 tracks B-cell maturation from GC to post-GC.

Double-Hit, Triple-Hit, and Double Expressor

These are confusing but important.

Double expressor (IHC): MYC >40% AND BCL2 >50% by IHC. Intermediate prognosis. Worse than standard DLBCL, better than double-hit.

Double-hit / triple-hit (FISH): MYC rearrangement PLUS BCL2 and/or BCL6 rearrangement by FISH.

WHO5: MYC+BCL2 counts as double-hit; MYC+BCL6 alone doesn’t count (BCL6 alone has better prognosis)
ICC: both count
Now called “High-grade B-cell lymphoma with MYC and BCL2 and/or BCL6 rearrangements”
Poor prognosis; requires intensive therapy (DA-EPOCH-R)
Usually GCB type (counterintuitive exception to GCB = better)

Strategy: screen with IHC (MYC >40%, BCL2 >50%) → if positive, order FISH → reclassify as high-grade B-cell lymphoma if confirmed. Not all double expressors are double-hit (protein overexpression without gene rearrangement possible).

Primary Mediastinal Large B-Cell Lymphoma (PMBL)

A distinct DLBCL subtype arising from thymic (medullary) B-cells.

Clinical: young adults (often female) with anterior mediastinal mass.

Diagnosis of exclusion: must rule out systemic DLBCL with secondary mediastinal involvement. Staging (PET-CT) essential.

PMBL vs. CHL (both present as mediastinal masses): PMBL retains the B-cell program - OCT2 and BOB1 positive, CD45+, strong CD20+. CHL loses B-cell program (OCT2-/BOB1-/CD45-/CD20 variable). PMBL can have RS-like cells and be CD30+, so OCT2/BOB1 are what save you diagnostically.

Mediastinal gray zone lymphoma (MGZL): a real entity for cases that fall between PMBL and CHL. Two scenarios: (1) morphology of B-cell lymphoma but stains like CHL; (2) morphology of CHL but stains like B-cell lymphoma.

Primary B-Cell Lymphoma of the Testis

Testicular cancer in older men (>60). Previously termed testicular DLBCL. Young men with testicular mass = germ cell tumor. Older men = lymphoma. The testis is immune-privileged → propensity for CNS involvement and contralateral relapse.

T-Cell/Histiocyte-Rich Large B-Cell Lymphoma (THRLBCL)

A DLBCL subtype with limited scattered popcorn-like B-cells in an abundant T-cell and histiocyte background. Looks similar to NLPHL but behaves aggressively.

NLPHL vs. THRLBCL:

NLPHL: nodular architecture, CD57/PD1+ T-cell rosettes around LP cells
THRLBCL: diffuse growth pattern, no TFH rosettes

If there’s no nodularity and no TFH rosettes, it’s THRLBCL regardless of how NLPHL-like the large cells look. NLPHL may progress to THRLBCL (indolent → aggressive).

Primary Effusion Lymphoma (PEL)

HHV-8-associated large B-cell neoplasm
Involves body cavity effusions (pleural, pericardial, peritoneal) without solid masses
HIV/AIDS patients
Cells are B-cells but lack typical B-cell markers (CD19, CD20, CD22, PAX5 negative)
Express terminal differentiation markers: CD30, CD38, CD138, MUM1
Diagnosed by HHV-8 (LANA-1) positivity
Can be confused with T-cell lymphoma or ALCL

Extracavitary PEL variant: solid masses (vs. effusions) with starry-sky pattern. Still HHV-8+. HHV-8 testing distinguishes from Burkitt.

HHV-8 also causes Kaposi sarcoma and multicentric Castleman disease - all HIV-associated.

31.10 Lymphoplasmacytic Lymphoma and Waldenstrom Macroglobulinemia

Lymphoplasmacytic Lymphoma (LPL)

Diffuse interfollicular proliferation of light-chain-restricted plasma cells
Dutcher bodies (intranuclear Ig pseudoinclusions) - characteristic
Cell spectrum: lymphocytes → lymphoplasmacytoid cells → plasma cells
The majority are IgM

Dutcher vs. Russell bodies: Dutcher = intranuclear Ig pseudoinclusions (LPL). Russell = cytoplasmic Ig inclusions.

Molecular: ~90% have MYD88 L265P mutation. ~30-40% have CXCR4 mutations. MYD88 is both diagnostic (distinguishes from other small B-cell lymphomas) and therapeutic (activates BTK → ibrutinib). LPL without MYD88 has worse prognosis and doesn’t respond to ibrutinib - these typically have CXCR4.

MYD88 L265P is to LPL what BRAF V600E is to HCL - a near-universal defining mutation.

Waldenstrom Macroglobulinemia (WM)

WM = LPL + bone marrow involvement + IgM M-protein (any concentration). All three required. LPL can involve lymph nodes without meeting WM criteria.

Clinical: visual and neurologic deficits from hyperviscosity (IgM pentamers sludge blood). WITHOUT CRAB symptoms (those are myeloma features).

Hyperviscosity symptoms: blurred vision, headache, epistaxis, confusion.

Treatment of symptomatic hyperviscosity: plasma exchange (plasmapheresis) - physically removes IgM pentamers. IgM is mainly intravascular (due to size), making plasmapheresis highly effective.

31.11 Plasma Cell Neoplasms

MGUS Overview

Two types of MGUS:

IgM MGUS (lymphoid/lymphoplasmacytic) → precursor to LPL/WM
Non-IgM MGUS (plasma cell) → precursor to smoldering myeloma → multiple myeloma

The Ig class determines the precursor pathway.

IgM MGUS criteria:

IgM M-protein <3 g/dL
Clonal BM lymphoplasmacytic infiltrate <10%
No symptoms (no anemia, hyperviscosity, lymphadenopathy, hepatosplenomegaly)

~50% of IgM MGUS cases have MYD88 mutation (same as LPL).

Non-IgM MGUS criteria:

IgG, IgA, or IgD M-protein <3 g/dL (cannot be IgM)
Clonal BM plasma cells <10%
No CRAB symptoms
~1% per year progression risk

Light-chain MGUS: M-protein is light chains only (kappa or lambda). Detected by serum free light chain assay with abnormal ratio. SPEP may be normal since free light chains are small.

Smoldering Myeloma

Between MGUS and active myeloma.

M-protein ≥3 g/dL and/or clonal BM plasma cells 10-60%
No CRAB symptoms
~10% per year progression risk in first 5 years

Absence of CRAB is what distinguishes from active myeloma.

Multiple Myeloma

Diagnostic criteria - either:

Option A: plasmacytoma OR clonal BM plasma cells >10%, PLUS one of:

CRAB symptoms, or
Free light chain ratio involved/uninvolved ≥100, or
MRI with >1 focal lesion

Option B (SLiM): >60% clonal plasma cells in BM (no CRAB needed; such high tumor burden requires treatment)

Myeloma-defining events (CRAB + SLiM):

Calcium elevation (>11 mg/dL)
Renal insufficiency (creatinine >2 mg/dL)
Anemia (Hgb <10 g/dL)
Bone lesions (lytic, on X-ray, CT, or PET-CT)
Sixty percent clonal plasma cells in marrow
Light chain ratio involved/uninvolved ≥100
MRI with >1 focal lesion

Laboratory Findings

M-protein on SPEP (usually IgG > IgA; IgM rare in myeloma)
Abnormal serum free light chain ratio
Bence Jones proteinuria (free light chains in urine; small enough to pass through glomerulus; can cause cast nephropathy)
Rouleaux on blood smear (RBCs stacked like coins; paraprotein reduces negative charge between RBCs; also elevates ESR)

Multiple myeloma: Rouleaux formation on peripheral smear (RBCs stacked like coins) due to paraprotein coating red cells.

Bone disease: Osteolytic lesions (myeloma cells produce RANKL, inhibit OPG → osteoclast activation). Bone scan often negative (no osteoblastic activity; use X-ray skeletal survey, CT, or PET-CT).

SPEP and Immunofixation

SPEP is the screening method. Five peaks from anode (+) to cathode (-): Albumin → Alpha-1 → Alpha-2 → Beta → Gamma. Immunoglobulins travel in the gamma region (some IgA and IgM can migrate in beta - pitfall).

A monoclonal peak = M protein (M-spike, paraprotein) - sharp, narrow peak representing a clonal Ig-producing process.

SPEP limitations: can miss small M-proteins and light-chain-only disease. Serum free light chain assay complements SPEP for complete screening.

Immunofixation (IFE): confirmatory/typing step after SPEP detects an M-protein. Uses specific antibodies (anti-IgG, anti-IgA, anti-IgM, anti-kappa, anti-lambda) to identify exactly which immunoglobulin class and light chain the M-protein is.

SPEP = detects and quantifies. IFE = identifies and types. Both are needed. IgM → think LPL/Waldenstrom. IgG/IgA → think myeloma.

Immunophenotype of Myeloma Cells

CD138 (syndecan-1) positive, CD38 bright
CD19 negative (abnormal - normal plasma cells are CD19+; loss of CD19 is the key aberrancy)
CD56 positive (aberrant)
CD20 usually negative
Surface Ig negative
Cytoplasmic Ig light-chain restricted

Flow cytometry usually underestimates plasma cell numbers. Daratumumab (anti-CD38) therapy causes loss of CD38 expression - alternative gating markers include CD319 and p63.

Cytogenetics in Myeloma

FISH is the gold standard (not karyotype - plasma cells have low mitotic activity in vitro).

IGH translocations most common (~60%). Break-apart probe for IGH is best screening. Many partners possible.

Most common IGH partner: t(11;14) (~25%).

Prognostic cytogenetics:

Abnormality	Prognosis	Features
t(11;14)	Good	Lymphoplasmacytic morphology, BCL1+/CD20+ (mimics MCL!), responds to venetoclax
t(4;14)	Poor	Involves FGFR3/MMSET, blastoid appearance, FGFR3+ by IHC
t(14;16)	Poor	Involves IGH/MAF
del(17p)	Poor	TP53 loss
Gain 1q	Poor	CKS1B amplification
Hyperdiploid	Good	Trisomies of odd chromosomes (3,5,7,9,11,15,19,21, except 13)
Hypodiploid	Poor	Genomic instability

Memory: “odd chromosomes = oddly good prognosis.”

High-risk cytogenetics in R-ISS: t(4;14), t(14;16), del(17p).

Myeloma with t(11;14) pitfall: can be Cyclin D1+/CD20+ - mimics MCL. Distinguish by plasma cell markers (CD138+, CD38+, cytoplasmic Ig) vs. B-cell markers.

Ig type in myeloma: most commonly IgG kappa. Light chain amyloidosis is more commonly lambda. Memory: myeloma = kappa, amyloid = lambda.

Plasma Cell Leukemia

Circulating clonal plasma cells >5% of WBCs. Primary (de novo) or secondary (after known plasma cell neoplasm). Very aggressive. The cells have left the marrow niche.

Plasmacytoma

Solitary mass of monotypic plasma cells. Osseous (bone) or extraosseous. Must rule out systemic myeloma before calling it solitary.

Most common site of extraosseous plasmacytoma: upper respiratory tract (~80% - nasopharynx, sinuses, oropharynx). Better prognosis than osseous.

Amyloidosis

Extracellular deposition of abnormally folded proteins forming beta-pleated sheets. Primary (AL = light chain) or secondary (AA = serum amyloid A from chronic inflammation). Many types by precursor protein.

AL (light chain) amyloidosis is the most common in the US. Caused by deposition of Ig light chains from a clonal plasma cell proliferation (most commonly lambda).

Detection:

Histology: waxy, amorphous, homogeneous eosinophilic material
Congo red stain → apple-green birefringence under polarized light
IHC: stains positive for serum amyloid P component (SAP) - universal component of all amyloid regardless of type
Typing gold standard: mass spectrometry (identifies exact precursor protein; superior to IHC-based typing)

Treatment depends entirely on amyloid type, so correct typing matters.

31.12 Mature T-Cell and NK-Cell Neoplasms

T-cell lymphomas are less common than B-cell lymphomas but often more aggressive. Classification is complicated by site - the same immunophenotype can represent different entities depending on where it presents.

Key concepts up front:

Pan T-cell markers: CD2, CD3, CD5, CD7. Most T-cell lymphomas have loss of ≥1 pan T-cell marker. CD7 is most commonly lost.
T-cell clonality proven by TCR gene rearrangement (TCR-gamma most common target) - the T-cell equivalent of light chain restriction for B-cells
Site matters: skin (MF/Sezary, primary cutaneous ALCL, SPTCL); nodes (PTCL-NOS, AITL, ALCL); GI (EATL, MEITL); nasal (NK/T-cell); liver/spleen (hepatosplenic); blood (T-LGL, ATLL, Sezary)
Prognosis spectrum: ALK+ ALCL (best) > ALK-/DUSP22+ ALCL > MF/SPTCL (indolent) >>> PTCL-NOS, AITL, hepatosplenic, EATL, ATLL (poor, 5-year survival 20-30%)

Peripheral T-Cell Lymphoma, Not Otherwise Specified (PTCL-NOS)

Most common T-cell lymphoma in the West (~30% of TCLs). Wastebasket category.

Nodal T-cell lymphoma of mature cells that can’t be classified elsewhere
Diagnosis of exclusion - not ALCL (no hallmark cells/CD30/ALK), not AITL (no TFH markers), not other specific entities
T-cell equivalent of DLBCL-NOS
Poor prognosis (~30% 5-year survival)
Treatment typically CHOP or CHOP-like but outcomes inferior to B-cell lymphomas

Immunophenotype: CD4+ usually (helper T-cell derived), most pan T-cell markers retained, with loss of at least 1 pan T-cell marker (CD7 most commonly). Must be negative for TFH markers (CD10, BCL6, ICOS, PD1, CXCL13) - if positive, reclassify as AITL or nodal TFH lymphoma.

Majority show clonal TCR gene rearrangement.

Lennert (lymphoepithelioid) variant: clusters of epithelioid histiocytes obscuring atypical T-cells. Histiocytes are reactive. Can be confused with granulomatous conditions or Hodgkin. Associated with trisomy 3.

PTCL-NOS molecular subclassification (like GCB/ABC for DLBCL):

Th1 subtype: TBX21 (T-bet) positive, better prognosis
Th2 subtype: GATA3 positive, worse prognosis

GATA3 in PTCL = worse prognosis. Don’t confuse with GATA1 (Down syndrome AML).

Angioimmunoblastic T-Cell Lymphoma (AITL)

Now called TFH cell lymphoma, angioimmunoblastic type (WHO5/ICC).

Clinical: older adults with characteristic presentation:

B symptoms
Hepatosplenomegaly
Generalized lymphadenopathy
Skin rash
Effusions
Autoimmune phenomena

Clinical picture is so classic that deviation should raise diagnostic skepticism.

Histology:

Prominent arborizing high endothelial venules (the characteristic “prominent vasculature”)
Atypical T-cells with clear cytoplasm clustered around vessels
Mixed inflammatory background (eosinophils, plasma cells, histiocytes, often EBV+ immunoblasts)
Expanded FDC meshworks (CD21/CD23)

Cell of origin: T-follicular helper (TFH) cells. TFH normally resides in germinal centers helping B-cells with class switching and affinity maturation.

Immunophenotype: CD4+, most pan T-cell markers retained with loss of ≥1 (usually CD7), PLUS TFH markers: CD10, BCL6, ICOS, PD1, CXCL13. At least 2-3 TFH markers should be positive. T-cells are EBER-negative; background B-cells may be EBER-positive (reactive immunoblasts expanded due to TFH help).

Molecular: TET2 (~80%), DNMT3A (~30%), IDH2 (~20-30%, R172 variant - highly specific), RHOA G17V (~70%). TET2 and DNMT3A are also seen in CHIP/MDS suggesting common early mutation.

IDH2 R172 is the most specific mutation for AITL (distinct from IDH2 R140 in AML). IDH2 produces 2-hydroxyglutarate which inhibits TET2 - converging pathways.

TET2 mutations are shared between AITL and other TFH-origin lymphomas (including TFH-type PTCL).

TCR rearrangement: most cases clonal. ~30% also show clonal IGH rearrangement (associated B-cell expansion, not concurrent B-cell lymphoma) - these B-cells can occasionally progress to overt EBV+ DLBCL.

Unique feature: AITL has a background B-cell proliferation that can be clonal and EBER-positive, due to continued TFH help. This drives polyclonal hypergammaglobulinemia and can occasionally evolve into EBV+ DLBCL.

Adult T-Cell Leukemia/Lymphoma (ATLL)

Caused by HTLV-1 (Human T-lymphotropic virus type 1). HTLV-1 integrates into host genome and drives T-cell proliferation through the Tax protein (transactivates viral and cellular genes). Only ~5% of HTLV-1 carriers develop ATLL, usually after long latency (>20 years). Transmitted via breast milk, sexual contact, blood transfusion. (Don’t confuse with HIV - HTLV-1 and HIV are both retroviruses that infect CD4+ T-cells.)

Endemic regions: Japan (especially Kyushu), Caribbean, intertropical Africa, Middle East, South America, Papua New Guinea.

Clinical features:

Widespread lymphadenopathy
Peripheral blood involvement (leukemia)
Skin rash (papules, plaques, erythroderma)
Lytic bone lesions with hypercalcemia (from PTHrP and osteoclast-activating factors)

ATLL vs. myeloma: both cause lytic lesions and hypercalcemia. ATLL causes a rash; myeloma does not. Also: ATLL has flower cells and is CD4+ T-cell; myeloma has rouleaux and is CD138+ plasma cell. ATLL has lymphadenopathy; myeloma typically doesn’t. ATLL has no M-protein; myeloma does.

Morphology: flower cells (cloverleaf cells) - large lymphocytes with markedly multilobated nuclei. Morphologic hallmark.

Immunophenotype: CD4+, CD2+, CD3+, CD5+, bright CD25, CD7 negative, CD8 negative. ATLL cells also express CCR4, the target of mogamulizumab.

Clonal TCR rearrangement present. Clonal HTLV-1 proviral integration detectable by PCR (distinguishes from polyclonal carrier state).

Subtypes: acute (most common, most aggressive, median survival ~6 months), lymphoma, chronic, and smoldering - defined by pattern of organ involvement, LDH/calcium, degree of leukemic involvement.

Opportunistic infections: Pneumocystis, Strongyloides hyperinfection (characteristic association), CMV, fungal. Immunodeficiency resembles HIV (both retroviruses infect CD4+ T-cells).

Anaplastic Large Cell Lymphoma (ALCL)

ALCL is characterized by large cells with abundant cytoplasm and horseshoe-shaped nuclei (hallmark cells). All cases are CD30 positive (strong, uniform).

Clinical: children and young adults (ALK+); older adults (ALK-). B symptoms + lymphadenopathy. May involve BM, skin, bone, soft tissue.

Morphology: ranges from small cells to large anaplastic cells. Hallmark cells are ALWAYS present regardless of variant - horseshoe/reniform nuclei with eosinophilic paranuclear region. Can form perivascular rosettes.

Immunophenotype:

CD30+ (strong, uniform) - essential
CD3 negative (unusual for T-cell lymphoma)
EMA+
Clusterin+
ALK+ in ALK-positive subtype
Variable other T-cell markers (CD2, CD4, CD5, CD7)

CD30+/CD3-/EMA+ pattern distinguishes ALCL from CHL (which is CD30+/CD15+/EMA-).

Majority show clonal TCR rearrangement despite CD3 negativity by flow. Cytoplasmic CD3 may be detected by IHC.

ALCL Subtypes

ALK-positive ALCL:

Contains ALK translocation
Most commonly t(2;5) NPM1-ALK fusion
Children and young adults
Cytoplasmic AND nuclear ALK staining = t(2;5) (because NPM1 shuttles between nucleus and cytoplasm, bringing ALK along)
Cytoplasmic-only ALK = variant translocations (TPM3, ATIC, etc.)
Favorable prognosis (>80% 5-year survival)
Crizotinib is a therapeutic option

ALK-negative ALCL:

Older adults
Worse prognosis than ALK+ (~50% 5-year survival)
Still second-best T-cell lymphoma prognosis (after ALK+ ALCL)
Brentuximab vedotin (anti-CD30 ADC) effective
DUSP22 rearrangement → good prognosis (similar to ALK+)
TP63 rearrangement → very poor prognosis
Molecular hierarchy: ALK+ (best) ≥ ALK-/DUSP22+ (good) > ALK-/triple negative > ALK-/TP63+ (worst)

Breast implant-associated ALCL (BIA-ALCL):

Fibrous capsule around breast implants, especially textured implants
Always ALK-negative
Presents 8-10 years post-implant with late-onset seroma
Diagnosis: cytology of periprosthetic seroma fluid - hallmark cells, CD30+, EMA+, ALK-negative
JAK/STAT pathway mutations (STAT3, JAK1)
Capsulectomy alone may be curative if confined to capsule
Led to recall of certain textured implants

Small cell variant: worst prognosis among ALCL variants (counterintuitive - “small” but worse because easily missed, often advanced at diagnosis). TP63-rearranged is molecularly worst.

Gamma/Delta T-Cell Lymphomas

Gamma/delta T-cell phenotype: CD3+, CD56+, double-negative for CD4 and CD8, CD5-negative. Innate-like T-cell subset (vs. conventional alpha/beta). Normally in mucosal surfaces and spleen.

Neoplastic counterparts: hepatosplenic TCL, cutaneous gamma/delta TCL, MEITL.

Hepatosplenic T-Cell Lymphoma

Gamma/delta T-cell lymphoma involving liver, spleen, and bone marrow.

Clinical: young males with hepatosplenomegaly and cytopenias. Classic association: chronic immunosuppression, particularly thiopurine + anti-TNF therapy for IBD. Also post-solid organ transplant.

Course: aggressive, median survival ~1 year, resistant to standard chemo. Allogeneic SCT only potentially curative.

Histology:

Spleen: atrophic/absent white pulp, red pulp infiltration (sinusoidal)
Liver: sinusoidal pattern (CD3 IHC shows linear chains lining sinusoids)
Bone marrow: sinusoidal pattern (intrasinusoidal T-cells)

T-LGL leukemia also shows sinusoidal BM pattern but is CD8+ and indolent.

Immunophenotype: CD3+, CD56+, CD4-, CD5-, CD8- (classic gamma/delta double-negative). Rim of clear cytoplasm.

Cytotoxic profile: nonactivated - TIA-1+, granzyme B-negative, perforin-negative (“armed but not firing”).

Cytogenetics: isochromosome 7q (i(7q)) in most cases. Trisomy 8 also common.

Cutaneous Gamma/Delta T-Cell Lymphoma

Skin involvement by gamma/delta T-cell neoplasm. Presents with skin ulcers, plaques, or subcutaneous nodules.

Immunophenotype: CD3+, CD56+, CD4-, CD5-, CD8-.

Cytotoxic profile: activated - TIA-1+, granzyme B+, perforin+. All three positive (explains tissue destruction and ulceration).

Prognosis: aggressive, <20% 5-year survival. Worse than MF/Sezary or SPTCL.

Distinguished from SPTCL which is alpha/beta, CD8+, CD56-.

Cytotoxic profile cheat sheet: gamma/delta in skin = activated. Gamma/delta in liver/spleen = nonactivated.

Subcutaneous Panniculitis-like T-Cell Lymphoma (SPTCL)

Alpha/beta T-cell lymphoma (NOT gamma/delta). Presents as superficial soft tissue mass - subcutaneous nodules mimicking panniculitis.

Histology:

T-cell infiltrate confined to subcutaneous fat (if dermis involved, consider cutaneous gamma/delta TCL)
Rimming of adipocytes by neoplastic T-cells
Extensive karyorrhexis
“Bean bag” cell appearance

Immunophenotype: CD3+, CD8+, CD56-. Distinguishes from cutaneous gamma/delta TCL.

Cytotoxic profile: activated (TIA-1+, granzyme B+, perforin+). Activated granules cause fat cell destruction.

Prognosis: better than gamma/delta (~80% 5-year survival) - WHO separated them for this reason (same location, very different prognosis).

Molecular: HAVCR2 mutations (encoding TIM-3). Germline HAVCR2 mutations → loss of TIM-3 function → inability to terminate immune responses → chronic T-cell activation.

Associated condition: hemophagocytic lymphohistiocytosis (HLH) - major cause of morbidity/mortality. Activated cytotoxic T-cells release cytokines triggering macrophage activation. High ferritin, cytopenias, hepatosplenomegaly, hemophagocytosis.

Intestinal T-Cell Lymphomas

Two historically related entities, now separate:

EATL (formerly type I EATL):

Alpha/beta T-cell lymphoma
Associated with celiac disease
Progression: celiac → refractory celiac (type II with aberrant IELs) → EATL
Gluten-free diet decreases risk
Usually jejunum with large ulcerated masses, often perforating
HLA-DQ2 and HLA-DQ8 (same as celiac - chronic gluten-driven inflammation)
Immunophenotype: cytoplasmic CD3+, CD30+, CD4-, CD8-

Progression IEL phenotype:

Normal IEL: surface CD3+, CD8+
Refractory celiac II: cytoplasmic CD3+, CD8 lost
EATL: cytoplasmic CD3+, CD8 lost, CD30+

MEITL (Monomorphic Epitheliotropic Intestinal T-cell Lymphoma) (formerly type II EATL):

Gamma/delta T-cell lymphoma
NOT associated with celiac
More common in East Asian populations (celiac is rare there)
Monomorphic (uniform small-medium cells) and epitheliotropic
Aberrant CD8 expression (unusual for gamma/delta which is normally double-negative)
Aggressive

Extranodal NK/T-Cell Lymphoma, Nasal Type

Clinical: midline destructive nasal or palate mass in a patient from East Asian, Indigenous American, or Central/South American populations. Previously called “lethal midline granuloma.”

Pathophysiology: EBV-driven (virtually all cases EBER+). NK-cell origin (most) or cytotoxic T-cell origin.

Histology: angioinvasive (tumor cells invade vessel walls → vessel destruction → ischemic necrosis) with extensive coagulative necrosis. Angiodestructive pattern characteristic.

Immunophenotype: cytoplasmic CD3+ (surface CD3 negative because NK cells don’t express surface CD3), CD56+, EBER+. Cytotoxic molecules (granzyme B, perforin, TIA-1) positive.

Leukemic counterpart: aggressive NK-cell leukemia. Same biology (EBV+, CD56+, cytoplasmic CD3+) but leukemic presentation. More common in East Asian populations. Extremely aggressive (weeks to months survival). DIC and HLH complications. Distinguish from chronic NK-cell lymphoproliferative disorder (indolent).

T-Cell Large Granular Lymphocytic Leukemia (T-LGL)

Definition: persistent (>6 months) increase in large granular lymphocytes with aberrant phenotype and evidence of T-cell clonality.

Must rule out reactive LGL expansions: post-splenectomy, post-transplant, viral infections, solid tumors.

Clinical: leukocytosis with neutropenia (expanded cytotoxic T-cells suppress granulopoiesis). Recurrent infections. Associated with rheumatoid arthritis (often pre-existing) and pure red cell aplasia. Splenomegaly common but mild.

Immunophenotype: CD8+, CD2+, CD3+, CD57+, CD16+, CD4-, dim/negative CD5 and/or CD7. CD57 and CD16 are NK-associated markers co-expressed on these cytotoxic effector T-cells.

Clonal TCR rearrangement essential for diagnosis (distinguishes neoplastic from reactive).

Molecular: STAT3 mutations (~40-50%), STAT5B less common. JAK-STAT activation. Potential JAK inhibitor target.

Bone marrow pattern: sinusoidal (like hepatosplenic TCL, but T-LGL is CD8+ and indolent).

Cutaneous T-Cell Lymphoma (Mycosis Fungoides/Sezary Spectrum)

Spectrum of the same disease:

CTCL (skin only)
MF (skin + lymph nodes)
Sezary syndrome (skin + lymph nodes + blood)

All are CD4+ T-cell neoplasms of skin-homing T-cells.

Immunophenotype: CD4+, CD2+, CD3+, CD5+, CD7 negative. CD26 also typically negative. CD7 loss is the most characteristic aberrant finding.

Histology (CTCL/MF):

Epidermotropism (neoplastic T-cells migrate into epidermis)
Small to medium cells with hyperchromatic, cerebriform (convoluted) nuclei
Perinuclear halos
Plaque stage: Pautrier microabscesses (epidermal clusters of malignant T-cells - pathognomonic)

Mycosis Fungoides Clinical Progression

The name “fungoides” predates understanding of lymphoma - has nothing to do with fungi, refers to mushroom-like tumor appearance in late stage.

Patch stage: flat erythematous scaly patches, sun-protected areas (“bathing suit distribution”). Subtle histology.
Plaque stage: raised indurated plaques. Pautrier microabscesses prominent.
Tumor stage: large ulcerating nodules. Dermal infiltrate with cerebriform nuclei.

Generally indolent in early stages despite the ominous name.

Sezary Syndrome

Leukemic variant - progression from skin-limited to systemic.

Clinical triad: erythroderma, lymphadenopathy, circulating Sezary cells (CD4+ with cerebriform nuclei).

Morphology: Sezary cells have characteristic cerebriform (convoluted) nuclei with deep grooves and folds (brain-like). Medium-large lymphocytes on smear.

Diagnostic criteria: skin + LN + blood involvement of clonal T-cells PLUS one or more of:

Absolute Sezary cell count >1000/uL
CD4:CD8 ratio >10
Loss of ≥1 T-cell antigen (most commonly CD7)

CD4:CD8 >10 (well above the normal 3-4:1) strongly suggests clonal CD4+ expansion.

Treatment: skin-directed for early MF (phototherapy, topical steroids/chemotherapy). Systemic for advanced disease: photopheresis (extracorporeal photochemotherapy), systemic chemotherapy, mogamulizumab.

31.13 Hodgkin Lymphoma

Hodgkin lymphoma is fundamentally different from non-Hodgkin lymphomas - the malignant cells (Reed-Sternberg cells) comprise only a tiny fraction of the tumor mass, which is predominantly reactive inflammatory cells. This explains the B symptoms (fever, weight loss, night sweats) common in Hodgkin - the inflammatory milieu produces cytokines.

Hodgkin is a B-cell neoplasm of germinal center origin. Two main subtypes that are fundamentally different diseases sharing the “Hodgkin” name:

Classical HL (95%) - Reed-Sternberg cells, loss of B-cell program
Nodular Lymphocyte Predominant HL (NLPHL) (5%) - popcorn cells, retains B-cell program

The Reed-Sternberg Cell: A Bizarre B-Cell

The Reed-Sternberg (RS) cell is the diagnostic hallmark of classical HL. Large (20-50 um) with bilobed or multilobed nucleus, each lobe containing a prominent eosinophilic nucleolus creating the classic “owl eye” appearance.

Reed-Sternberg cell in Hodgkin lymphoma: Large cell with bilobed nucleus and prominent “owl eye” nucleoli. CD30+, CD15+, CD45-.

RS cells were long mysterious because they lack typical B-cell markers. Molecular studies revealed they derive from germinal center B-cells with clonal Ig gene rearrangements AND “crippling” somatic mutations (stop codons, frameshifts) that should trigger apoptosis. They escape death through EBV infection (~40% of cases; LMP1 mimics CD40 signaling → NF-kB activation) and constitutive NF-kB activation.

RS cells have Ig gene rearrangements but do NOT produce functional immunoglobulin.

Immunophenotype of classic RS cells:

CD30 positive - ALWAYS; essential for diagnosis
CD15 positive (~85%)
CD45 negative - unlike most hematologic malignancies
CD20 negative or variable - despite B-cell origin
PAX5 weakly positive - only remnant of B-cell program
OCT2 and BOB1 negative (lost due to epigenetic silencing)
EMA negative

CD30 is not specific for HL - also in ALCL, some DLBCL, embryonal carcinoma, and activated lymphocytes (immunoblasts). CD15 with CD30 increases specificity.

RS cell cytokine production (why B symptoms and specific histologies):

IL-5 → eosinophilia (especially mixed cellularity)
IL-13 → fibrosis (nodular sclerosis)
IL-6, IL-9 → B symptoms

If you see eosinophilia + lymphadenopathy on a board vignette, think Hodgkin (especially mixed cellularity).

RS cell vs. immunoblast (both large, CD30+): RS cells lose OCT2 and BOB1. Immunoblasts retain OCT2 and BOB1. Critical IHC distinction when differentiating CHL from reactive immunoblast hyperplasia.

CHL with aberrant T-cell expression: CHL RS cells can aberrantly express T-cell markers (CD2, CD3, CD4, CD7), mimicking ALCL. Distinguish by:

CHL: PAX5 weak+, EMA-, Ig gene rearrangement+
ALCL: EMA+, ALK+ or ALK-, TCR rearrangement+, hallmark cells
CD15+ favors CHL

CHL with aberrant T-cell expression has worse prognosis (higher grade/stage).

Can You Karyotype These?

Neither myeloma nor Hodgkin lymphoma is amenable to karyotype. Myeloma plasma cells have low proliferative rates and rarely enter metaphase (use FISH). RS cells comprise <1-2% of HL tumor mass - background cells dominate any karyotype.

Classical Hodgkin Lymphoma Subtypes (95% of HL)

All share the RS cell immunophenotype (CD30+/CD15+/CD20-/CD45-). They differ in background composition, architecture, and EBV association.

Prognosis principle: more lymphocytes = better prognosis (reflects host immune response).

Subtype	% of CHL	Classic Features	EBV
Nodular sclerosis	~70% (most common)	Young adults, mediastinal mass, collagen bands, lacunar cells	Low
Mixed cellularity	~20-25%	Older adults, HIV+, mixed inflammatory background	High
Lymphocyte-rich	~5%	Nodular pattern, abundant lymphocytes, best prognosis of CHL	Variable
Lymphocyte-depleted	<5%	Elderly, HIV+, few lymphocytes + histiocytes + fibrosis, worst prognosis	High

Lymphocyte-rich = best prognosis. Lymphocyte-depleted = worst.

Most important prognostic factor in CHL: stage (Ann Arbor).

Ann Arbor staging:

Stage I: single lymph node region
Stage II: ≥2 regions, same side of diaphragm
Stage III: both sides of diaphragm
Stage IV: disseminated (extranodal organs)
“B” suffix: B symptoms present

Early stage (I-II) = abbreviated chemo + radiation. Advanced (III-IV) = full ABVD.

CHL is highly curable - even advanced stage has >70% cure rate with modern therapy.

Nodular Sclerosis CHL

Most common CHL subtype in the US. Classic presentation: young woman with anterior mediastinal mass.

Histology: collagen bands dividing node into nodules; lacunar cells (RS variants with cytoplasm retracting during formalin fixation → clear lacuna around cell).

Mediastinal mass DDx (young woman):

Nodular sclerosis CHL (most common)
Primary mediastinal large B-cell lymphoma (PMBL)
T-lymphoblastic lymphoma (adolescent males)
Thymoma
Germ cell tumor

Biopsy with immunophenotyping is key. CD30+/CD15+/CD20-/CD45- → CHL.

Grading: not required for nodular sclerosis (stage matters more than grade).

EBV association: low.

Mixed Cellularity CHL

~20-25% of CHL. Mixed inflammatory background (eosinophils, plasma cells, histiocytes, lymphocytes) driven by RS cell cytokines. More common in older adults and HIV+.

EBV strongly associated - detected by EBER ISH in most cases. LMP1 → NF-kB activation → survival.

Lymphocyte-Rich CHL

~5% of CHL. Nodular pattern with abundant small lymphocytes, scattered RS cells. Best prognosis of classical subtypes.

Often confused with NLPHL and with nodular sclerosis CHL (all three can be nodular). Distinctions:

Lymphocyte-rich CHL: nodules formed by lymphocytes, no fibrous bands, RS cells at the RIMS of nodules, classic CHL immunophenotype (CD30+/CD15+), CD3+ T-cell rosettes
Nodular sclerosis CHL: nodules created by collagen bands, lacunar cells
NLPHL: popcorn cells in CENTERS of nodules, CD57+/PD1+ TFH rosettes, CD20+/CD45+/CD30-/CD15-

The different T-cell rosette types reflect different microenvironments - NLPHL LP cells reside in GC-like areas.

Lymphocyte-Depleted CHL

<5%. Rarest and most aggressive classical subtype.

Histology: few lymphocytes, prominent histiocytes and fibrosis, numerous RS cells or bizarre pleomorphic variants. Diffuse fibrosis variant or reticular variant (many bizarre RS cells).

Demographics: elderly and HIV+. HIV-associated HL is most often mixed cellularity, but lymphocyte-depleted is enriched in HIV compared with the general population. Loss of protective T-cell surveillance → EBV-driven RS cells proliferate unchecked.

EBV strongly associated.

Can mimic ALCL or carcinoma due to large bizarre cells. CD30/CD15 positivity with weak PAX5 confirms CHL.

Nodular Lymphocyte-Predominant Hodgkin Lymphoma (NLPHL)

Distinct from CHL, treated more like an indolent B-cell lymphoma.

Malignant cell: LP (lymphocyte-predominant) cells, aka popcorn cells - large cells with multilobated, folded nuclei resembling popcorn. Less prominent nucleoli than RS cells.

Immunophenotype - retains B-cell program:

CD20 strongly positive
CD45 positive
CD30 negative, CD15 negative
BCL6+
OCT2+ and BOB1+
EMA+
PAX5 strong

Some NLPHL cases show focal CD30 OR CD15 (not both). If both positive, reclassify as CHL.

Architectural features:

Nodular pattern
Popcorn cells in CENTERS of nodules (reflecting GC-like microenvironment)
CD57+/PD1+ TFH rosettes around LP cells (vs. CD3+ rosettes in lymphocyte-rich CHL)

Clinical: indolent with excellent prognosis. Risk of late relapses and transformation to THRLBCL/DLBCL.

Rituximab works (because LP cells are CD20+) - unlike CHL.

IgD+ NLPHL variant: associated with Moraxella catarrhalis. Almost exclusively young males. Analogous to H. pylori driving gastric MALT - chronic antigenic stimulation drives GC B-cell proliferation.

PTGC: benign mimic of NLPHL (ill-defined enlarged GCs with mantle zone infiltration). PTGC is associated with NLPHL but is NOT a risk factor - just associated. Can precede, follow, or coexist.

Quick Reference Comparison

Feature	Classical HL	NLPHL
Malignant cell	Reed-Sternberg	Popcorn (LP) cells
CD30	+	-
CD15	+	-
CD20	-/variable	Strong +
CD45	-	+
PAX5	Weak	Strong
OCT2/BOB1	-	+
EMA	-	+
Rosettes	CD3+ (lymphocyte-rich)	CD57+/PD1+
B-cell program	Lost	Retained
Rituximab	Ineffective	Effective
Clinical course	Curable with chemo	Indolent; may transform

Chapter 32: Flow Cytometry

Flow cytometry is essential for diagnosing and classifying hematologic malignancies. Understanding the principles helps interpret results.

32.1 Principles

How it works:

Cells in suspension pass single-file through a laser beam
Light scatter and fluorescence are detected
Forward scatter (FSC): Correlates with cell size
Side scatter (SSC): Correlates with internal complexity (granularity, nuclear lobulation)
Fluorescence: Antibodies conjugated to fluorophores detect specific antigens

Gating: Sequential selection of cell populations for analysis

Initial gate based on FSC vs. SSC identifies cell populations
Subsequent gates refine analysis to population of interest

32.2 Interpreting Flow Cytometry

Key patterns to recognize:

Pattern	Diagnosis
CD5+, CD19+, CD23+, dim CD20	CLL
CD5+, CD19+, CD23-, Cyclin D1+	Mantle cell lymphoma
CD10+, CD19+, CD5-	Follicular lymphoma, Burkitt, B-ALL
CD19+, TdT+, dim CD45	B-ALL
CD3+, TdT+, CD4/CD8 variable	T-ALL
CD38++, CD138+, CD19-, abnormal PC markers	Plasma cell myeloma

Light chain restriction: Normal B-cells show polyclonal κ and λ light chains (κ:λ ratio ~2:1). Monoclonal B-cell population shows either κ or λ only - evidence of clonality.

Chapter 33: Bone Marrow Examination

The bone marrow examination is fundamental to hematopathology. Understanding how to interpret the aspirate and biopsy is essential. This chapter covers the mechanics of the procedure and then walks through the production disorders you will be expected to diagnose from the marrow - iron deficiency, megaloblastic anemias, anemia of inflammation, sideroblastic anemias, congenital dyserythropoietic anemias, inherited bone marrow failure syndromes (Fanconi, dyskeratosis congenita), pure red cell aplasia, cyclic neutropenia, and aplastic anemia. The final section covers non-hematolymphoid lesions of the spleen, which often land in the hematopathology sign-out queue.

33.1 Procedure and General Interpretation

Indications for Bone Marrow Examination

Unexplained cytopenias or cytoses
Suspected hematologic malignancy (staging, diagnosis)
Fever of unknown origin
Staging of lymphoma or solid tumors
Evaluation of iron stores (when noninvasive workup is ambiguous)
Monitoring treatment response
Evaluation of abnormal peripheral blood findings

Specimen Types

Aspirate: Liquid marrow obtained by suction. Best for:

Cell morphology (individual cell detail)
Differential count (myeloid:erythroid ratio)
Iron stain
Flow cytometry
Cytogenetics
Molecular studies

Core biopsy: Solid cylinder of bone and marrow. Best for:

Overall cellularity assessment
Architecture (fibrosis, granulomas, metastatic tumor)
Focal lesions that may be missed by aspiration
Reticulin/collagen staining

Touch preparations (imprints): Biopsy touched to slide before fixation. Useful when aspirate is a “dry tap.”

Cellularity Assessment

Cellularity is the percentage of marrow space occupied by hematopoietic cells (vs. fat).

Normal cellularity by age: Approximately 100 minus age (±10%)

Infant/child: 80-100%
Young adult: 60-70%
Elderly: 30-40%

Hypercellular marrow (>expected for age):

Reactive: Infection, hemolysis, recovery from chemotherapy
Neoplastic: Leukemia, MPN, MDS

Hypocellular marrow (<expected for age):

Aplastic anemia
Hypoplastic MDS
Post-chemotherapy
Some infections (parvovirus B19)

The Myeloid:Erythroid (M:E) Ratio

Normal M:E ratio is approximately 2:1 to 4:1 (myeloid cells outnumber erythroid precursors).

M:E Ratio	Interpretation
Increased (>4:1)	Myeloid hyperplasia (infection, CML, G-CSF), erythroid hypoplasia (pure red cell aplasia)
Decreased (<2:1)	Erythroid hyperplasia (hemolysis, blood loss, B12/folate recovery), myeloid hypoplasia

Maturation Assessment

Normal myeloid maturation: Progressive stages visible - blast → promyelocyte → myelocyte → metamyelocyte → band → neutrophil. A “left shift” means increased immature forms.

Normal erythroid maturation: Pronormoblast → basophilic normoblast → polychromatic normoblast → orthochromatic normoblast → reticulocyte. Look for synchronous nuclear and cytoplasmic maturation.

Dysplasia: Abnormal maturation features (see MDS section). Must involve ≥10% of a lineage to be significant.

Iron Stores Assessment

The iron stain (Prussian blue, also called Perls stain) on the aspirate assesses two things: storage iron in macrophages and sideroblastic iron in erythroid precursors. Microscopic examination of marrow is the gold standard for iron stores, but in practice it is reserved for cases where noninvasive tests are ambiguous (especially concurrent inflammation plus suspected iron deficiency).

Storage iron: Iron in macrophages. Graded 0-4+ (absent to markedly increased).

Absent: Iron deficiency
Increased: Anemia of chronic disease, sideroblastic anemia, hemochromatosis, transfusion iron overload

Sideroblasts: Erythroid precursors with iron granules. Normal marrow has scattered fine granules in 20-50% of normoblasts.

Ring sideroblasts: ≥5 iron granules encircling ≥1/3 of the nucleus. These represent iron-laden mitochondria and are pathologic. The threshold for MDS-RS classification is ≥15% ring sideroblasts.

Common Patterns

Finding	Consider
Hypercellular, left-shifted myelopoiesis	CML, infection, G-CSF effect
Hypercellular with dysplasia	MDS
Hypercellular with blasts ≥20%	Acute leukemia
Hypocellular with fatty replacement	Aplastic anemia
Dry tap with teardrop cells on smear	Myelofibrosis
Paratrabecular lymphoid aggregates	Follicular lymphoma, other B-NHLs
Interstitial lymphoid infiltrate	CLL, other low-grade lymphomas
Non-caseating granulomas	Sarcoidosis, infections (TB, fungal), drugs
Metastatic carcinoma (clusters)	Breast, prostate, lung most common

33.2 Iron Deficiency Anemia

Pathophysiology and Iron Handling

Iron is absorbed predominantly in the duodenum (and proximal jejunum). Heme iron (from meat) is absorbed intact by HCP1. Non-heme iron (Fe3+ in plants) must be reduced to Fe2+ by duodenal cytochrome B (DcytB) before being taken up by DMT1. Hepcidin, made by the liver, is the master regulator - it degrades ferroportin (the basolateral iron exporter), blocking iron absorption when stores are adequate or during inflammation.

Body iron distribution: ~65% in hemoglobin, ~10% in myoglobin and oxidative enzymes (cytochromes, catalase), and ~25% in storage as ferritin (soluble, readily mobilized) and hemosiderin (insoluble, partially degraded ferritin in macrophages, stains with Prussian blue). Serum ferritin reflects intracellular ferritin and total body stores.

Quantitatively: 1 mL of whole blood contains ~0.5 mg iron; 1 mL of packed RBCs contains ~1 mg iron. One unit of PRBCs (~200 mL of RBCs) carries ~200 mg iron. This is why chronically transfused patients (thalassemia major, sickle cell, MDS) eventually need chelation with deferasirox, deferoxamine, or deferiprone.

Common causes of iron deficiency (bleeding, poor intake, poor absorption, increased demand):

Infants and children: decreased intake; increased use with inadequate intake (growth spurts)
Adults: blood loss (colon cancer, menses); decreased intake (strict vegetarianism); decreased absorption (celiac sprue, small bowel resection); increased use with poor intake (pregnancy, lactation)

Children with iron deficiency are more likely to have elevated lead levels. Iron deficiency upregulates DMT1 in the duodenum, and DMT1 also transports lead. Add pica (eating paint chips) and you get a vicious cycle: iron deficiency increases lead absorption, and lead further inhibits heme synthesis.

Laboratory Profile

Iron deficiency develops in stages: (1) storage iron depletion (low ferritin, normal Hb), (2) iron-deficient erythropoiesis (low transferrin saturation, high TIBC, normal Hb), (3) iron deficiency anemia (low Hb, microcytic hypochromic).

The earliest finding in iron deficiency is a decrease in serum ferritin - it is the most sensitive test. Ferritin reflects total body iron stores. The catch: ferritin is an acute phase reactant and is falsely elevated in inflammation, infection, malignancy, and hepatic insufficiency (impaired clearance). A normal ferritin does not rule out iron deficiency in a patient with concurrent inflammation.

Classic iron panel in IDA: decreased transferrin saturation, increased TIBC, decreased serum iron, decreased ferritin.

% Transferrin saturation = (serum iron / TIBC) × 100. Normal is 20-50%. In iron deficiency the saturation is typically <15%. Percent saturation and TIBC are inversely correlated. TIBC rises in iron deficiency (more unsaturated transferrin) and falls in inflammation (transferrin is a negative acute phase reactant).

Zinc protoporphyrin (ZPP) and free erythrocyte protoporphyrin (FEP) are increased when iron is unavailable for heme synthesis - zinc substitutes for iron in protoporphyrin IX. Elevated in iron deficiency, anemia of inflammation, and lead toxicity. Normal in thalassemia (heme synthesis is fine; the problem is globin).

Soluble transferrin receptor (sTfR) is shed from erythroid precursor membranes in proportion to erythropoietic activity and cellular iron demand. sTfR is increased in IDA and normal in anemia of inflammation. This makes sTfR the best single marker for distinguishing true iron deficiency from AI when ferritin is ambiguous. The sTfR/log ferritin ratio (Thomas plot) is even better for the combined-deficiency case.

Peripheral Smear and RDW

IDA shows hypochromic microcytic RBCs, but a more specific finding is pencil cells (thin elliptocytes) - long narrow RBCs that look like pencils or cigars. These are uncommon in thalassemia or anemia of inflammation.

IDA has a higher RDW (often >17%) than thalassemia or AI. The elevated RDW reflects anisocytosis - a mix of older, normally sized RBCs made before stores depleted and newer microcytic RBCs. In thalassemia, RDW is normal to mildly elevated because every cell is uniformly small. The shortcut: high RDW + microcytosis = iron deficiency; normal RDW + microcytosis = thalassemia trait.

IDA and thalassemia are both microcytic and can show target cells. AI is more likely to be normocytic without target cells.

33.3 Megaloblastic Anemia: Folate and Vitamin B12 Deficiency

Folate and B12 Metabolism

Folate is ingested in green vegetables, beans, and citrus. It is absorbed in the jejunum after deconjugation from the polyglutamate form by intestinal conjugase, and released from enterocytes as 5-methyl-tetrahydrofolate (methyl folate, 5-MTHF). Body stores last only ~4 months, so dietary folate deficiency develops fast.

Methyl folate (5-MTHF) is converted to the active form tetrahydrofolate (THF) by methionine synthase, a B12-dependent methyltransferase. In this reaction, B12 transfers a methyl group from methyl-THF to homocysteine, producing methionine and THF. In B12 deficiency, methyl folate accumulates - the classic “methyl trap.” This causes functional folate deficiency even when folate intake is adequate, and it explains why B12 and folate deficiencies have identical hematologic findings.

THF is the cofactor for methyl transfer in DNA synthesis. One key reaction: THF donates a one-carbon unit to convert deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) via thymidylate synthase. Without adequate THF, dUMP is misincorporated into DNA, causing strand breaks and impaired DNA synthesis. Folate deficiency impairs DNA (not RNA or protein) synthesis, so nuclear maturation arrests while cytoplasmic maturation continues - this produces megaloblastic changes with nuclear-cytoplasmic dyssynchrony.

Vitamin B12 comes from animal products. Absorption involves a relay:

Food-bound B12 released by gastric acid
B12 binds R factor (haptocorrin) in the stomach
Pancreatic enzymes degrade R factor in the duodenum, releasing B12
B12 binds intrinsic factor (IF) produced by gastric parietal cells
B12-IF complex is absorbed in the terminal ileum via the cubilin receptor
In enterocytes, B12 is bound to transcobalamin I and II and exported to the bloodstream

Transcobalamin II (TCII) carries ~20% of serum B12 and is the metabolically active transport form that delivers B12 to tissues. Transcobalamin I (haptocorrin, ~80% of serum B12) delivers it to the liver for storage. TCII deficiency causes megaloblastic anemia despite normal serum B12 (B12 is present but not delivered).

B12 is also a cofactor for methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA in mitochondria. In B12 deficiency, methylmalonic acid (MMA) accumulates. This reaction has nothing to do with folate, which is why MMA is specific for B12 deficiency.

B12 body stores last 3-5 years (vs. folate’s 4 months), so dietary B12 deficiency takes years to develop.

Causes

Dietary:

Folate deficiency is common and occurs in alcoholics (poor diet plus direct folate antagonism by alcohol plus impaired absorption)
B12 deficiency from diet is rare and occurs in strict vegetarians/vegans only (and takes years)

Malabsorption:

Folate: celiac sprue, tropical sprue, Crohn’s affecting jejunum
B12: pernicious anemia (anti-IF antibodies, most common cause in developed countries), Crohn disease affecting terminal ileum or surgical resection of terminal ileum, pancreatic insufficiency (cannot degrade R factor), post-gastrectomy (no IF), Diphyllobothrium latum infestation (fish tapeworm that competes for B12)

Increased demand (both folate and B12):

Pregnancy - folate supplementation (400 mcg/day) prevents neural tube defects
Chronic hemolysis (SCD, thalassemia, HS) - folate supplementation recommended

Drugs/medications:

Folate: methotrexate (DHFR inhibitor), trimethoprim, phenytoin, sulfasalazine
B12: phenytoin (Dilantin), metformin (reduces ileal absorption), PPIs/H2 blockers (reduced acid impairs release of food-bound B12), nitrous oxide (oxidizes B12 cobalt and irreversibly inactivates it - watch for this after repeated anesthesia)

Peripheral Smear and Marrow

Classic smear in megaloblastic anemia: oval macrocytes (larger and more oval than the round macrocytes of liver disease or alcohol), anisopoikilocytosis, hypersegmented neutrophils, large platelets, and pancytopenia when severe. MCV is often >115 fL in fully developed disease. Hypersegmented neutrophils are the earliest and most specific peripheral blood finding (≥5 lobes in one cell or ≥5% of cells with 5+ lobes is diagnostic).

Bone marrow: hypercellular with striking megaloblastic changes - nuclear-cytoplasmic dyssynchrony (large immature nuclei with maturing cytoplasm), erythroid hyperplasia (reversed M:E ratio), left shift, giant metamyelocytes and band forms, and megakaryocytes with hypersegmented nuclei. The marrow can mimic leukemia or MDS because of the dramatic left shift and dysplasia. Drugs that impair DNA synthesis (hydroxyurea, azathioprine) can cause identical megaloblastic changes without vitamin deficiency.

Many megaloblastic erythroblasts die in the marrow (ineffective erythropoiesis), producing intramedullary hemolysis: high LDH (often >1000), high indirect bilirubin, and low haptoglobin. The combination of pancytopenia + high LDH + low haptoglobin can mislead toward a microangiopathic hemolytic anemia diagnosis. Check MCV and smear first.

Distinguishing Folate from B12 Deficiency

Test	Folate Deficiency	B12 Deficiency
Serum folate	Low	Normal/high
RBC folate	Low	Often low (methyl trap, 2/3 of cases)
Serum/urine MMA	Normal	Elevated
Homocysteine	Elevated	Elevated
Neurologic disease	No	Yes
Urinary FIGLU (histidine load)	Elevated	Normal

RBC folate is low in both folate deficiency AND in 2/3 of B12 deficiency cases (because of the methyl trap). This limits its specificity. B12 deficiency can falsely lower RBC folate but does not affect serum folate.

Elevated MMA + elevated homocysteine = B12 deficiency. Normal MMA + elevated homocysteine = folate deficiency. MMA is the most specific marker for B12 because the methylmalonyl-CoA mutase reaction does not involve folate.

FIGLU test: folate is needed for histidine metabolism. In folate deficiency, formiminoglutamic acid (FIGLU) accumulates and is excreted after a histidine load. Specific for folate but rarely used clinically - serum and RBC folate plus homocysteine have replaced it.

B12 level pitfalls: falsely low in HIV infection, pregnancy, and on oral contraceptives. Falsely normal/high despite true deficiency in hepatic, renal, and myeloproliferative diseases (released hepatocyte B12, impaired renal clearance, elevated TCII). When suspicion is high despite borderline B12, check MMA and homocysteine.

B12 Neurologic Disease and Pernicious Anemia

Only B12 deficiency (not folate) causes neurologic disease: subacute combined degeneration of the spinal cord (posterior columns and lateral corticospinal tracts). Symptoms: peripheral neuropathy/paresthesias, loss of proprioception and vibratory sense, ataxia, spastic paraparesis. Mechanism: impaired myelin synthesis, likely from MMA-driven abnormal fatty acid incorporation. Giving folate alone to a B12-deficient patient can correct the hematologic picture but not the neurologic damage - a classic board trap.

Anti-IF antibody is the most specific serologic marker for pernicious anemia (sensitivity ~50-70%, specificity ~98%). Two types: Type I (blocking) prevents B12-IF binding; Type II (binding/precipitating) prevents the IF-B12 complex from binding the ileal receptor. Anti-parietal cell antibodies are more sensitive (~90%) but less specific. Pernicious anemia also increases risk of gastric carcinoid and gastric adenocarcinoma.

Schilling test (historical, no longer performed but conceptually important):

Stage 1: oral radioactive B12 + IM unlabeled B12 (to saturate binding proteins). Low urinary radioactivity = malabsorption
Stage 2: repeat with IF added. Correction = pernicious anemia. No correction = ileal disease
Stage 3: repeat after antibiotics. Correction = bacterial overgrowth
Stage 4: repeat after pancreatic enzymes. Correction = pancreatic insufficiency

Replaced by anti-IF antibody testing and MMA levels.

33.4 Anemia of Inflammation (AI / ACD)

Chronic systemic inflammation alters marrow iron utilization, suppresses EPO secretion, and suppresses RBC sensitivity to EPO. Three mechanisms drive anemia of inflammation (anemia of chronic disease):

Hepcidin upregulation by IL-6, which degrades ferroportin and traps iron in macrophages (functional iron deficiency)
Reduced EPO secretion (inflammation blunts the renal EPO response to hypoxia)
Reduced RBC sensitivity to EPO (cytokines directly inhibit erythroid progenitors)

Hepcidin is the central mediator - the link between inflammation and iron-restricted erythropoiesis.

Associated with rheumatoid arthritis and other collagen vascular diseases, chronic infection (osteomyelitis, bronchiectasis), and malignancy.

Laboratory Profile

AI is characterized by low-normal (15-20%) transferrin saturation, normal to decreased TIBC, normal to decreased serum iron, and normal to increased ferritin. The TIBC behavior is the key distinguisher from IDA: transferrin is a negative acute phase reactant, so TIBC falls in AI but rises in IDA.

Finding	IDA	AI
Serum iron	Low	Low
TIBC	High	Low/normal
% transferrin saturation	Very low (<15%)	Low-normal (15-20%)
Ferritin	Low	Normal/high
sTfR	Elevated	Normal
RDW	>17%	Normal/mild
Marrow macrophage iron	Absent	Increased
Marrow sideroblasts	Absent	Decreased

AI typically presents as a normocytic anemia but can become microcytic with chronicity. The reticulocyte count is inappropriately low (hypoproliferative). Mild anemia (Hb 8-11 g/dL) is characteristic.

Marrow findings in AI are the opposite of IDA: increased stainable iron in macrophages (iron is trapped because hepcidin has degraded ferroportin) but decreased iron in erythroid precursors (sideroblasts). This “iron block” pattern - plenty of storage iron but inadequate delivery to erythroid precursors - is the histologic signature.

Treatment targets the underlying inflammatory condition. EPO-stimulating agents help in CKD and cancer-related anemia.

33.5 Sideroblastic Anemias

Sideroblastic anemias are disorders with anemia and ring sideroblasts on marrow biopsy. The defect is in heme synthesis or mitochondrial function, so iron accumulates in perinuclear mitochondria rather than being incorporated into heme.

Peripheral Smear and Iron Studies

Inherited sideroblastic anemia is typically microcytic (defective heme synthesis in developing RBCs). Acquired sideroblastic anemia (MDS-RS, drugs, alcohol) is typically macrocytic. A dimorphic RBC population (hypochromic + normochromic) is suggestive.

Inherited X-linked sideroblastic anemia (XLSA) often shows a bimodal RBC volume distribution on the automated analyzer - one population of normal RBCs and one of microcytic hypochromic sideroblastic RBCs. In X-linked carrier females, lyonization creates two distinct populations.

Peripheral smear often shows basophilic stippling (aggregates of ribosomes/RNA) and Pappenheimer bodies (iron-laden siderotic granules visible on Wright stain as fine blue-purple dots; confirmed as iron by Prussian blue). Pappenheimer bodies are also seen post-splenectomy (the spleen normally pits iron inclusions out of RBCs).

Iron studies in sideroblastic anemia look like hemochromatosis: increased transferrin saturation, normal/decreased TIBC, increased iron, increased ferritin. Ineffective erythropoiesis and intramedullary hemolysis add hyperbilirubinemia, high LDH, and low haptoglobin. The ring sideroblasts on marrow biopsy distinguish this from hemochromatosis.

Marrow Findings

Ring sideroblasts: erythroid precursors with ≥5 iron granules encircling ≥1/3 of the nucleus on Prussian blue stain. Iron accumulates in perinuclear mitochondria because heme synthesis is defective or mitochondrial function is impaired. The marrow also shows increased iron stores and erythroid hyperplasia with mild dyserythropoiesis. ≥15% ring sideroblasts is the threshold for MDS-RS.

Causes

Acquired:

MDS-RS: SF3B1 mutation in >80% (splicing factor, best prognosis among MDS subtypes)
Medications: isoniazid (inhibits pyridoxine-dependent ALAS2, correctable with B6), chloramphenicol, chemotherapy
Irradiation
Copper and pyridoxine (B6) deficiencies - copper is a cofactor for cytochrome c oxidase; excess zinc causes copper deficiency
Lead toxicity (inhibits ferrochelatase, the enzyme that inserts iron into protoporphyrin)
Alcohol abuse (direct mitochondrial toxicity)

Inherited (rare):

X-linked recessive, most commonly from ALAS2 mutations
ALAS2 (5-aminolevulinate synthase 2) catalyzes the first and rate-limiting step of erythroid heme synthesis: glycine + succinyl-CoA → ALA, with pyridoxal phosphate (B6) as cofactor. Some ALAS2 mutations respond to high-dose pyridoxine
Autosomal forms: SLC25A38 (mitochondrial glycine transporter), GLRX5 (iron-sulfur cluster assembly)
Pearson syndrome: inherited sideroblastic anemia with pancreatic insufficiency

33.6 Congenital Dyserythropoietic Anemias (CDA)

The CDAs are inherited defects of erythropoiesis with dyserythropoietic features in the marrow. Three classical types; type II is most common.

CDA Type II (HEMPAS)

CDA type II is the most common type, inherited autosomal recessive. Also known as HEMPAS: Hereditary Erythroblastic Multinuclearity with Positive Acidified Serum test. Caused by SEC23B mutations (COPII vesicle coat protein).

Marrow features: multinucleate erythroid precursors (2-7 nuclei), abnormal mitotic figures, increased karyorrhexis. Binucleate normoblasts are the hallmark. The nuclei fail to separate properly because defective COPII vesicular transport disrupts cytokinesis.

Electron microscopy: abnormally abundant endoplasmic reticulum running parallel to the cell membrane, creating a “double membrane” appearance. This is pathognomonic. The SEC23B defect blocks ER-to-Golgi vesicle budding, so ER accumulates beneath the plasma membrane.

Positive Ham test with heterologous serum only (not autologous). The abnormal membrane glycosylation makes CDA II cells susceptible to lysis by anti-I antibody present in ~1 in 3 normal donor sera. This is the classical distinction from PNH.

Iron overload is common despite no transfusion requirement.

CDA Type I

CDA I: characterized by dysplastic erythroid precursors with frequent internuclear chromatin bridging between adjacent erythroblasts. This is the light microscopy hallmark. CDAN1 or C15orf41 mutations. Autosomal recessive. Moderate anemia. Responsive to interferon-alpha therapy.

On electron microscopy, CDA I shows a pathognomonic spongy or “Swiss-cheese-like” heterochromatin appearance - the nuclear chromatin has an abnormal lacey, porous texture with multiple electron-lucent areas. This EM finding is diagnostic and distinguishes CDA I from CDA II.

Iron overload develops over time.

CDA Type III

CDA III is the rarest type, autosomal dominant (unlike I and II, which are AR). KIF23 mutations. Pronounced multinucleation with up to 12 nuclei per erythroblast (gigantoblasts). Large erythroblasts with lobulated nuclei. Relatively mild anemia. Associated with monoclonal gammopathy and myeloma in some families.

Shared CDA Features

CDA RBCs show increased density of both I and i antigens on their surface. The i antigen normally converts to I during the first 2 years of life; persistent i antigen on adult RBCs suggests CDA, thalassemia, or other stress erythropoiesis.

Positive acidified serum (Ham) test can occur in both CDA II and PNH. The distinction: CDA II is positive with heterologous serum only; PNH is positive with both autologous AND heterologous serum. The mechanism differs - CDA II cells are lysed by anti-I in donor serum because of abnormal membrane glycosylation; PNH cells lack GPI-anchored complement regulators (CD55, CD59) and are lysed by any complement source. Sucrose hemolysis test is negative in CDA, positive in PNH. Flow cytometry for GPI-anchored proteins has replaced the Ham test for PNH diagnosis.

33.7 Inherited Bone Marrow Failure Syndromes

Fanconi Anemia

Fanconi anemia is an autosomal recessive chromosomal breakage syndrome that is complicated by aplastic anemia, usually by age 10.

Classic triad:

Pancytopenia (progressive aplastic anemia)
Congenital anomalies: absent or hypoplastic thumbs, absent radii, café au lait spots, short stature, microcephaly, renal malformations
Cancer predisposition

Elevated HbF is a clue in a child with cytopenias. FA can present initially with isolated macrocytic anemia or isolated thrombocytopenia, progressing through anemia → thrombocytopenia → neutropenia → frank aplastic anemia. Median age of aplastic anemia onset is ~7 years.

Genetics: FANCA, FANCC, and FANCG are the most commonly mutated. FANCA ~65%, FANCC ~10%, FANCG ~10%. Over 20 FANC genes are known - all function in the Fanconi anemia DNA repair pathway, which resolves DNA interstrand crosslinks (ICLs). BRCA2 is FANCD1, linking the BRCA pathway to FA. All subtypes are autosomal recessive except FANCB (X-linked). Carrier frequency ~1/300.

Epidemiology: high incidence in individuals of Afrikaner ancestry due to a founder FANCA deletion - the most common single FA mutation worldwide. Also individuals with Ashkenazi Jewish ancestry (FANCC c.456+4A>T founder mutation).

Cancer risk: >500-fold increased AML risk (usually with monocytic differentiation, often preceded by MDS). Squamous cell carcinomas of head/neck, esophagus, and anogenital region by the 3rd-4th decade. Also hepatocellular carcinoma, gastric carcinoma, cutaneous SCC. Without the FA pathway, cells accumulate chromosomal breaks and translocations that drive malignant transformation.

Screening test: chromosomal breakage test using DNA crosslinking agents - patient lymphocytes cultured with diepoxybutane (DEB) or mitomycin C (MMC) show dramatically increased chromosomal breaks, radial figures, and translocations. FA cells are hypersensitive to these agents because they cannot repair the induced ICLs. Highly sensitive and specific. Somatic mosaicism (reversion in blood cells) can cause false negatives - test skin fibroblasts if suspicion is high.

HSCT is the only cure for the marrow failure. Conditioning must be reduced-intensity because FA cells are hypersensitive to alkylating agents and radiation. Post-transplant SCC risk remains high.

Dyskeratosis Congenita (Zinsser-Engman-Cole Syndrome)

DC is a telomere biology disorder. Most commonly X-linked recessive (DKC1 gene, encoding dyskerin), but autosomal dominant (TERC, TERT) and autosomal recessive (NOP10, NHP2, WRAP53) forms exist. All affected genes encode components of the telomerase complex or telomere maintenance machinery. The result: critically short telomeres, stem cell exhaustion, and progressive marrow failure.

DKC1 encodes dyskerin, a component of the H/ACA ribonucleoprotein complex that stabilizes TERC (telomerase RNA component), which provides the RNA template for telomere repeat synthesis. Mutations in either gene impair telomerase function, producing progressive telomere shortening with each division. When telomeres become critically short, cells undergo senescence or apoptosis. This explains progressive disease and anticipation (earlier, more severe disease in successive generations). Telomere length by flow-FISH is the screening test.

Classic clinical features:

Pancytopenia (progressive aplastic anemia)
Reticulated skin hyperpigmentation (lacy, net-like)
Nail dystrophy (ridging, splitting, absent nails)
Oral leukoplakia
Lacrimal duct atresia (overflow lacrimation)
Testicular atrophy
Pulmonary fibrosis
Esophageal strictures, liver fibrosis, premature graying, dental abnormalities, osteoporosis

DC typically presents later than FA (median marrow failure in 2nd decade) and often with isolated cytopenia (thrombocytopenia or macrocytic anemia) before pancytopenia develops. DC should be in the differential for any young patient with aplastic anemia plus pulmonary fibrosis.

Mortality: bone marrow failure (~60-70%), pulmonary disease (~10-15%), malignancy (~10%). X-linked DKC1 has the worst prognosis. HSCT cures the marrow failure but does not prevent pulmonary fibrosis or cancer risk. Like FA, reduced-intensity conditioning is required (DC cells are hypersensitive to DNA damage). Cancer risk: MDS/AML, head/neck SCC.

33.8 Pure Red Cell Aplasia and Transient Erythrocytopenia of Childhood

Congenital: Diamond-Blackfan Anemia (DBA)

Congenital pure red cell aplasia = Diamond-Blackfan anemia, caused by RPS19 mutations (most common, ~25% of cases). Other genes: RPL5, RPL11, RPS26, RPS10 - all ribosomal protein genes. DBA is a ribosomopathy: impaired ribosome biogenesis triggers p53-mediated apoptosis of erythroid progenitors, causing selective red cell aplasia. Autosomal dominant inheritance in ~45%.

Marrow: erythroid precursors absent. Macrocytic anemia. Leukocytes and platelets unaffected.

Presentation: typically 3 months of age (median), >90% diagnosed by age 1. Macrocytic anemia with inappropriately low reticulocytes is the hallmark. About 50% have congenital anomalies: craniofacial (Pierre Robin, cleft palate), thumb anomalies (triphalangeal thumb), short stature, cardiac septal defects, urinary tract.

Labs: overexpression of i antigen on RBCs, elevated erythrocyte adenosine deaminase (eADA) activity (~85% of cases, useful biomarker), and elevated HbF. eADA helps distinguish DBA from TEC (TEC has normal eADA). Note: eADA can also be elevated in hereditary spherocytosis and after transfusion, so it is supportive, not pathognomonic.

Treatment: ~75% respond to corticosteroids (prednisone suppresses p53-mediated apoptosis of erythroid precursors). ~25% require chronic transfusions. ~20% achieve spontaneous remission, usually by adolescence. HSCT is curative but reserved for transfusion-dependent patients who fail steroids.

Acquired Pure Red Cell Aplasia

Acquired PRCA has multiple causes:

~1 in 3 cases are associated with thymoma (especially the spindle cell/medullary/type A variant). Mechanism: T-cell mediated suppression of erythroid precursors or anti-EPO antibodies from aberrant thymic lymphocytes. Thymoma-associated PRCA may improve after thymectomy.
Parvovirus B19 (cytopathic effect on erythroid progenitors via the P antigen receptor)
CLL, T-cell large granular lymphocytic leukemia
Collagen vascular diseases
Medications
After EPO therapy: anti-EPO antibodies can develop and neutralize both exogenous and endogenous EPO, causing transfusion-dependent anemia. Most cases were associated with a specific Eprex formulation (polysorbate 80 leachates from rubber stoppers acted as adjuvants). Treatment: immunosuppression (cyclosporine, cyclophosphamide). Patients must discontinue all EPO products - switching to another ESA is contraindicated because antibodies cross-react.

Transient Erythrocytopenia of Childhood (TEC)

TEC is a self-limiting red cell aplasia in previously healthy children aged 1-4 years, characterized by reticulocytopenia, normochromic normocytic anemia, and thrombocytosis. Marrow is hypocellular from erythroid hypoplasia. Often follows a viral infection; some cases are driven by parvovirus B19. Spontaneous recovery within 1-2 months.

Feature	TEC	DBA
Age	1-4 years	Typically <1 year
MCV	Normal	Macrocytic
eADA	Normal	Elevated
HbF	Normal	Elevated
Congenital anomalies	No	~50%
Course	Self-limited	Chronic

33.9 Cyclic Neutropenia

Cyclic neutropenia is characterized by severe periodic neutropenia with regular ~21-day oscillations of the ANC. Caused by mutations in ELA2 (ELANE), the gene encoding neutrophil elastase. The mutant elastase misfolds and triggers the unfolded protein response in myeloid precursors, leading to apoptosis and cyclic arrest of granulopoiesis.

During nadirs (typically 3-5 days), the ANC drops to near zero, and patients are at risk for severe infection. Oral ulcers, gingivitis, and aphthous stomatitis are classic during neutropenic troughs. Between cycles, the ANC returns spontaneously into the normal range.

Treatment: G-CSF prevents severe infections and can dampen the cycling.

33.10 Aplastic Anemia

Aplastic anemia is pancytopenia with a hypoproliferative (hypocellular) marrow. Most cases are idiopathic (60-70%), with the rest attributable to medications/toxins, viral infection, or inherited causes.

Causes:

Idiopathic (majority)
Medications/toxins (chloramphenicol, NSAIDs, sulfonamides, gold, benzene, chemotherapy)
Viral: hepatitis-associated aplastic anemia occurs 2-3 months after an episode of seronegative hepatitis (Hep A, B, C are typically negative - the hepatitis is likely immune-mediated, and the same immune process attacks the marrow)
Inherited (FA, DC, Shwachman-Diamond)
Pregnancy-associated (may resolve after delivery)
Trisomy 21 (Down syndrome)

Mimickers to rule out before calling it idiopathic aplastic anemia:

Paroxysmal nocturnal hemoglobinuria
Hairy cell leukemia
T-cell large granular lymphocytic leukemia
Hypoplastic MDS
Hypoplastic AML

PNH and AA are closely related: ~50% of AA patients have small PNH clones, and PNH can evolve into AA. Clonal evolution to MDS/AML occurs in ~15% of AA patients over 10 years.

Treatment: HSCT in younger patients with a matched donor; immunosuppression (ATG + cyclosporine) otherwise. Eltrombopag has been added to IST regimens.

33.11 Spleen: Non-Hematolymphoid Lesions

Splenic specimens frequently arrive at hematopathology sign-out. Many splenic lesions are non-lymphoid - vascular tumors, cysts, pseudotumors, and congenital abnormalities. This section covers the non-heme lesions worth recognizing.

Congenital and Miscellaneous

Accessory spleen is a nodule of splenic tissue that arises congenitally apart from the main body of the spleen. Found in ~10-30% of people, most commonly in the splenic hilum or pancreatic tail. Clinical traps:

After splenectomy for ITP or AIHA, an accessory spleen can hypertrophy and cause disease recurrence
An accessory spleen in the pancreatic tail can mimic a pancreatic neoplasm on imaging
Nuclear medicine scan with heat-damaged RBCs or sulfur colloid can identify splenic tissue

Gamna-Gandy bodies (siderotic nodules) are fibrotic, iron- and calcium-encrusted foci in the spleen - yellow-brown or rust-colored on gross exam. They form from organizing hemorrhagic foci and contain hemosiderin, calcium, and fibrous tissue. Prussian blue positive. Seen in portal hypertension/congestive splenomegaly (most common), sickle cell disease (repeated infarcts), and splenic trauma. On MRI: low signal on T1 and T2 (iron and calcium).

Immune thrombocytopenic purpura (ITP) spleen shows follicular hyperplasia with markedly expanded marginal zones and attenuated mantle zones, plus foamy macrophages forming parafollicular diffuse sheets. The expanded marginal zones contain the IgG-producing B cells and plasma cells that make antiplatelet antibodies. The red pulp macrophages are what destroy antibody-coated platelets. This explains why splenectomy works in ITP: it removes both the site of platelet destruction and a major site of autoantibody production.

Myelolipoma is a tumor-like mass that resembles bone marrow - mature adipose tissue intermixed with hematopoietic cells (all three lineages). Most common in the adrenal but occurs in the spleen. Benign, usually incidental. Distinguish from extramedullary hematopoiesis (diffuse, associated with myelofibrosis/MPNs, not forming a discrete mass).

Inflammatory pseudotumor (IPT, inflammatory myofibroblastic tumor) is a reactive splenic mass of spindle cells (myofibroblasts), lymphocytes, plasma cells, histiocytes, and fibrosis. Characteristic mixture of small lymphocytes, plasma cells, histiocytes, and plump spindle cells without atypia. IgG4-related disease should be excluded (IgG4/IgG ratio, serum IgG4). Some cases (especially pediatric) harbor ALK rearrangements. EBV-associated IPT occurs in immunocompromised patients. Splenectomy is curative.

Post-chemotherapy histiocyte-rich pseudotumor - a residual splenic mass after chemotherapy for DLBCL (or other lymphomas) may consist predominantly of histiocytes/macrophages clearing necrotic tumor, not residual lymphoma. Key distinction by IHC: CD68+ histiocytes vs. CD20+ lymphoma cells. PET-CT may show FDG uptake in the histiocyte-rich areas (false positive for residual disease).

Splenic Cysts

Cyst Type	Lining	Key Features
Epithelial (mesothelial)	Flattened mesothelial cells (calretinin+, CK+)	True cyst, unilocular, clear/straw-colored fluid, thin-walled
Epithelial (epidermoid)	Squamous epithelium (CK+, calretinin-)	True cyst, elevated CA 19-9 and CEA in fluid
Pseudocyst (false cyst)	No epithelial lining	Most common splenic cyst; post-traumatic hematoma degeneration; fibrous wall, hemosiderin-laden macrophages

Epithelial cysts of the spleen are true cysts with an epithelial lining; most are unilocular with fibrous trabeculations. Mesothelial-lined cysts likely arise from mesothelial invaginations during splenic development. Epidermoid cysts arise from ectopic squamous epithelial rests and secrete CA 19-9 and CEA (a board trap - elevated tumor markers in fluid, but the cyst is benign).

Splenic pseudocysts lack an epithelial lining and are the most common splenic cyst. Most result from degeneration of a post-traumatic hematoma. History of trauma is often present.

Vascular Lesions: Non-Neoplastic

Peliosis is characterized by blood-filled, cyst-like spaces within splenic parenchyma (also occurs in liver). The spaces are not lined by endothelium - this distinguishes peliosis from hemangiomas. Associated with anabolic steroids, oral contraceptives, immunosuppression (HIV, post-transplant), and Bartonella henselae infection (bacillary peliosis). Risk of rupture and hemorrhage.

Sclerosing angiomatoid nodular transformation (SANT) is a benign vascular lesion of the spleen. Gross: a single well-circumscribed mass with peripheral nodularity and central sclerosis. Histology: multiple angiomatoid nodules surrounded by dense sclerotic stroma. The nodules recapitulate the three normal splenic vascular elements: capillaries (CD34+), sinusoidal channels (CD8+, CD31+), and small veins (CD31+). IgG4-related disease may contribute. Benign, no malignant potential, usually incidental.

Splenic hamartoma is a benign tumor composed exclusively of red pulp with disorganized sinuses and sometimes atypical stromal cells and extramedullary hematopoiesis. White pulp is absent - this is the distinguishing feature. The sinusoidal channels are lined by CD8+ littoral-type endothelium. Well-circumscribed but unencapsulated. Distinguish from SANT: hamartomas lack the sclerotic stroma and three-vessel nodular pattern.

Vascular Lesions: Benign Neoplastic

Hemangiomas are the most common primary benign tumor of the spleen. Thin-walled benign vascular channels lined by flattened endothelium (CD34+, CD31+, factor VIII+). Most common subtype is cavernous (large dilated spaces). Cavernous hemangioma histology: large blood-filled spaces separated by thin fibrous septa, lined by flat endothelial cells without atypia.

On imaging: well-circumscribed, hypoattenuating on CT with progressive centripetal enhancement; T2-bright on MRI (fluid-filled spaces). Usually asymptomatic and incidental; large ones risk rupture, and Kasabach-Merritt phenomenon is a rare complication.

Differential: littoral cell angioma (CD8+, CD68+), angiosarcoma (atypical endothelium), peliosis (no endothelial lining).

Lymphangiomas are benign proliferations of lymphatic channels filled with proteinaceous fluid (not blood). IHC: D2-40 (podoplanin)+, CD31+, CD34 variable, factor VIII negative (distinguishing from hemangioma). Multicystic with pink proteinaceous fluid in the lumina. Can be cystic, cavernous, or capillary. May be part of generalized lymphangiomatosis.

Littoral cell angioma (LCA) is a benign vascular tumor unique to the spleen, arising from littoral cells (the sinus-lining endothelium). Usually incidental but can cause cytopenias.

Histology shows two distinct endothelial populations:

Tall endothelial cells lining the luminal surface with papillary architecture and erythrophagocytosis - these have histiocytic differentiation
Flat/basilar endothelial cells at the base of the channels - these have conventional endothelial differentiation

IHC profile (key board topic):

Cell Type	IHC Profile
Tall (luminal) cells	CD68+, CD163+, CD21+, factor VIII+, CD34-, CD31-
Flat (basilar) cells	CD34+, CD31+, factor VIII+, CD68-, CD163-

This two-layered pattern distinguishes LCA from hemangioma and angiosarcoma and recapitulates the normal splenic sinus architecture (littoral cells on top, basement membrane/endothelium underneath). The CD21 positivity in the tall cells is notable.

CD8 highlights normal splenic sinuses but not littoral cell angioma (LCA tall cells are variably CD8-negative or weak). This distinguishes LCA from other vascular lesions like hamartoma and SANT. Loss of CD8 in a vascular proliferation that otherwise resembles littoral cells supports LCA over normal sinus tissue. Combined with the CD68+/CD163+/CD21+ tall cell profile and erythrophagocytosis, this confirms the diagnosis.

LCA is benign, but it may be associated with malignancies in other organs - a reported, if uncertain, association worth flagging.

Vascular Lesions: Malignant Neoplastic

Angiosarcoma is the most common non-hematologic malignant tumor of the spleen (lymphomas remain the most common splenic malignancy overall). Highly aggressive vascular neoplasm with atypical endothelial cells forming irregular, anastomosing vascular channels with slit-like spaces. High mitotic rate, necrosis, nuclear pleomorphism, solid or papillary growth.

IHC: CD31+ (most sensitive), ERG+, CD34+ (variable), factor VIII+ (variable). Associated with prior radiation, thorotrast, and vinyl chloride (hepatic angiosarcoma).

Presentation: splenomegaly, spontaneous splenic rupture in ~25%, hemoperitoneum, anemia, thrombocytopenia, DIC. Metastasizes widely (liver, lung, bone). Prognosis is dismal - most patients present with rupture or disseminated disease, and median survival is measured in months.

Hemangioendothelioma is a vascular neoplasm with intermediate malignant potential between hemangioma and angiosarcoma. Epithelioid or spindle endothelial cells forming vascular channels with mild atypia and low mitotic rate. Subtypes: epithelioid (WWTR1-CAMTA1 fusion), retiform, and others. Local recurrence is common; metastasis is less frequent than with angiosarcoma. Splenectomy with long-term follow-up.

Kaposi sarcoma (KS) in the spleen: thin-walled slit-like blood vessels and hyaline (eosinophilic) globules (PAS-positive, representing phagocytosed RBC fragments). HHV-8 (KSHV) driven. IHC: HHV-8 LANA-1 positive (nuclear, pathognomonic), CD31+, CD34+, D2-40+ (lymphatic marker). Four clinical forms:

Classic (elderly Mediterranean men)
Endemic (African)
Immunosuppression-related (transplant)
Epidemic (HIV/AIDS)

Splenic involvement is common in disseminated KS.

Chapter 34: Post-Transplant Lymphoproliferative Disorder (PTLD)

PTLD represents a spectrum of lymphoid proliferations occurring in immunosuppressed transplant recipients. Understanding PTLD requires appreciating the role of EBV and the balance between immune surveillance and malignant transformation.

Formally, PTLD is defined as a lymphoid or plasmacytic proliferation that develops as a consequence of immunosuppression in a recipient of any transplant (solid organ or hematopoietic stem cell). The spectrum runs from benign-looking reactive hyperplasia all the way to frank, aggressive lymphoma. What ties the spectrum together is the underlying mechanism: immunosuppression knocks out T-cell surveillance, and EBV-infected B-cells get to do whatever they want.

Pathophysiology

In immunocompetent individuals, EBV infects B-cells and drives their proliferation, but T-cells control this proliferation. In transplant recipients on immunosuppression, T-cell surveillance is impaired, allowing EBV-infected B-cells to proliferate unchecked. Most PTLD is EBV-driven, though EBV-negative cases occur (usually later post-transplant, more aggressive).

The mechanism is worth spelling out because it explains everything downstream. EBV infects B-cells and expresses latent membrane proteins that mimic normal activation signals: LMP1 mimics CD40 signaling and LMP2A mimics B-cell receptor signaling. These signals drive B-cell proliferation. Normally cytotoxic T-cells eliminate the infected, proliferating B-cells. Immunosuppression removes that brake. Without T-cell control, EBV-driven B-cell proliferation can progress from reactive polyclonal expansion to oligoclonal to monoclonal malignancy.

Roughly 20 to 30% of PTLD is EBV-negative. These cases tend to occur later post-transplant and carry a worse prognosis because they behave more like de novo lymphomas that happen to arise in an immunosuppressed host, rather than reversible EBV-driven proliferations.

Risk Factors

EBV seronegativity at transplant (highest risk - primary EBV infection post-transplant)
Type of transplant: Higher risk with more intense immunosuppression
Highest: Intestinal, multivisceral
Then: Lung
Then: Heart
Then: Liver
Lowest: Kidney, pancreas
Intensity of immunosuppression: T-cell depleting agents (ATG, OKT3) increase risk
Time post-transplant: Most cases occur in first year (EBV+ cases); EBV- cases occur later
Pediatric recipients (more likely EBV-naïve)

The single most important risk factor is an EBV-seronegative recipient receiving an organ from an EBV-seropositive donor (D+/R-). This is the setup for primary EBV infection in a host with zero pre-existing EBV-specific memory T-cells, while that host is pharmacologically immunosuppressed. The virus has nothing standing in its way. That is why children have the highest PTLD risk - they are more likely to be EBV-naive at the time of transplant simply because they haven’t had time to pick up EBV yet. In pediatric transplant, EBV viral load monitoring by PCR is standard, and a rising load triggers pre-emptive reduction of immunosuppression.

Transplant type tracks directly with immunosuppression intensity. Highest risk: intestinal / multivisceral, then lung, then heart, then liver, then kidney. Gut has enormous lymphoid tissue and requires the most aggressive immunosuppression to prevent rejection. Kidneys are the easy end of the spectrum. T-cell depleting agents (ATG, OKT3) carry specific PTLD risk because they eliminate the exact cells that are supposed to keep EBV-infected B-cells in check.

Most PTLD occurs within the first year post-transplant, when immunosuppression is most intense and primary EBV infection is most likely. Early PTLD (less than 1 year) is typically EBV-positive and often responds to reduction of immunosuppression. Late PTLD (greater than 1 year) is more often EBV-negative, more aggressive, and less responsive to IS reduction.

WHO Classification of PTLD

The WHO classification (and its parallel ICC version) divides PTLD into four categories based on histology, clonality, and architecture. The scheme represents a spectrum of severity from reactive proliferation to frank lymphoma.

1. Non-destructive PTLD (early lesions)

Plasmacytic hyperplasia
Infectious mononucleosis-like
Florid follicular hyperplasia
Preserve nodal architecture; polyclonal or oligoclonal

2. Polymorphic PTLD

Mixed population of lymphocytes, plasma cells, immunoblasts
Destructive (efface architecture)
Usually EBV+, often clonal
May regress with reduced immunosuppression

3. Monomorphic PTLD (most common)

Fulfills criteria for a lymphoma diagnosis
Usually B-cell (DLBCL-like most common)
Less commonly T-cell or NK-cell
Clonal, often EBV+

4. Classical Hodgkin lymphoma-type PTLD

Rare; resembles classical HL
EBV+ in most cases

The current WHO approach uses an integrated classification with three axes: histologic diagnosis (hyperplasia, polymorphic lymphoproliferation, mucocutaneous ulcer, or lymphoma), oncogenic viral association (EBV or KSHV/HHV8), and immune deficiency / dysregulation context (HIV, inborn error of immunity, post-transplant, autoimmune, iatrogenic, or immune senescence). The ICC retains PTLD as a distinct subgroup of iatrogenic lymphoid lesions because the clinical management is specific.

Non-destructive PTLD

Non-destructive PTLD preserves architecture with a florid proliferation of polytypic cells. This is the earliest, most benign end of the spectrum. The tissue structure (lymph node follicles, germinal centers, tonsillar architecture) is intact. The proliferating cells are polyclonal or oligoclonal - no single dominant malignant clone yet. These lesions often regress completely with reduction of immunosuppression alone. Catching PTLD at this stage is the goal of EBV viral load monitoring.

The three subtypes of non-destructive PTLD are infectious mononucleosis-like, plasmacytic hyperplasia, and florid follicular hyperplasia.

Infectious mononucleosis-like PTLD. Intact architecture with florid proliferation of lymphocytes, plasma cells, and immunoblasts. The histology is indistinguishable from infectious mononucleosis in an immunocompetent host - in a normal patient, this would just be called IM. The difference is context: it’s happening in an immunosuppressed transplant recipient.
Plasmacytic hyperplasia. Intact architecture with a florid expansion of polytypic plasma cells (both kappa and lambda light chains present, proving the plasma cell population is not monoclonal). Often found in tonsils, adenoids, or lymph nodes. Mildest form of PTLD. Excellent response to IS reduction.
Florid follicular hyperplasia. Looks like regular reactive follicular hyperplasia morphologically, but with extensive EBER positivity. This is the key diagnostic point - normal reactive follicular hyperplasia is EBER-negative or only focally positive. When you see widespread EBER positivity across hyperplastic follicles in a transplant patient, it is non-destructive PTLD, not just a reactive node. This is why EBER testing is essential in any transplant patient with lymphadenopathy.

Polymorphic PTLD

Polymorphic PTLD has architectural destruction that looks like lymphoma, but the infiltrate is a mixture of immunoblasts, plasma cells, and small lymphocytes - it does not meet criteria for any specific lymphoma. The key features are: (1) destruction of normal architecture (unlike non-destructive PTLD), and (2) a heterogeneous “polymorphic” cell population (unlike monomorphic PTLD, which looks like a defined lymphoma). It is usually EBV+ and often clonal by molecular studies, but morphologically it hasn’t committed to a single lymphoma phenotype. Polymorphic PTLD may still regress with reduction of immunosuppression because it hasn’t fully progressed to a committed malignant program.

Monomorphic PTLD

Monomorphic PTLDs are neoplasms that fulfill the criteria for a conventional B- or T/NK-cell neoplasm outside of a transplant setting. This is the most common PTLD type. Most are DLBCL-like; less commonly Burkitt lymphoma, plasmablastic lymphoma, plasmacytoma, or T-cell lymphomas. These are named as the specific lymphoma type plus “post-transplant lymphoproliferative disorder” (e.g., “DLBCL, EBV+, PTLD”). They meet every diagnostic criterion for that lymphoma outside the transplant setting; the transplant context is the modifier.

Low-grade B-cell lymphomas (CLL/SLL, follicular, mantle cell, marginal zone) are NOT considered monomorphic PTLD - they are treated as incidental / coincidental lymphomas unrelated to immunosuppression. The one exception is MALT lymphoma, which IS classified as PTLD if it is EBV-positive. The logic: MALT lymphoma is normally H. pylori-driven, not EBV-driven, so EBV positivity in a MALT lymphoma specifically implicates the immunosuppression / EBV axis.

Classical Hodgkin lymphoma-type PTLD

CHL-PTLDs are neoplasms that fulfill strict criteria for conventional classical Hodgkin lymphoma outside of a transplant setting. The word “strict” is load-bearing here. Diagnosis requires RS cells that are CD30+, CD15+, and CD20-. Most cases are EBV+.

CHL-PTLD can be confused with polymorphic PTLD because polymorphic PTLD frequently contains large, RS-like cells in a mixed inflammatory background - visually similar to Hodgkin. The fix is strict immunophenotyping:

CHL-PTLD: large cells are CD30+, CD15+, CD20-
Polymorphic PTLD with RS-like cells: large cells are often CD20+ or CD15-

If the large cells are CD20+ or CD15-, it’s polymorphic PTLD, not CHL-PTLD. This distinction matters because treatment differs (CHL-PTLD may be managed with ABVD-like regimens; polymorphic PTLD starts with IS reduction).

EBER can help differentiate CHL-PTLD from polymorphic PTLD by the size of the cells it’s positive in:

CHL-PTLD: EBER positive only in the large cells (the RS cells - a defined neoplastic clone)
Polymorphic PTLD: EBER positive in a mixture of cell sizes (small, medium, and large - reflecting the polyclonal / oligoclonal reactive-like pattern)

The pattern makes sense mechanistically. CHL has a single defined neoplastic clone (the large RS cells) surrounded by reactive inflammatory cells, so only the RS cells carry EBV. Polymorphic PTLD is a more chaotic proliferation of variably-sized EBV-infected cells, so EBER lights up cells of many sizes.

Clinical Presentation

May involve any site (nodal or extranodal)
Allograft involvement is common and concerning
GI tract, CNS, and lungs commonly involved
Often presents with fever, lymphadenopathy, or graft dysfunction
May mimic rejection in the allograft

Diagnosis

Tissue biopsy is essential
EBV testing: EBER in situ hybridization (most sensitive for EBV in tissue)
EBV viral load monitoring: Rising EBV DNA in blood may precede clinical PTLD
Clonality studies: Immunoglobulin gene rearrangement

Treatment

Reduction of immunosuppression (RI): First-line for many cases; allows immune reconstitution. Balances PTLD control vs. rejection risk.

Rituximab: Anti-CD20 antibody for CD20+ B-cell PTLD. Often combined with RI.

Chemotherapy: R-CHOP or similar regimens for aggressive or rituximab-refractory disease.

EBV-specific cytotoxic T lymphocytes: Adoptive immunotherapy; restores EBV-specific immunity.

Surgery/radiation: For localized disease.

The mainstay treatment of monomorphic PTLD is immunosuppression reduction plus R-CHOP. In practice the approach is stepwise: first reduce IS, then add rituximab if CD20+, then add chemotherapy if needed. The trade-off is always the same - reducing IS helps fight PTLD but raises the risk of graft rejection.

Prognosis

Prognosis tracks directly with the WHO category.

Non-destructive PTLD: excellent prognosis. Most cases regress with reduction of immunosuppression alone. These are polyclonal / oligoclonal reactive proliferations that haven’t progressed to clonal malignancy. With restored immune surveillance, T-cells regain control of EBV-infected B-cells. Early detection (EBV viral load monitoring, prompt biopsy of lymphadenopathy) is what gets you here.
Polymorphic PTLD: intermediate. Often responds to IS reduction plus rituximab.
Monomorphic PTLD: poor prognosis. This is essentially a lymphoma (usually DLBCL) in an already-compromised host. These patients face both the lymphoma and the transplant - ongoing immunosuppression needs, comorbidities, and risk of graft rejection if IS is reduced. EBV-negative monomorphic PTLD has even worse prognosis than EBV-positive, because the EBV-negative form is less responsive to IS reduction and behaves more like a de novo aggressive lymphoma.

Chapter 35: Mastocytosis

Mastocytosis is a clonal proliferation of mast cells. The clinical spectrum ranges from indolent cutaneous disease to aggressive systemic mastocytosis with organ damage.

Pathophysiology

KIT D816V mutation is found in >90% of adult systemic mastocytosis. This mutation causes constitutive activation of the KIT receptor tyrosine kinase, driving mast cell proliferation and survival. The mutation is important therapeutically - it confers resistance to imatinib but sensitivity to midostaurin and avapritinib.

Classification

Cutaneous mastocytosis (CM): Mast cell infiltration limited to skin

Urticaria pigmentosa: Most common; reddish-brown macules/papules that urticate when stroked (Darier sign)
Diffuse cutaneous mastocytosis: Rare; diffuse skin thickening
Mastocytoma of skin: Solitary lesion; usually in children

Systemic mastocytosis (SM): Mast cells infiltrate extracutaneous organs (bone marrow, liver, spleen, GI tract)

Indolent SM: No organ dysfunction; good prognosis
Smoldering SM: Higher mast cell burden but no organ damage
SM with associated hematologic neoplasm: SM + MPN, MDS, or other myeloid neoplasm
Aggressive SM: Organ dysfunction (cytopenias, hepatomegaly, malabsorption)
Mast cell leukemia: ≥20% mast cells in bone marrow aspirate or ≥10% in blood

Diagnostic Criteria for Systemic Mastocytosis

Major criterion: Multifocal dense aggregates of ≥15 mast cells in bone marrow or other extracutaneous organ

Minor criteria:

25% of mast cells are spindle-shaped or atypical
KIT D816V mutation
Mast cells express CD25 and/or CD2 (aberrant; normal mast cells don’t express these)
Serum tryptase persistently >20 ng/mL

Diagnosis requires: Major + 1 minor, OR 3 minor criteria

Clinical Features

Mediator-related symptoms (from mast cell degranulation):

Flushing, pruritus, urticaria
Hypotension, syncope (anaphylactoid reactions)
GI symptoms (cramping, diarrhea)
Triggered by heat, exercise, alcohol, certain medications (NSAIDs, opioids)

Organ infiltration symptoms:

Hepatosplenomegaly
Cytopenias
Bone pain, pathologic fractures (mast cell bone lesions)
Malabsorption

Laboratory Findings

Serum tryptase: Elevated (>20 ng/mL in SM); useful for monitoring
Bone marrow: Aggregates of atypical mast cells
Immunophenotype: CD117+ (KIT), CD25+ and/or CD2+ (aberrant)
KIT D816V mutation: By PCR or sequencing

Treatment

Mediator symptoms: H1 and H2 antihistamines, cromolyn sodium, leukotriene inhibitors, epinephrine for anaphylaxis

Cytoreductive therapy (for advanced SM): Midostaurin (KIT inhibitor approved for advanced SM), avapritinib, cladribine, interferon-α

Chapter 36: Histiocytic and Dendritic Cell Neoplasms

Histiocytic and dendritic cell neoplasms are rare, but they show up on the boards because the immunophenotype differences are clean and the morphology is distinctive. The key to this chapter is keeping the cell lineages straight. Once you know what cell a neoplasm came from, the markers and the clinical behavior mostly follow.

36.1 Histiocyte Lineage and Normal Biology

Before any of the neoplasms make sense, you have to pin down the normal cells.

The two histiocyte cell types. Histiocyte is an umbrella term for two cell types: macrophages and dendritic cells. Both derive from myeloid stem cells in the bone marrow. The neoplasms you’ll see are organized by which of these two cells got transformed.

Macrophages: job is phagocytosis. They engulf and destroy pathogens, dead cells, and debris using lysozyme, reactive oxygen species, and acid hydrolases. They do some antigen presentation too, but poorly.
Dendritic cells: job is antigen presentation to T-cells. They capture antigen in tissues, process it, migrate to lymph nodes, and present via MHC to initiate the adaptive response. They’re the most potent APCs in the body.

Macrophage markers: CD68, CD163, lysozyme. CD68 is a lysosomal marker. CD163 is a scavenger receptor. Both are sensitive macrophage markers.

Tissue-specific macrophage names (you already know these but worth listing): Kupffer cells (liver), alveolar macrophages (lung), microglia (brain), osteoclasts (bone).

Monocyte to macrophage transition: a circulating monocyte that enters tissue becomes a macrophage. Monocytes in blood carry CD14 and CD64. Once in tissue they differentiate to macrophages with CD68 and CD163 and specialize based on the organ.

Dendritic cell subtypes:

Langerhans cells: in the epidermis and mucosal epithelium. Resident “macrophages” in the skin are actually Langerhans cells - historically called macrophages, but they function as dendritic cells (antigen presentation). Identified by CD1a, langerin (CD207), and S100. On EM they contain Birbeck granules.
Interdigitating dendritic cells (IDCs): in lymph node T-cell zones. These are Langerhans cells that migrated to the node.
Dermal dendritic cells: in the dermis (below the epidermis where Langerhans cells live). Distinct from Langerhans cells - no Birbeck granules, no CD1a, no langerin. Instead they express CD68, CD163, and Factor XIIIa.
Follicular dendritic cells (FDCs): in germinal centers. These are the odd ones out - they’re mesenchymal-derived, not myeloid. They trap antigen-antibody complexes on their surface for B-cell selection.
Plasmacytoid dendritic cells (pDCs): in lymph nodes, morphologically resemble plasma cells (eccentric nucleus, moderate cytoplasm). Their job is producing type I interferons (IFN-α/β) during viral infection. Markers: CD123, CD303 (BDCA2), CD4. Their neoplasm is BPDCN (blastic plasmacytoid dendritic cell neoplasm).

The Langerhans cell lifecycle matters for understanding the neoplasms:

Starts as a Langerhans cell in the skin
Captures antigen - goes back to circulation as a maturing dendritic cell
Arrives in lymph node as an interdigitating dendritic cell (IDC)

This is why IDC sarcoma retains macrophage markers (CD163, lysozyme) - its Langerhans cell ancestor was historically a “skin macrophage.”

Lineage exception: all histiocytes derive from myeloid stem cells, except follicular dendritic cells and fibroblastic reticulum cells, which derive from mesenchymal stem cells. This is why FDC sarcoma has a completely different immunophenotype (CD21, CD23, CD35, clusterin; no CD68 or CD163) from the rest.

Skin layer summary:

Epidermis: Langerhans cells (CD1a+, langerin+, S100+)
Dermis: dermal dendritic cells (CD68+, CD163+, Factor XIIIa+)

Their respective neoplasms are LCH (Langerhans) and juvenile xanthogranuloma (dermal dendritic).

36.2 Langerhans Cell Histiocytosis (LCH)

LCH is a clonal proliferation of Langerhans cells. Despite being derived from myeloid precursors, LCH cells express CD1a and langerin (CD207), which are the defining Langerhans cell markers.

Molecular pathogenesis: The MAPK pathway is activated in virtually all cases:

BRAF V600E mutation: ~50-60% of cases
MAP2K1 mutations: many BRAF-negative cases
This has transformed treatment - BRAF inhibitors are effective

All forms of LCH - even the mildest unifocal form - are now recognized as clonal neoplasms, not reactive processes. The presence of BRAF V600E or other driver mutations confirms neoplastic nature.

Historical terminology (still board-tested despite all three being “LCH” now):

Old name	Extent	Classic age
Eosinophilic granuloma	Unifocal LCH (single lesion, usually bone)	Adolescents / young adults
Hand-Schuller-Christian	Multifocal bone involvement	Children > 3 years
Letterer-Siwe	Multifocal, multisystem	Infants < 2 years

General rule: younger age plus more widespread disease equals worse prognosis.

Eosinophilic granuloma (unifocal LCH): mildest form. Single lytic bone lesion - skull, ribs, vertebrae most common. Langerhans cells with abundant eosinophils (hence the old name). Excellent prognosis. Treatment: curettage, intralesional steroids, or even observation.

Hand-Schuller-Christian disease (multifocal bone LCH): classic triad of lytic skull lesions, diabetes insipidus, and exophthalmos. The DI comes from LCH infiltration of the posterior pituitary/hypothalamus disrupting ADH - this can be the first symptom. Exophthalmos is from orbital infiltration. Full triad is rarely seen but heavily board-tested. Requires systemic chemotherapy. It is a malignant (clonal) proliferation of Langerhans cells, even though it sits in the middle of the severity spectrum.

Letterer-Siwe disease (multisystem LCH, the most aggressive): involves bone, skin, liver, spleen, bone marrow, lungs. Risk organ involvement (liver, spleen, marrow) predicts poor outcome. Without treatment, rapidly fatal. It is a malignant proliferation of Langerhans cells. Board trick: if a question gives you scalp rash + lytic skull lesions + diabetes insipidus + exophthalmos all together, the answer is Letterer-Siwe, not Hand-Schuller-Christian, because only Letterer-Siwe involves everything (bone + skin + pituitary). The scalp rash is a seborrheic-dermatitis-like eruption.

Single-system vs multisystem in current terminology:

Single-system LCH:

Bone (most common): lytic “punched-out” lesions, especially skull, ribs, vertebrae
Skin: seborrheic dermatitis-like rash, especially scalp
Lymph node: usually cervical

Multisystem LCH:

Involvement of 2+ organ systems
“Risk organs” (liver, spleen, bone marrow) involvement predicts worse prognosis
Diabetes insipidus from pituitary involvement (classic presentation)
Pulmonary LCH: almost exclusively in adult smokers; cystic lung disease

Histopathology:

Langerhans cells with characteristic grooved, folded, or “coffee-bean” nuclei
Mixed inflammatory background with eosinophils (hence “eosinophilic granuloma”)
Birbeck granules on electron microscopy: tennis-racket-shaped cytoplasmic inclusions. Pathognomonic but rarely needed for diagnosis now (CD1a/langerin IHC has replaced EM). They’re formed by the langerin protein during endocytosis. If the board shows you an EM image with tennis-racket structures, it’s LCH.

Immunophenotype: CD1a+, langerin (CD207)+, S100+.

Lymphoid neoplasm associations (rare but testable):

LCH is associated with acute lymphoblastic leukemia (ALL) - concurrent or sequential. Mechanism unclear; possibly shared progenitor or shared MAPK alterations.
LCH is associated with follicular lymphoma through transdifferentiation. A follicular lymphoma cell with t(14;18) BCL2-IGH can transdifferentiate into a Langerhans cell. The resulting LCH carries the t(14;18) translocation but expresses CD1a and langerin. This is a striking example of lineage plasticity.

Treatment:

Single bone lesion: curettage, intralesional steroids, or observation
Multisystem/risk organ: chemotherapy (vinblastine + prednisone standard)
BRAF-mutated refractory disease: BRAF inhibitors (vemurafenib)

Langerhans cell sarcoma: the high-grade malignant form. Overtly sarcomatous with high-grade cytology and high mitotic rate. Retains Langerhans cell markers (CD1a+, langerin+, S100+) but with frankly malignant morphology. Can arise de novo or by transformation from pre-existing LCH. Very rare, poor prognosis. Treatment: aggressive chemotherapy, BRAF inhibitors if mutated.

36.3 Juvenile Xanthogranuloma (JXG)

Juvenile xanthogranuloma is the neoplasm of dermal dendritic cells - the counterpart to LCH but from dermis instead of epidermis.

Clinical: skin-colored to yellowish papules/nodules, usually in infants and children. Usually self-limited and regresses spontaneously. Rarely involves the eye (especially iris) - can be associated with neurofibromatosis type 1 plus juvenile myelomonocytic leukemia (JMML), the classic NF1 triad. (JMML was historically called “juvenile CML,” but JMML is the correct current WHO term and the board answer.)

Histology: abundant dendritic cells with foamy (lipidized) cytoplasm (“xantho” means yellow, referring to the gross appearance) and Touton giant cells. A Touton giant cell is a multinucleated giant cell with a central ring of nuclei surrounded by foamy cytoplasm and peripheral eosinophilic cytoplasm. The Touton giant cell is the pathognomonic finding.

Immunophenotype: CD68+, CD163+, Factor XIIIa+. CD1a and langerin are negative (this is how you separate it from LCH). S100 is typically negative or weakly positive (unlike LCH, which is strong S100+). Factor XIIIa positivity is shared with Erdheim-Chester disease - both are dermal dendritic cell-derived.

36.4 Erdheim-Chester Disease (ECD)

ECD is the systemic histiocytosis that looks like JXG under the microscope but behaves totally differently clinically.

Histology: xanthomatous appearance with giant cells - foamy histiocytes, looks a lot like JXG. Stains similarly too: CD68+, CD163+, Factor XIIIa+.

Location is what makes ECD different from JXG: JXG is skin-limited; ECD is systemic.

Radiographic signature: symmetric osteosclerosis of the diaphysis and metaphysis of lower extremity long bones (especially tibiae and femora). This is pathognomonic. It’s the opposite of LCH, which causes lytic punched-out lesions. The sclerosis in ECD results from xanthogranulomatous infiltration and reactive bone formation. Board rule: symmetric sclerotic long bone lesions equals Erdheim-Chester.

Systemic involvement: ECD almost always involves the bones, and about half of cases involve the great cardiac vessels (“hairy aorta” - periaortic fibrosis coating the thoracic and abdominal aorta). Other sites:

Retroperitoneum (retroperitoneal fibrosis, ureteral obstruction)
Orbits (bilateral orbital masses)
Skin
CNS
Pituitary (diabetes insipidus, like LCH)

ECD can coexist with LCH in the same patient - both share BRAF V600E.

Molecular: ECD most commonly has BRAF V600E mutations (~50-60%), like LCH. Other MAPK mutations (MAP2K1, NRAS, KRAS) account for additional cases. BRAF inhibitors (vemurafenib) are FDA-approved for BRAF-mutated ECD - a real breakthrough given ECD’s historically limited treatment options.

36.5 Rosai-Dorfman Disease (RDD)

Rosai-Dorfman disease is also called sinus histiocytosis with massive lymphadenopathy. “Sinus” refers to the lymph node sinuses where the histiocytes accumulate. “Massive lymphadenopathy” describes the clinical presentation - painless, bilateral cervical lymph node enlargement that can be very impressive in size.

Clinical: most commonly affects children and young adults. Usually self-limited. Can rarely involve extranodal sites (skin, soft tissue, orbit, CNS). Not a true malignancy - considered a reactive/non-neoplastic histiocytosis (though some cases have been shown to be clonal).

The diagnostic feature is emperipolesis: RDD shows large dendritic cells with prominent nucleoli and striking emperipolesis, which is engulfment of live lymphocytes.

Emperipolesis vs phagocytosis (important distinction): phagocytosis kills the engulfed cell using lysozyme; emperipolesis preserves the engulfed cell alive. The lymphocyte can actually leave the histiocyte unharmed after emperipolesis. On histology, you see lymphocytes sitting in clear spaces within histiocyte cytoplasm - as if they’re in little halos. S100 stain highlights the histiocytes and makes emperipolesis easier to see.

36.6 Follicular Dendritic Cell (FDC) Sarcoma

Remember: FDCs are mesenchymal-derived, not myeloid. That’s why their neoplasm has a unique marker profile.

Normal FDCs: present in lymph node germinal centers. Morphology: some cytoplasm, sharply defined oval nucleus, single small nucleolus, slightly eosinophilic. They form the scaffold of germinal centers and trap antigen-antibody complexes on their surface for B-cell selection.

FDC sarcoma histology: storiform appearance (whorled/pinwheel pattern) with fascicles of plump to spindle cells with bland cytology. Deceptively bland-looking for a sarcoma. Can mimic meningioma, thymoma, or other spindle cell neoplasms.

Location: usually lymph nodes (especially cervical) or extranodal sites. FDC sarcoma can arise within a Castleman disease lesion (specifically the hyaline-vascular type). Intermediate-grade malignancy with risk of recurrence.

Immunophenotype: CD21, CD23, CD35, and clusterin. All four are FDC markers (CD21/23/35 are complement receptors that trap immune complexes). Order all four because some may be negative in any given case. FDC sarcoma is negative for macrophage markers (CD68, CD163) and negative for Langerhans markers (CD1a, langerin).

Note: CD21 and CD23 are also expressed by CLL, but CLL is a lymphoid neoplasm with completely different morphology - no confusion in practice.

36.7 Interdigitating Dendritic Cell (IDC) Sarcoma

IDC sarcoma is the look-alike for FDC sarcoma that you distinguish by immunophenotype.

Morphology: looks like FDC sarcoma (spindle cells in a lymph node) - same storiform pattern.

Immunophenotype is the separator: IDC sarcoma is negative for CD21, CD23, CD35, and clusterin, but positive for S100, CD163, and lysozyme.

Why? IDCs derive from Langerhans cells, which are modified macrophages. They retain macrophage markers (CD163, lysozyme) from their lineage. The S100 positivity links them to the Langerhans cell origin.

36.8 Histiocytic Sarcoma

Histiocytic sarcoma is a malignant neoplasm of tissue macrophages. Very rare and extremely aggressive.

Immunophenotype: CD68+, CD163+, PU.1+, lysozyme+. PU.1 is a myeloid transcription factor. These are all macrophage markers. Negative for CD1a/langerin (not LCH), negative for CD21/CD23/CD35 (not FDC sarcoma), negative for B/T-cell markers (not lymphoma).

Origin: can arise de novo or by transdifferentiation from a pre-existing lymphoid neoplasm (follicular lymphoma, CLL). The lymphoma cell effectively reprograms into a macrophage, again showing lineage plasticity.

Prognosis: poor.

36.9 Hemophagocytic Lymphohistiocytosis (HLH)

HLH is not a neoplasm. It’s a syndrome of immune dysregulation with excessive activation of macrophages and T-cells causing a cytokine storm. It’s included here because it’s in the differential of histiocytic disorders and it’s critical to recognize - rapidly fatal without treatment.

Pathophysiology: HLH is caused by abnormal function of NK and T-cells leading to macrophage proliferation and activation. Normally, NK cells and cytotoxic T-cells kill infected or abnormal cells and then undergo apoptosis - this terminates the immune response. In HLH, this “off switch” fails:

Defective cytotoxic function → ineffective killing → persistent antigen stimulation
Uncontrolled T-cell and macrophage activation
Massive cytokine release (IFN-γ, TNF-α, IL-6, IL-18)
Macrophages phagocytose blood cells (hemophagocytosis)

Classification:

Primary (familial) HLH: genetic defects in the cytotoxic pathway - perforin (PRF1), MUNC13-4 (UNC13D), syntaxin-11 (STX11). The mutated genes encode proteins essential for NK/cytotoxic T-cell killing:

Perforin: forms pores in target cell membranes
MUNC13-4: primes cytotoxic granules for fusion with cell membrane
Syntaxin-11: mediates granule-membrane fusion

Without functional cytotoxic machinery, the immune response can’t be terminated. Usually presents in infancy.

Secondary (acquired) HLH: infection-related or autoimmune-associated. Triggers:

Infections (most common trigger): EBV is the #1 viral trigger. Also CMV, HSV, HIV, bacterial, fungal.
Malignancy: especially T-cell/NK-cell lymphomas, Hodgkin lymphoma.
Autoimmune/autoinflammatory diseases: when HLH occurs in this context it’s called macrophage activation syndrome (MAS). Classic association is juvenile rheumatoid arthritis (Still disease). Also seen in SLE.

Diagnostic criteria (HLH-2004): need 5 of 8 criteria, OR molecular diagnosis of a familial HLH mutation is sufficient.

The 8 criteria:

Fever
Splenomegaly
Cytopenias (≥2 lineages): Hgb <9, platelets <100K, ANC <1000
Hypertriglyceridemia (≥265 mg/dL) and/or hypofibrinogenemia (≤150 mg/dL)
Hemophagocytosis in bone marrow, spleen, or lymph nodes
Low or absent NK-cell activity
Ferritin ≥500 ng/mL (often massively elevated, >10,000)
Elevated soluble CD25 (sIL-2R) ≥2400 U/mL

Clinical findings (criteria 1 and 2): fever and hepatosplenomegaly. The fever is characteristically high and persistent, not responsive to antibiotics (from massive cytokine release). Splenomegaly can be massive and comes from macrophage infiltration and extramedullary hematopoiesis.

Hematologic findings (criteria 3, 5, 6, 8): cytopenias (>2 lineages), hemophagocytosis, low NK cell activity, and increased soluble CD25.

Cytopenias: from macrophage phagocytosis of blood cells and marrow suppression
Hemophagocytosis: macrophages engulfing intact RBCs/WBCs/platelets in BM. Present in ~80% but NOT required for diagnosis. Also not sensitive or specific - can be seen in sepsis, transfusion reactions, and other conditions. Don’t wait for a BM biopsy to diagnose HLH.
Low NK activity: reflects the underlying cytotoxic defect
Soluble CD25 (sIL-2R): reflects massive T-cell activation

Serologic findings (criteria 4 and 7): hypertriglyceridemia, hyperferritinemia, and hypofibrinogenemia.

Hypertriglyceridemia: cytokines inhibit lipoprotein lipase so triglycerides accumulate
Hyperferritinemia: often massively elevated (>10,000 is highly specific for HLH in the right clinical context); ferritin is released from activated macrophages
Hypofibrinogenemia: macrophages produce plasminogen activator, driving fibrinolysis, which consumes fibrinogen

The lab triad of high ferritin + high triglycerides + low fibrinogen is the classic HLH signature.

Treatment:

Identify and treat the trigger (infection, malignancy)
Immunosuppression: dexamethasone (CNS-penetrating), etoposide
Supportive care: transfusions, antimicrobials
Emapalumab: anti-IFN-γ antibody for refractory disease
Stem cell transplant: for familial HLH or refractory disease

Prognosis: high mortality without treatment. Even with treatment, mortality remains significant, especially for malignancy-associated HLH.

PART IV: MEDICAL MICROBIOLOGY

Microbiology is unique among the clinical laboratory sciences in requiring identification of living organisms and understanding their pathogenic mechanisms. This section provides comprehensive coverage of bacteria, viruses, fungi, and parasites with emphasis on organism identification and clinical correlations.

Taxonomic Overview of Human Pathogens

The following hierarchy provides a roadmap for the organisms covered in this section. Understanding where each pathogen fits in this classification aids in predicting its characteristics, staining properties, culture requirements, and antimicrobial susceptibilities.

I. BACTERIA

A. GRAM-POSITIVE

Cocci

Catalase-Positive –> STAPHYLOCOCCUS
Coagulase-positive –> S. aureus (MSSA, MRSA)
Coagulase-negative –> S. epidermidis, S. saprophyticus, S. lugdunensis
Catalase-Negative –> STREPTOCOCCUS / ENTEROCOCCUS
Beta-hemolytic: Group A (S. pyogenes), Group B (S. agalactiae)
Alpha-hemolytic: Optochin-sensitive (S. pneumoniae), Optochin-resistant (Viridans streptococci)
Gamma-hemolytic: Bile esculin +, 6.5% NaCl + (Enterococcus); Bile esculin +, 6.5% NaCl - (S. gallolyticus)

Bacilli (Rods)

Spore-forming: Aerobic (Bacillus) vs Anaerobic (Clostridium)
Non-spore-forming: Listeria monocytogenes, Corynebacterium
Branching/Filamentous: Actinomyces (anaerobic), Nocardia (aerobic, partially acid-fast)

Acid-Fast Bacilli

Mycobacterium tuberculosis complex
Nontuberculous mycobacteria (MAC, M. kansasii, M. abscessus)
Mycobacterium leprae

B. GRAM-NEGATIVE

Cocci

Neisseria gonorrhoeae (ferments glucose only)
Neisseria meningitidis (ferments glucose + maltose)
Moraxella catarrhalis

Enterobacteriaceae (Oxidase-NEGATIVE)

Lactose Fermenters (pink on MacConkey): E. coli, Klebsiella, Enterobacter
Non-Lactose Fermenters (colorless): Salmonella, Shigella, Proteus, Yersinia, Serratia

Non-Fermenters (Oxidase-POSITIVE)

Pseudomonas aeruginosa, Acinetobacter baumannii, Stenotrophomonas maltophilia, Burkholderia cepacia

Fastidious Gram-Negative Bacilli

Haemophilus influenzae, Legionella pneumophila, Bordetella pertussis
HACEK organisms, Brucella, Francisella tularensis, Pasteurella, Bartonella

Curved/Spiral Bacteria

Vibrio, Campylobacter jejuni, Helicobacter pylori

C. ATYPICAL / GRAM-INDETERMINATE

Spirochetes: Treponema pallidum, Borrelia burgdorferi, Leptospira interrogans
Obligate Intracellular: Rickettsia, Coxiella, Anaplasma, Ehrlichia, Chlamydia
Cell Wall-Deficient: Mycoplasma pneumoniae

II. FUNGI

A. YEASTS (unicellular, reproduce by budding)

Candida species (germ tube test, CHROMagar for speciation)

C. albicans - most common; oral thrush, vulvovaginitis, esophagitis (AIDS), candidemia; germ tube POSITIVE
C. glabrata - second most common; intrinsically fluconazole-resistant, elderly/diabetics
C. parapsilosis - TPN lines, neonates, hand carriage by healthcare workers
C. tropicalis - neutropenic patients, hematologic malignancy
C. krusei - intrinsically fluconazole-resistant, bone marrow transplant patients
C. auris - emerging MDR threat, healthcare outbreaks, difficult to identify, often pan-resistant

Cryptococcus (encapsulated, urease positive, India ink visualization)

C. neoformans - meningitis in AIDS (CD4 <100), “soap bubble” lesions in brain, pigeon droppings
C. gattii - immunocompetent hosts, Pacific Northwest, pulmonary and CNS disease

Pneumocystis jirovecii (atypical fungus, cannot be cultured)

PCP pneumonia in AIDS (CD4 <200), ground-glass opacities on CT
Diagnosed by methenamine silver or DFA on induced sputum/BAL
Prophylaxis and treatment: TMP-SMX

Malassezia furfur (lipophilic, requires oil overlay for culture)

Tinea versicolor (hypo/hyperpigmented patches, “spaghetti and meatballs” on KOH)
Folliculitis, catheter-related fungemia (TPN with lipids)

B. MOLDS (multicellular, grow as hyphae)

Aspergillus (septate hyphae, 45-degree acute angle branching)

A. fumigatus - most common pathogen; invasive aspergillosis (neutropenia), aspergilloma (“fungus ball”), ABPA (allergic bronchopulmonary aspergillosis in asthmatics/CF)
A. flavus - sinusitis, produces aflatoxin (hepatocellular carcinoma)
A. niger - otomycosis (ear infection), black conidia
A. terreus - intrinsically amphotericin-resistant

Mucormycetes (pauciseptate/ribbon-like hyphae, 90-degree wide angle branching)

Rhizopus, Mucor, Lichtheimia (Absidia), Cunninghamella
Rhinocerebral mucormycosis - diabetic ketoacidosis, iron overload, deferoxamine use
Angioinvasive → vessel thrombosis, tissue necrosis, black eschar
Treatment: amphotericin B + surgical debridement; resistant to voriconazole

Dermatophytes (keratinophilic - infect skin, hair, nails only)

Trichophyton - all three sites (skin, hair, nails); T. rubrum most common cause of tinea pedis/onychomycosis
Microsporum - skin and hair only; M. canis from cats/dogs (fluoresces under Wood’s lamp)
Epidermophyton - skin and nails only (NOT hair); E. floccosum
Clinical: Tinea capitis (scalp), corporis (body/ringworm), cruris (groin/jock itch), pedis (foot/athlete’s foot), unguium (nails/onychomycosis)

Fusarium (septate, banana-shaped macroconidia)

Keratitis (contact lens), disseminated disease in neutropenia, onychomycosis
Often resistant to amphotericin; voriconazole preferred

Scedosporium/Lomentospora

S. apiospermum - near-drowning pneumonia, mycetoma
L. prolificans - disseminated disease, highly drug-resistant

C. DIMORPHIC FUNGI (mold at 25°C/environment, yeast at 37°C/body - “mold in the cold, yeast in the heat”)

Histoplasma capsulatum

Geography: Ohio and Mississippi River valleys, bat/bird droppings (caves, chicken coops)
Small intracellular yeast within macrophages
Acute pulmonary histoplasmosis (often asymptomatic), chronic cavitary disease, disseminated (AIDS, immunocompromised) - hepatosplenomegaly, pancytopenia, oral ulcers
Diagnosis: urine/serum antigen, culture, histopath showing macrophages with yeast

Blastomyces dermatitidis

Geography: Great Lakes, Ohio/Mississippi valleys, southeastern US
Broad-based budding yeast (“figure 8”), thick double-refractile cell wall
Pulmonary disease, skin lesions (verrucous/ulcerative), osteomyelitis, prostatitis
Diagnosis: culture, histopath; antigen cross-reacts with Histoplasma

Coccidioides immitis/posadasii

Geography: Southwestern US (Arizona, California), Mexico - “Valley fever”
Spherules containing endospores (unique - does NOT convert to yeast)
Primary pulmonary infection (60% asymptomatic), erythema nodosum (“desert rheumatism”), disseminated disease (meningitis, skin, bone) - risk factors: pregnancy, Filipino ancestry, African ancestry, immunosuppression
Diagnosis: serology (IgM then IgG), culture (highly infectious - BSL-3)

Paracoccidioides brasiliensis

Geography: South America (Brazil, Colombia, Venezuela)
“Captain’s wheel” / “pilot wheel” - large yeast with multiple peripheral buds
Chronic pulmonary disease, mucocutaneous lesions (mulberry-like oral lesions), lymphadenopathy

Sporothrix schenckii

“Rose gardener’s disease” - inoculation from thorns, sphagnum moss, cats
Lymphocutaneous sporotrichosis: painless nodule at inoculation site → ascending nodular lymphangitis
“Cigar-shaped” yeast at 37°C
Treatment: itraconazole; amphotericin for disseminated

Talaromyces (Penicillium) marneffei

Geography: Southeast Asia, Southern China - endemic mycosis of AIDS
Intracellular yeast with central septum (divides by fission, not budding)
Disseminated disease: skin papules with central umbilication (molluscum-like), fever, hepatosplenomegaly

III. PARASITES

A. PROTOZOA (single-celled eukaryotes)

Blood and Tissue Protozoa:

Plasmodium (malaria - Anopheles mosquito vector)

P. falciparum - most severe, cerebral malaria, no hypnozoites, banana-shaped gametocytes, high parasitemia, knobs on RBCs
P. vivax - benign tertian, hypnozoites (relapse), Duffy antigen required, enlarged RBCs, Schuffner dots
P. ovale - benign tertian, hypnozoites (relapse), oval RBCs with fimbriated edges
P. malariae - quartan malaria, “band forms,” nephrotic syndrome, no hypnozoites
P. knowlesi - Southeast Asia, zoonotic (macaques), can cause severe disease, 24-hour cycle
Diagnosis: thick and thin smears, rapid antigen tests, PCR

Babesia microti (Ixodes tick - same as Lyme)

Northeast US, “Maltese cross” tetrad form in RBCs
Hemolytic anemia, asplenic patients at highest risk
Co-infection with Lyme, Anaplasma common

Toxoplasma gondii

Cat definitive host (oocysts in feces), undercooked meat (tissue cysts)
Immunocompetent: usually asymptomatic or mononucleosis-like
AIDS (CD4 <100): ring-enhancing brain lesions, encephalitis
Congenital: chorioretinitis, hydrocephalus, intracranial calcifications (diffuse)
Diagnosis: serology (IgG/IgM), PCR of CSF/amniotic fluid

Trypanosoma

T. brucei gambiense - West African sleeping sickness, chronic, Winterbottom sign (posterior cervical lymphadenopathy)
T. brucei rhodesiense - East African sleeping sickness, acute/severe
Both: tsetse fly vector, trypomastigotes in blood, antigenic variation, CNS involvement (sleeping, behavioral changes)
T. cruzi - Chagas disease, reduviid bug (“kissing bug”), Central/South America
Acute: Romana sign (unilateral periorbital edema), chagoma
Chronic: dilated cardiomyopathy, megacolon, megaesophagus
Amastigotes in tissue (heart, GI)

Leishmania (sandfly vector)

Visceral leishmaniasis (kala-azar) - L. donovani, hepatosplenomegaly, pancytopenia, hypergammaglobulinemia, fever
Cutaneous leishmaniasis - L. tropica, L. major, L. mexicana; painless ulcer with raised borders
Mucocutaneous leishmaniasis - L. braziliensis; destructive nasal/oral lesions
Amastigotes within macrophages (Leishman-Donovan bodies)

Naegleria fowleri

Primary amebic meningoencephalitis (PAM), warm freshwater (lakes, hot springs)
Enters through cribriform plate, rapidly fatal
Motile trophozoites in CSF

Acanthamoeba

Granulomatous amebic encephalitis (GAE) - immunocompromised, subacute
Keratitis - contact lens wearers (homemade saline)
Cysts and trophozoites in tissue

Intestinal and Urogenital Protozoa:

Giardia lamblia (intestinalis)

“Camper’s diarrhea,” fecal-oral, contaminated water (beavers)
Bloating, foul-smelling fatty stools (malabsorption), no blood/WBCs
Trophozoites: “owl face” / falling leaf motility; cysts: 4 nuclei
Diagnosis: stool antigen, O&P (cysts), string test (trophozoites)

Entamoeba histolytica

Amebiasis: bloody diarrhea (flask-shaped ulcers), liver abscess (“anchovy paste”)
Trophozoites with ingested RBCs, cysts with 4 nuclei and chromatoid bars
Must distinguish from non-pathogenic E. dispar (antigen testing)

Cryptosporidium

Severe watery diarrhea in AIDS (CD4 <100), self-limited in immunocompetent
Acid-fast oocysts in stool, organisms on brush border
Waterborne outbreaks (resistant to chlorine)

Cyclospora cayetanensis

Prolonged watery diarrhea, imported berries/produce
Acid-fast oocysts (larger than Cryptosporidium), autofluoresce
Treatment: TMP-SMX

Cystoisospora (Isospora) belli

Chronic diarrhea in AIDS
Large acid-fast oocysts, Charcot-Leyden crystals (eosinophils)
Treatment: TMP-SMX

Microsporidia (Enterocytozoon, Encephalitozoon)

Chronic diarrhea in AIDS, keratitis
Tiny spores (1-2 μm), modified trichrome or Gram stain

Trichomonas vaginalis

STI, “strawberry cervix,” frothy green-yellow discharge
Motile trophozoites on wet mount (jerky motility), no cyst stage
Treat patient AND partner with metronidazole

B. HELMINTHS (multicellular worms - eosinophilia is common)

Nematodes (Roundworms) - cylindrical, complete GI tract

Intestinal Nematodes:

Ascaris lumbricoides - largest intestinal roundworm

Fecal-oral (eggs), larval migration through lungs (Loeffler syndrome - eosinophilia, pulmonary infiltrates)
Intestinal obstruction, biliary/pancreatic obstruction
Eggs: fertilized (round, mammillated) vs unfertilized (elongated)

Enterobius vermicularis (pinworm)

Most common US helminth, fecal-oral, perianal itching (nocturnal)
Scotch tape test for eggs, no eosinophilia (no tissue invasion)

Trichuris trichiura (whipworm)

Fecal-oral, “whip-shaped” worm, rectal prolapse in heavy infection
Barrel-shaped eggs with bipolar plugs

Hookworm - skin penetration (walking barefoot)

Necator americanus (Americas), Ancylostoma duodenale (Old World)
Larval lung migration, iron deficiency anemia (blood loss from intestinal attachment)
Eggs: thin-shelled, oval, with developing embryo

Strongyloides stercoralis

Skin penetration, autoinfection cycle (can persist for decades)
Hyperinfection syndrome in immunocompromised (steroids!) - disseminated, gram-negative sepsis
Larvae (not eggs) in stool - rhabditiform (feeding) vs filariform (infectious)

Trichinella spiralis

Undercooked pork/game meat, encysted larvae in muscle
Periorbital edema, myalgias, eosinophilia, splinter hemorrhages
Diagnosis: serology, muscle biopsy showing coiled larvae

Tissue Nematodes (Filarial worms): - all transmitted by arthropod vectors

Wuchereria bancrofti / Brugia malayi

Lymphatic filariasis, mosquito vector
Elephantiasis (lymphedema), hydrocele
Microfilariae with nocturnal periodicity (blood smear at night)

Onchocerca volvulus

“River blindness,” blackfly vector, West Africa/Central America
Subcutaneous nodules (adult worms), skin changes (“leopard skin”), blindness (microfilariae in eye)
Skin snip for microfilariae

Loa loa

“African eye worm,” deerfly (Chrysops) vector
Migration across conjunctiva, Calabar swellings (transient subcutaneous edema)
Microfilariae with diurnal periodicity

Dracunculus medinensis (Guinea worm)

Contaminated water (copepods), near eradication
Female worm emerges through skin (usually leg), wound wrapped around stick for slow extraction

Toxocara canis/cati

Dog/cat roundworm, children (pica), humans are dead-end host
Visceral larva migrans (hepatomegaly, eosinophilia) or ocular larva migrans (unilateral vision loss)

Cestodes (Tapeworms) - flat, segmented (proglottids), no GI tract, scolex attachment

Taenia solium (pork tapeworm)

Intestinal infection: eating undercooked pork (cysticerci) → adult worm
CYSTICERCOSIS: eating eggs (fecal-oral, autoinfection) → larvae encyst in tissues
Neurocysticercosis: seizures, “Swiss cheese” brain, ring-enhancing or calcified cysts
Proglottids: <13 uterine branches (vs >13 in T. saginata)

Taenia saginata (beef tapeworm)

Intestinal infection only (NO cysticercosis)
Undercooked beef, largest tapeworm (up to 25 meters)
Proglottids: >13 uterine branches

Diphyllobothrium latum (fish tapeworm)

Raw freshwater fish (sushi, gefilte fish)
Vitamin B12 deficiency (worm competes for B12) → megaloblastic anemia
Operculated eggs

Echinococcus granulosus

Dog definitive host, sheep intermediate host, humans accidental
Hydatid cyst disease - liver (most common), lung; anaphylaxis if cyst ruptures
“Water lily sign” / daughter cysts / hydatid sand (scolices)
Do NOT aspirate without anti-helminthic coverage

Echinococcus multilocularis

Fox definitive host, rodents intermediate
Alveolar echinococcosis - invasive, “parasitic cancer” of liver

Hymenolepis nana (dwarf tapeworm)

Most common tapeworm in US, direct fecal-oral (no intermediate host needed)
Autoinfection possible, institutionalized children

Trematodes (Flukes) - flat, leaf-shaped, unsegmented; complex life cycles with snail intermediate hosts

Schistosoma (blood flukes) - cercariae penetrate skin in freshwater

S. mansoni - Africa, South America, Middle East; lateral spine on egg; hepatosplenic disease, “pipestem fibrosis”
S. japonicum - Asia; small round egg, no spine; most eggs in tissue → severe hepatosplenic disease
S. haematobium - Africa, Middle East; terminal spine; urinary schistosomiasis, bladder cancer (squamous cell)
Swimmer’s itch (cercarial dermatitis), Katayama fever (acute schistosomiasis)

Clonorchis sinensis / Opisthorchis viverrini (liver flukes)

Raw freshwater fish, Asia
Biliary obstruction, cholangiocarcinoma
Operculated eggs with “sitting Buddha” appearance

Fasciola hepatica (sheep liver fluke)

Watercress, sheep/cattle, worldwide
Biliary disease, eosinophilia, “liver rot”
Large operculated eggs

Fasciolopsis buski (intestinal fluke)

Water chestnuts, water bamboo, Asia
Intestinal obstruction, malabsorption

Paragonimus westermani (lung fluke)

Raw freshwater crabs/crayfish, Asia/Americas
Hemoptysis, lung cysts (mimics TB), eosinophilia
Operculated eggs in sputum/stool

C. ECTOPARASITES

Sarcoptes scabiei (scabies mite)

Intense pruritus (worse at night), burrows in web spaces/wrists/genitals
Norwegian (crusted) scabies in immunocompromised - highly contagious
Diagnosis: skin scraping showing mites, eggs, or scybala (feces)

Lice (Pediculosis)

Pediculus humanus capitis - head lice, nits on hair shafts
Pediculus humanus corporis - body lice, live in clothing, vector for typhus/trench fever/relapsing fever
Phthirus pubis - pubic lice (“crabs”), also eyelashes

Demodex

D. folliculorum - hair follicle mite, blepharitis, rosacea

IV. VIRUSES

A. DNA Viruses (all dsDNA except Parvovirus which is ssDNA)

Herpesviruses (dsDNA, enveloped, establish lifelong latency)

HSV-1 - oral herpes, herpes encephalitis (temporal lobe), herpetic whitlow, keratitis
HSV-2 - genital herpes, neonatal herpes (vertical transmission)
VZV (HHV-3) - varicella (chickenpox), zoster (shingles), Ramsay Hunt syndrome
EBV (HHV-4) - infectious mononucleosis, Burkitt lymphoma, nasopharyngeal carcinoma, PTLD, oral hairy leukoplakia
CMV (HHV-5) - mononucleosis-like syndrome, congenital CMV (hearing loss, intracranial calcifications), retinitis/colitis in AIDS, transplant rejection
HHV-6 - roseola infantum (exanthem subitum), febrile seizures
HHV-7 - roseola (less common cause)
HHV-8 - Kaposi sarcoma, primary effusion lymphoma, multicentric Castleman disease

Hepadnavirus

Hepatitis B virus (HBV) - acute/chronic hepatitis, cirrhosis, hepatocellular carcinoma; uses reverse transcriptase

Papillomaviruses (dsDNA, non-enveloped, circular genome)

HPV 1, 2, 4 - common warts (verruca vulgaris)
HPV 6, 11 - anogenital warts (condyloma acuminata), laryngeal papillomatosis; LOW-risk
HPV 16, 18, 31, 33, 45 - cervical/anal/oropharyngeal carcinoma; HIGH-risk (E6 inhibits p53, E7 inhibits Rb)

Polyomaviruses (dsDNA, non-enveloped)

JC virus - progressive multifocal leukoencephalopathy (PML) in immunocompromised; demyelination
BK virus - hemorrhagic cystitis (BMT patients), polyomavirus nephropathy (renal transplant)

Adenovirus (dsDNA, non-enveloped, icosahedral)

Pharyngoconjunctival fever, epidemic keratoconjunctivitis (“pink eye”)
Acute respiratory disease (military recruits)
Gastroenteritis (serotypes 40, 41)
Hemorrhagic cystitis (immunocompromised)

Parvovirus (ssDNA - the ONLY ssDNA human virus, non-enveloped, smallest DNA virus)

Parvovirus B19 - erythema infectiosum (fifth disease, “slapped cheek”), aplastic crisis (sickle cell), hydrops fetalis, pure red cell aplasia

Poxviruses (dsDNA, enveloped, largest DNA virus, replicates in CYTOPLASM)

Variola (Smallpox) - eradicated; synchronous rash, Guarnieri bodies
Molluscum contagiosum - umbilicated papules, Henderson-Patterson bodies
Monkeypox - zoonotic, lymphadenopathy distinguishes from smallpox
Vaccinia - used for smallpox vaccine

B. RNA Viruses

POSITIVE-SENSE ssRNA (genome can directly serve as mRNA)

Picornaviruses (small, non-enveloped, icosahedral; “PicoRNAvirus”)

Poliovirus - poliomyelitis (anterior horn cell destruction, flaccid paralysis)
Coxsackievirus A - hand-foot-mouth disease, herpangina
Coxsackievirus B - myocarditis, pericarditis, pleurodynia (Bornholm disease)
Echovirus - aseptic meningitis, nonspecific febrile illness
Rhinovirus - common cold (>100 serotypes, acid-labile)
Hepatitis A virus (HAV) - acute hepatitis (fecal-oral, no chronicity), “infectious hepatitis”

Flaviviruses (enveloped, icosahedral)

Hepatitis C virus (HCV) - chronic hepatitis (80% chronicity), cirrhosis, HCC; RNA hypervariable region
Dengue virus - dengue fever, dengue hemorrhagic fever/shock syndrome; Aedes mosquito
Yellow fever virus - hemorrhagic fever, jaundice, Councilman bodies; Aedes mosquito
West Nile virus - meningoencephalitis, flaccid paralysis; Culex mosquito, bird reservoir
Zika virus - congenital microcephaly, Guillain-Barre; Aedes mosquito
St. Louis encephalitis - encephalitis; Culex mosquito
Japanese encephalitis - encephalitis in Asia; Culex mosquito, pig reservoir

Togaviruses (enveloped)

Rubella virus - German measles (3-day measles), congenital rubella syndrome (cataracts, deafness, PDA, “blueberry muffin” rash)
Eastern equine encephalitis (EEE) - severe encephalitis, high mortality
Western equine encephalitis (WEE) - milder encephalitis
Venezuelan equine encephalitis - flu-like illness, encephalitis
Chikungunya - severe polyarthralgia (“bending up disease”); Aedes mosquito

Coronaviruses (enveloped, largest RNA genome, “crown” appearance)

Common cold coronaviruses (229E, OC43, NL63, HKU1)
SARS-CoV - severe acute respiratory syndrome (2003)
MERS-CoV - Middle East respiratory syndrome; camel reservoir
SARS-CoV-2 - COVID-19; ACE2 receptor entry

Caliciviruses (non-enveloped)

Norovirus - viral gastroenteritis (“cruise ship virus,” “winter vomiting disease”), highly contagious, 24-48 hour incubation
Sapovirus - pediatric gastroenteritis

Hepeviruses

Hepatitis E virus (HEV) - acute hepatitis (fecal-oral), high mortality in pregnant women, pig reservoir

NEGATIVE-SENSE ssRNA (must carry RNA-dependent RNA polymerase; genome must be transcribed to + sense first)

Orthomyxoviruses (enveloped, SEGMENTED genome - 8 segments)

Influenza A - pandemics and epidemics; avian/swine reservoirs, antigenic shift AND drift
Influenza B - epidemics only (no animal reservoir), antigenic drift only
Influenza C - mild respiratory illness

Paramyxoviruses (enveloped, NON-segmented, contain fusion protein)

Measles (Rubeola) - cough/coryza/conjunctivitis, Koplik spots, maculopapular rash (starts at head), SSPE, giant cell pneumonia, Warthin-Finkeldey cells
Mumps - parotitis, orchitis (sterility risk), aseptic meningitis, pancreatitis
Parainfluenza - croup (barking cough, steeple sign), bronchiolitis; types 1-4
RSV (Respiratory syncytial virus) - bronchiolitis/pneumonia in infants, palivizumab prophylaxis; syncytia formation
Human metapneumovirus - RSV-like illness
Nipah/Hendra viruses - encephalitis, bat reservoir (emerging threat)

Rhabdoviruses (bullet-shaped, enveloped)

Rabies virus - fatal encephalitis, Negri bodies (cytoplasmic inclusions in Purkinje cells/hippocampus), hydrophobia, travels retrograde via peripheral nerves

Filoviruses (filamentous, enveloped)

Ebola virus - viral hemorrhagic fever, high mortality, fruit bat reservoir
Marburg virus - viral hemorrhagic fever, African fruit bat reservoir

Bunyaviruses (enveloped, segmented - 3 segments)

Hantavirus - hantavirus pulmonary syndrome (Sin Nombre virus, deer mouse), hemorrhagic fever with renal syndrome (Old World hantaviruses)
California encephalitis virus / La Crosse virus - pediatric encephalitis
Rift Valley fever virus - hemorrhagic fever, livestock reservoir
Crimean-Congo hemorrhagic fever - tick-borne hemorrhagic fever

Arenaviruses (enveloped, segmented - 2 segments, “sandy” appearance from ribosomes)

Lymphocytic choriomeningitis virus (LCMV) - aseptic meningitis; mouse/hamster reservoir
Lassa fever virus - hemorrhagic fever in West Africa; rat reservoir
Junin, Machupo viruses - South American hemorrhagic fevers

DOUBLE-STRANDED RNA

Reoviruses (non-enveloped, segmented - 10-12 segments, “REO” = Respiratory Enteric Orphan)

Rotavirus - #1 cause of severe pediatric gastroenteritis worldwide, winter peaks, vaccine available
Coltivirus - Colorado tick fever

RETROVIRUSES (diploid +ssRNA genome, uses reverse transcriptase to make DNA that integrates into host genome)

Lentiviruses (slow viruses)

HIV-1 - AIDS worldwide; CD4+ T cell tropism, CCR5/CXCR4 coreceptors
HIV-2 - AIDS in West Africa, more indolent course

Deltaretrovirus

HTLV-1 - adult T-cell leukemia/lymphoma, HTLV-associated myelopathy (HAM)/tropical spastic paraparesis; transmitted via breast milk, sex, blood

Key Classification Principles:

Domain	Key Distinguishing Features	Empiric Treatment Considerations
Gram-positive bacteria	Thick peptidoglycan, purple on Gram stain	β-lactams, vancomycin
Gram-negative bacteria	Thin peptidoglycan + outer membrane (LPS), pink on Gram stain	Broader β-lactams, fluoroquinolones
Mycobacteria	Mycolic acid cell wall, acid-fast	Rifamycins, isoniazid, ethambutol, pyrazinamide
Atypical bacteria	Lack cell wall or obligate intracellular	Macrolides, tetracyclines, fluoroquinolones
Fungi	Eukaryotic, ergosterol membrane, chitin/glucan wall	Azoles, echinocandins, amphotericin B
Parasites	Eukaryotic, complex life cycles	Varies widely by organism
Viruses	Obligate intracellular, DNA or RNA genome	Antivirals specific to viral targets

Chapter 37: General Microbiology Principles

Clinical microbiology sits at the intersection of bench work and bedside impact. The work is deceptively simple looking - a colony, a stain, a sensitivity plate - but each step encodes a lot of pathophysiology and a lot of decisions. This chapter walks through the core framework: how specimens are collected, how organisms are visualized and isolated, how identification happens (phenotypic and molecular), and how you interpret results in the context of the major clinical syndromes (pneumonia, UTI, meningitis, endocarditis, diarrhea, prosthetic joint). Organism-specific chapters build on this foundation.

37.1 Specimen Collection and Transport

The success of microbiological diagnosis depends entirely on proper specimen collection.

General principles:

Collect before antibiotics when possible
Collect from the actual site of infection (not surrounding colonized areas)
Use appropriate containers and transport media
Minimize transport time
Label specimens properly

Common specimen types and considerations:

Specimen	Collection Considerations
Blood cultures	2-3 sets from separate venipunctures; 20-30 mL total; before antibiotics; aseptic technique
Urine	Clean-catch midstream or catheterized; transport quickly or refrigerate
CSF	Aseptic lumbar puncture; never refrigerate (N. meningitidis dies in cold)
Respiratory	Lower respiratory preferred (sputum quality matters); induced sputum or BAL for some pathogens
Stool	Fresh specimen; transport medium for culture; special containers for ova/parasites
Wound/abscess	Avoid swabs if possible; aspirate pus; anaerobic transport for deep wounds

Blood Cultures

Blood cultures are the single most important specimen in sepsis workup and they are unforgiving of sloppy collection. The standard is 2 to 3 sets drawn from separate venipuncture sites, 20 to 30 mL total per set (10 mL into each aerobic and anaerobic bottle). Drawing from an existing line adds contamination risk and should be reserved for line-infection workup (paired peripheral and line cultures looking for differential time to positivity).

Contamination is the main pitfall. A single positive for coagulase-negative Staphylococcus, Corynebacterium, Cutibacterium (formerly Propionibacterium), or Bacillus species in one of several bottles usually reflects skin flora, not true bacteremia. The same organism in multiple sets from separate sticks is the evidence you need to call it real.

Broth bottles contain resins (SPS, sodium polyanethol sulfonate, plus resin beads in some systems) that neutralize antibiotics and bind host antimicrobial factors. SPS is the classic culprit for inhibiting Neisseria, Peptostreptococcus, and Gardnerella - if you suspect those, a separate lytic or SPS-free bottle is needed. Continuous-monitoring systems (BacT/Alert, BACTEC) detect CO2 produced by growing organisms and flag bottles as positive, usually within 24 to 72 hours. Fastidious organisms (HACEK, Brucella, nutritionally-variant streptococci) may take up to 5 days; hold cultures longer if suspected.

Urine

Urine specimens must cross a colonized urethra and perineum, so the trick is minimizing contamination and getting the specimen to the lab fast. Clean-catch midstream is standard for outpatients; straight catheterization is used when the patient cannot void cleanly; suprapubic aspiration is the gold standard in neonates and the occasional puzzling case. Do not culture from indwelling Foley bag urine - the bag collects biofilm and is meaningless.

Transport within 2 hours at room temperature or within 24 hours refrigerated. Warm urine is a culture medium in itself - skin flora can bloom to “significant” counts if a specimen sits out, producing false positives. Boric acid transport tubes preserve counts at room temperature.

CSF

CSF is sterile, so any organism is potentially meaningful, but yields are often low because cell numbers are small. Collect at least 3 tubes; the microbiology tube is typically tube 2 or 3 (tube 1 has the highest blood contamination from the tap itself). Never refrigerate CSF for culture - Neisseria meningitidis and Haemophilus die at cold temperatures. Keep at room temperature or warm during transport. The lab centrifuges CSF and uses the pellet for Gram stain and culture to maximize sensitivity.

Respiratory Specimens

Expectorated sputum is the most common and the most problematic respiratory specimen because it passes through the oropharynx. The Bartlett or Murray-Washington criteria judge sputum quality on Gram stain: a good specimen has more than 25 PMNs and fewer than 10 squamous epithelial cells per low-power field. Excess squames mean the specimen is mostly saliva and should be rejected. BAL and protected-brush specimens bypass contamination and are preferred for immunocompromised patients or when TB, PJP, or atypical pathogens are suspected.

Wound and Deep Tissue

Swabs are the lowest-yield specimens. Aerobes grow fine, but anaerobes and fastidious organisms may not survive. Aspirated pus or tissue biopsy is always better. For deep or anaerobic infections (abdominal abscess, necrotizing fasciitis, gas gangrene), use anaerobic transport vials or pre-reduced anaerobic media; exposure to room air for even a few minutes can kill strict anaerobes like Bacteroides and Clostridium.

Stool

Stool is heavily colonized, so culture uses selective media (MacConkey, Hektoen enteric, XLD, SS) and specific pathogen workups (C. difficile, Campylobacter, Yersinia). Routine “stool culture” typically targets Salmonella, Shigella, and Campylobacter; E. coli O157:H7 requires separate screening (sorbitol-MacConkey). Viral and parasitic workups need separate specimens and different transport.

37.2 Microscopy and Staining

The microscope was the first tool to reveal the microbial world, and staining remains foundational to microbiology. Understanding why stains work allows you to interpret results and troubleshoot problems.

The Gram Stain: The Most Important Test in Clinical Microbiology

The Gram stain, developed by Hans Christian Gram in 1884, divides bacteria into two major groups based on fundamental differences in cell wall architecture. This simple test, performed in minutes, immediately narrows the differential and guides empiric antibiotic therapy.

Why the Gram stain works: The difference lies in cell wall structure.

Gram-positive bacteria have a thick peptidoglycan layer (20-80 nm) external to the cell membrane. This thick meshwork of cross-linked sugars and amino acids acts like a sponge, absorbing crystal violet dye and holding it tightly. When decolorizer (alcohol or acetone) is applied, the thick peptidoglycan dehydrates and contracts, trapping the crystal violet-iodine complexes inside. The cells remain purple.

Gram-negative bacteria have a thin peptidoglycan layer (2-7 nm) sandwiched between two membranes - the inner (cytoplasmic) membrane and an outer membrane containing lipopolysaccharide (LPS). When decolorizer is applied, it dissolves the outer membrane lipids, and the thin peptidoglycan cannot retain the crystal violet. The dye washes out, leaving the cells colorless until the pink safranin counterstain is applied.

The clinical power of morphology: Within seconds of examining a Gram stain, the morphology tells you which organisms to consider.

Gram-positive cocci in clusters are virtually always Staphylococcus - the cells divide in multiple planes and stick together. Gram-positive cocci in chains or pairs are Streptococcus or Enterococcus - they divide in one plane and remain attached.

Gram-negative diplococci in a CSF specimen immediately suggest Neisseria meningitidis. The same morphology in a urethral discharge is Neisseria gonorrhoeae. These organisms characteristically appear as kidney-bean-shaped pairs with adjacent sides flattened.

Gram-negative rods are a large group, but the clinical context narrows the differential - Enterobacteriaceae dominate in urinary and intra-abdominal infections, while Pseudomonas is the concern in nosocomial pneumonia.

Technical execution - the four-step sequence: The Gram stain has four steps in a fixed order, and each step has a specific role.

Crystal violet is the primary stain. It binds peptidoglycan and stains all bacteria purple.
Gram iodine is the mordant. It forms large crystal violet-iodine (CV-I) complexes inside the peptidoglycan that are too large to wash out easily. Without iodine, crystal violet alone would rinse off both gram-positives and gram-negatives.
Decolorizer is an acetone-alcohol mix. Acetone is more aggressive than alcohol alone. In gram-negatives, decolorizer dissolves the outer membrane and the thin peptidoglycan cannot retain the CV-I complexes. In gram-positives, the thick peptidoglycan dehydrates and contracts, trapping the CV-I complexes. This is the time-critical step and the most common source of error.
Safranin is the pink-red counterstain. It stains the decolorized gram-negatives; gram-positives are already dark purple and unaffected. Carbol fuchsin can substitute for safranin when you want to pick up weakly staining gram-negatives like Brucella, Legionella, and some anaerobes.

Slides are first air-dried, then fixed with heat or methanol. Methanol fixation is preferred for body fluids (CSF, blood culture bottles) because it preserves WBC morphology better; heat fixation is fine for colonies but can distort morphology if overdone.

Quality control: Gram stains must be read at 1000x total magnification (100x oil-immersion objective). Lower magnifications cannot resolve bacterial morphology. The single most useful internal control is host neutrophils - they should appear pink (gram-negative) because they lack peptidoglycan. If PMNs look blue or purple, the slide is under-decolorized and every gram reaction on it is suspect; repeat the stain.

Indications and limits: Gram stain works on cultures or direct specimens from sterile sites (CSF, synovial fluid, pleural fluid, peritoneal fluid, positive blood culture bottles). Direct Gram stains from non-sterile sites (stool, skin, wound swabs) are less useful because normal flora overwhelms the picture.

Acid-Fast Staining: Detecting Mycobacteria

Mycobacteria possess cell walls unlike any other bacteria - up to 60% of their wall mass consists of mycolic acids, long-chain fatty acids that create a waxy, hydrophobic barrier. This barrier makes mycobacteria impermeable to many antibiotics, resistant to desiccation, and able to survive inside macrophages. It also makes them resistant to routine Gram staining and requires a specialized approach.

The Ziehl-Neelsen stain exploits the same property that makes mycobacteria pathogenic. Carbol fuchsin (a red dye mixed with phenol) is applied with heat, which opens the waxy cell wall and allows dye penetration. Once inside, the dye becomes trapped. When the slide is washed with acid-alcohol - a mixture strong enough to decolorize virtually any other organism - mycobacteria retain the red dye because their waxy coat prevents the decolorizer from penetrating. Hence the term “acid-fast.”

Organisms that appear as red rods against a blue (methylene blue counterstain) background are acid-fast bacilli (AFB). In the right clinical context - a patient with chronic cough, night sweats, and upper lobe cavitary disease - this finding is diagnostic of tuberculosis until proven otherwise.

Fluorescent acid-fast staining (auramine-rhodamine) uses a fluorescent dye that binds to mycolic acids. It is more sensitive than conventional staining because fluorescent organisms are easier to spot against a dark background. However, positive results should be confirmed with a conventional stain because some artifacts can fluoresce.

Modified acid-fast staining uses a weaker decolorizer and detects organisms that are “partially” acid-fast - they have some mycolic acids but less than true mycobacteria. This includes Nocardia (important in immunocompromised patients), and the coccidian parasites Cryptosporidium, Cyclospora, and Cystoisospora.

Kinyoun vs Ziehl-Neelsen: Both are acid-fast stains using carbol fuchsin. Ziehl-Neelsen requires heating to drive the dye into the waxy cell wall (the “hot” stain). Kinyoun uses a higher carbol-fuchsin concentration with phenol and does not require heating (the “cold” stain). Both decolorize with acid-alcohol; both counterstain with methylene blue. Kinyoun is more convenient; ZN may be marginally more sensitive.

Modified acid-fast (Fite) stains use 1% sulfuric acid as the decolorizer instead of 3% HCl-alcohol. The weaker decolorizer preserves the stain in “partially” acid-fast organisms: Nocardia, Rhodococcus, Legionella micdadei, Cryptosporidium, Cyclospora, Cystoisospora. Standard AFB stains will miss these. True mycobacteria are “strongly” acid-fast and positive on both standard and modified stains.

Special Stains and Their Rationale

India ink is a negative stain that doesn’t stain the organism - it stains the background. Cryptococcus neoformans produces a thick polysaccharide capsule that excludes the ink particles, creating a clear halo around the yeast cell. In CSF from an AIDS patient with meningitis, this finding strongly suggests cryptococcal infection, though the cryptococcal antigen test is now preferred for diagnosis.

Calcofluor white binds to chitin in fungal cell walls and fluoresces under UV light. It detects all fungi - yeasts and molds alike - and is more sensitive than potassium hydroxide (KOH) preparations alone.

Silver stains (Gomori methenamine silver, GMS) deposit silver on fungal cell walls, turning them black. GMS is particularly important for detecting Pneumocystis jirovecii in lung tissue and bronchoalveolar lavage. It also highlights other fungi and some bacteria like Legionella.

Methylene blue highlights metachromatic (volutin/polyphosphate) granules in Corynebacterium diphtheriae - the granules appear reddish-purple against the blue background. This is also called the Albert stain or Loeffler methylene blue. The granules are polymerized inorganic polyphosphate and serve as energy storage. Methylene blue is also used for rapid detection of bacteria in CSF before culture results.

Acridine orange is a fluorescent stain that intercalates with DNA and RNA and is more sensitive than Gram stain for detecting bacteria, especially in blood culture bottles with low organism load. Organisms fluoresce bright orange against a dark background. The key limitation: AO cannot differentiate gram-positive from gram-negative - it only tells you an organism is there. If AO is positive but Gram is negative, repeat the Gram on a thinner preparation or from subculture.

Direct fluorescent antibody (DFA) stains use fluorescein-labeled antibodies against specific bacterial antigens. DFA has been largely replaced by PCR/NAAT, which is more sensitive, does not require viable organisms, and supports multiplex panels. DFA still has niche uses (Legionella, Chlamydia, Bordetella) but most labs now run PCR.

37.3 Culture Media

Culture media are carefully formulated mixtures that provide the nutrients and conditions bacteria need to grow. Understanding why certain media are used for certain purposes helps you order appropriate cultures and interpret results.

Non-selective Media: Supporting Growth of Most Organisms

Blood agar (usually sheep blood) is the workhorse of the microbiology laboratory. It provides the proteins, carbohydrates, and growth factors that most bacteria require. But beyond mere support of growth, blood agar reveals hemolytic patterns that help identify organisms.

Hemolysis reflects bacterial production of enzymes (hemolysins) that damage red blood cell membranes. Beta-hemolysis produces a clear zone around colonies - the RBCs have been completely lysed. This pattern is seen with Streptococcus pyogenes, Staphylococcus aureus, and Listeria monocytogenes, among others. Alpha-hemolysis produces a greenish zone - the hemoglobin has been oxidized to methemoglobin, but the cells aren’t fully lysed. Streptococcus pneumoniae and the viridans streptococci show this pattern. Gamma-hemolysis means no hemolysis at all.

Chocolate agar is blood agar that has been heated until the red cells lyse, releasing their contents. The brown color (like chocolate) comes from denatured hemoglobin. The critical difference from regular blood agar is that heating releases two growth factors from RBCs: hemin (factor X) and NAD (factor V). Haemophilus influenzae cannot synthesize these factors and will not grow on regular blood agar - it absolutely requires chocolate agar. Neisseria species also grow better on chocolate agar.

Selective Media: Isolating Specific Organisms

When you culture a specimen that contains many different bacteria (like stool or a wound swab), the organisms you’re looking for may be overgrown by normal flora. Selective media contain inhibitors that suppress unwanted organisms while allowing pathogens to grow.

MacConkey agar revolutionized enteric microbiology. It contains bile salts and crystal violet, which inhibit gram-positive bacteria while allowing gram-negative enterics to grow. It also contains lactose and a pH indicator - lactose fermenters produce acid, turning colonies pink, while non-fermenters remain colorless. This immediately distinguishes lactose-fermenting coliforms (E. coli, Klebsiella) from non-fermenters like Salmonella and Shigella, the usual pathogens sought in stool cultures.

Thayer-Martin agar is designed to isolate pathogenic Neisseria (gonorrhoeae and meningitidis) from sites with normal flora, like the genital tract or nasopharynx. It contains antibiotics - vancomycin (kills gram-positives), colistin (kills gram-negatives except Neisseria), and nystatin (kills fungi). Only Neisseria can grow.

Mannitol salt agar exploits the halotolerance of Staphylococcus. The 7.5% salt concentration inhibits most bacteria, but staphylococci thrive. The medium also contains mannitol and a pH indicator - S. aureus ferments mannitol and turns the agar yellow, while coagulase-negative staphylococci typically don’t.

Buffered charcoal yeast extract (BCYE) is the medium for Legionella. These organisms have unusual nutritional requirements - they need L-cysteine and iron supplementation that BCYE provides. The charcoal absorbs toxic substances from the agar. Legionella will not grow on routine media, so BCYE must be specifically requested when Legionnaires’ disease is suspected. On BCYE, L. pneumophila appears as gray-white to blue-green colonies with a ground-glass appearance after 3-5 days. Nocardia also grows on BCYE, and appears as chalky-white colonies. Key diagnostic clue: if an organism grows on BCYE but not on blood agar or MacConkey, think Legionella.

Eosin methylene blue (EMB) is the sibling of MacConkey - both contain inhibitors of gram-positives (bile salts for MacConkey; eosin and methylene blue for EMB) and both are differential for lactose fermentation. On EMB, lactose fermenters produce dark purple/black colonies; E. coli characteristically produces a metallic green sheen, virtually pathognomonic, from vigorous lactose fermentation that precipitates the dye complex. Non-fermenters are translucent/colorless.

Colistin-nalidixic acid (CNA) agar is a selective blood agar that suppresses gram-negative rods (colistin disrupts the outer membrane; nalidixic acid inhibits gram-negative DNA gyrase). Gram-positives grow freely and hemolysis patterns are still visible. CNA is used to pull gram-positives out of mixed wound cultures.

Mueller-Hinton agar is the standardized medium for antimicrobial susceptibility testing (disk diffusion / Kirby-Bauer and MIC testing). It is chosen for lot-to-lot reproducibility, low sulfonamide/trimethoprim/tetracycline inhibitors, and support of most non-fastidious pathogens. Mueller-Hinton with 5% sheep blood is used for fastidious organisms (streptococci). MH broth is used for broth microdilution MIC testing.

Bile esculin agar contains 4% bile (oxgall) that inhibits most gram-positives plus esculin (a sugar). Only Enterococcus species and group D streptococci (S. bovis / gallolyticus) can grow in bile and hydrolyze esculin. Bacteroides fragilis is also bile-resistant and esculin-positive and grows black. Positive result: the medium turns black from the reaction of esculetin (the esculin hydrolysis product) with ferric citrate.

Enteric pathogen media: Three specialized agars differentiate Salmonella from Shigella in stool cultures.

Hektoen enteric (HE) is green. Salmonella appears blue-green with black centers (H2S production). Shigella appears green/translucent.
Xylose-lysine-deoxycholate (XLD) is red. Salmonella appears red with black centers. Shigella is red without black centers.
Salmonella-Shigella (SS) agar is beige/light pink. Salmonella is colorless with black centers; Shigella is colorless without black centers. SS is the most inhibitory of the three and some Salmonella strains can be suppressed, so it is paired with HE or XLD.

All three exploit the same two features: Salmonella produces H2S (black centers); Shigella does not. Lactose, sucrose, and salicin fermenters (E. coli, Klebsiella) appear yellow-orange on HE/XLD. Shigella sonnei is a late lactose fermenter - it is the most common Shigella species in the US but can turn pink on MacConkey after 48 to 72 hours, causing identification confusion.

Sorbitol MacConkey (SMAC) substitutes sorbitol for lactose. Most E. coli ferment sorbitol (pink colonies); E. coli O157:H7 does NOT ferment sorbitol (colorless colonies). This is how you screen for the Shiga toxin-producing strain that causes hemorrhagic colitis and HUS. Non-O157 STEC strains may ferment sorbitol, so direct Shiga toxin EIA and PCR supplement culture.

CIN agar (cefsulodin-irgasan-novobiocin) selects for Yersinia enterocolitica. Y. enterocolitica grows as a distinctive bull’s-eye colony - a deep red center (mannitol fermentation) surrounded by a clear translucent outer zone. Mnemonic: “yer-CIN-ia.” Y. enterocolitica is psychrophilic (grows at 4°C), which also enables cold enrichment for stool samples.

Campylobacter agar (Skirrow or Campy-CVA) contains cefoperazone, vancomycin, and amphotericin to suppress competing flora. Campylobacter is microaerophilic (5 to 10% O2, 10% CO2) and thermophilic (grows best at 42°C, not 37°C). The high temperature plus reduced oxygen are the selective conditions - most other enteric organisms cannot grow under these conditions.

Selenite broth is a liquid enrichment medium selective for Salmonella. Sodium selenite suppresses coliforms during the first 6 to 12 hours, allowing Salmonella to grow preferentially. Subcultured afterwards to HE, XLD, or SS. Useful for low-count specimens like carrier screening.

Lim broth is Todd-Hewitt broth with gentamicin and nalidixic acid, used to enrich for group B streptococcus (S. agalactiae) from vaginal/rectal swabs of pregnant women. CDC recommends universal GBS screening at 36 to 37 weeks gestation; Lim broth enrichment increases sensitivity. Subculture to blood agar or run PCR.

Thioglycolate broth is a liquid enrichment medium that supports aerobes, anaerobes, microaerophiles, and facultative anaerobes simultaneously. Thioglycolate is a reducing agent that maintains a low redox at the bottom of the tube. Aerobes grow at the top; obligate anaerobes at the bottom; facultative anaerobes throughout; microaerophiles in the middle. The layered growth pattern characterizes the organism’s oxygen requirements.

Tinsdale and Hoyle agar contain potassium tellurite, which selects for Corynebacterium diphtheriae by inhibiting normal pharyngeal flora. C. diphtheriae reduces tellurite to metallic tellurium, producing characteristic dark gray-black colonies with a brown halo (the halo comes from H2S production).

Bordet-Gengou and Regan-Lowe are used to isolate Bordetella pertussis from nasopharyngeal specimens. Bordet-Gengou is potato-glycerol blood agar; Regan-Lowe is charcoal blood agar with cephalexin. B. pertussis grows slowly (3 to 7 days) as small, shiny, “mercury droplet” or “bisected pearl” colonies. PCR has largely replaced culture for pertussis diagnosis.

Atmospheric and Temperature Requirements

Organisms are classified by their oxygen needs.

Obligate aerobes require oxygen for survival (grow only at the top of thioglycolate broth or in aerobic atmospheres). Examples: Mycobacterium tuberculosis, Pseudomonas aeruginosa, Nocardia, Bordetella pertussis.
Facultative anaerobes grow with or without oxygen. They use aerobic respiration when O2 is available and fermentation when it is not. This includes E. coli, Staphylococcus, most Streptococcus, Enterococcus, Salmonella, Klebsiella, Listeria - most clinically important bacteria.
Obligate anaerobes cannot tolerate oxygen (they lack superoxide dismutase and catalase). Examples: Clostridium, Bacteroides fragilis, Actinomyces, Fusobacterium, Prevotella, Peptostreptococcus. Specimens must be transported in anaerobic containers.
Aerotolerant anaerobes do not use oxygen but tolerate it. Clostridium tertium, some Lactobacillus.
Microaerophiles need low oxygen (5 to 10%) and often elevated CO2. Examples: Campylobacter, Helicobacter pylori, Borrelia burgdorferi.

Anaerobes - Flora, Media, and Clinical Concepts

Anaerobes are the predominant flora in several body sites - colon (>99% of bacteria are anaerobes, outnumbering aerobes 1000:1), oral cavity, vagina, and skin. Most anaerobic infections arise when a mucosal barrier is disrupted and endogenous anaerobes invade normally sterile sites.

Key site-specific anaerobes.

Skin: Cutibacterium acnes, Peptostreptococcus.
Oral cavity: Actinomyces, Prevotella, Porphyromonas, Fusobacterium, Peptostreptococcus, Veillonella, Eubacterium.
Vagina: Lactobacillus (dominant, keeps pH ~4.5 via lactic acid), Prevotella, Porphyromonas.
Colon: Bacteroides fragilis (most clinically significant anaerobe in intra-abdominal infection), Clostridium, Bifidobacterium.

Obligate anaerobes lack superoxide dismutase and catalase, so oxygen generates toxic reactive species they cannot detoxify. This is why anaerobic specimens require oxygen-free transport.

Anaerobic media:

LKV (laked blood with kanamycin and vancomycin) - selective for Prevotella and Bacteroides. Kanamycin inhibits facultative gram-negatives; vancomycin inhibits gram-positives; laked (lysed) blood provides hemin and vitamin K.
PEA (phenylethyl alcohol) agar - inhibits facultative gram-negative rods while allowing anaerobes (both gram-positive and gram-negative) to grow. Selective, not differential.
CCFA (cycloserine-cefoxitin-fructose agar) for C. difficile (not covered in sampled cards but standard).
Bile esculin agar (already covered) also grows B. fragilis group because of its bile resistance.

Prevotella and Porphyromonas fluoresce red/brick-red under Wood’s lamp UV light because of protoporphyrin IX production - a rapid presumptive identification tool.

Aerotolerance testing distinguishes true obligate anaerobes from facultative organisms after isolation. Subculture to both an anaerobic plate and an aerobic plate with 5% CO2 at 35 to 37°C. Growth only under anaerobic conditions = obligate anaerobe. Growth in both = not obligate. Read at 24 and 48 hours.

Empiric treatment rule of thumb.

Above the diaphragm (head/neck, pulmonary): clindamycin - oral flora, excellent oral/lung penetration.
Below the diaphragm (intra-abdominal, pelvic): metronidazole - excellent against B. fragilis and gut anaerobes. Metronidazole is activated by anaerobic electron transport, generating toxic intermediates. Aerobes do not activate it, which is why it spares normal aerobic flora.

Broader-coverage alternatives: carbapenems, piperacillin-tazobactam, ampicillin-sulbactam, cephamycins (cefoxitin, cefotetan).

Temperature requirements are mostly 37°C at ambient air, but know the exceptions: Campylobacter at 42°C microaerophilic, Yersinia cold-enrichment at 4°C then 25°C, Listeria growth at 4°C (enables foodborne spread in refrigerated soft cheeses and deli meats), mycobacteria at 37°C (M. tuberculosis) or 30-33°C (some NTM) requiring weeks of incubation.

Fastidious organisms require supplemented media because they cannot grow on standard blood or nutrient agar. Examples: Haemophilus (X and V factors, chocolate agar), Legionella (cysteine and iron, BCYE), Neisseria (CO2, chocolate/TM), Bordetella pertussis (Bordet-Gengou), Francisella tularensis (cysteine-supplemented media).

Mycobacterial Culture Media and Processing

Mycobacteria are obligate aerobes, nonmotile, non-spore-forming, and slow-growing (generation time 12 to 24 hours for M. tuberculosis vs 20 minutes for E. coli). Their lipid-rich cell wall (up to 60% mycolic acid) makes them impermeable to many stains, antibiotics, and disinfectants.

Specimen processing:

3 sputum specimens 8 to 24 hours apart; at least one early morning (highest yield from overnight accumulation). Single smear ~50 to 60% sensitive; three specimens reach ~70 to 80%.
Gastric aspirate is used in children and others unable to expectorate; early morning collection; neutralize with sodium bicarbonate to protect mycobacteria from gastric acid.
NALC-NaOH digestion-decontamination: N-acetyl-L-cysteine (NALC) is the mucolytic that breaks disulfide bonds in mucus; NaOH kills contaminating flora. Mycobacteria survive because of their resistant cell wall. Over-decontamination kills mycobacteria and reduces sensitivity.
Concentration by centrifugation (>3,000g for 15 minutes) after decontamination concentrates mycobacteria in the pellet.

Microscopy: Gram staining is unreliable (weakly gram-positive, beaded or ghost-like). Use acid-fast stains.

Carbol fuchsin is the primary stain; acid-alcohol is the decolorizer; methylene blue is the counterstain. Bacilli appear slender and beaded; cording pattern (parallel alignment resembling twisted rope) from cord factor (trehalose dimycolate) is suggestive of M. tuberculosis complex.
Ziehl-Neelsen (hot) vs Kinyoun (cold) - covered above. Kinyoun is the most commonly used conventional stain in US labs.
Fluorochrome stains (auramine-O, auramine-rhodamine) are ~10% more sensitive than conventional AFB and can be read at 40x objective (vs 100x oil for ZN/Kinyoun), so slides are screened faster. Positive fluorochrome results should be confirmed by ZN or Kinyoun because non-mycobacterial artifacts can autofluoresce.

NAAT: CDC recommends NAAT on at least 1 respiratory specimen from every patient with TB symptoms, regardless of smear result. Xpert MTB/RIF detects MTB complex DNA in ~2 hours and simultaneously screens for rifampin resistance. NAAT sensitivity ~130 organisms/mL vs 5,000 to 10,000/mL for smear. Positive NAAT confirms TB; negative does not rule it out (culture still required).

Culture: Specimens are inoculated into both solid and broth media to maximize recovery and speed.

Egg-based solid media: Lowenstein-Jensen (LJ) - whole eggs, glycerol, asparagine, malachite green to inhibit contaminants. M. tuberculosis produces buff, rough, dry colonies in 3 to 8 weeks.
Agar-based synthetic solid media: Middlebrook 7H10 and 7H11 - transparent, earlier microcolony detection; 7H11 adds casein hydrolysate for improved recovery of drug-resistant strains.
Broth media (Middlebrook 7H9, MGIT 960) - faster (1 to 3 weeks). MGIT uses a fluorescent O2-sensing compound at the bottom of the tube; oxygen quenches fluorescence, so as mycobacteria consume O2, fluorescence rises and the instrument flags the tube positive.
BACTEC 460TB (legacy) was radiometric, using 14C-palmitic acid and detecting 14CO2. Replaced by non-radiometric MGIT because of radioactive waste.
Septi-Check is a biphasic system with 7H9 broth and attached agar paddles - largely historical.

Temperature exceptions: most mycobacteria grow at 37°C, but know the exceptions.

30 to 32°C (cooler-body-site organisms, “Heat Makes Us Mad”): M. haemophilum, M. marinum, M. ulcerans, M. chelonae (also M. malmoense).
42°C: M. xenopi (thermophilic; associated with hospital hot-water systems, causes TB-like cavitary pneumonia). Labs should culture suspected M. xenopi at 42°C alongside 37°C.

M. haemophilum requires hemin/iron supplementation (the name means blood-loving). Standard LJ and 7H10 lack sufficient iron; use chocolate agar, hemin-supplemented media, or factor-X disks. Causes skin nodules/ulcers and lymphadenitis in immunocompromised patients (HIV, transplant).

Rapid growers (Runyon Group IV): visible colonies within 7 days. Includes M. fortuitum, M. chelonae, M. abscessus. May grow on routine blood or chocolate agar. Resistant to first-line TB drugs but often susceptible to macrolides (clarithromycin), fluoroquinolones, amikacin, and carbapenems.

Identification of mycobacteria:

Chemiluminescent 16S rRNA probes (AccuProbe): M. tuberculosis complex, M. avium, M. intracellulare, M. kansasii, M. gordonae. Hours, not weeks.
HPLC of mycolic acid profiles - reference-lab method, largely superseded.
DNA sequencing of 16S rRNA, Hsp65, or rpoB - reference standard; rpoB also picks up rifampin resistance.
MALDI-TOF with a specialized mycobacterial database and an inactivation extraction protocol.

Biochemical differentiation within MTB complex:

M. tuberculosis: niacin-positive, nitrate-positive. Niacin accumulates because M. tuberculosis lacks the enzyme to convert it to NAD.
M. bovis and M. africanum: niacin-negative, nitrate-negative. M. bovis is pyrazinamide-resistant (important because PZA is first-line).
MTB complex produces a heat-labile catalase (loses activity at 68°C); most NTM produce a heat-stable catalase. The 68°C catalase test separates them.

37.4 Identification Methods

Once an organism grows in culture, identification is the next step. The methods available range from simple phenotypic tests that have been used for over a century to sophisticated molecular techniques that can identify organisms in minutes.

Biochemical Testing

For most of microbiology’s history, organisms were identified by what they could metabolize. Can this organism ferment lactose? Does it produce urease? Can it grow without oxygen? These metabolic fingerprints, combined with Gram stain morphology, allow identification of most common pathogens.

The power of biochemical testing lies in its simplicity and low cost. Sugar fermentation tubes show acid production (color change) and gas production (bubbles in a Durham tube). The oxidase test - a drop of reagent that turns purple if cytochrome c oxidase is present - takes seconds and immediately narrows the differential among gram-negative rods. The catalase test distinguishes staphylococci (positive) from streptococci (negative). These tests require only basic supplies and no expensive equipment.

The limitation is time. Traditional biochemical testing requires overnight incubation, sometimes longer for slow-growing organisms. Automated systems (Vitek, MicroScan, Phoenix) miniaturize and accelerate the process but still require pure cultures and several hours of incubation.

Catalase: detects the enzyme that splits hydrogen peroxide into water and oxygen. Add 3% H2O2 to a colony on a glass slide - immediate bubbling is positive. Staphylococcus is catalase-positive; Streptococcus and Enterococcus are catalase-negative. This is the first test used to split gram-positive cocci in clusters (Staph) from chains (Strep/Enterococcus). Other catalase-positives include Listeria, Bacillus, Corynebacterium, Pseudomonas, and most Enterobacteriaceae. Do NOT perform the catalase test on colonies from blood agar - RBC catalase gives false positives. Use a wooden stick rather than a metal loop; metal can catalyze H2O2 breakdown.

Coagulase: detects the staphylococcal enzyme that converts fibrinogen to fibrin (the central virulence marker of S. aureus). Two formats.

Slide coagulase detects bound coagulase (clumping factor) on the bacterial surface. A drop of rabbit plasma is mixed with a colony; clumping within 10 to 15 seconds is positive. Rapid but can be falsely negative (strains lacking bound coagulase) or falsely positive (S. lugdunensis and S. schleiferi have surface clumping factors).
Tube coagulase detects free (secreted) coagulase and is the gold standard. Colonies are inoculated into rabbit plasma (with EDTA to prevent calcium-driven false clotting) and incubated at 37°C. Read at 4 hours, recheck at 24 hours if negative. A true gel that does not flow when tilted is positive. Beyond 24 hours, staphylokinase (fibrinolysin) can dissolve the clot and create a false negative.

Staphylococcus lugdunensis is a classic board pitfall: coagulase-negative on the tube test but positive on the slide test. It acts more like S. aureus clinically (aggressive endocarditis, bone/joint infection, skin abscess).

Oxidase: detects cytochrome c oxidase, the terminal enzyme in the aerobic electron transport chain. A reagent (tetramethyl-p-phenylenediamine) is applied to a colony on filter paper. Blue/purple color within 10 to 30 seconds is positive. All Enterobacterales are oxidase-negative - this is the key branch point for gram-negative identification. Oxidase-positive gram-negatives include Pseudomonas, Neisseria, Vibrio, Campylobacter, Helicobacter, Aeromonas, Plesiomonas, Pasteurella. Do NOT test from MacConkey colonies - acid production can give false negatives.

PYR (pyrrolidonyl arylamidase): detects L-pyrrolidonyl aminopeptidase. Three PYR-positive organisms to remember: Streptococcus pyogenes (Group A Strep), Enterococcus, and Staphylococcus lugdunensis. The test distinguishes S. aureus (PYR-) from S. lugdunensis (PYR+), Enterococcus (PYR+) from group D Strep like S. bovis (PYR-), and Group A Strep (PYR+) from other beta-hemolytic streps.

Urease: detects the urease enzyme that splits urea to ammonia and CO2. Ammonia raises pH and turns a phenol red indicator from yellow to pink. Urease-positive organisms: Proteus (rapid, strong), Helicobacter pylori, Ureaplasma, Cryptococcus neoformans, some Klebsiella and Enterobacter. The rapid urease (CLO) test for H. pylori on gastric biopsies uses this principle.

Hugh-Leifson oxidative-fermentative (O-F) medium: determines whether an organism ferments or merely oxidizes a sugar. Two tubes of the same medium and sugar are inoculated: one open to air (aerobic), one sealed with mineral oil (anaerobic). Acid production turns the indicator yellow.

Both tubes yellow = fermenter (acid produced aerobically and anaerobically; typical of Enterobacterales).
Only open tube yellow = oxidizer (acid only with oxygen; typical of non-fermenting gram-negatives like Pseudomonas).
Neither yellow = non-utilizer.

The O-F test is the classical separator of fermenters (Enterobacterales) from oxidizers (non-fermenters), though MALDI-TOF has largely displaced it.

Enterobacterales - Defining Features and Biochemical Patterns

All Enterobacterales share four defining features:

Facultative anaerobes.
Oxidase negative (key split from Pseudomonas, Neisseria, Vibrio).
Ferment glucose (distinguishes from non-fermenters).
Reduce nitrate to nitrite.

Biochemical patterns help narrow within the order.

H2S producers (black on TSI/KIA/HE/XLD): Salmonella, Edwardsiella, Citrobacter freundii, Proteus.
Strongly urease-positive and phenylalanine-deaminase-positive (the “Tribe Proteeae” - Proteus, Morganella, Providencia). Combination is specific to this group. Klebsiella is weakly urease-positive, PDA-negative.
Strictly non-motile: Shigella and Klebsiella. All others motile via peritrichous flagella. Yersinia species are motile at 25°C but not 37°C, except Y. pestis (always nonmotile).
Voges-Proskauer (VP) positive (detects acetoin in butanediol fermentation): KESH - Klebsiella, Enterobacter, Serratia, Hafnia (plus Pantoea). VP+ and MR are usually opposite.

Antigenic structure:

O antigen - outermost LPS polysaccharide; endotoxin; basis for serogrouping (E. coli O157, Salmonella groups). Lipid A is the toxic moiety.
K antigen - capsular polysaccharide. E. coli K1 = neonatal meningitis. Klebsiella K1/K2 associated with invasive liver abscess syndrome.
H antigen - flagellar protein (flagellin). Used in serotyping (E. coli O157:H7). Non-motile organisms lack H antigen.

TSI and KIA

Triple sugar iron (TSI) and Kligler iron agar (KIA) are slant media used to characterize gram-negative rods.

TSI contains glucose:lactose:sucrose = 1:10:10; KIA contains only lactose + glucose. Both contain phenol red (yellow with acid, red when alkaline) and ferrous sulfate (black with H2S).
Poured as a slant with an aerobic upper surface and anaerobic butt; inoculated by stab-and-streak.
Glucose is fermented first in small amounts; if it is the only sugar metabolized, the butt stays yellow but the slant reverts to red in 18 to 24 hours as alkaline amines from peptone catabolism accumulate.

Classic patterns.

K/K (red slant, red butt) = no fermentation. Non-fermenters: Pseudomonas, Acinetobacter, Stenotrophomonas, other non-fermenting GNR.
K/A (red slant, yellow butt) = glucose only. Shigella, Salmonella, some Serratia.
A/A (yellow slant, yellow butt) = glucose + lactose/sucrose. E. coli, Klebsiella, Enterobacter.
H2S positive (black butt) - Salmonella, Proteus, Citrobacter freundii.
Gas (bubbles, cracks, agar displacement) - most fermenters produce some gas; Shigella does not.

Beta-Lactamase Hierarchy

Understanding beta-lactamases drives empiric antibiotic choice for gram-negative infections.

Penicillinase - narrow-spectrum, hydrolyzes penicillin only; inhibited by clavulanate. Classic in plasmid-encoded S. aureus and many Enterobacterales. Amoxicillin-clavulanate overcomes it.
ESBL (extended-spectrum beta-lactamase) - class A serine beta-lactamase, plasmid-encoded (CTX-M most common globally, plus TEM, SHV variants). Hydrolyzes penicillins, all cephalosporins (including 3rd/4th gen), and aztreonam. Does NOT hydrolyze carbapenems. Inhibited by clavulanic acid (CLSI double-disk synergy test: clavulanate adjacent to ceftazidime increases zone by >=5 mm). Treat with carbapenems.
AmpC - class C cephalosporinase, chromosomally encoded, inducible. Classic organisms: SPICE / ESCAPPM - Serratia, Pseudomonas, Indole-positive Proteus (P. vulgaris), Citrobacter freundii, Enterobacter cloacae; extended mnemonic includes Aeromonas, Providencia, Morganella. Low baseline expression (organism appears ceftriaxone-susceptible), induced high expression during therapy -> clinical failure. AmpC hydrolyzes penicillins, 1st-3rd gen cephalosporins, cephamycins (cefoxitin); does NOT hydrolyze cefepime or carbapenems. Not inhibited by clavulanic acid. Treat with carbapenem or cefepime.
Carbapenemase (KPC, NDM, OXA-48) - hydrolyzes all beta-lactams including carbapenems. Found in Enterobacterales (CRE = carbapenem-resistant Enterobacterales). KPC (class A serine) is most common in US, inhibited by avibactam. NDM and VIM (class B metallo-beta-lactamases) require zinc, inhibited by EDTA but not avibactam. OXA-48 (class D) common in Middle East/North Africa. Detection: modified Hodge test (cloverleaf indentation), now largely replaced by CarbaNP and molecular tests. Treatment options are few: ceftazidime-avibactam, meropenem-vaborbactam, polymyxins (colistin), tigecycline.

ESBL vs AmpC summary: both confer 3rd-gen cephalosporin resistance; the clavulanic acid inhibition test separates them. ESBL is inhibited by clavulanate; AmpC is not.

MALDI-TOF Mass Spectrometry

MALDI-TOF MS has revolutionized clinical microbiology. What once took 24-48 hours now takes minutes. This technology identifies organisms by their protein fingerprint, particularly ribosomal proteins that vary among species.

The principle is elegant: a colony is smeared onto a target plate, covered with a matrix chemical, and hit with a laser. The laser causes proteins to be ejected and ionized. These ions accelerate through a vacuum tube; smaller proteins travel faster than larger ones. The pattern of arrival times - the mass spectrum - is unique to each species. Software compares the unknown spectrum to a database and provides an identification.

The impact on clinical microbiology has been profound. Blood culture bottles that turn positive at 3 AM can be Gram-stained and identified by the time the microbiologist arrives in the morning. Same-day identification of most organisms has become routine. The technology is cost-effective - once the instrument is purchased, per-test costs are minimal.

Limitations exist: MALDI-TOF requires a pure culture (though direct testing from positive blood cultures is increasingly used), the database must contain the organism (unusual species may not be identified), and closely related species may have indistinguishable spectra.

Key MALDI-TOF limitations to know.

Works best on cultured, isolated colonies. Direct identification from clinical specimens (blood, urine) is being developed but is less reliable because host proteins and mixed flora create background interference.
Mixed cultures are problematic - combined protein spectra from multiple organisms create composite fingerprints that do not match any single database entry. Each distinct colony type must be picked and tested separately.
Database dependence: organisms not in the reference database will be misidentified or returned as “no identification.” Rare, new, and environmental species are vulnerable; labs sometimes build custom local entries.
Poor discrimination of closely related organisms with nearly identical spectra: E. coli vs Shigella (greater than 99% genomic similarity; often misidentified - clinically important because management differs), Streptococcus pneumoniae vs S. mitis, Brucella vs Ochrobactrum.
No susceptibility data - MALDI-TOF gives identification only, not antibiograms.

Blood culture bottles that turn positive overnight can be processed (lysis-centrifugation or serum separator tubes) for direct MALDI-TOF, giving identification to the treating team while routine subcultures are still incubating. This is a major workflow advance.

Molecular Methods

When biochemical testing and MALDI-TOF fail - or when identification is urgent - molecular methods provide the answer.

16S ribosomal RNA gene sequencing is the universal bacterial identifier. All bacteria possess ribosomes, and the genes encoding ribosomal RNA contain both conserved regions (allowing universal PCR primers) and variable regions (allowing species differentiation). When an organism cannot be identified by conventional methods, 16S sequencing provides the answer. It’s the reference standard for describing new species.

Syndrome-based multiplex PCR panels have transformed rapid diagnosis of infectious syndromes. A respiratory panel can simultaneously test for 15-20 pathogens (bacteria and viruses) from a single nasopharyngeal swab in about an hour. Meningitis/encephalitis panels test CSF for bacterial, viral, and fungal causes of CNS infection. GI panels test stool for common causes of infectious diarrhea. These panels are expensive but provide rapid results that guide therapy and infection control.

The limitation of molecular methods is that they detect nucleic acid, not viable organisms. A positive PCR doesn’t necessarily mean viable pathogen - residual DNA from prior infection or colonization may be detected. Clinical correlation is essential.

37.5 Urinary Tract Infections

UTI is a syndrome-based diagnosis. The urinary tract is sterile proximal to the urethra; the distal urethra is normally colonized by Lactobacillus, coagulase-negative staphylococci, Corynebacterium, and Enterococcus. UTI occurs when organisms ascend from the urethra into the normally sterile upper tract.

Definitions: UTI encompasses urethritis (urethra), cystitis (bladder), and pyelonephritis (kidney/renal pelvis). Lower UTI = urethritis and cystitis; upper UTI = pyelonephritis.

Uncomplicated UTI = acute cystitis in a healthy, non-pregnant woman with normal genitourinary anatomy. Treat with nitrofurantoin, TMP-SMX, or fosfomycin.
Complicated UTI = UTI in pregnancy, men, children, anatomic anomalies, diabetes, stones, spinal cord injury, indwelling catheter, immunocompromise, recent instrumentation, or healthcare-associated setting. Broader spectrum and longer duration.
Asymptomatic bacteriuria = >=10^5 CFU/mL of a single organism in a properly collected specimen from a patient without UTI symptoms. Common in the elderly, diabetics, and catheterized patients. Do not treat except in pregnant women and patients undergoing urologic instrumentation. Treating asymptomatic bacteriuria promotes resistance without clinical benefit (major antimicrobial stewardship principle).

Causative organisms:

E. coli - most common cause overall (~80% of uncomplicated, ~50% of complicated). Uropathogenic E. coli (UPEC) strains have P-fimbriae, type 1 fimbriae, hemolysin, and K capsule.
Staphylococcus saprophyticus - second most common in young, sexually active women. Coagulase-negative Staph that is novobiocin-resistant (the key lab ID feature distinguishing it from S. epidermidis, which is novobiocin-sensitive).
Klebsiella, Proteus, Enterobacter - next most common Enterobacterales. Proteus produces urease, alkalinizes urine, and causes struvite (magnesium ammonium phosphate) stones.
Enterococcus - common in older men with obstructive uropathy (BPH), after urogenital instrumentation, and in catheter-associated UTI.
Pseudomonas aeruginosa - catheter-associated, nosocomial.
Candida - most common cause of fungal UTI, in catheterized, diabetic, immunosuppressed, and broad-spectrum-antibiotic-exposed patients. C. albicans most common; non-albicans (C. glabrata, C. tropicalis) increasing. Candiduria may represent colonization, lower UTI, or disseminated candidiasis with renal involvement. Remove the catheter; treat with fluconazole if symptomatic.
Culture-negative UTI: think Chlamydia, Mycoplasma, Ureaplasma in sexually active patients with urethritis symptoms and negative standard cultures. These are missed on routine urine culture (Mycoplasma/Ureaplasma lack cell walls; Chlamydia is obligate intracellular).

Hemorrhagic cystitis causes: adenovirus types 11 and 21, and BK virus in immunosuppressed patients (particularly post-bone-marrow transplant; BK in kidney transplant more typically causes tubulointerstitial nephritis or ureteric stenosis, not hemorrhagic cystitis). Non-infectious: cyclophosphamide/ifosfamide via the acrolein metabolite, prevented with MESNA.

Laboratory approach:

Clean-catch midstream urine is standard. Initial flow flushes urethral flora; the midstream portion is what goes to the lab.
>=10^5 CFU/mL of a single organism is the classic threshold for significant bacteriuria. 10^4 to 10^5 CFU/mL may be significant in symptomatic patients (correlate clinically).
Only culture symptomatic patients, except pregnancy and pre-urologic procedure. Screening asymptomatic patients is how you end up treating bacteriuria that was never going to hurt them.

Dipstick/UA surrogates:

Blood (hemoglobin) - nonspecific but may support UTI in the right context.
Leukocyte esterase - indicates WBCs (pyuria).
Nitrite - indicates nitrate-reducing bacteria (E. coli, Klebsiella, Proteus). Enterococcus and S. saprophyticus do NOT reduce nitrate and are nitrite-negative on dipstick.
Pyuria on microscopy (WBCs in urine). Eosinophiluria suggests acute interstitial nephritis, not UTI.

Leukocyte esterase plus nitrite together has high specificity for UTI.

37.6 Pneumonia

Pneumonia is parenchymal lung infection. The differential depends on the setting: community-acquired, hospital-acquired, ventilator-associated, aspiration, and immunocompromised each have distinct organisms. Specimen quality matters - a sputum Gram stain is only useful when it is truly sputum (>25 PMNs, <10 squames per LPF). BAL and protected brush specimens bypass oropharyngeal contamination and are preferred in immunocompromised patients.

Classification

Pneumonia is divided by the setting in which it developed, which dictates the expected pathogens and therefore empiric therapy.

Community-acquired pneumonia (CAP): acquired outside healthcare settings.
Hospital-acquired pneumonia (HAP): develops >=48 hours after hospitalization.
Ventilator-associated pneumonia (VAP): develops >=48 hours after intubation.

CAP itself is subdivided into typical (lobar) and atypical by presentation.

Typical (lobar): acute onset, productive cough, high fever, lobar consolidation on CXR. Main organisms: S. pneumoniae (most common overall), H. influenzae, S. aureus, Klebsiella, Moraxella catarrhalis.
Atypical: gradual onset, dry cough, patchy interstitial infiltrates, extrapulmonary symptoms. Main organisms: Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila.

Imaging is not reliable for splitting typical from atypical. The distinction is most useful for directing empiric coverage.

Viral CAP is increasingly recognized (up to 30% when molecular testing is used). Major pathogens: influenza, SARS-CoV-2, RSV, rhinovirus, adenovirus, parainfluenza, human metapneumovirus.

Host-Specific Pathogens

Special circumstances change the differential.

COPD: S. pneumoniae, H. influenzae, Moraxella catarrhalis. Pseudomonas in severe COPD with frequent antibiotics.
Aspiration: oral anaerobes (Bacteroides, Fusobacterium, Peptostreptococcus) plus aerobic oral flora, usually polymicrobial. Classically affects right lower lobe (gravity-dependent when supine).
Alcoholism: S. pneumoniae, Klebsiella pneumoniae (classic “currant-jelly sputum” cavitary pneumonia), anaerobes, gram-negative aerobic bacilli.
Injection drug use: S. aureus, particularly MRSA; often from septic pulmonary emboli off right-sided endocarditis.
Neutropenia: gram-negative aerobic bacilli (Pseudomonas, E. coli, Klebsiella) and fungi (Aspergillus). Empiric anti-pseudomonal beta-lactam for febrile neutropenia.
Cystic fibrosis - children: S. aureus (including MRSA), H. influenzae. Adults: Pseudomonas aeruginosa (mucoid phenotype), Burkholderia cepacia complex, Stenotrophomonas, Achromobacter. Burkholderia cepacia can cause rapid clinical decline and is a transplant contraindication at some centers.
Chronic granulomatous disease: catalase-positive organisms. CGD patients have defective NADPH oxidase and cannot produce reactive oxygen species; catalase-positives neutralize their own H2O2. Key pathogens: Aspergillus (most common cause of death), S. aureus, Serratia marcescens, Burkholderia cepacia, Nocardia.
AIDS with low CD4: Pneumocystis jirovecii (CD4 <200), Cryptococcus neoformans, Mycobacterium tuberculosis, CMV, endemic mycoses.

Animal and Environmental Exposures

Cattle, goats, sheep (placental aerosols): Coxiella burnetii (Q fever).
Psittacine birds (parrots, parakeets): Chlamydophila psittaci (psittacosis).
Bat/bird droppings (Ohio/Mississippi river valleys): Histoplasma capsulatum.
Rodent urine and feces (southwest US): Hantavirus (hantavirus pulmonary syndrome).
Rabbits, ticks: Francisella tularensis (pneumonic tularemia).
Sandstorms, desert southwest: Coccidioides immitis / posadasii (valley fever).

Hospital-Acquired Pneumonia

HAP/VAP pathogens include more drug-resistant organisms than CAP because of healthcare exposure, prior antibiotics, and debilitated hosts. The core concern is MRSA and gram-negative bacilli (Pseudomonas, Acinetobacter baumannii, Klebsiella, Enterobacter). Empiric coverage: anti-pseudomonal beta-lactam (e.g., cefepime or piperacillin-tazobactam) plus vancomycin or linezolid.

Diagnostic Specimens

Nasopharyngeal swab - best for viruses (multiplex PCR: influenza, RSV, SARS-CoV-2, adenovirus, parainfluenza, metapneumovirus) and Bordetella pertussis. Does not diagnose bacterial pneumonia reliably - NP colonization does not equal lower tract infection.
Sputum - expectorated, preferably morning before eating (most concentrated overnight secretions). Rinse mouth first. Quality: >25 PMNs and <10 squames per LPF; >10 squames means oral contamination and the specimen should be rejected.
Bronchoalveolar lavage (BAL) - gold standard for ventilated and immunocompromised patients. Quantitative culture threshold >=10^4 CFU/mL.
Protected specimen brush - another bypass of the upper airway.

Ancillary Diagnostics

Urinary antigen testing: Legionella pneumophila (serogroup 1 only, ~70 to 80% of Legionella pneumonia) and Streptococcus pneumoniae (C-polysaccharide cell wall antigen). Rapid (15 to 30 minutes), highly specific, stay positive early even after antibiotics. Also used for Histoplasma and Blastomyces (fungal antigens).
Procalcitonin: elevated (>0.5 ng/mL) suggests bacterial infection; low values support de-escalation or stopping antibiotics. Not for initial empiric decisions alone but useful for stewardship.

37.7 Meningitis

Bacterial meningitis is a medical emergency. CSF is sterile, so any organism on Gram stain or culture is meaningful. The classic CSF profile is neutrophilic pleocytosis, elevated protein, and decreased glucose, with organisms visible on Gram stain. Never refrigerate CSF - N. meningitidis and Haemophilus die cold.

Meningitis vs Encephalitis

Meningitis is inflammation of the meninges and presents with headache, neck stiffness, photophobia, and fever. CSF shows pleocytosis.

Encephalitis is inflammation of the brain parenchyma and presents with altered mental status, seizures, focal deficits, and personality/behavioral changes. Imaging shows parenchymal abnormalities.

Many infections involve both (meningoencephalitis).

Acute Bacterial Meningitis

Typical CSF profile: neutrophilic pleocytosis, elevated protein, decreased glucose, with organisms on Gram stain.

Age-stratified pathogens (pure board memorization).

Neonates (<1 month): Group B Streptococcus (S. agalactiae), E. coli (K1 capsule), Listeria monocytogenes. Mnemonic: “GEL.” Reflects organisms acquired from the maternal genital tract at delivery.
Infants/children/young adults (2 months to ~18 years): S. pneumoniae, N. meningitidis, H. influenzae type b. S. pneumoniae is now the most common at all pediatric ages post-Hib vaccine. N. meningitidis causes outbreaks in dorms and barracks.
Older adults (>=50 years, immunocompromised, pregnant): S. pneumoniae, N. meningitidis, Listeria monocytogenes, plus gram-negative bacilli in healthcare settings. Listeria’s reappearance is why empiric meningitis treatment in patients over 50 adds ampicillin to the standard ceftriaxone + vancomycin.
Postsplenectomy or functional asplenia (including sickle cell): encapsulated organisms - S. pneumoniae (most common, ~50%), H. influenzae type b, N. meningitidis (OPSI). Vaccination plus prophylactic penicillin are the preventive strategies.

Aseptic (Viral) Meningitis

Aseptic meningitis = lymphocytic meningitis with negative routine bacterial cultures. The CSF profile is lymphocytic pleocytosis, normal-to-mildly elevated protein, normal glucose. Main causes.

Enteroviruses (Coxsackie, Echovirus, Poliovirus) - most common cause of viral meningitis overall.
HSV-2 causes recurrent lymphocytic meningitis (Mollaret meningitis). HSV-1 causes encephalitis (temporal lobes); HSV-2 causes meningitis - classic pitfall to keep straight.
Arboviruses (West Nile, EEE, WEE), mumps, LCMV, HIV (primary infection).

Subacute Meningitis

Gradual onset over days to weeks. Think:

Mycobacterium tuberculosis - basilar meningitis with cranial nerve palsies and very low CSF glucose.
Cryptococcus neoformans - HIV/AIDS (CD4 <100), diagnosed with India ink and cryptococcal antigen.
Other fungi (Coccidioides, Histoplasma), Brucella, neurosyphilis, Lyme.

Encephalitis

Usually viral. The most important cause is HSV-1, which causes hemorrhagic necrosis of the temporal lobes (MRI: temporal lobe hyperintensity). Empiric acyclovir should be started immediately while HSV testing is pending - delay worsens outcomes.

Other viral causes: HHV-6, VZV (especially immunocompromised), CMV, rabies, arboviruses (West Nile is the most common arboviral cause in the US, plus Eastern/Western equine, St. Louis, La Crosse).

Laboratory Workup

Both CSF and blood cultures are sent. Blood cultures are positive in 50 to 80% of bacterial meningitis. CSF tests to consider.

Chemistry: glucose, protein.
Cell count with differential. PMNs = bacterial; lymphocytes = viral/TB/fungal.
Gram stain and culture.
Special stains: India ink and cryptococcal antigen (CrAg) for Cryptococcus, AFB smear and culture or MTB PCR for TB.
Multiplex meningitis/encephalitis PCR panels cover HSV-1/2, VZV, enterovirus, CMV, HHV-6, Cryptococcus, S. pneumoniae, N. meningitidis, H. influenzae, E. coli K1, GBS, Listeria.
Cytology for malignancy; fungal culture if indicated.

37.8 Endocarditis

Endocardial infection requires 3 sets of blood cultures from 3 separate sites over 24 hours to maximize sensitivity (>95%). The organisms depend on the substrate: native valve vs prosthetic valve vs IV drug use vs healthcare-associated. Think about slow-growers (HACEK, Bartonella, Brucella, Coxiella) when standard cultures are negative.

Acute vs Subacute

Acute - infection of previously normal native valves by virulent organisms. Rapid onset, high fever, rapid valve destruction. S. aureus is the most common cause.
Subacute - infection of previously abnormal valves by less virulent organisms. Indolent over weeks to months, low-grade fever, gradual valve damage. Viridans streptococci are the single most common subacute native-valve cause; also Enterococcus, HACEK.

Valve Involvement

Most common valve overall: mitral.
Classic valve in IV drug users: tricuspid (right-sided, from injection of skin flora into the venous system). Septic pulmonary emboli on CXR (multiple bilateral nodular infiltrates). Left-sided IE sends emboli to brain, kidneys, spleen.

Organism-Setting Map

Acute native valve: S. aureus (including MRSA).
Subacute native valve: viridans streptococci (S. mitis, S. sanguinis), Enterococcus, HACEK, group D streptococci (S. bovis / gallolyticus - think of concurrent colon cancer screening).
Prosthetic valve - very early (<2 months): S. epidermidis, S. aureus, gram-negative bacilli, Candida - reflects perioperative contamination.
Prosthetic valve - early (2 to 12 months): still S. epidermidis and S. aureus dominated.
Prosthetic valve - late (>12 months): same organisms as native valve (viridans strep, Enterococcus, S. aureus, HACEK).

The early CoNS predominance on prosthetic valves reflects biofilm formation on prosthetic material.

HACEK Organisms

HACEK is a mnemonic for the slow-growing oropharyngeal gram-negatives that cause subacute endocarditis and are classic culprits in culture-negative endocarditis.

Haemophilus spp. (H. parainfluenzae, H. aphrophilus)
Aggregatibacter actinomycetemcomitans
Cardiobacterium hominis
Eikenella corrodens
Kingella kingae

They previously required prolonged incubation up to 14 days for culture recovery, but modern continuous-monitoring systems (BacT/Alert, BACTEC) usually pick them up within 5 days. PCR is another route when cultures are negative.

Culture-Negative Endocarditis

Organisms difficult to recover by standard culture. Mnemonic: “Cardiac Bacteria Cleverly Try Hiding” covers the main five.

Coxiella burnetii (Q fever) - serology or PCR on valve tissue.
Bartonella spp. (B. henselae, B. quintana) - serology or PCR.
Chlamydophila (uncommon).
Tropheryma whipplei - Whipple disease; PAS stain on valve tissue, PCR.
HACEK.

Also consider Brucella (exposure history), fungi (especially Candida on prosthetic valves or long-line patients), and prior antibiotic exposure blunting cultures.

Noninfectious (“Sterile”) Endocarditis

No organism is present, so cultures are negative by definition.

Libman-Sacks - lupus. Sterile vegetations on both surfaces of the mitral valve.
Nonbacterial thrombotic (marantic) endocarditis - sterile fibrin-platelet vegetations associated with malignancy, DIC, hypercoagulable states.
Loeffler endocarditis - eosinophilic, endomyocardial fibrosis, restrictive cardiomyopathy.

Blood Culture Protocol and Duke Criteria

At least 3 sets from 3 separate venipuncture sites spaced over time (traditionally over 24 hours; in critical illness the sets can be collected closer together before starting empiric therapy). Draw before antibiotics - a single dose can clear cultures for days.
Each set = aerobic + anaerobic bottle; 4 to 6 bottles total across 2 to 3 sets.
True endocarditis typically shows continuous bacteremia (all sets positive with the same organism); intermittent positivity with skin flora suggests contamination.

Duke criteria define definite IE when:

2 major, or
1 major + 3 minor, or
5 minor.

Major criteria: (1) typical organism from 2+ separate blood cultures, or persistent bacteremia; (2) endocardial involvement on echo (vegetation, abscess, prosthetic dehiscence) or new valvular regurgitation.

Minor criteria: predisposing cardiac condition or IV drug use, fever >38°C, vascular phenomena (Janeway lesions, mycotic aneurysm, conjunctival hemorrhages, septic pulmonary infarcts), immunologic phenomena (Osler nodes, Roth spots, glomerulonephritis, positive rheumatoid factor), and microbiologic evidence not meeting major criteria.

37.9 Infectious Diarrhea

Stool is heavily colonized, so diagnosis relies on selective media (MacConkey, HE, XLD, SS, CIN, sorbitol MacConkey, Campy-CVA), toxin assays (C. difficile), and molecular panels. The differential depends on exposure (travel, food, water, antibiotic use, immune status) and clinical pattern (watery vs inflammatory/bloody vs enteric fever).

Epidemiology

Viruses are the most common cause overall, especially in children. Main viral causes:

Norovirus - most common cause of epidemic gastroenteritis, all ages, cruise-ship and nursing-home outbreaks.
Rotavirus - formerly the leading pediatric cause; vaccine has dramatically reduced incidence.
Enteric adenovirus (serotypes 40, 41), astrovirus, sapovirus.

Most common bacterial causes (roughly in order): E. coli (various pathotypes), Salmonella, Shigella, Campylobacter. Campylobacter is the most common bacterial cause in developed countries; Salmonella dominates in developing countries.

Inflammatory vs Noninflammatory Pattern

The stool pattern narrows the differential.

Inflammatory: fever, fecal WBCs, bloody stool, severe abdominal pain, tenesmus. Mechanism: mucosal invasion or cytotoxin. Organisms: Shigella, non-typhoidal Salmonella, Campylobacter, EHEC, C. difficile, Entamoeba histolytica. Fecal lactoferrin/leukocytes positive; RBCs present.
Noninflammatory: watery diarrhea without WBCs or blood. Mechanism: enterotoxin (secretory) or osmotic. Organisms: viruses, Vibrio cholerae, ETEC, Cryptosporidium, Giardia, S. aureus toxin, B. cereus toxin. Fecal WBCs negative.

Time-to-Onset After Exposure (Foodborne)

Time from exposure to symptoms narrows the differential for foodborne illness.

<6 hours - preformed toxin: S. aureus, Bacillus cereus. Rapid onset because the toxin is already in the food - no incubation needed.
~1 day (6 to 72 hours): Salmonella, Clostridium perfringens, Clostridium botulinum.
>=1 day (1 to 7 days) - invasive or infectious-dose organisms: ETEC, Vibrio vulnificus, EHEC, Campylobacter, Shigella.

The board-testable pattern: symptoms within 6 hours = preformed toxin.

EHEC / Shiga Toxin-Producing E. coli (STEC)

Classic transmission: undercooked ground beef, contaminated milk, fruit, vegetables.
Very low infectious dose (~10 to 100 organisms), so person-to-person spread occurs in households, daycares, and nursing homes.
Produces Shiga toxin, which damages intestinal and renal vascular endothelium. Causes hemorrhagic colitis and hemolytic uremic syndrome (HUS) in 5 to 10%, especially in children.
Antibiotics are contraindicated - they increase toxin release and HUS risk.
Most common strain: E. coli O157:H7. Grows as colorless (sorbitol non-fermenting) colonies on sorbitol MacConkey, whereas normal E. coli ferments sorbitol and appears pink.
Non-O157 STEC strains (O26, O111, O103, O145) collectively cause more disease than O157:H7 and do ferment sorbitol, so they are missed by SMAC alone. Shiga toxin EIA or PCR (stx1/stx2) should be run on all diarrheal stools to pick up both O157 and non-O157.

Laboratory Workflow

Standard “stool culture” targets Salmonella, Shigella, Campylobacter (some labs also include Aeromonas and Plesiomonas). Culture has been largely replaced by multiplex PCR panels (e.g., BioFire FilmArray GI), which detect 20+ bacterial, viral, and parasitic targets in 1 to 3 hours.

Organisms not picked up by routine stool culture and requiring specific requests or media.

EHEC: sorbitol MacConkey + Shiga toxin testing.
Vibrio: TCBS (thiosulfate-citrate-bile salts-sucrose) agar.
Yersinia: CIN agar + cold enrichment.

Stool ova and parasites (O&P) is indicated for diarrhea persisting >7 to 14 days, immunocompromised patients, travel, and daycare outbreaks. Common parasitic causes: Giardia, Cryptosporidium, Cyclospora, Entamoeba histolytica. PCR panels are increasingly replacing O&P.

Multiplex PCR advantages: rapid, broad-range (bacteria/viruses/parasites), high sensitivity, detects fastidious organisms. Limitations: no susceptibility data, may detect colonization/shedding rather than active infection, expensive.

37.10 Prosthetic Joint Infection

PJI is a slow, often biofilm-driven process. Periprosthetic tissue and joint fluid cultures are higher yield than swabs, and multiple samples (at least 3 to 5) are recommended because the organism load is low and many of the organisms are low-virulence skin flora (coagulase-negative Staph, Cutibacterium) that are hard to call as pathogens from a single culture. Sonication of the explanted hardware dislodges biofilm organisms and improves yield.

Organism Map

Most joints (hip, knee): coagulase-negative staphylococci (S. epidermidis) most common, followed by S. aureus, streptococci, enterococci, gram-negative bacilli, and anaerobes.
Shoulder prostheses: Cutibacterium acnes (formerly Propionibacterium acnes). The shoulder is dense in sebaceous glands and hair follicles where C. acnes lives, leading to frequent intraoperative contamination. Causes low-grade, indolent infections that may not manifest for months and may present as persistent shoulder pain without classic signs (afebrile, normal CRP).

Cutibacterium acnes

Anaerobic (or aerotolerant anaerobe), gram-positive rod with diphtheroid/club-shaped morphology.
Normal skin flora - sebaceous glands and hair follicles.
Causes: shoulder prosthesis infection (most common cause), neurosurgical shunt infection, breast implant infection, spinal hardware infection, acne vulgaris; rarely endocarditis.
Slow-growing, requires extended incubation up to 14 to 21 days on anaerobic media. Standard blood cultures (5 to 7 days) may miss it.
Sonication of the explanted prosthesis dislodges biofilm and improves yield.

Diagnostic Approach

Synovial fluid culture alone is insensitive (~60 to 80%) because of biofilm organisms, prior antibiotics, and low bacterial concentrations. Synovial fluid cell count and differential are more useful than culture for screening.
At least 3 intraoperative periprosthetic tissue cultures, held 14 to 21 days under anaerobic conditions, improve sensitivity.
Sonication of explanted hardware disrupts biofilm and releases organisms for culture.
Alpha-defensin synovial fluid biomarker supports diagnosis.
Intraoperative frozen section: >=5 neutrophils per HPF in at least 1 HPF of periprosthetic tissue suggests infection (do not count neutrophils within fibrin, within vessels, or in surface inflammatory infiltrates). Specificity ~95%, sensitivity variable. Guides single-stage vs two-stage revision.

37.11 Vector-Borne Infections

Vectors (ticks, mosquitoes, lice, fleas, sandflies) deliver organisms that are often hard to culture, so diagnosis often relies on serology, PCR, and blood smears. Geographic and exposure history is the key to narrowing the differential.

Mosquitoes

Anopheles (night-biting): Malaria (Plasmodium), lymphatic filariasis (Wuchereria bancrofti, Brugia malayi/timori - with Culex), dog heartworm (Dirofilaria immitis). Resting position at 45° to the surface, spotted wings.
Aedes (day-biting, black-and-white striped): Dengue, yellow fever, chikungunya, Zika. A. aegypti is the primary vector; A. albopictus is a secondary.
Culex: West Nile virus, St. Louis encephalitis, lymphatic filariasis (W. bancrofti in urban areas), Japanese encephalitis.

Ticks

Ixodes (I. scapularis east, I. pacificus west): Lyme disease (Borrelia burgdorferi), Babesiosis (Babesia microti), Anaplasmosis (Anaplasma phagocytophilum), Powassan virus, Tick-borne encephalitis virus (I. ricinus in Europe, I. persulcatus in Asia). Ixodes must feed for 36 to 48 hours to transmit B. burgdorferi. Co-infection with Lyme, Babesia, and Anaplasma from a single tick bite is well-documented.
Dermacentor (wood tick): Rocky Mountain spotted fever (Rickettsia rickettsii), Colorado tick fever (Coltivirus, a reovirus), tularemia. Found in western US/Canada at high elevations.
Amblyomma (lone star tick): Ehrlichia chaffeensis (human monocytic ehrlichiosis), tularemia, alpha-gal syndrome (red meat allergy), Heartland virus.

Colorado tick fever causes a classic saddleback fever pattern (fever, defervescence, fever) and infects erythroid precursors in bone marrow.

Tick-borne encephalitis virus (TBEV) is a flavivirus endemic in Europe and Asia. Transmitted by Ixodes ticks and occasionally via unpasteurized dairy.

Fleas

Rat flea (Xenopsylla cheopis): Yersinia pestis (plague) and Rickettsia typhi (endemic/murine typhus). The flea becomes “blocked” by Y. pestis in its foregut and regurgitates bacteria during feeding. Plague also spreads via direct contact with infected animals and via respiratory droplets in the pneumonic form.
Cat and dog flea (Ctenocephalides): Dipylidium caninum (double-pored tapeworm). The flea is the intermediate host; humans (usually children) get infected by accidentally swallowing fleas during close contact with pets.

Lice

Body louse (Pediculus humanus corporis): Epidemic typhus (Rickettsia prowazekii), Epidemic relapsing fever (Borrelia recurrentis), trench fever (Bartonella quintana). Crushing the louse on abraded skin releases the organism. Associated with crowding, poverty, and war.

Epidemic relapsing fever causes recurring 5 to 7 day febrile episodes because of antigenic variation of variable major proteins (VMPs). Jarisch-Herxheimer reaction is common after starting treatment.

Flies

Sandfly (Phlebotomus in Old World, Lutzomyia in New World): Leishmaniasis and Bartonellosis (Carrion disease - B. bacilliformis). Small (2-3 mm) nocturnal-feeding flies. Leishmania promastigotes are injected during blood feeding, phagocytosed by macrophages, and convert to amastigotes intracellularly.
Black fly (Simulium): Onchocerciasis (Onchocerca volvulus - river blindness). Black flies breed in fast-flowing rivers (the name). Day-biting. Mass ivermectin distribution has drastically reduced disease burden in Africa and the Americas.
Deerfly / mango fly (Chrysops): Loiasis (Loa loa, African eyeworm). Day-biting fly in West/Central African rainforests. Loa loa has diurnal periodicity of microfilariae (blood draw during day). Calabar swellings and subconjunctival worm migration.
Tsetse fly (Glossina): African trypanosomiasis (Trypanosoma brucei - sleeping sickness). Large (6-14 mm), day-biting, scissor-like resting wing position. Both sexes transmit the parasite.
Dung fly (Musca sorbens): mechanical transmission of Chlamydia trachomatis serovars A, B, C causing trachoma, the leading infectious cause of blindness worldwide. Also spread by direct contact (eye-to-eye, fingers, fomites).

Mites

Chigger (larval mite, Leptotrombidium): Scrub typhus (Orientia tsutsugamushi). Asia-Pacific (tsutsugamushi triangle). Eschar at bite site is pathognomonic. Fever, maculopapular rash, lymphadenopathy. Treat with doxycycline.

True Bugs

Reduviid / triatomine / “kissing bug”: Chagas disease (Trypanosoma cruzi). Central and South America. The bug defecates near the bite wound; the patient scratches feces into the wound or a mucous membrane. Acute: chagoma, Romaña sign (unilateral periorbital swelling). Chronic: cardiomyopathy, megacolon, megaesophagus.

Multi-Route: Tularemia and Others

Francisella tularensis is transmitted by ticks (Dermacentor, Amblyomma), deer flies (Chrysops), direct contact with rabbits (rabbit fever), contaminated water/food, and aerosol inhalation. It is highly infectious (10 to 50 organisms) and is a Category A bioterrorism agent. Ulceroglandular form is most common.

37.12 Syndrome-Organism Quick Reference

This section collects the classic “most common cause of X” and “X after Y exposure” associations that are high-yield on boards. They are organized by body system. Details and mechanisms are in the organism-specific chapters; this is a memorization scaffold.

Cardiovascular

Myocarditis (US/Europe, viral): Parvovirus B19 (most common by PCR on endomyocardial biopsy), Coxsackie B, Adenovirus. Also HHV-6, EBV, CMV, influenza, SARS-CoV-2. Non-viral: Trypanosoma cruzi (Chagas, leading cause in Latin America), Borrelia (Lyme carditis).
Bacteremia with colon cancer: Clostridium septicum and Streptococcus bovis (biotype 1 / S. gallolyticus). S. bovis bacteremia or endocarditis always prompts colonoscopy (even if asymptomatic). S. bovis biotype 2 is associated with neonatal meningitis.

Skin and Soft Tissue

Impetigo: S. aureus (most common, bullous form is always S. aureus via exfoliative toxin), S. pyogenes (non-bullous).
Cellulitis/erysipelas: S. pyogenes (erysipelas), S. aureus (purulent cellulitis/abscess). Erysipelas = sharply demarcated, raised, bright red, upper dermis; cellulitis = deeper, less demarcated.
Hot tub / whirlpool folliculitis: Pseudomonas aeruginosa.
Cat scratch: Bartonella henselae (self-limited lymphadenopathy).
Cat bite (rapid cellulitis <24 h): Pasteurella multocida.
Dog bite: Pasteurella canis / multocida; Capnocytophaga canimorsus in asplenic/immunocompromised (overwhelming sepsis, DIC, purpura fulminans).
Freshwater wound: Aeromonas hydrophila (wound/necrotizing fasciitis), Mycobacterium marinum (fish tank granuloma, sporotrichoid nodules, grows at 30°C).
Saltwater / raw oysters: Vibrio vulnificus - rapidly progressive cellulitis with hemorrhagic bullae, especially in liver disease (iron overload). High mortality.
Butcher/fisherman with dorsal hand lesion: Erysipeloid (Erysipelothrix rhusiopathiae). Purplish, well-demarcated lesion. Penicillin-sensitive, resistant to vancomycin (unusual for a gram-positive).
Erythrasma: Corynebacterium minutissimum. Coral-red fluorescence under Wood’s lamp (porphyrin production). Resembles tinea cruris.
Toxic shock syndrome: S. aureus (TSST-1, tampon-associated classically) and S. pyogenes (streptococcal pyrogenic exotoxins, often with nec fasc). Diffuse erythroderma, hypotension, multiorgan failure. S. aureus TSS has desquamation of palms/soles during recovery.
Necrotizing fasciitis: polymicrobial (Type I, diabetics/post-surgical) most common overall; S. pyogenes (Type II) in healthy young adults, rapid progression, toxic shock; Clostridium perfringens (gas gangrene); V. vulnificus (saltwater).
Botryomycosis: S. aureus (also Pseudomonas). Chronic suppurative infection forming bacterial granules with Splendore-Hoeppli material - mimics actinomycosis but cultures staph.
Rhinoscleroma: Klebsiella rhinoscleromatis. Chronic granulomatous nose infection with Mikulicz cells (foamy macrophages) and Russell bodies.
Glanders: Burkholderia mallei. Horses/mules primarily; rare in humans. B. mallei is the only non-motile Burkholderia. Category B bioterrorism agent.
Melioidosis: Burkholderia pseudomallei. Tropical soil/water (SE Asia, northern Australia). Most common presentation: pneumonia, can mimic TB. Diabetes is the key risk factor.
Mycetoma (Madura foot): actinomycotic caused by Actinomadura, Nocardia, Streptomyces (bacterial); eumycotic caused by Scedosporium (US most common) or Madurella mycetomatis (worldwide most common).
Sporotrichosis: Sporothrix schenckii. Rose gardener’s disease - lymphocutaneous nodules tracking along lymphatics from thorn prick.
Chromoblastomycosis: Phialophora verrucosa, Cladophialophora carrionii, Fonsecaea pedrosoi. Muriform / sclerotic / Medlar bodies (“copper pennies”) in tissue are pathognomonic.
Lobomycosis: Lacazia loboi (Amazon basin, also dolphins). Keloid-like nodules; cannot be cultured.
Cutaneous larva migrans: Ancylostoma braziliense (dog/cat hookworm). Serpiginous pruritic tracks on bare feet; dead-end in humans.
Rat-bite fever: Streptobacillus moniliformis (North America, also Haverhill fever from food/water) or Spirillum minus (Asia, Sodoku). Fever, migratory polyarthritis, maculopapular rash.

Head, Eyes, ENT

Pharyngitis (bacterial): S. pyogenes (Group A Strep). Complications: rheumatic fever, post-strep glomerulonephritis.
Whooping cough: Bordetella pertussis. Three phases (catarrhal, paroxysmal with inspiratory whoop, convalescent). PCR on NP swab; culture on Regan-Lowe or Bordet-Gengou. Lymphocytosis characteristic.
Acute epiglottitis: Haemophilus influenzae type b (pre-Hib); post-Hib, more often adults, and S. pneumoniae/S. aureus/GAS. Thumbprint sign on lateral neck X-ray. Tripod position, drooling, hot-potato voice.
Otitis media: S. pneumoniae, non-typeable H. influenzae, Moraxella catarrhalis.
Chancroid: Haemophilus ducreyi. Painful genital ulcer + painful unilateral inguinal lymphadenopathy (distinguish from painless syphilitic chancre). Chocolate agar with vancomycin.
Lymphogranuloma venereum: Chlamydia trachomatis serovars L1, L2, L3. Primary painless papule, then painful inguinal lymphadenopathy (groove sign: nodes above and below the inguinal ligament).
Croup (acute laryngotracheobronchitis): Parainfluenza types 1-3 (type 1 most common). Barking cough, inspiratory stridor, steeple sign on CXR.
Trachoma: C. trachomatis serovars A, B, C. Leading infectious cause of blindness worldwide. Transmitted by direct contact and Musca sorbens dung flies.
Fungal otitis externa (otomycosis): Aspergillus niger (~90%), Candida albicans.
Juvenile periodontitis: Aggregatibacter actinomycetemcomitans (HACEK).

GI and Peritoneum

Pseudomembranous colitis: Clostridioides difficile. Toxins A (enterotoxin) and B (cytotoxin). Risk factors: antibiotics (clindamycin, fluoroquinolones, cephalosporins), hospitalization, PPIs, age >65.
Preformed-toxin food poisoning in <6 hours: S. aureus, Bacillus cereus (emetic form from leftover rice/pasta - cereulide). B. cereus diarrheal form (8-16 h) is from heat-labile enterotoxin produced in vivo.
Balantidium coli: only ciliated protozoan human pathogen; pig-feces-contaminated water; dysentery.
Primary (spontaneous) bacterial peritonitis: E. coli most common in cirrhosis/ascites adults, followed by Klebsiella and S. pneumoniae (S. pneumoniae in pediatric nephrotic syndrome). SBP is monomicrobial with ascitic PMN >250/uL.
Secondary bacterial peritonitis: polymicrobial (E. coli, Enterococcus, B. fragilis, other enterics) - bowel perforation.

Musculoskeletal

Osteomyelitis overall: S. aureus (50-70%). CoNS on prosthetic joints, gram-negatives in diabetic foot, Pseudomonas in puncture wounds through sneakers.
Osteomyelitis in sickle cell disease: Salmonella (typhi/paratyphi) classically, plus S. aureus. Functional asplenia impairs clearance of intracellular organisms.
Monoarticular septic arthritis (children and adults): S. aureus.
Septic arthritis in IV drug users: MRSA. Unusual joints may be involved (sacroiliac, sternoclavicular, pubic symphysis).
Polyarticular septic arthritis in young adults: Neisseria gonorrhoeae (disseminated gonococcal infection - migratory polyarthritis/tenosynovitis, pustular dermatitis, then septic monoarthritis).

Respiratory

Viral pneumonia in infants <12 months: RSV.
Viral pneumonia in adults: Influenza (orthomyxovirus). Oseltamivir/zanamivir within 48 hours.

CNS

HSV-1 encephalitis: temporal lobe hemorrhagic necrosis; start acyclovir empirically.
HSV-2 meningitis (Mollaret recurrent lymphocytic meningitis).
Fungal meningitis: Cryptococcus neoformans (HIV, CD4 <100). India ink + CrAg.
SSPE (subacute sclerosing panencephalitis): reactivated measles 7 to 10 years post-infection. Progressive neurologic deterioration, periodic high-amplitude EEG complexes. No treatment.
PML: JC virus in immunocompromised (AIDS CD4 <200, natalizumab, rituximab).
Primary amebic meningoencephalitis (PAM): Naegleria fowleri. Warm freshwater through nasal cavity to brain. >95% mortality. Motile trophozoites in CSF.
Granulomatous amebic encephalitis (GAE): Acanthamoeba, Balamuthia mandrillaris. Subacute/chronic in immunocompromised; trophozoites AND cysts in brain.

Obstetric/Gynecologic

Septic abortion / postpartum toxic shock: Clostridium sordellii. Absence of fever (or hypothermia), leukemoid reaction (WBC >50,000), hemoconcentration, refractory hypotension. High mortality.

Viral Diseases with Named Rashes

Measles: Rubeola virus (Paramyxovirus). 3 Cs + Koplik spots, then rash head to toe. Complications: pneumonia (leading cause of death), encephalitis, SSPE.
German measles: Rubella virus (Togavirus). Posterior auricular/postoccipital lymphadenopathy. Congenital rubella syndrome: sensorineural deafness, cataracts, PDA.
Erythema infectiosum (fifth disease / slapped cheek): Parvovirus B19. Aplastic crisis in sickle cell, hydrops fetalis in pregnancy, chronic pure red-cell aplasia in immunocompromised. Infects erythroid progenitors via P antigen.
Hand-foot-and-mouth: Coxsackie A (A16; also Enterovirus 71).

Human Herpesviruses Numbering

HHV-1 = HSV-1 (oral).
HHV-2 = HSV-2 (genital).
HHV-3 = VZV (chickenpox/shingles).
HHV-4 = EBV (mono, Burkitt, nasopharyngeal carcinoma).
HHV-5 = CMV (congenital, transplant).
HHV-6 and HHV-7 = Roseola (exanthem subitum).
HHV-8 = KSHV (Kaposi sarcoma, primary effusion lymphoma, multicentric Castleman).

All herpesviruses establish latency.

Visceral and Parasitic

Visceral larva migrans: Toxocara canis/cati (dog/cat roundworm). Children ingest eggs from contaminated soil. Hepatomegaly, pulmonary symptoms, ocular larva migrans (retinal granuloma, mimics retinoblastoma). Eosinophilia and elevated IgE. Albendazole.
San Joaquin Valley fever: Coccidioides immitis (southwest US, northern Mexico). Inhaled arthroconidia; spherules with endospores in tissue.
Carrion disease (Oroya fever + verruga peruana): Bartonella bacilliformis, Andean South America, sandfly vector. Acute hemolytic anemia (infects RBCs directly), chronic warty skin lesions.
Plague: Yersinia pestis - bipolar “safety pin” staining. Bubonic, pneumonic, septicemic forms. Rat flea vector; also droplet and direct contact.
Leprosy: Mycobacterium leprae. Cannot be cultured on artificial media (obligate intracellular). Grows at 30-33°C (skin, peripheral nerves, nasal mucosa). Tuberculoid (paucibacillary, strong CMI) vs lepromatous (multibacillary, weak CMI). Multidrug therapy.

Brucellosis (Undulant Fever) by Animal Exposure

Brucella species cause undulant fever - cycling fevers, night sweats, hepatosplenomegaly, arthritis, sacroiliitis. Transmitted through contact with infected animals and unpasteurized dairy.

Cattle and bison: B. abortus (most common cause of human brucellosis worldwide).
Pigs: B. suis (mnemonic: suis = swine). Higher rates of abscess formation.
Goats, sheep, camels: B. melitensis (most virulent species, most severe human disease). Common in Mediterranean, Middle East, Central/South America; unpasteurized goat cheese/milk.
Dogs: B. canis (milder, less common; occupational risk for vets and breeders).

Congenital / Perinatal (TORCH)

TORCH infections are transmitted vertically (transplacental, intrapartum, or postpartum via breastmilk). Common features: hepatosplenomegaly, jaundice, thrombocytopenia, IUGR, microcephaly, rash.

T = Toxoplasmosis (diffuse intracranial calcifications, chorioretinitis).
O = Other - syphilis, hepatitis B, Zika, HIV, varicella, parvovirus B19.
R = Rubella (cataracts, PDA, sensorineural deafness).
C = Cytomegalovirus (periventricular calcifications).
H = Herpes simplex (intrapartum exposure is main route).

Transmission route affects timing and which organisms matter.

Transplacental (in utero): rubella, CMV, toxoplasmosis, syphilis, parvovirus B19, Zika.
Intrapartum: HSV, HIV, HBV, GBS.
Postpartum (breastfeeding): HIV, HTLV-1, CMV.

First-trimester maternal infections tend to produce the most severe fetal effects (active organogenesis).

Live-Attenuated Vaccine Notes

MMR (live attenuated): mild measles-like rash and fever 7 to 12 days post-vaccination in 5 to 15% of recipients. Normal, self-limited, not a contraindication to the second dose.
Varicella (Oka strain, live attenuated): vaccine virus establishes latency in dorsal root ganglia and can reactivate as mild shingles later in life. Zoster risk is lower than after natural chickenpox. Adults >=50 should still get Shingrix regardless of prior varicella immunization status.
Oral polio (Sabin, OPV) can revert to virulence during GI replication, causing vaccine-associated paralytic polio (VAPP, ~1 per 2.4 million doses) and circulating vaccine-derived poliovirus (cVDPV) outbreaks in under-immunized populations. The US uses IPV (Salk, killed) exclusively; OPV still used globally for mucosal immunity in eradication efforts.

Rickettsia and Relatives - Intracellular Lifestyle

Rickettsiae (includes Rickettsia, Ehrlichia, Anaplasma, Bartonella) are obligate intracellular gram-negative organisms with leaky membranes that cannot maintain coenzyme pools without host cytoplasmic contents. They import host ATP and NAD via specific transporters.

Entry: induced phagocytosis, then rapid escape from the phagosome to reside in the host cytoplasm. Contrast: Chlamydia stays in an inclusion body; Coxiella burnetii thrives in the phagolysosome.
Replication: binary fission in the cytoplasm. Rickettsia uses actin-based motility (like Listeria and Shigella) to push into adjacent cells, avoiding antibodies. Rickettsia can briefly survive extracellularly; Chlamydia cannot.
Staining: too small for Gram stain (~0.3 to 0.5 um). Giemsa stains them purple-blue; immunofluorescence (DFA/IFA) and Warthin-Starry silver are alternatives; PCR is the most sensitive diagnostic.

Chapter 38: Gram-Positive Cocci

The gram-positive cocci include some of the most important human pathogens. Two genera dominate clinical practice: Staphylococcus (catalase-positive) and Streptococcus/Enterococcus (catalase-negative). This simple catalase test - dropping hydrogen peroxide on colonies and watching for bubbles - is the first branch point in identifying any gram-positive coccus. All Staphylococcus spp. are catalase-positive and all Streptococcus spp. (including Enterococcus) are catalase-negative. A common board question: what single test differentiates Staphylococcus from Streptococcus? Catalase.

38.1 Staphylococcus

Staphylococci are gram-positive cocci that divide in multiple planes, producing irregular clusters that resemble bunches of grapes (the name comes from the Greek “staphyle,” meaning grape). All staphylococci produce catalase, the enzyme that breaks down hydrogen peroxide into water and oxygen. This distinguishes them from streptococci, which are catalase-negative. Staphylococci are non-motile, non-spore-forming, and facultatively anaerobic. They’re hardy - they survive on environmental surfaces (doorknobs, stethoscopes) for a long time, which matters for nosocomial spread.

The next critical distinction is coagulase, an enzyme that clots plasma. Staphylococcus aureus is coagulase-positive; all other staphylococci are coagulase-negative. This matters because S. aureus is far more virulent and requires different treatment considerations. Two exceptions deserve mention: S. intermedius and S. pseudintermedius are also coagulase-positive but are primarily animal pathogens (rare in human disease). S. lugdunensis is coagulase-negative by tube but often positive by slide, which trips up identification - more on this below.

Colonization epidemiology

S. aureus colonizes the anterior nares in ~50% of individuals (persistently in 20-30%, intermittently in up to 50%). Nasal carriage is a risk factor for surgical site infections, nosocomial infections, and recurrent skin infections. Pre-surgical decolonization with mupirocin nasal ointment plus chlorhexidine body washes reduces post-op S. aureus infection rates, particularly in cardiac and orthopedic surgery. For context on other bacterial colonization rates worth memorizing:

S. pneumoniae colonizes the oropharynx of ~5% of adults and up to 50% of children (especially daycare attendees). Carriage precedes invasive disease and goes up when host defenses drop (viral URI, asplenia, immunodeficiency).
GBS colonizes the rectum/vagina of ~20% of pregnant women (literature ranges 10-40%). This is why universal screening at 35-37 weeks exists.
N. meningitidis colonizes the oro/nasopharynx of ~5-15% of the population. Carriage is usually asymptomatic but provides the reservoir for outbreaks.
C. difficile colonizes the colon of ~30% of healthy infants (their gut lacks toxin receptors, so carriage is asymptomatic), less than 3% of healthy adults, and up to 20% of long-term care residents.

Staphylococcus aureus: The Superbug

S. aureus is the single most important bacterial pathogen in clinical medicine. It causes disease ranging from minor skin infections to life-threatening bacteremia and endocarditis. Its name comes from the golden (aureus) pigment its colonies produce.

Why S. aureus is so pathogenic: This organism possesses an extraordinary array of virulence factors that allow it to evade immune defenses, adhere to tissues, invade, and cause damage.

The coagulase enzyme that defines S. aureus contributes to its pathogenicity by coating bacteria with fibrin, hiding them from phagocytes. Coagulase is positive on both slide (bound coagulase / clumping factor) and tube (free coagulase) tests for S. aureus - that double positivity is what separates it from S. lugdunensis, which is slide+ but tube- (more below). Protein A binds the Fc portion of antibodies in the wrong orientation, preventing opsonization. The golden pigment (staphyloxanthin) is actually an antioxidant that protects against neutrophil-generated reactive oxygen species. S. aureus also frequently produces a polysaccharide capsule (types 5 and 8 are ~75% of clinical isolates) that is anti-phagocytic - encapsulated strains are more virulent and capsule contributes to biofilm formation on prosthetic devices.

For tissue destruction, S. aureus produces numerous toxins. S. aureus is beta-hemolytic, mediated primarily by alpha-hemolysin (also called alpha-toxin, Hla). The nomenclature is confusing: the alpha-toxin is what produces beta-hemolysis. Alpha-hemolysin inserts heptameric pores into host cell membranes - RBCs, platelets, monocytes, endothelial cells - and is the single most important cytotoxin of S. aureus. Note: all other Staphylococci are gamma-hemolytic (non-hemolytic), so beta-hemolysis in a catalase+, coagulase+ GPC fits. S. aureus also makes beta-hemolysin (a sphingomyelinase - this is what CAMP factor potentiates in the GBS CAMP test), delta-hemolysin, and gamma-hemolysin, plus proteases, lipases, and hyaluronidase that break down tissue barriers. Panton-Valentine leukocidin (PVL) destroys neutrophils and is associated with the severe necrotizing pneumonia and skin infections seen with community-acquired MRSA strains.

Some S. aureus strains produce superantigen toxins that cause specific toxin-mediated diseases. Toxic shock syndrome toxin-1 (TSST-1) causes toxic shock syndrome. As a superantigen, TSST-1 cross-links MHC class II on APCs with the Vbeta region of the T-cell receptor, bypassing normal antigen processing and non-specifically activating up to 20% of T cells. The resulting cytokine storm (TNF-alpha, IL-1, IL-2, IFN-gamma) drives the classic clinical picture: fever, diffuse macular (“sunburn-like”) erythroderma, hypotension, and multiorgan failure, with desquamation of palms and soles 1-2 weeks later. Classic presentation is a menstruating woman with acute fever, myalgias, diarrhea, progressing to shock and diffuse rash. Original cases were associated with retained superabsorbent tampons (menstrual TSS), but now most cases are non-menstrual TSS from surgical wound infections, burns, and nasal packing, which carries higher mortality. Important gotcha: blood cultures are typically negative in TSS because it is toxin-mediated, not bacteremia. Treatment: fluid resuscitation, remove the foreign body, anti-staphylococcal antibiotics plus clindamycin to shut off toxin production.

Exfoliative toxins A and B (also called epidermolytic toxins) cause staphylococcal scalded skin syndrome (SSSS). These are serine proteases that specifically cleave desmoglein-1 in the granular layer of the epidermis, causing intraepidermal splitting with widespread bullae and sheets of epidermal exfoliation. Nikolsky sign is positive. SSSS primarily affects neonates and young children (immature renal clearance of toxin); adults can develop SSSS if immunocompromised or in renal failure. Key histologic distinction from TEN: SSSS splits superficially at the granular layer with no necrotic keratinocytes (TEN = full-thickness epidermal necrosis at the dermal-epidermal junction).

Enterotoxins A-E cause staphylococcal food poisoning. Enterotoxin A is the most common. These toxins are preformed in food, heat-stable (survive cooking that kills the bacteria), and resistant to GI proteases. They are superantigens that stimulate the vagus nerve and vomiting center. Rapid onset (1-6 hours) of prominent vomiting, cramps, watery diarrhea, no fever, self-limited within 24 hours. Classic sources are hand-prepared foods held at room temperature (potato salad, custard, cream pastries, ham). That very short incubation period is what distinguishes S. aureus food poisoning from most other bacterial gastroenteritis.

Identification in the laboratory: S. aureus grows readily on blood agar, producing beta-hemolytic colonies, often with a golden pigment (though not all strains are pigmented, and you can’t rely on color alone). The key tests are catalase (positive, like all staphylococci) and coagulase (positive, distinguishing it from coagulase-negative staphylococci). The tube coagulase test is definitive - mixing colonies with rabbit plasma causes visible clotting within 4 hours. The slide coagulase test detects clumping factor, a surface-bound protein - it’s faster but less specific.

Mannitol salt agar (MSA) exploits two S. aureus features: tolerance of high salt (7.5% NaCl, which inhibits most bacteria) and fermentation of mannitol (which turns the pH indicator yellow). Yellow colonies on MSA are presumptively S. aureus.

The clinical spectrum - from trivial to fatal: S. aureus causes infections ranging from pimples to endocarditis. S. aureus is the #1 cause of right-sided endocarditis (IVDU-associated), osteomyelitis, septic arthritis, and toxin-mediated diseases. It is also the most common cause of healthcare-associated bloodstream infections, skin and soft tissue infections (abscesses, cellulitis, wound infections), and the second most common cause of nosocomial pneumonia. Skin and soft tissue infections are most common: folliculitis (hair follicle infection), furuncles (boils - deeper infections), carbuncles (interconnected furuncles), impetigo (honey-crusted lesions - also caused by GAS), and cellulitis. S. aureus is also the most common cause of lactational mastitis and of surgical wound infections. These can be trivial or can progress to invasive disease.

When S. aureus enters the bloodstream, it causes bacteremia that metastasizes to distant sites with frightening efficiency. S. aureus endocarditis can destroy heart valves within days. Osteomyelitis typically involves the vertebrae or long bones. Septic arthritis destroys joints. Pneumonia, particularly following influenza, is necrotizing and often fatal. The triad of bacteremia, spine pain, and elevated inflammatory markers should prompt urgent imaging for vertebral osteomyelitis. Clinical pearl: S. aureus bacteremia always requires echocardiography to rule out endocarditis and a minimum of 4-6 weeks of IV antibiotics.

Beta-lactam mechanism (quick reminder): Beta-lactam antibiotics (penicillins, cephalosporins, carbapenems, monobactams) bind penicillin-binding proteins (PBPs / transpeptidases) that cross-link peptidoglycan chains. By inhibiting transpeptidation (the final step of cell wall synthesis), they weaken the cell wall and cause osmotic lysis of actively growing bacteria. Bactericidal.

MRSA - the beta-lactam resistant epidemic: Methicillin-resistant S. aureus carries the mecA gene (less commonly mecC), which sits on the staphylococcal cassette chromosome mec (SCCmec) and encodes an altered penicillin-binding protein called PBP2a. Normal PBPs are the targets of beta-lactam antibiotics. PBP2a has a structurally modified active site with very low affinity for beta-lactams but retains transpeptidase activity, so cell wall synthesis continues even at high antibiotic concentrations. The result is resistance to essentially all beta-lactams (penicillins, cephalosporins, carbapenems), with ceftaroline and ceftobiprole (5th-gen cephalosporins with PBP2a activity) as the exceptions. MRSA is typically susceptible to vancomycin, daptomycin, linezolid, TMP-SMX (skin infections), and doxycycline.

Two distinct epidemics occurred. Healthcare-associated MRSA (HA-MRSA) emerged in hospitals in the 1960s and spread globally. These strains are often resistant to multiple non-beta-lactam antibiotics as well. Community-associated MRSA (CA-MRSA) emerged in the 1990s in people without healthcare exposure. CA-MRSA strains often carry Panton-Valentine leukocidin (PVL), a pore-forming toxin that destroys neutrophils and is associated with severe skin infections and necrotizing pneumonia in otherwise healthy young people.

Laboratory detection of MRSA:

Cefoxitin disk (30 µg) diffusion test is the CLSI-recommended phenotypic method. Cefoxitin is a better inducer of mecA expression than oxacillin and has replaced the old oxacillin (1 µg) disk. Zone ≤21 mm = MRSA, ≥22 mm = MSSA.
Mannitol salt agar supports growth (yellow colonies) but is not specific for MRSA.
Chromogenic MRSA media (CHROMagar MRSA, BBL CHROMagar) contain cefoxitin plus chromogenic substrates - MRSA colonies appear mauve/pink. Used for rapid nasal surveillance.
Latex agglutination for PBP2a: latex beads coated with anti-PBP2a monoclonal antibodies agglutinate when mixed with a colony extract of MRSA. Results in minutes. Used for rapid confirmation.
PCR for mecA (or the SCCmec-orfX junction) is the molecular gold standard. Real-time PCR (Cepheid GeneXpert, BD MAX, Roche LightCycler) detects MRSA in nasal swabs within 1-2 hours and is widely used for pre-op screening, ICU admission screening, and outbreak investigation.

VISA and VRSA - the vancomycin problem: Reduced vancomycin susceptibility comes in two flavors.

VISA (vancomycin-intermediate S. aureus) has vancomycin MIC 4-8 μg/mL. The mechanism is progressive cell wall thickening: increased peptidoglycan layers trap vancomycin in the outer cell wall before it can reach its D-Ala-D-Ala target on the inner membrane. VISA typically emerges after prolonged vancomycin therapy. Treat with daptomycin, linezolid, or TMP-SMX per susceptibilities.

VRSA (vancomycin-resistant S. aureus) has vancomycin MIC ≥16 μg/mL. Mechanism is the vanA gene acquired from VRE via conjugative plasmid transfer in patients co-colonized with both organisms. vanA modifies the peptidoglycan precursor terminus from D-Ala-D-Ala to D-Ala-D-Lac, which vancomycin cannot bind. VRSA is extremely rare but important to recognize. Both VISA and VRSA strains are usually also MRSA.

Inducible clindamycin resistance and the D-test: Both macrolides (erythromycin, azithromycin) and lincosamides (clindamycin) bind the 23S rRNA of the 50S ribosomal subunit. The erm gene (erythromycin ribosome methylase) methylates that shared binding site and confers resistance to macrolides, lincosamides, and streptogramin B - the MLSB phenotype. erm can be constitutively expressed (resistant to all three outright) or inducible (erythromycin induces expression during therapy, causing clindamycin to fail mid-treatment even though the isolate appeared susceptible).

The D-test detects inducible resistance. Place an erythromycin (15 µg) disk and a clindamycin (2 µg) disk 15-20 mm apart on a lawn of the organism on Mueller-Hinton agar. After overnight incubation, if the clindamycin zone is flattened (D-shaped) on the side facing the erythromycin disk, the test is positive - report clindamycin as resistant. A round clindamycin zone = D-test negative = clindamycin is safe to use. A positive D-test means erythromycin has induced erm expression in the organisms near the interface, and treating with clindamycin would select for constitutive MLSB mutants.

GPC identification algorithms - standard results for the common staphylococci:

Organism	Catalase	Slide coag	Tube coag	PYR / ornithine decarb	Novobiocin
S. aureus	+	+	+	Negative	Sensitive
S. lugdunensis	+	+	Negative	Positive	Sensitive
S. epidermidis	+	Negative	Negative	Negative	Sensitive
S. saprophyticus	+	Negative	Negative	Negative	Resistant

Coagulase-Negative Staphylococci (CoNS)

The coagulase-negative staphylococci present a clinical conundrum: they’re the most common organisms isolated from blood cultures, yet most of the time they’re contaminants. Distinguishing true infection from contamination - and knowing when contamination itself has become infection - requires understanding both the organisms and the clinical context.

Staphylococcus epidermidis is the prototypical CoNS and the most common species isolated from clinical specimens. It’s a normal skin inhabitant and almost always reaches culture bottles through skin contamination during phlebotomy. But S. epidermidis is also a legitimate pathogen in specific settings: it’s the leading cause of prosthetic device infections, including prosthetic valve endocarditis, vascular graft infections, and infected orthopedic hardware.

The key to S. epidermidis pathogenicity is biofilm formation. The organism produces polysaccharide intercellular adhesin (PIA) that allows it to stick to plastic and metal surfaces and form multilayered communities. Within biofilms, bacteria are protected from antibiotics and immune defenses. This is why device infections are so difficult to cure without device removal - antibiotics can suppress planktonic bacteria but can’t eradicate biofilm.

Interpreting positive cultures: When CoNS grows from a blood culture, consider: Was there a single positive bottle or multiple? Were the same species and antibiogram present in separate cultures? Does the patient have a central line, prosthetic heart valve, or other hardware? A single positive blood culture from a patient without devices is almost certainly contamination. Multiple positive cultures with the same organism from a patient with a central line probably represents true catheter-related bloodstream infection.

Staphylococcus saprophyticus is unique among CoNS because it has a well-defined clinical niche: urinary tract infections in young, sexually active women (15-25 years). It’s the second most common cause of uncomplicated UTI in this demographic after E. coli. The organism has specific adherence to urogenital epithelium and produces urease (may contribute to stone formation). Classic setup: “honeymoon cystitis” after sexual activity. S. saprophyticus is novobiocin-resistant; most other CoNS (including S. epidermidis) are novobiocin-sensitive, so a simple novobiocin disk test nails the identification. S. saprophyticus is also intrinsically resistant to fosfomycin - important for treatment choice. Standard therapy: TMP-SMX, nitrofurantoin, or a fluoroquinolone.

Staphylococcus haemolyticus is normal skin flora and the second most frequently isolated CoNS from blood cultures after S. epidermidis. It causes line-associated infections (catheter bacteremia, UTI from catheters) and shares S. epidermidis’ biofilm tropism for prosthetic surfaces. The concerning feature is its high rate of antibiotic resistance, including frequent methicillin resistance and emerging reduced vancomycin susceptibility (heterogeneous VISA phenotype), which can make treatment difficult. When isolated from a sterile site with clinical signs, treat as a real pathogen - don’t dismiss as contaminant.

Staphylococcus lugdunensis deserves special attention because it’s a CoNS that behaves like S. aureus. It causes aggressive native valve endocarditis, deep-seated abscesses, and skin infections with virulence matching S. aureus. It produces a bound coagulase (clumping factor) that can cause positive slide coagulase tests, potentially leading to misidentification as S. aureus - but the tube coagulase test is negative. S. lugdunensis is also PYR-positive and ornithine decarboxylase-positive (both negative in S. aureus), which is how the lab confirms the distinction. Unlike most CoNS, S. lugdunensis is usually beta-lactam susceptible. Treat it as aggressively as S. aureus.

Non-Staphylococcus catalase-positive GPC

When a catalase-positive GPC does not fit the Staphylococcus algorithm (wrong colony morphology, oxidase-positive, unusual arrangement), consider these organisms. MALDI-TOF resolves the identification quickly.

Micrococcus spp. are large gram-positive cocci arranged in tetrads (packets of 4) or irregular clusters, produced by division in two perpendicular planes. Colonies are bright yellow, smooth, non-hemolytic - which is exactly why they can be mistaken for S. aureus (also yellow on MSA, golden on blood agar). Key differentiators:

Micrococcus is strictly aerobic; S. aureus is facultatively anaerobic. The anaerobic O-F glucose test is the most reliable tiebreaker - Micrococcus does not produce acid from glucose anaerobically, S. aureus does.
Micrococcus is oxidase-positive; S. aureus is oxidase-negative.
Micrococcus is coagulase-negative.
Bacitracin: Micrococcus is susceptible; Staphylococcus is resistant.

Micrococcus is part of normal skin/mucosal flora and is almost always a contaminant when isolated from clinical specimens. Rare true infections occur in immunocompromised hosts (endocarditis, peritonitis, CSF shunt meningitis). A single positive blood culture bottle with no clinical picture = contamination.

Rothia mucilaginosa (formerly Stomatococcus mucilaginosus) is a large gram-positive coccus that is normal oral and respiratory flora. Its trademark is mucoid, slimy, dome-shaped colonies that adhere firmly to agar (you can barely pick them with a loop). It is weakly catalase-positive, which is a common source of misclassification: a weak positive may be called negative (pushing identification toward Streptococcus) or positive (toward Staphylococcus/Micrococcus). The mucoid morphology plus gram-positive coccus plus weak catalase should make you think Rothia. Confirm with MALDI-TOF. Rothia causes opportunistic bacteremia, endocarditis, meningitis, and peritonitis in dialysis patients.

Summary - catalase-positive GPC that are NOT Staphylococcus: Micrococcus (tetrads, yellow, strictly aerobic, oxidase+), Rothia mucilaginosa (mucoid, weakly catalase+), and Kocuria (related to Micrococcus).

38.2 Streptococcus

Streptococci are gram-positive cocci that divide in one plane, producing chains or pairs (the name comes from “streptos,” meaning twisted chain). The key feature distinguishing them from staphylococci is catalase - streptococci are catalase-negative. Hydrogen peroxide dropped on colonies produces no bubbles.

Classification: Hemolysis and Lancefield Groups

Streptococci are classified by two complementary systems:

Hemolysis on blood agar provides immediate visual classification. Beta-hemolysis (complete clearing around colonies) indicates complete RBC lysis - the most pathogenic streptococci (Groups A, B) are beta-hemolytic. Alpha-hemolysis (greenish discoloration) indicates partial hemolysis with hemoglobin oxidation - S. pneumoniae and the viridans group produce this pattern. Gamma-hemolysis (no change) means no hemolysis.

Lancefield grouping classifies beta-hemolytic streptococci by cell wall carbohydrate antigens. Rebecca Lancefield developed this serological scheme in the 1930s, and it remains clinically useful: Group A (S. pyogenes), Group B (S. agalactiae), Groups C and G (various species), and so on.

Quick identification reference for the common streptococci:

Test	GAS (pyogenes)	GBS (agalactiae)	Group C/G (dysgalactiae)	S. pneumoniae	Viridans	Enterococcus	Group D (gallolyticus)
Hemolysis	β	β (narrow)	β	α	α (or γ)	γ/α	γ/α
PYR	+	Negative	Negative	-	-	+	Negative
Bacitracin	Sensitive	Resistant	Resistant	-	-	-	-
CAMP	-	+	-	-	-	-	-
Hippurate	-	+	-	-	-	-	-
Optochin	-	-	-	Sensitive	Resistant	-	-
Bile solubility	-	-	-	Soluble	Insoluble	-	-
Bile esculin	-	-	-	-	-	+	+
6.5% NaCl	-	-	-	-	-	+	Negative

Streptococcus pyogenes (Group A Streptococcus, GAS)

GAS is one of the most important human pathogens, causing diseases ranging from trivial pharyngitis to rapidly fatal necrotizing fasciitis. It’s exclusively a human pathogen - transmission is person-to-person.

Laboratory identification: GAS produces beta-hemolysis on blood agar. Two simple tests provide presumptive identification: bacitracin-sensitive (a small “A” disk, 0.04 U, inhibits growth) and PYR-positive (pyrrolidonyl arylamidase, which is also positive in enterococci - enterococci are distinguished by bile esculin and 6.5% NaCl growth). Together PYR+ and bacitracin-sensitive have ~99% sensitivity/specificity for GAS; bacitracin alone is ~95% sensitive. Lancefield grouping (Group A carbohydrate antigen) or rapid antigen tests confirm. Heads up: S. anginosus group and S. dysgalactiae can cross-react with Lancefield Group A antisera, mimicking GAS - colony size helps (S. anginosus group = pinpoint; S. dysgalactiae = large; GAS = large).

Virulence factors - why GAS is so dangerous: The M protein is the most important virulence factor. This surface-anchored coiled-coil protein extends from the cell wall like hair and binds complement regulatory proteins (Factor H, C4BP), preventing C3b deposition and opsonization. The hypervariable N-terminus generates over 200 M types. Type-specific antibodies are protective, but the antigenic diversity means one M-type infection doesn’t protect against others. Certain M types (M1, M3, M12, M18, M49) are associated with specific diseases (rheumatic fever, glomerulonephritis).

The hyaluronic acid capsule provides additional antiphagocytic protection and has a clever twist - it’s chemically identical to human hyaluronic acid (molecular mimicry), so the immune system doesn’t recognize it as foreign. Both M protein and the capsule are antiphagocytic, but only M protein is antigenic.

GAS produces two hemolysins:

Streptolysin O is oxygen-labile (active only anaerobically, produces subsurface hemolysis on blood agar). It is a cholesterol-dependent cytolysin and is immunogenic - the ASO (antistreptolysin O) titer measures antibodies against it and documents recent GAS infection (peaks 3-5 weeks after infection).
Streptolysin S is oxygen-stable, active aerobically, and produces the visible surface beta-hemolysis on blood agar plates. It is a small peptide (~2.7 kDa) and is non-immunogenic (no antibody response, cannot be used serologically).

The streptococcal pyrogenic exotoxins (SpeA, SpeB, SpeC) are superantigens that cross-link MHC Class II molecules to T-cell receptors non-specifically, causing massive T-cell activation and cytokine storm. They are carried on lysogenic bacteriophages (e.g., phage T12 carries speA) - phage conversion is what makes a harmless GAS strain into a scarlet-fever or STSS strain. SpeA is responsible for both scarlet fever and streptococcal toxic shock syndrome. Scarlet fever = GAS pharyngitis in a patient without anti-toxin antibodies, producing a diffuse sandpaper-textured erythematous rash that starts on the trunk, circumoral pallor, “strawberry tongue,” and Pastia’s lines (accentuation in skin folds). Additional spreading factors: streptokinase, hyaluronidase, and DNase.

The spectrum of GAS disease:

Suppurative infections range from minor to catastrophic. Pharyngitis (“strep throat”) is the most common - fever, exudative pharyngitis, tender cervical lymphadenopathy. Scarlet fever is pharyngitis plus a diffuse erythematous rash (from pyrogenic exotoxins), with sandpaper texture and pastia lines (accentuation in skin folds). Impetigo is superficial skin infection with honey-crusted lesions. Cellulitis and erysipelas (sharply demarcated, raised cellulitis) are deeper infections. Necrotizing fasciitis is the dreaded “flesh-eating” disease - rapid spread along fascial planes with tissue destruction, often requiring extensive surgical debridement. Streptococcal toxic shock syndrome combines soft tissue infection with shock and multiorgan failure.

Post-infectious sequelae - the immune aftermath: GAS infection can trigger immune-mediated diseases weeks later. Two to remember:

Acute rheumatic fever occurs 2-4 weeks after pharyngitis ONLY (not skin infection). The mechanism is molecular mimicry - antibodies and T-cells directed against M protein cross-react with cardiac myosin and valvular glycoproteins. Jones criteria major manifestations: carditis, migratory polyarthritis, Sydenham chorea, erythema marginatum, and subcutaneous nodules. Rheumatic heart disease: acutely, mitral regurgitation (leaflet inflammation/edema prevents closure); chronically, mitral stenosis (fibrosis, calcification, and commissural fusion over years of recurrent episodes). Mitral valve is affected >90% of the time, followed by aortic. Histologic hallmark: Aschoff body (granuloma with Anitschkow “caterpillar” cells - macrophages with wavy chromatin). Rheumatic fever CAN be prevented by promptly treating GAS pharyngitis with antibiotics, and secondary prophylaxis with monthly penicillin G benzathine IM prevents recurrences.

Post-streptococcal glomerulonephritis (PSGN) follows either pharyngeal OR skin GAS infection, 1-3 weeks later. Immune complexes deposit in glomeruli causing a classic nephritic syndrome: hematuria (smoky/cola-colored urine with RBC casts), hypertension, periorbital and lower-extremity edema, and proteinuria. Biopsy: diffuse proliferative GN with subepithelial “humps” on electron microscopy and granular IgG/C3 along the GBM (“starry sky” pattern) on immunofluorescence. Elevated ASO and anti-DNase B titers confirm recent GAS. Treatment is supportive. PSGN CANNOT be prevented by treating the GAS infection - by the time symptoms appear, immune complexes are already deposited.

Note: Group C and Group G streptococci (S. dysgalactiae) also produce streptolysin O, so they can elevate ASO titers and be mistaken for GAS. Importantly, Group C/G are NOT associated with rheumatic fever (but can cause PSGN). Don’t call rheumatic fever based on elevated ASO if the original infection was Group C/G.

Diagnosis and treatment: Rapid antigen detection tests on throat swabs provide quick results but have lower sensitivity than culture - a negative rapid test should be confirmed with culture in children. Treatment: penicillin V (oral) or amoxicillin for pharyngitis; penicillin G IV for serious infections. Clindamycin is added for necrotizing fasciitis and STSS because it shuts off toxin production by blocking ribosomal protein synthesis. Penicillin-allergic: azithromycin or clindamycin. Starting treatment within 9 days of symptom onset prevents rheumatic fever. GAS has never developed penicillin resistance.

Streptococcus agalactiae (Group B Streptococcus, GBS)

Group B strep is a commensal organism that colonizes the vaginal and gastrointestinal tracts of about 25% of healthy women. In adults, it rarely causes disease. The reason GBS matters so much in medicine is what it does to newborns.

The neonatal catastrophe: When a colonized mother gives birth vaginally, the baby passes through a birth canal teeming with GBS. Most babies clear the organism without incident, but some develop invasive infection - and when they do, it’s often devastating. GBS is the leading cause of neonatal sepsis and meningitis in developed countries.

The three major neonatal complications to remember: sepsis, pneumonia, and meningitis.

Early-onset disease (within the first week of life, usually first 24-48 hours) presents as sepsis, pneumonia, or meningitis. The baby was exposed during delivery, and bacteria that entered the respiratory tract or bloodstream multiply rapidly. Risk factors for early-onset disease: maternal GBS colonization, prolonged rupture of membranes (>18 hours), prematurity, and prior GBS-affected infant.

Late-onset disease (1 week to 3 months) presents primarily as meningitis and may be acquired from the mother, healthcare workers, or the community. Not prevented by intrapartum antibiotics.

Prevention through screening: Because colonization is common but disease is uncommon, we can’t treat every colonized mother. Instead, universal screening at 35-37 weeks gestation identifies colonized women, who receive intrapartum antibiotic prophylaxis (penicillin or ampicillin during labor). This dramatically reduces early-onset disease - one of the great successes of evidence-based obstetric practice. Late-onset disease is not prevented by intrapartum antibiotics.

Laboratory identification: GBS produces medium-sized grayish-white mucoid colonies with a narrow zone of beta-hemolysis on blood agar (much narrower than the wide hemolysis of GAS). The narrow zone is sometimes subtle enough to look alpha-hemolytic on first pass. Biochemically:

PYR negative (distinguishes from GAS and Enterococcus, which are PYR+)
Bacitracin resistant (GAS is bacitracin-sensitive)
Catalase negative (all strep are) - this is the key test to distinguish GBS from Listeria monocytogenes, which has a similar presentation (neonatal infection, CAMP-positive, small beta-hemolytic zone) but is catalase-positive and a gram-positive rod
CAMP test positive - see below
Hippurate hydrolysis positive - GBS produces hippuricase, which cleaves hippuric acid to glycine + benzoic acid; detected by ninhydrin turning purple. Positive hippurate + positive CAMP + Lancefield Group B = definitive GBS.

The CAMP test: GBS produces CAMP factor (named for Christie, Atkins, Munch-Petersen), which enhances the activity of S. aureus beta-hemolysin (sphingomyelinase). Streak GBS perpendicular to S. aureus on blood agar and an arrowhead-shaped zone of enhanced hemolysis appears where the streaks meet. Important gotcha: Listeria monocytogenes is also CAMP-positive - the catalase test separates them (Listeria+, GBS-).

GBS in adults: While neonatal disease gets the attention, GBS increasingly causes invasive infections in adults, particularly diabetics and the elderly. Skin and soft tissue infections, bacteremia, and bone/joint infections occur. These strains may represent an aging population with more comorbidities or possible changes in GBS epidemiology.

Streptococcus pneumoniae (Pneumococcus)

The pneumococcus is a formidable pathogen that has shaped the history of infectious disease. Before antibiotics, pneumococcal pneumonia killed one-third of those infected. Even today, with vaccines and antibiotics, it remains a leading cause of pneumonia, meningitis, and otitis media worldwide.

What makes the pneumococcus dangerous - the capsule: S. pneumoniae possesses a thick polysaccharide capsule that is absolutely essential for virulence and is its primary virulence factor. Strains without capsules are avirulent. The capsule prevents phagocytosis by inhibiting C3b deposition on the bacterial surface, letting the bacteria multiply unchecked.

There are over 90 (some sources: >100) capsular serotypes, and immunity to one serotype doesn’t protect against others. Serotypes 1, 3, 4, 5, 6A/B, 7F, 9V, 14, 18C, 19A/F, and 23F cause most invasive disease. Type-specific anticapsular antibodies are protective and are the basis for the pneumococcal vaccines.

The characteristic morphology: S. pneumoniae appears as lancet-shaped (pointed) gram-positive diplococci, with the pointed ends facing outward and broad ends facing each other. The cells often have a “halo” on Gram stain - that’s the unstained capsule. In CSF this morphology is essentially pathognomonic for pneumococcal meningitis.

Laboratory identification: On blood agar, pneumococci produce alpha-hemolysis - a greenish zone of partial hemolysis from H2O2-mediated oxidation of hemoglobin to methemoglobin. Colonies often show a characteristic draughtsman (checkers-piece) morphology - a central dimple from autolysis. Two key tests distinguish S. pneumoniae from viridans streptococci (which are also alpha-hemolytic):

Optochin sensitive (ethylhydrocupreine disrupts the pneumococcal cell membrane ATPase). Zone of inhibition ≥14 mm around a 6 mm disk = sensitive. ~1-2% of S. pneumoniae strains are optochin-resistant, so confirm with bile solubility when equivocal.
Bile soluble - sodium deoxycholate activates the pneumococcal autolysin (LytA), which degrades peptidoglycan. In broth: S. pneumoniae turns clear (cells lyse); viridans remain turbid. On a colony: a drop of 10% deoxycholate dissolves an S. pneumoniae colony within 15 minutes.

The quellung reaction: type-specific anticapsular antiserum causes capsular swelling (visualized by microscopy) and provides serotyping. Rarely performed routinely. The urinary pneumococcal antigen test (BinaxNOW) is rapid, remains positive even after antibiotics have been started, but stays positive for weeks after infection - useful but not a tool for recent-vs-old distinction.

Clinical syndromes - remember C-MOPS for the leading pneumococcal diseases:

Conjunctivitis
Meningitis (most common cause of bacterial meningitis in adults)
Otitis media (most common cause in children)
Pneumonia (most common cause of community-acquired bacterial pneumonia)
Sinusitis

S. pneumoniae is also the most common cause of spontaneous bacterial peritonitis in children with nephrotic syndrome. Classic pneumonia presentation: sudden shaking chills, high fever, pleuritic chest pain, and productive cough with rusty sputum (blood-tinged). Chest X-ray shows lobar consolidation. Risk factors for invasive disease: asplenia (highest risk), hypogammaglobulinemia, HIV, and extremes of age.

Vaccines represent a triumph of immunology: The polysaccharide capsule is the vaccine target.

PPSV23 (Pneumovax) contains purified capsular polysaccharides from 23 serotypes. It generates a T-cell-independent response, which works in adults but fails in children <2 (their immature immune systems can’t respond to pure polysaccharides without T-cell help).
Conjugate vaccines (PCV13, PCV15, PCV20) link the polysaccharides to a protein carrier (e.g., diphtheria toxoid CRM197), converting them into T-cell-dependent antigens. This generates robust immunity even in infants and has dramatically reduced invasive pneumococcal disease in children.

Antibiotic resistance: Pneumococcal penicillin resistance is not through beta-lactamase production; it is through altered PBPs with reduced penicillin affinity. Important board fact: penicillin resistance breakpoints differ by infection site. For meningitis: MIC ≤0.06 susceptible, ≥0.12 resistant (very strict because CSF penetration is limited). For non-meningitis (pneumonia, bacteremia): MIC ≤2 susceptible, ≥8 resistant. A strain with MIC 1 is resistant for meningitis but susceptible for pneumonia - that’s why antibiograms report pneumococcal susceptibility twice (meningitis vs. non-meningitis).

Group C and Group G Streptococci

Group C and G streptococci (S. dysgalactiae subspecies equisimilis is the main human species) are large-colony beta-hemolytic streptococci that can cause pharyngitis, skin infections, and bacteremia clinically similar to GAS. Two things to remember:

They produce streptolysin O, so ASO titers can be elevated in Group C/G infection. Don’t use elevated ASO alone to call GAS.
They can cross-react with Lancefield Group A antisera, which is why colony size and biochemistry matter.

Importantly, Group C/G are NOT associated with rheumatic fever (though they may cause PSGN). A patient with elevated ASO and carditis whose original infection was Group C/G does not have rheumatic fever.

Lab features: PYR negative, bacitracin resistant, Lancefield antigen C or G. Large colonies (distinguishes from S. anginosus group).

Viridans Group Streptococci

The viridans streptococci are a heterogeneous collection of species that share key characteristics: on Gram stain they appear as gram-positive cocci in long chains. They are alpha-hemolytic (or gamma-hemolytic), optochin-resistant, and bile-insoluble - the two mirror-image results that distinguish them from S. pneumoniae (optochin-sensitive, bile-soluble). They’re called “viridans” from the Latin for green, referring to alpha-hemolysis.

The main species to know:

S. mitis - most common cause of viridans strep bacteremia in neutropenic patients and can cause “viridans strep shock syndrome.” Can be penicillin-resistant.
S. mutans - dental caries.
S. sanguinis - subacute bacterial endocarditis.
S. salivarius - rare infections, probiotic use.
S. anginosus group - deep abscesses (see below).

These organisms are commensals of the mouth and upper respiratory tract. They play a beneficial role by occupying niches that might otherwise be taken by pathogens. But they can cause serious disease when they reach sites they shouldn’t.

Infective endocarditis - the classic viridans disease: Dental procedures and even routine tooth brushing cause transient bacteremia with oral flora, including viridans streptococci. In people with normal heart valves, these bacteria are rapidly cleared. But damaged or abnormal valves - rheumatic heart disease, congenital defects, degenerative changes, or prosthetic valves - provide surfaces where bacteria adhere to platelet-fibrin vegetations and proliferate.

Viridans streptococcal endocarditis is the prototype of subacute bacterial endocarditis: insidious onset over weeks to months, low-grade fever, malaise, weight loss, new murmur, and classic peripheral manifestations (splinter hemorrhages, Osler nodes, Janeway lesions) from immune complex deposition and septic emboli. Without treatment, fatal; with treatment, cure rates are high because these organisms generally remain penicillin-susceptible. Any viridans strep in blood cultures should trigger evaluation for endocarditis (multiple blood culture sets, echocardiography).

S. mutans and dental caries: S. mutans is the principal agent of tooth decay. It metabolizes sucrose to lactic acid, which demineralizes enamel, and synthesizes glucans (sticky polysaccharides) that let it form biofilm on tooth surfaces. S. mutans thrives in the acidic environment it creates. Preventing S. mutans biofilm - oral hygiene, fluoride, limited dietary sugar - is the foundation of caries prevention.

The anginosus group (S. anginosus, S. intermedius, S. constellatus - formerly the “S. milleri group”) has a distinctive tendency to form deep abscesses: brain, liver, lung (empyema), intra-abdominal, peritonsillar. Distinguishing features:

Tiny pinpoint colonies (distinguishes from large-colony GAS, GBS, S. dysgalactiae)
Caramel/butterscotch odor on the plate
Hemolysis is variable (beta, alpha, or gamma - unreliable for classification)
Can carry Lancefield antigens A, C, F, or G, which cross-reacts with GAS and Group C/G antisera
Biochemistry: Voges-Proskauer (VP) positive (acetoin production - unusual among streptococci), arginine hydrolysis positive, sorbitol fermentation negative. Can hydrolyze esculin.

When a streptococcus comes from pus in a deep abscess, think S. anginosus group. Treatment: surgical drainage + prolonged penicillin or ampicillin; polymicrobial abscesses often need broader coverage.

Streptococcus gallolyticus (formerly S. bovis)

This organism deserves special attention because of its strong clinical association with colorectal pathology - a classic boards topic.

Classification: S. gallolyticus is part of the Streptococcus bovis/equinus complex (SBSEC), which was reclassified in recent years. The clinically important point is that S. gallolyticus (previously S. bovis biotype I) is strongly associated with colorectal neoplasia.

Identification:

Group D streptococcus (Lancefield grouping)
γ-hemolytic (non-hemolytic) or α-hemolytic
Bile esculin positive (like Enterococcus)
6.5% NaCl negative (distinguishes from Enterococcus)
PYR negative

The colon cancer connection: When S. gallolyticus is isolated from blood cultures (bacteremia) or endocarditis, colonoscopy is mandatory to evaluate for colorectal carcinoma or adenomatous polyps - even if the patient is asymptomatic. 25-80% of patients with S. gallolyticus bacteremia have underlying colorectal pathology. The organism adheres to colonic mucosa (especially neoplastic tissue expressing heparan sulfate proteoglycans) and translocates through disrupted mucosal barriers. Also associated with meningitis in Strongyloides hyperinfection.

The classic teaching: S. bovis in the blood, cancer in the colon. Compare to Clostridium septicum, also associated with colon cancer but a gram-positive rod.

Clinical pearl: Any patient with S. gallolyticus bacteremia or endocarditis needs a GI workup, even if asymptomatic. Missing a colon cancer in this setting is a preventable error.

38.3 Enterococcus

Enterococci are hardy organisms that inhabit the human gastrointestinal tract. They’re gram-positive cocci that form chains or pairs and are catalase-negative - features they share with streptococci, from which they were historically classified (as “Group D streptococci”). But enterococci are distinct organisms with a unique clinical profile, characterized most notably by their intrinsic resistance to many antibiotics and their remarkable ability to acquire resistance to almost everything else.

The enterococcal paradox: These organisms are not particularly virulent. They cause disease primarily in debilitated, hospitalized patients who have been treated with broad-spectrum antibiotics. The very antibiotics meant to help these patients eliminate normal flora competition and select for enterococci - which then cause infection.

Normal flora: Enterococcus is normal flora of the GI and GU tract, which explains its association with UTIs, biliary infections, and intra-abdominal infections. These organisms are extreme survivors - they tolerate bile, 6.5% NaCl, 40% bile, pH 9.6, and 60°C for 30 minutes. Hardiness plus intrinsic antibiotic resistance = ideal nosocomial pathogen.

Hemolysis: Usually gamma-hemolytic, but some (especially E. faecalis) can be alpha-hemolytic.

Two species dominate human disease: E. faecalis causes 80-90% of enterococcal infections and, while resistant to many antibiotics, generally remains susceptible to ampicillin. E. faecium causes most of the remainder and is far more concerning - it’s often resistant to ampicillin and more likely to be vancomycin-resistant. Rough resistance numbers: ~80% of E. faecium isolates are VRE vs. ~5-10% of E. faecalis; E. faecalis is more common overall. When a lab reports “Enterococcus species,” knowing whether it’s faecalis or faecium matters a lot for empiric treatment.

Laboratory identification: Enterococci are catalase-negative, gamma- or alpha-hemolytic GPC. The identification triad:

PYR positive (distinguishes from Group D strep, which is PYR-)
Bile esculin positive (hydrolyzes esculin in the presence of bile, producing a black precipitate from esculetin + ferric citrate) - shared with Group D strep
Growth in 6.5% NaCl - the key feature separating Enterococcus from Group D strep (which does NOT grow in 6.5% NaCl)

Compare the same three tests in Group D strep (S. gallolyticus/bovis): PYR negative, bile esculin positive, no growth in 6.5% NaCl.

Intrinsic resistance creates therapeutic challenges: Enterococci are naturally resistant to cephalosporins (all of them), clindamycin, TMP-SMX, and low levels of aminoglycosides. The aminoglycoside resistance is overcome when aminoglycosides are combined with a cell wall-active agent (ampicillin or vancomycin) - the cell wall agent allows aminoglycoside entry, producing synergistic killing. This is why enterococcal endocarditis is traditionally treated with ampicillin plus gentamicin.

Acquired resistance - the VRE problem: Vancomycin-resistant enterococci (VRE) are a major hospital pathogen. Resistance is mediated by transferable plasmids carrying van gene clusters that modify the peptidoglycan precursor terminus from D-Ala-D-Ala (high vancomycin affinity) to D-Ala-D-Lac or D-Ala-D-Ser (1000-fold reduced affinity).

Gene	Resistance	Teicoplanin	Transferable	Typical host
vanA	High-level vanco (MIC >64)	Yes - resistant	Yes (plasmid)	E. faecium > E. faecalis
vanB	Moderate-high vanco	No - susceptible	Yes (plasmid)	E. faecalis, E. faecium
vanC	Low-level vanco (MIC 2-32)	No	No (chromosomal)	E. gallinarum, E. casseliflavus

vanA is the most common and most concerning - it’s been the source of VRSA when transferred to S. aureus. vanA is inducible by both vancomycin and teicoplanin (both drugs fail). vanB is inducible by vancomycin only (teicoplanin does not induce expression, so teicoplanin remains clinically useful in countries where it’s available).

E. gallinarum and E. casseliflavus are distinct: vanC is intrinsic and chromosomal (not transferable), so these organisms are not considered true VRE for infection control. They are the only motile Enterococcus species. E. casseliflavus also produces a characteristic yellow pigment.

Treatment of VRE: linezolid or daptomycin.

Clinical infections - Enterococcus classically causes:

UTI (most common - especially catheter-associated)
Endocarditis (subacute; third most common cause after viridans strep and S. aureus)
Biliary tract infections (cholangitis, cholecystitis - ascends via biliary tree as normal bowel flora)
Bacteremia, intra-abdominal infections (often polymicrobial), wound infections, rare meningitis

Enterococci frequently participate in polymicrobial infections - they’re part of the mix in complicated intra-abdominal and wound infections - but their pathogenic contribution in those settings is debated.

Chapter 39: Gram-Positive Bacilli

This chapter walks through the gram-positive bacilli grouped by clinical behavior rather than strict taxonomy. We start with the aerobic spore-formers (Bacillus), then Listeria, the diphtheroids (Corynebacterium and the related pleomorphic rods - Erysipelothrix, Arcanobacterium, Rhodococcus, Tropheryma), the anaerobic spore-formers (the Clostridia), the branching filaments (Actinomyces and Nocardia), and finally a grab-bag of other anaerobes that are co-studied with this group on the boards - including a few gram-negative anaerobes (Fusobacterium, Bacteroides) that share clinical contexts and are easier to learn alongside their gram-positive neighbors.

39.1 Bacillus

The genus Bacillus comprises large, gram-positive, spore-forming rods. Most are harmless environmental organisms, but two species cause significant human disease through completely different mechanisms: B. anthracis through invasive infection with potent toxins, and B. cereus through food poisoning.

Bacillus anthracis

Anthrax occupies a unique place in the history of microbiology. It was the first disease for which a bacterial etiology was proven (by Robert Koch in 1876), the first disease against which animals were vaccinated (by Louis Pasteur in 1881), and it remains the prototypical bioterrorism agent. The 2001 anthrax letter attacks in the United States killed five people and created widespread panic, demonstrating the terror potential of this organism.

What makes B. anthracis special: Unlike most Bacillus species, B. anthracis is non-motile and non-hemolytic. It produces two plasmid-encoded virulence factors that are both required for full pathogenicity: a poly-D-glutamate capsule (unusual because it’s a polypeptide rather than a polysaccharide) that is antiphagocytic, and the tripartite anthrax toxin.

The anthrax toxin is a remarkable three-component system: Protective antigen (PA) is the binding component that delivers the two enzymatic components into host cells. Edema factor (EF) is an adenylate cyclase that causes massive increases in intracellular cAMP, leading to edema. Lethal factor (LF) is a metalloprotease that cleaves MAP kinase kinases, disrupting cell signaling and causing cell death. Neither EF nor LF can enter cells alone - they require PA. This understanding is critical because the anthrax vaccine targets PA, blocking both toxic activities.

Cutaneous anthrax accounts for 95% of naturally occurring cases. Spores enter through a break in the skin, germinate, and multiply locally. The initial lesion is a painless papule that progresses to a vesicle and then ulcerates. What’s distinctive is the black eschar (from Greek for coal - hence “anthrax”) - a painless, depressed black scab surrounded by edema. Without treatment, cutaneous anthrax has about 20% mortality; with antibiotics, death is rare.

Inhalation anthrax is far more deadly and is the form most feared in bioterrorism scenarios. Spores are inhaled and reach the alveoli, where they’re engulfed by macrophages and transported to mediastinal lymph nodes. There they germinate, multiply, and produce toxin. The clinical presentation is initially flu-like, then rapidly progresses to hemorrhagic mediastinitis, sepsis, and shock. Chest imaging shows a widened mediastinum from hemorrhagic lymphadenopathy. Even with aggressive treatment, mortality exceeds 50%.

Laboratory considerations: B. anthracis is a CDC select agent, and laboratory protocols reflect biosafety concerns. Large, non-hemolytic, non-motile gram-positive rods that form Medusa head colonies (ground-glass appearance with irregular comma-shaped projections) should be handled with extreme caution and referred to public health laboratories for confirmation.

Bacillus cereus

B. cereus is an environmental organism that causes food poisoning through two distinct mechanisms - one resembling staphylococcal food poisoning, the other resembling C. perfringens.

The emetic syndrome results from ingestion of preformed cereulide toxin. The classic scenario is reheated fried rice: B. cereus spores survive cooking, germinate when rice is left at room temperature, and produce the heat-stable cereulide toxin. Reheating doesn’t destroy the toxin. Within 1-6 hours of eating, patients develop nausea and vomiting. The illness is brief and self-limited.

The diarrheal syndrome results from enterotoxins produced in the intestine after ingestion of vegetative organisms or spores. Onset is 8-16 hours after eating, with watery diarrhea and cramps. This presentation is essentially identical to C. perfringens food poisoning and is also self-limited.

Beyond food poisoning: In immunocompromised patients, particularly those with indwelling catheters or injection drug users, B. cereus can cause serious invasive infections. Endophthalmitis following penetrating eye trauma is a particularly devastating manifestation - the organism produces toxins that rapidly destroy ocular tissues, often resulting in loss of the eye even with prompt treatment.

Distinguishing B. cereus from B. anthracis matters: B. cereus is beta-hemolytic and motile; B. anthracis is non-hemolytic and non-motile. This distinction is critical when a Bacillus species grows from a clinical specimen, particularly in the context of possible bioterrorism.

39.2 Listeria monocytogenes

Listeria is one of the most important foodborne pathogens, not because it causes common disease - it doesn’t - but because it causes devastating disease in vulnerable populations: pregnant women, newborns, the elderly, and the immunocompromised.

Why Listeria is unique among foodborne pathogens: Most bacteria that contaminate food are killed by refrigeration. Listeria is psychrotolerant - it grows at refrigerator temperatures (4°C). This means refrigerated foods that might seem safe can actually allow Listeria to multiply to dangerous levels. The classic high-risk foods are soft cheeses (particularly those made with unpasteurized milk), deli meats, hot dogs, and smoked seafood - all foods that are kept refrigerated and eaten without further cooking.

The intracellular lifestyle: Listeria is a facultative intracellular pathogen that has evolved remarkable mechanisms to survive and spread inside host cells. After being ingested by a macrophage, Listeria escapes from the phagosome into the cytoplasm using listeriolysin O, a pore-forming toxin that punches holes in the phagosomal membrane. Once in the cytoplasm, Listeria hijacks the host cell’s actin machinery using a surface protein called ActA. ActA recruits host actin to polymerize at one pole of the bacterium, creating an “actin rocket tail” that propels the bacterium through the cytoplasm. When the bacterium reaches the cell membrane, it pushes outward, creating a protrusion that is engulfed by neighboring cells - allowing Listeria to spread from cell to cell without ever entering the extracellular space where antibodies could neutralize it.

Clinical presentations: In healthy adults, Listeria typically causes a self-limited febrile gastroenteritis. The real danger is in vulnerable populations. In pregnancy, Listeria can cross the placenta and infect the fetus, causing miscarriage, stillbirth, or neonatal sepsis and meningitis. In the elderly and immunocompromised, Listeria causes meningitis with a mortality rate of 20-30%. The meningitis often has a subacute presentation - developing over days rather than hours - and should be suspected when CSF shows gram-positive rods.

Laboratory identification: Listeria appears as small gram-positive rods that can look almost coccoid. Key features include catalase positivity (distinguishing it from streptococci), beta-hemolysis (narrow zone), and the distinctive tumbling motility at room temperature. At 37°C, the motility disappears. The CAMP test is positive, but unlike group B strep (which produces an arrowhead shape), Listeria produces a rectangular zone of enhanced hemolysis.

Treatment considerations: Ampicillin + gentamicin for serious infections (ampicillin alone for less severe disease). A critical point: Listeria is intrinsically resistant to cephalosporins. This matters because empiric meningitis coverage typically includes ceftriaxone - which won’t cover Listeria. In any patient at risk for Listeria meningitis (elderly, immunocompromised, pregnant), ampicillin must be added to the empiric regimen.

39.3 Corynebacterium

The corynebacteria are gram-positive rods with a distinctive appearance - club-shaped cells that fail to separate completely after division, leaving them arranged in “V” and “L” formations or parallel rows called palisades. This Chinese letters arrangement is characteristic and helps identify the genus on Gram stain.

Corynebacterium diphtheriae

Diphtheria was once a leading cause of childhood death. The disease is now rare in vaccinated populations, but understanding its pathogenesis illuminates fundamental principles of bacterial toxin biology and the power of vaccination.

The toxin is the disease: C. diphtheriae causes local infection in the throat, but the systemic disease - the myocarditis, the neuropathy, the death - is caused entirely by a single toxin. This toxin is not even encoded in the bacterial chromosome; it’s carried by a lysogenic bacteriophage. Only strains infected with this phage (lysogenized strains) produce toxin and cause clinical diphtheria.

Diphtheria toxin is the prototype A-B toxin. The B subunit binds to a receptor on host cells, allowing the A subunit to enter. Once inside, the A subunit performs a devastatingly specific biochemical reaction: it ADP-ribosylates elongation factor 2 (EF-2), a protein absolutely essential for ribosome function. With EF-2 inactivated, the cell cannot synthesize proteins and dies. Because a single toxin molecule can inactivate many EF-2 molecules (it acts catalytically), even tiny amounts of toxin are lethal to cells.

Clinical presentation: Respiratory diphtheria begins as pharyngitis, but the hallmark is the pseudomembrane - a thick, gray, adherent membrane composed of dead epithelial cells, inflammatory cells, fibrin, and bacteria. The membrane is tenacious; attempting to remove it causes bleeding. As local disease progresses, the pseudomembrane can extend and obstruct the airway. Massive cervical lymphadenopathy produces the classic bull neck appearance.

The systemic effects of absorbed toxin appear after the acute illness. Myocarditis at 2-3 weeks is the leading cause of death. Peripheral neuropathy - often affecting cranial nerves first (palatal paralysis, diplopia) then peripheral nerves - may develop weeks to months later.

Prevention through vaccination: The diphtheria toxoid vaccine contains inactivated toxin. By stimulating antibody production against the toxin, vaccination prevents systemic disease even if colonization occurs. This is why diphtheria has virtually disappeared from vaccinated populations - and why it resurges wherever vaccination rates drop.

Corynebacterium jeikeium

This species deserves mention because it represents the “diphtheroids” - corynebacteria other than C. diphtheriae that are usually dismissed as skin contaminants but can cause real disease in immunocompromised patients. C. jeikeium causes catheter-related bloodstream infections and is notoriously antibiotic-resistant - often susceptible only to vancomycin. When corynebacteria grow from blood cultures in a patient with an indwelling catheter, don’t automatically dismiss them as contamination.

39.4 Erysipelothrix rhusiopathiae

E. rhusiopathiae is a thin, pleomorphic, non-spore-forming gram-positive rod that causes erysipeloid, a localized skin infection classically associated with occupational exposure. The disease is sometimes called fish handler’s disease or seal finger because it’s acquired by fishermen, butchers, veterinarians, and anyone handling raw meat or fish. Organisms enter through skin abrasions on the hands.

The classic presentation is a violaceous, well-demarcated, painful plaque on the hand (usually a finger), slowly spreading proximally. Unlike streptococcal erysipelas (which it superficially resembles by name), the lesion is purplish rather than bright red, and the patient is typically not systemically ill. The infection is usually self-limited but can progress to systemic disease with endocarditis (classically aortic valve, and notoriously difficult to diagnose - mean delay from onset to diagnosis can be months).

Lab identification: H2S producer on TSI (one of the few gram-positive rods that does this). Catalase negative, alpha-hemolytic on sheep blood agar. Intrinsically resistant to vancomycin - an important distinction from Listeria and other gram-positive rods.

Treatment: Penicillin is the drug of choice.

39.5 Arcanobacterium (Trueperella) haemolyticum

A. haemolyticum causes an exudative pharyngitis in adolescents and young adults that clinically mimics streptococcal pharyngitis. The distinguishing feature is a scarlatiniform rash that develops in roughly half of cases - unlike strep’s scarlet fever, the Arcanobacterium rash spares the face. On Gram stain, the organism appears as small, slightly curved gram-positive rods. Colonies show narrow beta-hemolysis and characteristic “opacity halos” that develop after 48 hours.

Think of Arcanobacterium when a young adult has pharyngitis plus rash but rapid strep is negative.

39.6 Rhodococcus equi

R. equi is a partially acid-fast gram-positive coccobacillus (often mistaken for mycobacteria or Nocardia) that causes cavitary pneumonia almost exclusively in severely immunocompromised hosts, particularly transplant recipients and patients with advanced HIV. The organism lives in soil contaminated by horse manure (hence “equi”). Equine exposure isn’t always in the history; environmental spores are enough.

The classic presentation is a cavitary upper-lobe pneumonia that mimics tuberculosis. On culture, colonies are initially salmon-pink (then white, then red) on Mueller-Hinton or Loeffler agar. Modified Kinyoun acid-fast staining is positive, which can confuse the picture with Nocardia.

Treatment: Requires prolonged combination therapy, often macrolide + rifampin + a third agent (quinolone or linezolid). Mortality is high despite treatment.

39.7 Tropheryma whipplei

T. whipplei is the causative agent of Whipple disease, a rare multisystem infection that classically affects middle-aged men (male-to-female ratio about 8:1). The organism is an actinomycete but doesn’t grow on standard bacterial culture - it was historically identified only by tissue biopsy showing PAS-positive foamy macrophages in the lamina propria of the small intestine. Modern diagnosis uses PCR on tissue or CSF.

Clinical tetrad: chronic diarrhea with malabsorption, weight loss, migratory arthralgias (often predating GI symptoms by years), and lymphadenopathy. Cardiac involvement (culture-negative endocarditis) and CNS involvement (dementia, supranuclear ophthalmoplegia, oculomasticatory myorhythmia - pathognomonic) can occur and are particularly serious.

Key pathology: Small bowel biopsy shows blunted villi and lamina propria packed with foamy macrophages filled with PAS-positive, diastase-resistant bacteria. Electron microscopy shows the characteristic trilaminar cell wall.

Treatment: Ceftriaxone followed by prolonged TMP-SMX maintenance (at least 1 year). Relapse is common, particularly with CNS disease, because TMP-SMX doesn’t adequately penetrate the CNS - some authorities prefer doxycycline plus hydroxychloroquine for CNS or refractory disease.

39.8 Anaerobic Gram-Positive Bacilli

Clostridioides (Clostridium) difficile

C. difficile is the most important cause of healthcare-associated diarrhea and one of the most urgent antibiotic resistance threats. Understanding its pathogenesis explains why it’s so difficult to eradicate and why it keeps coming back.

The ecology of C. difficile infection: C. difficile lives in the gut - it’s present in about 3% of healthy adults and up to 30% of hospitalized patients. In a healthy gut, the normal microbiota outcompetes C. difficile and keeps its numbers low through “colonization resistance.” Antibiotics destroy this competition. When broad-spectrum antibiotics decimate the normal flora, C. difficile proliferates unchecked and produces toxins that cause disease. This is why C. difficile infection (CDI) is fundamentally a disease of disrupted microbiota.

The spore problem: C. difficile forms spores, and this single feature explains most of the infection control challenges. Spores are metabolically dormant, encased in a protective coat, and resistant to almost everything - heat, drying, alcohol-based hand sanitizers, and most disinfectants. When a patient with CDI has diarrhea, they shed millions of spores that contaminate the environment and can persist on surfaces for months. Healthcare workers pick up spores on their hands, carry them to the next patient, and the cycle continues. The only way to remove spores from hands is physical washing with soap and water - alcohol doesn’t kill them.

The toxin-mediated disease: C. difficile doesn’t invade tissue. All disease manifestations result from two large toxins: toxin A (TcdA) and toxin B (TcdB). Both toxins enter colonic epithelial cells and inactivate Rho GTPases by glucosylation. Rho GTPases regulate the cytoskeleton; when they’re inactivated, the cytoskeleton collapses, tight junctions break down, and cells die. The result is fluid secretion, inflammation, and mucosal damage.

In severe cases, the damaged mucosa becomes covered with pseudomembranes - plaques of fibrin, mucus, inflammatory cells, and debris that look like yellow-white patches on colonoscopy. Fulminant colitis can progress to toxic megacolon and perforation.

Hypervirulent strains: The NAP1/BI/027 strain emerged in the early 2000s and caused devastating outbreaks. This strain produces more toxin, has additional virulence factors including a binary toxin (CDT), and is associated with fluoroquinolone resistance - which gave it a selective advantage as fluoroquinolone use increased.

Diagnostic approach: The challenge in diagnosis is that detecting C. difficile isn’t the same as diagnosing CDI. Many hospitalized patients carry C. difficile without symptoms (colonization). Only test patients with significant diarrhea (≥3 unformed stools in 24 hours). NAAT is highly sensitive - it detects the toxin genes - but it can’t distinguish colonization from infection. The two-step algorithm uses GDH screen + toxin EIA. This approach balances sensitivity and specificity.

Treatment principles: The first step is stopping the inciting antibiotic if possible. Oral vancomycin or fidaxomicin (not IV vancomycin, which doesn’t reach the gut lumen) are preferred. For recurrent infection, which occurs in 20-30% of patients because spores survive in the gut and germinate when treatment stops, fecal microbiota transplantation (FMT) restores colonization resistance by repopulating the gut with healthy bacteria - it’s remarkably effective, with cure rates exceeding 90%.

Clostridium perfringens

C. perfringens is one of the fastest-growing bacteria known - its doubling time of 10 minutes means that what starts as a few contaminating organisms can become an overwhelming infection in hours. This speed, combined with a potent array of tissue-destroying toxins, makes C. perfringens the agent of gas gangrene, one of the most feared infections in surgery and trauma.

The organism: C. perfringens is a large gram-positive rod and an obligate anaerobe. Unlike most clostridia, it rarely forms spores in clinical specimens - but spores are abundant in soil, dust, and the GI tract, explaining how wounds become contaminated. On blood agar, colonies show a distinctive double zone of hemolysis: complete hemolysis close to the colony (from α-toxin) and partial hemolysis further out (from θ-toxin).

The toxins do the damage: C. perfringens produces over a dozen toxins, but α-toxin is the key player in gas gangrene. α-toxin is a lecithinase (phospholipase C) that destroys cell membranes by cleaving lecithin, a major membrane phospholipid. When α-toxin attacks muscle cells, they die and lyse, providing more nutrients for bacterial growth. The fermentation of muscle sugars produces gas, which dissects along tissue planes.

Gas gangrene (clostridial myonecrosis) is a surgical emergency. It typically follows traumatic injury or surgery where muscle is contaminated with spores. The wound becomes exquisitely painful - pain out of proportion to examination is an early warning sign. The skin develops a bronze discoloration, then blisters. Crepitus (a crackling sensation from gas in the tissues) may be felt. A thin, foul-smelling “dishwater” discharge seeps from the wound. Systemically, the patient deteriorates rapidly with shock and multiorgan failure. Without emergent surgical debridement - often requiring amputation - death follows within hours. Even with surgery and antibiotics (penicillin plus clindamycin), mortality is substantial.

Food poisoning is a much more benign manifestation. C. perfringens type A produces an enterotoxin in the intestine. When spores in contaminated food (typically meat dishes held at warm temperatures) survive cooking, they germinate and multiply. After ingestion, sporulation in the gut releases enterotoxin, causing watery diarrhea and cramps 8-16 hours later. The illness is self-limited and rarely requires treatment.

Clostridium tetani

Tetanus has been known since antiquity - Hippocrates described the characteristic muscle spasms. The disease is entirely preventable by vaccination, yet it still kills tens of thousands annually in regions with inadequate immunization, particularly neonates in developing countries.

The organism and its niche: C. tetani is an obligate anaerobe that forms terminal spores (drumstick / tennis racket appearance). Spores are ubiquitous in soil and remarkably durable - they can survive for decades. The organism itself is not invasive; it remains localized at the wound site. All clinical manifestations result from its toxin.

Tetanospasmin - a toxin of exquisite specificity: Tetanospasmin is the second most potent toxin known (after botulinum toxin). It’s an A-B toxin that enters peripheral nerve terminals and travels retrograde along axons to the spinal cord and brainstem. There, the A subunit cleaves synaptobrevin (VAMP), a protein essential for neurotransmitter release. But tetanospasmin specifically targets inhibitory neurons - those releasing GABA and glycine. Without inhibitory input, motor neurons fire unopposed, and muscles contract uncontrollably.

The clinical picture is unforgettable: Generalized tetanus begins with trismus (lockjaw) - spasm of the masseter muscles makes it impossible to open the mouth. The facial muscles contract, producing risus sardonicus - a grimacing “sardonic smile.” As the disease progresses, spasms spread to the neck, back, and extremities. Opisthotonus is the extreme hyperextension of the spine from paraspinal muscle spasm. Spasms are excruciatingly painful and can be triggered by minor stimuli - a noise, a touch, a light. Respiratory failure from laryngeal spasm or diaphragmatic involvement is the usual cause of death.

Localized tetanus, with rigidity confined to muscles near the wound, may occur and can progress to generalized disease. Neonatal tetanus, from infection of the umbilical stump in babies born to unimmunized mothers, remains a significant cause of infant mortality in developing countries.

Prevention is everything: The tetanus toxoid vaccine is one of medicine’s great successes. The toxoid (inactivated toxin) induces protective antibodies, and boosters maintain immunity. DTaP for children and Tdap for adults include tetanus toxoid. For contaminated wounds in inadequately immunized individuals, tetanus immune globulin provides passive protection.

Clostridium botulinum

Botulinum toxin is the most potent biological toxin known - the estimated lethal dose in humans is approximately 1 nanogram per kilogram body weight. This extraordinary potency reflects the toxin’s mechanism: it blocks neuromuscular transmission, causing flaccid paralysis that can paralyze the diaphragm and kill by respiratory arrest.

The toxin mechanism is the mirror image of tetanus: Both tetanospasmin and botulinum toxin are zinc metalloproteases that cleave SNARE proteins required for synaptic vesicle fusion. But while tetanospasmin blocks inhibitory neurons in the CNS, botulinum toxin blocks acetylcholine release at the NMJ. The result is the opposite of tetanus: flaccid paralysis rather than spastic paralysis.

Seven serotypes (A-G) are recognized, with types A, B, and E causing most human disease. The toxin is an A-B structure - the B component binds to receptors on presynaptic nerve terminals, allowing the A component to enter and cleave SNAP-25 or synaptobrevin (depending on serotype).

Foodborne botulism results from ingesting preformed toxin in food. Classic sources are home-canned vegetables with low acid content, where spores survive inadequate canning and germinate under anaerobic conditions. Commercial canning failures and fermented foods are other sources. Symptoms begin 12-36 hours after ingestion with cranial nerve dysfunction: diplopia, dysphagia, dysarthria, and dilated pupils. Descending symmetric paralysis follows - the hallmark that distinguishes botulism from Guillain-Barré syndrome (which ascends). Respiratory failure may develop. Importantly, patients remain alert - the toxin doesn’t cross the blood-brain barrier.

Infant botulism is the most common form in the United States. Infants under one year ingest spores (the classic source is honey, which is why honey should never be given to infants under 1 year), which germinate in the immature intestine where normal flora has not yet established colonization resistance. Toxin is produced in vivo. The baby becomes hypotonic (“floppy baby syndrome”), feeds poorly, has a weak cry, and may develop respiratory failure. Unlike foodborne botulism, infant botulism is treated with botulism immune globulin (BIG), not antitoxin.

Wound botulism has emerged as a disease of injection drug users, particularly those injecting black tar heroin. Spores contaminate the drug, germinate in the wound, and produce toxin locally.

Treatment: For foodborne botulism, equine antitoxin neutralizes circulating toxin but cannot reverse toxin already bound to nerve terminals - hence the importance of early administration. Supportive care, often including prolonged mechanical ventilation, is essential. Recovery occurs as new nerve terminals sprout, a process that can take months.

39.9 Gram-Positive Branching Filamentous Rods

Actinomyces and Nocardia are gram-positive bacteria that form branching filaments, giving them a fungus-like appearance on microscopy. They’re often confused with each other and with true fungi, but they’re distinct organisms with very different pathogenesis and treatment. The key to distinguishing them lies in their oxygen requirements and acid-fast staining properties.

Actinomyces israelii

Actinomyces species are normal inhabitants of the human mouth, living in the crevices between teeth and in tonsillar crypts. They’re strict anaerobes and cause no harm in their usual niches. Disease occurs when these organisms breach their normal boundaries - following dental procedures, facial trauma, aspiration, or, classically, in association with intrauterine devices.

Understanding actinomycosis: Unlike most bacterial infections, which present acutely, actinomycosis is a chronic disease that evolves over weeks to months. The organisms grow slowly, forming dense masses of branching filaments surrounded by inflammatory tissue. As the infection progresses, it forms multiple draining sinus tracts that discharge pus containing characteristic sulfur granules - yellow, grainy particles that are actually dense colonies of organisms (not sulfur at all; the name refers to their appearance).

Cervicofacial actinomycosis (“lumpy jaw”) is the most common form. It typically follows dental work or tooth extraction, or occurs in patients with poor dental hygiene. A painless indurated mass develops in the jaw or neck, eventually forming sinus tracts that drain through the skin. The infection ignores tissue planes, crossing fascial boundaries and even eroding bone - behavior that often leads clinicians to suspect malignancy rather than infection.

Thoracic actinomycosis results from aspiration of oral contents. The pulmonary infection mimics lung cancer or tuberculosis, with mass lesions, cavitation, and chest wall involvement. Abdominal and pelvic actinomycosis are associated with surgery, trauma, and IUDs - the copper or plastic provides a surface for biofilm formation and anaerobic conditions.

Diagnosis requires suspicion: Because the organisms are slow-growing anaerobes, they’re often missed on routine culture. The sulfur granules in pus are highly suggestive but not pathognomonic (Nocardia and some fungi can produce similar structures). Histopathology showing branching gram-positive filaments in a background of chronic inflammation supports the diagnosis.

Treatment: Actinomyces is exquisitely sensitive to penicillin, which remains the drug of choice. The challenge is duration - because the organisms grow slowly and form dense colonies protected from antibiotics, treatment must continue for 6-12 months. Surgical debridement of large abscesses accelerates recovery.

Nocardia Species

Where Actinomyces is an anaerobic commensal that causes disease by breaching normal barriers, Nocardia is an aerobic environmental organism that causes disease in patients with impaired immunity. This fundamental difference explains their different epidemiology and clinical presentations.

Ecology and host factors: Nocardia species live in soil and decaying organic matter. Infection occurs by inhalation of dust containing the organisms. In immunocompetent people, the infection is contained; clinical nocardiosis is predominantly a disease of the immunocompromised, particularly those on chronic corticosteroids (which impair macrophage function), transplant recipients, and patients with advanced HIV.

The critical distinction - partial acid-fastness: Nocardia has a waxy cell wall that makes it weakly / partially acid-fast (modified Kinyoun positive). Using a modified acid-fast stain (with a weaker decolorizer than the standard Ziehl-Neelsen), Nocardia stains red while Actinomyces does not. This simple test differentiates the two organisms when you see branching gram-positive filaments.

Pulmonary nocardiosis is the most common presentation. Patients develop nodular or cavitary pneumonia that often mimics tuberculosis or malignancy. The infection has a propensity to disseminate, and brain imaging should be performed in all cases of pulmonary nocardiosis - CNS involvement occurs in 30-50% of cases, often without neurological symptoms. Brain abscesses are frequently multiple.

Treatment differs fundamentally from actinomycosis: Nocardia is not susceptible to penicillin. TMP-SMX is the traditional drug of choice, though species-level identification with susceptibility testing is important because resistance patterns vary. Treatment duration is prolonged (months to years in immunocompromised patients), and the mortality rate remains significant despite appropriate therapy.

Feature	Actinomyces	Nocardia
Oxygen requirement	Obligate anaerobe	Obligate aerobe
Acid-fast	Negative	Weakly positive
Source	Normal oral flora	Environmental (soil)
Host	Normal hosts (trauma/surgery)	Immunocompromised
Characteristic	Sulfur granules, sinus tracts	Pulmonary + CNS dissemination
Treatment	Penicillin	TMP-SMX

39.10 Other Anaerobes Commonly Studied with Gram-Positive Bacilli

This section bundles a few organisms that are routinely co-studied with the anaerobic gram-positives even though some are technically gram-negative. Fusobacterium and the Bacteroides / Prevotella / Porphyromonas group are gram-negative anaerobes; Cutibacterium and Lactobacillus are gram-positive. They’re grouped here because boards questions on anaerobic infections (aspiration pneumonia, intra-abdominal abscesses, oral / dental disease, prosthetic device infections) cut across the gram-positive / gram-negative line.

Fusobacterium

Despite the name “Fuso” implying shape, Fusobacterium is actually classified with the anaerobic gram-negative rods - but it’s often studied alongside the anaerobic gram-positives because of overlapping clinical presentations. It’s a long, thin, spindle-shaped (fusiform) anaerobe of the oral and GI flora.

F. necrophorum causes Lemierre syndrome, the classic “forgotten disease” that has resurged with antibiotic stewardship. A young, healthy patient has pharyngitis, then roughly a week later develops fever, neck pain, and septic pulmonary emboli from septic thrombophlebitis of the internal jugular vein. The diagnosis is often missed because pharyngitis “should get better.” Imaging shows the jugular thrombus and cannonball-like lung nodules.

F. nucleatum is a player in periodontal disease and has been implicated as a cofactor in colorectal cancer.

Treatment: Beta-lactam/beta-lactamase inhibitor combinations or metronidazole.

Cutibacterium (Propionibacterium) acnes

A slow-growing anaerobic diphtheroid of the skin, best known as a dismissable contaminant in blood cultures. The caveat: it’s a real pathogen in prosthetic joint infections (especially shoulder), post-neurosurgical CSF shunt infections, and endocarditis on prosthetic valves. Diagnosis requires patience - cultures may take up to 14 days to grow, so routine 5-day cultures miss it.

When Cutibacterium grows from multiple sets of blood cultures in a patient with a prosthetic device, it’s almost always real.

Lactobacillus

Normal flora of the vagina (Döderlein bacilli) and GI tract. Almost never pathogenic, but can cause endocarditis in severely immunocompromised patients or after GI procedures. Of practical importance, Lactobacillus is intrinsically resistant to vancomycin - a trap when vancomycin-resistant gram-positive rods are reported from blood culture and assumed to be VRE.

Anaerobic Gram-Negative Rods (Bacteroides, Prevotella, Porphyromonas)

Strictly these are gram-negative but they are tagged with gram-positive anaerobes in many study resources because they share clinical contexts (polymicrobial abscesses, aspiration pneumonia, intra-abdominal infections).

Bacteroides fragilis is the dominant anaerobe of the colon and the most common cause of anaerobic bacteremia. Its capsule is an unusual virulence factor - it’s zwitterionic and actually stimulates abscess formation (most capsules are antiphagocytic; Bacteroides is special). B. fragilis is intrinsically resistant to penicillin due to chromosomal beta-lactamase. Treatment: metronidazole, carbapenems, or beta-lactam/beta-lactamase inhibitor combos.

Prevotella and Porphyromonas are oropharyngeal anaerobes that cause aspiration pneumonia, lung abscesses, and oral/dental infections, often polymicrobial. Historically pigmented (“black-pigmented Bacteroides”) before being reclassified.

Chapter 40: Gram-Negative Cocci

40.1 Neisseria

Gram-negative diplococci (kidney bean / coffee-bean shaped with flat concave sides facing each other); catalase-positive and oxidase-positive; nonmotile (no flagella, though Type IV pili give them “twitching motility” on surfaces). Fastidious - grow at 35-37°C in 5% CO2 on enriched media (chocolate agar) or selective media (Thayer-Martin).

The two clinically important species are N. gonorrhoeae and N. meningitidis. Both are exclusively human pathogens. Nonpathogenic Neisseria (N. sicca, N. mucosa, N. flavescens, N. lactamica) live as normal flora of the upper respiratory tract - benign, but worth knowing because they can contaminate cultures and share the same Gram stain appearance.

Thayer-Martin selective media contains VCN: Vancomycin (suppresses gram-positives), Colistin (suppresses gram-negatives other than Neisseria), Nystatin (suppresses fungi), plus trimethoprim (suppresses swarming Proteus). This cocktail lets fastidious Neisseria grow out of specimens loaded with mixed genital, pharyngeal, or rectal flora.

The sugar utilization panel is the classic biochemical discriminator among gram-negative diplococci:

N. gonorrhoeae: glucose only
N. meningitidis: glucose AND maltose (Mnemonic: MeNingitidis - Maltose)
N. lactamica: glucose + maltose + lactose (nonpathogen)
M. catarrhalis: non-saccharolytic (ferments nothing)

These tests are being displaced by MALDI-TOF in modern labs, but the pattern still shows up on boards.

Neisseria are catalase-positive and oxidase-positive. Among gram-negative diplococci, oxidase+ / catalase+ narrows you to Neisseria or Moraxella - sugar utilization, DNase, and colony morphology take you the rest of the way.

IgA protease is a shared Neisseria virulence factor that cleaves secretory IgA1 at the hinge region, separating the Fab portions from the Fc. This disables mucosal antibody defense. It’s worth knowing that S. pneumoniae and H. influenzae also produce IgA protease - three organisms sharing the respiratory mucosal niche have converged on the same immune evasion trick.

Neisseria gonorrhoeae (Gonococcus)

Neisseria gonorrhoeae is a gram-negative diplococcus that causes gonorrhea, one of the most common sexually transmitted infections worldwide. The organism is exclusively a human pathogen - it infects only humans and has no animal reservoir. Understanding how gonococcal virulence factors function mechanistically explains both the clinical manifestations and the challenges of prevention and treatment.

Laboratory Identification: On Gram stain, gonococci appear as gram-negative diplococci that are often intracellular within neutrophils. This intracellular location reflects both the organism’s ability to be phagocytosed and its remarkable ability to survive inside phagocytes. Gram stain of urethral discharge showing intracellular gram-negative diplococci within PMNs is >95% sensitive and specific for gonococcal urethritis in symptomatic men. The organism is oxidase positive (like all Neisseria) and ferments glucose only - not maltose, lactose, or sucrose. This carbohydrate utilization pattern distinguishes it from N. meningitidis, which ferments both glucose and maltose. Gonococci are fastidious and require enriched media (chocolate agar) or selective media (Thayer-Martin, Martin-Lewis) containing antibiotics that suppress competing flora while allowing Neisseria growth.

Virulence Factors and Their Pathophysiological Mechanisms

The gonococcus has evolved a sophisticated arsenal of virulence factors that enable it to establish mucosal infection, evade immunity, and persist despite the inflammatory response it provokes.

Worth noting upfront: N. gonorrhoeae lacks a polysaccharide capsule (unlike N. meningitidis). This is a major difference between the two. Gonorrhea’s virulence rests on pili, Opa proteins, LOS, and IgA protease rather than an anti-phagocytic capsule. The tradeoff: gonococci are excellent at sticking around the mucosa but rarely cause fulminant bacteremic disease on the scale of meningococcemia.

Pili (Type IV pili) - The First Point of Contact: Gonococcal pili are hair-like protein appendages that extend from the bacterial surface and are essential for initial attachment to non-ciliated columnar epithelium of the urethra, endocervix, rectum, pharynx, and conjunctiva. Without pili, gonococci cannot establish infection. The pili bind to CD46 and other receptors on epithelial cells, anchoring the bacterium close enough to deploy additional adhesins. Pili also help the organism resist neutrophil killing.

But pili do something more remarkable: they undergo antigenic variation. The gonococcal genome contains one expressed pilin gene (pilE) and multiple silent pilin gene copies (pilS). Through recombination, portions of pilS genes can be copied into pilE, generating a new antigenic variant. A single strain can generate millions of different pilin variants. This variation allows the organism to escape antibody responses - by the time the host mounts effective anti-pilin antibodies, the bacterium has changed its coat. This is why immunity to gonorrhea is not protective against reinfection - each exposure may bring a pilin variant the host has never seen, and it has been a major obstacle to vaccine development.

Opa (Opacity) Proteins - Mediating Intimate Attachment and Invasion: Opa proteins are outer membrane proteins that mediate closer attachment to host cells and can promote cellular invasion. They’re named for their effect on colony morphology - Opa-expressing colonies appear opaque. Gonococci possess multiple opa genes, and each gene can independently switch on or off through phase variation - slipped-strand mispairing during replication changes the reading frame, turning protein expression on or off. This creates a population of bacteria with different combinations of Opa proteins expressed.

Different Opa proteins bind different host receptors. Some bind CEACAM (carcinoembryonic antigen-related cell adhesion molecule) receptors on epithelial cells, facilitating invasion. Some bind heparan sulfate proteoglycans. This repertoire allows the organism to adapt to different tissue niches and evade Opa-specific antibodies.

LOS (Lipooligosaccharide) - Endotoxin Without the O-Antigen: Gonococci produce lipooligosaccharide rather than the lipopolysaccharide of enteric bacteria - it lacks the repeating O-antigen polysaccharide. Like LPS, LOS has a lipid A component that activates innate immunity via TLR4, triggering the inflammatory response responsible for the purulent discharge characteristic of symptomatic gonorrhea. But LOS also undergoes phase variation and can be sialylated using host-derived CMP-NANA (cytidine monophosphate-N-acetylneuraminic acid). Sialylated LOS mimics host glycoconjugates and helps the organism resist complement-mediated killing.

IgA Protease - Disabling Mucosal Defense: The mucosal surfaces of the genital tract, pharynx, and rectum are protected by secretory IgA antibodies. Gonococci produce an IgA1 protease that cleaves human IgA1 antibodies at the hinge region, separating the antigen-binding Fab portions from the Fc portion needed for effector functions. This neutralizes mucosal antibody defenses at the site of infection.

Mechanisms of Iron Acquisition: Iron is essential for bacterial growth but is sequestered by host proteins (transferrin, lactoferrin, hemoglobin). Gonococci express surface receptors (TbpA/TbpB for transferrin, LbpA/LbpB for lactoferrin, HpuA/HpuB for hemoglobin-haptoglobin) that directly strip iron from these host proteins - a sophisticated system that reflects the organism’s adaptation to the iron-limited human environment.

Clinical Syndromes Reflect the Organism’s Mucosal Tropism

Urethritis in men is typically symptomatic - the inflammatory response to gonococcal infection produces purulent discharge and dysuria. Symptoms develop 2-5 days after exposure. The discharge is thick, purulent, and yellow-green (compare to Chlamydia, which gives thin, mucoid, clear discharge). Only ~10% of infected men are asymptomatic, so most men seek treatment, limiting transmission. Untreated urethritis can ascend to cause epididymitis (unilateral testicular pain/swelling) and prostatitis. In sexually active men under 35, N. gonorrhoeae and C. trachomatis are the leading causes of acute epididymitis; over 35, E. coli takes over. Repeated infection can scar the epididymis or vas deferens and cause male infertility.

Cervicitis in women is often asymptomatic or causes only mild symptoms (mucopurulent discharge, cervical friability, bleeding). ~50% of infected women are asymptomatic, which is the crux of the public health problem. The endocervix (non-ciliated columnar epithelium) is the preferred site of attachment. Unrecognized infection leads to ongoing transmission. More critically, untreated cervicitis can ascend to cause pelvic inflammatory disease (PID) - infection of the endometrium, fallopian tubes, and peritoneum. PID is often polymicrobial (gonorrhea + chlamydia + vaginal anaerobes). The inflammatory damage causes tubal scarring - roughly 10-20% tubal factor infertility after a single PID episode, rising with each recurrence. Consequences: chronic pelvic pain, ectopic pregnancy, and infertility.

Fitz-Hugh-Curtis syndrome is perihepatitis complicating PID - right upper quadrant pain from inflammation of the liver capsule, with classic “violin-string” fibrinous adhesions between the liver surface and the anterior abdominal wall. Both N. gonorrhoeae and C. trachomatis cause it.

Pharyngeal and rectal gonorrhea are common among men who have sex with men and are usually asymptomatic (>90% of pharyngeal cases), serving as hidden reservoirs for transmission. Rectal gonorrhea presents as proctitis (pain, discharge, tenesmus) when symptomatic. Pharyngeal infection is harder to eradicate than urogenital infection - requires higher antibiotic tissue concentrations, and it’s a worry spot for resistance evolution because gonococci can exchange DNA with commensal pharyngeal Neisseria. NAAT of rectal and pharyngeal swabs is recommended in MSM screening.

Ophthalmia neonatorum - gonococcal conjunctivitis in neonates - develops within 5 days of birth from exposure to infected maternal genital secretions during passage through the birth canal. Purulent, severe, can progress to corneal ulceration and blindness if untreated. Distinguish from chlamydial neonatal conjunctivitis, which appears later (5-14 days). Routine erythromycin ophthalmic ointment at birth is the prevention standard.

Disseminated gonococcal infection (DGI) occurs in 0.5-3% of infections when gonococci enter the bloodstream. The classic triad is dermatitis + tenosynovitis + migratory polyarthralgia - pustular skin lesions on the distal extremities (often hemorrhagic), inflammation of tendon sheaths producing “sausage fingers” typically at the hands/wrists, and joint pain that moves around before settling. DGI can progress to frank septic arthritis, typically monoarticular in a large joint. N. gonorrhoeae is the most common cause of septic arthritis in sexually active young adults, and the knee is the most commonly involved joint. Synovial fluid shows >50,000 WBC/μL with >80% neutrophils; Gram stain is positive only ~25% of the time, so NAAT on synovial fluid helps. Risk factors include complement deficiencies (especially terminal components C5-C9, needed for bactericidal membrane attack complex formation against Neisseria), menstruation, and pregnancy. DGI is more common in women than men, because women’s infections are more often asymptomatic and go untreated long enough to disseminate.

Diagnosis and Treatment: NAAT (nucleic acid amplification testing) is the preferred diagnostic method - it’s highly sensitive and can be performed on urine or swabs, facilitating screening. Culture is essential when antimicrobial susceptibility testing is needed, which is increasingly important given resistance trends.

Treatment has become challenging due to antimicrobial resistance. Gonococci have progressively developed resistance to sulfonamides, penicillins, tetracyclines, and fluoroquinolones. Current CDC regimen: ceftriaxone 500 mg IM single dose (1 g if body weight ≥150 kg). Single-dose IM ensures compliance. Dual therapy with azithromycin was previously recommended but was dropped as azithromycin resistance rose. Because Chlamydia coinfection is common (~40%) and is not detected by gonococcal cultures, add doxycycline 100 mg BID × 7 days if Chlamydia cannot be excluded. Test of cure with repeat NAAT 1-2 weeks post-treatment is recommended for pharyngeal infections.

Neisseria meningitidis (Meningococcus)

Neisseria meningitidis is one of the most feared bacterial pathogens because of its ability to cause fulminant disease in previously healthy people. A person can go from perfect health to death within 24 hours. Understanding the pathophysiology of meningococcal disease explains why it can be so devastating and why certain populations are at particular risk.

Laboratory Identification: Meningococci appear as gram-negative diplococci, morphologically indistinguishable from gonococci. The key laboratory distinction is carbohydrate fermentation: meningococci ferment both glucose AND maltose, while gonococci ferment glucose only. Like all Neisseria, meningococci are oxidase positive. Unlike gonococci, meningococci are encapsulated - the polysaccharide capsule is visible as a halo around diplococci on Gram stain and is essential for virulence.

Serogroups and Epidemiology: The capsular polysaccharide defines 13 serogroups, but five cause nearly all disease: A, B, C, W, Y. Mnemonic for the US-relevant serogroups: YWCA (Y, W, C, A) - the nonprofit. Geographic distribution varies - serogroup A causes epidemics in the African “meningitis belt,” serogroup B predominates in the US and Europe, and C, W, Y fill in the rest. Serogroup B is unusual - its capsular polysaccharide resembles human neural cell adhesion molecule (NCAM), so the host sees it as self, making polysaccharide-based vaccines against B unworkable.

About 5-15% of adults carry N. meningitidis asymptomatically in the nasopharynx at any given time. Rates climb in close-contact environments - 25-40% in college dormitories, up to 40% in military barracks. Transmission is by respiratory droplets, requiring close contact. Most carriers never develop invasive disease. Carriage actually induces immunity to the colonizing strain; invasive disease occurs when a new virulent strain is acquired by a non-immune person during the brief window before immunity develops. This is why close contacts of cases (household members, military recruits, college freshmen) are the highest-risk group.

Pharyngitis - meningococci can cause mild pharyngitis during the colonization-to-invasion transition, though most nasopharyngeal colonization is entirely asymptomatic. Whether pharyngitis progresses to meningococcemia depends on bacterial virulence factors (capsule serogroup, LOS structure) and host factors (complement status, prior immunity). Pharyngitis alone does not require specific treatment.

Virulence Factors and How They Cause Disease

The Polysaccharide Capsule - Essential for Bloodstream Survival: The capsule is absolutely essential for invasive disease. Unencapsulated strains can colonize the nasopharynx but cannot cause systemic infection. The capsule works by inhibiting complement-mediated killing and phagocytosis. The polysaccharide’s negative charge repels phagocytes, and the capsule prevents complement component C3b from depositing directly on the bacterial surface where it could promote opsonization or membrane attack complex formation.

This explains why patients with complement deficiencies - particularly deficiencies of the terminal complement components C5-C9 (which form the membrane attack complex) - are at dramatically increased risk of meningococcal disease. In these patients, even if complement is activated, they cannot form the pore-forming complex that directly kills gram-negative bacteria. Patients with properdin deficiency (an alternative pathway regulator) are also at high risk. Asplenic patients are vulnerable because the spleen is critical for clearing encapsulated bacteria that have resisted complement killing.

Lipooligosaccharide (LOS) - The Endotoxin That Drives Shock: Meningococcal LOS is among the most potent endotoxins known. When meningococci reach the bloodstream and are killed by immune defenses, they release massive amounts of LOS (also called endotoxin or lipid A). This LOS binds to TLR4 on macrophages and endothelial cells, triggering an explosive cytokine response - TNF-α, IL-1, IL-6, and others.

The consequences of this cytokine storm explain the clinical syndrome of meningococcemia. Vascular endothelium becomes “activated” and procoagulant. Capillary leak causes fluid extravasation and hypotension. Diffuse intravascular coagulation (DIC) develops, consuming clotting factors and platelets while depositing microthrombi throughout the circulation. The petechial rash that is the hallmark of meningococcal disease represents microvascular thrombosis and hemorrhage - platelets consumed by DIC can’t plug the small vessel leaks.

Purpura Fulminans - When DIC Becomes Visible: The most terrifying manifestation of meningococcemia is purpura fulminans - large, rapidly expanding areas of hemorrhagic necrosis of the skin and underlying tissue. These represent complete thrombotic occlusion of dermal blood vessels with subsequent tissue death. Purpura fulminans carries high mortality and, in survivors, often requires amputation of gangrenous extremities.

Waterhouse-Friderichsen Syndrome: Bilateral adrenal hemorrhage occurs in fulminant meningococcemia because the adrenal glands are particularly susceptible to hemorrhagic necrosis during DIC. The resulting acute adrenal insufficiency compounds the shock state. This is why glucocorticoid replacement is part of the treatment of suspected meningococcemia.

Pili and Adhesins - Initiating Colonization: Like gonococci, meningococci use Type IV pili for initial attachment to nasopharyngeal epithelium. The pili bind to CD46 on epithelial cells. Opa proteins provide additional attachment. Interestingly, meningococcal pili also undergo antigenic variation, though in a different pattern than gonococcal pili.

Clinical Syndromes

Meningococcal Meningitis presents with the classic triad of fever, headache, and neck stiffness, plus the usual meningitis exam findings (Kernig sign, Brudzinski sign, nuchal rigidity). It may progress rapidly from initial symptoms to obtundation. The petechial/purpuric rash is the critical diagnostic clue that separates it from other bacterial meningitides - S. pneumoniae and H. influenzae meningitis do not produce this rash. Bacterial meningitis with petechiae is meningococcal until proven otherwise. A rapidly spreading petechial rash in a febrile patient is a true medical emergency. CSF shows purulent findings (neutrophilic pleocytosis, high protein, low glucose). CSF Gram stain shows gram-negative diplococci (intracellular and extracellular, often within neutrophils) with roughly 60-80% sensitivity. Blood cultures are positive in ~50% of meningitis cases and ~80% of meningococcemia.

Meningococcemia Without Meningitis can be even more fulminant - septic shock with DIC develops so rapidly that the patient may die before meningeal inflammation develops. The rash starts as petechiae (small, non-blanching spots) and can progress to purpura (larger hemorrhagic areas) and purpura fulminans within hours.

Diagnosis and Treatment: The rapidity of disease mandates empiric treatment on clinical suspicion. Blood cultures and CSF (if lumbar puncture is safe) should be obtained, but antibiotic therapy must not be delayed. Gram stain of CSF showing diplococci is rapid and specific. PCR can detect meningococcal DNA in CSF and blood.

Treatment is with high-dose IV penicillin G (the drug of choice when susceptibility is confirmed) or ceftriaxone (preferred for empiric therapy because it also covers S. pneumoniae, the other common cause of bacterial meningitis). Duration is 7 days for meningococcal meningitis. Dexamethasone given before or with the first antibiotic dose reduces mortality and neurological sequelae - primary benefit is in pneumococcal disease, but it’s often given empirically before the organism is confirmed.

A note on penicillin resistance: N. meningitidis can develop reduced penicillin susceptibility through altered penicillin-binding proteins (PBP2 and PBP3), not via β-lactamase. This is mechanistically different from N. gonorrhoeae, which can acquire a plasmid-mediated β-lactamase. Altered PBPs reduce penicillin affinity; MICs of 0.12-1.0 mg/L are intermediate and still treatable with high-dose penicillin. The drift toward resistance is another reason ceftriaxone has become the go-to empiric agent.

Prevention

Vaccines: Meningococcal conjugate vaccines (MenACWY - Menactra, Menveo) contain capsular polysaccharides conjugated to protein carriers, making them T-cell dependent. This produces memory responses, works in young children, and reduces nasopharyngeal carriage (herd effect). Routinely recommended at age 11-12, with a booster at 16. Also recommended for: college freshmen in dorms, military recruits, travelers to the meningitis belt, complement-deficient patients, asplenic patients, and microbiologists handling N. meningitidis.

Serogroup B vaccines (MenB - Trumenba, Bexsero) use different technology. Because the serogroup B polysaccharide mimics human NCAM, earlier polysaccharide-based B vaccines risked triggering autoimmune cross-reactions and didn’t work. Modern MenB vaccines use recombinant outer membrane proteins (factor H binding protein, NadA, NHBA, PorA). MenB is recommended for ages 16-23 (preferably 16-18) under shared clinical decision-making, and for high-risk individuals or outbreak control.

Chemoprophylaxis for close contacts (household members, kissing contacts, daycare contacts, healthcare workers with direct respiratory exposure such as intubation without a mask) should be given within 24 hours to eliminate nasopharyngeal carriage and prevent secondary cases. Options: rifampin (600 mg BID × 2 days), ciprofloxacin (single dose), or ceftriaxone (single IM dose - preferred in pregnancy).

Laboratory handling note: N. meningitidis is fragile outside the human host. CSF specimens should never be refrigerated - the organism dies rapidly in the cold. Specimens should be transported promptly to the laboratory at room temperature.

40.2 Moraxella catarrhalis

Moraxella catarrhalis was long considered a harmless commensal of the upper respiratory tract, but it’s now recognized as the third most common bacterial cause of otitis media and sinusitis in children (after S. pneumoniae and H. influenzae) and a significant cause of lower respiratory tract infections in adults with chronic lung disease. Understanding its niche in respiratory infections and its nearly universal β-lactamase production is important for appropriate empiric therapy.

Laboratory Identification: M. catarrhalis is a gram-negative diplococcus, kidney-bean shaped and morphologically identical to Neisseria on Gram stain - intra- and extracellular organisms within neutrophils in a sputum from a COPD patient should raise the suspicion. It was formerly classified as Neisseria catarrhalis (and before that, Branhamella catarrhalis). It is catalase-positive and oxidase-positive, nonmotile, and non-spore-forming.

Distinguishing M. catarrhalis from Neisseria on the bench:

Non-saccharolytic - ferments no sugars (glucose, maltose, lactose, sucrose, fructose all negative). Neisseria always ferments at least glucose.
DNase positive (Neisseria is DNase-negative)
Tributyrin (butyrate esterase) positive
Grows well on blood agar at 35-37°C without a CO2 requirement (Neisseria needs CO2 and chocolate/Thayer-Martin)
Does not grow on MacConkey agar (distinguishing it from Enterobacterales - M. catarrhalis is a respiratory organism, not an enteric one)
Characteristic “hockey puck” colonies: round, opaque, cohesive, and slide intact across the agar surface when pushed with a loop, because the colonies are self-adherent but don’t stick to the agar

MALDI-TOF gives definitive identification quickly in modern labs.

Pathogenesis and Host-Pathogen Interaction

M. catarrhalis colonizes the oropharynx/nasopharynx in healthy individuals. Carriage rates are highest in young children (50-75% in daycare settings) and lower in adults (1-5%). Colonization is a prerequisite for infection but does not always lead to disease. M. catarrhalis possesses several adherence factors - it is encapsulated and has pili, plus outer membrane proteins UspA1, UspA2, and OMP CD that bind to respiratory epithelial receptors and help resist complement-mediated killing. These OMPs are candidate vaccine targets.

Unlike the major respiratory pathogens S. pneumoniae and H. influenzae type b, M. catarrhalis is rarely invasive. Its LOS is less potent than meningococcal LOS, and it lacks the polysaccharide capsule and iron-acquisition machinery needed for sustained bacteremia. Pathogenicity is limited to the respiratory mucosa, where it acts as an opportunist - disease emerges when local defenses are impaired: young children with immature immunity (otitis media), COPD patients with damaged airways (exacerbations), or elderly/immunocompromised patients (pneumonia, rare bacteremia).

Clinical Syndromes

Otitis media and sinusitis in children account for most M. catarrhalis disease. The organism is the third most common cause of acute otitis media, responsible for about 15-20% of cases. It tends to cause disease during winter months, often following viral upper respiratory infections that disrupt mucociliary function and promote bacterial overgrowth.

COPD exacerbations are triggered by M. catarrhalis in a significant proportion of cases. Patients with chronic bronchitis have altered airway defenses and chronic bacterial colonization; shifts in the colonizing population or acquisition of new strains can trigger acute exacerbations with increased sputum production, purulence, and dyspnea.

Pneumonia occurs primarily in elderly patients and those with underlying lung disease. It’s a community-acquired pathogen in these populations.

The β-Lactamase Problem - Why Ampicillin Doesn’t Work

The most clinically important feature of M. catarrhalis is that >90% of strains produce β-lactamase (approaching 100% in many surveys). This wasn’t always the case - β-lactamase production was rare until the 1970s but spread rapidly through the population, a classic example of antibiotic selection pressure driving resistance evolution.

The β-lactamases produced by M. catarrhalis are typically BRO-1 or BRO-2 enzymes. These efficiently hydrolyze penicillin and ampicillin, making these antibiotics useless. This is critically important for empiric therapy of otitis media and sinusitis, where M. catarrhalis is one of the common pathogens - amoxicillin alone will not reliably cover M. catarrhalis.

Empiric treatment of suspected M. catarrhalis infections (or mixed infections where M. catarrhalis is a possible pathogen) should use antibiotics stable to these β-lactamases: amoxicillin-clavulanate, second- or third-generation cephalosporins (cefuroxime, etc.), macrolides (azithromycin), fluoroquinolones, or TMP-SMX. The β-lactamase inhibitor clavulanate effectively neutralizes M. catarrhalis β-lactamase, making amoxicillin-clavulanate an excellent empiric choice for respiratory tract infections.

Non-catarrhalis Moraxella - Eye Infections

A few other Moraxella species are worth recognizing. M. lacunata and M. nonliquefaciens cause ocular infections - conjunctivitis and keratitis. M. lacunata is the classic cause of angular blepharoconjunctivitis (infection at the lateral canthus). These are normal conjunctival flora that cause opportunistic disease. Morphologically they differ from M. catarrhalis: the non-catarrhalis Moraxella species are gram-negative coccobacilli (rod-ish), not diplococci. They generally don’t produce β-lactamase, so they remain susceptible to penicillins. Other non-catarrhalis species: M. osloensis and M. phenylpyruvica.

Chapter 41: Enterobacteriaceae (Enterobacterales)

The Enterobacteriaceae family includes many of the most important human pathogens - E. coli, Klebsiella, Salmonella, Shigella, and others. They share defining features that unify the family: gram-negative bacilli, facultative anaerobes, glucose fermenters, nitrate reducers, and oxidase negative. This last feature - oxidase negativity - is the single most useful test distinguishing Enterobacteriaceae from other gram-negative rods like Pseudomonas (oxidase positive).

On MacConkey agar, Enterobacteriaceae are divided by whether they ferment lactose. Lactose fermenters produce acid, turning the pH indicator pink/red. Non-fermenters remain colorless. This simple distinction immediately narrows the differential: lactose fermenters include E. coli, Klebsiella, and Enterobacter; non-fermenters include Salmonella, Shigella, and Proteus.

The IMViC panel is a classic biochemical scheme that still shows up on boards. Four tests: Indole, Methyl Red, Voges-Proskauer, Citrate. The pattern for E. coli is ++-- (indole+, MR+, VP-, citrate-). The pattern for Klebsiella/Enterobacter is --++. MALDI-TOF has replaced this in most real labs, but the panel remains a high-yield set of discriminators.

41.1 Lactose Fermenters

Escherichia coli

E. coli is simultaneously the most common commensal gram-negative bacterium in the human gut and one of the most important pathogens. This paradox reflects the fact that different strains have very different capabilities. The E. coli living harmlessly in your intestine lacks the virulence factors that make pathogenic strains dangerous.

Laboratory features: E. coli is indole positive (tryptophan deaminase splits tryptophan, Kovac’s reagent turns red), IMViC ++--, and commonly beta-hemolytic on blood agar - especially uropathogenic strains (UPEC) and certain diarrheagenic strains. The hemolysis is mediated by alpha-hemolysin (HlyA), a pore-forming RTX-family cytotoxin. On MacConkey E. coli grows as pink/red flat dry colonies (lactose fermenter). On EMB agar it produces the classic green metallic sheen.

E. coli causing disease outside the gut: Commensal E. coli becomes pathogenic when it escapes its normal niche. Urinary tract infections - the most common bacterial infection in humans - occur when fecal E. coli ascend the urethra to the bladder. Uropathogenic E. coli (UPEC) accounts for ~90% of community-acquired UTIs. UPEC isn’t random commensal E. coli - it carries specific virulence factors:

Type 1 fimbriae (FimH adhesin) bind mannose residues on uroplakin III of bladder epithelium - important for cystitis
P fimbriae (PapG adhesin) bind globoseries glycolipids on renal epithelium - important for pyelonephritis
Alpha-hemolysin for tissue damage, siderophores for iron acquisition, K1 capsule for serum resistance

In neonates, E. coli is the second most common cause of neonatal meningitis (after GBS), transmitted during passage through the birth canal. About 70-80% of E. coli meningitis isolates carry the K1 capsule, a polysaccharide of polysialic acid (alpha-2,8-linked N-acetylneuraminic acid). K1 is anti-phagocytic, blocks alternative-pathway complement activation, and mimics host neural cell adhesion molecule (NCAM), so it’s poorly immunogenic. The K1 capsule is chemically identical to the capsule of Neisseria meningitidis serogroup B - both polysialic acid, both molecularly mimicking NCAM. That similarity is why serogroup B meningococcal vaccines are protein-based (OMV/factor H binding protein) rather than polysaccharide - you can’t mount a good response against a self-mimicking polysaccharide. CSF in neonatal E. coli meningitis shows neutrophilic pleocytosis, high protein, low glucose, gram-negative rods on stain. E. coli meningitis also occurs in immunocompromised adults and in patients with compromised gut integrity (colorectal cancer, Strongyloides hyperinfection).

E. coli causing intestinal disease: Diarrheal disease requires acquisition of specific virulence factors - toxins, adhesins, or invasion machinery - that distinguish pathogenic strains. Understanding the different pathotypes explains the different clinical presentations. Six pathotypes matter: ETEC, EHEC, EPEC, EIEC, EAEC, and DAEC.

Enterotoxigenic E. coli (ETEC) is the leading cause of traveler’s diarrhea. ETEC is non-invasive and afebrile - the toxins act on intact epithelium without mucosal damage. No fever, no blood, no WBCs in stool. Two toxins:

Heat-labile toxin (LT) is an AB5 toxin identical in mechanism to cholera toxin. It ADP-ribosylates Gs-alpha, locking adenylate cyclase on, driving intracellular cAMP up. Elevated cAMP opens CFTR chloride channels, pulling chloride and water into the lumen. LT is destroyed at 60°C (hence “heat-labile”) and is immunogenic.
Heat-stable toxin (ST) activates guanylyl cyclase C, increasing intracellular cGMP. cGMP also opens CFTR, same end result. ST is a small peptide, survives boiling, and is poorly immunogenic (no vaccine target). Fun mechanistic tie-in: linaclotide (Linzess), a constipation drug, works by the same GC-C/cGMP pathway.

Prevention is “boil it, cook it, peel it, or forget it.”

Enterohemorrhagic E. coli (EHEC) causes a completely different - and far more dangerous - disease. The prototype is E. coli O157:H7. These strains produce Shiga toxin (also called verotoxin because it kills Vero cells in culture), identical to the toxin made by Shigella dysenteriae. Shiga toxin is encoded by a lambdoid bacteriophage - an example of lysogenic conversion. Antibiotics trigger the bacterial SOS response, inducing phage lysis and massive toxin release. This is the molecular reason you don’t give antibiotics in EHEC.

Mechanistically, Shiga toxin is an AB5 toxin. The B5 pentamer binds Gb3 (globotriaosylceramide) on endothelial cells, especially glomerular endothelium. The A subunit is an N-glycosidase that cleaves an adenine from 28S rRNA, halting protein synthesis and killing the cell. The result: hemorrhagic colitis and thrombotic microangiopathy.

EHEC produces two syndromes that can occur singly or together:

Hemorrhagic colitis: bloody diarrhea, severe abdominal cramps, and characteristically no fever (a useful discriminator from invasive bacterial diarrhea).
Hemolytic uremic syndrome (HUS): develops 5-10 days after diarrhea onset in ~5-15% of cases, most often in children. The triad: microangiopathic hemolytic anemia (schistocytes, high LDH, low haptoglobin), thrombocytopenia, and acute kidney injury. ADAMTS13 activity is normal in HUS (this distinguishes it from TTP, where ADAMTS13 is severely deficient).

Transmission is foodborne (undercooked ground beef, unpasteurized milk/juice, produce, petting zoos - cattle are the reservoir), person-to-person (low infectious dose, 10-100 organisms, comparable to Shigella), and waterborne. Management is supportive: IV fluids, renal monitoring, transfusion as needed. No antibiotics, no antimotility agents (loperamide) - both worsen outcomes.

Laboratory identification of EHEC exploits a metabolic quirk: O157:H7 doesn’t ferment sorbitol, so it produces colorless colonies on sorbitol-MacConkey (SMAC) agar while other E. coli produce pink colonies. Modern labs use Shiga toxin EIA or PCR, which also catches non-O157 STEC.

E. coli O104:H4 is a notable hybrid strain that caused the 2011 German outbreak (>4,000 cases, 54 deaths). It was an enteroaggregative strain that acquired the Stx2 phage, giving it both prolonged aggregative adherence and Shiga toxin production. The combination drove an unusually high HUS rate (~25%).

Enteropathogenic E. coli (EPEC) is moderately invasive and produces a low-grade febrile diarrhea, typically in infants and children in developing countries. EPEC uses a type III secretion system to inject effector proteins that destroy microvilli and establish “attaching and effacing” (A/E) lesions - the bacteria sit tightly on a pedestal of reorganized host actin. No Shiga toxin, no LT/ST.

Enteroinvasive E. coli (EIEC) produces an illness clinically identical to shigellosis: invasive, high fever, dysentery with fecal WBCs. EIEC and Shigella share the large pINV virulence plasmid that encodes their invasion machinery - EIEC basically is Shigella dressed as E. coli. Biochemically EIEC mimics Shigella too: non-lactose-fermenting, non-motile, lysine decarboxylase negative. Distinguishing them requires molecular methods or serotyping.

Enteroaggregative E. coli (EAEC) causes watery, sometimes persistent diarrhea. Three populations to remember: HIV patients (persistent diarrhea), infants/children in developing countries (persistent diarrhea >14 days), and travelers (less common than ETEC but increasingly recognized). On HEp-2 cell culture, EAEC adheres in a characteristic “stacked brick” pattern mediated by aggregative adherence fimbriae (AAF/I, AAF/II, AAF/III). The bacteria form a mucus biofilm and produce enterotoxins.

Klebsiella pneumoniae

Klebsiella pneumoniae is characterized by its thick polysaccharide capsule, which gives colonies a distinctive mucoid, glistening appearance and contributes to virulence by inhibiting phagocytosis. Understanding Klebsiella requires recognizing that it now exists in three clinical contexts: classic community-acquired infection, hypervirulent strains causing invasive disease, and highly resistant strains threatening healthcare.

Laboratory identification: Klebsiella on Gram stain shows short, plump gram-negative rods surrounded by a wide clear halo - the capsule shows up as a negative impression on Gram, and as a prominent unstained halo on India ink. On MacConkey agar, colonies are pink/red and distinctively mucoid (slimy, string-like) - the ropy consistency reflects the K antigen capsule. The string test (touching a loop to a colony and pulling away) is positive if a mucoid string ≥5 mm forms; a positive string test marks the hypermucoviscous strains linked to invasive liver abscess syndrome.

Key biochemical features:

Nonmotile - among Enterobacterales only Klebsiella and Shigella are strictly nonmotile (no flagella, no swarming)
Indole negative for all species except K. oxytoca, which is indole positive - K. oxytoca is notable because it causes antibiotic-associated hemorrhagic colitis, distinct from C. difficile colitis
IMViC --++ (indole-, MR-, VP+, citrate+)
Urease positive, which alkalinizes urine during UTI and can drive struvite stone formation, similar to Proteus (though Klebsiella’s urease is weaker)

Carrier state: Healthy community adults carry K. pneumoniae in the GI tract at ~5-10%, but hospitalized patients carry it at up to 35%. Hospital acquisition happens through healthcare worker hands and environmental surfaces. Prior antibiotics, indwelling devices, and ICU stays all raise risk.

Classic Klebsiella pneumonia: Historically, K. pneumoniae was known for causing a severe, necrotizing pneumonia in alcoholics, diabetics, and COPD patients - patients with impaired cough reflex and neutrophil function. The classic description includes “currant jelly” sputum (thick, bloody, gelatinous - the jelly consistency comes from massive capsule production, the blood from necrotizing tissue destruction) and upper lobe cavitation. Chest imaging can show a “bulging fissure sign” from a heavy, swollen consolidated lobe displacing the adjacent fissure. Mortality is historically around 50% even with treatment. Don’t confuse “currant jelly sputum” (Klebsiella pneumonia) with “currant jelly stool” (intussusception) - different entities. While this classic alcoholic-pneumonia picture still occurs, Klebsiella’s bigger footprint today is as a nosocomial pathogen causing pneumonia, UTI, and bloodstream infections in hospitalized patients.

Other infections: K. pneumoniae is a common nosocomial cause of UTI (especially in catheterized patients) and causes pyogenic liver abscesses, particularly in patients with East or Southeast Asian ancestry (see hypervirulent strains below).

Hypervirulent K. pneumoniae: In the 1980s, a distinct syndrome emerged in Asia: community-acquired pyogenic liver abscess in otherwise healthy patients, caused by Klebsiella strains with enhanced capsule production (K1/K2 serotypes) and other virulence factors. These hypervirulent strains are associated with metastatic infection - the bacteria seed from the liver abscess to the eyes (endophthalmitis, often causing blindness), CNS, or other sites. This syndrome is reported more commonly in East and Southeast Asian populations and in patients with diabetes. The string test is positive on these isolates.

Intrinsic resistance and carbapenem-resistant K. pneumoniae (CRKP): K. pneumoniae is intrinsically resistant to ampicillin and ticarcillin because of a chromosomal SHV-1 beta-lactamase. This means ampicillin/amoxicillin alone should never be used empirically for Klebsiella (unlike E. coli, which may be ampicillin-susceptible). On top of intrinsic resistance, Klebsiella is a major carrier of acquired ESBLs (plasmid-mediated CTX-M) and carbapenemases. The most concerning development is the emergence of strains resistant to carbapenems - our “last-line” antibiotics for serious gram-negative infections. These strains produce carbapenemases, enzymes that hydrolyze virtually all beta-lactams. The most common in the United States is KPC (Klebsiella pneumoniae carbapenemase). Treatment options are severely limited - polymyxins (which have significant toxicity), tigecycline, and newer agents like ceftazidime-avibactam and meropenem-vaborbactam.

Other Klebsiella species

Klebsiella rhinoscleromatis causes rhinoscleroma, a chronic granulomatous infection of the upper respiratory tract (nose, sinuses, pharynx, larynx), endemic in tropical regions. The pathognomonic histologic finding is Mikulicz cells - large macrophages with vacuolated cytoplasm containing intracellular bacteria, visible on H&E. You may also see Russell bodies (eosinophilic inclusions in plasma cells). Treatment is prolonged antibiotics (fluoroquinolones, TMP-SMX).

Klebsiella granulomatis (formerly Calymmatobacterium granulomatis) causes granuloma inguinale (Donovanosis), a sexually transmitted disease endemic in tropical regions. Presentation: painless genital ulcers with beefy-red, friable, granulating tissue that bleeds easily and progressively destroys local tissue. No lymphadenopathy (this distinguishes it from syphilis, LGV, and chancroid). Quick differential for genital ulcers:

Painless: syphilis (chancre), granuloma inguinale (Donovanosis)
Painful: chancroid (H. ducreyi), HSV

K. granulomatis cannot be cultured on standard media. Diagnosis is by Giemsa or Wright stain of a tissue crush preparation showing Donovan bodies - intracellular bacteria within macrophages, with a characteristic bipolar “safety pin” appearance. The term “Donovan bodies” refers specifically to the intracellular bacteria, not the macrophages themselves. Treatment: azithromycin or doxycycline for at least 3 weeks.

Enterobacter

Enterobacter species are opportunistic gram-negative pathogens that have become increasingly important causes of healthcare-associated infections. What makes Enterobacter particularly challenging is not its virulence - it’s relatively low-virulence compared to organisms like Pseudomonas - but its intrinsic resistance mechanism that can turn initial susceptibility into treatment failure. Understanding the molecular basis of this inducible resistance is essential for clinical practice.

Laboratory Identification: Enterobacter species are lactose fermenters, producing pink/red colonies on MacConkey agar. They are motile (unlike Klebsiella) and Voges-Proskauer positive (they produce acetoin from glucose metabolism - a feature useful for distinguishing them from similar Enterobacteriaceae). Multiple species exist, with E. cloacae and E. aerogenes (now reclassified as Klebsiella aerogenes) being the most clinically important.

Clinical Significance: Enterobacter causes healthcare-associated infections in hospitalized patients, particularly those who have received prior antibiotics, have indwelling devices, or are in the ICU. Common infections include ventilator-associated pneumonia, catheter-associated urinary tract infections, bloodstream infections (often from catheter or intra-abdominal sources), and wound infections. Enterobacter is not a particularly aggressive pathogen - it primarily infects patients whose defenses are already compromised.

The Inducible AmpC β-Lactamase - Why Cephalosporins Fail

The key to understanding Enterobacter is its chromosomal AmpC β-lactamase. All Enterobacter carry the ampC gene, which encodes a β-lactamase capable of hydrolyzing most cephalosporins and penicillins. The critical point is that this gene is inducible - under normal conditions, it’s expressed at low levels, and the organism may test susceptible to third-generation cephalosporins like ceftriaxone. But when exposed to certain β-lactams, ampC expression is induced to high levels, and resistance emerges.

The molecular mechanism: AmpC expression is regulated by a complex system involving the ampR regulator, AmpD (a cytoplasmic amidase), and AmpG (a permease). Normally, cell wall recycling products (muropeptides) are processed by AmpD, keeping AmpC expression low. When β-lactam antibiotics disrupt cell wall synthesis, the recycling pathway is overwhelmed, muropeptides accumulate, and they trigger AmpR to activate ampC transcription. The result is rapid β-lactamase production.

The clinical trap: A patient with Enterobacter bacteremia may have cultures showing susceptibility to ceftriaxone. The physician starts ceftriaxone, and the patient initially improves. But the antibiotic selects for constitutive AmpC overproducers (mutants that express AmpC at high levels even without induction - often through mutations in ampD). Within days, the patient relapses with now-resistant Enterobacter. This “emergence of resistance on therapy” occurs in up to 20% of patients treated with third-generation cephalosporins for Enterobacter infections.

The SPACE organisms: Enterobacter is one of the “SPACE” organisms - Serratia, Pseudomonas, Acinetobacter, Citrobacter (specifically C. freundii), and Enterobacter - all of which carry inducible AmpC β-lactamases and are at risk for the same phenomenon. The mnemonic helps remember which organisms should NOT be treated with third-generation cephalosporins even if they test susceptible.

Treatment Implications: Because of the risk of resistance emergence, Enterobacter infections should be treated with agents stable to AmpC:

Carbapenems (meropenem, imipenem) are stable to AmpC and are first-line for serious Enterobacter infections
Cefepime (fourth-generation cephalosporin) is relatively stable to AmpC and can be used, though resistance can still emerge
Fluoroquinolones are not affected by β-lactamases but resistance is increasing
Aminoglycosides can be used in combination
Third-generation cephalosporins (ceftriaxone, ceftazidime) should be avoided even when tested susceptible

The Emerging Carbapenem Resistance Threat: Beyond AmpC, some Enterobacter strains have acquired carbapenemases - enzymes that hydrolyze even carbapenems. These carbapenem-resistant Enterobacter (CRE) strains are designated urgent threats by the CDC. When carbapenems fail, options become very limited: ceftazidime-avibactam, meropenem-vaborbactam, polymyxins, or tigecycline, depending on the specific resistance mechanism.

Citrobacter

Citrobacter species are lactose-fermenting, motile (peritrichous flagella) gram-negative rods and opportunistic nosocomial pathogens. The genus is named for its ability to use citrate as a sole carbon source (citrate positive) on Simmons citrate agar - the medium turns blue from alkaline pH. Citrate positivity is shared with Klebsiella, Enterobacter, and Serratia; E. coli is citrate negative, which helps differentiate it.

Two clinically important species:

C. freundii: H2S positive on TSI and Hektoen agar - which means it must be distinguished from Salmonella, also H2S positive. The key difference: C. freundii ferments lactose (pink on MacConkey), Salmonella does not (colorless on MacConkey). On TSI, C. freundii gives a yellow (acid) slant with a yellow+black butt, while Salmonella gives a red (alkaline) slant with yellow+black butt. C. freundii is ONPG-positive (beta-galactosidase+) while most Salmonella are ONPG-negative. C. freundii is also one of the SPICE/SPACE organisms (inducible AmpC beta-lactamase), so avoid third-generation cephalosporins even when susceptibility testing suggests otherwise.
C. koseri (formerly C. diversus): H2S negative, notable for causing neonatal brain abscesses.

Citrobacter causes UTIs, pneumonia, and bloodstream infections, mostly in hospitalized or otherwise compromised patients.

41.2 Non-Lactose Fermenters

MacConkey agar: Colorless colonies

Salmonella

Salmonella is divided into two distinct clinical categories that behave very differently: non-typhoidal Salmonella (NTS), which causes gastroenteritis that stays in the gut, and typhoidal Salmonella (S. Typhi and S. Paratyphi), which causes a systemic illness. Understanding the pathogenesis explains this clinical divergence.

Nomenclature: Salmonella enterica is the only clinically relevant species, with more than 2,500 serovars classified by O (somatic), H (flagellar), and Vi (capsular) antigens. The major serovars fall into two groups:

Typhoidal: S. Typhi, S. Paratyphi (cause typhoid/enteric fever)
Non-typhoidal (NTS): S. Typhimurium, S. Enteritidis, and many others (cause gastroenteritis)

The formally correct name “Salmonella enterica serovar Typhimurium” is conventionally abbreviated “Salmonella Typhimurium” or “S. Typhimurium” (with the serovar name capitalized and not italicized).

Non-typhoidal Salmonella gastroenteritis: NTS serovars (Enteritidis, Typhimurium predominating) cause one of the most common foodborne infections. Transmission routes:

Food (most important): poultry, eggs, dairy, undercooked produce
Fecal-oral
Reptile pets: turtles, iguanas, snakes, lizards harbor Salmonella in their GI tracts. The CDC recommends against reptile pets in households with children under 5.
Other pet contact (dogs, cats, rodents)

Incubation is 6-48 hours. After ingestion, Salmonella invades the intestinal epithelium, triggering an inflammatory response with neutrophil recruitment. The result is an inflammatory diarrhea - watery to bloody, with fever, cramps, and nausea.

In immunocompetent patients, NTS gastroenteritis is self-limited (3-7 days). Antibiotics are not indicated for uncomplicated NTS - they don’t shorten illness, may prolong fecal shedding/carrier state, and risk selecting resistance. Treatment is supportive (oral rehydration). Antibiotics are indicated in specific scenarios: severe/invasive disease, bacteremia, extremes of age, immunocompromise, and patients with prosthetic joints or vascular grafts.

In immunocompromised patients (HIV with low CD4, malignancy, chemotherapy, organ transplant), the elderly, and those with structural abnormalities like sickle cell disease, NTS can escape the gut and cause bacteremia and metastatic infection - high mortality, often with chills, fever, and hypotension. Patients with sickle cell disease are particularly prone to Salmonella osteomyelitis. Sickled RBCs infarct bone, creating a vascular niche; functional asplenia impairs clearance of intracellular organisms; and gut ischemia from sickling allows Salmonella translocation. S. aureus is still the overall most common cause of osteomyelitis, but the classic board association “sickle cell + osteomyelitis” points at Salmonella.

Typhoid fever is caused by S. Typhi and S. Paratyphi. Unlike NTS, S. Typhi is exclusively human-adapted - there is no animal reservoir, which is why transmission is strictly fecal-oral, through human fecal contamination of food or water. Instead of causing localized intestinal inflammation, S. Typhi invades the gut silently, penetrates to mesenteric lymph nodes, enters the bloodstream, and disseminates to the reticuloendothelial system (liver, spleen, bone marrow). The clinical presentation reflects this systemic infection: step-wise rising fever over days, relative bradycardia (pulse lower than expected for the fever), hepatosplenomegaly, and rose spots - faint salmon-colored macules on the abdomen. Counterintuitively, constipation is more common than diarrhea in the first week.

The danger of typhoid comes from the location of bacteria: they replicate in Peyer’s patches in the intestinal wall. As these patches swell and necrose, they can perforate or hemorrhage - life-threatening complications that occur in the third week if untreated. Other complications: endocarditis, splenic/liver abscess.

Chronic carriage occurs in ~1-5% of patients. Bacteria persist in the gallbladder (often on gallstones), shed intermittently in stool. The most famous carrier was “Typhoid Mary” (Mary Mallon), a cook who infected dozens before public health authorities forcibly isolated her. Cholecystectomy is sometimes required to eliminate the carrier state.

Vi antigen: S. Typhi carries the Vi (virulence) capsular antigen, a polysaccharide of N-acetylgalactosaminuronic acid. It is anti-phagocytic and anti-complement. Vi is nearly unique to S. Typhi (some S. Paratyphi C strains also carry it) and has three uses to know:

Serologic identification
Vi polysaccharide vaccine (Typhim Vi)
Vi can mask O antigen, causing false-negative O agglutination in serotyping - the organism must be heated to destroy Vi before O typing

Typhoid in the U.S.: Most U.S. typhoid cases are imported, associated with travel history to endemic areas (South/Southeast Asia especially, Africa, Latin America).

Laboratory identification:

Salmonella does not ferment lactose or sucrose - colorless/pale colonies on MacConkey
On Hektoen enteric (HE) agar: blue-green colonies with black centers (H2S+)
On xylose-lysine-deoxycholate (XLD): red colonies with black centers
On Salmonella-Shigella (SS) agar: colorless colonies ± H2S
Citrate positive (NTS serovars), but S. Typhi is citrate negative - a useful exception within the genus
Lysine decarboxylase (LDC) positive and ornithine decarboxylase (ODC) positive - classic pattern
H2S production on TSI: most NTS produce copious H2S (heavy blackening). S. Typhi produces only scant H2S, a thin black line at the slant-butt junction - the “mustache” pattern. S. Paratyphi does not produce H2S at all, which makes it look like Shigella on TSI (K/A, no gas, no H2S); motility distinguishes them - S. Paratyphi is motile, Shigella is not.

Blood culture is key for typhoid diagnosis; bone marrow culture is even more sensitive and remains positive even after antibiotics are started.

Shigella

Shigella causes bacillary dysentery - bloody diarrhea with mucus, fever, and painful straining (tenesmus). Understanding Shigella’s pathogenesis explains why so few organisms cause disease and why the disease is so locally destructive.

Shigella is genetically and biochemically very similar to E. coli - genomically, Shigella is effectively E. coli, but it’s maintained as a separate genus for clinical convention. Key distinguishing features of Shigella vs. E. coli: Shigella is nonmotile, non-lactose-fermenting, does not produce gas from glucose, does not produce H2S, and is LDC negative.

The most infectious bacterial pathogen: The infectious dose for Shigella is remarkably low - as few as 10-100 organisms can cause disease, compared to ~10^5-108 for Salmonella and ~10^8 for Vibrio cholerae. The low dose reflects Shigella’s acid resistance (survives gastric pH). This explains why Shigella spreads so efficiently person-to-person fecal-oral, particularly in settings with poor sanitation, daycare centers, MSM, and institutions. Contaminated food and water also transmit. No animal reservoir - humans and some primates are the only hosts. The “4 F’s” of Shigella transmission: Fingers, Flies, Food, Feces.

Invasion without dissemination: Shigella’s strategy is invasion of the colonic epithelium. The bacteria are taken up by M cells, escape into the lamina propria, are engulfed by macrophages, and then induce apoptosis to escape. They enter colonic epithelial cells from the basolateral side and use actin-based motility (like Listeria) to spread from cell to cell. This invasion destroys the epithelium, causing ulceration, bleeding, and the characteristic bloody mucoid stool. Unlike S. Typhi, Shigella almost never disseminates to the bloodstream - the disease remains localized to the colon.

Shiga toxin: S. dysenteriae type 1 produces Shiga toxin (stx1), the original Shiga toxin. Mechanism is identical to EHEC’s: AB5 toxin, B5 binds Gb3, A cleaves 28S rRNA, inhibits protein synthesis, damages endothelium, can trigger HUS. The other Shigella species don’t produce this toxin, which is why S. dysenteriae causes the most severe disease.

Key treatment distinction: HUS caused by S. dysenteriae should be treated with antibiotics (fluoroquinolones, azithromycin, or ceftriaxone), unlike EHEC-HUS where antibiotics are contraindicated. The paradox: severe invasive shigellosis carries enough morbidity and mortality that treatment is necessary despite the theoretical toxin-release risk. The untreated Shigella dysentery is worse than the treated one.

The four species: Mnemonic “Dysentery Follows Bad Supper” = D, F, B, S = groups A, B, C, D:

S. dysenteriae (group A) - most virulent, produces Shiga toxin, causes HUS; uncommon overall
S. flexneri (group B) - predominates in developing countries; associated with post-infectious reactive arthritis (formerly Reiter syndrome) in HLA-B27+ patients (arthritis + urethritis + conjunctivitis, 1-3 weeks after GI infection). Other GI triggers of HLA-B27 reactive arthritis: Campylobacter, Yersinia, Salmonella, Chlamydia trachomatis.
S. boydii (group C) - rare, mainly India
S. sonnei (group D) - most common in the U.S. and developed countries; generally mildest disease (watery diarrhea that may become bloody). Outbreaks in daycare, schools, institutions. S. sonnei is the only Shigella species that is ONPG-positive - it can be a late lactose fermenter.

Laboratory identification:

Does not ferment lactose - colorless on MacConkey (in contrast to E. coli)
Does not produce gas from sugar fermentation (in contrast to E. coli, which produces abundant gas)
Does not produce H2S (distinguishing from Salmonella)
Strictly nonmotile (Salmonella is motile) - among Enterobacterales, only Klebsiella and Shigella are strictly nonmotile
On TSI: K/A without gas, without H2S - the “quiet” TSI pattern
On HE agar: green transparent colonies (no H2S, no lactose fermentation)
On XLD: red/pink colonies (no H2S)

Complications:

HUS (S. dysenteriae type 1 only)
Post-infectious reactive arthritis, HLA-B27 associated (especially S. flexneri)

Proteus and Tribe Proteeae

Tribe Proteeae groups three genera: Proteus, Morganella, Providencia. All share two biochemical hallmarks: strong urease positivity and phenylalanine deaminase (PDA) positivity. All three cause UTIs, especially complicated/catheter-associated UTIs, and the urease activity alkalinizes urine to promote struvite stone formation. Proteus is by far the most clinically significant genus.

Proteus species are non-lactose fermenters notable for two distinctive laboratory features: swarming motility and powerful urease production. Both have clinical relevance.

Swarming motility is impossible to miss - on blood agar, Proteus colonies spread outward from the inoculation point in successive concentric waves/rings, producing a “bull’s eye” pattern that covers the entire plate. Proteus alternates between short vegetative “swimmer” cells (in liquid) and elongated, hyperflagellated “swarmer” cells (on solid surfaces). The swarming can obscure other organisms on mixed cultures, so selective media or the Dienes line technique are used to contain it. P. mirabilis has the most dramatic swarming. Clostridium septicum also swarms but isn’t a typical UTI pathogen.

Urease and stone formation: Proteus produces the most potent urease of any bacterium. The enzyme hydrolyzes urea to ammonia (NH3) and CO2, alkalinizing the urine. When the pH rises above 7, magnesium ammonium phosphate (struvite) precipitates, forming stones. Proteus - especially P. mirabilis - is the classic cause of “struvite” or “infection stones” - large, branching calculi called staghorn calculi because they fill the renal pelvis and calyces like antlers. These stones harbor bacteria within their matrix, making eradication impossible without surgical removal. Other urease-positive, struvite-forming UTI organisms: Klebsiella, Morganella, Providencia, and Corynebacterium urealyticum - but Proteus is the most strongly associated.

Biochemical profile:

Catalase positive
Nitrate positive
Urease strongly positive
PDA positive
H2S positive on TSI, HE, and XLD (black precipitate - distinguish from Salmonella by swarming)
On TSI: K/A or A/A (depending on sucrose fermentation) with gas and H2S

P. mirabilis vs. P. vulgaris:

P. mirabilis is indole negative (the mnemonic “Mira-BILE is INDO-lent”). It is the most common Proteus species in clinical UTIs, generally susceptible to ampicillin.
P. vulgaris is indole positive, typically more antibiotic-resistant (a SPICE/ESCAPPM organism with inducible AmpC). P. vulgaris was historically used in the Weil-Felix test: the OX-19, OX-2, and OX-K antigens of P. vulgaris cross-react with Rickettsia antibodies, providing a crude serologic test for rickettsial infections before specific tests existed.

The indole test uses Kovac’s reagent (p-dimethylaminobenzaldehyde), added to organisms grown in tryptophan-containing broth (e.g., SIM medium). Positive = red/pink ring at the top where the reagent sits; negative = no color change (stays yellow). A rapid “spot indole” can be done directly from colonies.

Clinical infections: Proteus primarily causes urinary tract infections, particularly in catheterized patients and those with structural abnormalities. It also causes wound infections and bacteremia.

Morganella and Providencia: Both cause UTIs in patients with indwelling catheters - long-term catheterized nursing home residents, spinal cord injury patients. Both are urease-positive and PDA-positive. Both are intrinsically resistant to many antibiotics (AmpC producers). Morganella morganii is also associated with post-surgical wound infections. Providencia stuartii is the most commonly isolated Providencia from catheter-associated UTIs.

Yersinia

The genus Yersinia includes three human pathogens of vastly different importance: Y. enterocolitica (self-limited gastroenteritis), Y. pseudotuberculosis (rare, enterocolitica-like illness), and Y. pestis (plague).

Yersinia enterocolitica

Y. enterocolitica causes a febrile diarrheal illness that can mimic appendicitis because of prominent mesenteric lymphadenopathy and right lower quadrant pain. This has led to unnecessary appendectomies.

Clinical syndrome: Most infections are self-limited gastroenteritis - watery or bloody diarrhea, abdominal pain, fever - resolving in 1-3 weeks without antibiotics. Most common in children and young adults, and more common in Scandinavia and Northern Europe. Antibiotics are reserved for severe infection, bacteremia, or immunocompromise.

Pathogenesis - mesenteric lymphadenitis and pseudoappendicitis: Y. enterocolitica replicates in Peyer’s patches of the terminal ileum, then spreads to mesenteric lymph nodes, causing mesenteric lymphadenitis with right lower quadrant pain, tenderness, fever, and leukocytosis. The presentation mimics acute appendicitis - “pseudoappendicitis”. At surgery, the appendix is normal and mesenteric nodes are grossly enlarged and inflamed. Classic board scenario: young patient with RLQ pain taken for appendectomy, appendix is normal, mesenteric lymph nodes are enlarged. Other causes of mesenteric lymphadenitis to keep in mind: M. tuberculosis, Salmonella, and Y. pseudotuberculosis.

Transmission: Fecal-oral route, direct contact with animals (especially pigs - the primary reservoir; also dogs, cats, livestock), contaminated water, and blood transfusion (see below). High-risk foods: raw or undercooked pork (chitterlings), unpasteurized milk, contaminated water.

Iron and Yersinia: Y. enterocolitica is siderophilic (iron-loving) - more common and more virulent in patients with iron overload, particularly hereditary hemochromatosis. Iron is essential for bacterial metabolism and also impairs macrophage bactericidal function. Y. enterocolitica has a siderophore receptor (FoxA) that scavenges host iron. Deferoxamine (iron chelator) paradoxically increases Yersinia risk by acting as a bacterial siderophore. Vibrio species are similarly siderophilic, and the fungi Mucor/Rhizopus (Mucorales) also thrive in iron overload.

Transfusion-associated sepsis: Y. enterocolitica is the most common cause of transfusion-related sepsis from packed RBC transfusions. It can grow and produce endotoxin at 4°C (RBC storage temperature), multiplying during the 42-day storage period. Contamination happens during donor collection (donor bacteremia or skin contamination). Transfusion of heavily contaminated units causes acute sepsis - fever, rigors, hypotension, DIC. Prevention: limiting RBC storage time, pathogen reduction technology, bacterial detection testing.

Laboratory features:

Motile at 25°C but nonmotile at 37°C (peritrichous flagella expressed at cold temperatures; at body temperature the organism switches to expressing virulence factors - Yops - via type III secretion). Test motility at 25°C. Mnemonic: “runs in the cold to warm itself up.”
Urease positive
Grows on CIN (cefsulodin-irgasan-novobiocin) agar with characteristic “bull’s eye” colonies (dark red/purple center, translucent border)
Cold enrichment at 4°C enhances recovery from stool
On TSI: K/A or A/A (some ferment sucrose)

Yersinia pseudotuberculosis

Y. pseudotuberculosis rarely infects humans but causes a disease similar to Y. enterocolitica: mesenteric lymphadenitis/pseudoappendicitis, enterocolitis, and septicemia. The namesake feature is necrotizing granulomas (resembling tuberculosis lesions histologically) in mesenteric nodes and liver. Like Y. enterocolitica, it is motile at 25°C and nonmotile at 37°C. Transmission: contaminated food/water and contact with animals (rodents, birds, pigs, rabbits, deer, farm animals). Evolutionarily, Y. pseudotuberculosis is the ancestor of Y. pestis - Y. pestis evolved from it by acquiring the pMT1 and pPCP1 virulence plasmids and losing flagellar genes.

Yersinia pestis

Y. pestis caused the Black Death of the 14th century, which killed one-third of Europe’s population, and has caused at least two other pandemics. Plague persists today in wild rodent populations worldwide, including the American Southwest. Y. pestis is a CDC Tier 1 select agent - laboratory protocols reflect these biosafety concerns.

Microbiology and Gram stain: Y. pestis is a short, ovoid gram-negative rod with characteristic bipolar “safety pin” staining on Wright or Giemsa stain. It is:

Oxidase negative (as an Enterobacterales member)
Catalase positive, indole negative, urease negative
Facultative anaerobe
Nitrate reduction positive
Grows on MacConkey agar as a non-lactose fermenter (colorless colonies), small and translucent, may take 48 hours
Nonmotile at all temperatures. This is a KEY distinguishing feature from Y. enterocolitica and Y. pseudotuberculosis (both motile at 25°C). The nonmotility reflects loss of flagellar genes during evolution from Y. pseudotuberculosis.
TSI: K/A, no H2S, no gas
Fried egg colony morphology on blood agar after 48-72 hours: raised opaque center with flat translucent periphery. Also called “hammered copper” appearance. Don’t confuse with Mycoplasma (also fried egg colonies) - clinical context and GNR with bipolar staining distinguish Y. pestis.

Transmission and reservoir: Classic cycle is flea bite - Xenopsylla cheopis (Oriental rat flea) and other fleas transmit between rodents and to humans. In the U.S., prairie dogs are the most common reservoir (Cynomys species), along with ground squirrels, rock squirrels, and wood rats. Most U.S. plague cases occur in rural western states (New Mexico, Arizona, Colorado, California). Domestic cats can also acquire plague by hunting infected rodents and transmit it to humans.

Virulence factors:

F1 capsular antigen - a polysaccharide/protein capsule, anti-phagocytic, expressed at 37°C but not at 25°C (temperature-regulated). F1 is used for rapid diagnosis (immunochromatographic dipstick), DFA, and vaccine development (F1-V recombinant).
Fibrinolysin (plasminogen activator, Pla) - a surface protease that activates host plasminogen to plasmin, dissolving fibrin clots. This lets the organism break away from the bite site and disseminate to lymph nodes and blood.
Coagulase (Ymt, Yersinia murine toxin) - clots blood in the flea’s gut (proventriculus), blocking the flea. The “blocked” flea, unable to feed normally, regurgitates bacteria-laden blood into each new bite wound. Coagulase-mediated blocking is essential for flea-to-mammal transmission.
Yops (Yersinia outer proteins) injected by type III secretion system (YopE, YopH, YopJ, YopM, YopT, YpkA) inhibit phagocytosis, disrupt cytoskeleton, and trigger apoptosis. PhoP/PhoQ system supports intracellular survival. These factors allow intracellular survival within macrophages, where the organism multiplies before escaping to cause extracellular infection.

Three clinical forms:

Bubonic plague (80-90%, most common): from a flea bite. Sudden fever, headache, myalgias, and exquisitely tender, swollen lymph nodes (buboes) - 1-10 cm, firm, painful, may suppurate. Most commonly inguinal (bite on lower extremity). Progresses to septicemic plague in 50-60% of untreated cases. Untreated mortality ~60%, treated ~10%. Diagnosed by lymph node (bubo) aspiration showing GNR with bipolar staining; culture on blood agar (BSL-3) confirms. Rapid tests: F1 antigen immunochromatographic dipstick, DFA for F1, PCR. All Y. pestis isolates must be immediately reported to public health authorities.
Septicemic plague: bloodstream infection with high fever, shock, DIC, peripheral extremity gangrene (blackened fingers, toes, nose, ears from thrombotic necrosis) - the origin of the name “Black Death”. Can occur primarily or secondary to bubonic. Essentially 100% mortality without treatment.
Pneumonic plague: pneumonia from inhalation (primary) or hematogenous spread from bubonic/septicemic (secondary). Bloody or watery sputum, chest pain, dyspnea, cyanosis. Incubation 1-3 days. The only form transmissible person-to-person (respiratory droplets) - most concerning for bioterrorism. Essentially 100% mortality without treatment.

Treatment for any form is an aminoglycoside - streptomycin or gentamicin - or doxycycline; early treatment (within 24 hours) dramatically improves survival.

Serratia marcescens

Serratia is a nosocomial pathogen that first came to attention because of a striking feature: some strains produce a bright red pigment called prodigiosin. The red color made Serratia useful as a marker organism in the pre-antibiotic era - it was even dispersed in biowarfare experiments before it was recognized as pathogenic. We now know it causes serious hospital-acquired infections.

Prodigiosin is produced optimally at 25-30°C and not at 37°C, so clinical isolates from human infection often look non-pigmented on initial plates, while ambient-temperature growth (bathrooms, sink drains, lab benches left overnight) shows the classic pink-orange-red discoloration. The pink stain around bathroom fixtures and shower tiles is almost always Serratia. Prodigiosin has been investigated for anticancer properties.

Clinical significance: Serratia causes pneumonia (especially ventilator-associated), urinary tract infections (catheter-associated), wound infections, and bacteremia, primarily in hospitalized patients - particularly in the ICU and those on prolonged antibiotics. It’s been implicated in outbreaks traced to contaminated solutions, medications, and equipment - multi-dose saline vials, IV fluids, hand lotions, disinfectants. Serratia is one of the SPICE/ESCAPPM organisms with inducible AmpC beta-lactamase, so the same “don’t trust third-generation cephalosporin susceptibility testing” rule from Enterobacter applies here.

Laboratory identification: Serratia is a non-lactose fermenter, though some strains ferment lactose slowly. The red pigment is helpful when present, but many clinical strains are non-pigmented. DNase positivity helps distinguish it from other Enterobacteriaceae.

Chapter 42: Non-Fermentative Gram-Negative Bacilli

The non-fermenters are a metabolically distinct group of gram-negative bacilli that cannot ferment glucose - they either oxidize it or are metabolically inert. This metabolic limitation reflects their environmental origins: they’re water and soil organisms, not adapted to the nutrient-rich mammalian gut. But in hospitals - with their artificial airways, catheters, and immunocompromised patients - these organisms have found a niche. They’re now among the most important causes of healthcare-associated infections, and their intrinsic and acquired resistance makes them therapeutic nightmares.

42.1 Pseudomonas aeruginosa

Pseudomonas aeruginosa is the quintessential opportunistic pathogen. In healthy people, it causes only trivial infections - swimmer’s ear, hot tub folliculitis. But in hospitalized patients, particularly those who are immunocompromised, mechanically ventilated, or catheterized, it causes serious, often fatal infections. The organism’s intrinsic resistance to many antibiotics, combined with its ability to rapidly acquire additional resistance mechanisms, makes every Pseudomonas infection a therapeutic challenge.

The laboratory gives immediate clues: P. aeruginosa is oxidase-positive - a drop of oxidase reagent on a colony turns purple within seconds. This single test distinguishes it from the Enterobacteriaceae, which are oxidase-negative. When you pick up a non-lactose-fermenting GNR on MacConkey, oxidase is the first triage test: oxidase-negative puts you back in Enterobacterales; oxidase-positive opens the door to Pseudomonas, Aeromonas, Vibrio, Neisseria. P. aeruginosa is also catalase-positive.

The organism produces a distinctive grape-like or corn tortilla-like odor, often noticed before the plate is even opened. The smell comes from 2-aminoacetophenone, a metabolite some people describe as fruity, others as fresh cornflakes. When a wound dressing smells like grapes, think Pseudomonas. On TSI, P. aeruginosa gives K/K (alkaline slant, alkaline butt) because it doesn’t ferment any sugar - it’s a pure oxidizer. That makes it an obligate aerobe (it needs oxygen to grow).

Pseudomonas is motile via a single polar (monotrichous) flagellum. That gives it fast, darting motility rather than the peritrichous tumbling you see in Enterobacterales. The flagellum matters clinically too: flagellin activates TLR5 and drives airway inflammation.

Colonies frequently show pigmentation from three phenazine pigments you should know by color:

Pyocyanin - blue-green, specific to P. aeruginosa, generates reactive oxygen species and contributes to tissue damage
Pyoverdine (fluorescein) - yellow-green, fluorescent under UV light, a siderophore that scavenges iron from the host
Pyorubin - red-brown, less commonly seen

The combination of pyocyanin plus pyoverdine is what produces the classic blue-green color in sputum, wound drainage, and on agar. Not all strains are pigmented, but when present, the color is diagnostic.

One more lab feature worth memorizing: P. aeruginosa grows at 42°C. Most other Pseudomonas species (P. fluorescens, P. putida) don’t. Thermotolerance is also why it colonizes hot tubs. Incubation at 42°C on cetrimide agar is a classic selective approach.

Why Pseudomonas is so dangerous: This organism is armed with an array of virulence factors that explain its pathogenicity. Exotoxin A, like diphtheria toxin, ADP-ribosylates elongation factor 2, blocking protein synthesis and killing host cells. A type III secretion system acts like a molecular syringe, injecting toxins directly into host cells. Biofilm formation allows Pseudomonas to coat catheters, ventilator tubing, and damaged airways with a protected bacterial community that resists both antibiotics and immune defenses.

Pseudomonas thrives in moist environments: hospital water systems, humidifiers, respiratory therapy equipment, sinks, showers, flower vases, hot tubs, swimming pools, cosmetics, even the inside of sneakers. This ubiquity in wet places is the unifying thread behind nearly every Pseudomonas syndrome - wherever there’s moisture and a portal of entry, Pseudomonas finds it.

In cystic fibrosis patients, Pseudomonas undergoes a phenotypic switch to a mucoid phenotype driven by mutations in the mucA gene, which normally represses alginate biosynthesis. The result is abundant alginate exopolysaccharide forming a structured biofilm. On culture, mucoid colonies look glistening and wet. The transition is almost pathognomonic for CF-related Pseudomonas, signals chronic colonization, and drives the progressive lung destruction that kills most CF patients. Early in CF, non-mucoid strains predominate; mucoid variants emerge and take over as the disease advances. P. aeruginosa is the leading cause of pneumonia in CF patients from adolescence onward.

Clinical syndromes reflect the settings where Pseudomonas thrives:

Nosocomial pneumonia, especially VAP - P. aeruginosa is the leading gram-negative cause of ventilator-associated pneumonia. Mortality runs 30 to 50%. Risk factors: prolonged mechanical ventilation, prior antibiotic exposure, ICU admission, immunosuppression.
Catheter-associated UTIs and bloodstream infections - colonized urinary and vascular catheters.
Burn wound infections - Pseudomonas is the leading cause of burn wound sepsis. The organism produces proteases and phospholipases that liquefy burned tissue. Green discoloration of the dressing is the classic clue.
Hot tub folliculitis - itchy papulopustular rash in the distribution of a swimsuit (occluded, wet skin), from under-chlorinated hot tubs.
Ecthyma gangrenosum - necrotic skin lesion with a black eschar surrounded by a red halo, seen in neutropenic patients with Pseudomonas bacteremia. The pathology is bacterial invasion of blood vessel walls with thrombosis and hemorrhagic infarction. Nearly pathognomonic for Pseudomonas sepsis. Treatment: anti-pseudomonal antibiotics plus debridement.
Puncture wound through a sneaker - a nail through a tennis shoe into the foot classically causes Pseudomonas osteomyelitis (often osteochondritis of a metatarsal head or the calcaneus). The moist shoe interior is the reservoir; the nail inoculates the bacteria into bone.
Ear infections - three patterns: swimmer’s ear (otitis externa from water exposure), malignant (necrotizing) otitis externa in diabetics (see below), and chronic suppurative otitis media.
Contact lens keratitis - P. aeruginosa is the leading cause of bacterial keratitis in contact lens wearers. Extended wear, sleeping in lenses, and contaminated lens solution are the risk factors. Rapidly progressive - corneal perforation can occur within 24 to 48 hours. Treat with topical fluoroquinolone plus fortified aminoglycoside drops.

Malignant otitis externa deserves special mention. The classic presentation is a diabetic (usually elderly) patient with persistent severe ear pain, purulent drainage, and granulation tissue in the ear canal. What begins as swimmer’s ear can become an invasive infection that spreads through cartilage and bone to involve the skull base, cranial nerves (CN VII / facial nerve most commonly, producing facial palsy), and brain. P. aeruginosa is the cause in ~90% of cases. Diagnosis requires CT or MRI showing bone erosion. Treatment is prolonged (6 to 8 weeks) IV anti-pseudomonal antibiotics plus surgical debridement. The term “malignant” doesn’t mean cancer - it means aggressive and life-threatening.

Antibiotic selection is limited by intrinsic and acquired resistance: Pseudomonas is intrinsically resistant to many antibiotics including ampicillin, first- and second-generation cephalosporins, and most tetracyclines. Treatment requires “anti-pseudomonal” agents: piperacillin-tazobactam, ceftazidime, cefepime, carbapenems (except ertapenem), aminoglycosides, and fluoroquinolones. For serious infections, combination therapy is often used both for synergy and to prevent emergence of resistance - Pseudomonas can become resistant to any single agent during the course of therapy. A typical empiric regimen for suspected Pseudomonas sepsis in a neutropenic patient is piperacillin-tazobactam plus tobramycin, or meropenem if extended-spectrum resistance is suspected.

42.2 Acinetobacter baumannii

Acinetobacter has emerged as a global threat, particularly in intensive care units and military hospitals. The organism first gained notoriety during the Iraq and Afghanistan wars, when wounded soldiers developed infections with pan-resistant strains dubbed “Iraqibacter”. But Acinetobacter is now a worldwide problem, one of the most difficult hospital-acquired pathogens to treat. It’s also a classic cause of wound infections in natural disaster victims - contaminated soil and debris inoculate traumatic wounds.

What makes Acinetobacter dangerous isn’t virulence - it’s resistance and persistence: Compared to Pseudomonas, Acinetobacter is not particularly virulent. It primarily infects patients who are already critically ill. But it possesses two features that make it a hospital nightmare. First, it can acquire resistance genes with remarkable efficiency, and many strains are now resistant to virtually all conventional antibiotics. Second, it survives on dry surfaces for weeks to months, contaminating equipment and hands and spreading inexorably through ICUs. The organism is ubiquitous in nature and in the hospital environment - soil, water, ventilator circuits, bed rails, curtains, central lines, healthcare worker hands. Once it’s in a unit, it’s hard to get out.

Laboratory identification: Acinetobacter is a gram-negative coccobacillus, often pleomorphic. The cells are plump and round, and during stationary phase they shorten and round up further into coccoid forms, which can be mistaken for gram-positive cocci or for Neisseria (gram-negative diplococci). During rapid growth (log phase), they look more rod-shaped. This shape variation is a real source of diagnostic confusion on Gram stain. Notably, A. baumannii can retain some crystal violet and appear purple despite being a gram-negative organism.

The biochemical profile is the clean diagnostic:

Oxidase-negative (key feature; distinguishes from Pseudomonas)
Catalase-positive
Obligate aerobe
Non-motile (no flagella; contrast with motile Pseudomonas)
Encapsulated (polysaccharide capsule resists phagocytosis, complement, and desiccation, and promotes biofilm formation on devices)
Non-pigmented
Non-lactose fermenter and a non-fermenter entirely (K/K on TSI)

On MacConkey, Acinetobacter grows as colorless to pale pink colonies; some strains oxidize lactose weakly and give a faint pink that can be confused with a lactose fermenter. The combination of oxidase-negative, non-fermenting, non-motile GNR/coccobacillus is the textbook profile.

Clinical syndromes: A. baumannii most commonly causes pneumonia in the hospital, especially ventilator-associated pneumonia. It’s the second most common gram-negative cause of VAP after Pseudomonas in many ICUs. Mortality for A. baumannii VAP ranges from 30 to 75%. Other syndromes: catheter-related bloodstream infections, wound infections (as above), UTIs, and post-neurosurgical meningitis.

Treatment of MDR Acinetobacter has become an exercise in desperation. A. baumannii is commonly multi-drug resistant (MDR), extensively drug resistant (XDR), or pan-drug resistant (PDR). Resistance mechanisms worth knowing:

OXA-type carbapenemases (OXA-23, OXA-24, OXA-58) are the big one
Efflux pumps
Loss of outer membrane porins
AmpC beta-lactamase
Aminoglycoside-modifying enzymes

When carbapenems fail, options dwindle to toxic agents like polymyxins (colistin), which damages the bacterial outer membrane but causes significant nephrotoxicity, plus tigecycline, or newer agents like cefiderocol. Phage therapy is under active investigation for PDR strains. Infection control - hand hygiene, environmental cleaning, contact precautions - is essential to prevent spread, because once Acinetobacter establishes itself in an ICU, eradication is extremely difficult.

42.3 Stenotrophomonas maltophilia

Stenotrophomonas occupies a peculiar niche: it’s inherently resistant to carbapenems, the antibiotics of last resort for many gram-negative infections. This resistance is conferred by two chromosomal beta-lactamases: L1 (a metallo-beta-lactamase) and L2 (a serine beta-lactamase). That means the broad-spectrum carbapenem therapy used for severe hospital-acquired infections actually selects for Stenotrophomonas - prior carbapenem exposure is one of the strongest risk factors for S. maltophilia emergence.

Laboratory identification: S. maltophilia is a non-fermenting gram-negative rod with a clean biochemical signature:

Oxidase-negative (distinguishes from Pseudomonas; same as Acinetobacter)
DNase-positive (distinguishes from Acinetobacter, which is DNase-negative)
Lysine decarboxylase-positive (unusual among non-fermenters)
Oxidizes glucose and maltose - that second one is the name of the species (maltophilia = maltose-loving). Despite oxidizing these sugars it’s classified as a non-fermenter because it’s purely oxidative on O-F glucose.

Colonies on blood agar often show a yellow to yellow-green pigment, sometimes with lavender-green discoloration of the agar around the growth. This pigment can be mistaken for Pseudomonas pyoverdine, but the oxidase test sorts it out fast: Pseudomonas is oxidase-positive, Stenotrophomonas is oxidase-negative. Some strains give off an ammonia-like smell. On MacConkey, colonies are pale to colorless (non-lactose fermenter).

Environmental reservoir: S. maltophilia is ubiquitous - soil, water, plant rhizospheres, and throughout hospital water systems. It colonizes taps, dialysis machines, respiratory equipment, and indwelling catheters, and it forms biofilms on plastic. When you find it in a clinical specimen, the question is often colonization versus true pathogen; correlation with the clinical picture matters.

Clinical significance: Stenotrophomonas causes nosocomial pneumonia and bacteremia, primarily in immunocompromised patients and those with prolonged ICU stays. It’s an increasingly recognized cause of ventilator-associated pneumonia, especially in mechanically ventilated patients who’ve been on broad-spectrum antibiotics. It’s also recognized as a pathogen in cystic fibrosis patients. The irony is that the patients most likely to receive carbapenem therapy - those with resistant gram-negative infections - are the same patients likely to develop Stenotrophomonas superinfection.

Treatment paradox: Because of carbapenem resistance, the treatment of choice is trimethoprim-sulfamethoxazole (TMP-SMX) - an old, inexpensive drug that remains reliably active. Alternatives: ticarcillin-clavulanate, fluoroquinolones (levofloxacin, moxifloxacin), minocycline, ceftazidime (some strains), and cefiderocol. This is one of the few gram-negative infections where knowing the organism immediately changes therapy away from carbapenems rather than toward them.

42.4 Burkholderia

Burkholderia is a genus with two clinically distinct arms worth knowing separately. B. pseudomallei causes melioidosis - a tropical, potentially bioterror-associated infection that’s the great mimicker of medicine. B. cepacia complex is a CF and CGD pathogen that has reshaped infection control in cystic fibrosis care. They share genus-level features (gram-negative rods, often oxidase-positive) but the clinical and epidemiologic contexts are completely different.

Burkholderia pseudomallei (melioidosis)

B. pseudomallei is the cause of melioidosis, a serious infection endemic in Southeast Asia (Thailand, Malaysia, Myanmar, Vietnam, Cambodia, Laos) and Northern Australia. The organism is a saprophyte living in soil and surface water in tropical regions - rice paddies, streams, and ponds are the classic exposure. Infection occurs through percutaneous inoculation, inhalation, or ingestion. It’s a Tier 1 select agent (bioterrorism potential), so any suspected isolate must be handled at BSL-3, worked up only to a preliminary level in a sentinel lab, and referred to an LRN reference lab with proper notification.

Risk factors for symptomatic disease: diabetes is the most important, followed by renal disease, alcoholism, thalassemia, and occupational soil/water exposure (rice paddy farmers).

Laboratory identification: B. pseudomallei is:

Motile (peritrichous flagella)
Oxidase-positive
Catalase-positive
Bipolar (“safety pin”) staining on Gram, Wright, or Giemsa - stain concentrates at the poles, sparing the center. The classic safety pin trio to remember: B. pseudomallei, Yersinia pestis, and Klebsiella granulomatis (Donovan bodies).
Produces characteristic wrinkled (rugose), dry, metallic colonies after 48 to 72 hours on agar, from biofilm formation. On Ashdown’s agar (crystal violet plus gentamicin, selective medium), colonies appear wrinkled and violet.

Clinical syndromes: Melioidosis is the “great mimicker” because it can present in many ways. Pneumonia is the most common presentation, ranging from acute fulminant pneumonia to a chronic cavitary disease that mimics tuberculosis. Other presentations: septicemia with multiple organ abscesses, localized skin and soft tissue infections, osteomyelitis, prostatic abscess (common in endemic areas), and CNS infection.

Latency - the “Vietnam time bomb”: B. pseudomallei can establish latency, surviving intracellularly in macrophages within granulomas for years to decades, and reactivate under immunosuppression or diabetes. Cases have presented more than 20 years after the patient left an endemic area. The behavior parallels Mycobacterium tuberculosis.

Burkholderia cepacia complex

Burkholderia cepacia complex (Bcc) occupies a unique and feared position in the world of cystic fibrosis - it’s an organism that can transform a stable CF patient into a critically ill one within weeks, and its ability to spread from patient to patient has led to strict infection control measures that have fundamentally changed CF care. Understanding why Bcc is so dangerous in CF patients, while being a relatively minor pathogen elsewhere, requires understanding both the organism’s characteristics and the unique CF airway environment.

Laboratory Identification: Bcc organisms are gram-negative bacilli that are weakly oxidase-positive (note: B. pseudomallei is strongly oxidase-positive; Bcc is weaker - a useful phenotypic contrast). They are motile, catalase-positive, lysine decarboxylase-positive, ONPG-positive, and they oxidize both glucose and lactose. They produce a green-yellow pigment on agar that can resemble Pseudomonas pyoverdine and cause initial identification confusion. The weaker oxidase reaction, growth on B. cepacia selective agar, and MALDI-TOF or molecular ID distinguish them.

Bcc organisms require selective media for isolation from respiratory specimens because they grow slowly and are easily overgrown by other bacteria. BCSA (Burkholderia cepacia selective agar) contains polymyxin B plus other antibiotics that suppress Pseudomonas and competing flora while allowing Bcc to grow - the trick works because Bcc is intrinsically resistant to polymyxin B, which the plate exploits as a selective pressure. OFPBL agar is another option. In CF microbiology, selective Bcc media is routinely included when processing respiratory specimens. Importantly, “Burkholderia cepacia complex” is not a single species but a group of at least 20 closely related species (genomovars), with B. cenocepacia and B. multivorans being the most clinically important in CF.

Environmental reservoir and outbreaks: Bcc was originally identified as a plant pathogen causing onion rot (the name cepacia comes from Latin cepa = onion). It’s found in soil, water, and on plants, and it thrives in moist environments. In hospitals, Bcc has been isolated from contaminated disinfectants (including chlorhexidine and quaternary ammonium compounds - it can grow in some of them), nebulizer solutions, and intravenous fluids, which has caused outbreaks. Its ability to survive disinfection makes it a persistent environmental threat.

Two patient populations matter: Bcc causes pneumonia most importantly in cystic fibrosis (second most important CF pathogen after P. aeruginosa) and in chronic granulomatous disease (CGD). CGD patients have defective NADPH oxidase in their neutrophils and can’t mount an oxidative burst against catalase-positive organisms - and Bcc is catalase-positive, which is exactly why it’s on the CGD pathogen list alongside Staph aureus, Serratia, Nocardia, and Aspergillus.

Why Bcc Is a Cystic Fibrosis Pathogen

The CF lung provides an environment ideally suited for Bcc colonization. The thick, dehydrated mucus, chronic inflammation, and damaged epithelium create a niche where Bcc can establish itself. The organism possesses multiple features that enable it to thrive:

Metabolic versatility: Bcc can utilize an extraordinarily wide range of carbon sources, including compounds that are toxic to other bacteria. This metabolic flexibility allows it to survive in nutrient-poor environments and to persist in contaminated solutions (this property has caused outbreaks from contaminated antiseptics and nebulizer solutions).

Biofilm formation: Like Pseudomonas, Bcc forms robust biofilms in the CF airway. Within these biofilms, bacteria are protected from antibiotics and immune defenses.

Intrinsic antibiotic resistance: Bcc is naturally resistant to many antibiotics, including aminoglycosides and - critically - the polymyxins (colistin). This last point is significant because colistin is the inhaled antibiotic commonly used to suppress Pseudomonas in CF; it has no activity against Bcc. The mechanisms of intrinsic resistance include altered LPS structure (making colistin unable to bind), efflux pumps, and β-lactamases.

Cepacia Syndrome - The Nightmare Scenario

While many CF patients with Bcc have a chronic colonization pattern similar to Pseudomonas, a subset develop “cepacia syndrome” - a fulminant, often fatal illness characterized by rapidly progressive necrotizing granulomatous pneumonia with bacteremia and septic shock. It can develop suddenly in patients who were previously stable with chronic Bcc colonization, with clinical deterioration unfolding over days to weeks. Even with aggressive treatment, mortality approaches 100% in severe cases. The mechanism is not fully understood, but it appears to involve a switch from chronic colonization to invasive disease, possibly triggered by host immunologic factors or bacterial quorum sensing signals.

Cepacia syndrome occurs predominantly with certain genomovars, particularly B. cenocepacia, which has epidemiologic markers associated with transmissibility and virulence.

Person-to-Person Transmission - Why Infection Control Matters

Bcc can spread directly from CF patient to CF patient through respiratory droplets or fomites. This was demonstrated dramatically in the 1990s when certain epidemic strains spread through CF clinics, causing outbreaks of colonization and death. This epidemiological pattern is different from Pseudomonas, which is primarily acquired from the environment rather than from other patients.

The transmission risk has led to strict infection control policies in CF care:

CF patients with Bcc are segregated from other CF patients
Separate clinic times or rooms are used for Bcc-positive patients
Many CF centers maintain lists of patients’ Bcc status
Social contact between CF patients (including attendance at CF camp or support groups) is now discouraged

These measures have reduced transmission but have also created social isolation for Bcc-positive CF patients - a reminder that infection control policies have human costs as well as benefits.

Treatment Challenges: Because of intrinsic resistance, treatment options are limited and must be guided by susceptibility testing. Combinations of antibiotics are typically required. Agents that may have activity include TMP-SMX, minocycline, meropenem (some strains), and ceftazidime (some strains), but resistance is common and unpredictable. Lung transplantation for CF patients with Bcc has historically been controversial because of concerns about post-transplant outcomes, though approaches vary by center and species.

Beyond CF: Bcc can occasionally cause infection in non-CF patients, particularly hospitalized patients with indwelling catheters or immune compromise. Outbreaks have occurred from contaminated solutions, including antiseptics (chlorhexidine) and nebulizer solutions, reflecting the organism’s ability to survive in these environments.

Chapter 43: Fastidious Gram-Negative Bacilli

This chapter covers the fastidious gram-negative bacilli - organisms that don’t grow on routine media, produce distinctive clinical syndromes, and show up repeatedly on boards precisely because their fastidiousness creates a diagnostic trap. If your lab is running standard blood agar and MacConkey and nothing grows, yet the patient clearly has an infection, many of the organisms here are on the shortlist.

The theme tying them together is the same: ordinary growth conditions miss them. You have to know they’re possible, alert the lab, and request appropriate media. Several of these organisms (Brucella, Francisella) are BSL-3 laboratory-acquired infection risks - when you suspect them, notify the lab.

43.1 Haemophilus

Haemophilus species are small gram-negative coccobacilli. The genus name means blood-loving, reflecting their requirement for growth factors found in blood. Within the genus, H. influenzae is by far the most important pathogen - encapsulated strains cause invasive disease, nontypeable strains cause mucosal infections, and the Hib conjugate vaccine is one of the triumphs of modern immunization.

Basic identification

Haemophilus organisms are small, pleomorphic gram-negative coccobacilli. They are oxidase-positive, catalase-variable, non-motile, non-spore-forming, and facultatively anaerobic. Like Neisseria and S. pneumoniae, they produce IgA protease for mucosal immune evasion.

Haemophilus transmits person-to-person via respiratory droplets. It colonizes the upper respiratory tract of 20-80% of healthy people (mostly nontypeable strains). Encapsulated type b (Hib) was the major invasive disease cause before vaccination.

Growth requirements - the X and V factor framework

The defining feature of Haemophilus is its requirement for growth factors present in blood:

X factor = hemin (iron-containing porphyrin)
V factor = NAD/NADP

X factor is used for cytochrome synthesis. V factor is used for electron transport. The requirement profile differs by species, and this is both a boards question and a practical ID tool:

Species	X factor	V factor	Porphyrin test	Disease
H. influenzae	Yes	Yes	Negative	Invasive disease (Hib), mucosal infections (NTHi)
H. parainfluenzae	No	Yes	Positive	HACEK endocarditis
H. ducreyi	Yes	No	Positive	Chancroid
H. aphrophilus (now Aggregatibacter)	No	No	-	HACEK endocarditis

Mnemonic: H. influenzae needs Roman numeral 10 - X (10) and V (5), both factors.

Factor testing with XV disks: You place X, V, and XV disks on a cysteine-containing agar and observe where colonies grow. H. influenzae grows only around the XV disk. H. parainfluenzae grows around V and XV. H. ducreyi grows around X and XV.

The porphyrin test distinguishes H. influenzae from H. parainfluenzae. The test provides delta-aminolevulinic acid (ALA) and asks whether the organism can synthesize porphyrins/heme from it. H. influenzae can’t - it needs exogenous hemin - so it’s porphyrin-negative. H. parainfluenzae can synthesize heme and is porphyrin-positive (it only needs external V factor).

Media and the satellite phenomenon

Haemophilus won’t grow on routine media - a critical boards point:

Does NOT grow on MacConkey agar (too selective, inhibits fastidious organisms)
Does NOT grow on regular blood agar (V factor is trapped inside intact RBCs, plus NADases from lysed cells destroy it)
Grows on chocolate agar (heating to 80C lyses RBCs releasing X and V factors, and inactivates NADases)

Satelliting is the classic demonstration of the V factor requirement. Streak Haemophilus on blood agar alongside Staphylococcus aureus. S. aureus is beta-hemolytic and lyses the surrounding RBCs, releasing V factor. Tiny Haemophilus colonies appear only immediately adjacent to the S. aureus streak - satellite colonies. This phenomenon is essentially pathognomonic. A similar satelliting occurs with Abiotrophia/Granulicatella (nutritionally variant streptococci), which also need V factor.

H. ducreyi requires specialized media - Mueller-Hinton chocolate with IsoVitaleX and vancomycin at 33C in CO2. It’s notoriously hard to culture.

The Hib story - encapsulated vs. nontypeable

H. influenzae strains fall into two categories with completely different clinical behavior.

Encapsulated strains possess a polysaccharide capsule that is the major virulence factor for invasive disease. Six capsular serotypes exist (a through f), but serotype b (Hib) is by far the most important human pathogen - it caused over 95% of invasive Haemophilus disease in the pre-vaccine era.

The type b capsule is polyribosyl ribitol phosphate (PRP), a polymer of ribose, ribitol, and phosphate. Like capsules of pneumococcus and meningococcus, it inhibits complement deposition and phagocytosis, allowing survival in the bloodstream.

Nontypeable H. influenzae (NTHi) strains lack capsules and cannot cause invasive disease in immunocompetent hosts - they can’t survive in the bloodstream. They are instead highly adapted for mucosal colonization: surface adhesins, biofilm formation, IgA protease. Post-Hib vaccine, NTHi has become the predominant H. influenzae pathogen, causing otitis media, sinusitis, bronchitis, and COPD exacerbations.

Pathogenesis of invasive Hib disease

Before vaccination, invasive H. influenzae type b disease was devastating. The organism colonized the nasopharynx, and in susceptible children - particularly those under 5 whose immune systems couldn’t mount an effective antibody response to polysaccharide antigens - the bacteria invaded the bloodstream.

Meningitis was the most feared complication and was the leading cause of bacterial meningitis in children aged 6 months to 3 years. Mortality was 5%, and 25-35% of survivors had permanent sequelae - deafness, developmental delay, seizure disorders.

Epiglottitis is a life-threatening airway emergency where the epiglottis becomes infected, swollen, and inflamed, potentially occluding the airway within hours. Classic presentation: a toxic-appearing child sitting forward, drooling, with stridor and a muffled “hot potato” voice. Direct visualization shows a cherry red swollen epiglottis. Do NOT attempt to examine the throat or lay the child down - secure the airway first.

Other invasive Hib diseases: pneumonia, septic arthritis (large joints), facial cellulitis (often with sinusitis), and bacteremia.

The Hib vaccine

The Hib conjugate vaccine has essentially eliminated invasive Hib disease (>99% reduction). The PRP capsular polysaccharide is conjugated to a protein carrier (diphtheria toxoid, tetanus toxoid, CRM197, or meningococcal outer membrane protein complex).

This conjugation is essential because pure polysaccharides are T-cell-independent antigens - they stimulate B cells but don’t induce memory and don’t work in children under 2. Conjugation to protein converts the response to T-cell-dependent, producing durable memory even in infants. The vaccine protects only against type b, so other serotypes (a, c-f) and NTHi remain possible pathogens.

NTHi mucosal disease

Otitis media: one of the top three causes (with S. pneumoniae and M. catarrhalis). Organism ascends the Eustachian tube from the nasopharynx.
Sinusitis: same mechanism via nasopharynx, especially with viral URI impairing mucociliary clearance.
COPD exacerbations: NTHi chronically colonizes damaged COPD airways and proliferates to trigger acute exacerbations.

Antibiotic resistance: Approximately 30-40% of H. influenzae produce beta-lactamase (usually a TEM-type enzyme), rendering them resistant to ampicillin and amoxicillin. Empiric treatment often uses amoxicillin-clavulanate or a cephalosporin (stable to this beta-lactamase).

H. ducreyi and chancroid

H. ducreyi causes chancroid, a sexually transmitted disease with painful genital ulcers and painful inguinal lymphadenopathy (buboes that may suppurate). This distinguishes it from syphilis and granuloma inguinale (Klebsiella granulomatis), both of which cause painless ulcers. Chancroid facilitates HIV transmission via mucosal disruption.

Mnemonic: you acquire Haemophilus ducreyi after you do X-rated stuff (H. ducreyi needs X factor only).

Diagnosis: Gram stain of ulcer material may show the classic schools of fish pattern - parallel chains of gram-negative coccobacilli aligned in the same direction, as if swimming together. Gram stain sensitivity is only about 50%, and culture is also insensitive (maximum ~75%) and requires specialized media. PCR where available is the most sensitive method. Treatment: azithromycin 1g PO single dose.

Differential for painful genital ulcers: chancroid (H. ducreyi), HSV (vesicles that evolve to ulcers), and LGV (C. trachomatis L1-L3). Donovanosis (Klebsiella granulomatis) is painless with macrophages full of safety-pin Donovan bodies.

H. parainfluenzae - the HACEK member

H. parainfluenzae requires only V factor (porphyrin-positive). It’s normal upper respiratory and oral flora and is one of the most common HACEK organisms causing endocarditis. Less pathogenic than H. influenzae in general, but important when the question involves endocarditis after dental work.

43.2 Legionella pneumophila

Legionella causes Legionnaires’ disease, a severe pneumonia that emerged dramatically in 1976 when 221 attendees of an American Legion convention in Philadelphia developed pneumonia and 34 died. Investigation of the hotel air conditioning system led to discovery of the organism.

Ecology and transmission

Legionella is a gram-negative rod that lives naturally in freshwater at 25-45C, where it parasitizes free-living amoebae (Acanthamoeba, Naegleria). This intracellular lifestyle in amoebae pre-adapted it to survive inside human macrophages and also protects it from chlorination.

In built environments, Legionella colonizes water systems - cooling towers, hot water tanks, decorative fountains, hospital water supplies, whirlpool spas, air conditioning evaporative condensers. Transmission is via inhalation of contaminated aerosols. The organism can survive 14 months in water with only minor viability decrease.

Critically, Legionella cannot be transmitted person-to-person. Outbreak control means environmental investigation and water-system remediation, not patient isolation. Standard precautions suffice - no respiratory isolation is needed.

Classic epidemiologic scenarios: multiple patients from the same hotel develop pneumonia; hospital water system contamination producing nosocomial cases; elderly with COPD or immunocompromise (especially on corticosteroids).

Growth requirements and staining

Legionella has two key laboratory quirks:

Growth requires cysteine and iron. The standard medium is BCYE agar (buffered charcoal yeast extract with L-cysteine and ferric pyrophosphate). The charcoal absorbs toxic metabolites. Legionella does NOT grow on blood agar, MacConkey, or chocolate agar. If you don’t specifically request BCYE, the lab won’t set it up and you’ll miss it.
Stains poorly on Gram stain due to high branched-chain fatty acid content in the cell wall. Classic Gram stain of sputum shows abundant neutrophils with no visible organisms. The organisms are better seen with Dieterle or Warthin-Starry silver stain, or by direct fluorescent antibody (DFA). On silver stain, they appear as dark brown/black rods, typically within alveolar macrophages (facultative intracellular pathogen).

The other bacterium with a cysteine growth requirement worth knowing is Francisella tularensis - same BCYE can grow it. Clinical context differentiates.

Intracellular lifestyle and treatment

Once inhaled, Legionella is phagocytosed by alveolar macrophages. It prevents phagosome-lysosome fusion using a Dot/Icm type IV secretion system, establishing a replicative niche (the Legionella-containing vacuole). Defense requires cell-mediated immunity - activated macrophages and T cells - not antibodies.

This intracellular lifestyle determines treatment:

Beta-lactams are ineffective (don’t penetrate cells)
Use fluoroquinolones (levofloxacin) or macrolides (azithromycin) - both penetrate macrophages

Risk factors for severe disease: advanced age, smoking, COPD, immunosuppression (especially corticosteroids).

Clinical syndromes

Legionella causes two distinct clinical entities:

Legionnaires’ disease - severe atypical pneumonia. High fever, nonproductive cough, GI symptoms (diarrhea, nausea), hyponatremia (SIADH), relative bradycardia, elevated transaminases. Chest X-ray shows consolidation (lobar) or interstitial infiltrates - either pattern is possible, may be unilateral or bilateral, with or without pleural effusion. Rounded opacities can mimic mass lesions. Radiographic progression despite appropriate therapy is common in the first 48-72 hours. ICU admission rates 30-50%. Mortality 10-15%.
Pontiac fever - self-limited flu-like illness without pneumonia (fever, myalgias, headache), resolving in 2-5 days. Same organism, high attack rate (90-100%), no mortality. Mechanism unclear - possibly hypersensitivity to toxin.

Diagnosis

Urinary antigen test: rapid (~15 minutes), specific, detects L. pneumophila serogroup 1 only (which causes ~80% of cases). Remains positive for weeks, useful even after antibiotics started.
PCR: detects all Legionella species and serogroups from respiratory specimens. Increasing availability.
BCYE culture: gold standard, detects all serogroups, but slow (3-5 days).
DFA: rapid but lower sensitivity.

The preferred combination is urinary antigen plus PCR. Always request BCYE culture specifically when Legionella is suspected.

43.3 Bordetella pertussis

Pertussis - whooping cough - remains a significant cause of morbidity despite vaccination. The disease is particularly dangerous in infants too young to be vaccinated, which is why immunizing pregnant women in the third trimester has become standard practice.

Microbiology and virulence factors

B. pertussis is a small gram-negative coccobacillus (ovoid/short rod), a strict human pathogen with no animal reservoir. It’s fastidious, oxidase-positive, catalase-positive, urease-negative, non-motile, non-fermenting, and does NOT grow on MacConkey agar.

It attaches specifically to ciliated respiratory epithelium without tissue invasion - the bacteria stay on the surface. Disease is toxin-mediated.

Attachment factors:

Filamentous hemagglutinin (FHA): the major adhesin, binds sulfated glycolipids on ciliated epithelium
Pertactin (PRN): outer membrane protein that promotes adherence
Fimbriae (FIM)

FHA and pertactin are the components of the acellular pertussis vaccine (DTaP).

Toxins:

Tracheal cytotoxin (TCT): a peptidoglycan fragment that inhibits DNA synthesis in ciliated epithelial cells, causing ciliostasis and ciliated cell death. Loss of the mucociliary escalator means mucus accumulates, triggering the paroxysmal cough. Produced constitutively.
Pertussis toxin (PT): the key systemic toxin, unique to B. pertussis. An AB5 toxin that ADP-ribosylates the Gi alpha subunit, locking inhibitory G proteins in an inactive state. Result: adenylate cyclase can’t be turned off, so cAMP increases.

Pertussis toxin effects:

Lymphocytosis - strikingly high WBC counts (20,000-100,000/μL, >60% lymphocytes). Mechanism: PT prevents lymphocyte response to chemokines that normally signal extravasation, so lymphocytes remain trapped in the bloodstream. Nearly pathognomonic in the right clinical context.
Increased insulin secretion (hypoglycemia in infants)
Immune suppression

The three stages

Total illness duration is 6-10 weeks - hence the Chinese name “hundred-day cough.”

Catarrhal stage (1-2 weeks): mild URI symptoms - rhinorrhea, mild cough, low-grade fever. Indistinguishable from a cold. Most contagious period (highest bacterial shedding), but nobody suspects pertussis yet, so no isolation is taken and close contacts are exposed. Best time to culture - bacterial burden is highest.
Paroxysmal stage (2-8 weeks): severe paroxysms of rapid, consecutive coughs followed by a sudden forceful inspiration producing the classic whoop (air rushing through narrowed glottis). Post-tussive vomiting is common. Lymphocytosis is present. Patients may turn red or blue during paroxysms. Triggered by eating, crying, activity. Difficult to isolate organism via culture at this stage.
Convalescent stage (weeks to months): gradual improvement in cough frequency and severity.

Paroxysmal complications: pneumothorax, rib fractures, subconjunctival hemorrhage, hernias, urinary incontinence, CNS hemorrhage.

The danger to infants

Infants often do not whoop - their airways are too small to generate the sound. Instead, they may present with apnea, cyanosis, or bradycardia during paroxysms. Most pertussis deaths occur in infants under 3 months, before they can complete their primary vaccine series. Pertussis is highly contagious - attack rate of 80-100% in susceptible household contacts.

In adolescents and adults with waning immunity, pertussis often presents as prolonged cough without classic whoop.

Diagnosis

PCR is the preferred method (real-time RT-PCR targeting IS481 or ptxS1). Nasopharyngeal swab or aspirate is the optimal specimen. Most sensitive during the first 3 weeks of cough. Remains positive after antibiotic initiation.
Culture on Bordet-Gengou (BG) agar (potato-glycerol-blood agar) or Regan-Lowe agar (charcoal agar with cephalexin). Specific but insensitive. Sensitivity drops dramatically after the catarrhal stage or after antibiotic treatment. Colonies appear after 3-7 days as small, shiny, dome-shaped colonies resembling drops of mercury. PCR has largely replaced culture for clinical diagnosis.
Serology (anti-PT IgG) useful for late-presenting cases (>3-4 weeks of cough).

Identification after culture: colony morphology, DFA staining, agglutination with specific antisera, PCR.

Treatment and prophylaxis

Macrolides are first-line:

Azithromycin (preferred, 5 days)
Clarithromycin (7 days)
Erythromycin (14 days)

For macrolide-allergic: TMP-SMX. Antibiotics are most effective during the catarrhal stage (reduce severity and duration). In the paroxysmal stage, toxin-mediated damage is established, so antibiotics have little effect on cough but do reduce contagiousness.

Close contacts should receive prophylactic macrolides regardless of vaccination status. Post-exposure prophylaxis is most effective within 21 days of the index case’s cough onset.

Vaccine history

The whole-cell pertussis vaccine (DPT) was highly effective but caused frequent adverse reactions. The current acellular pertussis vaccine (DTaP) contains pertussis toxoid, FHA, pertactin, and fimbriae. It’s better tolerated but provides less durable immunity, which is why pertussis has resurged in vaccinated populations. Immunity wanes after 5-10 years.

Tdap booster at age 11-12 and every 10 years thereafter. Pertussis in vaccinated populations is most likely in teenagers who haven’t yet had their Tdap booster. Tdap in pregnancy at 27-36 weeks each pregnancy allows transplacental antibody transfer and protects newborns until primary immunization.

Other Bordetella species

B. bronchiseptica causes kennel cough in dogs and atrophic rhinitis in pigs. Rarely causes pertussis-like illness in immunocompromised humans. Key differences from B. pertussis: motile, urease-positive, grows on MacConkey, grows faster (1-2 days vs 3-7).
B. parapertussis also causes pertussis-like symptoms in humans.

43.4 HACEK Organisms

HACEK is a group of fastidious gram-negative bacteria classically associated with culture-negative endocarditis. They share three properties: all are gram-negative rods, all are normal oral flora, and all require CO2 for optimal growth (capnophilic).

The acronym:

Haemophilus spp. (H. parainfluenzae, H. aphrophilus)
Aggregatibacter (A. actinomycetemcomitans, A. aphrophilus - formerly Haemophilus aphrophilus)
Cardiobacterium hominis
Eikenella corrodens
Kingella kingae

Why “culture-negative”?

Historically, HACEK organisms were missed because they grow slowly and require CO2-enriched atmosphere. Modern automated blood culture systems (BacT/Alert, BactEC) provide CO2-enriched atmosphere in the bottles and typically detect HACEK within the standard 5-day incubation. Still, when endocarditis is suspected with negative initial cultures, the lab should be alerted to hold cultures longer and use appropriate conditions. Molecular methods (16S rRNA sequencing, PCR) can identify HACEK organisms directly from blood or valve tissue when cultures remain negative. MALDI-TOF provides rapid identification from isolates.

Mechanism and clinical course

HACEK organisms enter the bloodstream during dental procedures, with poor oral hygiene, or after mucosal disruption. They adhere to non-bacterial thrombotic endocarditis (NBTE) vegetations - sterile fibrin-platelet deposits on damaged valve endothelium. HACEK endocarditis typically affects previously damaged or prosthetic valves.

Course is subacute - weeks to months of low-grade fever, fatigue, weight loss, new or changing murmur. Vegetations are large and friable, with high embolization rates (stroke, mycotic aneurysm, splenic infarct). Remember: most endocarditis is gram-positive (Strep, Staph, Enterococcus). When a gram-negative rod grows from blood in an endocarditis case, especially one that’s slow-growing and CO2-requiring, think HACEK.

Growth: best on chocolate agar in 5-10% CO2 at 35-37C. Growth is slow (2-7 days). May grow on blood agar in CO2 but often requires enriched media.

Treatment: ceftriaxone 2g IV daily for 4 weeks - HACEK organisms are generally susceptible to third-generation cephalosporins. Ampicillin-sulbactam or fluoroquinolones are alternatives.

Aggregatibacter aphrophilus

Formerly classified as Haemophilus aphrophilus. Normal upper respiratory flora. Causes bacteremia, endocarditis, brain abscess, and meningitis, frequently after dental procedures. Uniquely for a former Haemophilus, it does NOT require factor V or factor X and therefore grows on blood agar (unlike true Haemophilus species). Like all HACEK, requires CO2.

Eikenella corrodens

Two distinctive features make Eikenella easy to spot in the lab:

Agar pitting - colonies create visible depressions in the blood agar surface, as if pressed in. The name “corrodens” refers to this “corroding” of the agar.
Bleach-like (hypochlorite) odor - some describe it as musty or like wet cement.

Biochemical profile: catalase-negative, oxidase-positive, nitrate-positive, indole-negative, non-motile, facultatively anaerobic.

Clinical associations:

Subacute endocarditis (HACEK)
Human bite wounds (fight bites): classic scenario is a patient who punches someone in the mouth and develops a wound infection of the hand after striking teeth. These clenched-fist injuries can cause deep space infection, osteomyelitis, and septic arthritis of the MCP joint. Treatment: amoxicillin-clavulanate or ampicillin-sulbactam (covers Eikenella and oral anaerobes).
IV drug users who lick their needles - the classic “skin-popping” abscess. Oral flora contaminates the injection site, often polymicrobial with oral anaerobes. Can cause necrotizing fasciitis and osteomyelitis.

Eikenella is resistant to clindamycin and metronidazole - important for choosing empiric coverage.

Kingella kingae

Pediatric pathogen of increasing importance. Colonizes the respiratory tract of young children. Causes:

Subacute endocarditis (HACEK)
Pediatric septic arthritis and osteomyelitis, especially in children under 4

Improved culture techniques (inoculating synovial fluid into blood culture bottles) and molecular methods have revealed K. kingae is much more common than previously recognized as a cause of joint and bone infection in young children.

43.5 Capnocytophaga

Capnocytophaga species are capnophilic (CO2-requiring), fusiform (spindle-shaped) gram-negative rods with gliding motility on agar. They split into two clinically distinct groups based on where they live:

Human oral flora group

C. ochracea, C. gingivalis, and C. sputigena are normal human oral flora. They cause:

Periodontitis (gum disease)
Bacteremia in neutropenic patients (especially hematologic malignancies, post-chemotherapy mucositis)
Endocarditis

These organisms are catalase-negative and oxidase-negative.

Animal oral flora - C. canimorsus

Capnocytophaga canimorsus is normal dog and cat oral flora - present in saliva of ~25% of dogs. It’s transmitted via dog bites (most common), cat bites, or contact with dog/cat saliva (e.g., licking open wounds).

Clinical syndrome: 1-8 days after a bite, high fever, purpura fulminans (DIC), and rapidly progressive fulminant sepsis. Mortality ~25-30%.

High-risk hosts: asplenic patients (highest risk - spleen clears encapsulated bacteria), alcoholics, liver disease, immunosuppression. Asplenic patients should be counseled about dog/cat bite risk and consider prophylactic antibiotics after bites.

Distinguishing features: C. canimorsus is catalase-positive and oxidase-positive - the opposite of human oral Capnocytophaga species. The epidemiologic history (dog/cat bite) plus a fusiform gram-negative rod narrows the ID. MALDI-TOF provides definitive identification.

Gram stain clue: Capnocytophaga is often found intracellularly within macrophages/WBCs on Gram stain of blood or tissue. Fusiform morphology plus intracellular location is suggestive. Buffy coat examination can increase yield.

Blood cultures may take 5-7 days to grow. Treatment: penicillin or ampicillin-sulbactam (generally susceptible to beta-lactams). In penicillin allergy: carbapenems or clindamycin.

Classic board scenario: 3 days after a dog bite, asplenic or immunocompromised patient presents with sepsis; Gram stain shows fusiform GNR, peripheral smear shows organisms within WBCs = C. canimorsus.

43.6 Brucella

Brucellosis is one of the most common zoonotic infections worldwide, though rare in countries with pasteurized dairy and effective livestock programs. Brucella species are small gram-negative coccobacilli (so small they can be confused with cocci), non-motile, non-spore-forming, and facultatively intracellular.

Species and reservoirs

The four human-pathogenic species, in rough order of virulence:

Species	Animal reservoir	Virulence
B. melitensis	Sheep and goats	Most virulent in humans
B. suis	Pigs	2nd most virulent; bioterrorism concern (Tier 1)
B. abortus	Cattle (cows)	Less virulent; abortion in cattle
B. canis	Dogs	Mild human disease, serology may miss it

B. melitensis is named after Malta, where it was first identified in British soldiers. Classic transmission route: unpasteurized goat cheese (queso fresco). B. suis was the first biological weapon produced by the US military (1950s, since destroyed).

B. abortus and the erythritol story: B. abortus causes spontaneous abortion in cattle because of its tropism for erythritol, a sugar alcohol concentrated in bovine placenta. The organism grows preferentially in the placenta, causing placentitis and fetal death. In humans, erythritol is not similarly concentrated in placenta, so Brucella does NOT specifically cause abortions in humans.

Transmission

Humans acquire Brucella through:

Ingestion of unpasteurized milk/dairy products (most common route in endemic areas - Mediterranean, Middle East, Central/South America)
Contact with animal carcasses or body fluids (occupational: veterinarians, farmers, slaughterhouse workers)
Inhalation of aerosolized bacteria (laboratory workers - one of the most common laboratory-acquired infections, BSL-3 organism)

Person-to-person transmission is extremely rare. Pasteurization is the single most effective public health intervention.

There is no approved human vaccine for brucellosis.

Biochemistry and laboratory safety

Most Brucella species are oxidase-positive and catalase-positive. Also urease-positive. Exceptions and nuances:

B. ovis is oxidase-negative AND urease-negative (board-testable oddity). B. ovis infects sheep but rarely causes human disease.
B. suis gives a rapid urease-positive reaction within 15 minutes; other Brucella species take hours (up to 24h). Useful biotyping test.

Additional biotyping: CO2 requirement, H2S production, growth in the presence of thionin and basic fuchsin dyes, monospecific antisera agglutination.

On culture, Brucella grows slowly - requires 5-21 days, CO2 supplementation, producing small, smooth, non-hemolytic colonies. The lab MUST be notified when brucellosis is suspected. Brucella is one of the most common laboratory-acquired infections - simply handling plates on the open bench or sniffing them can infect workers. BSL-3 precautions are required.

Diagnosis is often serologic (agglutination tests) because culture is slow and hazardous. B. canis can be missed by standard agglutination tests (different surface antigens).

Pathogenesis - intracellular life

Brucella is a facultative intracellular pathogen that survives within macrophages by preventing phagosome-lysosome fusion and creating a modified replicative vacuole (Brucella-containing vacuole). This intracellular location:

Protects from antibodies and complement
Requires cell-mediated immunity for clearance
Necessitates antibiotics that penetrate cells (doxycycline, rifampin)
Explains chronicity of untreated infection

Dissemination occurs through the reticuloendothelial system - macrophages carrying the organism to liver, spleen, bone marrow, and lymph nodes. This explains why these are the organs that become enlarged and granulomatous.

Clinical features

Classic presentation is undulant fever - a febrile illness where the fever rises and falls in a cyclical, wave-like pattern. Also called Malta fever.

Other hallmark features:

Profuse night sweats with a characteristic malodorous (musty/hay-like) smell. Mechanism not fully understood.
Fatigue, weight loss, generalized aching
Hepatosplenomegaly
Arthralgia, myalgia

The illness can be acute (months) or chronic (years) if untreated. Chronic brucellosis features persistent fatigue, depression, arthritis.

Focal complications include bone/joint disease (the most common focal complication - sacroiliitis, vertebral osteomyelitis, septic arthritis), endocarditis, and neurobrucellosis.

Histology: disseminated infection produces non-caseating granulomas in liver, spleen, and bone marrow. Differential for non-caseating hepatic granulomas: sarcoidosis, Q fever, tularemia, fungal infections, drug reaction. Clinical context (animal exposure, travel, undulant fever) narrows it down.

Treatment

Prolonged combination therapy is required due to intracellular persistence:

Doxycycline + rifampin for 6 weeks (WHO standard)
Alternative: doxycycline + streptomycin for 6 weeks
Antibiotics that penetrate cells are essential: doxycycline, rifampin, aminoglycosides

43.7 Francisella tularensis

Francisella causes tularemia, a zoonotic infection acquired from rabbits, rodents, and arthropod bites. It’s one of the most infectious bacteria known - infectious dose as low as 10 organisms via inhalation - and is a CDC Category A (Tier 1) select agent for bioterrorism.

Microbiology and growth

F. tularensis is a small, pleomorphic gram-negative coccobacillus/bacillus. A facultative intracellular pathogen that survives within macrophages.

Growth requirements: F. tularensis requires cystine or cysteine for growth (cystine is two cysteines joined by a disulfide bond). It grows on cysteine-enriched media:

Chocolate agar with IsoVitaleX (contains cysteine) - preferred
Cysteine-heart blood agar
BCYE agar (the same medium for Legionella)

The two bacteria you must know that require cysteine for growth: Legionella pneumophila (cysteine only) and Francisella tularensis (cysteine or cystine). Clinical context differentiates - Legionella is pneumonia with water exposure; Francisella is ulceroglandular disease with animal exposure.

Francisella does NOT grow on MacConkey agar. It may grow on blood agar (doesn’t require factor V or X, unlike Haemophilus) but growth is slow and unreliable. Grows slowly at 35C, poorly at 28C. Hold plates 5-7 days.

Colonies are tiny, pinpoint (1-2mm), smooth, gray-white, and non-hemolytic after 2-4 days.

Distinguishing from Haemophilus: both are small GN coccobacilli. Francisella is satellite-test negative (no satelliting around S. aureus) and does not require X or V factors - it requires cysteine instead. Haemophilus satellites around S. aureus and requires X and/or V factors. MALDI-TOF is definitive.

Laboratory safety

Like Brucella, Francisella is a BSL-3 pathogen and one of the most common laboratory-acquired infections. Workers have become infected simply by sniffing culture plates. When tularemia is suspected, notify the lab immediately. Sentinel labs should refer suspect isolates to Laboratory Response Network (LRN) reference labs. The combination of low infectious dose + easy aerosolization + high morbidity is why it’s a Tier 1 select agent.

Tularemia cannot be transmitted person-to-person. No respiratory isolation needed for patients - standard precautions suffice. Transmission routes are all environmental or occupational.

Transmission and reservoirs

Reservoirs: rabbits (most common US source), ticks, deer flies, squirrels, rodents, cats.

Routes of infection:

Tick or deer fly bites (most common in US)
Handling infected animals (skinning/dressing rabbits or squirrels)
Inhalation (laboratory exposure; mowing over infected animal carcasses; landscaping; farming)
Ingestion of contaminated water or food
Laboratory accidents

Clinical forms

Route of entry determines syndrome:

Ulceroglandular tularemia (75-85% of cases, most common form): papule at inoculation site evolves to a punched-out ulcer with black eschar base, accompanied by tender regional lymphadenitis (may suppurate). Presentation: headache, fever, myalgias, plus the ulcer and lymphadenopathy. Typically ulcer is on the hand/arm (handling rabbits) or lower extremity (tick bite). Incubation 3-5 days.
Glandular: lymphadenopathy without visible ulcer.
Oculoglandular: conjunctivitis plus preauricular lymphadenopathy (inoculation via eye).
Typhoidal: septic presentation without ulcer.
Pneumonic: inhalation route. Most severe form, highest mortality. The bioterrorism presentation would be pneumonic tularemia from aerosolized release.

Without treatment, ulceroglandular mortality is 5-15%.

Treatment

Aminoglycosides are drug of choice - streptomycin or gentamicin. Alternatives: doxycycline or ciprofloxacin.

43.8 Pasteurella multocida

Pasteurella is the organism to think of when someone presents with a rapidly progressive wound infection after a cat or dog bite. Cats are the classic source - their long, sharp teeth inoculate bacteria deep into tissue, and their mouths harbor high concentrations of Pasteurella.

Microbiology

P. multocida is a gram-negative coccobacillus/rod (pleomorphic). Normal oral/respiratory flora of:

Cats (~90% carriage)
Dogs (~50-66% carriage)

Grows readily on blood agar and chocolate agar. Does NOT grow on MacConkey. Colonies are gray, non-hemolytic, with a characteristic musty/mousy odor.

Biochemical profile: oxidase-positive, catalase-positive, indole-positive. Non-motile, non-spore-forming.

The ID shortcut: animal bite wound + GN coccobacillus + oxidase+/indole+ + musty odor = Pasteurella multocida.

Clinical picture

Cellulitis and osteomyelitis following cat/dog bites. Cat bites are higher risk because:

Cat teeth are thin and sharp, creating deep puncture wounds that seal over and create an anaerobic environment.
Cat saliva has higher P. multocida carriage.

Infection develops rapidly within 12-24 hours of the bite - much faster than Eikenella fight-bite infections, which is a useful timing distinction. Presentation: rapid cellulitis, purulent drainage, lymphangitis. Can progress to osteomyelitis, septic arthritis, bacteremia, sepsis, or meningitis. Can also cause respiratory infections.

Treatment

Amoxicillin-clavulanate (oral) or ampicillin-sulbactam (IV) is standard. Most Pasteurella produce beta-lactamase, so combine with a beta-lactamase inhibitor. These combinations also cover oral anaerobes and streptococci, which is what you want for a mixed animal bite wound.

Important resistance point: First-generation cephalosporins (cephalexin) should NOT be used for cat/dog bites - P. multocida may be resistant. P. multocida is also resistant to clindamycin and erythromycin. Penicillin alone is active against Pasteurella but misses anaerobes.

IV alternative: ertapenem. Fluoroquinolones are options for penicillin-allergic patients.

43.9 Bartonella henselae (Cat Scratch Disease)

Bartonella henselae causes cat scratch disease, one of the most common causes of chronic lymphadenopathy in children and young adults. Transmitted by cat scratches or bites, with kittens being the most common source (they have higher levels of bacteremia than adult cats). Cat fleas maintain the organism in cat populations.

Typical course: a papule or pustule develops at the scratch site within days. Weeks later - often after the initial lesion has healed - regional lymph nodes draining the scratch site become enlarged and tender. Axillary and epitrochlear nodes are common (arms); cervical nodes (face). Lymphadenopathy can persist for months before gradually resolving.

In immunocompromised patients, Bartonella causes something different: bacillary angiomatosis - a proliferative vascular disease resembling Kaposi sarcoma, with red-purple papular skin lesions and visceral involvement. Primarily in AIDS patients. Responds to antibiotic therapy.

43.10 Anaplasma phagocytophilum and Ehrlichia chaffeensis

These obligate intracellular bacteria cause clinically similar tick-borne diseases recognized only since the 1990s. Both infect white blood cells and produce characteristic intracytoplasmic inclusion bodies called morulae (Latin for mulberry, describing the stippled appearance).

Anaplasma phagocytophilum causes human granulocytic anaplasmosis (HGA). Transmitted by Ixodes ticks - the same ticks that transmit Lyme and Babesia, so coinfection is possible. Anaplasma infects neutrophils; morulae within neutrophils on peripheral smear are diagnostic when present.
Ehrlichia chaffeensis causes human monocytic ehrlichiosis (HME). Transmitted by Amblyomma americanum (Lone Star tick), dominant in the southeastern US. Ehrlichia infects monocytes; morulae appear within monocytes.

Presentation: fever, headache, myalgia, malaise about a week after tick bite. Lab findings often striking - leukopenia, thrombocytopenia, elevated liver enzymes. This triad in a patient with tick exposure should prompt empiric doxycycline immediately, before confirmatory testing returns. Both diseases can progress to respiratory failure, meningitis, and death. Doxycycline is highly effective early; delayed treatment increases mortality.

Chapter 44: Curved and Spiral Bacteria

This chapter covers bacteria with curved or spiral morphology - Vibrio, Campylobacter, Helicobacter, and the spirochetes (Treponema, Borrelia, Leptospira, plus a few odds-and-ends like Spirillum and Brachyspira). Despite sharing similar shapes, they cause very different diseases. A handful of freshwater cousins (Aeromonas, Plesiomonas) get covered here too because they come up in the same differential.

44.1 Vibrio

The vibrios are curved gram-negative rods that live in aquatic environments. They are halotolerant (salt-tolerant, often called halophilic), which explains their association with seafood and seawater. The genus includes three clinically important species, each causing distinct disease. V. parahaemolyticus is the most salt-tolerant of the group, growing in up to 8-10% NaCl.

On the bench, a few features let you pick Vibrio out of a stool culture quickly:

Oxidase positive (this single feature separates Vibrio from Enterobacterales, which are oxidase negative)
Curved (comma-shaped) gram-negative rods, larger than Campylobacter
Motile via a single polar flagellum, giving classic darting (“shooting star”) motility on wet mount
Facultative anaerobes
On TSI slant, A/A (yellow butt, yellow slant) without gas or H2S from glucose and sucrose fermentation. The A/A pattern can look like E. coli or Klebsiella - oxidase is the key differentiator.

TCBS agar (thiosulfate-citrate-bile-sucrose) is the selective/differential medium for Vibrio. Bile salts and alkaline pH suppress normal stool flora. Sucrose fermentation changes the indicator color:

V. cholerae forms YELLOW colonies on TCBS (ferments sucrose)
V. parahaemolyticus and V. vulnificus form GREEN/blue-green colonies (do not ferment sucrose)
V. alginolyticus is also sucrose positive (yellow), like V. cholerae

The lab has to be notified when Vibrio is suspected - TCBS is not a routine stool plate. Once yellow colonies are seen, confirmation for V. cholerae includes the oxidase test, the string test (mucoid string forms when a colony is mixed with 0.5% sodium deoxycholate), O1/O139 serogrouping with specific antisera, and PCR for the cholera toxin gene (ctxAB).

V. cholerae and epidemic cholera: Cholera remains one of the great epidemic diseases of human history, still causing outbreaks where sanitation fails. Understanding the mechanism explains everything about the clinical presentation.

Only V. cholerae serogroups O1 and O139 produce cholera toxin and cause epidemic cholera. O1 is unencapsulated; O139 (Bengal) is encapsulated and emerged in 1992 in India and Bangladesh. O1 has two biotypes (classical and El Tor - El Tor is the current pandemic strain) and two serotypes (Ogawa, Inaba). Non-O1/non-O139 strains can cause mild gastroenteritis but not epidemic cholera (no CT). CT is encoded by the CTX bacteriophage, a filamentous phage - another example of lysogenic conversion (like diphtheria toxin, Shiga toxin).

Cholera toxin is the prototype AB5 toxin - five B subunits bind GM1 ganglioside on intestinal epithelium and deliver the A subunit inside. The A subunit ADP-ribosylates the Gs alpha subunit, permanently activating adenylate cyclase. cAMP rises, CFTR chloride channels open, and chloride (with sodium and water behind it) floods into the intestinal lumen. The result is massive secretory diarrhea - patients can lose 10-20 liters per day, with stool that looks like rice water (the flecks are bits of mucus). The mechanism is identical to ETEC LT toxin.

The critical clinical point: cholera is not an inflammatory disease. There’s no fever, no blood in the stool, no fecal leukocytes. The bacteria don’t invade - they simply produce toxin. Death comes from dehydration and electrolyte imbalance, which means treatment is fundamentally about fluid replacement. Oral rehydration solution (ORS) exploits the fact that glucose-sodium cotransport is preserved - glucose in the gut lumen enhances sodium (and water) absorption, partially compensating for secretory losses. This simple intervention has saved millions of lives.

V. cholerae is unusual among Vibrio in that it can survive in both salt and fresh water. That matters for transmission: cholera outbreaks ride on contaminated freshwater supplies (wells, rivers, city water). The 2010 Haiti outbreak (introduced by UN peacekeepers) caused more than 800,000 cases. V. parahaemolyticus and V. vulnificus are strictly marine.

V. parahaemolyticus causes the most common Vibrio gastroenteritis worldwide. Raw or undercooked shellfish (oysters, shrimp, crab, sushi) is the vehicle. Onset is 4-48 hours after ingestion: explosive watery diarrhea, cramps, nausea, vomiting. Self-limited over 2-3 days. The virulence factor is thermostable direct hemolysin (TDH), demonstrated as the Kanagawa phenomenon - beta-hemolysis on Wagatsuma blood agar. Treatment is supportive; antibiotics are rarely needed.

V. vulnificus is far more dangerous. Two syndromes:

Wound infections: seawater contaminates cuts or abrasions, producing rapidly progressive cellulitis that can advance to necrotizing fasciitis with hemorrhagic bullae within hours. Mortality around 25% overall, up to 50% in liver disease patients.
Primary septicemia: people with liver disease or iron overload eat raw oysters; the bacteria cross from the gut into the bloodstream, producing septic shock with characteristic bullous skin lesions. Mortality exceeds 50%.

The combination of liver disease + saltwater exposure + hemorrhagic bullae + rapid clinical deterioration is virtually pathognomonic for V. vulnificus. Treatment: doxycycline + ceftriaxone (or cefotaxime) plus surgical debridement. Patients with cirrhosis should never eat raw oysters.

Why liver disease patients? Vibrio is siderophilic (iron-loving), and cirrhosis raises transferrin saturation and free iron. Add impaired hepatic clearance of bacteria (from portal-systemic shunting) and impaired complement function and you have a setup for rapid, fatal invasion. Mucorales share this iron-loving behavior.

V. vulnificus is the only Vibrio that ferments lactose - on MacConkey it is pink (lactose positive), while other Vibrio species are colorless. This is a useful differential on mixed plates.

V. alginolyticus causes soft-tissue infections (wound, otitis externa/media, keratitis in contact lens wearers) after saltwater exposure. It is the most salt-tolerant Vibrio species but generally less severe than V. vulnificus.

Why shellfish? Bivalves are filter feeders - a single oyster filters about 50 liters of water per day, concentrating Vibrio to levels far above the surrounding water. Warm months (May through October) are peak season because warmer water supports proliferation.

Aeromonas and Plesiomonas shigelloides

These two deserve brief mention here because they sit next to Vibrio in the differential for oxidase-positive, gram-negative rods from water exposure.

Aeromonas species (A. hydrophila, A. caviae, A. veronii) are found in fresh water (lakes, rivers, ponds, aquaria, tap water) and soil - this is the key distinction from Vibrio (salt water). Oxidase positive, facultative anaerobes. They can resemble Enterobacterales on initial testing. Clinical syndromes:

Wound infections (from freshwater exposure) - can progress to necrotizing fasciitis or myonecrosis, especially A. hydrophila
Gastroenteritis (usually self-limited watery diarrhea; can be bloody)
Bacteremia in immunocompromised hosts (hepatobiliary disease, malignancy)
Infections after medical leech (Hirudo medicinalis) therapy - Aeromonas is a gut symbiont of the leech and is inoculated during feeding. Prophylactic fluoroquinolones or TMP-SMX are standard when leeches are used for venous congestion after flap surgery.

Treatment: fluoroquinolones or TMP-SMX (Aeromonas produces beta-lactamase).

Plesiomonas shigelloides is found in tropical/subtropical freshwater and causes gastroenteritis after eating raw shellfish (oysters) or drinking contaminated water. It is the only oxidase-positive Enterobacterales (recently reclassified from Vibrionaceae). It shares an O antigen with Shigella sonnei, producing cross-reactivity on serotyping. Usually self-limited; fluoroquinolones or TMP-SMX if needed.

44.2 Campylobacter jejuni

Campylobacter is the most common cause of bacterial gastroenteritis in the United States (about 1.5 million cases per year - more than Salmonella and Shigella combined). It’s a curved or S-shaped gram-negative rod that appears as gull wings on Gram stain.

The poultry connection: Campylobacter colonizes the GI tract of birds - chickens, turkeys - whose body temperature is 42°C. The organism grows optimally at this temperature, which is a feature you exploit in the lab. Contaminated poultry is the major source of human infection. Cross-contamination in the kitchen (cutting vegetables on a board used for raw chicken) is a common route. Other reservoirs include cattle, sheep, pigs, and dogs; unpasteurized milk and contaminated water are secondary vehicles.

Microaerophilic metabolism: Campylobacter requires a special atmosphere - 5% O2, 10% CO2, 85% N2 - to grow. This microaerophilic requirement means routine aerobic cultures won’t recover it. The lab sets up specific Campylobacter culture on selective media (Campy-BAP, Skirrow, or CCDA - all contain antibiotics that suppress normal flora) at 42°C in a microaerobic atmosphere. Incubation is 48-72 hours. Campylobacter does NOT grow on MacConkey.

On the bench:

Oxidase positive, catalase positive
Curved GNR with darting/corkscrew motility on wet mount (a single polar flagellum at each end - amphitrichous)
C. jejuni hydrolyzes hippurate (hippurate positive), which separates it from C. coli (the second most common species, hippurate negative). MALDI-TOF has largely replaced biochemical testing for species ID.

Clinical presentation: Campylobacter causes an inflammatory enteritis - watery diarrhea that can become bloody, with fever and crampy abdominal pain (can be severe enough to mimic appendicitis - “pseudo-appendicitis”). Incubation is 2-5 days. Fecal WBCs are present (it’s invasive). The illness is self-limited over 5-7 days in most cases. Antibiotics are reserved for severe/bloody disease, high fever, immunocompromised patients, bacteremia, or prolonged illness. Azithromycin is first-line. Fluoroquinolone resistance is increasing (greater than 20% in the US, much higher in Asia), so cipro is not empiric anymore.

Post-infectious sequelae - the reason the boards love Campylobacter:

Guillain-Barré syndrome (GBS): ascending demyelinating polyneuropathy 1-3 weeks after infection. Campylobacter is the most commonly identified preceding infection in GBS. The mechanism is molecular mimicry - antibodies against C. jejuni lipooligosaccharide cross-react with GM1 and GD1a gangliosides on peripheral nerve myelin. About 1 in 1,000 Campylobacter infections triggers GBS. Antibiotic treatment does NOT prevent GBS - the immune process is already underway.
Reactive arthritis (Reiter syndrome): occurs in HLA-B27-positive patients 1-3 weeks after infection. Asymmetric oligoarthritis of large lower-extremity joints, often with urethritis and conjunctivitis. Other enteric triggers in HLA-B27 hosts: Shigella flexneri, Salmonella, Yersinia enterocolitica. Chlamydia trachomatis is the other major (genitourinary) trigger. The arthritis is sterile - immune-mediated, not direct infection.

44.3 Helicobacter pylori

Helicobacter pylori has transformed our understanding of gastric disease. For centuries, peptic ulcers were blamed on stress, diet, and excess acid. The discovery that most ulcers are caused by a bacterial infection - and can be cured with antibiotics - earned Barry Marshall and Robin Warren the 2005 Nobel Prize (the discovery itself was 1982; Marshall famously drank a culture to prove causation). About 50% of the world’s population carries H. pylori; only 10-20% develop clinical disease.

How it survives in acid: The stomach should be an impossible environment for bacteria. H. pylori has evolved a remarkable solution: it produces abundant urease, which converts urea to ammonia and CO2. The ammonia neutralizes gastric acid locally, creating a micro-environment where the bacterium can survive. This urease production is so characteristic that it’s the basis for several diagnostic tests.

On the bench, H. pylori is:

Gram-negative curved/spiral rod
Strongly urease positive, catalase positive, oxidase positive
Microaerophilic

H. pylori doesn’t invade tissue - it lives in the mucus layer overlying the gastric epithelium, primarily in the antrum. But chronic infection induces persistent inflammation (chronic active gastritis), which over decades can lead to serious complications.

The spectrum of H. pylori disease: Nearly everyone infected develops chronic gastritis, but only a minority develop complications. Why some get ulcers or cancer while others remain asymptomatic involves bacterial virulence factors (CagA cytotoxin), host genetics, and environmental factors.

Peptic ulcer disease (10-15% of infected): H. pylori causes more than 90% of duodenal ulcers and about 70% of gastric ulcers - not NSAIDs or stress. Eradication cures the disease and prevents recurrence.
Gastric adenocarcinoma (~1-2% of infected, over decades): intestinal type. Progression is chronic gastritis → atrophic gastritis → intestinal metaplasia → dysplasia → carcinoma. H. pylori is a WHO Group 1 carcinogen.
Gastric MALT lymphoma: chronic antigenic stimulation drives clonal lymphoid proliferation. Remarkably, early-stage MALT lymphomas regress in about 75% of cases with H. pylori eradication alone - curing cancer with antibiotics.

Diagnostic approaches:

Non-invasive:

Urea breath test (UBT): patient ingests 13C- or 14C-labeled urea. H. pylori urease cleaves it and labeled CO2 is detected in exhaled breath. Detects active infection.
Stool antigen test (ELISA using monoclonal antibodies): detects active infection. Useful for diagnosis and test-of-cure.
Serology: indicates exposure but cannot distinguish active from past infection - not clinically useful once seropositivity is known.

PPIs must be stopped for 2 weeks and antibiotics for 4 weeks before UBT or stool antigen testing - both suppress organism burden and cause false negatives.

Invasive (during upper endoscopy):

Rapid urease test (CLO test): biopsy placed in urea-containing medium with pH indicator; urease produces ammonia, color changes (usually yellow to red/pink). Fast, cheap, high sensitivity.
Histology: H&E shows organisms along surface epithelium. Special stains when the H&E is subtle or the organism burden is low:
- Immunohistochemistry (now most common - highest sensitivity/specificity)
- Giemsa (organisms blue/purple)
- Warthin-Starry silver (organisms black)
- Methylene blue
Culture: microaerobic, 37°C, 3-7 days. Rarely needed clinically.

Treatment requires combination therapy: H. pylori has developed resistance to individual antibiotics, so treatment requires multiple drugs simultaneously. Standard regimens:

Triple therapy: PPI + clarithromycin + amoxicillin (or metronidazole)
Bismuth quadruple therapy: PPI + bismuth + metronidazole + tetracycline

Rising clarithromycin resistance has shifted the default empiric regimen toward bismuth quadruple therapy or fluoroquinolone-based alternatives in resistant regions. Test of cure (UBT or stool antigen) should be performed 4 weeks after completing therapy, off PPIs.

H. heilmannii is a zoonotic Helicobacter (reservoirs: cats, dogs, pigs) that rarely infects humans. Compared to H. pylori it is longer (5-9 um vs. 2.5-5 um) and more tightly spiral-shaped. Also causes gastritis and MALT lymphoma, but much less common. The tighter, longer spirals are the morphologic tip-off on biopsy.

44.4 Spirochetes

The spirochetes are a distinctive group of bacteria with a unique morphology - long, thin, helical cells that move by rotating around their long axis in a corkscrew motion. This motility is powered by endoflagella (axial filaments / periplasmic flagella) that run between the inner and outer membranes. Spirochetes are technically gram-negative (have inner and outer membranes) but stain poorly by Gram stain because they are too thin - darkfield microscopy, silver stains, or specific IHC are needed to see them.

Three medically important genera cause human disease:

Treponema - syphilis, yaws, bejel, pinta
Borrelia - Lyme disease, relapsing fever
Leptospira - leptospirosis

Each has distinctive morphology on darkfield:

Treponema: tightly, regularly coiled, no hooks
Borrelia: loosely, irregularly coiled, largest (visible on Giemsa/Wright stain)
Leptospira: tightly coiled with hooked (question-mark shaped) ends

Treponema pallidum (Syphilis)

Syphilis is the great imitator - a sexually transmitted infection that can mimic almost any disease. It progresses through defined stages over years to decades, and if untreated, can ultimately destroy the cardiovascular and nervous systems. T. pallidum cannot be cultured in vitro (only propagated in rabbit testes), so diagnosis relies on serology, darkfield microscopy, silver stains, PCR, or IHC.

Transmission: sexual contact (most common - organism penetrates intact mucous membranes or abraded skin), transplacental (congenital syphilis), or transfusion (rare - T. pallidum does not survive more than 72 hours in stored blood at 4°C). The infectious dose is remarkably low - on the order of ~57 organisms.

Pathogenesis: T. pallidum disseminates via lymphatics and blood within hours of inoculation. The hallmark histologic lesion is obliterative endarteritis with a plasma cell-rich perivascular infiltrate. Endothelial swelling and fibrosis narrow vessel lumens. This vasculitis underlies every stage: chancre, rash, gummas, cardiovascular and neurologic damage. Plasma cell-rich infiltrates in any tissue should raise suspicion for syphilis.

Primary syphilis appears 10-90 days after exposure (average 21 days, or about 3 weeks) as a chancre at the site of inoculation - usually genitals, but also mouth, anus, or fingers. The chancre is:

Painless
Indurated
Clean-based (no exudate), with raised rolled borders
Single (usually; multiples occur in HIV coinfection)
Accompanied by bilateral non-tender regional lymphadenopathy
Teeming with spirochetes - highly infectious

Contrast with chancroid (H. ducreyi): PAINFUL, ragged, purulent. Mnemonic: syphilis = painLESS, chancroid = painFUL.

The chancre heals spontaneously within 3-6 weeks without treatment, but the infection persists - the organism has already disseminated.

Secondary syphilis develops 6-8 weeks after the chancre heals (most infectious stage - high spirochete burden in lesions):

Diffuse maculopapular rash classically involving palms and soles (remember the palm/sole rash differential: syphilis, Rocky Mountain spotted fever, hand-foot-mouth disease)
Condylomata lata - moist, flat, gray-white warty lesions in intertriginous areas (distinguish from condylomata acuminata from HPV, which are dry and pointed). Highly infectious.
Mucous patches (painless gray oral erosions)
Patchy (“moth-eaten”) alopecia
Generalized lymphadenopathy, fever, malaise, hepatitis, glomerulonephritis

Resolves spontaneously; the infection continues into latent disease.

Latent syphilis is the asymptomatic period with positive serology. Early latent (infection within the past year) may still be infectious sexually; late latent (more than a year, or unknown duration) is not (but can still transmit vertically). The distinction matters for treatment: early latent gets a single dose of IM benzathine penicillin G; late latent gets three weekly doses.

Tertiary syphilis develops in 25-40% of untreated patients, years to decades later:

Gummas: large necrotizing granulomas with central caseous necrosis, epithelioid histiocytes, giant cells, and plasma cell-rich infiltrate. Anywhere - skin, liver (hepar lobatum), bone, nasal septum (saddle nose), palate. Relatively avascular (underlying endarteritis) but respond rapidly to penicillin.
Cardiovascular syphilis: obliterative endarteritis of the vasa vasorum of the ascending aortic root. Destruction of the elastic media produces aortic aneurysm of the ascending aorta, aortic regurgitation, and coronary ostial stenosis. Intima has a characteristic tree-bark appearance. Was the most common cause of aortic aneurysm before antibiotics.
Neurosyphilis - can occur at any stage but classically late. Two forms:
- Meningovascular: CNS vasculitis with stroke-like symptoms (“stroke in a young patient - think syphilis”)
- Parenchymatous: tabes dorsalis (posterior column and dorsal root degeneration - loss of proprioception/vibration, sensory ataxia, lightning/lancinating pains, Charcot joints) and general paresis (progressive dementia, personality change, psychosis)
Argyll Robertson pupil: small, irregular pupils that accommodate but do not react to light (“prostitute’s pupils - accommodates but doesn’t react”). Virtually pathognomonic for neurosyphilis.

Congenital syphilis occurs when T. pallidum crosses the placenta, particularly in early maternal infection.

Placental findings: large, pale, edematous (hydropic) placenta; chronic villitis with plasma cell-rich infiltrates; and necrotizing funisitis - the “barber pole” pattern of umbilical cord inflammation with alternating zones of necrosis and inflammation spiraling around Wharton jelly. Necrotizing funisitis is highly suggestive of congenital syphilis.

Early manifestations (infant): rhinitis (“snuffles”), hepatosplenomegaly, maculopapular rash.

Late manifestations (after age 2):

Hutchinson triad: notched central incisors (Hutchinson teeth) + interstitial keratitis + CN VIII sensorineural deafness
Mulberry molars
Saddle nose (nasal septum destruction)
Saber shins (anterior tibial bowing from periostitis)

HIV coinfection alters the picture: more frequent ocular syphilis (anterior/posterior uveitis, optic neuritis - can lead to blindness), accelerated progression to neurosyphilis, more aggressive course, potentially false-negative serology (impaired antibody production in advanced HIV), and multiple chancres instead of single. All syphilis patients should be tested for HIV, and vice versa.

Diagnostic methods

Direct detection (for primary syphilis / tissue):

Darkfield microscopy (historically primary): fresh chancre exudate on a slide under darkfield illumination - the corkscrew spirochetes glow against a dark background. Sensitivity ~80% in experienced hands. Cannot be used on oral lesions - commensal oral spirochetes cause false positives. Requires immediate examination.
Spirochete IHC on tissue biopsy (skin rash, placenta, gummas): monoclonal anti-T. pallidum antibodies. Higher sensitivity/specificity than silver stains. Cross-reacts with other spirochetes like Brachyspira (intestinal spirochetosis).
Silver stains (Warthin-Starry, Dieterle)
PCR - increasingly the go-to for direct detection

Serology - the workhorse, split into two categories:

Nontreponemal tests (RPR, VDRL) detect antibodies against cardiolipin-cholesterol-lecithin released from damaged host cells and T. pallidum membrane.

RPR (Rapid Plasma Reagin): macroscopic flocculation on a card
VDRL (Venereal Disease Research Laboratory): microscopic flocculation on a slide
Sensitive but nonspecific - false positives with antiphospholipid syndrome, SLE, pregnancy, IV drug use, infectious mononucleosis (EBV), hepatitis, malaria, endocarditis, leprosy, and old age
Titers correlate with disease activity and decline with successful treatment. A 4-fold decline (e.g., 1:32 → 1:8) within 6-12 months = adequate treatment. Rising titers = relapse or reinfection. “Serofast” = persistent low-titer positivity (1:1 or 1:2) despite adequate treatment, not failure.
Prozone (hook) effect: in secondary syphilis, extremely high antibody concentrations inhibit lattice formation and produce a FALSE NEGATIVE. Occurs in 1-2% of secondary syphilis cases. Request serial dilutions when clinical suspicion is high but RPR is negative.

Treponemal tests (FTA-ABS, TP-PA, EIA/CIA) detect antibodies against specific T. pallidum antigens.

FTA-ABS: patient serum absorbed with nonpathogenic Reiter treponeme extract (to remove cross-reactive antibodies), then applied to a slide with fixed T. pallidum; detected with fluorescein-labeled anti-human IgG under fluorescence microscopy.
TP-PA / MHA-TP: patient serum mixed with particles (gelatin for TP-PA, sheep RBCs for MHA-TP) coated with T. pallidum antigen. Agglutination = positive. TP-PA has replaced MHA-TP and is the most common confirmatory test.
EIA / CIA (IgG enzyme/chemiluminescent immunoassay): automated, high-throughput, uses recombinant T. pallidum antigens. Now the most common screening test in the reverse algorithm.
Specific; confirm the diagnosis.
Remain positive for life after infection - “serologic scar.” Cannot monitor treatment or distinguish active from past infection.

Treponemal tests become positive BEFORE nontreponemal tests (IgM ~1-2 weeks after chancre; IgG at 3-4 weeks; RPR/VDRL at 4-6 weeks after exposure). Very early primary syphilis can have a positive treponemal test but negative RPR.

Testing algorithms:

Traditional algorithm: screen with nontreponemal (RPR/VDRL) → confirm positives with treponemal.
Reverse algorithm (now widely used by large labs): screen with automated treponemal EIA/CIA → confirm with quantitative RPR. Discordant (EIA+, RPR-) → perform a second treponemal test (TP-PA) to resolve. EIA+/TP-PA+/RPR- usually means previously treated, very early infection, or late latent.

Neurosyphilis diagnosis requires CSF examination. CSF-VDRL is specific but insensitive (misses 30-50% of cases). CSF pleocytosis (lymphocytic) and elevated protein support the diagnosis.

Treatment: Penicillin G for all stages.

Primary / secondary / early latent: benzathine penicillin G 2.4 million units IM × 1
Late latent / tertiary (non-neuro): benzathine penicillin G 2.4 million units IM weekly × 3
Neurosyphilis: IV aqueous crystalline penicillin G × 10-14 days
Non-pregnant penicillin-allergic: doxycycline
Pregnant penicillin-allergic: must undergo penicillin desensitization - no alternative is acceptable for preventing congenital syphilis

Jarisch-Herxheimer reaction: Within hours of the first antibiotic dose, patients develop fever, rigors, hypotension, tachycardia, and worsening of skin lesions. Caused by release of bacterial lipoproteins from dying spirochetes, triggering a cytokine storm (TNF-alpha, IL-6, IL-8). Peaks at 2-8 hours, resolves by 24 hours. Most common in secondary syphilis (highest organism burden). Also seen with treatment of Lyme, relapsing fever, and leptospirosis. Supportive care - don’t stop antibiotics. Can be dangerous in late pregnancy or severe disease; warn patients.

The other treponematoses (endemic, non-venereal) are serologically and histologically indistinguishable from venereal syphilis. They produce positive RPR/VDRL and FTA-ABS and the same plasma cell-rich granulomatous inflammation on biopsy. Differentiation is by clinical presentation and epidemiology.

Yaws - T. pallidum subsp. pertenue - tropical (West Africa, Southeast Asia, Pacific Islands). Skin-to-skin contact, non-venereal. Primary “mother yaw” papilloma, secondary papillomas (including “crab yaws” on soles), tertiary destructive bone/cartilage lesions. No cardiovascular, neurologic, or congenital disease. Treatment: single-dose azithromycin (WHO campaigns).
Bejel (endemic syphilis) - T. pallidum subsp. endemicum - arid regions (Middle East, Sub-Saharan Africa). Transmitted by shared utensils and cups (oral-oral, especially children). Oral mucous patches, skin lesions, late gummatous destruction. No congenital, CV, or CNS disease.
Pinta - Treponema carateum - Central and South America. Skin-only. Primary scaly papule, secondary hyperchromic patches, late depigmented (achromic) patches resembling vitiligo. No visceral disease.

Borrelia burgdorferi (Lyme Disease)

Lyme disease is the most common vector-borne disease in the United States (about 35,000 reported cases per year, likely more than 400,000 actual). B. burgdorferi (North America) and related species (Europe) are transmitted by Ixodes scapularis (eastern US, deer tick) and Ixodes pacificus (western US). These are the same ticks that carry Anaplasma and Babesia, making coinfection common. The white-footed mouse is the primary reservoir; the white-tailed deer is the preferred host for adult ticks.

The tick must feed for 36-48 hours to transmit infection, which is why daily tick checks after outdoor activities in endemic areas are effective prevention. Endemic areas: Northeast (Connecticut to Virginia), upper Midwest (Wisconsin, Minnesota), and northern California. Seasonal peak is May-August - the nymphal stage does most transmission because the nymphs are tiny (hard to notice) and feed during summer.

Ixodes coinfections to suspect when Lyme features are atypical:

Babesiosis (Babesia microti) - hemolytic anemia, parasitemia on thin smear
Anaplasmosis (Anaplasma phagocytophilum) - leukopenia, thrombocytopenia, morulae in granulocytes
Borrelia miyamotoi (relapsing fever)
Powassan virus

Stage 1 - Early localized disease (3-30 days after bite):

Erythema migrans (EM) is the characteristic rash - an expanding erythematous annular lesion, often (but not always) with central clearing producing the bull’s-eye / target appearance. At least 5 cm across, expands over days to weeks (can exceed 30 cm). EM is pathognomonic for Lyme - treat clinically, no serology needed. Occurs in about 70-80% of cases. Serology is often negative this early.

Stage 2 - Early disseminated (weeks to months):

Hematogenous spread produces:

Multiple secondary EM lesions
Facial nerve palsy (Bell palsy) - Lyme is a common cause, especially bilateral
Lymphocytic meningitis
Radiculopathy
Cardiac Lyme (AV block, can be complete, requiring temporary pacing), myocarditis

Stage 3 - Late disease (months to years):

Lyme arthritis: monoarticular or oligoarticular inflammatory arthritis of large joints, especially the knee. Intermittent episodes. Some cases persist despite antibiotics - likely autoimmune (molecular mimicry between OspA and LFA-1).
Chronic encephalopathy (memory, concentration, fatigue)
Acrodermatitis chronica atrophicans (ACA) - blue-red skin discoloration with progressive atrophy, primarily European strains

Diagnosis:

Clinical (EM rash) for early localized disease - no serology
Two-tier serologic testing for later stages:
- First tier: EIA or IFA for Lyme IgM/IgG
- If positive/equivocal, confirm with Western blot - IgM (2 of 3 bands: 23/OspC, 39, 41) and IgG (5 of 10 bands: 18, 23, 28, 30, 39, 41, 45, 58, 66, 93)
- After 4-6 weeks of symptoms, only IgG is relevant - IgM can persist and cause false positives
Modified two-tier testing (MTTT): two different EIAs in sequence, replacing Western blot. Faster and more objective, increasingly common.
Early serology is insensitive (only about 40% positive in the first 2 weeks)
Culture on BSK (Barbour-Stoenner-Kelly) medium is possible but rarely needed clinically

B. burgdorferi is loosely coiled (3-10 wide coils) and is the only spirochete reliably visible on Giemsa or Wright-Giemsa stain - though in Lyme disease bacteremia is too low for reliable smear detection. In relapsing fever, Borrelia is visible on peripheral smear (see below).

Borrelia Relapsing Fever

Relapsing fever is caused by different Borrelia species that have evolved a remarkable immune evasion strategy: antigenic variation of variable major proteins (VMPs). Just as the host mounts an antibody response, the bacteria switch their surface proteins, escaping immune clearance. The result is recurring febrile episodes separated by afebrile intervals.

Two epidemiologic forms:

Epidemic (louse-borne) relapsing fever - B. recurrentis - transmitted by the human body louse (Pediculus humanus corporis). Conditions of overcrowding and poor hygiene - war, famine, refugee camps. Endemic East Africa and South America. Rare today but explosive outbreaks possible.
Endemic (tick-borne) relapsing fever - B. hermsii (and other species) - transmitted by soft-bodied Ornithodoros ticks. Western US/Canada, mountain cabins and caves. Soft ticks feed rapidly (minutes, often at night) - patients often don’t recall a tick bite.

Clinical pattern: Sudden high fever, headache, myalgia, sometimes hepatosplenomegaly. Each episode lasts 3-7 days, then resolves abruptly - often with drenching sweats. Days later, fever recurs. Without treatment, multiple relapses occur, each typically milder as the immune system catches up to the current antigenic variant.

Diagnosis: Relapsing fever Borrelia reach high enough blood concentrations to be visible on Wright- or Giemsa-stained peripheral blood smear during febrile episodes. This is unusual for spirochetes and aids rapid diagnosis.

Jarisch-Herxheimer is common and can be severe with relapsing fever treatment.

Leptospira interrogans (Leptospirosis)

Leptospirosis is the most widespread zoonotic disease in the world. Morphologically, Leptospira are tightly coiled with hooked (question-mark shaped) ends - unique among spirochetes and distinctive on darkfield. Pathogenic species is L. interrogans (more than 250 serovars); L. biflexa is saprophytic.

Transmission: Infected animals - especially rats, but also dogs, cattle, pigs, wildlife - shed leptospires in their urine for months to years. The organisms survive in warm, moist freshwater, mud, and soil. Humans are infected when contaminated water or soil contacts abraded skin or mucous membranes.

Risk factors:

Occupational: farmers, veterinarians, sewer workers, military
Recreational: swimming, kayaking, wading, white-water rafting, triathlons (famous outbreak clusters)
Flooding events in endemic areas
Most common in Hawaii within the US (tropical climate, feral animal reservoirs); globally most prevalent in Southeast Asia, Central/South America, sub-Saharan Africa. Doxycycline prophylaxis can prevent infection in high-risk exposures.

Clinical spectrum - classically biphasic:

Anicteric leptospirosis (90% of cases, self-limited flu-like illness):

Initial bacteremic phase (week 1): sudden fever, severe headache, myalgia (classically severe calf muscle pain)
Immune phase (week 2, IgM antibodies appearing): recurrent fever
Conjunctival suffusion - conjunctival redness without purulent discharge - is a classic, distinctive finding

Weil disease (5-10% of cases, severe) - triad:

Hepatic injury: jaundice, hepatomegaly, elevated bilirubin and LFTs - typically without major hepatocellular necrosis (transaminases only modestly elevated)
Renal injury: AKI, often non-oliguric with hypokalemia (potassium wasting), usually reversible
Hemorrhagic diathesis: thrombocytopenia, petechiae, pulmonary hemorrhage (severe pulmonary hemorrhagic syndrome [SPHS] is the most lethal manifestation)

Mortality 5-15%; SPHS much higher.

Diagnosis:

Clinical + epidemiology → empiric treatment
Microscopic agglutination test (MAT) is the reference standard - 4-fold rise between acute and convalescent sera, or single titer >=1:800
IgM ELISA for rapid screening
PCR of blood (week 1) or urine (week 2+)
Blood cultures on EMJH or Fletcher medium at 28-30°C are possible but too slow (weeks) for clinical use
Darkfield microscopy has low sensitivity, not recommended

Treatment: Doxycycline for mild disease; IV penicillin G for severe (though the evidence base is thin). Jarisch-Herxheimer can occur. Doxycycline prophylaxis 200 mg weekly for high-risk exposures.

Chapter 45: Mycobacteria

The mycobacteria are a distinctive group of bacteria defined by their unusual cell wall - rich in mycolic acids that make them impermeable to many stains and antibiotics. This waxy coat is responsible for their acid-fast staining, their slow growth (nutrients can’t get in quickly), and their ability to survive inside macrophages. The genus includes the two great human pathogens - M. tuberculosis and M. leprae - as well as numerous environmental species that cause opportunistic infections.

45.1 Mycobacterium tuberculosis

Tuberculosis has killed more people than any other infectious disease in human history. Even today, it infects one-quarter of the world’s population and kills over a million people annually. Understanding TB requires grasping the delicate balance between pathogen and host immunity that determines whether infection leads to disease.

The unique cell wall determines everything about TB: The mycobacterial cell wall is 60% lipid by weight, dominated by mycolic acids - long-chain fatty acids linked to arabinogalactan. This waxy barrier makes TB impermeable to many antibiotics, resistant to desiccation (allowing airborne transmission), and invisible to routine Gram staining. Cord factor (trehalose dimycolate) contributes to virulence and causes the characteristic serpentine cording pattern seen on culture.

The M. tuberculosis complex

“M. tuberculosis” on the board exam often means the whole complex, not just the type species. The MTB complex includes M. tuberculosis, M. bovis, and M. africanum, plus M. microti (voles), M. canettii (rare), and M. caprae (goats). All members cause clinical tuberculosis and all are treated with standard TB regimens, with one critical exception discussed below. Molecular typing (spoligotyping, MIRU-VNTR) distinguishes them when needed.

M. africanum is found only in West Africa, where it causes roughly half of TB cases. Clinically indistinguishable from M. tuberculosis. Biochemically it sits intermediate between M. tuberculosis and M. bovis (niacin-variable, nitrate-variable). Responds to standard therapy.

M. bovis infects cattle and is acquired by drinking unpasteurized milk. Before widespread milk pasteurization (63°C for 30 min, or 72°C for 15 sec), M. bovis was responsible for most TB in developed countries. Because the route of entry is oral rather than respiratory, M. bovis causes predominantly extrapulmonary, GI-centered TB - terminal ileum infection (mimics Crohn disease), mesenteric lymphadenitis, peritoneal and hepatic TB. Biochemically: niacin-negative, nitrate-negative. The attenuated M. bovis strain is what we call BCG (Bacillus Calmette-Guérin), the live TB vaccine used globally.

M. bovis is intrinsically resistant to pyrazinamide. This matters for the treatment regimen. The resistance is due to a point mutation in pncA (which normally encodes pyrazinamidase, the enzyme that activates pyrazinamide to pyrazinoic acid). When M. bovis is identified, drop pyrazinamide and substitute another agent (often a fluoroquinolone).

Species	Niacin	Nitrate	PZA
M. tuberculosis	Positive	Positive	Sensitive
M. bovis	Negative	Negative	Resistant
M. africanum	Variable	Variable	Sensitive

Transmission and the initial encounter: M. tuberculosis and M. africanum are transmitted by airborne droplet nuclei - particles 1-5 μm, small enough to remain suspended in air and reach the alveoli. A single cough from a patient with cavitary pulmonary TB can generate thousands of infectious particles. Hospitalized patients with suspected or confirmed pulmonary TB need airborne isolation: negative-pressure room, N95 respirator. After inhalation, bacilli are engulfed by alveolar macrophages.

Intracellular survival - the phagosome hijack: Unlike most bacteria, M. tuberculosis replicates inside macrophages. It does this by blocking phagosome maturation. Cord factor (trehalose-6,6’-dimycolate) and sulfatides prevent phagosome-lysosome fusion and block phagosomal acidification. LAM (lipoarabinomannan) contributes as well. The bacilli essentially convert the macrophage into a Trojan horse - they survive inside and get ferried to lymph nodes.

Cord factor has another role: it triggers macrophages to produce TNF-alpha, which drives granuloma formation. On AFB smear of culture, cord factor causes the bacilli to stick together in rope-like serpentine aggregates - the “cording” pattern is suggestive of MTB complex, though not 100% specific (some NTM cord as well).

The granuloma

Over 2-8 weeks, cell-mediated immunity develops. Macrophages produce IL-12, which drives TH1 cells to secrete IFN-gamma, which activates macrophages into epithelioid cells and Langhans giant cells (horseshoe arrangement of nuclei). These surround a core of caseating necrosis, wrapped in a cuff of lymphocytes and fibroblasts. Caseating granulomas are the pathognomonic histologic finding of TB - other granulomatous diseases (sarcoidosis, most fungal infections) typically produce non-caseating granulomas.

In most people, containment is successful. The bacteria persist dormant inside the granuloma, producing latent TB infection (LTBI): positive skin test or IGRA, no symptoms, normal CXR, not infectious. About 5-10% of LTBI patients eventually reactivate, usually within the first two years but sometimes decades later.

Primary vs. reactivation TB

Primary TB produces a Ghon focus - a subpleural granuloma in the middle/lower lung lobes (where inhaled droplet nuclei first deposit). Ghon focus + ipsilateral hilar/mediastinal lymphadenopathy = Ghon complex. When the Ghon complex calcifies (resolved primary TB), it becomes a Ranke complex (calcified parenchymal lesion plus calcified node) - seen as a dense nodule on CXR in patients who were exposed and contained the infection.

Reactivation (secondary) TB prefers the upper lobes because of the high oxygen tension - M. tuberculosis is a strict aerobe. Classic presentation: fever, weight loss, night sweats (“B symptoms”), chronic productive cough, sometimes hemoptysis, fatigue. CXR shows bilateral apical infiltrates with cavitation. Cavitary disease is highly infectious because the bacillary burden is enormous.

Reactivation risk factors

Reactivation occurs when cell-mediated immunity falters. HIV is the most powerful risk factor. Others: TNF-alpha inhibitors (infliximab, adalimumab, etanercept, golimumab, certolizumab), immunosuppressive therapy, diabetes, renal failure, malnutrition, advancing age. TNF-alpha is essential for granuloma maintenance - blocking it lets dormant bacilli escape. All patients starting a TNF-alpha inhibitor need LTBI screening (TST or IGRA plus CXR) before initiating therapy, and if LTBI is detected, treatment must begin before the biologic.

Disseminated and extrapulmonary TB

Miliary TB results from hematogenous dissemination. The term refers to the CXR appearance: tiny (1-3 mm), uniform, diffusely distributed nodules resembling millet seeds. Miliary TB typically occurs in immunocompromised patients, young children, and the elderly. It’s often paucibacillary - sputum AFB smears may be negative, and diagnosis can require bronchoscopy, bone marrow, or liver biopsy.

Common extrapulmonary sites (in rough order of frequency):

Lymph nodes (scrofula) - most common extrapulmonary form
Pleura - exudative, lymphocytic, high ADA, low glucose
Bone - vertebral (Pott disease) - vertebral body destruction, paravertebral abscess, kyphosis
Basilar meninges - TB meningitis - CSF with lymphocytic pleocytosis, very high protein, very low glucose
Adrenal gland - adrenal TB causing Addison disease
Kidneys - renal TB causing sterile pyuria
Epididymis, fallopian tubes

TB pleural effusion has elevated adenosine deaminase (ADA). ADA is released by activated T cells and macrophages. An ADA >40 U/L in a lymphocytic exudative pleural effusion has ~90% sensitivity and specificity for TB pleuritis. Pleural fluid AFB smear is usually negative (only ~10% positive); pleural biopsy with culture is more sensitive.

Diagnostic testing depends on the question

For LTBI, we test the immune response to TB antigens. Two options:

Tuberculin skin test (TST / Mantoux / PPD) measures delayed-type hypersensitivity. Purified protein derivative is injected intradermally. Previously sensitized T cells recognize PPD antigens, release cytokines, recruit macrophages, and produce induration (a palpable, raised, hardened area). Read at 48-72 hours. Measure induration (not erythema) in millimeters of transverse diameter.

Interpretation thresholds vary by risk:

≥5 mm positive: HIV, close TB contacts, organ transplant recipients, immunosuppressed
≥10 mm positive: healthcare workers, high-incidence countries, IV drug users, other high-risk groups
≥15 mm positive: low-risk individuals

TST has well-known false positives and false negatives.

False-positive TST: prior BCG vaccination and NTM infection. BCG is the most common cause worldwide - it contains attenuated live M. bovis that cross-reacts with PPD antigens. BCG-related false positivity wanes over 10-15 years. NTM share PPD antigens with M. tuberculosis. Technical error (wrong injection depth, misread) is a third cause.

False-negative TST causes:

Very early/recent infection - takes 2-12 weeks after exposure for T-cell sensitization. Contacts of TB patients may have negative initial TST and need repeat testing 8-10 weeks after last exposure.
Age <6 months - immature immune system
Immunocompromise, especially HIV - can’t mount a DTH response. The lower the CD4, the more likely the TST is falsely negative (“anergy”).
Sarcoidosis - granulomas sequester T cells peripherally, causing cutaneous anergy. Sarcoid patients may be anergic to multiple skin test antigens.
Long-standing TB - waning immune response. Two-step testing (repeat TST 1-3 weeks after a negative first test) can boost the response by recalling memory T cells. Important in healthcare worker screening.

Interferon-gamma release assays (IGRAs) are the blood-based alternative. They measure T-cell release of IFN-gamma after in vitro exposure to M. tuberculosis-specific antigens, which are ESAT-6 and CFP-10. These antigens are encoded in the RD1 (region of difference 1) genomic region, which is present in M. tuberculosis but absent from BCG and from most NTM (exceptions: M. kansasii, M. marinum, M. szulgai all share RD1). This is the structural reason IGRAs aren’t fooled by BCG vaccination.

Two FDA-approved IGRAs are available in the US: QuantiFERON-TB Gold (ELISA-based, measures IFN-gamma concentration in plasma) and T-SPOT.TB (ELISPOT-based, counts IFN-gamma-producing T cells as spot-forming units). T-SPOT may be slightly more sensitive in immunocompromised patients because it normalizes for cell count.

QuantiFERON-TB Gold Plus uses three tubes:

Nil tube (negative control): measures background IFN-gamma
TB antigen tube: contains ESAT-6 and CFP-10 to stimulate TB-specific T cells
Mitogen tube (positive control): contains phytohemagglutinin to confirm T-cell viability

If the mitogen tube is negative, the test is indeterminate - the patient is likely immunosuppressed or anergic. QFT-Plus actually uses two antigen tubes (one for CD4 stimulation, one for CD4+CD8).

IGRA advantages over TST: not affected by BCG, single blood draw (no return visit), objective result, no reader variability. Disadvantages: more expensive, requires specialized lab, indeterminate results in severe immunosuppression.

Neither TST nor IGRA distinguishes latent from active disease - both tell you only that immune sensitization exists.

For active TB, we need to find the organism:

AFB smear (Ziehl-Neelsen or fluorochrome on sputum): rapid but insensitive. Requires high bacterial burden. Cannot distinguish TB from NTM. Cording on AFB smear is suggestive of MTB complex.
Culture: gold standard. Solid media (Löwenstein-Jensen) or liquid (Middlebrook 7H10/7H11, MGIT). Takes 2-6 weeks. Provides organisms for drug susceptibility. M. tuberculosis on LJ produces buff-colored, dry, rough, raised colonies (“rough, tough, buff”) with cauliflower-like appearance. Nonchromogenic (no pigment).
NAAT (Xpert MTB/RIF): detects M. tuberculosis complex DNA and rifampin resistance simultaneously, directly from sputum, in ~2 hours. The assay targets the rpoB gene and specifically probes the 81-bp rifampin resistance-determining region. Sensitivity ~98% in smear-positive disease, ~67% in smear-negative. Xpert MTB/RIF Ultra has improved sensitivity for smear-negative and paucibacillary specimens.

Histologic detection of mycobacteria in tissue:

AFB stain (Ziehl-Neelsen on tissue sections)
FITE stain - modified acid-fast that uses peanut oil to protect mycolic acids from paraffin processing. More sensitive than standard ZN on paraffin-embedded tissue.
Mycobacterial immunohistochemistry - most sensitive histologic method
GMS (silver) does NOT detect mycobacteria - it’s for fungi

When is a TB patient no longer infectious

Historically, airborne isolation can be discontinued when ALL three criteria are met:

Adequately treated for ≥2 weeks with effective therapy
Clinically improved (reducing cough, afebrile)
3 consecutive negative AFB sputum smears collected 8-24 hours apart, with at least one early morning specimen

Culture conversion (negative culture) is the ultimate marker of treatment response but takes 2-3 months.

Treatment: RIPE

Standard treatment for drug-susceptible pulmonary TB uses RIPE: Rifampin, Isoniazid, Pyrazinamide, Ethambutol. Intensive phase is four drugs for 2 months, then continuation phase is Rifampin + Isoniazid for 4 more months (6 months total). CNS, bone, and drug-resistant TB require longer courses (9-12 months or more).

Moxifloxacin can substitute for ethambutol, and this allows a 4-month regimen in drug-susceptible disease. Previously streptomycin was used in place of ethambutol, but resistance is too common now. Rifapentine is preferred over rifampin in some regimens due to its longer half-life.

LTBI treatment: isoniazid for 9 months, or rifampin for 4 months, reduces reactivation risk by 60-90%.

The prolonged, multi-drug regimen is essential to prevent resistance. TB drug resistance arises from spontaneous chromosomal mutations, not plasmid transfer. Each resistance gene sits at a different chromosomal location. Resistance is NOT transmissible between organisms. Combination therapy works because the probability of a single bacillus being simultaneously resistant to four drugs is astronomically low.

Resistance genes (high-yield)

Rifampin resistance: rpoB gene, encoding the beta subunit of RNA polymerase. Over 95% of rifampin-resistant strains have mutations in the 81-bp RRDR region. Xpert MTB/RIF probes this region.
Isoniazid resistance: katG or inhA. katG encodes catalase-peroxidase, which activates isoniazid as a prodrug - katG mutations cause high-level resistance (~60-70% of INH resistance). inhA encodes enoyl-ACP reductase, the target of activated isoniazid in mycolic acid synthesis - inhA mutations cause low-level resistance (~20-35%).
Pyrazinamide resistance: pncA gene, encoding pyrazinamidase. M. bovis has an intrinsic pncA mutation. Pyrazinamide is unique among TB drugs: it is only active at acidic pH (inside acidified phagolysosomes), which is why it targets semi-dormant intracellular bacilli.

rpoB as a surrogate for MDR

>90% of rifampin-resistant M. tuberculosis isolates are also resistant to isoniazid. Therefore rpoB mutation detection functions as a surrogate marker for MDR-TB. This is what makes Xpert MTB/RIF so clinically useful: a positive rifampin-resistance result triggers immediate MDR workup and initiation of second-line therapy while full susceptibility results are still brewing in the lab.

Drug susceptibility testing

The reference standard for M. tuberculosis drug susceptibility is the agar proportion method. Standardized inocula are plated on Middlebrook 7H10/7H11 agar with and without the drug at a critical concentration. After 3 weeks, colony counts on drug vs. drug-free plates are compared. If growth on the drug-containing plate is ≥1% of growth on the drug-free plate, the strain is classified as resistant.

The 1% threshold reflects clinical reality: even in a “susceptible” population, some naturally resistant mutants exist at low frequency. Above 1%, the drug won’t work as monotherapy.

MDR-TB and XDR-TB

MDR-TB: resistance to at least rifampin AND isoniazid (the two most effective drugs). Treatment requires 18-24 months of second-line agents: fluoroquinolones, injectable aminoglycosides, linezolid, bedaquiline, pretomanid, delamanid.

XDR-TB: MDR-TB plus resistance to any fluoroquinolone AND at least one second-line injectable (kanamycin, capreomycin, amikacin). Pre-XDR and XDR have very high mortality and extremely limited treatment options.

45.2 Nontuberculous Mycobacteria (NTM)

The nontuberculous mycobacteria are environmental organisms found in soil and water worldwide. Unlike M. tuberculosis, they’re not transmitted person-to-person - humans acquire them from the environment. They cause disease primarily in two settings: structural lung disease (bronchiectasis, COPD, cystic fibrosis) where damaged airways can’t clear inhaled organisms, and immunocompromised states where the host can’t contain infection.

Distinguishing clinical relevance from colonization is critical: Because NTM are environmental, isolating them from respiratory specimens doesn’t necessarily mean disease. A patient might simply be colonized, or the specimen might be contaminated. The American Thoracic Society criteria for NTM lung disease require clinical symptoms, radiographic abnormalities, AND microbiologic evidence (multiple positive cultures or positive culture from a sterile site).

The Runyon classification

Runyon grouped NTM by pigmentation and growth rate. Still useful for the boards.

Group	Name	Pigment behavior	Growth rate	Examples
I	Photochromogens	Pigmented only with light exposure	Slow (>7 days)	M. kansasii, M. marinum
II	Scotochromogens	Pigmented regardless of light	Slow (>7 days)	M. scrofulaceum, M. gordonae
III	Nonchromogens	No pigment	Slow (>7 days)	MAC, M. xenopi, M. haemophilum, M. genavense, M. ulcerans
IV	Rapid growers	Variable	<7 days	M. abscessus, M. chelonae, M. fortuitum

The yellow/orange pigment in Groups I and II is from carotenoids - protective pigments against UV damage and oxidative stress. In photochromogens, light triggers carotenoid biosynthesis; in scotochromogens, production is constitutive.

M. szulgai is the weird one: photochromogen at 25°C, scotochromogen at 37°C. Unique temperature-dependent pigmentation. Mnemonic: lower numbers pair (Runyon I with 25°C; Runyon II with 37°C). At body temperature, M. szulgai always produces pigment. M. szulgai causes pulmonary disease, skin and soft tissue infections, bursitis, and tenosynovitis.

Mycobacterium avium complex (MAC)

MAC is the most common NTM pathogen worldwide. The complex includes M. avium and M. intracellulare, which are clinically indistinguishable and treated identically. M. avium is more common in AIDS-related disseminated disease; M. intracellulare is more common in pulmonary disease in immunocompetent patients. MAC is Runyon Group III (nonchromogen, slow-growing, 2-4 weeks). Colonies are smooth, dome-shaped, translucent to opaque, with no pigment. MAC is notable for producing multiple colony morphologies - smooth transparent (most virulent), smooth opaque, and rough variants - sometimes on the same plate.

MAC causes distinct clinical syndromes depending on host:

Cavitary lung disease in immunocompetent patients with COPD (or other chronic lung disease, prior TB, bronchiectasis, smoking). Upper-lobe cavities with cough, hemoptysis, weight loss - mimics TB.
Nodular/bronchiectatic disease in immunocompetent elderly women (Lady Windermere syndrome). Classic phenotype: tall, thin, elderly woman without prior lung disease. Theory: voluntary suppression of the cough reflex (“ladylike” refusal to cough) impairs mucociliary clearance in the right middle lobe and lingula (the anatomically dependent regions). Produces tree-in-bud nodules and bronchiectasis, not cavities.
Cervical lymphadenitis (scrofula) in immunocompetent children aged 1-5 years. MAC has surpassed M. scrofulaceum as the #1 cause. Painless, unilateral cervical or submandibular lymphadenopathy. Overlying skin may become violaceous. Treatment: complete surgical excision (not incision and drainage, which causes sinus tract formation). Antibiotics alone have limited efficacy for localized disease.
Tenosynovitis in immunocompetent patients - hand/wrist infection after minor trauma.
Hot tub lung - hypersensitivity pneumonitis (not true infection) in immunocompetent patients exposed to aerosolized MAC from contaminated hot tubs or whirlpools. Cough, dyspnea, fever after exposure; CXR with diffuse ground-glass opacities. Treatment: avoidance ± steroids.
Diffuse infiltrates in immunocompromised patients.
Disseminated disease in HIV/AIDS with CD4 <50. Fever, weight loss, anemia, elevated alkaline phosphatase. Small bowel biopsy may show foamy macrophages stuffed with AFB.

Treatment of pulmonary MAC: macrolide (clarithromycin or azithromycin) + rifampin (or rifabutin) + ethambutol for at least 12 months after culture conversion.

Disseminated MAC prophylaxis in HIV is indicated when CD4 <50. Preferred agent: azithromycin 1200 mg weekly (alternative: clarithromycin 500 mg BID). Must exclude active MAC disease with blood cultures before starting - otherwise monotherapy promotes macrolide resistance. Prophylaxis can be discontinued when CD4 >100 for ≥3 months on effective ART.

Mycobacterium kansasii

M. kansasii produces disease that clinically and radiographically mimics TB - upper-lobe cavitary disease, cough, weight loss, fever. It’s the NTM that most closely resembles tuberculosis and the second most common NTM cause of pulmonary disease after MAC.

M. kansasii on AFB stain: characteristically long bacilli with prominent cross-banding (alternating stained and unstained regions, “barber-pole” or “beaded” appearance). This morphology is distinctive and can give a presumptive ID before culture results. M. kansasii is also the largest NTM on AFB.

Runyon Group I photochromogen - nonpigmented in the dark, develops bright yellow-orange pigmentation 1-2 hours after light exposure. Treatment: rifampin + ethambutol + isoniazid (or azithromycin) for 12 months after culture conversion. Generally easier to treat than MAC.

Major risk factor for NTM pulmonary disease overall: prior pulmonary disease - COPD, bronchiectasis, prior TB, cystic fibrosis, alpha-1 antitrypsin deficiency, pneumoconiosis. Damaged airways can’t clear inhaled NTM.

Mycobacterium marinum

M. marinum causes skin infections in patients who work with water - aquarium handlers (“fish tank granuloma”), swimmers, fishermen. Enters through skin abrasions during contact with contaminated water. Grows optimally at 30°C (not 37°C), which is why it’s restricted to cooler extremities (hands, arms). Lesions may follow a sporotrichoid pattern (ascending nodules along lymphatics).

Swimming pool-associated M. marinum has become uncommon due to chlorination - the organism is sensitive to chlorine at standard pool concentrations. Home aquarium maintenance (cleaning tanks without gloves) is now the most common exposure.

Runyon Group I photochromogen. The laboratory must be told to incubate at 30°C when M. marinum is suspected - standard 37°C incubation may miss it. Treatment: clarithromycin + ethambutol or rifampin.

Mycobacterium gordonae

M. gordonae lives in tap water and is nearly always a contaminant when isolated from clinical specimens. It’s the most commonly isolated NTM contaminant overall. True infection is exceedingly rare - it happens only in severely immunocompromised patients. Isolation from sputum should prompt a review of specimen collection, not treatment. Runyon Group II scotochromogen (yellow/orange regardless of light). Identification confirmed by MALDI-TOF or molecular methods.

Mycobacterium scrofulaceum

M. scrofulaceum causes cervical lymphadenitis (scrofula) in immunocompetent children. The species name comes from “scrofula” (Latin scrofa = breeding sow, referring to the swollen-neck appearance). Painless unilateral cervical mass in a 1-5 year old, sometimes with violaceous skin discoloration and sinus tract drainage. Treatment: complete surgical excision (not I&D). Runyon Group II scotochromogen. MAC has overtaken M. scrofulaceum as the #1 cause of childhood NTM lymphadenitis.

Mycobacterium xenopi

M. xenopi grows in hot water - it’s thermophilic, optimal at 42-45°C. Found in hospital hot water systems, storage tanks, and plumbing. Causes TB-like upper-lobe cavitary pulmonary disease, predominantly in patients with COPD or other underlying lung disease. More common in Europe (especially UK) than North America. Runyon Group III nonchromogen. Characteristic “bird’s nest” colony morphology. Poor prognosis compared to MAC. Treatment: rifampin + ethambutol + clarithromycin ± moxifloxacin, 12-18 months.

Mycobacterium haemophilum

M. haemophilum requires hemin/iron supplementation of media to grow and grows at 30°C (preferentially cooler body sites). Causes papulonodular skin lesions, especially on extremities, and lymphadenitis in immunocompromised patients. Also increasingly recognized as a cause of cervical lymphadenitis in immunocompetent children. Runyon Group III.

Mycobacterium genavense

M. genavense is extremely fastidious and does NOT grow on standard solid mycobacterial media (Löwenstein-Jensen, Middlebrook 7H10/7H11) even when supplemented with hemin. Compare to M. haemophilum, which also needs hemin but will grow on these agars if it’s added. M. genavense may grow only in BACTEC MGIT broth after 4-8 weeks with supplementation, or may not grow at all. Diagnosis often relies on molecular methods (PCR, 16S rRNA sequencing) from tissue biopsies.

Classically causes disseminated disease in HIV/AIDS (similar clinical picture to MAC): fever, weight loss, hepatosplenomegaly, lymphadenopathy, and GI involvement (abdominal pain, diarrhea). Small bowel biopsy shows foamy macrophages with AFB, similar to Whipple disease and MAC. Runyon Group III.

Rapid growers: M. abscessus, M. chelonae, M. fortuitum

Runyon Group IV: rapid growers, colonies in <7 days. The three clinically important species are M. abscessus, M. chelonae, and M. fortuitum. Mnemonic: “Funny clowns Make Happy Unicorns Laugh” (fortuitum, chelonae, abscessus - pick your own ordering).

First-line treatment of all three: macrolides (clarithromycin preferred). Additional agents depending on species and susceptibility: amikacin, cefoxitin, imipenem, tobramycin, linezolid, tigecycline.

M. abscessus is the most problematic rapid grower. Highly drug-resistant. Has inducible macrolide resistance via the erm(41) gene - clarithromycin may appear susceptible on initial testing but becomes resistant after 14 days of exposure, so extended-incubation susceptibility testing is critical. The erm(41)-negative subspecies M. abscessus subsp. massiliense remains macrolide-susceptible, but the erm-positive M. abscessus subsp. abscessus requires combination therapy (amikacin + cefoxitin or imipenem, plus a macrolide if truly susceptible). Common in cystic fibrosis pulmonary disease.

M. chelonae causes widespread cutaneous disease in immunocompromised patients - multiple erythematous papules, nodules, and draining abscesses on extremities (especially legs). In immunocompetent patients, M. chelonae infections are usually a single localized wound infection.

M. fortuitum is associated with skin/wound infections in immunocompetent patients. Classic settings: post-pedicure foot infections from contaminated whirlpool footbaths (warm water + micro-abrasions from shaving), post-surgical wound infections (breast augmentation, LASIK), post-injection abscesses, tattoo-related infections. In HIV/immunocompromised hosts, M. fortuitum can disseminate and cause osteomyelitis, septic arthritis, and bacteremia.

M. fortuitum can be distinguished from M. chelonae and M. abscessus by the iron uptake test: when grown on LJ agar with ferric ammonium citrate, M. fortuitum takes up iron and colonies turn a rusty brown color. M. chelonae and M. abscessus don’t. Mnemonic: “Fortuitum = Ferric.”

Mycobacterium ulcerans

M. ulcerans causes Buruli ulcer (Bairnsdale ulcer in Australia) - a chronic, indolent, necrotizing skin and soft tissue infection sometimes called the “flesh-eating” ulcer. Third most common mycobacterial disease worldwide, after TB and leprosy.

Occurs mostly in West and Central Africa (highest burden in Ghana, Benin, Ivory Coast, Cameroon) but also Australia, Southeast Asia, and South America. Associated with stagnant water sources - swamps, riverbanks, slow-moving rivers. Environmental disruption (deforestation, dam construction, flooding) increases incidence. Detected in aquatic insects and biofilms. Exact transmission mechanism is unclear.

Lesions usually occur on lower extremities at sites of previous trauma. Starts as a painless nodule or papule, evolves into a large, painless ulcer with undermined/overhanging edges and a necrotic base with a white/yellow “cotton-wool” appearance. The painlessness is striking and is due to mycolactone, a polyketide toxin that destroys nerves and immune cells (also causes the tissue necrosis and local immunosuppression).

M. ulcerans is Runyon Group III (nonchromogen) and grows extremely slowly (3-12 months on culture), making culture impractical. PCR targeting the IS2404 insertion sequence is the preferred diagnostic method. Early cases treated with rifampin + clarithromycin for 8 weeks; advanced cases need surgery plus antibiotics.

Mycobacterium leprae (Hansen Disease)

Leprosy is covered here for completeness, though it’s clinically distinct. M. leprae has the longest incubation period of any bacterial infection: 2-20 years. Acquired from other humans (respiratory droplets from untreated lepromatous patients) or from armadillos (nine-banded armadillo, Dasypus novemcinctus) - armadillos are the only known non-human reservoir. In the southern US (Texas, Louisiana), armadillo contact is a recognized risk factor.

M. leprae has never been cultured in vitro - it grows only in living tissue. Armadillo footpads (research) and mouse footpads (Shepard model) are the experimental systems. The inability to culture is due to M. leprae’s extreme metabolic dependence on the host - it has the smallest genome of any mycobacterium. Diagnosis: skin biopsy with FITE stain, slit-skin smear, PCR, or clinical features.

M. leprae is the only bacterium that infects peripheral nerves. It lives within macrophages and Schwann cells. Schwann cell tropism is mediated by binding alpha-dystroglycan and PGL-1 on the Schwann cell surface. Neurotropism is what causes the peripheral neuropathy - sensory loss, motor weakness, autonomic dysfunction - that is the hallmark of leprosy. Like M. marinum, M. leprae prefers cooler body sites (earlobes, nose, fingers, toes, superficial nerves), which grow at ~30°C. Commonly affected nerves: ulnar, median, lateral popliteal (common peroneal), posterior tibial, greater auricular, facial.

Clinical hallmarks: hypopigmented/anesthetic skin lesions and enlarged (palpable) peripheral nerves.

The Ridley-Jopling spectrum runs from tuberculoid (paucibacillary) to lepromatous (multibacillary), reflecting the host immune response:

Tuberculoid leprosy (TT) = TH1 response. Strong cell-mediated immunity effectively contains infection. Few, well-defined hypopigmented/anesthetic skin lesions. Few bacilli (paucibacillary; AFB smear often negative). Prominent granulomas with epithelioid and giant cells on biopsy, often centered on nerves. Palpable, enlarged peripheral nerves. Lepromin test POSITIVE. The strong response limits bacterial spread but the granulomatous inflammation within nerves is what causes nerve damage.

Lepromatous leprosy (LL) = TH2 response. Ineffective cell-mediated immunity, poor containment. Numerous, diffuse, poorly-defined skin lesions. Leonine facies - nodular facial thickening giving a lion-like appearance, with prominent superciliary arches, madarosis (loss of eyebrows), and early saddle-nose deformity. Very high bacillary load (multibacillary; AFB smear strongly positive). Bacilli pack macrophages to form foamy macrophages, known as Virchow cells or lepra cells, sometimes containing globi (masses of bacilli within a capsule). Diffuse infiltration without well-formed granulomas. Widespread nerve involvement. Lepromin test NEGATIVE (anergic).

The Lepromin skin test differentiates tuberculoid from lepromatous leprosy. It’s read at 3-4 weeks (Mitsuda reaction) and detects cell-mediated reactivity to M. leprae antigens. Tuberculoid → positive (granuloma forms at injection site). Lepromatous → negative. The Lepromin test does NOT diagnose leprosy - it classifies the immune response in a known patient. Not a screening test.

Biopsy findings:

Tuberculoid: well-formed epithelioid granulomas with Langhans giant cells surrounding nerve bundles; few or no AFB on FITE.
Lepromatous: diffuse macrophage infiltration with foamy cytoplasm (lepra cells/Virchow cells), abundant AFB on FITE, poorly formed granulomas.

Granulomas centered along nerves are pathognomonic for leprosy.

Treatment: dapsone + rifampin + clofazimine in multidrug regimens. Paucibacillary disease is treated for ~6 months; multibacillary requires 12-24 months. Nerve damage is often irreversible once established, which is why early diagnosis matters.

Chapter 46: Atypical Bacteria (Gram-Indeterminate)

This chapter covers bacteria that don’t fit neatly into the gram-positive/gram-negative classification - either because they lack a cell wall entirely (Mycoplasma, Ureaplasma), live exclusively inside host cells (Rickettsia, Chlamydia, Coxiella, Ehrlichia, Anaplasma), or stain variably (Gardnerella, Bartonella). Despite their diversity, these organisms share important clinical features: they don’t grow on routine culture media, they don’t respond to cell wall-active antibiotics, and diagnosing them requires a high index of suspicion.

Two organizing principles help here. First, track the vector: tick-borne (Rickettsia, Ehrlichia, Anaplasma), louse-borne (R. prowazekii, Bartonella quintana), mite-borne (R. akari, Orientia), flea-borne (R. typhi), or none (Coxiella is inhaled; Chlamydia is sexually or respiratory transmitted; Mycoplasma is droplet). Second, track the target cell: endothelium (Rickettsia proper), WBCs (Ehrlichia in monocytes, Anaplasma in granulocytes), macrophage phagolysosome (Coxiella), epithelial cell inclusions (Chlamydia), or extracellular adherence (Mycoplasma). The cellular target drives the clinical picture.

46.1 Rickettsia Species

The rickettsiae are obligate intracellular gram-negative coccobacilli transmitted by arthropod vectors - ticks, lice, fleas, and mites. They’ve evolved to live inside vascular endothelial cells, and this tropism explains the clinical manifestations: these are diseases of small blood vessels.

When rickettsiae infect endothelium, they directly invade and multiply within vascular endothelial cells using actin-based motility to spread cell to cell. The resulting endothelial injury causes increased vascular permeability, microhemorrhages (the petechial rash), and thrombosis. The clinical result is a constellation of fever, rash, and - in severe cases - vascular leak leading to edema, hypotension, and end-organ damage. The rash in rickettsial diseases often involves the palms and soles, which should always trigger consideration of these infections.

A useful contrast: Rickettsia proper replicate free in the cytoplasm, while Ehrlichia and Anaplasma replicate inside a membrane-bound vacuole (the morula). Coxiella goes even further and thrives inside the phagolysosome. These subcellular differences track with how each organism looks on histology and why they cause different clinical syndromes.

Rickettsia rickettsii (Rocky Mountain Spotted Fever) [PREVIOUSLY TESTED]

Rocky Mountain spotted fever is a misnomer - most cases occur in the eastern two-thirds of the US, particularly North Carolina, Tennessee, Oklahoma, Arkansas, and Missouri (over 60% of cases come from five southeastern/south-central states). The name reflects where the disease was first described (Idaho/Montana, 1896), but the disease’s lethality if unrecognized persists everywhere.

The tick connection: R. rickettsii is transmitted by Dermacentor ticks - Dermacentor variabilis (American dog tick) in the East and Dermacentor andersoni (Rocky Mountain wood tick) in the West. Rhipicephalus sanguineus (brown dog tick) is the vector in the southwestern US and Mexico. The tick must be attached for 6-10 hours before transmission occurs, which is why careful tick checks after outdoor activities can prevent infection. The tick serves as both vector AND reservoir - transovarial transmission keeps the organism circulating in tick populations without requiring a mammalian reservoir. Seasonal peak: April-September.

Pathogenesis explains the clinical picture: R. rickettsii directly invades vascular endothelial cells throughout the body, spreading via actin-based motility. As endothelial cells are damaged, vessels become leaky. Fluid escapes into tissues, causing edema and hypovolemia. Damaged vessels thrombose, causing tissue ischemia. The brain, lungs, and kidneys are particularly vulnerable.

The classic triad is fever, headache, and rash - but this triad develops sequentially, not all at once. Fever and headache come first, often with myalgias so severe they’re described as “bone-breaking.” The rash typically appears on days 3-5 of illness, starting at the wrists and ankles and spreading centripetally toward the trunk. Initially macular, the rash becomes petechial/purpuric as vasculitis progresses. Involvement of the palms and soles is characteristic.

Important caveat: the “classic triad” of fever + rash + tick bite is present in only ~60% of cases at presentation. “Rocky Mountain Spotless Fever” - RMSF without rash - occurs in 10-15% of cases and is especially dangerous because it delays recognition.

Palms and soles differential: The palm/sole rash is a classic board clue. Infectious mimics include:

Secondary syphilis
Coxsackievirus (hand-foot-mouth disease)
Bacterial endocarditis (Janeway lesions)
Measles (late)
Meningococcemia
Kawasaki disease
Toxic shock syndrome
Reactive arthritis

Mnemonic: “You drive CARS with your palms and soles” - Coxsackie, AFTOSA (hand-foot-mouth), RMSF, Syphilis.

Severe disease and the G6PD association: Severe RMSF causes multi-organ failure: renal failure (ATN from vasculitis), meningoencephalitis (headache, confusion, seizures), DIC (widespread endothelial damage activates coagulation), non-cardiogenic pulmonary edema, and myocarditis. RMSF is rapidly fatal in patients with G6PD deficiency - G6PD-deficient endothelial cells can’t generate enough NADPH to neutralize the reactive oxygen species produced during infection, and vasculitis becomes fulminant.

The treatment imperative: RMSF has a mortality rate of 20-25% if untreated, but less than 5% with appropriate treatment. The critical point is that doxycycline must be started based on clinical suspicion - before laboratory confirmation. Serology doesn’t become positive until the second week of illness, and waiting for results is often fatal. Doxycycline is the treatment of choice even in children, where the risk of dental staining is far outweighed by the risk of death from untreated RMSF.

Diagnosis: Indirect immunofluorescence assay (IFA) on paired sera (acute + convalescent, looking for a 4x rise) is the current gold standard serologic test. Historically, the Weil-Felix reaction was used - patient serum agglutinates Proteus bacteria (OX-19, OX-2, OX-K antigens) due to cross-reactivity with rickettsial antigens. Weil-Felix is no longer considered reliable; it’s of historical interest and low-yield board trivia. PCR and immunohistochemistry on skin biopsy are available for early diagnosis. Again - do not wait for serology to treat.

Rickettsia prowazekii (Epidemic Typhus)

Epidemic typhus is a disease of human suffering - it flourishes wherever people are crowded together in conditions that promote louse infestation: wars, famines, refugee camps, and prisons. The body louse (Pediculus humanus corporis) is the vector; humans are the main reservoir (flying squirrels are a rare sylvatic reservoir in the US).

Transmission mechanics: The louse ingests R. prowazekii while feeding on an infected person. The organism multiplies in the louse gut. When the louse feeds on a new host, it defecates; the patient scratches the bite, inoculating Rickettsia from louse feces into the skin. The transmission cycle requires crowded, unsanitary conditions where body lice thrive.

The historical devastation: Typhus has killed millions throughout history, often determining the outcomes of wars. Napoleon’s army was decimated by typhus during the retreat from Moscow. The disease swept through concentration camps in World War II. Whenever social order collapses and people crowd together without clean clothing and bathing, typhus appears. R. prowazekii is a Category B bioterrorism agent.

Clinical features: After an incubation period of 1-2 weeks, patients develop abrupt high fever, severe headache, and myalgias. The rash appears around day 5, starting on the trunk and spreading centrifugally to the extremities - the opposite pattern from RMSF. The rash typically spares the face, palms, and soles. Severe cases progress to stupor (the word “typhus” comes from the Greek for “smoky” or “hazy,” describing the mental clouding), meningoencephalitis (CNS vasculitis; CSF shows lymphocytic pleocytosis), shock, and death. Mortality is 10-40% untreated.

Rash direction is high-yield: Typhus rash spreads trunk-to-extremities (centrifugal); RMSF rash spreads extremities-to-trunk (centripetal). Typhus rash rarely involves palms/soles; RMSF classically does.

Brill-Zinsser disease: R. prowazekii can persist for years in a dormant state, reactivating decades later when host immunity wanes. This recrudescent disease - Brill-Zinsser disease - is milder than primary infection but can serve as a source for new epidemics if lice are present. The latent patient is a reservoir.

Rickettsia typhi (Endemic/Murine Typhus)

Endemic typhus is maintained in a rat-flea-rat cycle; humans are incidental hosts infected by flea feces. Unlike epidemic typhus, it doesn’t require human crowding - just proximity to rats. The disease is milder than epidemic typhus, with a mortality rate under 5%, but it’s increasingly recognized as a cause of undifferentiated febrile illness in tropical and subtropical regions. Rash distribution mirrors epidemic typhus (centrifugal, trunk-predominant, sparing palms/soles). CNS involvement is less common than with R. prowazekii but can occur.

Rickettsia akari (Rickettsial Pox)

R. akari causes rickettsial pox, transmitted by the mouse mite (Liponyssoides sanguineus). Initially described in NYC in 1946. Outbreaks link to mouse-infested urban buildings.

Clinical sequence:

Papule at the mite bite site
Progresses to an eschar (painless, black, necrotic)
Then a generalized vesicular rash resembling varicella

The vesicular rash distinguishes rickettsial pox from all other rickettsial diseases (which cause maculopapular or petechial rashes). It’s mild and self-limited; treat with doxycycline anyway for speed.

Orientia tsutsugamushi (Scrub Typhus)

Formerly classified as a Rickettsia, Orientia tsutsugamushi causes scrub typhus. Endemic to the “tsutsugamushi triangle” (Southeast Asia, western Pacific, northern Australia). Expect it in a board vignette with recent travel to this region.

Vector and cycle: Transmitted by chiggers (larval Leptotrombidium mites). The mites feed on rodents (reservoir); humans are accidental hosts when they enter scrubby vegetation - hence “scrub” typhus. The chigger attaches at areas of tight clothing, feeds for several days, and transmits the organism. The chigger is nearly microscopic and usually unnoticed.

Clinical: Eschar at bite site (key diagnostic clue), fever, headache, regional lymphadenopathy, maculopapular rash. Severe disease: pneumonitis, meningoencephalitis, multi-organ failure. Treatment: doxycycline.

46.2 Ehrlichia and Anaplasma

Ehrlichia and Anaplasma are closely related obligate intracellular gram-negative coccobacilli in the family Anaplasmataceae. They’re tick-borne like the spotted fevers, but they differ from Rickettsia in a few key ways that are board-testable:

They infect white blood cells, not endothelium
They replicate in membrane-bound cytoplasmic vacuoles called morulae, not free in the cytoplasm
They rarely cause rash (unlike RMSF)
They characteristically produce leukopenia + thrombocytopenia + elevated transaminases

Morulae: The morula (Latin “morus” = mulberry) is a cluster of organisms within a membrane-bound vacuole inside a host WBC. On Wright or Giemsa-stained peripheral blood smear, morulae appear as basophilic (purple) inclusions in leukocyte cytoplasm. Finding morulae is diagnostic but has only ~20-30% sensitivity. PCR is more sensitive (>90%).

Ehrlichia chaffeensis - Human Monocytic Ehrlichiosis (HME):

Infects monocytes - look for morulae in monocytes
Vector: Lone Star tick (Amblyomma americanum) - recognizable by the white dot on the female’s back
Reservoir: white-tailed deer
Geography: southeastern and south-central US (matches the Lone Star tick range)
Clinical: fever, headache, myalgia, leukopenia, thrombocytopenia, elevated LFTs; rash uncommon (unlike RMSF)

Anaplasma phagocytophilum - Human Granulocytic Anaplasmosis (HGA):

Infects granulocytes (neutrophils) - look for morulae in neutrophils
Vector: Ixodes tick (I. scapularis in the East, I. pacificus in the West) - the same tick that transmits Lyme and babesiosis
Geography: upper Midwest and Northeast US
Watch for co-infections with Borrelia burgdorferi (Lyme) and Babesia microti - all three share the Ixodes vector

Treatment for both: doxycycline. Don’t wait for confirmation; treat on suspicion.

Lone Star tick extras: Amblyomma americanum also transmits Francisella tularensis and is the tick associated with alpha-gal syndrome (red meat allergy from tick saliva containing galactose-alpha-1,3-galactose). This is the tick that can make you allergic to steak.

Tick vector summary (high-yield):

Tick	Transmits
Dermacentor	RMSF (R. rickettsii), tularemia
Ixodes	Lyme (B. burgdorferi), anaplasmosis (A. phagocytophilum), babesiosis
Amblyomma (Lone Star)	Ehrlichiosis (E. chaffeensis), tularemia, alpha-gal syndrome
Rhipicephalus	RMSF (southwest US/Mexico)

46.3 Coxiella burnetii (Q Fever)

Coxiella burnetii is the black sheep of this family - it shares the obligate intracellular lifestyle but breaks all the other rules. It’s not transmitted by arthropod bite. It doesn’t primarily target endothelium. It doesn’t cause a rash. And it’s phenomenally resistant to environmental conditions.

An extraordinary organism: C. burnetii forms a spore-like small cell variant (SCV) that survives desiccation, heat, UV light, and disinfectants - persisting in dust, soil, and fomites for months to years. Only 1-10 organisms are needed to cause infection (extremely low infectious dose), making it one of the most infectious organisms known. These properties make it a potential bioterrorism agent and explain its epidemiology: infection occurs through inhalation of contaminated aerosols.

The phagolysosome trick: Most intracellular pathogens either escape the phagosome or block phagosome-lysosome fusion. Coxiella does the opposite - it thrives inside the phagolysosome of macrophages. The acidic pH (~4.5) actually activates its metabolic enzymes. This is unique and explains why it can be grown in acidified media in culture.

The animal connection: The primary reservoirs are cattle, sheep, and goats. The organism has remarkable tropism for the mammalian placenta, where it reaches up to 10^9 organisms per gram. During animal parturition, massive quantities of organisms aerosolize. Wind can carry contaminated aerosols several kilometers from farms. People at risk: farmers, veterinarians, abattoir workers, and laboratory personnel. The “Q” in Q fever stands for query - the disease was named before its cause was identified.

Two faces of Q fever:

Acute Q fever - most patients have a flu-like illness or are asymptomatic (~60% asymptomatic). About 5% require hospitalization. Major acute syndromes:

Atypical pneumonia (nonproductive cough, patchy infiltrates)
Granulomatous hepatitis with fibrin ring granulomas
Meningoencephalitis (rare)

Chronic Q fever is a different beast entirely. The organism persists intracellularly and causes culture-negative endocarditis, typically affecting patients with pre-existing valve abnormalities (especially prosthetic valves, bicuspid aortic valves) or immunosuppression. Treatment: doxycycline + hydroxychloroquine for at least 18 months, and even then, relapse is common.

Pregnancy: Q fever in pregnancy can cause spontaneous abortion, premature delivery, and low birth weight.

Fibrin ring granuloma: Fibrin ring granulomas (“doughnut granulomas”) in the liver or bone marrow are highly suggestive of disseminated Q fever. They have a central lipid vacuole surrounded by a ring of fibrin and epithelioid histiocytes. Not 100% pathognomonic (rarely seen in Hodgkin lymphoma, CMV, toxoplasmosis, EBV), but in the right clinical context (pneumonia + hepatitis + animal exposure) they’re essentially diagnostic.

Diagnosis: Serology is primary. C. burnetii undergoes phase variation - the organism expresses different surface antigens (phase I vs phase II) depending on conditions. In acute infection, phase II antibodies predominate. In chronic infection, phase I antibodies rise to high titers. This serologic pattern distinguishes acute from chronic disease. PCR is available; culture is possible but hazardous and requires BSL-3.

46.4 Bartonella

Bartonella species are gram-negative pleomorphic rods, facultative intracellular, and - unlike Rickettsia - they can be cultured on artificial media (chocolate or blood agar), though growth is slow (2-6 weeks). They’re oxidase-negative, catalase-negative, and urease-negative. Because growth is slow and fastidious, molecular methods (PCR, 16S rRNA sequencing) have largely replaced biochemical identification.

Bartonella henselae - Cat Scratch Disease and Bacillary Angiomatosis:

Two diseases, same organism, different hosts:

In immunocompetent hosts: cat scratch disease (CSD). A papule develops at the scratch/bite site, followed 1-3 weeks later by regional lymphadenopathy (usually unilateral, tender, may suppurate). Fever in ~30%. Self-limited; resolves over 2-4 months without antibiotics.

Complications: Parinaud oculoglandular syndrome (conjunctival granuloma + preauricular lymphadenopathy), encephalitis, hepatosplenic involvement.

In immunocompromised hosts (especially HIV with low CD4): bacillary angiomatosis (BA) and bacillary peliosis hepatis. These are angioproliferative vascular lesions:

Angiomatosis = skin (red-purple papulonodules, resemble Kaposi sarcoma)
Peliosis = visceral (blood-filled cystic spaces in liver/spleen)

Histology of CSD/LGV - the stellate microabscess: Lymph node biopsy in cat scratch disease reveals stellate (star-shaped) microabscesses with central neutrophilic necrosis surrounded by palisading epithelioid histiocytes and multinucleated giant cells. This pattern isn’t pathognomonic - it’s the same pattern seen in:

Lymphogranuloma venereum (Chlamydia L1-L3)
Tularemia (Francisella)

Clinical context (cat exposure vs. STI vs. water/animal exposure) differentiates these three.

Warthin-Starry stain: B. henselae organisms appear as small black rods within abscesses on Warthin-Starry silver impregnation stain. It’s fussy and hard to read. Other silver-stained organisms: H. pylori, Legionella, spirochetes (Treponema). IHC and PCR have largely taken over.

Histology of BA: Lobular proliferations of small blood vessels with plump/epithelioid endothelial cells, neutrophilic infiltrate, and granular amphophilic material (bacterial clumps). The key differential is Kaposi sarcoma:

Feature	Bacillary angiomatosis	Kaposi sarcoma
Vascular pattern	Lobular proliferation	Slit-like channels, spindle cells
Inflammatory cells	Neutrophils present	No neutrophils
Special finding	Granular bacterial clumps	Hyaline globules
Causative agent	Bartonella	HHV-8
Stain	Warthin-Starry +	HHV-8 IHC +

Bartonella quintana - Trench Fever:

Named for WWI soldiers in trenches
Vector: human body louse (Pediculus humanus corporis) - same vector as R. prowazekii
Modern setting: homeless/alcoholic populations, sometimes called “urban trench fever.” 5-10% of febrile homeless patients have B. quintana bacteremia.
Causes relapsing fever with 5-day (quintan) periodicity
Also causes culture-negative endocarditis (major cause in homeless patients - always consider it)
Can also cause bacillary angiomatosis (overlap with B. henselae, more often cutaneous)

Diagnosis: Extended-incubation blood cultures (2-6 weeks), serology, PCR. Lysis-centrifugation (Isolator) blood cultures improve yield by releasing intracellular organisms. Treatment: doxycycline +/- gentamicin; for endocarditis, prolonged combination therapy.

46.5 Chlamydia and Chlamydophila

The chlamydiae are obligate intracellular bacteria with a unique developmental cycle that makes them fundamentally different from other bacteria. They can’t synthesize their own ATP - they’re energy parasites that import ATP from the host via a bacterial ATP/ADP translocase. This is why they can’t grow on artificial media and require cell culture or molecular methods.

The Biphasic Life Cycle

Chlamydia exists in two distinct forms:

Elementary body (EB) - the infectious form:

Small (0.3 µm), metabolically inactive
Rigid outer membrane (disulfide cross-linked outer membrane proteins) allows limited extracellular survival
Binds host cells via MOMP and other adhesins (heparan sulfate proteoglycans as receptors)
Triggers its own receptor-mediated endocytosis
Prevents phagosome-lysosome fusion via Type III secretion system effectors - the inclusion body stays non-acidified

Reticulate body (RB) - the replicative form:

Larger (0.8-1 µm), metabolically active
Divides by binary fission inside the inclusion body
Cannot survive extracellularly (no rigid outer membrane)
Uses host ATP and scavenges amino acids, nucleotides, lipids

The cycle: EB attaches and enters -> reorganizes into RB within 6-8 hours -> RB replicates for 48-72 hours -> RBs condense back to EBs -> host cell ruptures -> released EBs infect new cells.

MOMP - major outer membrane protein: MOMP is the principal structural and antigenic protein of Chlamydia. It maintains outer membrane integrity, functions as a porin, and is the target of protective antibodies. Serovar differences in MOMP define C. trachomatis subtypes (A-C, D-K, L1-L3). MOMP is the target of direct fluorescent antibody (DFA) tests and many serologic assays.

Why they can’t be Gram stained: Chlamydiae have a cell wall, but it lacks peptidoglycan in the usual bacterial sense. This is why β-lactams are ineffective. Treat with tetracyclines or macrolides.

Chlamydia trachomatis

C. trachomatis is the most common bacterial STI in the US (~1.8 million reported cases annually; actual cases much higher given asymptomatic carriage) and the leading infectious cause of blindness worldwide. HPV is the most common STI overall; among bacterial STIs, chlamydia tops the list (more common than gonorrhea).

Serovar-disease associations are high-yield:

Serovars A, B, C - trachoma:

Endemic in developing countries with poor sanitation. Transmitted by direct contact, fomites, and flies. Repeated infections cause chronic follicular conjunctivitis -> conjunctival scarring -> inversion of eyelashes (trichiasis) -> corneal abrasion -> blindness. WHO “SAFE” strategy: Surgery, Antibiotics (azithromycin), Facial cleanliness, Environmental improvement.

Serovars D-K - urogenital infections:

Most common chlamydial disease. Infects columnar epithelium of urogenital tract and conjunctiva. Asymptomatic in ~25% of men and ~70% of women - this asymptomatic carriage drives transmission and is the rationale for screening all sexually active women under 25.

Clinical: mucopurulent discharge, dysuria, cervicitis, urethritis. Complications from ascending infection in women: pelvic inflammatory disease (PID), tubal scarring, ectopic pregnancy, infertility. In men: epididymitis.

C. trachomatis and N. gonorrhoeae are the two major sexually transmitted causes of PID. Coinfection is common (10-40% of gonorrhea patients also have chlamydia), which is why dual therapy is often given. Chlamydial PID tends to be more indolent but still destructive.

Reactive arthritis (formerly Reiter syndrome): Autoimmune response triggered by chlamydial infection. Classic triad: urethritis, conjunctivitis, arthritis - “can’t pee, can’t see, can’t climb a tree.” Occurs primarily in HLA-B27 positive individuals.

Neonatal infections (from vertical transmission during vaginal delivery, ~25-50% of infants from infected mothers):

Inclusion conjunctivitis: 5-14 days after birth (later than gonococcal ophthalmia, which appears 2-5 days)
Chlamydial pneumonia: 1-3 months of age, distinctive staccato cough, afebrile
Treatment: oral erythromycin or azithromycin - topical alone is insufficient

Serovars L1, L2, L3 - lymphogranuloma venereum (LGV):

These serovars are more invasive than D-K because they can infect macrophages and spread to regional lymph nodes. LGV has re-emerged among men who have sex with men.

Three-stage disease:

Primary: painless ulcer/papule at inoculation site (genitalia), heals spontaneously
Secondary (2-6 weeks later): painful unilateral inguinal lymphadenopathy (buboes), fever, malaise. The “groove sign” - lymphadenopathy above and below the inguinal ligament - is characteristic.
Tertiary: chronic inflammatory destruction -> fistulae, strictures (urethral/rectal), elephantiasis of genitalia, esthiomene

In MSM: rectal infection causes proctocolitis that can mimic Crohn disease.

LGV histopathology: stellate microabscesses with central neutrophilic necrosis surrounded by palisading histiocytes - same pattern as cat scratch disease and tularemia. Clinical context distinguishes them.

Treatment for LGV: doxycycline 100mg BID x 21 days (longer than uncomplicated chlamydia’s 7 days).

Chlamydia trachomatis - Diagnostic Methods

NAAT is the current standard: Nucleic acid amplification tests detect chlamydial DNA/RNA with >95% sensitivity and >99% specificity. Methods include PCR, strand displacement amplification, and transcription-mediated amplification. Non-invasive specimens are FDA-approved: urine, self-collected vaginal swabs. Multiplex platforms detect C. trachomatis and N. gonorrhoeae simultaneously. NAAT doesn’t require viable organisms, so transport is less critical.

Shell vial culture with McCoy cells - the gold standard historically, now the “legal standard” for medicolegal cases (sexual assault). McCoy cells (fibroblasts) on coverslips, inoculated and centrifuged to enhance attachment, then incubated 48-72 hours. Inclusions detected by IF staining (anti-MOMP). Near-100% specificity but only 70-85% sensitivity.

Direct fluorescent antibody (DFA) - historical; fluorescein-labeled anti-MOMP antibody applied directly to specimen. Required skilled microscopists, lower sensitivity than NAAT. Largely obsolete.

Serology - limited role in acute diagnosis of C. trachomatis urogenital infection (background seroprevalence). Useful for LGV and psittacosis.

Chlamydia trachomatis - Treatment

Uncomplicated D-K infection:

Doxycycline 100mg PO BID x 7 days (first-line, CDC preferred)
Azithromycin 1g PO single dose (alternative)

Other scenarios:

LGV: doxycycline x 21 days
Pregnancy: azithromycin single dose (doxycycline contraindicated)
Neonatal conjunctivitis: oral erythromycin x 14 days

Chlamydophila pneumoniae

C. pneumoniae (now Chlamydia pneumoniae) is a respiratory pathogen transmitted person-to-person via respiratory droplets - no intermediate host required. Accounts for ~10% of community-acquired pneumonia. Seroprevalence reaches 50-70% by adulthood, so serology is tricky - most adults have been exposed.

Clinical picture: Subacute, beginning with upper respiratory symptoms (pharyngitis, laryngitis, sinusitis) including hoarseness. Progresses over 1-2 weeks to a mild atypical/walking pneumonia. Fever usually low-grade, cough persistent. Bilateral infiltrates common on CXR. Severe disease can occur in elderly and immunocompromised. Epidemiologically linked (controversially) to atherosclerosis.

Diagnosis:

PCR/NAAT on respiratory specimens (nasopharyngeal swab, BAL, sputum) is now preferred - often part of multiplex respiratory panels alongside Mycoplasma, Legionella, and respiratory viruses
Serology: 4-fold rise in IgG between acute and convalescent sera (2-4 weeks apart) via microimmunofluorescence; limited by the paired-sample requirement and ubiquity of past exposure

Chlamydophila psittaci (Psittacosis)

C. psittaci causes psittacosis (parrot fever / ornithosis) - a zoonotic infection from birds. Organisms are shed in droppings, feather dust, and respiratory secretions of parrots, parakeets, pigeons, turkeys, ducks. Humans are infected by inhaling aerosolized dried bird excreta. Occupational risk: pet store workers, bird breeders/fanciers, poultry processors, veterinarians, zoo workers.

Board clue: pneumonia + bird exposure = psittacosis until proven otherwise.

Pathogenesis: Inhaled -> infects alveolar macrophages -> blood-borne spread to reticuloendothelial macrophages in liver and spleen (hepatosplenomegaly) -> secondary bacteremia seeds lungs -> pneumonia. The macrophage tropism in liver/spleen distinguishes psittacosis from C. pneumoniae (which primarily infects respiratory epithelium).

Clinical: High fever, severe headache (often the most prominent symptom), nonproductive cough, relative bradycardia (pulse-temperature dissociation), hepatosplenomegaly. CXR: patchy consolidation. Can be severe enough for ICU. Rare complications: endocarditis, meningoencephalitis, hepatitis. Incubation: 5-14 days. Human-to-human transmission is very rare.

Diagnosis: Serology (complement fixation or microimmunofluorescence) - 4x rise in titer confirms. PCR available but not fully standardized. Culture is hazardous (BSL-3; lab-acquired psittacosis has occurred); avoid culture unless in a BSL-3 facility. Psittacosis is nationally notifiable.

Treatment: Doxycycline (first-line) or macrolides.

46.6 Mycoplasma and Ureaplasma

Mycoplasma and Ureaplasma are the smallest free-living bacteria capable of self-replication (0.2-0.3 µm). They pass through 0.45 µm filters used for sterilization, which matters because they commonly contaminate cell cultures. Their small genomes (~580-1,300 genes) force them to depend on host cells for many nutrients.

The missing wall: Mycoplasma and Ureaplasma uniquely lack a cell wall. Their membranes contain sterols (cholesterol, scavenged from the host) for stability in the absence of peptidoglycan.

Clinical consequences of no cell wall:

No Gram stain (nothing for the dye to bind)
Inherently resistant to all cell-wall-targeting antibiotics - β-lactams, cephalosporins, carbapenems, vancomycin are all useless
Treatment requires macrolides, tetracyclines, or fluoroquinolones
Pleomorphic shape (no wall to hold form)

Culture: Requires complex media with serum for sterol supplementation (SP4, Friis, Frey, Hayflick media). Colonies show a characteristic “fried egg” appearance - dense center embedded in agar, thin transparent periphery. Growth is slow (1-3 weeks for M. pneumoniae). Ureaplasma colonies are tiny (15-60 µm; microscopic examination required). Most labs now use NAAT.

Extracellular, not intracellular: Unlike Chlamydia and Rickettsia, Mycoplasma and Ureaplasma live extracellularly, adhering to the surface of epithelial cells via specialized tip structures (P1 adhesin in M. pneumoniae). They damage host cells through:

Direct cytotoxicity (H2O2 production, nutrient competition)
Immune-mediated injury (molecular mimicry -> autoimmune responses)
Superantigen-like activity

Mycoplasma pneumoniae

Clinical presentation - “walking pneumonia”: M. pneumoniae is the archetypal atypical pneumonia. It predominantly affects children and young adults (ages 5-20), with epidemics in closed populations (military barracks, dorms, schools). Incubation 2-3 weeks. Gradual onset of tracheobronchitis that can progress to atypical pneumonia: malaise, low-grade fever, headache, and a dry, hacking cough persisting for weeks. Patients often feel relatively well despite CXR findings - hence “walking pneumonia.” CXR shows patchy interstitial infiltrates that look worse than the patient seems.

Distinctive complications:

Bullous myringitis (hemorrhagic blisters on tympanic membrane) - classic but uncommon
Erythema multiforme (targetoid skin lesions)
Guillain-Barré syndrome
Encephalitis

Cold agglutinins / anti-I autoimmune hemolytic anemia: This is the famous extrapulmonary manifestation. M. pneumoniae triggers production of IgM autoantibodies against the I antigen on adult RBCs. These cold agglutinins bind at low temperatures, activate complement, and cause intravascular and extravascular hemolysis.

Key distinctions:

Anti-I (targets I on adult RBCs) is M. pneumoniae
Anti-i (targets i on fetal RBCs) is EBV

Clinical hemolysis occurs in a minority of infected patients. Cold agglutinins are present (detectable titers) in about half of M. pneumoniae pneumonia cases. Titers often exceed 1:64.

Lab pitfall: at 4°C, cold agglutinins cause RBC clumping that yields spurious CBC results - falsely elevated MCV, falsely low RBC count, falsely high MCHC (often >36 g/dL triggers an alert). Warming the specimen to 37°C before analysis corrects the error.

Diagnosis:

PCR of respiratory specimens is the method of choice
Serology (IgM, or 4x rise in IgG) is also used
Cold agglutinin test is nonspecific but supportive when positive

Treatment: Macrolides (azithromycin) or doxycycline. Remember: no β-lactams will work.

Mycoplasma hominis and Ureaplasma urealyticum

These are urogenital colonizers that cause disease in specific clinical contexts.

Clinical syndromes:

Postpartum fever / endometritis (classic M. hominis / U. urealyticum scenario)
Chorioamnionitis
Neonatal pneumonia and meningitis (vertical transmission)
Non-gonococcal, non-chlamydial urethritis (NGU) - Ureaplasma is the most common cause
Pelvic inflammatory disease
Surgical wound infections

Ureaplasma urealyticum: Named for its characteristic urease activity - hydrolyzes urea to ammonia + CO2, alkalinizing the medium (color-change indicator media). This distinguishes it from Mycoplasma (urease-negative).

Treatment quirks:

M. hominis is unique among Mycoplasma species in being resistant to macrolides (azithromycin, erythromycin) but susceptible to clindamycin
General coverage: doxycycline or fluoroquinolones
Ureaplasma: azithromycin or doxycycline

46.7 Gardnerella vaginalis and Bacterial Vaginosis

Bacterial vaginosis (BV) is not a true infection in the classical sense - there’s no single pathogen invading host tissue. Instead, it represents a disruption of the normal vaginal ecosystem: the protective lactobacilli that normally dominate the vaginal flora are replaced by an overgrowth of mixed anaerobes, including Gardnerella vaginalis, Mobiluncus, Prevotella, and others.

Gardnerella microbiology: Pleomorphic gram-variable rod (often reported as gram-negative despite classification ambiguity). Non-motile, catalase-negative, oxidase-negative, beta-hemolytic on HBT (human blood bilayer Tween) agar. Part of normal vaginal flora that overgrows when Lactobacillus is diminished.

The ecology of the vagina: A healthy vagina is dominated by Lactobacillus species, which produce lactic acid and maintain a low pH (3.8-4.5). This acidic environment suppresses many potential pathogens. In BV, lactobacilli disappear, pH rises above 4.5, and anaerobes flourish. What triggers this shift isn’t fully understood, but risk factors include douching, new sexual partners, and IUD use.

Clinical presentation: The hallmark is a thin, grayish-white discharge with a fishy odor. The odor intensifies after intercourse (semen is alkaline) and is released by adding 10% KOH - the “whiff test.” Unlike vaginitis from Trichomonas or Candida, BV typically doesn’t cause significant inflammation; pruritus and dysuria are absent.

Amsel criteria - 3 of 4 make the diagnosis:

Thin, homogeneous (grayish-white) discharge
Vaginal pH >4.5
Positive whiff test (fishy/amine odor with KOH)
Clue cells on wet mount

Clue cells are vaginal epithelial cells studded with adherent bacteria, giving them a stippled/granular appearance with obscured cell borders. They’re essentially pathognomonic for BV in context.

Nugent score on Gram stain is the laboratory gold standard - scores 0-10 based on relative quantities of Lactobacillus (large gram-positive rods) vs. Gardnerella/anaerobes. Score ≥7 = BV.

Why BV matters beyond symptoms: BV is associated with significant complications:

Preterm delivery
Increased susceptibility to STIs including HIV
Post-procedural infections following gynecologic surgery
PID

Treatment: Metronidazole (first-line; oral 500mg BID x 7 days or vaginal gel) or clindamycin (oral or vaginal). Treatment is recommended for symptomatic women and for asymptomatic pregnant women at high risk for preterm delivery. Recurrence is common (~50% within 12 months) - the fundamental dysbiosis tends to return.

Chapter 47: Fungi

47.1 Fungal Classification and Principles

Fungi are eukaryotes that occupy a kingdom distinct from bacteria, plants, and animals. This matters clinically because their eukaryotic cell biology makes them harder to treat than bacteria - their cellular machinery is similar to ours, limiting selective toxicity. Antifungal therapy must exploit the differences that do exist: the fungal cell membrane contains ergosterol (not cholesterol), and the cell wall contains unique components like β-glucan and chitin. Fungi are heterotrophs (lack chlorophyll, can’t photosynthesize) and are either aerobes or facultative anaerobes. Because they’re eukaryotes, antibacterial antibiotics are useless against them.

The fundamental morphologic division separates yeasts and molds. Yeasts are unicellular and reproduce by budding - a daughter cell (called a blastoconidium) pinches off from the mother cell. Molds are multicellular, composed of tubular filaments called hyphae that branch and interweave to form a mycelium. Some fungi exist as either form depending on conditions - these dimorphic fungi grow as molds in the environment (at 25-30°C) and convert to yeasts in tissue (at 37°C). This mold in the cold, yeast in the heat pattern is key to understanding the endemic mycoses. The classic dimorphs are Histoplasma, Blastomyces, Coccidioides, Sporothrix, Paracoccidioides, and Talaromyces (Penicillium) marneffei.

Pseudohyphae vs. true hyphae

When a budding daughter cell fails to detach completely, you get a chain of incompletely separated yeast cells called pseudohyphae. The key distinction from true hyphae: pseudohyphae have constrictions at cell junctions, giving them a sausage-link appearance, while true hyphae have parallel walls of uniform width. Candida species classically produce pseudohyphae. C. albicans is unusual in that it produces yeast, pseudohyphae, AND true hyphae.

Hyphal morphology provides diagnostic clues

Septate hyphae have cross-walls (septa) dividing them into compartments; Aspergillus, Fusarium, Scedosporium, and dermatophytes are the clinically important septate molds. Non-septate (aseptate or pauciseptate) hyphae lack frequent cross-walls and appear as broad, ribbon-like tubes; the Mucormycetes (Mucor, Rhizopus) show this pattern. The angle of hyphal branching also helps - Aspergillus branches at 45° acute angles, while Mucormycetes branch at right angles (90°).

Sporangial vs. conidial molds

Molds split into two broad categories. Sporangial molds produce spores inside a sac (sporangium) and lack septa - the Mucormycetes are the prototype. Conidial molds produce spores (conidia) on specialized structures called conidiophores and are septate - Aspergillus, Fusarium, and the dermatophytes fit here. A useful board shortcut: aseptate + sporangia = Mucorales; septate + conidia = Aspergillus/Fusarium/dermatophytes.

Conidial molds are further divided into:

Hyaline molds: clear/colorless hyphae, light-colored colonies. Examples: Aspergillus, Fusarium, Scedosporium.
Dematiaceous molds: pigmented (brown/black) hyphae due to melanin in the cell wall. Examples: Cladophialophora, Bipolaris, Alternaria, Exophiala, Lomentospora, Madurella.

The macroscopic tell for dematiaceous molds: brown/black pigment on BOTH the front AND reverse of the agar plate. If only the front is dark (conidia pigmented but hyphae clear), it’s still a hyaline mold. Aspergillus niger is the classic trap here - it has dark conidia but hyaline hyphae, so the reverse of the plate stays light.

Cell wall architecture

Two components matter most: chitin (polymer of N-acetylglucosamine, provides structural rigidity) and β-1,3-glucan (glucose polymer, crosslinked with chitin). β-1,3-glucan is the target of echinocandins (caspofungin, micafungin, anidulafungin) and the basis of the serum Fungitell (BDG) assay. Some fungi have a polysaccharide capsule surrounding the cell wall - Cryptococcus is the most important example, with its thick glucuronoxylomannan capsule that blocks phagocytosis, serves as the target of the CrAg test, and can be visualized with India ink (negative staining), mucicarmine, or Alcian blue.

Conidia are asexual spores

These are the environmental dispersal units. When you inhale Aspergillus, Histoplasma, or Coccidioides conidia from disturbed soil, you’re inhaling the infectious propagules that initiate disease.

47.2 Diagnostic Techniques in Mycology

Direct examination

KOH preparation: 10-20% potassium hydroxide dissolves host keratin (hair, skin, nails) while leaving fungal cell walls intact, making hyphae and yeasts visible. The rapid workhorse for dermatophytes, Malassezia, and Candida.

Calcofluor white: a fluorescent dye that binds chitin and cellulose. Fungi glow apple-green or white under UV. More sensitive than KOH alone. Also highlights Pneumocystis cysts. Limitation: non-specific - it’ll light up cotton fibers and plant material too.

Gram stain: yeasts stain gram positive (they retain crystal violet) and appear as large purple cells 3-8 μm. Gram-positive yeast in a sterile site (blood, CSF) always requires urgent workup.

Wright-Giemsa on bone marrow or peripheral smear: catches intracellular yeasts. Histoplasma is the classic - small 2-4 μm yeasts packed inside macrophages, each with a clear halo.

India ink: historical method for Cryptococcus in CSF. The ink particles can’t penetrate the capsule, so you see a clear halo around the yeast against a dark background. Sensitivity only ~50% (needs high organism burden), replaced by CrAg LFA.

Tissue stains

Stain	What it highlights	Color
H&E	Dematiaceous hyphae (direct); hyaline fungi stain faintly eosinophilic	Brown/black vs. pink
GMS (Grocott methenamine silver)	All fungi - most sensitive, stains dead/degenerate fungi too	Black on green
PAS (periodic acid-Schiff)	All fungi - better morphologic detail than GMS	Magenta/red
Mucicarmine	Cryptococcus capsule	Magenta
Fontana-Masson	Melanin in Cryptococcus cell wall and dematiaceous fungi	Black/brown
Alcian blue	Cryptococcus capsule (alternative)	Blue

Board shortcut: if the question asks “which stain highlights fungi?” the answer is GMS or PAS. For Cryptococcus specifically, mucicarmine shows the capsule - but capsule-poor strains may stain weakly or negatively, in which case Fontana-Masson (melanin in cell wall) or GMS/PAS (the yeast itself) can rescue the diagnosis. Capsule-poor Cryptococcus often elicits granulomatous rather than minimally inflammatory reactions, which can mimic sarcoidosis or TB.

Culture media

Fungi grow slowly. Cultures are incubated at 25-30°C for 4-6 weeks - much longer than bacterial cultures. Dimorphic fungi get a parallel 37°C plate for yeast conversion.

Sabouraud dextrose agar (SDA): general-purpose medium. Acidic pH (~5.6) inhibits most bacteria. Standard for yeasts, molds, and dermatophytes.
SABHI and potato flake agar: enriched media for fastidious fungi that grow poorly on SDA.
Brain-heart infusion (BHI) with blood: enhanced recovery of dimorphic fungi.
Inhibitory mold agar: contains chloramphenicol to suppress bacteria; favors dimorphic fungi but suppresses dermatophytes (don’t use for tinea workup).
Potato dextrose agar (PDA): used AFTER isolation to encourage fruiting structure formation for mold ID. Also enhances red pigment production in Trichophyton rubrum.
Cornmeal agar with Tween 80: nutrient-poor medium that brings out yeast morphology - pseudohyphae, true hyphae, and chlamydospores (only C. albicans and C. dubliniensis make these).
Dermatophyte test medium (DTM): contains cycloheximide (suppresses saprophytes) + antibiotics + phenol red. Positive = color change from yellow to red during days 7-14 (alkaline shift from dermatophyte protein metabolism). Not definitive; confirm microscopically.
Birdseed (niger/Staib) agar: Cryptococcus produces melanin by laccase acting on caffeic acid in niger seeds, turning colonies brown/black. Highly specific among clinically relevant yeasts.
CHROMagar Candida: chromogenic substrates cleaved by species-specific enzymes. C. albicans = green, C. tropicalis = blue with purple halo, C. krusei = pink/rough, C. glabrata = mauve.
Canavanine-glycine-bromothymol blue (CGB) agar: differentiates Cryptococcus species. C. neoformans stays yellow; C. gattii turns blue.

Microscopy of mold colonies

Lift the mold from the plate with clear tape (Scotch tape prep) and stain with lactophenol cotton blue. Preserves the spatial relationship of conidiophore, conidia, and hyphae.

Biochemical and rapid tests

Urease-positive fungi (mnemonic TCTC): Trichosporon, Cryptococcus, Trichophyton mentagrophytes, Candida krusei. Also Coccidioides. Urease differentiates Cryptococcus from Candida (Candida is urease-negative).
Rapid trehalose test: C. glabrata turns the solution yellow (rapid trehalose assimilation).
Germ tube test: incubate yeast in serum at 37°C for <4 hours. Germ tubes (tube-like outgrowths with NO constriction at origin, distinguishing from pseudohyphae) = C. albicans or C. dubliniensis. Don’t incubate >4 hours or other Candida species will form pseudohyphae and give false positives.

Serology - antibody detection

Complement fixation (CF) and immunodiffusion (ID) are the classic antibody methods. CF titers are quantitative and followable. Used for Histoplasma (H and M bands on ID), Blastomyces (A band), Coccidioides (F and TP bands; CF titer correlates with severity), and Aspergillus (precipitins in ABPA). Less reliable in immunocompromised patients who may not mount a response.

Serology - antigen detection

Galactomannan (Aspergillus): cell wall polysaccharide released during invasive infection. Sandwich ELISA (Platelia). False positives: piperacillin-tazobactam, amoxicillin-clavulanate, dietary galactomannan (guar gum, locust bean gum - think ice cream, cream cheese), Histoplasma/Penicillium/Fusarium cross-reactivity. BAL galactomannan has fewer false positives than serum. Sensitivity is better in BAL than serum.
Cryptococcal antigen (CrAg): detects glucuronoxylomannan. The immunochromatographic lateral flow assay (LFA) is >99% sensitive and >99% specific on serum or CSF. Quantitative titers follow treatment. Historical methods: latex agglutination (false positive with Trichosporon due to cross-reactive GXM), India ink.
Histoplasma antigen: urine or serum. Best for disseminated disease, less sensitive for localized pulmonary. Cross-reacts with Blastomyces, Coccidioides, Talaromyces. Useful for monitoring response.
1,3-β-D-glucan (BDG / Fungitell): pan-fungal screen. Positive in most conidial fungi (Aspergillus, Fusarium, Candida, Pneumocystis). NEGATIVE in Cryptococcus, Mucormycetes, Blastomyces (mnemonic: “CMB” or “Can’t Make β-glucan”). Malassezia is also BDG-negative. False positives: hemodialysis (cellulose membranes), IVIG, albumin, gauze packing, piperacillin-tazobactam, streptococcal bacteremia.

Molecular and mass spectrometry

PCR and sequencing target ribosomal DNA (18S, 28S, and the ITS region). ITS has the highest species-level discrimination for fungi (analogous to 16S rRNA for bacteria).

MALDI-TOF mass spectrometry: identifies fungi by their unique protein profiles (laser ionization, time-of-flight). Rapid, revolutionized yeast identification. Mold identification requires extraction protocols and extended databases.

47.3 Yeasts

Candida Species

Candida is the most important fungal pathogen in clinical medicine. Species of Candida are the 4th leading cause of healthcare-associated bloodstream infections and the most common cause of invasive fungal disease worldwide. The genus includes a handful of clinically important species, and knowing their differences matters for treatment.

The ecology of Candida shapes its pathogenesis: Candida species colonize human mucosal surfaces - mouth, gut, vagina, and skin - as commensals. Disease occurs when local or systemic immunity is compromised, or when mucosal barriers are breached. Antibiotics that eliminate competing bacterial flora allow Candida overgrowth. Central venous catheters breach the skin barrier. Neutropenia removes a critical defense.

Host defense matters for presentation:

Cellular (T-cell) immunity, especially Th17, defends against mucocutaneous candidiasis. T-cell deficiency (AIDS, DiGeorge) predisposes to thrush and esophagitis. Chronic mucocutaneous candidiasis (CMC) reflects defects in IL-17 signaling (mutations in IL-17RA/F, STAT1 gain-of-function, AIRE, ACT1) - persistent thrush, nail dystrophy, and skin lesions starting in childhood. CMC patients do NOT typically develop invasive disease because neutrophils are intact.
Neutropenia drives invasive candidiasis (candidemia, hepatosplenic candidiasis, deep-organ infection). Additional invasive risk: central catheters, broad-spectrum antibiotics, TPN, abdominal surgery, ICU stay. Candidemia mortality: 30-50%.

Species matter because susceptibility varies. The top 3 species by bloodstream infection prevalence: C. albicans (~40-50%) > C. glabrata (~20-25%) > C. parapsilosis (~10-15%).

C. albicans: most common. Reliably susceptible to fluconazole. Produces yeast + pseudohyphae + true hyphae. Colonies have characteristic peripheral “feet” (hyphae extending into agar). Grows within 1 day.
C. glabrata: now #2 in many centers. Smaller yeast (2-4 μm vs. 5-7 μm for C. albicans), grows ONLY as budding yeast (no pseudohyphae or hyphae). Grows slightly slower (2-3 days). Dose-dependent fluconazole susceptibility; echinocandins preferred. Mauve on CHROMagar. Haploid (unusual for Candida). Rapid trehalose positive.
C. parapsilosis: predilection for catheter-related infections and TPN - biofilm formation on plastic is key. Echinocandins have reduced activity, so fluconazole is often preferred when susceptibility permits.
C. krusei: intrinsic fluconazole resistance. Selected for in patients on fluconazole prophylaxis. Treat with echinocandins or voriconazole. Urease-positive (one of the TCTC organisms).
C. lusitaniae: intrinsic amphotericin B resistance.
C. auris: emerging public health threat. Multidrug-resistant (azoles, often echinocandins, sometimes amphotericin). Frequently misidentified by conventional methods. Persistent in healthcare environments; causes nosocomial outbreaks. CDC urgent threat.
C. dubliniensis: associated with oral candidiasis in HIV. Can develop fluconazole resistance with repeated exposure. Like C. albicans, it produces germ tubes and chlamydospores.

Superficial disease:

Oral thrush: creamy white patches on erythematous mucosa, scrapable to a raw base. Common in HIV/AIDS (often first manifestation - should prompt CD4 testing), on inhaled steroids, chemotherapy, and in infants.
Candidal vaginitis: vaginal burning, itching, and curd-like (cottage cheese) white discharge. Non-malodorous (unlike BV). Normal vaginal pH 4.0-4.5 (distinguishes from BV and trichomoniasis). KOH: budding yeast and pseudohyphae. Risk factors: diabetes, antibiotics, high-estrogen states (pregnancy, OCPs), immunosuppression.
Cutaneous intertrigo: moist skin folds (under breasts, groin, axillae, diaper area). Satellite pustules around main erythema are characteristic.

Laboratory identification:

Germ tube test: serum at 37°C <4 hours → germ tubes in C. albicans and C. dubliniensis.
Chlamydospores on cornmeal agar with Tween 80: large, thick-walled, round terminal spores. Only C. albicans and C. dubliniensis produce these.
CHROMagar colors as above.
Rapid trehalose: yellow = C. glabrata.
MALDI-TOF is now the standard for definitive identification.
BDG is positive in invasive Candida but non-specific.
T2Candida (magnetic resonance from blood) provides rapid species-level identification.

Tissue morphology: three forms simultaneously possible - yeast (blastoconidia), pseudohyphae, and true hyphae. The yeast-to-hyphal transition is a virulence mechanism (hyphae invade tissue; yeast disseminates).

Treatment depends on disease and species. Mucosal disease: topical or oral azoles. Invasive candidiasis: echinocandins first-line (caspofungin, micafungin, anidulafungin) because of fungicidal activity and broad coverage. Source control (catheter removal) is essential. Fluconazole is used for step-down once susceptibility is confirmed. Amphotericin B is reserved for refractory cases (nephrotoxicity).

Cryptococcus neoformans and C. gattii

Cryptococcus is the most important cause of fungal meningitis, particularly in AIDS. The thick polysaccharide capsule is both its defining feature and its major virulence factor.

Ecology and transmission:

C. neoformans reservoir: pigeon guano (nitrogen-rich droppings). Primarily causes disease in immunocompromised hosts, especially AIDS with CD4 <100.
C. gattii reservoir: eucalyptus trees. Originated in Australia/Papua New Guinea; Vancouver Island outbreak 1999-2003 brought it to North America. Can cause disease in immunocompetent patients and tends to produce larger cryptococcomas.

Infection is by inhalation of desiccated yeast cells or basidiospores. Primary pulmonary infection is often asymptomatic. Dissemination, especially to the CNS, occurs with T-cell immunodeficiency. Cryptococcus crosses the blood-brain barrier by Trojan horse (inside macrophages) and transcytosis.

The capsule as virulence factor: The polysaccharide capsule (glucuronoxylomannan) is antiphagocytic, interferes with antigen presentation and complement, and is visualized with India ink (negative stain - clear halo), mucicarmine (magenta), or Alcian blue.

Organs affected (4 main sites):

Lungs: pneumonia, nodules, rarely ARDS.
Skin: ulcerative papules/nodules; in AIDS may mimic molluscum contagiosum (umbilicated papules).
Bone/joints: osteolytic lesions (any bone, commonly vertebrae and skull).
CNS: basilar meningitis with cranial nerve involvement (especially CN VI abducens palsy and CN II papilledema from elevated ICP).

Clinical presentation of cryptococcal meningitis: Subacute - days to weeks. Headache, fever, altered mental status. Neck stiffness may be subtle. A characteristic and dangerous feature is elevated intracranial pressure - capsule material clogs arachnoid granulations. Management: serial LPs or lumbar drain for pressure control.

CSF: elevated opening pressure (often >20-25 cmH2O), lymphocytic pleocytosis (may be minimal in AIDS), elevated protein, low glucose, CrAg >99% sensitive, India ink ~50% sensitive.

Diagnosis:

Cryptococcal antigen (CrAg) lateral flow assay: >99% sensitive/specific. Serum or CSF. Point-of-care. Quantitative titers follow treatment. Trichosporon cross-reacts on the older latex test (less on LFA).
India ink historically. Capsule excludes ink → clear halo.
Mucicarmine stains capsule magenta. Fontana-Masson stains cell wall melanin (rescues capsule-poor strains).
On H&E: round yeasts with a clear halo (unstained capsule) - “soap bubble” appearance. Variable size 4-10 μm with narrow-based budding (Crypto = narrow; Blasto = Broad).
PCR on CSF increasingly used.

Culture:

Grows on routine media (SDA). Mucoid colonies when capsule is prominent (capsule-poor strains are not mucoid).
Urease positive (one of TCTC).
Cryptococcus produces phenol oxidase (laccase), which converts caffeic acid to melanin. On birdseed (niger/Staib) agar, colonies turn brown/black - specific for Cryptococcus among common yeasts. Melanin is a virulence factor (scavenges oxidants).
Wet prep: budding yeast with NO pseudohyphae or hyphae (never forms hyphae, unlike Candida).
CGB agar differentiates species: C. neoformans stays yellow (can’t use glycine, sensitive to canavanine). C. gattii turns blue (uses glycine, resists canavanine).
BDG is NEGATIVE for Cryptococcus (capsule obscures glucan).

Cryptococcus on birdseed (niger seed) agar: Melanin pigment production gives brown colonies. The thick capsule is visualized with India ink.

Treatment: Staged.

Induction: amphotericin B + flucytosine for ≥2 weeks (synergistic - rapidly sterilizes CSF).
Consolidation: fluconazole 400-800 mg/day for 8 weeks.
Maintenance/secondary prophylaxis: fluconazole 200 mg/day for ≥1 year (lifelong in AIDS until immune reconstitution with ART pushes CD4 >100-200 sustained).

Malassezia

Malassezia (formerly Pityrosporum) is the most common fungus on human skin. It’s a lipophilic yeast that lives on sebaceous areas (scalp, face, upper trunk), requiring exogenous long-chain fatty acids because it can’t synthesize them. This explains its predilection for adolescents/adults (active sebaceous glands), neonates (vernix is lipid-rich), and patients on lipid-containing TPN.

Tinea (pityriasis) versicolor: infection of the stratum corneum, producing hypo- or hyperpigmented patches on the trunk (“versicolor” = changing colors). Hypopigmentation predominates on darker skin; hyperpigmentation on lighter skin. The organism produces azelaic acid, which inhibits melanocyte tyrosinase (hypopigmentation) and causes mild inflammation (hyperpigmentation). Lesions are more apparent after sun exposure.

KOH: “spaghetti and meatballs” - short thick hyphae (spaghetti) mixed with round yeast clusters (meatballs). Essentially pathognomonic. Budding Malassezia shows a characteristic collarette at the budding site - resembles bowling pins or Mickey Mouse ears.

Seborrheic dermatitis: inflammatory reaction to Malassezia in sebaceous areas (scalp → dandruff, eyebrows, nasolabial folds, central chest). Driven by lipase activity generating irritating fatty acids. More severe in HIV (often an early sign), Parkinson disease, and immunosuppression.

Malassezia folliculitis: acneiform papules/pustules on the trunk (commonly misdiagnosed as acne). Exacerbated by humidity and sweating.

Systemic Malassezia: rare but occurs in neonates and adults on lipid-containing TPN through central catheters. The lipid infusion provides exactly what the organism needs; catheter biofilm forms; organisms can embolize to the lungs (septic emboli). Treatment: remove the catheter and give azoles. Echinocandins are intrinsically ineffective (Malassezia lacks β-1,3-glucan synthase - same reason BDG is negative).

Diagnosis of systemic disease: culture the catheter tip with sterile olive oil overlay on SDA (Dixon agar or Leeming-Notman also work). Without lipid supplementation the organism won’t grow. Communicate clinical suspicion to the lab.

Treatment of superficial disease: topical selenium sulfide or ketoconazole shampoo; zinc pyrithione. Extensive or recurrent disease may need oral azoles.

White and Black Piedra

Superficial hair shaft infections.

White piedra (Trichosporon spp.): soft white/yellow nodules loosely attached to hair (pubic, axillary, beard, eyelash). Treatment: hygiene/shaving. But Trichosporon can cause disseminated infection in neutropenic patients: fungemia, pneumonia, renal failure. Important: Trichosporon is resistant to echinocandins (use voriconazole). It’s urease-positive (part of TCTC) and its glucuronoxylomannan cross-reacts with the Cryptococcal latex antigen test (false positive).
Black piedra (Piedraia hortae): hard dark brown/black nodules that surround and penetrate the hair shaft (primarily scalp). Piedraia is dematiaceous (pigmented on H&E, dark colonies on agar). Treatment: shaving + topical salicylic acid or antifungal shampoo (because nodules are firmly adherent, unlike white piedra).

Both are more common in tropical/temperate climates with long hair, poor hygiene, or heavy use of hair oils.

Geotrichum candidum

Yeast-like fungus found in water, soil, plants, and normal flora. Causes catheter-related bloodstream infections, wound infections, and oral/bronchial disease in immunocompromised patients. Produces arthroconidia (rectangular cells from hyphal fragmentation) like Trichosporon, but is urease-negative (distinguishes it). Generally susceptible to azoles and amphotericin.

1,3-β-D-Glucan (BDG) Assay - Summary

Pan-fungal cell wall marker; positive in most invasive fungal infections (Candida, Aspergillus, Pneumocystis).

Key NEGATIVES (organisms that do NOT produce detectable BDG) - mnemonic CMB (Can’t Make β-glucan):

Cryptococcus: thick polysaccharide capsule obscures glucan.
Mucormycetes (Rhizopus, Mucor): cell wall contains chitosan instead of glucan.
Blastomyces: cell wall has α-1,3-glucan, not β-1,3-glucan.
Malassezia also lacks β-1,3-glucan.

False positives: IV immunoglobulin, albumin, hemodialysis with cellulose membranes, piperacillin-tazobactam, gauze/surgical sponges, streptococcal bacteremia.

Pneumocystis jirovecii

Pneumocystis jirovecii (formerly P. carinii - the species name changed when it was recognized that the organism infecting humans is distinct from the one in rats) causes Pneumocystis pneumonia (PJP, formerly PCP), historically the most common AIDS-defining illness. Despite being classified as a fungus based on molecular analysis, it behaves very differently from other fungi and cannot be cultured.

A unique organism: Long classified as a protozoan based on morphology and response to antiprotozoal drugs. Molecular studies revealed it’s a fungus, but unusual. Its cell wall contains β-D-glucan (so BDG is positive) but lacks ergosterol, which is why azole antifungals don’t work. Two forms: trophic forms (small, thin-walled) and cysts (thick-walled, containing up to 8 intracystic bodies). Has never been cultured in vitro.

Who gets PJP: Most people are exposed by age 2-4 (seroconversion). Disease occurs in severe T-cell immunodeficiency: AIDS with CD4 <200, transplant recipients on immunosuppression, patients on high-dose corticosteroids, hematologic malignancies.

Clinical presentation: Subacute - days to weeks. Progressive dyspnea, nonproductive (dry) cough, fever. Hypoxemia often out of proportion to exam and CXR - patient looks okay but O2 sat is 80%. Elevated LDH (>90% of cases, nonspecific but useful for monitoring). Classic CXR: bilateral interstitial infiltrates in a perihilar (“bat wing/butterfly”) distribution with ground glass. CT: bilateral diffuse GGO centrally. Cystic changes and pneumothorax occur, especially with aerosolized pentamidine prophylaxis (which can also cause atypical upper-lobe disease).

Extrapulmonary PJP is rare but occurs with severe immunosuppression or in patients on aerosolized pentamidine (protects lungs only): liver, lymph nodes, bone marrow, spleen.

Histology: alveoli filled with frothy, eosinophilic intra-alveolar exudate (organisms + surfactant + debris). This can mimic pulmonary alveolar proteinosis - distinguish with GMS (PJP organisms are GMS-positive; PAP exudate is GMS-negative).

Diagnosis: Because Pneumocystis can’t be cultured, diagnosis requires visualizing the organism. Induced sputum first-line; if negative, BAL is more sensitive (~90-95%).

Stains:

GMS stains cyst walls black; cysts appear as cup-shaped, “crushed ping-pong balls,” or navicular (boat-shaped) forms, 5-8 μm.
DFA (direct fluorescent antibody): most sensitive cytologic method; stains both cysts and trophic forms.
Giemsa: stains trophic forms (not cysts).
Calcofluor white highlights cysts.

PCR is now routine - more sensitive than cytology but can detect colonization (quantitative cutoffs help distinguish disease from colonization). Useful when BAL cytology is negative but suspicion remains high, or for non-invasive specimens (induced sputum, oral wash) in non-HIV patients who may have lower burdens.

BDG is elevated (cyst walls contain β-glucan) - sensitive but not specific.

Treatment and prophylaxis: TMP-SMX is first line for both treatment and prophylaxis. For moderate-to-severe disease (PaO2 <70 or A-a gradient >35), adjunctive corticosteroids reduce mortality by dampening inflammation as organisms die. Prophylaxis when CD4 <200 in AIDS, and in other high-risk immunocompromised patients. Alternatives for sulfa-allergic patients: dapsone, atovaquone, pentamidine.

47.4 Molds

Molds are filamentous fungi composed of branching hyphae. The two most important molds in clinical practice are Aspergillus and the Mucormycetes. Distinguishing them is critical because they require different treatments.

Aspergillus

Aspergillus species are ubiquitous environmental molds - we all inhale Aspergillus spores daily. In immunocompetent people, innate immunity clears them. Disease occurs when host defenses are impaired or when structural lung abnormalities provide a niche.

The spectrum of aspergillus disease depends entirely on the host’s immune status.

ABPA (allergic bronchopulmonary aspergillosis) - IgE-mediated hypersensitivity in asthma/CF patients. The organism colonizes airways; the immune response causes damage. Diagnostic criteria: asthma + central bronchiectasis + elevated total IgE (>1000 IU/mL) + positive Aspergillus-specific IgE (RAST/ImmunoCAP) + Aspergillus precipitins (IgG by ID) + peripheral eosinophilia. Treatment: corticosteroids (mainstay - immune-mediated, not invasive). Itraconazole or voriconazole as steroid-sparing adjuncts. Total IgE tracks disease activity.

Allergic fungal sinusitis (AFS): the sinus version of ABPA. Chronic sinusitis with nasal polyps, thick “allergic mucin” containing Charcot-Leyden crystals and fungal hyphae, elevated IgE, eosinophilia. Caused by Aspergillus, Bipolaris, Curvularia, and other dematiaceous molds. Surgery + oral steroids.

Aspergilloma - Aspergillus fungal ball in a pre-existing lung cavity (from TB, sarcoidosis, emphysema, abscess). The fungal ball is a mass of matted hyphae + mucus + debris. CT: round mass in cavity with air crescent sign (mass moves with position). Usually asymptomatic; the major complication is massive hemoptysis from vessel erosion. Treatment: observation if asymptomatic, voriconazole/itraconazole (limited penetration), surgery (lobectomy) for definitive treatment especially with significant hemoptysis. Bronchial artery embolization for acute bleeding.

Invasive aspergillosis - in prolonged neutropenia (AML induction, HSCT), chronic steroids, solid organ transplant, CGD, advanced HIV/AIDS. Angioinvasive - hyphae penetrate blood vessels, causing thrombosis, hemorrhagic infarction, and hematogenous dissemination to brain, skin, etc. Originates in nose/sinuses or lungs (portal of entry is respiratory tract).

Pulmonary IA CT findings: nodules with halo sign (ground-glass surrounding a nodule = hemorrhage from angioinvasion). Later: air crescent sign as necrotic tissue retracts. Sinonasal IA: palatal or nasal eschar (similar to mucormycosis). Mortality >80% for disseminated disease.

Invasive aspergillosis classically has NEGATIVE blood cultures - despite angioinvasion, Aspergillus rarely grows from blood (unlike Fusarium, which does).

Aspergillus in the eye: keratitis and endophthalmitis after trauma, especially with topical steroid use (steroids create localized immunodeficiency). Feathery corneal infiltrate with satellite lesions and hypopyon. Treatment: topical natamycin or voriconazole; endophthalmitis needs intravitreal antifungals + vitrectomy.

Otomycosis: A. niger is the species classically associated with otitis externa (fluffy green/black growth in the ear canal with ear pain and impaired hearing). Also Candida albicans. Treatment: aural toilet + topical antifungals (clotrimazole, ciclopirox).

Histologic identification: Septate hyphae with acute-angle (45°) branching - “dichotomous branching,” often Y-shaped. Uniform narrow width (3-6 μm). Distinguishes from Mucormycetes on histopathology. BUT - tissue histology cannot distinguish Aspergillus from Fusarium or Scedosporium (all identical in tissue). Culture is required for definitive identification.

Reproductive structures are ONLY seen in cultures unless a fungal ball grows in an air-filled space (sinus or lung cavity - then the aspergilloma biopsy may show conidiophores with vesicles and phialides).

Aspergillus fumigatus: Blue-green colonies on culture. Histologically, septate hyphae with 45° acute angle branching.

Top three species ranked by clinical frequency: A. fumigatus > A. flavus > A. terreus.

Species	Colony (macroscopic)	Conidial head (microscopic)	Clinical pearl
A. fumigatus	Blue-green with “white apron”	Flask-shaped vesicle, uniseriate phialides on upper 2/3, columnar head	Most common (>90% IA). Thermotolerant (grows at 45-50°C)
A. flavus	Yellow-green/olive	Globose vesicle, biseriate phialides covering entire vesicle, radiate head	Produces aflatoxin: G→T transversion at p53 codon 249, causes hepatocellular carcinoma (synergistic with HBV)
A. terreus	Cinnamon-brown/orange	Globose vesicle, biseriate, columnar head. Produces aleurioconidia directly from hyphae	RESISTANT to amphotericin B - use voriconazole
A. niger	Dark brown/black forward, LIGHT reverse (only conidia pigmented; hyphae are hyaline)	Large globose vesicle, biseriate, radiate head with dark brown conidia	Most common cause of otomycosis; aspergilloma

Diagnosis:

Culture and histopathology remain important. Culture identifies species based on colony + conidial head morphology.
Galactomannan (Aspergillus cell wall polysaccharide) - useful for diagnosis and monitoring in neutropenic patients. False positives: piperacillin-tazobactam, amoxicillin-clavulanate (β-lactams produced from Penicillium), dietary galactomannan (guar/locust bean gum in ice cream), cross-reactivity with Histoplasma/Penicillium/Fusarium. BAL has fewer false positives than serum.
β-D-glucan elevated but nonspecific. False positives: dialysis, IVIG, albumin, gauze.
Aspergillus-specific IgE by RAST/ImmunoCAP for ABPA; Aspergillus precipitin IgG by ID also for ABPA.

Treatment:

Voriconazole is first-line for invasive aspergillosis (Herbrecht 2002 trial - superior to amphotericin B deoxycholate). Monitor trough levels (target 1-5.5 μg/mL). Side effects: visual disturbances, hepatotoxicity, skin photosensitivity (increased skin cancer with prolonged use), drug interactions (CYP2C19/3A4).
Isavuconazole and posaconazole are alternatives.
Fluconazole has NO activity against Aspergillus.
A. terreus: voriconazole (never amphotericin).
Reducing immunosuppression and recovery of neutrophil counts are essential.

Fusarium

A hyaline septate mold that is morphologically indistinguishable from Aspergillus on tissue histology (septate, acute-angle branching). Clinical context and culture separate them.

Disease:

Cutaneous infections, especially in burn patients and neutropenic patients. Lesions are painful red/purple papulonodules, often with central necrosis (ecthyma gangrenosum-like).
Keratitis - famous contact lens outbreak 2006 linked to a specific contact lens solution.
Disseminated fusariosis in chemotherapy/neutropenic patients - multiple skin lesions may be the first sign.

Key distinction from Aspergillus: Fusarium can be recovered from BLOOD CULTURES. Aspergillus essentially never grows from blood. This is because Fusarium produces adventitious conidia (yeast-like cells) that circulate. Mortality is very high (70-100%) in persistently neutropenic patients.

Culture: cottony colonies, color varies by species (white, pink, purple, salmon). Microscopy: characteristic banana-shaped or canoe-shaped multicelled macroconidia.

Scedosporium (and Lomentospora prolificans)

Septate, acute-angle branching hyphae in tissue - another Aspergillus look-alike.

Clinical:

Near-drowning pneumonia: classic association. Aspiration of polluted water introduces Scedosporium apiospermum (formerly Pseudallescheria boydii).
Most common cause of eumycotic mycetoma in the USA (white grains).
Sinusitis, disseminated disease in immunocompromised.

Colony: “house mouse gray” color. Microscopy: oval conidia (annelloconidia) on annellides, sometimes lollipop-shaped (don’t confuse with Blastomyces mold form).

Treatment: Scedosporium is RESISTANT to amphotericin B. Voriconazole is first line for S. apiospermum.

Lomentospora prolificans (formerly Scedosporium prolificans): a dematiaceous mold causing serious infection in the immunocompromised. Gray/black colonies on both sides of the plate. In tissue: septate with acute-angle branching (mimics Aspergillus). Produces annelloconidia. Pan-resistant - resistant to amphotericin B, echinocandins, and most azoles. Voriconazole + terbinafine may be synergistic. Mortality is very high.

Mucormycetes (Mucormycosis/Zygomycosis)

The Mucormycetes (order Mucorales, formerly “zygomycetes”) cause mucormycosis - a devastating infection in specific clinical contexts that requires urgent recognition because treatment must include surgical debridement. Genera: Rhizopus, Mucor, Lichtheimia (formerly Absidia), Cunninghamella, Rhizomucor, Saksenaea, Apophysomyces. Rhizopus is the most common human pathogen.

The host determines the risk: Mucormycosis occurs almost exclusively in patients with specific predisposing conditions:

Diabetes (especially DKA): the acidotic environment displaces iron from transferrin, increasing free iron available for Mucorales growth. Acidosis also impairs neutrophil function.
Iron overload: Mucormycetes are siderophilic (iron-loving). Hereditary hemochromatosis, chronic transfusions increase risk.
Deferoxamine therapy paradoxically increases risk - the deferoxamine-iron chelate is used BY Mucorales as an iron source. Newer chelators (deferasirox) do not have this problem.
Neutropenia, transplant, chronic steroids, trauma/burns.

Transmission: inhalation of sporangiospores from environment (soil, decaying organic matter, bread, fruit). Cutaneous inoculation via trauma/burns/contaminated dressings. GI acquisition possible from contaminated food.

Rhinocerebral disease - the classic presentation: Sinuses → orbit → brain. Facial pain, nasal congestion, fever. Angioinvasion → thrombosis → infarction → coagulative necrosis. The characteristic finding is a black eschar on the palate or nasal septum - necrotic tissue from vascular invasion. Orbit: proptosis, ophthalmoplegia, vision loss. Brain: cavernous sinus thrombosis, cerebral infarction. Mortality 50-80%. Progression is days, not weeks.

Distinguishing from Aspergillus: Both cause angioinvasive disease in neutropenic patients. Histopathology distinguishes them:

Feature	Mucormycetes	Aspergillus
Septation	Pauciseptate/aseptate	Septate
Width	Broad, 6-25 μm, ribbon-like	Narrow, 3-6 μm
Branching angle	90° (wide/obtuse)	45° (acute)
Hyphal pattern	Twisting, bending, convoluted	Parallel, uniform
Galactomannan	Negative	Positive
β-D-glucan	Negative (chitosan, not glucan)	Positive

A negative BDG and negative galactomannan in a DKA patient with black eschar should increase suspicion, not reassure.

Staining tip: PAS tends to stain Mucorales BETTER than GMS (unusual - for most fungi GMS is more sensitive). Mucorales hyphae have thin fragile walls that may collapse during processing, leading to poor GMS staining. Use both when suspected.

Culture:

Grow fast on SDA (48-72 hours) - so fast the mycelium fills the plate and pushes the lid up (“lid-lifter” colonies).
Forward color: initially white/fluffy, becoming gray/black as sporangia mature with pigmented sporangiospores. Reverse remains light (hyphae are hyaline; pigment is in spores).
Genus identification on wet prep (tape or tease mount):

Genus	Rhizoids	Sporangiophore origin
Rhizopus	Present, prominent	Directly opposite rhizoids (nodal)
Mucor	Absent	Random, from any point
Lichtheimia (Absidia)	Present	Between rhizoid nodes (internode)

Treatment requires surgery: Antifungal therapy alone is inadequate. Aggressive surgical debridement of necrotic tissue is essential - devascularized tissue won’t deliver drug, residual fungus progresses. Liposomal amphotericin B is the antifungal of choice; posaconazole or isavuconazole are alternatives/step-down. Echinocandins are NOT effective (no β-glucan target). Correct underlying conditions: treat DKA, stop deferoxamine, reduce immunosuppression.

Dermatophytes

The dermatophytes are a specialized group of fungi that have evolved to live on keratin - the structural protein of skin, hair, and nails. Three genera cause human disease:

Trichophyton: infects skin, hair, and nails.
Microsporum: skin and hair only.
Epidermophyton: skin and nails only (never hair).

Transmitted by direct contact with infected humans, animals, or fomites (shower floors, combs).

The diseases are named by body site - all called “tinea” (Latin for worm, reflecting the ring-like appearance) followed by the location:

Tinea capitis (scalp): primarily children; can cause scarring alopecia.
Tinea corporis (body): classic “ringworm” - expanding annular plaque with raised scaly border and central clearing.
Tinea cruris (groin, “jock itch”): top causes T. rubrum and Epidermophyton floccosum.
Tinea pedis (feet, “athlete’s foot”): interdigital maceration and scaling.
Tinea unguium / onychomycosis (nails): thickened discolored dystrophic nails.

Trichophyton species:

T. tonsurans: most common tinea capitis in the USA (>95%). Anthropophilic. Endothrix (grows inside hair shaft). Does NOT fluoresce under Wood lamp (only Microsporum fluoresces). “Black dot” tinea capitis - hairs break at scalp leaving black stumps.
T. rubrum: most common dermatophyte worldwide. Causes tinea pedis, corporis, cruris, unguium. Red pigment on BOTH forward AND reverse, especially on PDA. Cannot penetrate hair shafts (negative hair perforation test) - so rare cause of tinea capitis. Urease-negative. Microconidia teardrop-shaped along hyphae (“birds on a wire”). Macroconidia rare (pencil-shaped when present).
T. mentagrophytes: red pigment on REVERSE only (not front). Urease-positive (part of TCTC). Hair perforation-positive. Round microconidia in grape-like clusters.

Microsporum: yellow reverse pigment on agar. Produces large multicelled fusiform (spindle-shaped) macroconidia with thick rough walls. M. canis: bright yellow reverse, >6-celled macroconidia with terminal knob (zoophilic - cats/dogs). M. gypseum: thin-walled macroconidia with ≤6 cells (geophilic - soil). Don’t confuse Microsporum (dermatophyte) with Microsporidium (GI parasite).

Epidermophyton floccosum: olive-green to khaki suede-textured colonies. Only produces macroconidia - NO microconidia (unique among common dermatophytes). Macroconidia are smooth-walled, club-shaped (“beaver tail”), in clusters of 2-3. Infects skin and nails only, never hair.

Diagnosis:

KOH prep of scales/nail debris: dissolves keratin, reveals hyphae and arthroconidia.
Culture on SDA or DTM. DTM turns red if positive (days 7-14) but can have false positives - confirm microscopically.
PDA enhances pigment (especially T. rubrum red reverse).
Wood lamp: only Microsporum fluoresces (greenish). Trichophyton does not.

Treatment:

Tinea corporis, cruris, pedis: topical antifungals (terbinafine, azoles) usually sufficient.
Tinea capitis and onychomycosis: oral therapy (topicals don’t penetrate follicles or nail matrix). Oral terbinafine is first-line; itraconazole alternative. Terbinafine is an allylamine: inhibits squalene epoxidase (earlier in ergosterol synthesis than azoles), causing toxic squalene accumulation + ergosterol deficiency. Concentrates in keratin-containing tissues.
Onychomycosis duration: 3 months for fingernails, 4-6 months for toenails.

47.5 Dimorphic Fungi

The dimorphic fungi share a remarkable adaptation: they exist as molds in the environment and transform into yeasts (or yeast-like forms) at body temperature. Mold in the cold, yeast in the heat explains their pathogenesis - you inhale mold spores from the environment, and they transform into yeast forms that evade host defenses.

Unlike opportunistic fungi (Aspergillus, Candida, Cryptococcus), dimorphic fungi are primary pathogens - they cause disease in immunocompetent people. Severe/disseminated disease is more common in the immunocompromised.

Each dimorphic fungus is endemic to a specific geographic region. Geographic history is essential when considering these diagnoses.

Yeast size comparison in tissue (relative to RBC at ~7-8 μm):

Histoplasma: smaller than RBC (2-4 μm) - “His-to = Tiny”
Blastomyces: slightly larger than RBC (8-15 μm) - “Blasto = Bigger”
Coccidioides: much larger than RBC (spherules 20-80 μm) - “Cocci = Colossal”

Histoplasma capsulatum

Histoplasma is the most common endemic mycosis in the US. Despite the species name, it has no true capsule - the “halo” seen on staining is a cell shrinkage artifact.

Two varieties:

H. capsulatum var. capsulatum: Americas, especially Ohio and Mississippi River valleys (also St. Lawrence River, Central America, Caribbean). Small intracellular yeast 2-4 μm.
H. capsulatum var. duboisii: equatorial Africa. Larger yeast (8-15 μm), more skin/bone involvement, giant cell granulomas.

Ecology and transmission: Histoplasma thrives in soil enriched with nitrogen from bird or bat droppings (birds themselves are NOT infected; body temp too high). Caves with bat guano (“spelunker’s disease”), old chicken coops, and areas under bird-roosting sites are high-risk. Construction workers are at elevated occupational risk from soil disturbance during excavation, demolition, and renovation.

Infection: inhalation of microconidia (2-5 μm). Mold-form cultures are highly infectious (BSL-3).

Pathogenesis: Inhaled spores convert to yeast in the lungs. These small yeasts (2-4 μm) are engulfed by alveolar macrophages but survive intracellularly - facultative intracellular pathogen like M. tuberculosis, requiring Th1/cell-mediated immunity for containment. The yeasts spread via lymphatics to hilar lymph nodes and can disseminate hematogenously via the reticuloendothelial system to liver, spleen, bone marrow, adrenals.

In immunocompetent people, cell-mediated immunity controls the infection within weeks, often leaving granulomas and calcified lymph nodes visible on imaging (the “histoplasmoma”). In AIDS with CD4 <150, disseminated disease occurs.

Clinical syndromes:

Acute pulmonary histoplasmosis: flu-like illness (fever, cough, chest pain, fatigue, myalgia) resolving spontaneously in most immunocompetent patients. Heavy exposure (cleaning chicken coops, caves) can cause ARDS even in healthy people. Granulomatous inflammation with calcifying nodules and hilar lymphadenopathy mimics TB. Can even involve adrenals like TB.
Chronic pulmonary histoplasmosis: in patients with underlying lung disease (COPD, emphysema). Upper lobe cavitary disease with weight loss, cough, hemoptysis - mimics TB.
Fibrosing mediastinitis: Histoplasma is the most common cause in the US. Progressive fibrosis of mediastinal structures (SVC, PA/PV, airways, esophagus). Aberrant immune response to Histoplasma antigens. No effective treatment (antifungals don’t reverse fibrosis).
Disseminated histoplasmosis: in immunocompromised (especially AIDS with CD4 <150) and extremes of age. Fever, weight loss, generalized lymphadenopathy, hepatosplenomegaly, pancytopenia (bone marrow), skin lesions, oral ulcers, and adrenal involvement (hypotension/adrenal insufficiency). LDH often markedly elevated. Mimics miliary TB or lymphoma.

Diagnosis:

Histoplasma antigen (urine and serum): highly sensitive for disseminated disease (>90%) and useful for monitoring. Less sensitive for localized pulmonary (~40-70%). Cross-reacts with Blastomyces, Coccidioides, and Talaromyces (share galactomannan-like antigens).
Histopathology: small intracellular yeasts within macrophages on GMS/PAS/Wright-Giemsa. Classic look: 2-4 μm oval yeasts packed in macrophage cytoplasm with a peripheral clear halo (artifact, not a true capsule).
Mold-form culture: tuberculate macroconidia (large round thick-walled conidia with finger-like projections/knobs) + smooth microconidia. BSL-3.
Serology: CF titer, ID bands (H and M precipitins).

Histo look-alikes on skin biopsy: Leishmania (Middle East/Latin America travel), Talaromyces marneffei (HIV + Southeast Asia). Geography + history distinguishes.

Histoplasma capsulatum: Small (2-4 μm) intracellular yeasts within macrophages. Endemic to Ohio/Mississippi River valleys.

Treatment: Itraconazole for mild-to-moderate disease. Amphotericin B for severe or disseminated disease, followed by itraconazole for maintenance (often lifelong in AIDS until CD4 recovers).

Blastomyces dermatitidis

Blastomycosis is endemic to central and eastern USA - overlap with Histoplasma (Ohio/Mississippi River valleys) plus Great Lakes region, St. Lawrence River, and parts of Central Canada. Also parts of Africa and India. The ecologic niche: moist soil near waterways (rivers, lakes), decaying organic matter. Outbreaks linked to riverbank activities, construction near waterways, beaver dams. Less clearly associated with bird/bat guano than Histoplasma. Acquired by inhalation of conidia from contaminated soil. Dog blastomycosis is often a sentinel for human exposure.

Clinical syndromes: Unlike Histoplasma, Blastomyces commonly causes symptomatic disease even in immunocompetent hosts.

Pulmonary blastomycosis: pneumonia (fever, cough, chest pain). CXR can show masses mimicking lung cancer, miliary pattern, or infiltrates mimicking TB or bacterial pneumonia. Diagnosis often delayed.
Cutaneous blastomycosis: distinctive verrucous (warty) skin lesions with raised irregular borders. Can mimic squamous cell carcinoma (pseudoepitheliomatous hyperplasia of overlying skin). Skin is the most common extrapulmonary site (often represents dissemination but can occur independently).
Osteoarticular: osteomyelitis (vertebrae, ribs, long bones), septic arthritis.
Genitourinary: prostatitis, epididymo-orchitis.
CNS: meningitis, brain abscess (in immunocompromised).

Risk factors for disseminated disease include immunosuppression (HIV, transplant, anti-TNF therapy).

The characteristic yeast (diagnostic on direct examination or histopathology):

Large (8-15 μm), slightly larger than RBC.
Thick, refractile, “double-contoured” cell wall.
Broad-based budding (wide isthmus between parent and daughter - “figure-8” appearance).
Memory aid: Blasto = Broad/Big base; Crypto = narrow/Constricted.

Histology: pyogranulomatous inflammation - mixed microabscesses (neutrophils) + granulomas (epithelioid cells, giant cells). Large thick-walled yeasts with broad-based budding on GMS/PAS (often visible on H&E too because of the thick wall).

Mold form (25°C): white to tan cottony colonies. Microscopy: thin hyphae with small round/pyriform conidia on short lateral conidiophores (“lollipop” or “teardrop” shape). Not specific - resembles Chrysosporium and other saprophytes; confirm by yeast conversion at 37°C, DNA probe, or MALDI-TOF. Mold cultures are BSL-3.

Blastomyces: Large (8-15 μm) yeast with thick “double-contoured” cell wall and broad-based budding. Pathognomonic “figure-8” appearance.

Diagnosis: The large broad-based budding yeast is so distinctive that direct examination of clinical specimens (sputum, pus, wet mount) can provide rapid presumptive diagnosis. Antigen testing (urine) available but cross-reacts with Histoplasma. Culture confirms. BDG is negative (α-1,3-glucan, not β-glucan, in cell wall).

Treatment: Itraconazole for mild-moderate; amphotericin B followed by itraconazole for severe/disseminated.

Coccidioides immitis and C. posadasii

Coccidioidomycosis - Valley fever - is endemic to the desert Southwest (Arizona, California’s San Joaquin Valley, Nevada, Utah, New Mexico, west Texas) and northern Mexico/Central America. Two species:

C. immitis: San Joaquin Valley/southern California.
C. posadasii: rest of Southwest US, Mexico, Central/South America.

Both cause identical disease. The fungus lives in desert soil and is dispersed by wind, dust storms, and earthquakes. Potential bioterrorism concern - a single arthroconidium can cause infection; highly infectious, environmentally stable, mass exposure possible. [TODO: verify current select agent / CDC bioterrorism category status; Coccidioides was removed from the HHS select agents list in 2012.]

Transmission: inhalation of arthroconidia (2-5 μm barrel-shaped spores from fragmentation of hyphae). BSL-3 required in the lab - Coccidioides is among the most common causes of laboratory-acquired fungal infection.

A unique tissue form: Unlike other dimorphic fungi, Coccidioides doesn’t form budding yeast in tissue. It forms spherules (20-80 μm) - large round structures filled with endospores (2-5 μm each). Mature spherules rupture and release endospores, each developing into new spherules. Pathognomonic on histopathology. Culture yields only the mold form; tissue form doesn’t reliably grow in standard conditions.

Clinical spectrum:

~60% asymptomatic or mild flu-like illness.
Acute pulmonary coccidioidomycosis: pneumonia often accompanied by erythema nodosum (painful red nodules on shins) and arthralgias (“desert rheumatism”). Pneumonia + erythema nodosum + arthralgias = Valley fever (essentially diagnostic in endemic areas). Erythema nodosum and erythema multiforme are immune-mediated, not skin infection. CXR: infiltrates, hilar adenopathy, pleural effusion, thin-walled cavities.
Chronic pulmonary: cavitary lung disease mimicking TB.
Disseminated disease: ~1% overall, but higher in specific populations. Sites: skin (papules, verrucous lesions, sinuses), meninges (chronic basilar meningitis - requires lifelong fluconazole), bones/joints (osteomyelitis, septic arthritis). Higher dissemination risk: patients with African or Filipino ancestry, pregnant women (especially 3rd trimester), immunocompromised (HIV, transplant, anti-TNF).

Histology: granulomatous reaction (epithelioid cells, Langhans giant cells, lymphocytes) around spherules. Ruptured spherules releasing endospores elicit mixed suppurative + granulomatous response. Caseous necrosis can occur (TB mimic).

Mold form: barrel-shaped arthroconidia alternating with empty disjunctor cells. When hyphae fragment, arthroconidia are released as highly infectious airborne particles.

Diagnosis:

Serology is the primary diagnostic tool and is reliable even with a single elevated titer (more reliable for Coccidioides than other dimorphic fungi). IgM (TP precipitin) early; IgG (CF) later and correlates with disease burden. CF titer >1:32 suggests dissemination. Falling titers = improvement.
Histopathology: spherules with endospores = diagnostic.
Culture (BSL-3 mandatory).
Skin test (coccidioidin, spherulin) rarely used; negative in active disease (anergy) is a poor prognostic sign.

Treatment: Many patients with primary pulmonary disease don’t require treatment. Fluconazole is preferred for symptomatic disease. Amphotericin B for severe disease. Coccidioidal meningitis requires lifelong fluconazole - not curable, only suppressible.

Paracoccidioides brasiliensis

Endemic to South America (Brazil, Colombia, Venezuela, Argentina). Also called South American blastomycosis (not “coccidioidomycosis” - counterintuitive) because of clinical similarity to blastomycosis. The most common systemic mycosis in Latin America.

Acquired primarily by inhalation but can also be traumatic inoculation. Heavy male predominance (~10:1) - 17β-estradiol inhibits mold-to-yeast conversion, protecting women.

Characteristic morphology in tissue: large yeast (10-30 μm) with multiple peripheral buds arranged around the mother cell - the mariner’s wheel (ship’s wheel/captain’s wheel) appearance. Pathognomonic.

Clinical syndromes:

Acute/subacute (juvenile) form: disseminated disease with lymphadenopathy, hepatosplenomegaly; worse prognosis.
Chronic (adult) form (most common): progressive pulmonary disease + mulberry-like oral/nasal ulcers (stomatitis) + lymphadenopathy. Adrenal involvement common (like Histoplasma).

Treatment: Itraconazole for mild-moderate; amphotericin B for severe.

Sporothrix schenckii

Sporotrichosis is different from the other endemic mycoses - it’s acquired by traumatic inoculation, not inhalation. The organism lives on plants (especially sphagnum moss, rose thorns, hay) and in soil worldwide (not geographically restricted).

The classic presentation: a gardener or florist pricks their finger on a rose thorn. Days to weeks later, a papule appears at the inoculation site. New nodules appear along the draining lymphatic channels (lymphocutaneous / sporotrichoid spread). This ascending chain of nodules following lymphatics is the classic pattern.

Zoonotic transmission from cats (especially in Brazil) increasingly recognized.

Differential for lymphocutaneous spread: Sporothrix, Nocardia, atypical mycobacteria (M. marinum), Leishmania, tularemia.

Less common forms:

Pulmonary sporotrichosis: inhalation, typically in alcoholic men with COPD - chronic cavitary pneumonia mimicking TB.
Disseminated: widespread skin lesions, osteoarticular involvement, meningitis; in immunocompromised (HIV, alcoholism, anti-TNF, transplant).

Histology: pyogranulomatous inflammation (central suppuration surrounded by granulomatous reaction with giant cells) - three-zone pattern: central neutrophils, epithelioid middle, outer lymphocytes.

Organisms are characteristically sparse and difficult to find in tissue (unlike Blastomyces or Coccidioides). The Splendore-Hoeppli phenomenon (asteroid body) - radiating eosinophilic material around a central organism (antigen-antibody complexes and host debris forming a sunburst pattern) - is a helpful clue when organisms are sparse. Not specific to sporotrichosis but suggestive in this clinical setting.

Morphology:

Tissue yeast: 4-6 μm cigar-shaped yeasts (elongated oval, sausage-like).
Mold form (25°C): thin hyphae with small oval conidia arranged in a rosette at the tip of the conidiophore (“daisy” or “flower” pattern). Colonies: initially white, becoming dark brown/black with age.

Treatment: Itraconazole is effective for cutaneous and lymphocutaneous disease. Amphotericin B for severe or disseminated cases.

Talaromyces marneffei (formerly Penicillium marneffei)

Board pearl: HIV + Southeast Asia + umbilicated skin lesions = Talaromyces marneffei.

Third most common opportunistic infection in AIDS in Southeast Asia (Thailand, Vietnam, southern China, northeast India), after TB and Cryptococcus. Reservoir: bamboo rats. Acquired by inhalation.

Clinical: Mimics disseminated histoplasmosis. Fever, weight loss, umbilicated skin papules with central necrosis, lymphadenopathy, hepatosplenomegaly.

Mold form (25°C): diffusible red pigment into surrounding agar is classic and distinctive. Penicillium-like brush (penicillus) conidiophore structure - branched conidiophores with chains of conidia resembling “skeleton hands” or a paintbrush.

Yeast form (37°C/tissue): oval yeasts (2-5 μm) within macrophages with a distinctive central septum (cross-wall). This is the key distinguishing feature from Histoplasma on histology - Histoplasma has no septum. Also differs from Histoplasma in shape (sausage-shaped vs. round) and geography (Southeast Asia vs. Americas/Africa).

Histo mimics: also resembles Leishmania. Geography + history + the central septum distinguish.

47.6 Other Mycoses

Chromoblastomycosis

Chronic subcutaneous infection caused by dematiaceous fungi (Fonsecaea pedrosoi most common; Cladophialophora carrionii, Phialophora verrucosa) acquired by traumatic inoculation from soil/plant material. Tropical/subtropical distribution (Central/South America, Africa, Asia). Barefoot agricultural workers.

Presentation: slowly progressive verrucous (warty) cauliflower-like plaques, typically on the lower extremities. Develops over months to years. Does not disseminate systemically.

Pathognomonic: sclerotic bodies (Medlar bodies, “copper pennies”) - round, thick-walled, brown, muriform (cross-septate in two planes) cells 5-12 μm in the dermis within granulomatous/suppurative inflammation. Visible on H&E without special stains (brown melanin pigment).

Phaeohyphomycosis

Any infection caused by dematiaceous molds with pigmented hyphae in tissue - but WITHOUT sclerotic bodies (distinguishes from chromoblastomycosis) or grains (distinguishes from mycetoma).

Causes: Exophiala, Alternaria, Bipolaris, Cladophialophora, Curvularia, many others.

Clinical forms:

Subcutaneous (cyst, abscess at inoculation site).
Allergic fungal sinusitis (most common form; Bipolaris, Curvularia, Alternaria).
Cerebral phaeohyphomycosis: Cladophialophora bantiana is the most common cause of fungal brain abscess by a dematiaceous mold (neurotropic). Can occur in immunocompetent patients. High mortality. Treatment: surgical excision + voriconazole.
Disseminated (in immunocompromised).

Bipolaris: causes allergic fungal sinusitis and occasional subcutaneous/disseminated disease. Classic microscopic feature: bipolar germination - germ tubes emerging from both ends of elongated multicelled dark brown conidia on wet mount after 12-24h incubation. Hence the name.

Stain: Fontana-Masson confirms melanin.

Treatment: surgical excision + antifungals (itraconazole, voriconazole).

Eumycotic Mycetoma

Chronic subcutaneous infection presenting with the triad of tumefaction (swelling) + draining sinuses + grains (fungal microcolonies in discharge). Classic site: foot (“Madura foot”).

Two types of mycetoma:

Eumycotic (fungal): grains are fungal microcolonies.
Actinomycotic (bacterial - Nocardia, Actinomyces, Streptomyces): grains are bacterial microcolonies.

Grain color tells you the organism:

White grains = hyaline mold: Scedosporium, Acremonium, Fusarium, Aspergillus nidulans.
Black grains = dematiaceous mold: Madurella mycetomatis (most common worldwide), Madurella grisea, Exophiala.

Scedosporium is the most common cause of eumycotic mycetoma in the USA.

Treatment: prolonged antifungals + surgery (may require amputation in advanced disease).

Rhinosporidium seeberi

Causes rhinosporidiosis - polypoid mucosal mass, usually nasal (also conjunctival), in young men. Endemic in South Asia (India, Sri Lanka), acquired from stagnant fresh water (swimming, bathing in ponds/lakes). Natural reservoir: fish and aquatic insects.

Classification note: NOT a true fungus - reclassified as an aquatic protist (Mesomycetozoa, related to fish parasites). Still studied in mycology because of its tissue appearance. Cannot be cultured in vitro.

Tissue: spherules (sporangia) with endospores, morphologically similar to Coccidioides but MUCH LARGER (sporangia 100-300 μm vs. 20-80 μm for Coccidioides; endospores 6-10 μm). Clinical context (nasal polyp in a patient with residence in or exposure to endemic areas of South Asia) separates from Coccidioides (pulmonary disease in Southwest US).

Treatment: surgical excision is definitive (no effective medical therapy - antifungals and antibiotics don’t work because it’s neither fungus nor bacterium). Cauterize base to reduce recurrence. Dapsone tried adjunctively with variable results.

Lacazia loboi (Lobomycosis / Keloidal Blastomycosis)

Endemic in South/Central America (Amazon basin). Also causes natural infection in dolphins (bottlenose dolphins, coastal Florida and tropical waters) - a unique zoonotic link.

Acquired by traumatic inoculation. Chronic subcutaneous infection producing hard, painless, keloid-like nodules on the face and ears over years to decades. Does NOT disseminate systemically.

Histology: abundant thick-walled yeast cells (6-12 μm) connected in chains (“chains of sausages,” “string of pearls”) within the dermis - pathognomonic. Cannot be cultured.

Treatment: wide surgical excision.

Prototheca

Not a fungus - an achlorophyllous (colorless) green algae, the only algae known to cause human disease. P. wickerhamii is most common. Studied in mycology because it resembles fungi on culture and stains. Found worldwide in fresh/marine water, soil, sewage.

Transmission: direct inoculation through skin breaks (trauma, surgery, catheters).

Clinical forms:

Cutaneous/subcutaneous (most common): nodules, plaques, ulcers at inoculation site.
Olecranon bursitis: classic presentation - chronic non-healing elbow bursitis after trauma with exposure to contaminated water/soil. Necrotizing granulomas in the bursa wall.
Disseminated (immunocompromised): widespread skin lesions, fungemia.

Histology: morula-like sporangia with internal septations dividing into triangular endospores - “soccer ball” or “wagon wheel” pattern on GMS/PAS. Pathognomonic.

Culture: white yeast-like colonies on SDA (can be mistaken for Candida), but no pseudohyphae or germ tubes. Growth is inhibited by cycloheximide but not by chloramphenicol/gentamicin - so Prototheca will NOT grow on dermatophyte test medium (which contains cycloheximide).

Treatment: amphotericin B + surgical excision; reduce immunosuppression.

47.7 Antifungal Therapy

Four main classes target fungal-specific structures (ergosterol, β-glucan synthesis, nucleic acid synthesis). Because fungi are eukaryotes, selectivity is narrower than with antibacterials.

Polyenes

Amphotericin B

Mechanism: binds ergosterol in the fungal cell membrane, creating pores that disrupt membrane integrity → leakage and cell death. Selectivity comes from fungal ergosterol vs. mammalian cholesterol (lower affinity for cholesterol explains toxicity).
Activity: broad spectrum against most fungi. EXCEPTIONS: A. terreus, Scedosporium spp., C. lusitaniae (all intrinsically resistant). Trichosporon and Fusarium have variable/reduced susceptibility.
Side effects: nephrotoxicity (renal tubular acidosis, hypokalemia, hypomagnesemia), infusion reactions (fever, rigors), anemia. Liposomal formulations reduce nephrotoxicity and are preferred for most invasive infections.

Nystatin

Polyene (same mechanism as amphotericin) but too toxic for systemic use.
Topical only: oral suspension (swish/swallow for thrush), troches, vaginal tablets, cream.
Not absorbed from the GI tract.
Primary use: oral candidiasis (especially in infants); mild cases in adults.

Azoles

Mechanism: inhibit lanosterol 14-α-demethylase (CYP51), blocking ergosterol synthesis. Membrane destabilizes.
Fungistatic against most yeasts; fungicidal against some molds.
Two classes: imidazoles (ketoconazole, miconazole - older, more toxic) and triazoles (fluconazole, itraconazole, voriconazole, posaconazole, isavuconazole).
Major drug interactions: all are CYP450 inhibitors.

Azole	Key uses	Notes
Fluconazole	Candida (susceptible spp.), Cryptococcus consolidation/maintenance, coccidioidomycosis	No activity against Aspergillus. C. krusei intrinsically resistant, C. glabrata dose-dependent
Itraconazole	First-line azole for dimorphic fungi (Histoplasma, Blastomyces, Sporothrix, Paracoccidioides); ABPA adjunct	Capsule absorption requires acidic pH; solution does not. Target trough >1 μg/mL
Voriconazole	First-line for invasive aspergillosis; Scedosporium apiospermum; Fusarium	TDM target 1-5.5 μg/mL. Side effects: visual disturbances, hepatotoxicity, skin photosensitivity → skin cancer risk
Posaconazole	Mucorales (salvage after amphotericin); dematiaceous molds; prophylaxis in neutropenic/HSCT	Broad spectrum. TDM trough >1 μg/mL (prophylaxis), >1.25 (treatment)
Isavuconazole	Invasive aspergillosis; mucormycosis	Alternative to posaconazole with fewer drug interactions

Candida resistance patterns relevant to azole choice:

C. krusei: intrinsic fluconazole resistance → echinocandin or voriconazole.
C. glabrata: dose-dependent fluconazole susceptibility; often better treated with echinocandins. Emerging echinocandin resistance via FKS gene mutations.
C. dubliniensis: may develop fluconazole resistance with repeated exposure.

Echinocandins

Caspofungin, micafungin, anidulafungin.
Mechanism: inhibit 1,3-β-D-glucan synthase → weakened cell wall → cell lysis.
Fungicidal against Candida; fungistatic against Aspergillus.
NOT effective against: Mucorales (no β-glucan target), Cryptococcus, dimorphic fungi, Trichosporon, Fusarium. Also not Malassezia.
IV only (not orally absorbed).
First-line for candidemia in critically ill patients and azole-resistant Candida.
Resistance: FKS gene mutations (1,3-β-D-glucan synthase gene) - emerging especially in C. glabrata.

Flucytosine (5-FC)

Oral antifungal. Converted to 5-fluorouracil (5-FU) inside fungal cells by cytosine deaminase (human cells lack this enzyme → selectivity).
5-FU inhibits fungal DNA and RNA synthesis.
Primary use: combination with amphotericin B for cryptococcal meningitis (synergistic). Also severe Candida.
Never use as monotherapy - resistance develops rapidly.
Toxicity: bone marrow suppression (dose-related - monitor levels and CBC).

Allylamines - Terbinafine

Mechanism: inhibits squalene epoxidase (earlier in ergosterol synthesis than azoles). Causes toxic squalene accumulation + ergosterol deficiency.
First-line systemic treatment for onychomycosis (better cure rates than azoles for nails).
Also tinea capitis, refractory tinea corporis.
Concentrates in keratin-containing tissues (nails, skin, hair).
Generally well-tolerated; rare hepatotoxicity (monitor LFTs).

Treatment quick-reference

Organism	First-line
Candidemia (critically ill)	Echinocandin
Mucosal candidiasis	Fluconazole (topical/oral)
Invasive aspergillosis	Voriconazole
A. terreus	Voriconazole (never amphotericin)
Mucormycosis	Liposomal amphotericin B + surgical debridement
Cryptococcal meningitis induction	Amphotericin B + flucytosine
Cryptococcal meningitis consolidation/maintenance	Fluconazole
Histoplasmosis/Blastomycosis (mild-mod)	Itraconazole
Histoplasmosis/Blastomycosis (severe)	Amphotericin B → itraconazole
Coccidioidomycosis (symptomatic)	Fluconazole
Coccidioidal meningitis	Lifelong fluconazole
Sporotrichosis (lymphocutaneous)	Itraconazole
Scedosporium apiospermum	Voriconazole (not amphotericin)
Lomentospora prolificans	Voriconazole + terbinafine (pan-resistant)
Onychomycosis (systemic)	Terbinafine
PJP	TMP-SMX + steroids if severe
ABPA	Corticosteroids; itraconazole adjunct

Chapter 48: Parasitology

Parasitology covers protozoa (single-celled eukaryotes) and helminths (worms). These organisms have complex life cycles that explain their epidemiology, clinical presentations, and diagnostic approaches. The lab side matters as much as the clinical side: the right specimen, the right stain, and the right timing determine whether you catch the parasite at all.

48.1 Laboratory Methods

Before the organism chapters, a quick tour of the workhorse methods. Parasitology still leans heavily on morphology. Some of these methods feel old-school, but they remain the gold standard on boards.

Stool Ova and Parasites (O&P)

Stool O&P was historically the test of choice for identifying intestinal parasites, and though multiplex PCR (BioFire GI, etc.) is supplanting it in many labs, O&P still matters for organisms not on PCR panels and for morphologic confirmation.

The rules that show up on the exam:

3 specimens collected at least 24 hours apart to exclude infection (parasites shed intermittently; one specimen misses ~40% of infections).
Examine within 1 hour of collection, or fix in preservative. Preservative options: PVA (polyvinyl alcohol) for permanent staining, 10% formalin for concentration methods and wet preps, or SAF (sodium acetate-acetic acid-formalin). Trophozoites die fast at room temperature without fixative.
Two methods of analysis per specimen: a wet preparation (fresh or concentrated stool for motile trophozoites, cysts, eggs, larvae; iodine adds detail) and a permanent preparation (trichrome or iron-hematoxylin for nuclear morphology).

The coccidia staining trap: Cryptosporidium, Cyclospora, and Cystoisospora are easily missed on routine trichrome. They require a modified acid-fast stain (or modified safranin). Organisms appear pink/red against a blue-green background. Board tip: “acid-fast organism in stool” means coccidia, not mycobacteria.

Microsporidia (now classified as fungi) need yet another stain - modified trichrome (Weber stain), where spores appear pink with a belt-like stripe, or calcofluor white fluorescence. EM remains the gold standard for microsporidia.

Specialty Specimens

Not every parasite hides in stool. The right specimen matters:

Duodenal contents - for Giardia and Strongyloides when stool O&P is repeatedly negative. Either endoscopic aspirate/biopsy or the Entero-Test (string test): the patient swallows a gelatin capsule on a string, the capsule goes to the duodenum, and the retrieved mucus on the string is examined for trophozoites or larvae.
Perianal tape test - for Enterobius vermicularis (pinworm). Females deposit eggs perianally at night, not in the intestinal lumen, so routine stool O&P misses them. Press clear tape to the perianal skin in the morning before bathing or defecation, then mount on a slide. Three consecutive mornings for maximum sensitivity.
Blood smears (Giemsa-stained, thick and thin) - for Plasmodium and Babesia (intraerythrocytic), and extracellularly for microfilariae: Mansonella, Wuchereria bancrofti, Loa loa, Brugia spp. Onchocerca microfilariae live in skin, not blood, and need skin snips. Timing matters for microfilariae: Wuchereria/Brugia show nocturnal periodicity (collect at night); Loa loa shows diurnal periodicity (collect during day).
Respiratory (sputum) - for parasites that pass through or reside in lung: Paragonimus westermani (eggs), Strongyloides (larvae during hyperinfection), Ascaris (larvae during pulmonary migration), hookworm (larvae during migration). Rarely Echinococcus if a cyst ruptures into the airway.
Skin snips - for Onchocerca volvulus microfilariae. Bloodless dermal shavings incubated in saline for hours let microfilariae emerge. Multiple snips from different body sites (iliac crest, scapula, calf) boost yield.

Culture and Molecular

Parasite culture is mostly impractical, but there’s one useful exception: free-living amebae (Naegleria, Acanthamoeba, Balamuthia) grow on non-nutrient agar seeded with an E. coli lawn. The amebae feed on the bacteria and create visible feeding tracks as they migrate. Incubation at 37-42°C favors Naegleria (thermotolerant); Acanthamoeba grows at 25-30°C.

PCR, especially as multiplex panels, is replacing microscopy for GI parasites. The modern diagnostic hierarchy for GI parasites (best to worst): PCR > antigen EIA > stool O&P > serology. Serology is useful for Entamoeba liver abscess and a handful of other tissue invaders, but not for routine acute GI infections.

48.2 Protozoa

Blood and Tissue Protozoa

Plasmodium (Malaria) - the global killer

Malaria kills hundreds of thousands of people annually, primarily children in sub-Saharan Africa. Understanding the life cycle explains everything - the clinical features, the species differences, and why treatment is complex.

The life cycle drives the disease: An infected female Anopheles mosquito injects sporozoites during a blood meal. Sporozoites travel to the liver, invade hepatocytes, and multiply (exoerythrocytic schizogony). After 1-2 weeks, the liver cells rupture, releasing thousands of merozoites into the bloodstream. Merozoites invade red blood cells, multiply (erythrocytic schizogony), rupture the cells, and invade new RBCs. This cycle repeats every 48 hours (P. falciparum, P. vivax, P. ovale) or 72 hours (P. malariae). Non-mosquito routes exist but are rare: congenital transmission, blood transfusion, needle sharing.

The synchronized rupture of infected RBCs causes the classic cyclic fever. Fever cycles by species: P. vivax/ovale - 48h (tertian), P. malariae - 72h (quartan), P. falciparum - irregular or ~48h. In practice, P. falciparum periodicity is usually obscured.

Species infection windows: Each species is picky about which RBCs it infects, and this dictates how high parasitemia can rise.

P. falciparum - infects RBCs of any age (no restriction), so parasitemia can exceed 20%.
P. vivax and P. ovale - reticulocytes only (young RBCs), so parasitemia rarely exceeds 2%.
P. malariae - senescent (old) RBCs only, so parasitemia is very low.

Timeline: First week - asymptomatic (hepatic schizogony). After the first week - vague symptoms (headache, myalgia, malaise). After several weeks - classic cyclical fevers with paroxysms of chills, fever, and sweats coinciding with synchronous RBC lysis. Incubation varies: P. falciparum 7-14 days, P. vivax/ovale 12-18 days (or months to years via hypnozoites), P. malariae 18-40 days.

P. vivax and P. ovale have a twist: Some liver-stage parasites become dormant (hypnozoites) rather than immediately replicating. These can reactivate months to years later, causing relapses. Primaquine is the only drug that kills hypnozoites - it’s required for “radical cure” of P. vivax and P. ovale infections. Critically, primaquine causes hemolysis in G6PD-deficient patients, so G6PD testing is mandatory before primaquine. Tafenoquine is an alternative single-dose hypnozoite killer but still has the G6PD caveat.

Don’t confuse relapse with recrudescence. Relapse (only P. vivax and P. ovale) is reactivation of liver-stage hypnozoites. Recrudescence (any species, especially P. malariae) is the return of blood-stage parasitemia that was never fully cleared (inadequate treatment or resistance).

P. malariae - the slow burner: Associated with nephrotic syndrome from immune complex-mediated membranoproliferative glomerulonephritis. P. malariae can persist at extremely low parasitemia for decades and recrudesce years later.

Mixed infections: In ~5% of cases, patients are infected with more than one Plasmodium species. Most common combo: P. falciparum + P. vivax. If a P. vivax/ovale component exists, you still need primaquine after treating the P. falciparum.

P. knowlesi: Southeast Asia (Malaysia, Borneo). Natural host is macaque monkeys. 24-hour erythrocytic cycle (shortest of any human Plasmodium) means parasitemia climbs fast and can mimic P. falciparum. On smear it looks like P. malariae (band forms). PCR is needed to confirm.

P. falciparum - why it kills: P. falciparum is the most dangerous species for several reasons:

It can infect RBCs of any age (other species are restricted to young or old RBCs), leading to higher parasitemia. Infected RBCs express surface proteins (PfEMP1) that bind to endothelial receptors (CD36, ICAM-1), causing sequestration in microvasculature. This drives cerebral malaria (coma, seizures), ARDS, and renal failure. Sequestration also means peripheral parasitemia underestimates total body parasite burden - you may see only ring stages on smear because mature forms are sequestered. If you do see schizonts of P. falciparum on peripheral smear, that signals overwhelming parasitemia and poor prognosis.

Blackwater fever - classic P. falciparum complication: severe intravascular hemolysis causing fever + renal failure + hemoglobinuria (dark/black urine). Historically also linked to quinine in G6PD-deficient patients. The massive free hemoglobin filtered by kidneys can cause acute tubular necrosis.

Vulnerability: In endemic areas, young children and pregnant women bear the highest mortality risk. Adults in endemic areas develop partial immunity through repeated exposure (premunition). Outside endemic areas, everyone is equally susceptible - no one has acquired immunity.

Host genetics shape risk. Board-testable protective factors:

HbS (sickle) - protects against P. falciparum only (parasitized RBCs sickle and get cleared; parasite growth impaired in HbS RBCs).
HbC, HbE, α- and β-thalassemia, HPFH (persistent HbF), G6PD deficiency - protect against all Plasmodium species. These balanced polymorphisms are concentrated in historically endemic regions.
Duffy negative [Fy(a-b-)] - protects against P. vivax (Duffy is the invasion receptor). Explains why P. vivax is rare in West Africa and P. ovale fills that niche.
Hereditary (Southeast Asian) ovalocytosis - protects against cerebral malaria specifically (rigid RBCs resist cytoadherence).

Microscopic diagnosis - the gold standard: Thick and thin blood smears examined by an experienced microscopist remain the diagnostic standard. Giemsa stain at pH 7.2 is the reference stain (proper pH is critical for visualizing Schüffner’s dots).

Thick smear for screening - RBCs are lysed, concentrating parasites. Use it to detect infection.
Thin smear for species identification and parasitemia quantification - RBCs preserved intact.

Sensitivity and timing:

Thick and thin films can detect ~5 parasites/µL if you examine ~200 oil immersion fields (100 on thick, 300 on thin) - roughly 20-30 minutes of careful microscopy.
A single smear is not enough to exclude malaria. 3 smears collected 12-24 hours apart over 48-72 hours are needed to reasonably exclude infection.
Ideal timing: just before or during a fever spike (peak parasitemia from schizont rupture) - but never delay if malaria is clinically suspected.
Parasitemia is calculated on the thin smear. Parasitemia >2% signals severe disease and correlates with poor outcomes. P. falciparum can exceed 20%.
After starting treatment, parasitemia transiently increases as sequestered schizonts release merozoites. This is expected and is not treatment failure. True failure = no decline after 48-72 hours.

Species-specific smear features:

Species	Infected RBC	Trophozoite	Gametocyte	Schizont/Other
P. falciparum	Normal size	Thin delicate rings, multiple rings per RBC, appliqué (accolé) forms, Maurer’s clefts	Banana/crescent-shaped (pathognomonic)	Schizonts rarely seen (sequestered)
P. vivax	Enlarged	Ameboid, large	Round	12-24 merozoites, Schüffner’s dots in RBC
P. ovale	Enlarged, oval, fimbriated (ragged) edges	Compact	Round	6-12 merozoites, Schüffner’s dots
P. malariae	Normal or smaller	“Bird’s eye” ring, band form across RBC	Large oval filling RBC	Rosette/daisy-head schizont (6-12 merozoites around central pigment)

Rapid antigen tests (RDTs) are performed on whole blood and distinguish P. falciparum from non-falciparum infection (not species-level for non-falciparum). Targets: HRP-2 (histidine-rich protein 2) for P. falciparum only, and aldolase or pLDH as pan-Plasmodium markers. Pitfalls: HRP-2 persists for weeks after treatment (false positives), and some African P. falciparum strains have HRP-2 gene deletions (false negatives).

P. falciparum: Multiple ring forms per RBC, “appliqué” forms at cell edge, banana-shaped gametocytes. Normal-sized infected RBCs.

P. vivax/ovale: Enlarged infected RBCs with Schüffner’s dots, large ameboid trophozoites, all stages in peripheral blood.

Rapid diagnostic tests (RDTs) detecting parasite antigens and PCR are useful adjuncts, but microscopy remains essential for quantifying parasitemia and monitoring treatment response.

Treatment principles: Artemisinin-combination therapy (ACT) is the global standard for uncomplicated malaria - artemisinins rapidly clear parasites, and the partner drug prevents resistance. Severe malaria requires IV artesunate, which is superior to quinine. Chloroquine-sensitive areas are increasingly rare. Remember primaquine for radical cure of P. vivax/ovale (after confirming G6PD status).

Babesia - malaria’s tick-borne mimic

Babesia microti is transmitted by Ixodes scapularis, the same tick that transmits Lyme disease and anaplasmosis. Endemic in the northeastern US (Connecticut, Massachusetts, New York, Rhode Island) and upper Midwest (Wisconsin, Minnesota). Like malaria, Babesia infects red blood cells.

The white-footed mouse trinity: The white-footed mouse (Peromyscus leucopus) is the reservoir for three Ixodes-transmitted infections - Anaplasma phagocytophilum, Borrelia burgdorferi, and Babesia microti. Coinfection is common in endemic areas. Ehrlichia chaffeensis uses a different reservoir (white-tailed deer).

Clinical features and who’s at risk: In healthy people, babesiosis may be asymptomatic or cause a mild flu-like illness with fever and hemolytic anemia. In asplenic, immunocompromised, or elderly patients, babesiosis can be severe or fatal - massive hemolysis, high-grade parasitemia, DIC, and multi-organ failure. The spleen is critical for clearing infected RBCs; without it, parasitemia can exceed 10-20% and spiral out of control.

Distinguishing from malaria microscopically: Babesia also appears as intraerythrocytic ring forms, and returning travelers (malaria and babesiosis are both tick/mosquito endemic) require careful differentiation.

Key distinctions:

Babesia rings are smaller and more pleomorphic (1-2.5 µm) than P. falciparum rings.
No hemozoin pigment in Babesia (malaria parasites produce brown-black pigment from hemoglobin digestion).
The Maltese cross tetrad - four merozoites arranged in a cross pattern in a single RBC - is pathognomonic for Babesia but present in only ~20% of cases.
Extracellular parasites occur in Babesia (rare in malaria).
No schizonts or gametocytes on peripheral smear.

Diagnosis: PCR (most sensitive, can speciate) and Giemsa-stained thin/thick blood smears. Serology is also available but is tricky in endemic areas because past exposure is common - cutoff for active infection is a very high titer ≥1:1024. Titers ≤1:64 usually mean past infection only.

Babesia on peripheral smear: Ring forms and tetrads (Maltese cross) within RBCs. Unlike malaria, no hemozoin pigment.

Treatment: Atovaquone plus azithromycin for mild-moderate disease. Quinine plus clindamycin for severe disease. Exchange transfusion may be needed for high parasitemia (>10%).

Toxoplasma gondii

Toxoplasma is remarkable for causing three completely different clinical syndromes depending on the host’s immune status and the timing of infection.

The cat connection: Cats are the definitive host - Toxoplasma sexually reproduces only in the feline intestine, releasing oocysts in cat feces. Humans are intermediate hosts. Routes of infection:

Ingestion of undercooked meat (tissue cysts with bradyzoites in pork, lamb, venison) - the most common route in developed countries.
Ingestion of oocysts from cat feces (cat litter, soil, unwashed produce).
Transplacental/congenital transmission.
Organ transplant, blood transfusion, lab accident (rare).

The “T” in the TORCH infections stands for Toxoplasmosis. Indoor cats fed commercial food rarely shed oocysts (they acquire it by hunting).

Three faces of toxoplasmosis:

In immunocompetent adults, primary infection is usually asymptomatic or causes mononucleosis-like illness with posterior cervical lymphadenopathy, mild fever, and atypical lymphocytes. Lymph node biopsy shows a characteristic triad: follicular hyperplasia, clusters of epithelioid histiocytes encroaching on germinal centers, and monocytoid B-cell hyperplasia in sinuses. No treatment needed.

In AIDS (CD4 <100), reactivation of latent infection causes toxoplasmic encephalitis - the most common opportunistic CNS infection in AIDS. Patients present with focal deficits, seizures, or altered mental status. Imaging shows multiple ring-enhancing lesions, typically basal ganglia. Major differential: primary CNS lymphoma (also ring-enhancing in AIDS). Empiric treatment with clinical and radiographic improvement helps sort them out.

In pregnancy, primary maternal infection can cross the placenta. The classic congenital triad: chorioretinitis, hydrocephalus, intracranial calcifications (calcifications are diffuse/scattered, unlike CMV which has periventricular calcifications). Additional features: seizures, intellectual disability, hepatosplenomegaly. Gestational-age effect: 1st-trimester infection = lowest transmission but most severe when transmitted; 3rd-trimester = highest transmission but milder disease.

Diagnosis: Serology is the mainstay for immunocompetent patients. Active infection is suggested by positive IgM, seroconversion of IgG (negative to positive), or a 4-fold rise in paired IgG titers (or very high titers >1:1024). Caveats: IgM can persist for months to years (not always “acute”), and most adults in endemic areas are IgG-positive from remote exposure. In AIDS patients with encephalitis, positive IgG plus compatible imaging is presumptive; PCR of CSF confirms but is insensitive. In pregnancy, amniocentesis with PCR can detect fetal infection.

Treatment: The combination of pyrimethamine plus sulfadiazine targets folic acid synthesis in the parasite. Leucovorin (folinic acid) prevents pyrimethamine’s myelosuppressive effects on the host.

Trypanosoma brucei - African Sleeping Sickness

Two subspecies cause human disease: T. b. gambiense (West/Central Africa, chronic) and T. b. rhodesiense (East Africa, acute).

Transmission: Tsetse fly bite injects trypomastigotes (metacyclic form).

Morphology on smear: T. brucei is seen only as the trypomastigote form (extracellular, in blood/lymph/CSF). It is slightly curved with a small posterior kinetoplast, a prominent undulating membrane, and a long free flagellum. The kinetoplast is a mass of mitochondrial DNA at the base of the flagellum - a defining feature of Kinetoplastida.

Pathogenesis: Trypanosomes evade immunity through antigenic variation - constantly switching their variable surface glycoprotein (VSG) coat, staying ahead of the immune response. Each wave of parasitemia represents a new VSG variant.

Clinical stages:

Hemolymphatic stage: Chancre at bite site, fever, lymphadenopathy (Winterbottom’s sign = posterior cervical nodes)
Meningoencephalitic stage: CNS invasion → daytime somnolence, personality changes, eventually coma and death

Diagnosis: Trypomastigotes on blood smear; CSF exam determines stage (presence of trypanosomes and elevated WBC = stage 2)

Treatment varies by species and stage:

Stage 1 T.b. gambiense: Pentamidine
Stage 1 T.b. rhodesiense: Suramin
Stage 2 T.b. gambiense: NECT (nifurtimox-eflornithine combination)
Stage 2 T.b. rhodesiense: Melarsoprol (toxic, but only option)

Trypanosoma cruzi - Chagas Disease (American Trypanosomiasis)

Endemic in Latin America; a leading cause of heart failure in South and Central America and increasingly recognized in the US blood supply (screened by NAAT on all US donations).

Transmission: Reduviid (triatomine, “kissing”) bug feeds on the face at night and defecates near the bite wound; parasites in the feces enter through the scratched bite wound or mucous membranes. Most cases occur in housing built of mud, adobe, or thatch, which provides crevices where the bugs hide by day. Other transmission routes:

Vertical (congenital) transmission.
Oral ingestion of contaminated food/juice (acai, sugarcane juice - a major route in Brazil, and tends to cause more severe acute disease).
Blood transfusion and organ transplantation.
Laboratory accidents.

Life cycle and morphology: T. cruzi exists in two forms in humans:

Trypomastigotes in peripheral blood - C-shaped with a large posterior kinetoplast (the key morphologic difference from T. brucei’s small kinetoplast). The kinetoplast size is the most reliable way to distinguish T. cruzi from T. brucei.
Amastigotes in tissues - round, no flagellum, with nucleus + rod-shaped kinetoplast adjacent. Amastigotes fill cardiac myocytes and smooth muscle cells (pseudocysts), and are the intracellular replicating form.

Clinical phases:

Acute phase - often in children. Chagoma (indurated lesion) at entry site, fever, hepatosplenomegaly. If parasites enter through the conjunctiva: Romaña sign (unilateral painless periorbital edema and conjunctivitis, often with ipsilateral preauricular lymphadenopathy; seen in ~50% of acute cases).
Indeterminate phase - asymptomatic with positive serology, lasts 10-30 years.
Chronic phase - cardiomyopathy (dilated, arrhythmias, RBBB and left anterior fascicular block, apical aneurysm, mural thrombi) and megasyndromes (megaesophagus, megacolon) from destruction of the myenteric plexus. Chronic Chagas cardiomyopathy affects ~30% of infected individuals and is the leading cause of death.

Diagnosis: Acute phase - trypomastigotes on blood smear. Chronic phase - serology (parasitemia is too low to see). Heart biopsy in acute myocarditis shows amastigotes within cardiac myocytes with lymphocytic inflammation.

Treatment: Benznidazole or nifurtimox, most effective in the acute phase. Chronic cardiac disease is largely irreversible; heart transplantation carries the risk of reactivation with immunosuppression.

Leishmania species - Sand Fly-Transmitted Disease

Transmitted by sandfly bite (Phlebotomus in Old World, Lutzomyia in New World); parasites replicate as amastigotes within macrophages. Most prevalent in the Middle East and South America. Reservoirs: dogs (L. infantum), rodents (L. major), humans (L. donovani).

Three clinical forms, determined by species:

Cutaneous - caused by all Leishmania species except L. donovani. Old World: L. tropica, L. major. New World: L. mexicana complex, L. braziliensis, L. amazonensis. Solitary, self-limiting ulcer at the bite site (“volcano crater” with raised indurated borders, sometimes called “oriental sore”). Heals over months leaving a scar.
Mucocutaneous - caused only by L. braziliensis (New World). Months to years after a healed skin lesion, the parasite metastasizes to nasal/oral/pharyngeal mucosa causing destructive, disfiguring lesions. Does not resolve spontaneously; requires systemic therapy.
Visceral (kala-azar) - caused by L. donovani and L. infantum/chagasi. Parasites disseminate to liver, spleen, and bone marrow. Massive hepatosplenomegaly, pancytopenia, fever, wasting, hypergammaglobulinemia, and skin hyperpigmentation (kala-azar = “black fever” in Hindi). Fatal without treatment.

Diagnosis is best by biopsy. Giemsa or H&E staining shows intracellular amastigotes (Leishman-Donovan bodies) within macrophages. Each amastigote is 2-5 µm with a round nucleus and a rod-shaped kinetoplast adjacent to the nucleus. A frequent cue: the marquee sign - amastigotes arrayed along the inner periphery of the macrophage cytoplasm like lightbulbs around a marquee.

Intracellular organisms in skin/histiocyte biopsies raise a board-classic differential: Leishmania vs Histoplasma vs Talaromyces (Penicillium) marneffei.

Histoplasma: GMS-positive, narrow-based bud, no kinetoplast.
Leishmania: kinetoplast present (rod next to nucleus), small pink nucleus on Giemsa, does not stain with GMS.
Talaromyces marneffei: divides by fission, has a cross-wall that can mimic a kinetoplast, GMS-positive.

Bone marrow or splenic aspirate is preferred for visceral disease; skin biopsy or scraping from the lesion edge for cutaneous disease. PCR is the most sensitive method and can speciate. Culture is done on NNN (Novy-McNeal-Nicolle) medium.

Treatment varies by syndrome. Visceral leishmaniasis: liposomal amphotericin B is preferred. Cutaneous: often observation (self-healing) or local/systemic therapy depending on species and severity. Mucocutaneous: systemic amphotericin B or miltefosine. Pentavalent antimonials (sodium stibogluconate) were the longstanding mainstay but have significant toxicity.

Free-living amebae - an overview

Free-living amebae are environmental organisms that occasionally cause opportunistic infections in humans. Three genera matter:

Naegleria fowleri - causes primary amebic meningoencephalitis (PAM), fulminant and near-universally fatal.
Acanthamoeba spp. - causes keratitis (in contact lens wearers) and granulomatous amebic encephalitis (GAE).
Balamuthia mandrillaris - causes granulomatous amebic encephalitis (GAE) and skin granulomas.

Exposure sources: water (swimming pools, lakes, hot springs, AC systems, dialysis water, neti pots, contact lens solution) and soil. Free-living amebae do not require humans for survival.

Naegleria fowleri - Primary Amebic Meningoencephalitis

Naegleria fowleri is a free-living ameba that causes one of the most rapidly fatal infections known to medicine. Understanding the unusual route of infection explains how to suspect the diagnosis.

The organism thrives in warm, stagnant freshwater - lakes, ponds, hot springs, and inadequately chlorinated pools (thermophilic, grows optimally at 42-46°C). Summer months and southern states have highest incidence. Not found in saltwater. Also implicated: contaminated tap water used in neti pots and water heaters.

Route of entry is the key. Naegleria does not infect by ingestion - it enters through the nose during diving/swimming, contacts the olfactory mucosa, and invades through the cribriform plate along the olfactory nerves (CN I) into the frontal lobes. This anatomic route explains both the diagnostic suspicion and the rapid, destructive meningoencephalitis.

Clinical presentation: a previously healthy young person (typically a child or young adult) swims in warm freshwater, then 1-9 days later develops severe headache, fever, nausea, and vomiting progressing rapidly to altered mental status, seizures, and death. Course from symptom onset to death is usually less than a week.

Diagnosis - the lab pitfall: CSF shows purulent meningitis (neutrophilic pleocytosis, elevated protein, low glucose) that looks bacterial. Motile trophozoites are visible on fresh wet mount, but Naegleria trophozoites are often mistaken for macrophages - they are the same size (10-35 µm), ameboid, and may have vacuolated cytoplasm. They also have a single nucleus with a large central dense karyosome (no peripheral chromatin), but cytocentrifugation destroys the trophozoites.

Two critical pre-analytic rules for suspected Naegleria:

Do not refrigerate CSF - cold kills the trophozoites and obscures diagnosis.
Examine CSF by wet mount at room temperature immediately, and warn the lab so they do not just centrifuge the specimen.

Naegleria does not produce cysts in human tissue - only trophozoites and occasionally flagellated forms. Cysts form only in the environment. This contrasts with Acanthamoeba and Balamuthia, which produce both trophozoites and cysts in tissue (the cyst form resists treatment).

Culture: non-nutrient agar with an E. coli lawn at 37-42°C. Trophozoites feed on bacteria and create visible tracks. A distinctive confirmatory test: placing the culture in distilled water induces flagellate transformation (Naegleria trophozoites grow flagella and swim).

Treatment with amphotericin B plus miltefosine has led to rare survivals, but mortality exceeds 97%. Prevention: avoid warm freshwater exposure, use nose clips if exposure is unavoidable, use sterile/distilled water (not tap) for nasal irrigation.

Acanthamoeba and Balamuthia - Granulomatous Amebic Encephalitis

Unlike Naegleria’s fulminant PAM, Acanthamoeba and Balamuthia cause a subacute to chronic granulomatous amebic encephalitis (GAE) in immunocompromised patients. The indolent course (weeks to months) contrasts sharply with Naegleria (days). Both produce trophozoites and cysts in human tissue, and the cysts resist treatment, making GAE very difficult to cure.

Acquisition differs by organism:

Acanthamoeba - enters via contact lenses (contaminated lens solution, using tap water to rinse lenses, wearing lenses while swimming). First presentation is amebic keratitis: severe eye pain out of proportion to findings, photophobia, and a ring-shaped corneal infiltrate. Diagnosis: corneal scraping cultured on E. coli lawn, confocal microscopy. Treatment: prolonged topical PHMB + propamidine.
Balamuthia - enters via soil/dirt exposure (skin wounds, respiratory tract). Starts as a skin granuloma.

Both can then disseminate hematogenously to the brain, causing multiple ring-enhancing lesions on MRI with granulomatous inflammation. CSF: lymphocytic pleocytosis.

Morphology on tissue: Acanthamoeba cysts have a distinctive thick, wrinkled, double wall (crenellated outer, smooth inner, 12-25 µm). Trophozoites have spiny acanthopodia projections. Granulomatous inflammation (epithelioid histiocytes, multinucleated giant cells) surrounds the organisms. Both trophozoites and cysts are visible on biopsy.

Mortality is very high despite multi-drug regimens.

Trichomonas vaginalis - The STI Protozoan

Trichomoniasis is the most common curable sexually transmitted infection worldwide, affecting an estimated 150 million people annually. Though not life-threatening, it causes significant morbidity and increases susceptibility to other STIs including HIV.

Trichomonas vaginalis is a flagellated protozoan that exists only as a trophozoite - there’s no cyst form. This means it can’t survive long outside the human body and requires direct sexual contact for transmission. The organism infects squamous epithelium of the genitourinary tract.

In women, trichomoniasis causes vaginitis with a characteristic frothy, yellow-green, malodorous vaginal discharge. The odor is often described as fishy. Vulvovaginal irritation, dysuria, and dyspareunia occur. On examination, the cervix may show punctate hemorrhages - the strawberry cervix - though this is seen in only about 2% of cases. Vaginal pH is elevated above 4.5.

In men, the infection is usually asymptomatic, though some develop urethritis with discharge. The asymptomatic male partner serves as a reservoir for reinfection if not treated simultaneously.

Morphology: pear-shaped trophozoite (7-23 µm), four anterior flagella plus one recurrent flagellum forming an undulating membrane along half the body length, and a central axostyle (rigid microtubule rod) that runs through the center and protrudes posteriorly. The anterior-placed nucleus sits in front of the axostyle.

Diagnosis: wet mount of vaginal secretions shows motile trophozoites with a characteristic jerky, non-directional (“tumbling”) motility - distinct from Giardia’s smoother “falling leaf” motility. Wet mount sensitivity is only 60-70%. NAAT is the preferred method - much more sensitive. Antigen testing is also available.

Treatment with metronidazole or tinidazole is highly effective. Both sexual partners must be treated simultaneously to prevent reinfection.

Intestinal Protozoa

Giardia lamblia - the backpacker’s nemesis

Giardia is the most common intestinal parasite in the US. Despite its simple biology - it attaches to the intestinal epithelium but doesn’t invade - it causes significant morbidity.

Transmission and epidemiology: Giardia is transmitted by the fecal-oral route, primarily through contaminated water. The cysts are remarkably hardy, surviving in cold water for months and resisting chlorination at standard concentrations. Mountain streams contaminated by beaver feces (hence “beaver fever” as well as “backpacker’s diarrhea”) are a classic source. Daycare centers (fecal-oral spread among children) are another common setting.

Pathogenesis without invasion: Giardia trophozoites attach to the brush border of the small intestine using a ventral sucking disk but don’t invade the epithelium. The mechanism of diarrhea is not fully understood but involves villous blunting and disaccharidase deficiency, leading to malabsorption.

Clinical presentation: After an incubation period of 1-2 weeks, patients develop watery, foul-smelling diarrhea, bloating, flatulence, and abdominal cramps. Steatorrhea (fatty stools) occurs from fat malabsorption. Unlike invasive diarrheas, there’s no fever or blood in the stool. Symptoms can persist for weeks and become chronic if untreated, causing weight loss and nutritional deficiencies.

Morphology: The trophozoite is unmistakable - pear-shaped (teardrop) with bilateral symmetry, two nuclei giving it a face-like appearance (“owl eyes” or “monkey face”), four pairs of flagella, central axonemes (median bodies creating a “smile”), and a ventral sucking disk for adhesion to duodenal epithelium. Trophozoite is 9-21 µm. On wet mount, Giardia shows characteristic falling leaf motility (smooth tumbling and rotating, distinct from Trichomonas’s jerky motility). Cysts are oval with four nuclei when mature, with internal axonemes/fibrils visible. The cyst is the infectious form and is resistant to chlorination.

Host factor: Giardia frequently causes chronic, refractory infection in patients with B-cell (humoral) immunodeficiency - CVID, IgA deficiency, Bruton’s agammaglobulinemia, hyper-IgM syndrome. Mucosal IgA is the main defense against Giardia.

Diagnosis: Traditional stool O&P can identify cysts, but shedding is intermittent, requiring multiple samples. Giardia antigen tests (EIA, DFA) are more sensitive and are now standard. PCR (multiplex GI panels) is the most sensitive. If stool is persistently negative despite high suspicion, consider duodenal aspirate or the string test (Entero-Test).

Treatment: Metronidazole or tinidazole are highly effective.

Entamoeba histolytica - invasive amebiasis

Unlike Giardia, Entamoeba histolytica is a tissue-invasive pathogen that can cause severe disease.

Transmission: Cysts in food or water contaminated with human feces. Cysts are the infectious form (resistant to stomach acid); trophozoites cannot survive outside the body. Endemic in developing countries; in the US, seen in travelers, immigrants, institutionalized patients, and MSM.

The critical distinction from nonpathogenic amebae: E. histolytica is morphologically identical to E. dispar, which is 10 times more common and completely harmless. Microscopy cannot distinguish them on cyst morphology alone - you must use antigen testing or PCR specific for E. histolytica, not just “E. histolytica/dispar complex.” The one reliable microscopic feature that clinches E. histolytica is erythrophagocytosis - trophozoites with ingested RBCs in the cytoplasm. This indicates invasive disease.

Pathogenesis of invasion: E. histolytica trophozoites invade the colonic epithelium using contact-dependent killing (amebapore protein) and cysteine proteases. The parasite literally eats its way into the colonic wall, creating characteristic flask-shaped ulcers - narrow at the mucosal surface, wider at the submucosal base. Ulcers form typically in the cecum and ascending colon.

Clinical syndromes:

Amebic colitis - bloody, mucoid diarrhea (dysentery), abdominal pain, tenesmus. Fever is often absent (unlike bacterial dysentery). Complications: perforation, toxic megacolon, amoeboma (granulomatous mass mimicking cancer).
Amebic liver abscess - trophozoites invade mesenteric vessels and travel via the portal circulation to the liver. RUQ pain, fever, hepatomegaly, sometimes without preceding diarrhea. Imaging shows a usually-solitary abscess in the right lobe (portal drainage from cecum/ascending colon goes to the right lobe). Aspirated contents: anchovy paste - thick, chocolate-brown (necrotic hepatocytes and lysed RBCs, not pus), odorless and sterile (few neutrophils because ameba lyses inflammatory cells).

Morphology:

Trophozoite (12-60 µm): single nucleus with fine peripheral chromatin and a small central karyosome. Ingested RBCs in cytoplasm = pathognomonic for E. histolytica.
Cyst (10-20 µm): round, 1-4 nuclei when mature (stops at 4, never more), chromatoid bodies with rounded/cigar-shaped bars of crystallized ribosomes, glycogen mass in immature cysts.

Diagnosis: Stool antigen testing or PCR specific for E. histolytica. Serology is positive in >90% of liver abscess cases. Microscopy showing trophozoites with ingested RBCs is diagnostic but insensitive.

Treatment requires two drugs: Metronidazole kills tissue trophozoites but doesn’t reliably eliminate luminal cysts. Follow metronidazole with a luminal agent - paromomycin or iodoquinol - to eliminate the intestinal reservoir and prevent relapse or transmission.

Non-pathogenic intestinal amebae and flagellates - knowing what to ignore

Several intestinal protozoa are commensals that do not require treatment, but you must distinguish them from E. histolytica on microscopy. The distinctions are frequently tested.

E. dispar - morphologically identical to E. histolytica but no erythrophagocytosis, no invasive disease. PCR/antigen test needed for reliable separation.
E. coli (Entamoeba coli, not the bacterium) - up to 8 nuclei in mature cysts (vs max 4 in E. histolytica), coarse/irregular peripheral chromatin with a large eccentric karyosome, vacuolated cytoplasm containing ingested bacteria (not RBCs). Chromatoid bodies have splinter-shaped/pointed ends (vs rounded in E. histolytica). The 8-nucleus count on cysts is the most reliable distinguishing feature.
E. hartmanni - looks like a small version of E. histolytica (<12 µm for trophozoite, <10 µm for cyst). Non-pathogenic. Size is the key clue.
Iodamoeba bütschlii - single nucleus with a large central karyosome surrounded by achromatic granules (“basket of fruit”), no peripheral chromatin. Cysts have a prominent glycogen vacuole that stains dark brown/mahogany with iodine - the defining feature (hence “Iodamoeba”).
Dientamoeba fragilis - despite the name, it’s a flagellate (related to trichomonads), not an ameba. Binucleate (two nuclei with fragmented karyosomes), no cyst form. May be transmitted “piggyback” within Enterobius eggs. Can cause diarrhea and abdominal symptoms; pathogenicity is debated. Treatment if symptomatic: iodoquinol or paromomycin.
Chilomastix mesnili - non-pathogenic flagellate. Pear-shaped trophozoite with one nucleus and a prominent cytostome. Cyst is lemon-shaped with an anterior nipple/knob - the defining clue. Not treated.

The board-testable takeaways: (1) number of nuclei in cysts separates E. histolytica/dispar (≤4) from E. coli (up to 8); (2) ingested bacteria = E. coli, ingested RBCs = E. histolytica; (3) the cytostome/lemon shape = Chilomastix (ignore it); (4) binucleate ameboid with no cysts = Dientamoeba.

Coccidia and Microsporidia

The intestinal coccidia - Cryptosporidium, Cyclospora, and Cystoisospora (formerly Isospora) - share a board-testable feature: they are routinely missed on trichrome-stained O&P and require modified acid-fast (or modified safranin) staining. They cause self-limited watery diarrhea in immunocompetent hosts and devastating chronic diarrhea in AIDS. Microsporidia (now reclassified as fungi) behave similarly and are often lumped into the differential.

Comparative sizes and stains on stool:

Organism	Size	Stain	Notes
Cryptosporidium	4-6 µm	Acid-fast (uniform)	Smallest coccidia; chlorine-resistant oocysts
Cyclospora	8-10 µm	Variable acid-fast; autofluorescent (blue-green under UV)	Classic route: imported fresh produce
Cystoisospora	20-30 µm	Acid-fast (uniform)	Oval/ellipsoidal; largest coccidia
Microsporidia	1-3 µm (spores)	Modified trichrome (Weber); calcofluor white	Smallest; pink spores with belt-like stripe; EM = gold standard

All spread by contaminated water and food. Cryptosporidium is the classic waterborne outbreak organism (resistant to chlorination at standard water treatment levels; the 1993 Milwaukee outbreak affected ~400,000 people). Cyclospora is classically foodborne from imported fresh produce (raspberries, basil, cilantro). Cystoisospora is less common and mainly affects the immunocompromised.

Immunocompetent disease is self-limited (1-2 weeks). Immunocompromised disease (especially AIDS with CD4 <100) is chronic, profuse, cholera-like watery diarrhea (liters/day) with wasting and dehydration.

Cryptosporidium - the small spore with big consequences

Cryptosporidium is important because of its severity in AIDS patients and its resistance to chlorine disinfection.

A unique apicomplexan: Cryptosporidium completes its entire life cycle on the surface of intestinal epithelial cells. Oocysts are small (4-6 µm), immediately infectious when shed (no environmental maturation needed), and resistant to chlorine. Outbreaks have occurred from contaminated municipal water supplies.

Disease depends on immune status: In immunocompetent people, Cryptosporidium causes self-limited watery diarrhea lasting 1-2 weeks. In AIDS (CD4 <100), the infection is devastating - profuse, cholera-like watery diarrhea, wasting, and can infect the biliary tract (cholangitis).

Tissue morphology: On duodenal biopsy, Cryptosporidium appears as small dome-shaped basophilic structures attached to the luminal surface of enterocytes (brush border). The location is unique: intracellular but extracytoplasmic - enclosed within a host-derived vacuole at the apical surface, not within the cytoplasm. H&E shows small blue dots lining the mucosa; GMS highlights them.

Diagnosis: Acid-fast stain of stool shows 4-5 µm red spheres against a blue-green background. Antigen detection (DFA) and PCR (multiplex GI panels) are more sensitive.

Treatment limitations: Nitazoxanide is modestly effective in immunocompetent patients but doesn’t work well in AIDS. The only effective treatment for AIDS-related cryptosporidiosis is immune reconstitution with ART.

Cyclospora cayetanensis

Oocysts 8-10 µm, variably acid-fast (partial acid-fast pattern), and autofluorescent blue-green under UV - a useful confirmatory feature. Tissue biopsy shows organisms within parasitophorous vacuoles inside enterocytes, and frequently shows multiple developmental stages simultaneously (merozoites, microgametocytes, macrogametocytes, oocysts). Endemic foodborne outbreaks from imported produce. Treatment: TMP-SMX.

Cystoisospora belli (formerly Isospora)

Oocysts 20-30 µm, oval/ellipsoidal, the largest of the coccidia, uniformly acid-fast. On biopsy, organisms are truly intracellular within enterocytes (deep in cytoplasm, unlike Cryptosporidium at the surface). Crescent-shaped asexual forms and ovoid sexual forms within parasitophorous vacuoles. Peripheral/tissue eosinophilia is common (unique among coccidia - a helpful clue). Treatment: TMP-SMX.

Microsporidia (Enterocytozoon bieneusi, Encephalitozoon spp.)

Recently reclassified from protozoa to fungi based on molecular phylogenetics, but still commonly covered in parasitology. Very small spores (1-3 µm) within enterocytes. Key stains: modified trichrome (Weber stain) - spores appear pink with a characteristic belt-like stripe (coiled polar tubule); calcofluor white (fluorescence); EM is the gold standard and shows the distinctive polar tubule. Chronic diarrhea in AIDS with CD4 <100. Can also cause keratitis and disseminated disease.

Balantidium coli - the ciliated oddball

Balantidium coli is the only ciliated protozoan that parasitizes humans and is also the largest protozoan parasite of humans (up to 200 µm, visible to the naked eye). Primary reservoir: pigs. Transmission: fecal-oral via contaminated water or pig contact. Human-to-human transmission also occurs.

Most infections are asymptomatic. When symptomatic, Balantidium causes colitis with large, irregular ulcers that can mimic E. histolytica colitis. Microscopy is easy because the organism is so large:

Trophozoite covered in cilia (short hair-like projections over the entire surface).
Bean-shaped (kidney-shaped) macronucleus - a hallmark identifying feature. A small micronucleus nestles in the concavity but is often not visible.
Cytostome (“mouth-like” opening) visible.

Treatment: tetracycline (first-line) or metronidazole.

48.3 Helminths

Helminths are multicellular parasitic worms - much more complex than protozoa, with sophisticated life cycles often involving multiple hosts. Understanding these life cycles explains the clinical presentations, geographic distributions, and diagnostic approaches.

Nematodes (Roundworms)

Nematodes are round, unsegmented worms. They can be divided into those that primarily infect the intestine and those that primarily infect tissues.

Lung migration - a shared pattern (HAS mnemonic): Hookworms, Ascaris, and Strongyloides all migrate through lungs during their life cycle (larvae coughed up and swallowed), which is why all three can cause Löffler syndrome (transient pulmonary infiltrates + peripheral eosinophilia). The parasite differs in how it gets there: hookworms and Strongyloides penetrate skin first; Ascaris is ingested and hatches in the intestine before migrating.

Ascaris lumbricoides - the giant roundworm

Ascaris is the largest intestinal nematode (females 20-35 cm, males 15-30 cm) and the most common helminthic infection worldwide (~800 million to 1 billion infected), primarily in areas with poor sanitation.

Life cycle: Fertilized eggs ingested from contaminated soil or produce (unfertilized eggs exist but are not infectious). Eggs require 2-3 weeks in warm moist soil to embryonate and become infectious. Larvae hatch in the intestine, penetrate the intestinal wall, travel via portal circulation to the liver, then to the lungs via the pulmonary artery, penetrate alveoli (Löffler syndrome), are coughed up and swallowed, and mature to adults in the small intestine. This hepato-tracheal migration is shared with hookworms and Strongyloides.

Complications:

Bowel obstruction - tangled worm masses (most classic complication, especially in children with heavy worm burden).
Biliary obstruction - worm migrates into the bile duct, causing biliary colic, cholangitis, recurrent pancreatitis.
Intestinal perforation.

Most infections are asymptomatic with mild worm burden. Diagnosis: characteristic eggs on stool O&P, or patients passing adult worms. Fertilized eggs are round/oval (55-75 µm) with a thick brown mammillated (bumpy) outer shell. Unfertilized eggs are more elongated with irregular mammillations and no embryo. Decorticated eggs have lost the mammillated coat and appear smooth.

Enterobius vermicularis - pinworm

Pinworm is the most common helminthic infection in American children (~40 million US infections). More common in temperate climates. Risk factors: school-age children, crowded living, institutions.

Life cycle: Fecal-oral via eggs. Eggs are swallowed (from hands, fomites, bedding, dust - eggs are lightweight and airborne), hatch in the duodenum, and larvae mature in the ileum, cecum, or appendix. Gravid females migrate out the anus at night to deposit up to 11,000 eggs on the perianal skin. Eggs become infectious within hours. Auto-infection via hand-to-mouth after scratching, and retroinfection (eggs hatch on perianal skin and larvae migrate back into the intestine).

Because eggs are deposited perianally and not in stool, standard stool O&P often misses the diagnosis. The tape test (cellophane tape pressed to perianal skin in the morning before bathing or defecation, repeated for 3 consecutive mornings) is the diagnostic method of choice. Eggs are oval, flattened on one side (D-shaped/plano-convex), 50-60 µm, containing a coiled larva.

Clinical features: mostly anal pruritus, worse at night. Can rarely cause appendicitis or appendiceal colic, vulvovaginitis/salpingitis in girls (worms migrate into genitourinary tract), or enuresis. On histologic cross-section, Enterobius has distinctive lateral alae (wing-like cuticular projections). Female worms are ~8-13 mm with a long, pointed tail (hence “pinworm”); males are smaller with curved tails.

Treatment: mebendazole or albendazole, treat the entire household.

Trichuris trichiura - whipworm

Whipworm is named for its shape: a long thin anterior portion (the “whip”) that threads through the colonic mucosa, and a thicker posterior (the “handle”) that hangs into the lumen. Lives in the cecum and ascending colon. Soil-transmitted, endemic in tropical/subtropical areas.

Acquired by ingestion of embryonated eggs from contaminated soil or food (eggs need 2-3 weeks in warm moist soil to embryonate). No skin penetration, no lung migration.

Clinical: mostly asymptomatic. In young children with heavy infection, Trichuris dysentery syndrome - bloody/mucoid diarrhea and rectal prolapse (from straining with a heavy rectal worm burden; worms may be visible on the prolapsed mucosa), iron-deficiency anemia, growth retardation, finger clubbing.

Eggs are barrel/lemon-shaped (50 µm) with bipolar mucus plugs - pathognomonic on stool O&P. Treatment: mebendazole or albendazole.

Hookworm (Necator americanus, Ancylostoma duodenale) - the cause of anemia

Hookworm infection was once endemic in the rural American South. The larvae live in soil and actively penetrate skin (usually bare feet). The skin penetration causes “ground itch” at the entry site. Larvae migrate via blood to the lungs, are coughed up and swallowed, and attach to the intestinal mucosa.

The critical point is that hookworms feed on blood. Adults attach to duodenal/jejunal mucosa via mouthparts and ingest blood continuously. Heavy infections cause iron-deficiency anemia - the major clinical manifestation and the leading cause of iron deficiency in the developing world.

Mouthpart distinction (definitive species identification - eggs are identical):

Ancylostoma duodenale - teeth (4 hook-like teeth; “Old World,” Southern Europe, North Africa, Middle East, Asia). Each worm ingests ~0.15 mL blood/day.
Necator americanus - cutting plates (semilunar plates; “New World,” Americas, sub-Saharan Africa). Each worm ingests ~0.03 mL blood/day.

Hookworm eggs (indistinguishable between species) are oval, 56-75 µm, thin clear shell with a developing morula (segmented embryo, usually 4-8 cell stage in fresh stool) visible inside.

Strongyloides stercoralis - the hyperinfection threat

Strongyloides is unique among helminths because it can complete its entire life cycle within a single human host (autoinfection). Filariform larvae can penetrate the intestinal wall or perianal skin and re-infect the host without ever leaving the body. In immunocompetent people, this is kept in check and patients can harbor Strongyloides for decades.

Life cycle: Skin penetration of filariform (L3) larvae, blood to lungs (Löffler possible), cough and swallow, mature in small intestine. Adult females burrow into the intestinal crypts of the duodenum/jejunum. Eggs hatch inside the intestine, so rhabditiform larvae (not eggs) are what you find in stool - this is unique among intestinal helminths. On wet mount, rhabditiform larvae have a short buccal cavity and a prominent genital primordium (compare: hookworm rhabditiform larvae have a long buccal cavity).

Hyperinfection syndrome: The danger comes with immunosuppression - particularly corticosteroids and HTLV-1 coinfection. Autoinfection accelerates uncontrollably, producing millions of larvae disseminating throughout the body. Larvae penetrating the gut wall carry enteric bacteria into the bloodstream. Patients develop gram-negative sepsis and meningitis (classically E. coli and Streptococcus bovis), respiratory failure, and multi-organ failure. Board pearl: gram-negative meningitis in an immunosuppressed patient with travel history = consider Strongyloides hyperinfection. Mortality exceeds 70%.

This is why screening for Strongyloides before starting corticosteroids is essential in anyone from an endemic area.

Diagnosis: challenging because larvae are shed intermittently. Options:

Stool wet mount for motile rhabditiform larvae.
Agar plate culture is the most sensitive single stool method - stool is placed on nutrient agar and larvae migrate through, carrying bacteria and leaving visible larval tracks with bacterial colonies along the furrows.
Serology is more sensitive than stool microscopy for detection.
Duodenal biopsy can show larvae/adults within crypts of Lieberkühn with surrounding eosinophilia.
String test (Entero-Test) is another option.

S. fuelleborni kellyi: Endemic to Papua New Guinea only. Unlike S. stercoralis (larvae in stool), S. fuelleborni kellyi sheds eggs in stool. Causes “Swollen Belly Syndrome” in infants; can be transmitted via breast milk.

Trichinella spiralis - from undercooked pork

Trichinella is acquired by eating undercooked meat containing encysted larvae - classically pork, also wild game (bear, boar, walrus). Cooking to >71°C (160°F) or freezing kills larvae in domestic pork, but arctic/wild game strains are freeze-resistant, so freezing does not reliably kill those.

Life cycle: larvae released by digestion in the stomach, mature in the small intestine. Female worms release larvae that migrate hematogenously to skeletal (striated) muscle - diaphragm, masseter, tongue, extraocular muscles, intercostals, deltoid, biceps - where they encyst within a modified nurse cell.

Clinical syndrome reflects the migration: initial GI symptoms, then muscle pain, periorbital edema (tropism for extraocular muscles), fever, eosinophilia, and elevated CPK. Severe cases: myocarditis, CNS involvement.

Diagnosis: serology is primary. Muscle biopsy shows encysted larvae coiled within nurse cells, with surrounding eosinophilic infiltrate. Over time, the cyst calcifies.

Anisakis - the sushi worm

Anisakis is acquired from raw or undercooked marine fish (salmon, herring, mackerel, squid) - common in Japan (sushi), Scandinavia (raw herring), and anywhere raw ocean fish is eaten. Cooking >63°C or freezing at -20°C for 7 days prevents transmission. Marine fish only, not freshwater.

Two phases of illness:

Acute (within hours): larvae penetrate the gastric/intestinal mucosa causing epigastric/abdominal distress, nausea, vomiting. Larvae may be coughed up or vomited.
Delayed (1-2 weeks): eosinophilic granulomatous response around larvae in the intestinal wall, causing diarrhea, abdominal pain, obstruction. Can mimic Crohn’s disease or appendicitis.

Endoscopy may visualize the larva attached to gastric mucosa and allow direct removal (curative).

Dirofilaria immitis - dog heartworm

Dirofilaria is the dog heartworm. Adult worms live in the pulmonary arteries and right ventricle of dogs, transmitted by mosquitoes. In humans, Dirofilaria cannot mature to adulthood. The immature worm dies in a small arteriole (usually pulmonary, sometimes subcutaneous) and causes a localized granulomatous reaction.

The classic presentation is an incidental pulmonary coin lesion on CXR or CT that mimics lung cancer or granulomatous disease. Diagnosis is made on histology of the excised nodule: coagulative necrosis with a degenerate worm surrounded by eosinophils and granulomatous inflammation.

Tissue Nematodes

Unlike the intestinal nematodes, tissue nematodes complete their development in tissues other than the GI tract. These filarial worms are transmitted by arthropod vectors, and their adult worms live for years in various tissue compartments - lymphatics, subcutaneous tissue, or the eye. Disease manifestations result from both the worms themselves and the host’s inflammatory response.

Identifying microfilariae on smears requires two features:

Sheath (a remnant of the egg membrane): sheathed = Wuchereria, Brugia, Loa loa; unsheathed = Mansonella, Onchocerca.
Tail nuclei pattern - whether nuclei extend to the tip of the tail or not (species-specific).

Periodicity matters too - nocturnal periodicity (draw blood at night, 10pm-2am) for Wuchereria and Brugia; diurnal for Loa loa; Onchocerca microfilariae live in skin, not blood.

Onchocerca volvulus - River Blindness

Onchocerciasis is a leading infectious cause of blindness, affecting millions in sub-Saharan Africa and parts of Central and South America. The name “river blindness” reflects the ecology: the blackfly vector (Simulium) breeds in fast-flowing rivers, and villages near these waterways have the highest rates of blindness.

The life cycle explains the geography and pathology: Blackflies deposit larvae when they bite. The larvae mature into adult worms that live in subcutaneous nodules, where they can survive for over a decade. The adults produce millions of microfilariae (larval offspring) that migrate through the skin and, critically, into the eyes.

Skin disease (onchodermatitis) results from the inflammatory response to microfilariae. Patients experience intense itching - so severe it can be debilitating. Chronic inflammation leads to skin changes: thickening, depigmentation (“leopard skin”), and premature aging (“sowda”). The skin may feel like sandpaper from the chronic scratching.

Blindness develops when microfilariae invade the cornea and other ocular structures. Dead microfilariae trigger inflammation that damages the cornea (sclerosing keratitis), iris, and retina. The blindness is irreversible.

Diagnosis relies on identifying microfilariae in skin snips - small biopsies incubated in saline, which allows the microfilariae to emerge and be visualized microscopically.

Adult worms coil within fibrotic onchocercomata - firm, non-tender, mobile subcutaneous nodules (1-3 cm) over bony prominences (iliac crest, scalp/head in Central America; trunk in Africa). Nodulectomy reduces microfilarial burden.

Treatment with ivermectin kills microfilariae and is given repeatedly over years. Ivermectin does NOT kill adult worms, which continue producing microfilariae - hence the need for repeated treatment. Dead microfilariae trigger the Mazzotti reaction (severe inflammation), so treatment must be carefully staged. Mass drug administration has dramatically reduced onchocerciasis.

Wuchereria bancrofti and Brugia - Lymphatic Filariasis (Elephantiasis)

Wuchereria bancrofti (~90% of cases) and Brugia spp. (B. malayi, B. timori) cause lymphatic filariasis, affecting ~120 million worldwide. The clinical hallmark - elephantiasis - results from years of lymphatic damage.

Vectors: Anopheles or Culex mosquitoes. Larvae deposited on skin during feeding enter through the bite wound and migrate to lymphatic vessels, where adults live for 5-15 years.

The lymphatic connection: Adult worms in lymphatic vessels cause inflammation, fibrosis, and obstruction. Dead/dying worms trigger granulomatous reaction. Chronic obstruction blocks interstitial fluid drainage, causing lymphedema. Over years, the edema becomes permanent: affected limbs or genitals (particularly scrotum - hydrocele) become massively enlarged with thickened, hardened skin resembling elephant hide. Secondary bacterial infections worsen the condition.

Acute episodes of lymphangitis and lymphadenitis punctuate the chronic course, often triggered by bacterial superinfection.

Diagnostic quirk - nocturnal periodicity: Microfilariae circulate in peripheral blood primarily at night (~10pm-2am), matching the feeding habits of the nocturnal mosquito vector. Blood samples must be collected at night. During the day, microfilariae sequester in pulmonary capillaries.

Treatment and prevention: Diethylcarbamazine (DEC) kills both adult worms and microfilariae. Mass drug administration programs combining ivermectin, albendazole, and DEC interrupt transmission in endemic areas. For established elephantiasis, management focuses on hygiene, limb elevation/exercise, and control of secondary infections.

Loa loa - The African Eye Worm

Loa loa is found in the rainforests of Central and West Africa. It’s the only filarial infection where you might actually see the worm - adult worms sometimes migrate across the conjunctiva (under the bulbar conjunctiva), creating the alarming but diagnostic finding of a worm visible in the eye. The worm can be surgically extracted from the subconjunctival space.

Vector: Mango fly (Chrysops, deer fly), which is a daytime biter.

Clinical features reflect the wandering adult worms. Most patients are asymptomatic. Symptomatic: Calabar swellings - transient, non-pitting, non-erythematous angioedema-like swellings (5-10 cm) that appear suddenly in one subcutaneous location, last hours to days, then resolve and reappear elsewhere as the worm moves. Common on forearms, wrists, hands. Eosinophilia is frequent.

Diurnal periodicity: Unlike W. bancrofti, Loa loa microfilariae circulate during the day, matching the day-biting habits of Chrysops. Blood samples during daytime hours.

Treatment caution: Patients with high microfilarial loads treated with DEC or ivermectin can develop fatal encephalopathy from massive microfilarial death. Microfilarial counts must be checked before treatment.

Dracunculus medinensis - The Guinea Worm

Guinea worm disease is on the verge of eradication - a remarkable public health achievement accomplished without vaccines or drugs, purely through behavior change and water treatment.

The life cycle is elegant and horrifying: Humans become infected by drinking water containing copepods (tiny crustaceans) harboring Dracunculus larvae. The larvae mature over a year into adult worms - females can reach 1 meter in length. When ready to release larvae, the female migrates to the skin, usually on the leg or foot, and creates an intensely painful blister. When the person seeks relief by immersing the affected limb in water, the worm emerges and releases thousands of larvae, completing the cycle.

Traditional treatment involves slowly winding the emerging worm around a stick, a few centimeters per day over several weeks. Pulling too quickly breaks the worm, causing severe inflammation and bacterial superinfection. No antiparasitic drug is effective against Dracunculus. This ancient treatment method may be the origin of the Rod of Asclepius (medical symbol).

Eradication efforts have reduced cases from millions annually to fewer than 20 by 2020. The strategy is simple: filter drinking water through cloth to remove copepods. Community education and provision of filter cloths has accomplished what no pharmaceutical intervention could.

Toxocara canis/cati - Visceral and Ocular Larva Migrans

Toxocara are roundworms of dogs (T. canis) and cats (T. cati). In their natural hosts, they complete a normal intestinal nematode life cycle. In humans - accidental hosts - the larvae cannot complete development. Instead, they wander through tissues, unable to reach the intestine, causing damage wherever they go.

How infection occurs: Dogs and cats pass Toxocara eggs in their feces. Eggs mature in soil and can remain infectious for years. Children playing in sandboxes or yards contaminated by pet feces ingest eggs from dirty hands. The eggs hatch, and larvae penetrate the intestinal wall, but instead of migrating to the lungs and back to the intestine (as they would in dogs), they wander aimlessly through tissues.

Visceral larva migrans occurs when larvae migrate through the liver, lungs, and other organs. Young children present with fever, hepatomegaly, pulmonary infiltrates, and - characteristically - extreme eosinophilia. The eosinophilia is often the first clue to diagnosis.

Ocular larva migrans occurs when a larva lodges in the eye. Older children typically present with unilateral visual loss. The inflammatory mass created by the larva can mimic retinoblastoma, and many eyes have been enucleated unnecessarily before the parasitic etiology was recognized. Whenever a child presents with a unilateral retinal mass, Toxocara serology should be checked before assuming malignancy.

Cestodes (Tapeworms)

Tapeworms are flat, segmented worms. Adults live in the intestine, absorbing nutrients through their body wall (no GI tract of their own). Tapeworm anatomy: a scolex (head - attachment organ with suckers and/or hooks), a neck (growth zone), and proglottids (body segments, each a self-contained reproductive unit). Think of it as a train - scolex is the engine, proglottids are boxcars. Gravid (egg-filled) proglottids shed in stool are often how species are identified.

The critical conceptual distinction for Taenia solium: infection with the adult worm (intestinal taeniasis) is different from infection with the larval stage (cysticercosis). They come from different exposures and have different consequences.

Taenia solium - the pork tapeworm with two faces

T. solium = pork. Two very different diseases, depending on what life stage you ingest.

Eating undercooked pork containing cysticerci (larval cysts) → intestinal tapeworm (taeniasis). Adult worm lives in the intestine and sheds eggs in stool. Usually asymptomatic or mild. Treatment: praziquantel or niclosamide.
Eating eggs (from contaminated food or hands, or auto-infection from an intestinal carrier) → cysticercosis. Eggs hatch, larvae penetrate the intestinal wall and migrate throughout the body, encysting in tissues. In humans, cysts in the brain = neurocysticercosis, the most common parasitic cause of seizures worldwide. Imaging: ring-enhancing lesions (live cysts) or calcifications (dead cysts). Non-meat eaters and vegetarians can get cysticercosis from an infected food handler - the exposure is eggs, not pork.

In the US, T. solium cysticercosis is most common in recent immigrants from Latin America; neurocysticercosis is the most common parasitic CNS disease in the US.

Taenia saginata - the beef tapeworm

T. saginata = beef. Unlike T. solium, T. saginata does not cause cysticercosis in humans - humans are dead-end for T. saginata eggs. Eating undercooked beef containing cysticerci causes only intestinal tapeworm infection. Adult worms can reach 4-12 m. Proglottids can actively crawl out of the anus.

Distinguishing T. solium from T. saginata morphologically:

Scolex: T. solium is armed - 4 suckers plus a rostellum with 2 rows of hooks. T. saginata is unarmed - 4 suckers only, no rostellum, no hooks. Mnemonic: solium = suckers + hooks.
Proglottid uterine branches: T. solium <13 branches per side; T. saginata >13 branches per side (15-30, more and thinner). Both proglottids are longer than wide.
Eggs: identical between the two species (round, 30-40 µm, thick radially striated embryophore with hexacanth embryo/6 hooklets). Species identification requires proglottid or scolex.

Diphyllobothrium latum - the fish tapeworm

Acquired from raw/undercooked freshwater fish (salmon, pike, perch, walleye). This tapeworm is the largest infecting humans (up to 10-12 m). Endemic in the extreme northern hemisphere (Scandinavia, Russia, Great Lakes region, Alaska, Japan).

Unique clinical feature: vitamin B12 deficiency → megaloblastic anemia. The worm has high affinity for B12 and absorbs it from the host’s intestine (80-100× the rate of host absorption), competing with the host for jejunal B12. Can also cause neurologic symptoms (peripheral neuropathy, subacute combined degeneration).

Distinguishing features:

Proglottid is wider than long (opposite of Taenia), with a central rosette-shaped uterus and a central genital pore (vs lateral in Taenia).
Egg is operculated (has a lid) with a small knob at the abopercular end, unembryonated, 55-75 µm. Can be confused with Paragonimus westermani eggs - Paragonimus is larger (~90 µm) with a thicker shell at the abopercular end and no knob.

Cooking to >56°C or freezing at -18°C for 24-48 hours kills the plerocercoid larvae. Marine fish do not carry D. latum (freshwater only).

Echinococcus granulosus - hydatid disease

Humans are accidental intermediate hosts. Definitive host: dogs. Intermediate hosts: sheep/cattle. Humans acquire infection by ingesting eggs from dog feces (contaminated food/water, hand-to-mouth in herding/pastoral communities). Once inside, oncospheres penetrate the gut wall, travel via portal circulation, and form hydatid cysts in:

Liver (most common, ~65-70% - right lobe predominance).
Lungs (~25%).
CNS (rare but serious).

Hydatid cyst appearance: well-defined, round, unilocular or multilocular. Contains numerous protoscoleces with hooklets that settle to the bottom of the cyst fluid - grossly looks like “hydatid sand.” Finding protoscoleces or hooklets in cyst fluid is diagnostic.

The critical management point: do not aspirate/biopsy a hydatid cyst without precautions. Cyst fluid is highly antigenic; rupture can cause:

Anaphylaxis (type I hypersensitivity to cyst antigens).
Secondary echinococcosis (spillage of protoscoleces seeds new cysts at spill sites).

Diagnosis is by imaging + serology, not aspiration. Treatment: surgical excision with care to prevent spillage, or the PAIR technique (Puncture, Aspiration, Injection of hypertonic saline or ethanol, Re-aspiration) with pre-treatment albendazole to reduce protoscolex viability.

Hymenolepis nana - the dwarf tapeworm

Smallest tapeworm infecting humans (15-40 mm), but the most common tapeworm infection worldwide. Unique features:

Does NOT require an intermediate host. Direct person-to-person transmission via fecal-oral route.
Autoinfection - eggs can hatch within the intestine, so worm burden can increase without re-exposure.
Also acquired by accidentally ingesting infected grain beetles or fleas (arthropod intermediate host).

Egg morphology: round/oval (30-47 µm), thin smooth outer shell, an inner membrane with two polar thickenings, and 4-8 polar filaments extending into the space between the shell and inner membrane. The polar filaments are the key diagnostic feature. Treatment: praziquantel.

Dipylidium caninum - the cat/dog flea tapeworm

Most common tapeworm in cats and dogs. Human infection is rare and requires accidental ingestion of an infected flea (the intermediate host, carrying cysticercoid larvae). Most human cases are in young children in close contact with pets. Symptoms: usually mild; proglottids (rice grain-like) may be visible in stool or on underwear. Egg morphology: eggs in egg packets (clusters of 5-30 enclosed in a membrane) - the packet arrangement is distinctive.

Treatment: praziquantel.

Trematodes (Flukes)

Flukes are flat, leaf-shaped worms. All require snail intermediate hosts, which explains their geographic distribution (where the appropriate snails exist). Most are hermaphroditic; the exception is the schistosomes, which have separate sexes.

Schistosoma - blood flukes with different addresses

Schistosomes are unique among flukes because adults live in blood vessels (venous plexuses), not the GI tract. They also have separate sexes - the male has a ventral groove (gynecophoral canal) in which the female resides during mating. The location of adult worms determines the clinical syndrome.

Life cycle: Humans are infected when cercariae (free-swimming larvae released by snails into freshwater) penetrate skin during water contact. Cercariae transform into schistosomulae, migrate through the vasculature, mature in the hepatic portal system, then migrate to species-specific venous plexuses where they mate and lay eggs. Eggs penetrate through vessel and organ walls into stool or urine. Eggs in freshwater hatch releasing miracidia that infect snails. Specific snail hosts: Biomphalaria (S. mansoni), Bulinus (S. haematobium), Oncomelania (S. japonicum).

Acute vs chronic disease:

Acute (Katayama fever): 2-8 weeks after cercarial penetration. Mediated by circulating immune complexes (type III hypersensitivity) against schistosomal antigens. Fever, urticaria, eosinophilia, hepatosplenomegaly, lymphadenopathy, cough. Self-limited; may be treated with steroids + praziquantel.
Chronic disease reflects tissue reaction to eggs (granulomatous inflammation, not the adult worms). Eggs trapped in tissues elicit Th2-mediated granulomas, leading to fibrosis and organ damage.

Species-specific syndromes:

Species	Adult worms live in	Eggs in	Chronic disease	Egg spine
S. mansoni	Mesenteric veins (also colon/rectum)	Stool	Periportal (pipestem) fibrosis → portal hypertension	Lateral (large, from the side)
S. japonicum	Portal/mesenteric veins	Stool	Periportal fibrosis → portal hypertension (most severe egg production)	Small/rudimentary lateral knob (often no visible spine)
S. haematobium	Vesical veins (bladder)	Urine	Hematuria, bladder squamous cell carcinoma	Terminal (at the tip)

S. mansoni and S. japonicum: Adults in mesenteric/portal veins. Eggs deposited in the intestinal wall pass into stool, but eggs that don’t escape get swept to the liver via portal circulation. Accumulated hepatic eggs cause granulomatous inflammation and Symmers’ pipestem (periportal) fibrosis, leading to presinusoidal portal hypertension with splenomegaly and esophageal varices. Liver function is relatively preserved (unlike alcoholic cirrhosis). Rectal biopsy showing eggs confirms diagnosis. Mnemonic: hae-MATOBIUM = terMINAL; MANS-oni = LATERAL; japonicum = barely any spine.

S. haematobium: Adults in vesical (bladder) venous plexus. Eggs deposited in the bladder wall cause hematuria (the cardinal sign), urinary frequency, and chronic inflammation. Chronic S. haematobium infection causes squamous cell carcinoma of the bladder (not the usual transitional cell carcinoma) - a major cause of bladder cancer in Egypt and sub-Saharan Africa. Urine collection best in late morning/afternoon for peak egg excretion.

Treatment: Praziquantel is effective against all schistosomes.

Paragonimus westermani - the lung fluke

Acquired by ingestion of undercooked crustaceans (crabs, crayfish). Larvae excyst in the intestine, penetrate the intestinal wall, migrate through the diaphragm into the pleural cavity, and encyst in the lung parenchyma. Endemic in East/Southeast Asia, West Africa, South America.

Symptoms: chronic cough, hemoptysis (bloody sputum), chest pain - often misdiagnosed as tuberculosis. Eggs are found in sputum (coughed up) or stool (swallowed). Egg morphology: operculated, golden-brown, thick shell, 80-120 µm (larger than D. latum), with no abopercular knob. Treatment: praziquantel.

Clonorchis sinensis and Opisthorchis species - the liver flukes

Acquired from eating raw or undercooked freshwater fish. Adults live in bile ducts causing chronic inflammation, biliary obstruction, cholangitis, cholelithiasis, and - most importantly - a significantly increased risk of cholangiocarcinoma (IARC Group 1 carcinogen). Endemic in East/Southeast Asia (China, Korea, Vietnam, Thailand).

Egg morphology: very small (30 µm), operculated, flask/vase-shaped, with shouldered operculum and a small abopercular knob. Found in stool or bile. Treatment: praziquantel.

Fasciola hepatica and Fasciolopsis buski - fluke from freshwater plants

Acquired by ingestion of freshwater plants (metacercariae encyst on the plant surface).

Fasciola hepatica (sheep liver fluke) - from watercress. Larvae penetrate the intestinal wall, migrate through peritoneum to liver, penetrate the hepatic capsule (eosinophilic inflammation), and enter bile ducts. Chronic infection: biliary obstruction, cholangitis, hepatic fibrosis. Not associated with cholangiocarcinoma (unlike Clonorchis). Treatment: triclabendazole (not praziquantel - important distinction).
Fasciolopsis buski (giant intestinal fluke) - from water chestnuts, water bamboo. Largest fluke in humans (up to 7.5 cm). Attaches to duodenal/jejunal mucosa, causing inflammation, abdominal pain, diarrhea, malabsorption. Heavy infections: edema, obstruction. Treatment: praziquantel.

Diagnosis and treatment summary: Stool examination for intestinal/liver flukes; urine for S. haematobium; sputum for Paragonimus. Praziquantel is effective against most flukes and all cestodes - except Fasciola hepatica, which needs triclabendazole.

Cross-cutting Pearls

Person-to-person transmission is the exception in parasitology. Most parasites need intermediate hosts, vectors, or environmental maturation. Those capable of direct person-to-person spread (worth remembering):

Enterobius vermicularis (eggs, fecal-oral, highly contagious within households).
Hymenolepis nana (eggs, fecal-oral; only tapeworm with direct transmission).
Taenia solium cysticercosis (eggs from an intestinal carrier cause cysticercosis in contacts).
Entamoeba histolytica, Giardia, Cryptosporidium (cysts/oocysts, fecal-oral).
Strongyloides (autoinfection; person-to-person is rare but possible).
Trichomonas vaginalis (sexual contact).
Balantidium coli (fecal-oral).

Parasites tested on sputum/respiratory specimens: Strongyloides (hyperinfection), Ascaris (migration), hookworm (migration), Paragonimus westermani, occasionally Echinococcus if a cyst ruptures into the airway.

Parasites that enter via skin penetration: Hookworms, Strongyloides, Schistosoma (all from soil or water).

Parasites with blood-borne microfilariae: Mansonella, Wuchereria bancrofti, Loa loa, Brugia. Onchocerca lives in skin, not blood.

Parasite - cancer associations (board classics):

S. haematobium - bladder squamous cell carcinoma.
Clonorchis sinensis / Opisthorchis - cholangiocarcinoma.

48.4 Ectoparasites

Ectoparasites live on or in the skin surface rather than inside the body. They cause disease directly through their feeding and burrowing activities, and indirectly by serving as vectors for other pathogens. Though often considered nuisances rather than serious medical conditions, ectoparasites cause substantial morbidity and, in the case of body lice, can transmit life-threatening infections.

Sarcoptes scabiei (Scabies)

Scabies is caused by a mite so small it’s nearly invisible to the naked eye, yet an infestation produces intense, maddening itch that’s among the most distressing symptoms in medicine. The condition has afflicted humans throughout recorded history and remains common worldwide.

The mite’s lifestyle creates the symptoms: The female Sarcoptes scabiei mite burrows into the stratum corneum (the outermost layer of skin), creating tunnels where she lays eggs. The mites themselves, their eggs, and their fecal pellets (scybala) are all highly antigenic. After initial infestation, it takes 4-6 weeks for sensitization to develop - this is why first-time infestations often aren’t itchy until weeks after exposure. With re-infestation, the immune system reacts immediately.

The itch is predominantly allergic, which explains several features: itching is worst at night (when the skin is warm and mites are most active), it’s out of proportion to the number of mites (a typical infestation involves only 10-15 mites), and it persists for weeks after successful treatment (until the antigenic material clears).

Distribution provides diagnostic clues: Mites prefer thin skin - the interdigital web spaces of the fingers, the wrists, the elbows, the axillae, the waistline, and the genitalia. In adults, the head and neck are typically spared (except in infants and immunocompromised patients). Finding linear burrows or papules in these characteristic locations should prompt consideration of scabies.

Crusted (Norwegian) scabies is a hyperinfestation occurring in immunocompromised patients or those unable to scratch (due to paralysis or dementia). Instead of a dozen mites, these patients harbor millions. The clinical picture is dramatically different: thick, crusted, hyperkeratotic plaques rather than subtle papules and burrows. Crusted scabies is extraordinarily contagious and has caused hospital and nursing home outbreaks.

Diagnosis is confirmed by skin scraping: mineral oil is applied to a burrow or papule, the area is scraped with a blade, and the material is examined microscopically for mites, eggs, or fecal pellets. In practice, scabies is often diagnosed clinically and treated empirically.

Treatment: Permethrin 5% cream applied from neck to toes, left on overnight, is first-line. All household members and close contacts must be treated simultaneously, and bedding and clothing should be washed in hot water. Oral ivermectin is an alternative, particularly useful for crusted scabies and institutional outbreaks. The itch may persist for weeks after successful treatment due to ongoing allergic response to dead mites and debris.

Lice (Pediculosis)

Three species of lice infest humans, each adapted to a specific body region. They’re obligate human parasites - they can’t survive long away from the human body, and they don’t infest other animals.

Pediculus humanus capitis (head louse) infests the scalp, primarily affecting school-age children. The lice live on the scalp, feeding on blood multiple times daily. Females cement their eggs (nits) to hair shafts near the scalp. Nits appear as tiny white or yellow-brown specks firmly attached to hairs - they can’t be brushed off like dandruff. Itching and secondary bacterial infection from scratching are the main symptoms. Though common and distressing to parents, head lice don’t transmit disease.

Pediculus humanus corporis (body louse) lives in clothing, visiting the skin only to feed. Unlike head lice, body lice are medically significant vectors, transmitting epidemic typhus (Rickettsia prowazekii), trench fever (Bartonella quintana), and relapsing fever (Borrelia recurrentis). Body louse infestation is associated with poverty, homelessness, and conditions where people can’t change or wash clothing - refugee camps, wars, and disasters. Control requires not just treatment of the person but washing or discarding infested clothing.

Phthirus pubis (pubic louse, “crabs”) infests coarse hair, primarily pubic hair but also eyelashes, eyebrows, and axillary hair. It’s transmitted through sexual contact and is considered an STI. The broad, crab-shaped body is distinctive under magnification.

Treatment for all lice involves topical pediculicides (permethrin, pyrethrins, or malathion) and mechanical removal of nits with a fine-toothed comb. Resistance to permethrin is increasing, making alternatives like ivermectin increasingly important. Close contacts must be treated, and attention to fomites (hats, combs, bedding) helps prevent reinfestation.

Chapter 49: Virology

Viruses are obligate intracellular parasites - they can’t replicate on their own and must commandeer host cell machinery. Understanding viral biology helps you understand disease patterns and treatment options.

This chapter organizes viruses by genome type (DNA vs RNA) and family, covers the diagnostic workup from cytopathic effect through molecular methods, and closes with prions and vaccines. For board purposes, keep a mental map of four things per virus: genome (ds/ss, DNA/RNA, +/- sense), envelope status, the classic clinical syndrome, and the preferred test.

Viral structure and classification basics

A virion is nucleic acid plus a protein capsid, with or without a lipid envelope derived from host membrane. The capsid is built from subunits called capsomeres and comes in three shapes: icosahedral (20 triangular faces, 12 vertices - adenovirus, parvovirus, picornavirus), helical (capsomeres spiral around nucleic acid - influenza, rabies, Ebola), and complex (pox bricks, bacteriophage head-tail).

Non-enveloped (naked) viruses are more resistant to heat, pH extremes, drying, and household disinfectants because they lack a fragile lipid envelope. This is why they transmit by fecal-oral route and survive on surfaces - think norovirus, rotavirus, adenovirus, HPV. Enveloped viruses are more fragile and spread by respiratory droplets, blood, or direct contact. Envelopes come from host membrane: plasma membrane (HIV, influenza), nuclear membrane (herpesviruses), or ER/Golgi.

Baltimore classification groups viruses by genome type and replication strategy:

Group I - dsDNA (adenovirus, herpes, pox): use host DNA polymerase
Group II - ssDNA (parvovirus): rolling circle replication
Group III - dsRNA (reovirus, rotavirus): must carry own RNA polymerase
Group IV - positive-sense ssRNA (picornavirus, flavivirus): genome acts as mRNA directly
Group V - negative-sense ssRNA (influenza, paramyxovirus, rabies): carry RNA polymerase in virion to make mRNA
Group VI - ssRNA-RT (retrovirus, HIV): RNA to DNA via reverse transcriptase, integrates
Group VII - dsDNA-RT (hepadnavirus, HBV): replicates through RNA intermediate

Genome shape can be linear (most common) or circular (HPV, HBV, polyomavirus). Segmented genomes (influenza, rotavirus, bunyavirus) consist of multiple separate RNA segments, enabling reassortment.

49.1 DNA Viruses

Herpesviruses: Latency Defines the Family

The herpesviruses share a defining characteristic: after primary infection, they establish lifelong latent infection. The virus persists in specific cell types, reactivating periodically to cause recurrent disease. There are eight human herpesviruses, and understanding where each hides and what it does when it awakens is the key to mastering this family.

Structure: all herpesviruses are large, enveloped, double-stranded DNA viruses with an icosahedral nucleocapsid. During latency the viral genome persists as a circular episome in the nucleus with minimal gene expression. Reactivation follows immunosuppression, stress, UV exposure, or fever.

Subfamilies and latency sites:

Alphaherpesvirinae (HSV-1, HSV-2, VZV): latent in sensory neurons (dorsal root / trigeminal ganglia)
Betaherpesvirinae (CMV, HHV-6, HHV-7): latent in myeloid/lymphoid cells (monocytes, CD4 T cells)
Gammaherpesvirinae (EBV, HHV-8): latent in lymphocytes (B cells mostly; HHV-8 also in endothelial cells)

A quick reference table for latency:

Virus	Latency site	Classic disease
HSV-1	Trigeminal ganglion	Oral herpes, encephalitis (temporal lobe)
HSV-2	Lumbosacral ganglia (S2-S5)	Genital herpes, Mollaret meningitis
VZV	Dorsal root ganglia	Chickenpox, shingles
EBV	B cells (via CD21)	Mono, Burkitt, NPC, PTLD
CMV	Monocytes, myeloid progenitors	Mono-like, transplant disease, congenital
HHV-6	CD4+ T cells	Roseola
HHV-7	CD4+ T cells	Roseola (less common)
HHV-8	B cells, endothelial cells	Kaposi sarcoma, PEL, MCD

HHV-6 can integrate into host chromosomes (telomeric integration) and be inherited vertically. About 1% of the population has chromosomally integrated HHV-6 (ciHHV-6), which causes persistently elevated blood PCR that can be misread as active infection - distinguish with hair follicle PCR.

HSV-1 and HSV-2 both cause vesicular lesions, but HSV-1 classically affects the orolabial area while HSV-2 classically affects the genital area (though either can infect either site). After primary infection, the viruses travel up sensory nerves and establish latency in ganglia - trigeminal ganglia for oral infection, sacral ganglia for genital infection. Reactivation produces the recurrent “cold sores” or genital outbreaks patients experience for life. HSV-1 also causes herpes keratitis (corneal infection that can lead to blindness) and is the most common cause of sporadic encephalitis in adults - a devastating infection that is treatable if recognized and treated promptly with IV acyclovir.

CNS syndromes are a high-yield distinction. HSV-1 causes encephalitis (temporal lobe predilection, the most common sporadic viral encephalitis in adults). HSV-2 causes meningitis, including Mollaret meningitis - a recurrent benign lymphocytic meningitis with episodes lasting 2-5 days separated by symptom-free intervals. Memory hook: HSV-1 = ONE brain (encephalitis, cerebral); HSV-2 = TWO meninges.

HSV-1 encephalitis classically involves bilateral necrotizing lesions of the anterior temporal lobes (and inferior frontal / insular cortex), thought to reflect spread from trigeminal ganglion along tentorial branches or from olfactory pathways. MRI shows T2/FLAIR hyperintensity with hemorrhagic necrosis. Presentation: fever, headache, confusion, aphasia, seizures, behavioral changes. Mortality without treatment exceeds 70%.

Other HSV clinical syndromes:

Herpetic whitlow - painful vesicular lesion on finger/hand, historically in dentists and respiratory therapists. Do not incise and drain - I&D is contraindicated because it spreads infection.
Erythema multiforme - HSV (usually HSV-1) and Mycoplasma pneumoniae are the two classic infectious triggers. EM appears 1-3 weeks after a herpes outbreak. Recurrent EM is almost always HSV-associated; prophylactic acyclovir prevents it.
Neonatal HSV - acquired at vaginal delivery from active maternal genital lesions. Three patterns: SEM (skin/eye/mouth), CNS, disseminated. Disseminated disease carries ~30% mortality even with treatment. Cesarean delivery is indicated if active genital lesions are present at delivery. HSV-2 is the most common cause, but neonatal HSV-1 is increasing.

HSV diagnostics:

CSF PCR is the gold standard for HSV encephalitis (>95% sensitivity, ~99% specificity). Can be negative very early (within 72 hours) - if suspicion is high, repeat and treat empirically with IV acyclovir.
Viral culture: HSV grows quickly, within 1-5 days, on MRC5 (human fetal lung fibroblasts), Vero, A549, or rabbit kidney cells. Replaced by PCR for diagnosis but still used for susceptibility testing.
Tzanck smear - scrape vesicle base, stain with Wright/Giemsa, look for multinucleated giant cells with the Three M’s: Moulding, Margination, Multinucleation. Cannot distinguish HSV from VZV. Sensitivity 60-70%.
Serology: not useful for diagnosis of acute lesions because seroprevalence is high (50-80% for HSV-1, 15-25% for HSV-2). Type-specific IgG (glycoprotein G-based) can distinguish HSV-1 from HSV-2 for counseling.

Treatment and resistance: Acyclovir and its prodrug valacyclovir are first-line. Acyclovir is a guanosine analog that must be activated by viral thymidine kinase (TK) and then inhibits viral DNA polymerase. Penciclovir/famciclovir work similarly. TK mutations are the most common mechanism of acyclovir resistance. For TK-resistant HSV/VZV, use foscarnet (pyrophosphate analog, directly inhibits DNA polymerase, TK-independent) or cidofovir.

Varicella-zoster virus (VZV) causes chickenpox (varicella) in primary infection - a generalized vesicular rash that appears in crops. The virus then establishes latency in dorsal root ganglia throughout the spine. Decades later, reactivation produces shingles (zoster) - a painful vesicular rash in a dermatomal distribution, limited to the distribution of one sensory nerve. Post-herpetic neuralgia - chronic pain persisting after the rash heals - is a common and debilitating complication.

Chickenpox distribution and stages: rash begins on face/scalp and spreads centrifugally to trunk and extremities, sparing palms and soles. The hallmark is lesions in different stages simultaneously (“crops”) - macules, papules, vesicles, pustules, and crusts all coexisting. The individual vesicle is classically described as a dewdrop on a rose petal. This asynchronous, trunk-predominant pattern distinguishes varicella from smallpox (synchronous, extremity/face predominant).

Zoster reactivation syndromes:

Classic zoster: unilateral dermatomal vesicles, most often thoracic (T3-L3). Prodromal burning/tingling precedes the rash.
Postherpetic neuralgia: neuropathic pain persisting >90 days after rash onset. Affects 10-20% of zoster patients, >50% in those over 60. Treatments: gabapentin, pregabalin, TCAs, lidocaine patches. The Shingrix vaccine (recombinant glycoprotein E + AS01B adjuvant) is >90% efficacious and recommended for adults >50.
Herpes zoster ophthalmicus - V1 distribution, sight-threatening.
Ramsay Hunt syndrome - VZV reactivation in the geniculate ganglion of CN VII. Triad: ipsilateral facial palsy, vesicles in the ear/canal/TM, and otalgia. Plus hearing loss, tinnitus, vertigo, dysgeusia. Worse prognosis than idiopathic Bell palsy; treat with acyclovir + corticosteroids.
Disseminated zoster - in immunocompromised, >2 dermatomes or visceral involvement.

VZV diagnostics:

PCR is the best test - vesicle fluid (swab the base of a vesicle), CSF for CNS disease, BAL for pneumonia, blood for disseminated disease.
VZV grows slowly in culture (1-4 weeks) on MRC5 fibroblasts - in contrast to HSV’s 1-5 days. VZV is cell-associated (doesn’t release much free virus), so specimens must be inoculated quickly; frozen specimens are suboptimal.
DFA on vesicle fluid is rapid but less sensitive than PCR.
IgG serology is useful for confirming immunity (healthcare workers, pregnant women with exposure); IgM is unreliable due to cross-reactivity with HSV.
Histology shows the same Three M’s (moulding, margination, multinucleation) and Cowdry type A inclusions as HSV - clinical context distinguishes them.

Epstein-Barr virus (EBV) infects B lymphocytes and causes infectious mononucleosis - fever, pharyngitis, lymphadenopathy, and the reactive T cells called “atypical lymphocytes.” EBV establishes latency in B cells. In immunocompromised patients, EBV-driven B cell proliferation can cause post-transplant lymphoproliferative disorder (PTLD) or lymphoma. EBV is also associated with nasopharyngeal carcinoma and Burkitt lymphoma.

Transmission and entry: EBV spreads mostly via saliva (the “kissing disease”), infecting oropharyngeal epithelium first, then B cells. It enters B cells via CD21 (complement receptor 2, CR2), which binds the viral gp350/220 glycoprotein. Shedding in saliva persists for months and then intermittently for life.

Infectious mononucleosis clinical features: fever, exudative pharyngitis, posterior cervical lymphadenopathy (posterior > anterior - a clue against bacterial pharyngitis), fatigue, hepatosplenomegaly. Spleen rupture is a feared complication - avoid contact sports for 3-4 weeks. A maculopapular rash after ampicillin/amoxicillin occurs in ~90% of mono patients given aminopenicillins and is a classic board association. Labs: lymphocytosis with >10% atypical lymphocytes, elevated transaminases.

Atypical lymphocytes (Downey type II cells) are reactive CD8+ T cells, not infected B cells. Large, abundant basophilic cytoplasm that scallops around adjacent RBCs. Also seen with CMV, toxoplasmosis, acute HIV, and drug reactions - most prominent with EBV.

Cold agglutinin association: EBV produces IgM anti-i (lowercase, fetal/cord antigen). Memory trick: “i” for Infectious mononucleosis. Contrast with Mycoplasma pneumoniae, which produces anti-I (uppercase) - “I have an atypical pneumonia.”

EBV-associated malignancies (reactivation, especially in immunocompromised):

PTLD (post-transplant lymphoproliferative disorder) - most common EBV-associated malignancy in transplant
Burkitt lymphoma (endemic/African form nearly 100% EBV+; sporadic less so) - jaw mass, t(8;14) c-MYC
Hodgkin lymphoma (mixed-cellularity subtype ~70% EBV+)
Nasopharyngeal carcinoma (southern China, Southeast Asia)
NK/T-cell lymphoma, nasal type (nearly 100% EBV+)
Primary CNS lymphoma in AIDS
Smooth muscle tumors in immunosuppressed children
Oral hairy leukoplakia (tongue, AIDS)

X-linked lymphoproliferative disease (Duncan disease): SH2D1A / SAP gene mutation causing fulminant primary EBV infection in young boys, with HLH, B-cell lymphoma, and dysgammaglobulinemia. SAP is needed for normal NK/T-cell cytotoxicity against EBV-infected B cells. Treatment: HSCT.

Heterophile antibody test (Monospot)

The Monospot detects heterophile IgM antibodies produced during EBV infection that cross-react with sheep/ox/horse RBC antigens. Modern method is a Davidsohn differential absorption: patient serum is split and absorbed with guinea pig kidney (removes Forssman antibodies but not EBV heterophiles) or beef RBC stroma (removes EBV heterophiles). Persistence after guinea pig absorption but removal by beef absorption = EBV heterophile antibody.

Sensitivity increases with age:

<4 years: <20%
4 years children: ~40%
Adults: ~80%

So a negative Monospot in a young child does NOT rule out EBV - use EBV-specific serology.

False positives: acute HIV, rubella, SLE, lymphoma, hepatitis, malaria (~2-3%).

EBV-specific serology (antibodies the patient makes against EBV):

Stage	VCA IgM	VCA IgG	EA	EBNA
Acute mono	+	+/-	+	-
Recent/convalescent	+/-	+	+	+/-
Past infection	-	+	-	+
Burkitt reactivation	-	+ (high)	+ (EA-R)	+
NPC	-	+ (high, IgA)	+ (EA-D)	+

Key points: EBNA appears last (6-12 weeks) and persists for life, so VCA IgM+ / EBNA- = acute infection (most reliable combination). VCA IgG persists lifelong. EA is transient in acute infection; its return suggests reactivation.

Early antigen has two patterns: EA-diffuse (nucleus + cytoplasm, associated with acute mono and NPC, not destroyed by methanol) and EA-restricted (cytoplasm only, associated with Burkitt and chronic/reactivated EBV, destroyed by methanol fixation). NPC is characterized by high VCA IgA reflecting mucosal involvement.

Tissue detection: EBV produces no cytopathic effect in culture - instead it immortalizes B cells into lymphoblastoid cell lines (which is how it was discovered). So culture is not used for diagnosis.

EBER-ISH (EBV-encoded RNA in situ hybridization) is the gold standard for tissue detection. EBER is expressed at 10^6-107 copies per cell in all latency patterns, making it extremely sensitive. Latency typing by immunohistochemistry:

Latency I: EBER+, EBNA1 only - Burkitt lymphoma
Latency II: EBER+, EBNA1 + LMP1, LMP2 - Hodgkin, NPC, NK/T
Latency III: EBER+, all EBNAs + all LMPs - PTLD, immunodeficiency-associated lymphomas

Quantitative blood EBV PCR is used for monitoring (PTLD risk in transplant, viral load for primary CNS lymphoma in AIDS).

Cytomegalovirus (CMV) is usually asymptomatic in immunocompetent people or causes a mononucleosis-like syndrome. It establishes latency in monocytes and lymphocytes. In immunocompromised patients - AIDS, transplant recipients - CMV reactivation causes retinitis (can cause blindness), colitis, esophagitis, and pneumonitis. Congenital CMV (the most common congenital infection) can cause hearing loss, microcephaly, and developmental delay.

CMV mononucleosis is heterophile-negative (distinguishing it from EBV mono), with less pharyngitis/lymphadenopathy and more hepatitis. Atypical lymphocytes still occur. Seroprevalence ranges from 40-100% depending on population.

CMV disease by host:

Transplant: CMV is the most important viral pathogen. D+/R- (donor positive, recipient negative) is the highest-risk mismatch for primary disease. Reactivation also common.
AIDS: disease emerges when CD4 <50 - classic presentations are CMV retinitis (blindness), colitis, esophagitis.
Congenital CMV: periventricular calcifications, microcephaly, hepatosplenomegaly, jaundice, petechiae (blueberry muffin baby), chorioretinitis, and sensorineural hearing loss (the leading infectious cause of childhood SNHL). Most severe with primary maternal infection in the first trimester.

Histology - the owl eye: CMV produces large basophilic intranuclear inclusions surrounded by a clear halo (owl eye) PLUS smaller granular basophilic cytoplasmic inclusions, all within a dramatically enlarged cell (cytomegaly). The combination of nuclear owl eye + cytoplasmic inclusions + cytomegaly is pathognomonic. CMV does NOT show the Three M’s.

CMV diagnostics:

Quantitative blood CMV DNA PCR is the standard for diagnosis and monitoring. Rising viral load in transplant patients triggers preemptive therapy.
Tissue biopsy with owl-eye inclusions confirms end-organ disease.
Urine or saliva PCR within the first 3 weeks of life diagnoses congenital CMV (after 3 weeks, positive results could represent postnatal acquisition).
pp65 antigenemia (historical) - stain peripheral leukocytes for the CMV tegument protein. Largely replaced by PCR. Labor-intensive, affected by neutropenia, requires processing within 6 hours.
Serology: limited utility because seroprevalence is high. Useful for pre-transplant screening (D/R status), diagnosing primary infection by seroconversion or 4-fold IgG rise, or IgG avidity (low avidity = recent primary infection - especially important in pregnancy). CMV IgM alone is unreliable (can persist months; cross-reactivity with EBV, rheumatoid factor).
Shell vial culture with fluorescent antibody is faster than conventional culture but has lower sensitivity than PCR.

CMV treatment:

Ganciclovir (IV) / valganciclovir (oral prodrug) - first line. Must be phosphorylated by viral UL97 kinase, then inhibits the CMV DNA polymerase (UL54). Main toxicity: myelosuppression (neutropenia, thrombocytopenia).
Foscarnet - pyrophosphate analog, directly inhibits UL54 polymerase, does NOT require UL97 activation. Nephrotoxicity, hypocalcemia, hypomagnesemia.
Cidofovir - nucleotide analog, also UL97-independent. Nephrotoxic.
Letermovir - CMV terminase inhibitor, used for prophylaxis in HSCT.
Maribavir - UL97 kinase inhibitor, an option for resistant CMV.

Resistance genetics:

==UL97 mutations == cause resistance to ganciclovir only (can’t activate it). Most common resistance mechanism. Switch to foscarnet.
UL54 mutations (DNA polymerase) can cause resistance to ganciclovir, foscarnet, and/or cidofovir depending on the specific mutation.
Resistance testing by NGS of UL97/UL54 from blood (requires viral load ≥1000 copies/mL). Indications: rising viral load or clinical progression despite therapy.

HHV-6 and HHV-7 cause roseola (exanthem subitum, sixth disease) in children 6 months to 2 years. Classic pattern: high fever (often >40C) for 3-5 days, then rash as fever resolves. Rose-pink maculopapular rash starts on trunk, spreads peripherally. HHV-6 is the most common cause of febrile seizures in children (~30% of first episodes). In HSCT patients, HHV-6 reactivation (30-50% of recipients, 2-4 weeks post-transplant) can cause limbic encephalitis (altered mental status, seizures, memory impairment) and delay engraftment.

HHV-8 (KSHV) primary infection is usually asymptomatic. Transmission: saliva (major route in endemic areas like sub-Saharan Africa), sexual contact (especially MSM), transplantation. Disease emerges with immunosuppression.

HHV-8 causes three malignancies:

Kaposi sarcoma - vascular neoplasm with four epidemiologic forms (classic/Mediterranean, endemic/African, iatrogenic/transplant, AIDS-associated). Histology: spindle cells forming slit-like vascular spaces with extravasated RBCs and hemosiderin. Diagnosis: HHV-8 LANA-1 immunohistochemistry (stippled nuclear staining) - distinguishes from bacillary angiomatosis, angiosarcoma, pyogenic granuloma.
Primary effusion lymphoma (PEL) - B-cell lymphoma presenting as body cavity effusions without a solid mass.
Multicentric Castleman disease (MCD) - B-cell lymphoproliferative disorder driven by viral IL-6 (vIL-6), presenting with fever, night sweats, lymphadenopathy, cytopenias, elevated CRP. First-line treatment: rituximab.

HHV-8 oncogenesis: LANA (latency-associated nuclear antigen) inactivates p53 and Rb, and HHV-8 encodes viral homologs of cellular genes: vCyclin, vFLIP (anti-apoptotic), vGPCR (constitutively active receptor driving angiogenesis), and vIL-6.

HSV and VZV diagnostic tests: PCR is the most sensitive method for detecting HSV in CSF (for encephalitis) and for typing genital lesions. Direct fluorescent antibody (DFA) and Tzanck smear are less sensitive but faster. The Tzanck smear - scraping the base of a vesicle, staining with Wright or Giemsa, and looking for multinucleated giant cells - can confirm herpesvirus infection but cannot distinguish HSV from VZV. Viral culture demonstrates characteristic cytopathic effect. Type-specific serology (IgG to HSV-1 or HSV-2) is useful for determining prior exposure but not for diagnosing acute lesions.

EBV and infectious mononucleosis diagnosis: The classic triad is fever, pharyngitis, and lymphadenopathy - but what makes the diagnosis is the peripheral blood smear. Atypical lymphocytes are reactive CD8+ T cells responding to EBV-infected B cells. They’re large with abundant basophilic cytoplasm that seems to hug adjacent red cells. The heterophile antibody test (Monospot) detects antibodies that agglutinate sheep or horse red cells - a curious phenomenon caused by polyclonal B cell activation. It’s positive in about 85% of cases but may be negative early in illness or in young children. EBV-specific serology provides definitive diagnosis: VCA-IgM indicates acute infection, VCA-IgG indicates past exposure, and EBNA (Epstein-Barr nuclear antigen) antibodies develop late and indicate past infection.

CMV diagnosis depends on the clinical context: In transplant patients, quantitative PCR (viral load) is the standard for monitoring - rising viral load triggers preemptive therapy. The pp65 antigenemia assay detects CMV protein in peripheral blood leukocytes and was widely used before PCR became standard. Histopathology showing the characteristic “owl’s eye” inclusions - large cells with both nuclear and cytoplasmic inclusions - is diagnostic in tissue specimens.

Hepatitis B Virus (HBV)

Hepadnavirus - an enveloped partially double-stranded DNA virus that replicates via an RNA intermediate using reverse transcriptase. The only DNA hepatitis virus (all others are RNA). After entry, the partially dsDNA genome is converted to covalently closed circular DNA (cccDNA) in the nucleus, which templates viral mRNA and pregenomic RNA. The pregenomic RNA is reverse-transcribed back to DNA by the viral polymerase. This is Baltimore Group VII. The persistent nuclear cccDNA is the reason HBV can never be fully cleared - it persists silently even after apparent recovery, creating reactivation risk with immunosuppression (rituximab, HSCT).

HBV produces three particle types in blood:

Dane particle (42 nm) - complete infectious virion with genome, polymerase, HBcAg core, enveloped in HBsAg
Spheres (22 nm) - HBsAg-only particles, non-infectious, outnumber Dane particles 1000:1 (act as immune decoys)
Filaments/tubules - elongated HBsAg particles, also non-infectious

Key proteins and their locations:

HBsAg - envelope (surface antigen)
HBcAg - core nucleocapsid, never free in serum (only detectable by IHC on tissue, nuclear and cytoplasmic staining)
HBeAg - secreted form of core antigen, free in serum, marker of active replication and high infectivity. Precore G1896A mutants cannot make HBeAg despite active replication.
HBV polymerase - core, has reverse transcriptase, DNA polymerase, and RNase H activities
HBx - regulatory transactivator, linked to HCC

Transmission (HBV is 50-100x more infectious than HIV and survives 7+ days on surfaces): sex, blood (transfusion, IVDU, needlestick), vertical (perinatal, most common in endemic areas).

Hepatitis D (delta) is a defective virus that requires HBV for its envelope (HBsAg). Enveloped, negative-sense ssRNA, smallest genome of any animal virus (~1.7 kb). Coinfection (simultaneous HBV + HDV) is usually self-limited. Superinfection of a chronic HBV carrier has high chronicity (~70-80%) and accelerated progression to cirrhosis - the most severe form of viral hepatitis.

Chronicity is inversely related to age at infection:

Neonates: ~90%
Children (1-5 years): ~30-50%
Adults: <10%

Chronic HBV = HBsAg positive for >6 months. Extrahepatic manifestations of chronic HBV include polyarteritis nodosa (immune complex vasculitis), membranous nephropathy (most common glomerular lesion), cryoglobulinemia (type II/III), and aplastic anemia.

Serologic timeline of acute infection:

HBV DNA - appears first (days after exposure)
HBsAg - ~4 weeks
HBeAg - shortly after HBsAg (high replication)
Anti-HBc IgM - 6-10 weeks, with symptoms
HBeAg loss and anti-HBe appearance
HBsAg disappears (~3-4 months)
Window period (anti-HBc IgM only positive marker)
Anti-HBs - last, immunity
Anti-HBc IgG persists lifelong

Serology interpretation (master this table):

Marker	Acute	Chronic (high infectivity)	Chronic (low infectivity)	Window	Resolved	Vaccinated
HBsAg	+	+	+	-	-	-
Anti-HBs	-	-	-	-	+	+
Anti-HBc IgM	+	-	-	+	-	-
Anti-HBc total	+	+	+	+	+	-
HBeAg	+	+	-	-	-	-
Anti-HBe	-	-	+	+	+	-
HBV DNA	+	high	low/none	+/-	-	-

Key diagnostic principles:

Vaccinated = anti-HBs positive, anti-HBc NEGATIVE (vaccine contains only recombinant HBsAg; no core exposure)
Natural immunity = anti-HBs positive AND anti-HBc positive
Anti-HBc IgM is the key marker of acute infection - stays positive through the window period when HBsAg has cleared but anti-HBs hasn’t yet appeared
HBeAg+ = high infectivity, high viral load (except in precore mutants)
Isolated anti-HBc positivity (anti-HBc+, HBsAg-, anti-HBs-) has four interpretations: window period, remote past infection with waning anti-HBs, occult low-level chronic HBV, or false positive. Workup: anti-HBc IgM and HBV DNA.

Quantitative HBV DNA is used for treatment monitoring. Current first-line antivirals are entecavir and tenofovir - nucleos(t)ide analogs that inhibit the HBV reverse transcriptase/polymerase. Because HBV RT and HIV RT are targeted by similar drugs, some HIV NRTIs (tenofovir, lamivudine, entecavir) also treat HBV - stopping these in HIV/HBV coinfected patients can cause a severe HBV flare.

Other DNA Viruses

Adenovirus - The Cause of Many Common Syndromes

Adenoviruses are non-enveloped, double-stranded linear DNA viruses with an icosahedral capsid. Characteristic fiber proteins project from the 12 vertices - these bind the Coxsackievirus-Adenovirus Receptor (CAR) on host cells for attachment. Over 70 serotypes exist, and different serotypes have tropism for different tissues. Being non-enveloped makes adenovirus environmentally resistant, enabling both respiratory and fecal-oral transmission.

Serotype to syndrome (high yield):

Serotypes 1-7: respiratory infections (pharyngitis, pneumonia). Types 3, 4, 7, 14 most common.
Serotypes 8, 19, 37: epidemic keratoconjunctivitis (EKC)
Serotypes 11 and 21: hemorrhagic cystitis
Serotypes 40 and 41: enteric, cause gastroenteritis
Serotypes 1, 2, 5: disseminated disease in immunocompromised (especially HSCT)

Respiratory disease: Adenovirus causes pharyngitis, often with conjunctivitis (pharyngoconjunctival fever), and can cause severe pneumonia, particularly in military recruits, infants, and immunocompromised patients. A live oral vaccine for types 4 and 7 is used in the military and has dramatically reduced outbreaks in crowded barracks.

Epidemic keratoconjunctivitis: highly contagious, bilateral (starts unilateral, spreads within days), watery discharge with preauricular lymphadenopathy. Subepithelial corneal infiltrates can develop 1-3 weeks later (immune-mediated, can persist months). Classically spreads through ophthalmology clinics via tonometers and hands, and through swimming pools.

Hemorrhagic cystitis: gross hematuria, dysuria. Other causes to remember: BK virus, cyclophosphamide/ifosfamide. Adenovirus hemorrhagic cystitis is usually self-limiting in immunocompetent hosts but can be severe and prolonged in transplant patients.

Gastroenteritis: enteric serotypes 40 and 41 are the second most common viral cause of childhood diarrhea after rotavirus. Adenovirus and rotavirus diarrhea can both precipitate intussusception via mesenteric lymphoid hyperplasia.

Disseminated disease in immunocompromised (especially HSCT, pediatric solid organ transplant): hepatitis (most common organ), pneumonitis, colitis, hemorrhagic cystitis, encephalitis. Mortality 50-80% in HSCT. Monitor with weekly blood adenovirus PCR. Treatment: cidofovir or brincidofovir, and reduce immunosuppression.

Diagnostics:

PCR is the primary method - often part of multiplex respiratory or GI panels (e.g., BioFire FilmArray). Quantitative blood PCR for monitoring viremia in transplant.
Culture: most serotypes grow on A549, HEK-293, or HeLa over 5-10 days. CPE looks like grape-like clusters of rounded, refractile cells. Serotypes 40 and 41 do not grow in standard culture - they need Graham 293 cells, which is why enteric adenovirus was underdiagnosed before PCR.
Histology: smudge cells - intranuclear inclusions with basophilic, homogeneous smudged chromatin that obliterates nuclear detail. Early lesions can show “two-toned” nuclei (eosinophilic center with basophilic rim). Seen in lung, liver, and bladder. No cytoplasmic inclusions, no syncytia.
DFA and rapid antigen: ~50-70% sensitivity, inferior to PCR.

Parvovirus B19 - The Red Cell Specialist

Parvovirus B19 is a small, non-enveloped, single-stranded DNA virus (the smallest DNA virus infecting humans) with a remarkably specific tropism: it infects and destroys erythroid progenitor cells. This tropism explains all of its clinical manifestations. Because it’s non-enveloped, it is NOT inactivated by solvent/detergent treatment of blood products - hence transfusion-transmitted risk. It requires dry heat or nanofiltration (15 nm) for inactivation.

Pathogenesis: Parvovirus B19 binds to the P antigen (globoside, Gb4) on red blood cell precursors in the bone marrow. Individuals with the rare p phenotype (lacking P antigen) are naturally resistant to parvovirus B19. The same P antigen is the target of the Donath-Landsteiner antibody in paroxysmal cold hemoglobinuria. The virus replicates in the nucleus of normoblasts/proerythroblasts and kills them as new virions are released. In normal individuals with healthy bone marrow, this destruction causes a temporary reticulocytopenia lasting 7-10 days, but the long lifespan of circulating RBCs (120 days) means this goes unnoticed - the marrow recovers before anemia develops.

Epidemiology: peak incidence at ages 3-7 years. Outbreaks occur in late winter and spring, with epidemics every 3-7 years. Patients are most infectious before the rash appears - by the time the slapped cheek rash is visible, viremia has cleared and the child is no longer contagious.

Erythema infectiosum (fifth disease) is the classic childhood illness - a mild febrile illness followed by the “slapped cheek” rash (bright red erythema of the cheeks) and then a lacy, reticular rash on the trunk and extremities. The rash represents the immune response to the virus and appears as viremia clears; by the time the rash appears, the child is no longer contagious.

Aplastic crisis occurs when parvovirus B19 infects someone with underlying hemolytic anemia (sickle cell disease, hereditary spherocytosis, thalassemia). These patients depend on accelerated red cell production to compensate for their shortened RBC lifespan. When parvovirus halts erythropoiesis, their hemoglobin plummets precipitously - a potentially life-threatening aplastic crisis requiring transfusion.

Hydrops fetalis occurs when a seronegative pregnant woman is infected during pregnancy. The fetus lacks immunity and cannot clear the virus. Massive destruction of fetal red cell precursors causes severe fetal anemia, leading to high-output heart failure, ascites, and hydrops. In severe cases, fetal death results. Intrauterine transfusion can be life-saving.

Chronic anemia in immunocompromised patients occurs because they cannot mount the antibody response needed to clear the virus. Persistent infection causes ongoing pure red cell aplasia and severe, chronic anemia requiring transfusions. Treatment with IVIG provides anti-parvovirus antibodies that clear the infection.

Bone marrow morphology is pathognomonic: giant pronormoblasts (“lantern cells”) with glassy eosinophilic nuclear inclusions surrounded by a clear zone, and absent downstream erythroid maturation. Diagnosis can also be made by PCR of blood or bone marrow (most sensitive), immunohistochemistry for VP1/VP2 capsid proteins, or in situ hybridization.

Serology: IgM appears first (persists 2-3 months, indicates acute); IgG confers lifelong immunity. Interpretation: IgM+/IgG- = acute; IgM+/IgG+ = recent; IgM-/IgG+ = past/immune. Hydrops fetalis occurs only with primary maternal infection.

Human Papillomavirus (HPV) - Warts and Cancer

HPV is a diverse family of over 200 genotypes that infect cutaneous and mucosal squamous epithelium. The clinical significance varies dramatically by type: some cause benign warts, while others cause cancer. Understanding the molecular basis of HPV-driven carcinogenesis explains why vaccines targeting high-risk types are so important. HPV is a non-enveloped, circular double-stranded DNA virus. HPV cannot be grown in standard cell culture (requires differentiating epithelium), so diagnosis is molecular/histologic.

Two infection patterns - master this distinction:

Permissive (episomal) infection - viral DNA stays circular and non-integrated, active virion production, cell lysis. E2 protein intact, suppressing E6/E7. Result: benign warts and condylomata. Associated with low-risk types (6, 11).
Non-permissive (integrated) infection - viral DNA integrates into host chromosome. Integration disrupts E2, releasing E6/E7 from suppression. Overexpression of E6/E7 drives dysplasia and cancer. Associated with high-risk types (16, 18, 31, 33, 45).

Low-risk types (6 and 11) cause genital warts (condyloma acuminata) - exophytic, cauliflower-like lesions on the anogenital epithelium. These types do not cause cancer but cause significant morbidity. They can also cause laryngeal papillomatosis when transmitted to neonates during delivery.

High-risk types (especially 16 and 18) are oncogenic. These types are responsible for the vast majority of cervical cancers, a substantial proportion of oropharyngeal (tonsil, base of tongue) cancers, and anal cancers. The mechanism of carcinogenesis involves two viral oncoproteins:

E6 protein binds to and promotes degradation of p53, the tumor suppressor that normally arrests the cell cycle when DNA damage is detected. Without p53, damaged cells continue to proliferate.

E7 protein inactivates the retinoblastoma protein (Rb), which normally prevents cells from entering S phase inappropriately. With Rb inactivated, cells divide uncontrollably.

Together, E6 and E7 override the two major tumor suppressor pathways, allowing accumulation of mutations and progression to cancer over years to decades of persistent infection.

The vaccine targets the capsid proteins of high-risk types (16, 18, and others in the 9-valent vaccine) and low-risk types (6, 11). By preventing infection with these types, the vaccine prevents the cancers and warts they cause - one of the few vaccines that prevents cancer.

HPV type to disease (memorize the mappings):

Common/plantar warts (verruca vulgaris) - HPV 1, 2, 4 (plus 7 for butcher warts)
Verruca plana (flat warts) - HPV 3, 10
Genital warts (condyloma acuminata) - HPV 6, 11 (low-risk)
Recurrent respiratory papillomatosis (laryngeal papillomatosis) - HPV 6, 11. Juvenile-onset acquired from infected birth canal; adult-onset sexually acquired.
Cervical HSIL/SCC and adenocarcinoma - HPV 16, 18 (plus 31, 33, 45, 52, 58). HPV 16 is most common in cervical SCC; HPV 18 is most common in cervical adenocarcinoma. Together 16+18 cause ~70% of cervical cancers.
HPV 16 also the dominant driver of oropharyngeal (tonsil, base of tongue) SCC, anal SCC, vulvar/vaginal/penile SCC.

Epidermodysplasia verruciformis (EV): autosomal recessive disease with TMC6/TMC8 (EVER1/EVER2) mutations conferring susceptibility to HPV 5 and 8. Widespread flat wart-like lesions and pityriasis versicolor-like plaques in sun-exposed areas; 30-60% develop SCC by age 40-50. Histology shows characteristic bluish-gray cytoplasmic staining of keratinocytes (viral protein accumulation) and prominent keratohyalin granules.

Diagnostics and IHC:

High-risk HPV shows block-positive p16 staining (diffuse strong nuclear + cytoplasmic). Mechanism: E7-mediated Rb inactivation drives compensatory p16 overexpression. Used for CIN2+, HPV-associated oropharyngeal SCC.
Cervical HPV testing: hrHPV molecular testing of cervical cytology specimens detects the 14 high-risk types. Cotest (cytology + hrHPV) or primary HPV testing are accepted screening paradigms.
Koilocytes - perinuclear halos with nuclear atypia - are the classic cytologic finding in productive infection.

Human Polyomaviruses - JC, BK, and Merkel

The human polyomaviruses are small, non-enveloped, circular dsDNA viruses. Primary infection occurs in childhood (usually asymptomatic, seroprevalence 70-90% by adulthood), with lifelong latency. Disease emerges with immunosuppression. The name “polyoma” comes from their ability to cause multiple tumors in experimental animals. Their early gene region encodes large T and small t antigens that interact with p53 and Rb - directly relevant to their oncogenic potential.

Three clinically important human polyomaviruses: JC virus (PML), BK virus (nephropathy, hemorrhagic cystitis), and Merkel cell polyomavirus (Merkel cell carcinoma).

JC Virus - The Demyelinating Polyomavirus

JC virus (named for John Cunningham, the patient from whom it was first isolated) is a polyomavirus that infects most people asymptomatically in childhood and establishes latent infection in the kidneys. In profoundly immunocompromised patients - particularly those with AIDS or on certain immunosuppressive therapies like natalizumab (for multiple sclerosis), rituximab, or other monoclonal antibodies - the virus can reactivate and cause progressive multifocal leukoencephalopathy (PML).

PML pathogenesis: JC virus reactivates and travels to the central nervous system, where it infects oligodendrocytes - the cells that produce myelin. Multifocal areas of demyelination appear throughout the brain. JCV also infects astrocytes, producing bizarre, pleomorphic reactive astrocytes on histology. The clinical presentation is subacute neurological decline with cognitive impairment, visual disturbances, weakness, and ataxia.

MRI and CSF findings - memorable because they’re “quiet”:

MRI: non-enhancing, non-mass-forming white matter lesions with T2/FLAIR hyperintensity, predilection for parieto-occipital regions. U-fibers involved (unlike MS). No ring enhancement (unlike toxoplasmosis or CNS lymphoma).
CSF: typically normal or near-normal glucose, protein, and cell count. This “bland” CSF contrasts with bacterial meningitis and viral encephalitis.
CSF JCV PCR - sensitivity 72-92%, specificity >99%. Negative PCR does not exclude PML.
Histology: enlarged oligodendrocytes with ground-glass nuclear inclusions, foamy macrophages.

PML is almost always fatal without immune reconstitution. In AIDS patients, antiretroviral therapy can restore immunity. In patients on immunosuppressive therapy, discontinuing the offending agent is essential.

BK Virus - The Kidney Transplant Threat

BK virus establishes latent infection in the kidneys (renal tubular epithelium, urothelium) and CNS after childhood acquisition. Low-level BK viruria is present in 5-10% of healthy immunocompetent adults. Disease emerges primarily in transplant recipients.

BK nephropathy (BKVN) occurs in 1-10% of kidney transplants. Tacrolimus > cyclosporine for risk; ureteral stent placement is another risk factor. The virus infects tubular epithelial cells, causing tubular necrosis, interstitial inflammation (which can mimic acute rejection), and potentially graft loss. Distinguishing BKVN from rejection is critical because treatment is opposite: reduce immunosuppression for BK, increase for rejection.

BK hemorrhagic cystitis occurs mainly in HSCT recipients (allogeneic > autologous).

Diagnostics:

Urine BK PCR - sensitive screening; viruria precedes viremia by weeks.
Plasma BK PCR - more specific for clinically significant disease. Plasma BK >10,000 copies/mL is presumptive BKVN - biopsy recommended. Monitoring protocol in renal transplant: urine BK PCR monthly x 6 months, then every 3 months for 2 years.
Urine cytology: decoy cells - enlarged nuclei with ground-glass intranuclear inclusions that mimic high-grade urothelial carcinoma. Clues to viral origin: smooth nuclear membranes, homogeneous chromatin.
Kidney biopsy: tubular epithelial cells with enlarged nuclei containing basophilic ground-glass intranuclear inclusions.
SV40 IHC - the gold-standard tissue stain. The anti-SV40 large T antigen antibody cross-reacts with both BK and JC virus (~70% homology). Cannot distinguish the two - context determines which.

Treatment: reduce immunosuppression. No proven antiviral. IVIG and cidofovir have been tried but are not standard of care.

Merkel Cell Polyomavirus (MCPyV)

MCPyV is clonally integrated in ~80% of Merkel cell carcinomas - aggressive neuroendocrine skin tumors of elderly, immunosuppressed, sun-exposed patients. The MCPyV large T antigen inactivates Rb (similar to HPV E7); small T antigen inhibits PP2A. Histology: small blue round cells with neuroendocrine features. IHC: CK20 paranuclear dot pattern, synaptophysin, chromogranin positive. MCPyV has also been associated with CLL/SLL.

TORCH Infections: Congenital Infection Overview

TORCH (or TORCHES) is the classic mnemonic for pathogens causing congenital infection. These infections can have devastating consequences for the fetus and newborn.

Pathogen	Classic Features	Diagnosis
Toxoplasma	Intracranial calcifications (diffuse), chorioretinitis, hydrocephalus	Serology, PCR
Other (Syphilis, VZV, Parvovirus B19)	Syphilis: snuffles, rash, bone abnormalities; Parvovirus: hydrops fetalis	Serology
Rubella	Cataracts, congenital heart defects (PDA), sensorineural deafness, “blueberry muffin” rash	Serology (IgM)
CMV	Periventricular calcifications, sensorineural hearing loss, microcephaly	Urine PCR (shell vial culture)
HSV	Skin vesicles, encephalitis, disseminated disease; often acquired during delivery	PCR, culture
EBV	Rare congenital infection	-
Syphilis	See “Other” above	RPR/VDRL, FTA-ABS

Key distinguishing feature: Toxoplasma causes diffuse/scattered intracranial calcifications; CMV causes periventricular calcifications. This is a classic boards distinction.

CMV is the most common congenital infection and the leading infectious cause of sensorineural hearing loss in children.

49.2 RNA Viruses

Picornaviruses: Small RNA Viruses with Big Impact

The Picornaviridae - the name means “small RNA viruses” - are among the simplest viruses that infect humans, yet they cause an enormous range of disease. They’re non-enveloped (naked) positive-sense ssRNA viruses, which makes them resistant to environmental conditions, detergents, and drying. They’re acid-stable, allowing them to survive passage through the stomach. Being positive-sense means their genome can be immediately translated by host ribosomes upon entry - no need for conversion or transcription first. They’re icosahedral, about 30 nm.

Clinically important picornaviruses:

Enteroviruses: poliovirus, coxsackievirus A and B, echovirus, enterovirus D68, enterovirus 71. Parechovirus is grouped here for board purposes.
Rhinovirus: most common cause of common cold
Hepatitis A (Hepatovirus): fecal-oral acute hepatitis

Enteroviruses cause ~75% of viral (aseptic) meningitis with peak incidence in summer and early fall. CSF profile: lymphocytic pleocytosis, normal glucose, normal to mildly elevated protein. Diagnosis: CSF enterovirus PCR.

Poliovirus - The Vaccine Success Story

Poliomyelitis has been virtually eliminated from the developed world through vaccination - one of medicine’s great triumphs. But understanding the disease remains important, both historically and because polio persists in some regions and remains a bioterrorism concern.

Pathogenesis explains the clinical spectrum: Poliovirus enters through the mouth, replicates in the oropharynx and gut-associated lymphoid tissue (particularly Peyer’s patches), and is shed in feces. In most people (95%), infection stops here - asymptomatic or a nonspecific febrile illness. In some, viremia carries the virus to the central nervous system.

The critical target is the anterior horn motor neurons of the spinal cord. The virus preferentially infects and destroys these cells. Because motor neurons are post-mitotic - they can’t regenerate - destruction is permanent. The result is lower motor neuron disease: asymmetric flaccid paralysis with muscle atrophy, fasciculations, and absent reflexes. Sensation is preserved because sensory neurons are spared. This constellation - asymmetric flaccid paralysis with intact sensation - is classic for polio and distinguishes it from Guillain-Barré syndrome (which affects peripheral nerves, causing sensory and motor findings).

Bulbar polio, involving cranial nerve motor nuclei, causes dysphagia and respiratory failure. The iron lung - an image that defines mid-20th century polio epidemics - provided artificial ventilation for patients with respiratory muscle paralysis.

Two vaccines, two strategies: Jonas Salk developed the inactivated polio vaccine (IPV), which uses killed virus given by injection. It induces systemic IgG antibodies that prevent viremia and thus paralytic disease - but it doesn’t induce mucosal immunity, so vaccinated individuals can still carry and transmit the virus.

Albert Sabin developed the oral polio vaccine (OPV), which uses live attenuated virus. Given orally, it replicates in the gut, inducing both systemic IgG and mucosal IgA. This prevents both disease and transmission. However, because it’s a live virus, OPV can rarely revert to virulence, causing vaccine-associated paralytic polio (VAPP, ~1 per 2.4 million doses). The US now uses only IPV; OPV remains important in global eradication campaigns because it’s cheaper, easier to administer, and induces mucosal immunity.

Memory hook: Salk is sulking because he’s dead (Salk = killed/inactivated, IM); Sabin = oral, live.

Coxsackievirus - The Heart and Hand Virus

Coxsackieviruses are divided into groups A and B based on their effects in newborn mice - but more importantly, based on the different clinical syndromes they cause.

Coxsackie A has tropism for skin and mucous membranes. Classic diseases:

Hand-foot-mouth disease (HFMD) - usually Coxsackie A16 or Enterovirus 71, vesicles on hands, feet, and oral mucosa.
Herpangina - painful vesicles/ulcers on the posterior oropharynx (soft palate, uvula, tonsils). Distinguishes from HSV gingivostomatitis, which affects the anterior mouth.
Acute hemorrhagic conjunctivitis
Aseptic meningitis

Coxsackie B has tropism for heart, pleura, and pancreas (“B is for Body”). Classic diseases:

Viral myocarditis and pericarditis - Coxsackie B is the most common viral cause. Can progress to dilated cardiomyopathy.
Pleurodynia (Bornholm disease) - sudden severe pleuritic chest pain, often the worst pain of the patient’s life. Self-limited but mimics MI/PE.
Aseptic meningitis

Both Coxsackie A and B can cause aseptic meningitis. Enterovirus culture (Rhesus monkey kidney, MRC-5, Vero, or rhabdomyosarcoma cells) takes 4-8 days with rounded refractile degenerating CPE but has been largely replaced by PCR.

Enterovirus D68 caused a 2014 US outbreak of severe respiratory illness in children (especially asthmatics) and was linked to acute flaccid myelitis, a polio-like syndrome with anterior horn cell damage. Enterovirus 71 causes HFMD with CNS complications.

Rhinovirus and Echovirus: Rhinovirus is the most common cause of the common cold (~50-80% of colds), with over 160 serotypes (making vaccination impractical). Replicates best at 33°C, the cooler temperature of the nasal mucosa, which is why it sticks to the upper airways. Transmitted by aerosols and fomites; survives on surfaces for hours. Can trigger asthma exacerbations. Diagnosis is by multiplex PCR, which often cannot distinguish rhinovirus from enterovirus due to sequence homology - they may be reported together as “rhinovirus/enterovirus.”

Echovirus (Enteric Cytopathic Human Orphan virus - “orphan” because initially no disease was linked) is a major cause of aseptic meningitis, with >30 serotypes. Also causes rash, conjunctivitis, respiratory illness, and severe neonatal sepsis-like illness.

Hepatitis A

A picornavirus (Hepatovirus) transmitted by fecal-oral route - contaminated water, shellfish, produce, person-to-person. Crowded settings in the US: daycares are a classic reservoir (children are often asymptomatic shedders who transmit to adults). Recent outbreaks: people who use drugs, homeless populations, MSM.

Natural history:

Incubation 2-6 weeks
Symptomatic acute hepatitis: jaundice, fever, malaise, transaminases often >1000
Usually lasts <2 months
NEVER becomes chronic. No carrier state.
~5% have a relapsing course
Fulminant hepatic failure <1%
Severity increases with age (adults more often symptomatic than children)

Diagnosis and prevention: Anti-HAV IgM indicates acute infection; anti-HAV IgG indicates past infection or vaccination and confers lifelong immunity.

The hepatitis A vaccine is killed (inactivated) whole-virus, two doses 6-12 months apart, >95% effective. Recommended for all children at age 1, travelers to endemic areas, MSM, people who use drugs, patients with chronic liver disease, homeless populations. Post-exposure prophylaxis: HAV vaccine preferred for healthy adults 1-40; immunoglobulin for infants, immunocompromised, older adults.

Paramyxoviruses: Respiratory Viruses with Fusion

The paramyxoviruses are large, enveloped, negative-sense ssRNA, non-segmented viruses that share a key feature: a fusion (F) protein that allows them to spread directly from cell to cell, creating multinucleated giant cells (syncytia). This cell-to-cell spread helps them evade antibodies in the extracellular space.

Because paramyxoviruses have a non-segmented genome, they do not undergo reassortment - no antigenic shift, which is why they cause epidemics but not pandemics (unlike influenza).

Family members:

Parainfluenza (types 1-4)
Mumps
Measles (Morbillivirus)
RSV (Orthopneumovirus)
Human metapneumovirus (hMPV)
Nipah and Hendra (Henipavirus, emerging zoonotic encephalitis from bats)

Measles (Rubeola) - One of the Most Contagious Diseases

Measles is extraordinarily contagious - an infected person can transmit the virus to 90% of susceptible contacts. Before vaccination, virtually everyone was infected in childhood. The virus spreads via respiratory droplets and aerosols, initially infecting respiratory epithelium and then spreading to lymphoid tissue. Viremia disseminates the virus to skin, respiratory tract, and other organs.

The clinical course is predictable and distinctive: After a 10-14 day incubation period, prodromal illness begins with the “3 Cs” plus Koplik spots - Cough, Coryza (runny nose), Conjunctivitis, and Koplik spots. Koplik spots are pathognomonic: tiny white/bluish spots on an erythematous base, appearing on the buccal mucosa opposite the molars. They look like “grains of salt on a red background” and appear 1-2 days before the rash.

The rash emerges as the fever peaks, starting on the face and spreading downward (cephalocaudal) over 3-4 days. It’s maculopapular, eventually becoming confluent. As the rash spreads downward, it begins fading on the face. The rash represents immune-mediated clearing of infected cells - immunocompromised patients may have no rash yet severe, disseminated infection.

Complications explain why measles matters: In developed countries with good nutrition, most cases resolve uneventfully. But complications can be devastating. Pneumonia - primary viral or secondary bacterial - is the most common cause of death. Encephalitis occurs in about 1 in 1000 cases. Most terrifying is subacute sclerosing panencephalitis (SSPE), a progressive, fatal neurodegenerative disease that appears 7-10 years after measles infection, caused by persistence of defective virus in the brain. Presentation: behavioral changes, myoclonic jerks, seizures, progressive cognitive decline. EEG shows periodic high-amplitude complexes; CSF has elevated measles antibody titers and oligoclonal bands. Median survival is 1-3 years after symptom onset.

Histology: Warthin-Finkeldey giant cells are pathognomonic - large multinucleated cells with up to 100 nuclei in lymphoid tissue (tonsils, lymph nodes, appendix, Peyer patches, thymus) representing virus-induced lymphocyte fusion. Can appear before the rash. Unlike HSV/VZV/CMV, measles is the only common virus with BOTH nuclear AND cytoplasmic inclusions.

Diagnosis: PCR of throat/nasopharyngeal swab or urine (most sensitive). Measles IgM appears with the rash; 4-fold rise in IgG between acute and convalescent sera confirms. Measles is a reportable disease - contact public health on suspicion. Airborne + contact isolation.

Vitamin A supplementation reduces measles complications and mortality, particularly in malnourished children. This is one of the most effective and cheapest interventions in global health.

Measles IgM Serology Caveats: IgM may be negative in first 72 hours of rash onset (false-negative window). If clinical suspicion is high, repeat in 3-5 days. Prozone effect (antibody excess) can cause false-negative results. False-positive IgM can occur with EBV infection and rheumatoid factor.

Mumps - The Swollen Gland Virus

Mumps was once a childhood rite of passage; vaccination has made it rare. The virus infects the upper respiratory tract, spreads via viremia, and has particular tropism for glandular and nervous tissue.

Parotitis is the hallmark: Painful swelling of the parotid glands, bilateral in about 70% of cases, gives patients the classic “chipmunk cheeks” appearance. The swelling obscures the angle of the jaw. The parotid duct opening (Stensen’s duct) may be red and edematous.

Orchitis is the feared complication in adult males: In post-pubertal males, mumps virus can infect the testes, causing painful swelling, usually unilateral. The testis becomes edematous within a confined capsule (the tunica albuginea), leading to pressure necrosis. Although orchitis causes testicular atrophy in about 50% of affected testes, sterility is rare because it’s usually unilateral and the contralateral testis compensates.

Aseptic meningitis occurs commonly but is usually benign. Pancreatitis and oophoritis are less common complications.

Parainfluenza - The Croup Virus

Parainfluenza viruses are the most common cause of croup (laryngotracheobronchitis), a distinctive syndrome seen in children 6 months to 3 years old. The subglottic area - just below the vocal cords - is the narrowest part of a child’s airway. When parainfluenza infection causes inflammation and edema here, the airway narrows further, causing the classic symptoms: inspiratory stridor (the high-pitched sound of air passing through a narrowed extrathoracic airway), a “barking” cough (compared to a seal’s bark), and hoarseness.

The X-ray shows the “steeple sign” - the normally squared-off subglottic area appears tapered, like a church steeple, due to edema. Treatment involves corticosteroids (to reduce inflammation) and nebulized epinephrine (to cause vasoconstriction and reduce edema) in moderate-severe cases.

RSV (Respiratory Syncytial Virus) is the leading cause of lower respiratory tract infection in infants, particularly bronchiolitis (infection of the small airways). RSV causes epidemics every winter. In most children, it causes cold-like symptoms, but in young infants - especially premature infants - it can cause severe disease requiring hospitalization. The F (fusion) glycoprotein mediates both entry and cell-cell fusion, creating the syncytia that give the virus its name.

Culture uses HEp-2 cells (human laryngeal carcinoma) - the optimal line for RSV - with characteristic syncytial CPE; sensitivity is ~50-70%, so PCR or rapid antigen has replaced it. Histology shows cytoplasmic inclusions and syncytia with no nuclear inclusions.

RSV prophylaxis (treatment is supportive; inhaled ribavirin in severe immunocompromised cases):

Palivizumab (Synagis) - monoclonal antibody against F protein, monthly IM during RSV season. For premature infants, chronic lung disease, hemodynamically significant CHD.
Nirsevimab (Beyfortus) - long-acting monoclonal, single dose covers the whole season.
Maternal RSV vaccine (Abrysvo) given during pregnancy protects neonates.

Parainfluenza: the leading cause of croup (laryngotracheobronchitis) in children 6 months to 3 years. Types 1 and 2 cause croup; type 3 causes bronchiolitis/pneumonia in infants. Barking cough, inspiratory stridor, steeple sign on X-ray. Treatment: corticosteroids (dexamethasone) and nebulized epinephrine for moderate-severe cases.

Mumps classically targets parotid glands, pancreas, gonads, and CNS:

Parotitis (bilateral in ~70%) - most common manifestation
Orchitis in post-pubertal males (~30%, usually unilateral - sterility rare)
Oophoritis (~5%)
Pancreatitis (~5%)
Aseptic meningitis (~10%), encephalitis rarely

Prevention: MMR. Diagnosis: PCR of saliva/buccal swab; IgM serology.

Human metapneumovirus (hMPV) - described in 2001, causes RSV-like illness (bronchiolitis, pneumonia, croup) in children, elderly, and immunocompromised. Peak late winter/spring (slightly later than RSV). Detection: PCR on multiplex respiratory panels. No antiviral, no vaccine.

Rabies Virus (Rhabdovirus) - The Fatal Encephalitis

Rabies is almost unique among infectious diseases: once symptoms appear, it’s essentially 100% fatal. Fewer than 20 people have ever survived symptomatic rabies. This makes prevention - through animal control, vaccination, and post-exposure prophylaxis - absolutely critical.

The virus is a bullet-shaped, enveloped, negative-sense RNA virus (rhabdo = rod/bullet in Greek), ~75 x 180 nm. The envelope is studded with G protein spikes that bind neuronal nicotinic acetylcholine receptors, NCAM, and p75NTR.

Reservoirs:

United States (>90% wildlife): bats, raccoons, skunks, foxes. Bats are the most common source of human rabies and bat bites may be unrecognized.
Worldwide: dogs (>95% of human rabies deaths, primarily Asia and Africa, mostly children). ~59,000 annual global deaths.

Pathogenesis explains the long incubation and fatal outcome: After a bite, the virus replicates locally in muscle tissue, then enters peripheral nerves. It travels retrograde along axons toward the central nervous system - a process that can take weeks to months depending on the bite’s distance from the brain. Once the virus reaches the CNS, it replicates explosively in neurons, particularly in the limbic system, hippocampus, and brainstem. By the time symptoms appear, the brain is extensively infected.

The clinical course is harrowing: The prodrome begins with paresthesias or pain at the bite site (often the first clue) - this reflects viral replication in dorsal root ganglia. Fever, malaise, and anxiety develop.

Furious rabies (80% of cases) is the classic form. Patients become agitated, confused, and hyperactive. The pathognomonic symptom is hydrophobia - painful spasms of the pharynx and diaphragm triggered by attempts to swallow, or even by the sight or sound of water. Aerophobia (spasms triggered by drafts of air) is similar. Hypersalivation is prominent; the combination of hypersalivation and inability to swallow produces the “foaming at the mouth” image. Periods of lucidity alternate with periods of agitation.

Paralytic rabies (20% of cases) presents as ascending paralysis mimicking Guillain-Barré syndrome - a diagnostic pitfall.

Both forms progress to coma and death, usually from respiratory failure, within 2 weeks of symptom onset.

Negri bodies - round-to-oval, eosinophilic cytoplasmic inclusions in neurons, particularly in hippocampal pyramidal cells and cerebellar Purkinje cells - are the histologic hallmark but present in only ~70-80% of cases, so absence does not exclude rabies. At autopsy, the brain may appear grossly normal; microscopically, perivascular lymphocytic cuffing and microglial nodules (Babes nodules) accompany the Negri bodies. The gold-standard diagnostic test is direct fluorescent antibody (DFA) on brain tissue at autopsy, or nuchal skin biopsy (containing hair follicles with nerve endings) for antemortem diagnosis.

Post-exposure prophylaxis is life-saving: The long incubation period provides a window for intervention. Wound cleaning with soap and water is critical - physical removal of virus reduces inoculum. Rabies immunoglobulin (RIG) provides immediate passive immunity and should be infiltrated around the wound. The rabies vaccine induces active immunity over subsequent weeks. Given promptly and correctly, PEP is virtually 100% effective.

Pre-exposure vaccination is recommended for high-risk groups: veterinarians, animal handlers, laboratory workers, cave explorers (bat exposure), and travelers to endemic areas where dog rabies is common and access to PEP might be limited.

Influenza

Influenza is an orthomyxovirus with a segmented RNA genome - a feature that enables the genetic reassortment responsible for pandemic emergence. The virus causes annual epidemics and occasional pandemics with massive global impact. Understanding how influenza’s virulence factors work at the molecular level explains its pathogenesis, transmission, and treatment.

The Two Key Surface Glycoproteins - How They Function

Two surface glycoproteins - hemagglutinin (HA) and neuraminidase (NA) - are not just immunologic targets; they are the functional machinery that allows the virus to infect cells, replicate, and spread. Understanding their mechanisms is essential.

Hemagglutinin (HA) - The Key to Entry: Hemagglutinin is a trimeric spike protein that mediates two critical steps in infection. First, HA binds to sialic acid (neuraminic acid) residues on the surface of respiratory epithelial cells. This receptor-binding function initiates infection - without it, the virus cannot attach to target cells. The specific sialic acid linkage matters: human influenza viruses preferentially bind α-2,6-linked sialic acid (found predominantly in human upper respiratory tract), while avian influenza viruses prefer α-2,3 linkages (found in avian intestinal and respiratory epithelia and in human lower respiratory tract). This receptor specificity determines species tropism and transmission efficiency.

Second, after the virus is internalized by endocytosis, HA mediates membrane fusion. As the endosome acidifies, HA undergoes a dramatic conformational change that exposes a fusion peptide, which inserts into the endosomal membrane and pulls the viral and cellular membranes together. This releases the viral genome into the cytoplasm. The HA must be cleaved by host proteases into HA1 and HA2 subunits for this fusion mechanism to work - this cleavage requirement is another determinant of virulence. Highly pathogenic avian influenza strains (like H5N1) have polybasic cleavage sites that can be cleaved by ubiquitous proteases, allowing systemic infection; low-pathogenic strains require trypsin-like proteases found only in the respiratory tract, limiting infection to the airways.

Neuraminidase (NA) - The Key to Release and Spread: Neuraminidase is a tetrameric enzyme that cleaves sialic acid residues. Its function becomes critical at the end of the viral life cycle. Newly synthesized viral particles bud from the infected cell’s surface, but HA on these new virions immediately binds back to sialic acid on the same cell surface - the virus essentially sticks to itself. Neuraminidase solves this problem by cleaving the sialic acid residues, releasing new virions to spread and infect more cells.

Neuraminidase also helps the virus penetrate respiratory mucus (which is rich in sialic acid-containing glycoproteins) to reach epithelial cells. Without functional NA, newly formed virions clump together on the cell surface and cannot disseminate. This is exactly why neuraminidase inhibitors (oseltamivir, zanamivir) are effective antivirals - by blocking NA activity, they prevent viral release and spread, limiting the infection if given early enough (within 48 hours, before the infection is too established).

How Influenza Causes Disease: The pathogenesis of influenza involves direct viral damage and immunopathology. The virus infects and kills ciliated respiratory epithelial cells, disrupting the mucociliary escalator that normally clears pathogens and debris. This denudation of the respiratory epithelium creates susceptibility to secondary bacterial pneumonia - historically the major cause of death in influenza pandemics. The host immune response - pro-inflammatory cytokines including IL-6, TNF-α, and interferons - causes the systemic symptoms of fever, myalgia, and malaise. In severe cases, a dysregulated “cytokine storm” contributes to acute respiratory distress syndrome (ARDS).

Antigenic Variation - Why Influenza Keeps Coming Back

Antigenic drift refers to the accumulation of point mutations in HA and NA genes over time. These gradual changes, concentrated in the antigenic sites that antibodies recognize, allow the virus to escape immunity generated by prior infection or vaccination. Because HA and NA are RNA polymerase products made by an error-prone enzyme (lacking proofreading), mutations occur constantly. This is why we need new flu vaccines each year - last year’s vaccine induces antibodies to last year’s HA and NA epitopes, which may no longer match this year’s circulating strains.

Antigenic shift is a sudden, major change in HA or NA, creating a novel subtype to which the population has no immunity. This occurs through reassortment - when two different influenza strains infect the same cell, their segmented genomes (8 separate RNA segments) can be packaged into progeny virions in new combinations. Pigs serve as “mixing vessels” because they can be infected by both avian and human strains, and their respiratory epithelium contains both α-2,3 and α-2,6-linked sialic acids. When a human virus and an avian virus co-infect a pig, reassortment can generate a virus with avian HA (to which humans have no immunity) but human-adapted internal genes (allowing efficient human-to-human transmission). The 1918 pandemic (H1N1), the 1957 pandemic (H2N2), and the 1968 pandemic (H3N2) all resulted from antigenic shift. The 2009 H1N1 “swine flu” pandemic arose through reassortment of avian, swine, and human influenza strains.

Types of Influenza Virus: Influenza A has 8 segments and infects multiple species (humans, birds, pigs, horses) - the only type that causes pandemics, because only influenza A has the animal reservoirs that allow antigenic shift. Classified by HA (H1-H18) and NA (N1-N11) subtypes. Influenza B has 8 segments but infects only humans/seals, undergoes only antigenic drift, and causes seasonal epidemics but not pandemics. Influenza C has 7 segments and causes mild respiratory illness (clinically minor).

Complications: the most common cause of death from influenza is post-influenza secondary bacterial pneumonia - from Staph aureus (including MRSA), Strep pneumoniae, H. influenzae, Strep pyogenes. Typical timeline: initial flu improves for 4-14 days, then recurrent fever and productive cough. Primary viral pneumonia, myocarditis, encephalitis, and Reye syndrome (with aspirin in children) are other complications.

Diagnostic specimens and methods:

Nasopharyngeal swab is the preferred specimen
PCR (RT-PCR) is the preferred method (most sensitive, can type/subtype)
Rapid antigen (RIDT): fast but ~50-70% sensitive; negative should be confirmed by PCR in high-risk patients
Hemadsorption - a historical culture-based test: RBCs are added to infected monolayers; if HA-expressing virus is present, RBCs stick to infected cells. Detects influenza, parainfluenza, and mumps (all HA-expressing). Largely replaced by PCR.

Treatment with neuraminidase inhibitors (oseltamivir, zanamivir, peramivir) works by blocking viral release. Baloxavir is a newer agent that inhibits the cap-dependent endonuclease required for viral mRNA synthesis. Treatment must be started within 48 hours of symptom onset. Amantadine/rimantadine (M2 channel blockers) are no longer used due to widespread resistance.

Influenza vaccines:

Intramuscular inactivated (killed) - standard seasonal flu shot, ≥6 months, quadrivalent
Intranasal live attenuated (LAIV/FluMist) - healthy non-pregnant ages 2-49; contraindicated in immunocompromised, pregnant, and young children

Coronaviruses

The Coronaviridae are enveloped, positive-sense ssRNA viruses with the largest RNA genome (~30 kb). Named for their crown-like spike proteins on EM. Human coronaviruses include seasonal common cold strains (229E, NL63, OC43, HKU1 - ~15-30% of colds, second to rhinovirus) and three epidemic/pandemic strains from bat reservoirs: SARS-CoV (2003), MERS-CoV (2012, intermediate host dromedary camels), SARS-CoV-2 (2019).

Structural proteins (SEMN):

S (Spike): trimeric glycoprotein that mediates receptor binding and fusion. Primary target of vaccines and neutralizing antibodies.
E (Envelope): small assembly/ion channel protein
M (Membrane): most abundant, shapes the envelope
N (Nucleocapsid): binds and packages the RNA genome

SARS-CoV-2 entry: spike protein binds ACE2 (angiotensin-converting enzyme 2), which is highly expressed on type II pneumocytes, enterocytes, endothelial cells, cardiac myocytes, and renal tubular cells. The host protease TMPRSS2 cleaves spike to enable membrane fusion. ACE2 is also the receptor for SARS-CoV (2003).

RNA replication: like all RNA viruses, coronaviruses use a virally encoded RNA-dependent RNA polymerase (RdRp), which host cells lack. This is the target of remdesivir (adenosine analog) and molnupiravir.

SARS-CoV-2 diagnostics:

PCR (RT-qPCR) - gold standard, NP/OP swab
Rapid antigen tests - detect N protein, sensitivity 50-80% vs PCR, best in symptomatic days 1-5. Negative antigen in a symptomatic patient should be confirmed by PCR.
Antibody tests - limited acute utility; useful for seroprevalence and past infection. Anti-Spike (anti-S) = past infection OR vaccination (all major vaccines target spike). Anti-Nucleocapsid (anti-N) = natural infection only (N is not in vaccines). So anti-S+/anti-N- = vaccinated only; anti-S+/anti-N+ = prior infection.

Vaccines all target the spike protein:

mRNA (Pfizer, Moderna) - lipid nanoparticle delivers spike mRNA. Does NOT enter nucleus or integrate into host DNA. Modified pseudouridine improves stability and reduces innate immune recognition.
Viral vector (J&J, AstraZeneca) - adenovirus vector carrying spike gene
Protein subunit (Novavax) - recombinant spike with adjuvant

HIV

HIV is a retrovirus (Retroviridae) - enveloped, positive-sense ssRNA, carries two copies of its RNA genome (diploid), and uses reverse transcriptase to create a DNA copy that integrates into the host genome. Understanding the molecular mechanisms of how HIV enters cells, how it destroys the immune system, and why it establishes an incurable latent reservoir is essential for understanding the disease and its treatment.

HIV belongs to the Lentivirus subfamily (“lenti” = slow). HIV-1 causes the global pandemic; HIV-2 is mostly West African, less pathogenic, slower progression, and less responsive to some antiretrovirals.

The Viral Structure and Key Proteins

HIV is an enveloped virus with a lipid membrane derived from the host cell. Embedded in this envelope are the spike proteins (gp120 and gp41) that mediate entry. Inside the virion is a cone-shaped capsid containing two copies of the RNA genome along with the enzymes reverse transcriptase, integrase, and protease - each a critical drug target.

Three main HIV genes:

Gag (Group-specific Antigen) - encodes structural proteins: p17 (matrix), p24 (capsid - used in antigen testing), p7 (nucleocapsid)
Pol (Polymerase) - encodes enzymes: reverse transcriptase, integrase, protease
Env (Envelope) - encodes gp160 precursor, cleaved into gp120 (surface, binds CD4/coreceptor) and gp41 (transmembrane, fusion)

Plus 6 regulatory/accessory genes: tat, rev, nef, vif, vpr, vpu.

How HIV Enters Cells - The Molecular Dance of Infection

HIV entry is a multi-step process requiring interaction with two receptors, and understanding these steps explains both viral tropism and drug development.

Step 1: gp120 binds CD4. The viral envelope protein gp120 recognizes and binds to the CD4 receptor on the surface of T helper cells, macrophages, and dendritic cells. CD4 is normally used by these cells to interact with MHC class II molecules during antigen presentation - HIV essentially hijacks this immune recognition molecule. The gp120-CD4 binding triggers a conformational change in gp120 that exposes a previously hidden region called the V3 loop.

Step 2: Coreceptor binding. The conformationally changed gp120 now binds to a coreceptor - either CCR5 or CXCR4. These are chemokine receptors that normally guide immune cell migration. R5-tropic viruses (using CCR5) predominate during early infection and are the variants typically transmitted. X4-tropic viruses (using CXCR4) emerge later in infection and are associated with more rapid CD4 decline. Some viruses are dual-tropic. This coreceptor requirement explains why people homozygous for the CCR5-Δ32 deletion mutation are highly resistant to HIV infection - without CCR5, R5-tropic viruses cannot enter their cells. Maraviroc, a CCR5 antagonist, exploits this by blocking the coreceptor.

Step 3: gp41-mediated fusion. Coreceptor binding triggers another conformational change, now in gp41. This protein contains a fusion peptide that inserts into the host cell membrane. gp41 then folds back on itself, pulling the viral and cellular membranes together until they fuse, releasing the viral core into the cytoplasm. Enfuvirtide is a fusion inhibitor that blocks this step.

Why CD4 Cells Die - Multiple Mechanisms of Destruction

The progressive loss of CD4+ T cells is the hallmark of HIV disease, but the mechanisms are more complex than simple viral killing.

Direct viral cytopathic effect: When HIV replicates in a CD4+ T cell, viral budding from the cell membrane can be directly cytotoxic. High levels of intracellular viral proteins can trigger apoptosis. Accumulation of unintegrated viral DNA can activate DNA damage responses.

Immune-mediated killing: Many infected cells are killed by the host’s own immune response. CD8+ cytotoxic T lymphocytes recognize viral peptides presented on MHC class I molecules and kill infected cells. This is actually protective early in infection but contributes to CD4 loss over time. Antibody-dependent cellular cytotoxicity (ADCC) also destroys infected cells.

Bystander killing: The majority of CD4+ T cells that die during HIV infection may not be productively infected. Chronic immune activation leads to exhaustion and apoptosis of uninfected cells. Pyroptosis - an inflammatory form of cell death triggered by incomplete viral infection of resting CD4 cells - may be a major mechanism. When HIV enters a resting CD4 cell but cannot complete reverse transcription, the accumulated cytoplasmic DNA triggers inflammasome activation, caspase-1 activation, and pyroptosis, which releases inflammatory cytokines that activate and draw in more CD4 cells to be infected.

The Latent Reservoir - Why HIV Cannot Be Cured

Here is the central tragedy of HIV biology: the virus integrates its DNA into the host genome, and once integrated, it becomes part of the cell’s genetic material. In activated CD4 cells that are actively dividing, the virus replicates and the cell dies. But if an infected cell transitions to a resting memory state before being killed, the viral DNA goes dormant along with the cell’s transcription machinery. This creates a latent reservoir - long-lived memory CD4+ T cells harboring silent but replication-competent HIV provirus.

These latently infected cells persist for decades. They are invisible to the immune system because they don’t express viral proteins. They are invisible to antiretroviral drugs because they aren’t replicating. When the cell is reactivated years later, the virus can emerge. This reservoir is established within days of infection - even patients treated during acute infection retain a latent reservoir - and it is the fundamental barrier to cure.

Acute HIV (acute retroviral syndrome) presents 2-4 weeks after exposure as a flu-like/mononucleosis-like illness: fever, pharyngitis, lymphadenopathy, maculopapular rash, myalgia, oral ulcers, diarrhea. Viral load is extremely high but antibodies may not yet be detectable (window period). Distinguishing from EBV mono: oral ulcers are more prominent in acute HIV; Monospot is typically negative (can be false-positive). p24 antigen or HIV RNA detects acute HIV before antibodies develop.

The Diagnostic Algorithm

HIV diagnosis uses fourth- or fifth-generation testing as the first step.

4th generation HIV Ag/Ab ELISA detects both HIV-1/2 antibodies AND p24 antigen (capsid protein from gag). Combined result - you can’t tell which component is positive. Narrows window period to ~2 weeks. Sensitivity >99.9%.
5th generation differentiates p24 antigen from HIV-1 antibodies from HIV-2 antibodies in a single assay. This lets you recognize acute HIV (Ag+/Ab-) immediately.

If the screening test is positive, an HIV-1/HIV-2 antibody differentiation immunoassay follows. This confirms the result and distinguishes HIV-1 from HIV-2.

If the differentiation assay is negative or indeterminate but screening was positive, HIV-1 RNA testing (viral load) is performed. This detects acute HIV before antibody seroconversion.

Western blot is no longer the confirmatory test (historically required 2 of 3 bands: p24, gp41, gp120/160) - it has been replaced by the differentiation assay, which is faster and more sensitive for acute infection.

Neonates born to HIV+ mothers are not screened by ELISA because maternal IgG crosses the placenta and persists up to 18 months. Use proviral DNA PCR (detects integrated HIV DNA in infant cells, unaffected by maternal antibodies). Testing at birth, 14-21 days, 1-2 months, and 4-6 months. Two positive DNA PCRs confirm infection.

CD4 Count - The Measure of Immune Damage

HIV infects CD4+ T cells, and progressive CD4 depletion defines disease progression. In untreated infection, after an initial rapid decline during acute infection (when viral loads peak), the immune system partially contains the virus and CD4 counts plateau. This is followed by years of gradual decline at roughly 50-80 cells/μL per year. When the CD4 count falls below 200 cells/μL, the risk of opportunistic infections rises dramatically, and AIDS is defined.

HIV Opportunistic Infections by CD4 Threshold:

CD4 Count	Opportunistic Infections	Prophylaxis
<500	Kaposi sarcoma, TB reactivation	-
<200	PCP (Pneumocystis), Candida esophagitis, Cryptococcus	TMP-SMX
<150	Histoplasma, Coccidioides (disseminated)	-
<100	Toxoplasma encephalitis, Cryptosporidium	TMP-SMX (also covers Toxo)
<50	MAC (Mycobacterium avium complex), CMV retinitis, CNS lymphoma, PML	Azithromycin (MAC)

AIDS-defining illnesses (occurs at any CD4, but defines AIDS):

PCP, Kaposi sarcoma, CMV disease, Cryptococcal meningitis
Esophageal candidiasis, CNS lymphoma, Toxoplasma encephalitis
Mycobacterial disease (TB, MAC), PML, Cryptosporidium/Isospora >1 month
HIV encephalopathy, wasting syndrome, invasive cervical cancer

Viral Load - The Measure of Viral Activity

HIV-1 RNA (viral load) measures the amount of virus in blood and is measured by reverse transcriptase PCR. It’s used to diagnose acute infection (before antibodies develop), monitor treatment response, and assess adherence and resistance. The goal of treatment is undetectable viral load (<20-50 copies/mL, depending on assay). The “set point” viral load - the level several months after acute infection, once equilibrium is reached - predicts the rate of disease progression.

CD4+ T cell count is measured by flow cytometry using anti-CD3/CD4/CD8/CD45 antibodies. Normal CD4: 500-1500 cells/μL.

HIV Genotyping: identifies resistance mutations in RT, protease, and integrase genes by Sanger sequencing or NGS. Perform at time of diagnosis (baseline - 10-15% of new diagnoses have transmitted resistance) and at treatment failure (before changing regimen). Test while on the failing regimen, because resistant variants are replaced by wild-type once drug pressure is removed. HIV monotherapy and non-adherence drive resistance because incomplete suppression allows resistant mutants to emerge under selective pressure. This is why modern ART is 3-drug combination (typically 2 NRTIs + integrase inhibitor).

HIV Diagnosis in Neonates: Maternal IgG persists in the infant for up to 18 months → standard antibody tests (ELISA, Western blot) give FALSE POSITIVE results. Must use HIV NAT (DNA PCR or RNA viral load) tested at birth, 14-21 days, 1-2 months, and 4-6 months. Two positive NAT results confirm infection. Start zidovudine (AZT) prophylaxis within 6-12 hours of birth. No breastfeeding in resource-rich settings.

Antiretroviral Therapy - Attacking the Viral Life Cycle

Understanding how each drug class works requires understanding the viral replication steps:

Entry inhibitors block the steps described above: maraviroc blocks CCR5 coreceptor binding; enfuvirtide blocks gp41-mediated fusion.

NRTIs (nucleoside/nucleotide reverse transcriptase inhibitors) are nucleoside analogs that are incorporated into the growing DNA chain during reverse transcription and cause chain termination because they lack the 3’-hydroxyl group needed for the next phosphodiester bond.

NNRTIs (non-nucleoside reverse transcriptase inhibitors) bind directly to reverse transcriptase at a site away from the active site, causing conformational changes that inhibit enzyme function.

Integrase inhibitors block the integration of viral DNA into the host genome - a critical step for establishing both productive and latent infection. These are now first-line therapy due to excellent efficacy and tolerability.

Protease inhibitors block the viral protease that cleaves polyprotein precursors into functional proteins during viral maturation. Without this cleavage, virions bud but are non-infectious.

Combination therapy with drugs from multiple classes - typically two NRTIs plus an integrase inhibitor - achieves viral suppression, prevents resistance emergence, and has transformed HIV from a fatal illness to a manageable chronic condition.

HTLV-1 (Human T-lymphotropic Virus Type 1)

HTLV-1 is a deltaretrovirus (oncogenic retrovirus) that infects CD4+ T cells and causes disease after decades of latency. ~10-20 million infected worldwide. Endemic in:

Japan (especially southwestern, Kyushu)
Caribbean (Jamaica, Trinidad)
Sub-Saharan Africa
South America (Brazil), Middle East (Iran)

US blood banks screen donors for HTLV-1/2 antibodies.

Transmission (HTLV is cell-associated, so free virus is less infectious than HIV):

Sex (male-to-female > female-to-male)
Blood exposure - transfusion, IVDU. Leukoreduction partially reduces risk.
Breastfeeding - the major vertical route in endemic areas (prolonged breastfeeding >6 months increases risk)
Transplacental (less common than breastfeeding)

Primary infection is usually asymptomatic. Only 2-5% of carriers develop disease after 20-40 years of latency.

Oncogenesis: the Tax protein activates NF-kB, promotes T-cell proliferation, inhibits DNA repair, and inactivates p53. This drives polyclonal expansion of infected cells; additional genetic hits over decades produce a monoclonal malignancy.

Late sequelae:

Adult T-cell leukemia/lymphoma (ATLL) - aggressive CD4+ T-cell malignancy. Latency 20-40 years. Lifetime risk ~5% if infected at <20 years old.
- Peripheral smear: flower (cloverleaf) lymphocytes - CD4+ T cells with deeply multilobated nuclei resembling flower petals
- Hypercalcemia (PTHrP from tumor cells, osteoclast activation) - “thirst”
- Elevated soluble IL-2 receptor (CD25)
- Skin lesions, lymphadenopathy, hepatosplenomegaly
- Despite elevated CD4 count, cells are dysfunctional (immunosuppression)
- IHC: CD4+, CD25+, CCR4+
- Acute subtype has median survival <1 year
HTLV-1-associated myelopathy / Tropical spastic paraparesis (HAM/TSP) - immune-mediated demyelination of thoracic spinal cord (lateral columns, corticospinal tracts). Female:male ~3:1. Shorter incubation than ATLL, especially in transfusion-acquired cases (1-3 years). Progressive spastic paraparesis, urinary dysfunction.
Strongyloides hyperinfection susceptibility - HTLV-1 impairs Th2 antihelminth defense. Screen HTLV-1 patients for Strongyloides and vice versa.
Infectious dermatitis, uveitis

Hepatitis C Virus (HCV)

Hepatitis C is a flavivirus that has achieved something remarkable in modern medicine: it went from untreatable to curable within a few decades. The development of direct-acting antivirals (DAAs) transformed HCV from a leading cause of liver transplantation to an infection that can be cured in 8-12 weeks with oral therapy.

Transmission: primarily through infected blood - IV drug use (most common in developed countries), blood transfusion (historical, pre-1992 screening), needlestick, non-sterile tattooing/piercing, hemodialysis. Sexual transmission is rare (much less efficient than HBV/HIV). Vertical transmission ~5%. NOT transmitted by casual contact, food, water, or breastfeeding.

Natural history - these numbers are high-yield:

~75% develop chronic infection (contrast with HBV adult, where <10% become chronic)
~15% of chronic HCV patients develop cirrhosis over decades
~5% of chronic HCV patients develop hepatocellular carcinoma

Diagnosis uses a two-step approach:

Anti-HCV antibody (ELISA/CLIA) - screening. Indicates exposure; persists even after the virus clears. Takes 4-10 weeks to appear after infection.
If antibody positive, reflex to HCV RNA (quantitative PCR) to confirm active infection.

Interpretation: Ab+ / RNA+ = active infection. Ab+ / RNA- = resolved (spontaneous clearance or post-treatment SVR). In acute HCV or immunosuppressed patients, HCV RNA is the primary diagnostic test.

HCV RNA also establishes baseline viral load, monitors treatment response, confirms SVR, and detects relapse. ==Sustained virologic response (SVR) = undetectable HCV RNA at 12 or 24 weeks post-treatment == is the current definition of cure (concordance with long-term outcomes >99%).

HCV genotyping is performed at baseline. Seven genotypes; genotype 1 is most common worldwide and in the US (~70%). Subtype 1a is more likely to develop resistance (NS5A Q30R) than 1b. Pan-genotypic regimens (sofosbuvir/velpatasvir) have made genotyping less clinically essential.

Fibrosis staging: liver biopsy is not required. Non-invasive options: transient elastography (FibroScan) measures liver stiffness by ultrasound; FibroTest (serum markers); APRI; FIB-4. Cirrhotic patients still require HCC surveillance even after SVR.

Treatment has been revolutionized: DAAs target specific viral proteins (NS3/4A protease, NS5B polymerase, NS5A) and achieve SVR rates exceeding 95% with 8-12 weeks of oral therapy. The medications are well-tolerated and effective across all genotypes. The barrier to treatment is now primarily cost and access, not efficacy.

Hepatitis G (GBV-C / pegivirus) is another Flaviviridae transmitted parenterally. Found in 1-4% of blood donors; does NOT appear to cause hepatitis or liver disease. Interestingly, GBV-C coinfection in HIV patients is associated with slower HIV progression. No treatment indicated.

Other RNA Viruses

Rubella (German Measles) - Togaviridae

Togaviruses are enveloped, positive-sense ssRNA viruses with two clinically relevant genera: Alphavirus (arboviral encephalitides, chikungunya) and Rubivirus (rubella). Rubella is NOT arthropod-borne - it spreads by respiratory droplets.

Rubella must not be confused with rubeola (measles) - they’re completely different viruses causing different diseases, though both produce a rash. In children and adults, rubella is trivial. In fetuses, it’s devastating. This dichotomy explains why rubella vaccination is such a public health priority.

In children and adults, rubella causes mild illness: low-grade fever, malaise, and a fine maculopapular rash that starts on the face and spreads downward, lasting about 3 days (hence the old name “three-day measles”). The distinctive feature is lymphadenopathy, particularly involving posterior auricular, suboccipital, and posterior cervical nodes - finding these nodes in a patient with a viral exanthem should make you think of rubella. Arthralgias and arthritis are common in adult women. Patients are infectious from 7 days before to 7 days after rash.

Congenital rubella syndrome is the catastrophe: When rubella infects a pregnant woman, particularly in the first trimester, the consequences for the fetus are severe. Risk of congenital defects is ~85% with infection in first 12 weeks. Classic triad:

Sensorineural deafness (most common, often the only manifestation, up to 80%)
Cataracts (plus microphthalmos, glaucoma, “salt and pepper” retinopathy)
Patent ductus arteriosus (plus peripheral pulmonary stenosis)

Other manifestations: microcephaly, intellectual disability, hepatosplenomegaly, thrombocytopenia with “blueberry muffin” rash, radiolucent bone disease. The tragedy is that this is entirely preventable through vaccination.

The MMR vaccine (measles, mumps, rubella) uses live attenuated virus and is contraindicated in pregnancy. All women of childbearing age should have rubella AND varicella IgG confirmed before pregnancy; vaccinate non-immune women at least 4 weeks before conceiving, or postpartum before discharge.

Alphaviruses - mosquito-borne:

Eastern equine encephalitis (EEE) - most severe, mortality 30-50%, Culiseta mosquitoes, bird reservoir, Atlantic/Gulf coast
Western (WEE) and Venezuelan (VEE) equine encephalitides - milder
Chikungunya - Aedes aegypti and albopictus mosquitoes. High fever, maculopapular rash, and severe polyarthralgia that can persist months to years. Name means “that which bends up” (stooped posture). Same vectors as dengue and Zika.

Rotavirus - Reoviridae

Rotavirus was once the leading cause of severe childhood diarrhea worldwide, responsible for hundreds of thousands of deaths annually, primarily in developing countries. The introduction of rotavirus vaccines has dramatically reduced this burden - one of the great public health achievements of the 21st century.

Reoviruses are non-enveloped, segmented double-stranded RNA viruses (10-12 segments; “Reo” = Respiratory Enteric Orphan). The double-shelled icosahedral capsid gives rotavirus its wheel-like appearance on EM (hence “rota”). Being non-enveloped, rotavirus is environmentally stable and resistant to many disinfectants (ethanol-based sanitizers have limited efficacy).

Besides rotavirus, Reoviridae includes Coltivirus / Colorado tick fever virus - tick-borne (Dermacentor andersoni, western US/Canada mountain areas), flu-like illness with leukopenia and “saddleback” fever pattern. Differential includes Rocky Mountain spotted fever (same tick, same area, but Rickettsia rickettsii has rash and needs doxycycline).

The disease hits young children hardest: Rotavirus causes severe watery diarrhea and vomiting in children under 5, with peak incidence at 6-24 months. The illness lasts about a week, and the main danger is dehydration - in settings without access to rehydration therapy, this can be fatal. In temperate climates, rotavirus has a striking winter seasonality.

Vaccination has transformed rotavirus epidemiology: The oral rotavirus vaccines (Rotarix, RotaTeq) are live attenuated and given in infancy. Hospitalizations have declined by >80% in vaccinated populations. There is a small risk of intussusception, but the benefits outweigh this risk.

Diagnosis: PCR is most sensitive; stool antigen EIA is widely used in clinical practice. Included in most multiplex GI panels.

Astrovirus (Astroviridae): non-enveloped +ssRNA with a characteristic star-shaped appearance on EM (“astron”). Causes gastroenteritis primarily in infants <1 year old, elderly, and immunocompromised. Usually milder than rotavirus or norovirus. Diagnosis by stool PCR.

Hepatitis E (Hepeviridae): non-enveloped (quasi-enveloped in blood) +ssRNA. Transmitted fecal-oral (contaminated water, genotypes 1 and 2) or zoonotic from undercooked pork, deer, boar (genotypes 3 and 4, developed countries). Endemic in Southeast Asia, Africa, Central America, India.

Usually self-limited acute hepatitis
Characteristically severe in pregnant women, with mortality up to 20-25% for genotype 1 in the third trimester (fulminant hepatic failure with coagulopathy, encephalopathy)
Can be chronic in immunosuppressed patients (genotype 3)
Board pearl: pregnant woman with hepatitis and travel to Southeast Asia = hepatitis E

Norovirus - Caliciviridae

Norovirus (previously “Norwalk virus” after a 1968 Norwalk, Ohio outbreak) is the most common cause of acute viral gastroenteritis across all age groups and is notorious for causing explosive outbreaks in closed settings - cruise ships, nursing homes, hospitals, schools, and restaurants. Responsible for >50% of global gastroenteritis outbreaks. The virus combines several features that make it extraordinarily contagious.

Caliciviruses are non-enveloped, positive-sense ssRNA viruses named for the cup-shaped (calyx) surface depressions on EM.

What makes norovirus so transmissible: The infectious dose is tiny - as few as 18 viral particles can cause illness. The virus is shed in enormous quantities in stool and vomit. It’s non-enveloped, so it resists environmental inactivation and survives on surfaces for days. It can be transmitted by aerosolized vomitus. And people remain contagious for days after symptoms resolve.

The clinical picture is dramatic but brief: Onset is sudden - within 12-48 hours of exposure, patients develop projectile vomiting, watery diarrhea, abdominal cramps, and sometimes low-grade fever. The illness is miserable but self-limited, typically resolving in 24-72 hours. In healthy adults, it’s a bad couple of days. In the elderly or debilitated, dehydration can be dangerous.

Outbreak control is challenging: The combination of low infectious dose (~18 particles), environmental persistence (surfaces for weeks, survives freezing and 60C), and continued shedding makes norovirus outbreaks difficult to control. Hand hygiene must include soap and water - alcohol-based sanitizers are less effective against non-enveloped viruses. Environmental disinfection with bleach is necessary. In healthcare settings, cohorting patients and excluding symptomatic staff are essential.

Diagnosis: RT-qPCR of stool is the gold standard. Antigen tests have lower sensitivity (~50-75%). Norovirus cannot be grown in standard cell culture.

Flaviviruses - Mosquito-Borne Disease

The flaviviruses are a family of enveloped, positive-sense ssRNA viruses, most of which are arboviruses transmitted by mosquitoes or ticks. The name comes from yellow fever (flavus is Latin for yellow), the prototype of the family. HCV is also a Flaviviridae member but is blood-borne (not arboviral).

Important flaviviruses:

Dengue - Aedes aegypti
West Nile - Culex, bird reservoir
Yellow fever - Aedes
Zika - Aedes
Japanese encephalitis - Culex, swine reservoir
St. Louis encephalitis - Culex
Tick-borne encephalitis, Powassan - ticks
HCV (Hepacivirus) - blood-borne

Dengue fever affects hundreds of millions of people annually in tropical and subtropical regions. Aedes mosquitoes (the same genus that transmits Zika and chikungunya) are the vectors. Primary infection causes “breakbone fever” - high fever, severe headache, retro-orbital pain, and myalgias/arthralgias so severe they feel like bones breaking. Most patients recover completely.

The danger comes with secondary infection by a different dengue serotype (there are four). Antibodies from the first infection don’t neutralize the new serotype - instead, they enhance viral uptake into cells, a phenomenon called antibody-dependent enhancement. The result can be dengue hemorrhagic fever or dengue shock syndrome, with plasma leakage, hemorrhage, and potentially fatal shock.

Yellow fever remains endemic in sub-Saharan Africa and South America despite the existence of an excellent vaccine. Biphasic course: Phase 1 (infection, ~3-6 days) with fever, headache, chills, myalgia, N/V (most recover); Phase 2 (intoxication, ~15-25%) with high fever, jaundice, hepatic failure, hemorrhage/DIC, renal failure, and the classic “black vomit” (hematemesis). Phase 2 mortality 20-50%. Histology: Councilman bodies - eosinophilic apoptotic hepatocytes in mid-zonal hepatic necrosis. The live attenuated 17D vaccine provides lifelong immunity and is required for travel to endemic areas.

West Nile virus reached the United States in 1999 and has since spread across the country, transmitted by Culex mosquitoes, bird reservoir (corvids - crows, jays, magpies - are sentinel species; dead bird surveillance is used). Humans and horses are dead-end hosts. Most infections are asymptomatic. About 20% cause a flu-like illness. Less than 1% develop neuroinvasive disease - meningitis or encephalitis - but this can be severe, particularly in the elderly. Acute flaccid paralysis resembling polio can occur.

Zika virus gained global attention in 2015-2016 when the epidemic in Brazil revealed its ability to cause congenital Zika syndrome. In adults, infection is usually mild or asymptomatic - low-grade fever, rash, conjunctivitis, arthralgias. But when pregnant women are infected, the virus can cause devastating fetal brain damage, including microcephaly and other neurologic abnormalities. Zika can also be sexually transmitted, unlike most arboviruses.

Flavivirus diagnostics - an important topic because cross-reactivity makes serology tricky:

PCR has a narrow window - viremia lasts only ~5-7 days after symptom onset. Use PCR in the first week; serology later. (Exception: Zika urine PCR may stay positive longer than blood.)
MAC-ELISA (IgM antibody capture) - detects virus-specific IgM 3-8 days after symptoms. Main limitation: extensive cross-reactivity among flaviviruses (shared envelope epitopes). Prior dengue infection or yellow fever vaccination can cause false positives for Zika, West Nile, etc.
Seroconversion or 4-fold IgM rise in paired sera 7-21 days apart is diagnostic.
PRNT (plaque reduction neutralization test) is the gold-standard confirmatory test - measures virus-specific neutralizing antibodies by serial dilution incubated with live virus on cell monolayers. The virus with the highest neutralizing titer (lowest PRNT90 titer) is the infecting virus. Slow, BSL-3/4 required for some viruses.
NS1 antigen rapid test is available for dengue.

Togaviruses - Alphavirus Encephalitides

Several alphaviruses cause encephalitis in the Americas, named for the equine hosts that amplify the viruses (horses, like humans, are dead-end hosts - they don’t transmit to mosquitoes efficiently).

Eastern equine encephalitis (EEE) is the most severe, with mortality rates around 30-50% and frequent neurologic sequelae in survivors. It’s found in the eastern United States and is fortunately rare because the mosquito vectors don’t commonly bite humans. Western equine encephalitis (WEE) and Venezuelan equine encephalitis (VEE) are less severe.

Chikungunya, also an alphavirus, has become globally important. It causes acute febrile illness with severe polyarthralgias - the joint pain is so characteristic that the name comes from a Tanzanian word meaning “to walk bent over.” Unlike dengue, chikungunya arthritis can persist for months to years in some patients.

Filoviruses - Ebola and Marburg

The filoviruses are among the most feared pathogens - their filamentous shape, high mortality, and dramatic hemorrhagic manifestations captured public attention during the 2014-2016 West African Ebola epidemic. Named for their filamentous (thread-like) morphology on EM - long pleomorphic filaments up to 14,000 nm in length. BSL-4 agents, CDC Category A bioterrorism. Reservoir: fruit bats.

Two genera: Ebolavirus (6 species, Zaire ebolavirus most lethal, up to 90% mortality) and Marburgvirus (first described 1967 in Marburg, Germany from imported African green monkeys).

Ebola and Marburg viruses are enveloped, negative-sense RNA viruses transmitted through contact with infected body fluids. In African outbreaks, transmission chains involve contact with sick family members, burial practices that involve touching corpses, and healthcare workers who lack adequate personal protective equipment. Not airborne transmitted.

Pathogenesis: the virus infects monocytes, macrophages, and dendritic cells, triggering massive cytokine release (“cytokine storm”) that causes endothelial dysfunction, vascular leak, and DIC.

The clinical course begins with nonspecific symptoms - fever, myalgia, headache - progressing to severe gastrointestinal symptoms with diarrhea and vomiting. Hemorrhagic manifestations (bleeding from multiple sites) occur late in the course and are not as universal as popular media suggests. Multi-organ failure and shock cause death; mortality varies by outbreak and viral strain but has ranged from 25% to 90%.

Treatment has improved dramatically. Supportive care - aggressive fluid resuscitation and electrolyte management - improves survival. Monoclonal antibody therapies (Inmazeb for Ebola Zaire) have shown mortality benefit and are now standard of care when available. The rVSV-ZEBOV (Ervebo) Ebola vaccine was approved in 2019.

Bunyaviruses - Hantavirus and Others

Bunyaviruses are enveloped, segmented (3 segments: L, M, S), negative-sense ssRNA viruses. Most are arthropod-borne; hantavirus is the notable exception (rodent-borne). Clinically important members: Hantavirus, Crimean-Congo hemorrhagic fever virus (tick-borne), Rift Valley fever virus (mosquito-borne), La Crosse encephalitis virus.

Hantaviruses are transmitted not by arthropods but by rodents. Humans become infected by inhaling aerosolized rodent urine, feces, or saliva - classically when cleaning out cabins or sheds with rodent infestations. There is no person-to-person transmission (rare exception: Andes virus). Different species carry different hantaviruses: deer mouse (Peromyscus maniculatus) carries Sin Nombre virus; striped field mouse carries Hantaan; bank vole carries Puumala.

In the Americas, hantaviruses cause hantavirus cardiopulmonary syndrome (HCPS) - Sin Nombre in the southwestern US (1993 Four Corners outbreak). After a prodromal illness with fever and myalgias, patients rapidly develop noncardiogenic pulmonary edema and cardiogenic shock from capillary leak. Mortality ~35%. ECMO may be life-saving.

Characteristic HCPS blood findings: neutrophilia without toxic granulation, thrombocytopenia, immunoblasts (>10% of lymphocytes), and hemoconcentration (elevated hematocrit from capillary leak). This triad of thrombocytopenia + immunoblasts + hemoconcentration in a patient with rapidly progressive pulmonary edema from a rural area is highly suggestive.

Old World hantaviruses (Hantaan, Seoul, Puumala) cause hemorrhagic fever with renal syndrome (HFRS) - fever, hemorrhage, acute kidney injury rather than pulmonary involvement.

California encephalitis virus (La Crosse virus) is a mosquito-borne bunyavirus causing encephalitis predominantly in children in the midwestern and eastern United States.

Arenaviruses - Rodent-Borne Viruses

Arenaviruses are enveloped, segmented ambisense RNA viruses (classified as negative-sense). Each segment (L and S) has both negative-sense and positive-sense coding regions. Named for the sandy granular appearance on EM (arena = sand in Latin) from incorporated host ribosomes. Two groups: Old World (LCMV, Lassa) and New World (Junin, Machupo, Guanarito - South American hemorrhagic fevers). Rodent reservoirs with chronic, asymptomatic infection.

Lymphocytic choriomeningitis virus (LCMV) is carried by the common house mouse (Mus musculus) and hamsters, transmitted through aerosolized excreta. Causes a biphasic illness: Phase 1 flu-like (fever, malaise, myalgia), then Phase 2 aseptic meningitis. CSF can show markedly low glucose - unusual for viral meningitis and can mimic TB meningitis. Most cases resolve. LCMV infection during pregnancy can cause congenital infection with severe neurologic damage, mimicking TORCH. In transplant recipients (organs from infected donors), LCMV causes fatal disease - a recognized cluster entity.

Lassa fever is a severe hemorrhagic fever caused by Lassa virus, endemic to West Africa (Sierra Leone, Liberia, Nigeria, Guinea). Reservoir: Mastomys rat (contact with urine/feces). Gradual onset fever, malaise, pharyngitis progressing to facial/neck edema, pleural effusion, hemorrhage. Mortality ~1% overall, 15-20% hospitalized. Deafness is a common sequela (~30%). Treatment: ribavirin (most effective when given early). Person-to-person transmission occurs. BSL-4.

Poxviruses - The Largest and Most Complex Viruses

Poxviruses are extraordinary in virology - they’re the largest viruses known, visible under light microscopy, and they possess a complexity that approaches that of small bacteria. They’re large enough to carry genes for their own DNA replication machinery, which is why they uniquely replicate entirely in the cytoplasm rather than requiring access to the nucleus like all other DNA viruses. This cytoplasmic replication creates the characteristic cytoplasmic inclusions seen on histopathology.

Smallpox (Variola Virus)

Smallpox holds a unique place in medical history as the only human infectious disease to be deliberately eradicated through vaccination - a campaign completed in 1980. The disease was devastating: Variola major mortality 30%; Variola minor (alastrim) ~1%. Survivors were often disfigured. The last natural case was in 1977 in Somalia. The virus now exists only in two WHO-authorized repositories: CDC (Atlanta) and VECTOR (Novosibirsk, Russia). It remains a CDC Category A bioterrorism agent.

CDC Category A bioterrorism agents (highest priority):

Variola (smallpox)
Bacillus anthracis (anthrax)
Clostridium botulinum toxin (botulism)
Francisella tularensis (tularemia)
Yersinia pestis (plague)
Filoviruses (Ebola, Marburg) - hemorrhagic fever
Arenaviruses (Lassa, Machupo) - hemorrhagic fever

Pathogenesis: Variola virus was transmitted by respiratory droplets and aerosols. After inhalation, the virus replicated in the upper respiratory tract, then spread to regional lymph nodes. Primary viremia seeded the reticuloendothelial system (spleen, liver, bone marrow), where massive replication occurred. Secondary viremia then disseminated the virus to the skin and mucous membranes, producing the characteristic rash.

The rash was distinctive: lesions evolved synchronously through stages (macule → papule → vesicle → pustule → crust), so at any given time, all lesions were at the same stage. This synchronous evolution distinguishes smallpox from chickenpox, where lesions appear in successive crops and exist in multiple stages simultaneously. The centrifugal distribution - denser on face and extremities than trunk, including palms and soles - was also characteristic. The rash classically starts as an enanthem in the oral cavity before skin lesions appear. Smallpox lesions are deep and firm; chickenpox lesions are superficial and thin-walled.

Feature	Smallpox	Chickenpox
Lesion stages	All same age (synchronous)	Different ages (crops)
Distribution	Centrifugal (face/extremities > trunk)	Centripetal (trunk > extremities)
Palms/soles	Involved	Spared
Depth	Deep, firm	Superficial, thin-walled

Guarnieri bodies are the pink eosinophilic cytoplasmic inclusions seen in poxvirus-infected cells - aggregates of viral replication machinery (viral factories) within the cytoplasm. Unique among DNA viruses, poxviruses replicate entirely in the cytoplasm (they carry their own DNA-dependent RNA polymerase). EM shows characteristic brick-shaped/ovoid particles with a dumbbell-shaped core.

Vaccinia is a closely related orthopoxvirus that produces mild disease and is the smallpox vaccine, cross-protective against variola and monkeypox. Administered by scarification with a bifurcated needle; a successful “take” produces a pustule that scars. Complications include progressive vaccinia (in immunocompromised), eczema vaccinatum, post-vaccinial encephalitis, and myocarditis.

Bioterrorism concern: Smallpox is classified as a Category A bioterrorism agent because of its transmissibility, high mortality in an unvaccinated population, and potential for public panic. Routine vaccination ended in the 1970s, leaving most of the world’s population susceptible. The Strategic National Stockpile maintains enough vaccine for emergency response.

Molluscum Contagiosum

Molluscum contagiosum virus causes a benign skin infection that contrasts starkly with the severity of smallpox - a reminder of how diverse poxvirus pathogenicity can be.

Pathogenesis: Molluscum spreads through direct skin contact and causes highly localized infection of the epidermis. The virus infects keratinocytes and induces them to proliferate, creating the characteristic lesion: a dome-shaped, waxy papule with a central umbilication (dimple). This umbilicated center contains the viral-laden central core. The virus produces proteins that interfere with interferon signaling and antigen presentation, allowing it to evade local immunity and persist for months.

Clinical patterns: In immunocompetent children, molluscum is a common, self-limited infection acquired by skin contact. Lesions resolve spontaneously over 6-12 months as cellular immunity eventually clears the infection. In adults, molluscum is often sexually transmitted, with genital distribution. In AIDS patients with advanced immunosuppression, molluscum can be extensive and disfiguring, with hundreds of large lesions - this extensive pattern can be an AIDS-presenting illness.

Henderson-Patterson bodies (also called molluscum bodies) are the diagnostic histopathologic finding: large, eosinophilic, intracytoplasmic inclusions that push the nucleus to the periphery of the cell. They represent viral inclusion bodies filled with maturing virions. Histologically, the lesion is a cup-shaped/crateriform epithelial proliferation with central umbilication, and the molluscum bodies enlarge progressively from basal to superficial layers (largest in the granular/cornified layers, where they are extruded through the umbilication). Keratohyalin granules are prominent and coarse.

Molluscum in different hosts:

Children (1-10 years): trunk/extremities/face, direct contact or fomites, self-limited in 6-18 months.
Adults: consider molluscum an STI when genital/inguinal. Test for other STIs and HIV.
HIV/AIDS with CD4 <200: giant molluscum (>1 cm), widespread, resistant to treatment, often on face. Differential for multiple facial umbilicated papules in HIV: molluscum, disseminated cryptococcosis, disseminated histoplasmosis, Penicillium (Talaromyces) marneffei. Giant facial molluscum in an adult should prompt HIV testing.

Treatment is not mandatory since lesions self-resolve, but can include curettage, cryotherapy, or topical agents to hasten clearance.

49.3 Viral Laboratory Diagnostics

Modern viral diagnostics lean heavily on molecular methods, with culture and serology preserved for specific indications. Know what test to send for each clinical question.

Molecular Methods

PCR (and RT-PCR for RNA viruses) is the mainstay of viral detection - high sensitivity and specificity, rapid turnaround, can quantify viral load, and can detect non-culturable viruses. Qualitative PCR detects presence/absence (diagnosis). Quantitative PCR (qPCR, “viral load”) measures copies/mL and is used for serial monitoring of HIV, HCV, HBV, CMV, BK, EBV. Rising viral load during treatment = failure or non-compliance. Multiplex panels (e.g., BioFire FilmArray) can detect 20+ respiratory or GI pathogens from a single specimen.

In situ hybridization (ISH) detects viral genomes directly in tissue sections using labeled complementary probes. Preserves architecture so you can see which cells are infected. EBER-ISH is the gold standard for EBV in tissue (PTLD, NPC, NK/T lymphoma).

Viral Culture

Viral culture uses monolayers of living cells; infected cells develop visible cytopathic effect (CPE). Different cell lines support different viruses. Culture takes days to weeks, so it has been largely replaced by PCR for routine diagnosis. It remains useful when the isolate is needed for susceptibility testing (e.g., HSV acyclovir resistance, CMV ganciclovir resistance), though molecular genotypic resistance testing is increasingly used.

Shell vial assay is a modified culture that shortens time to detection. The specimen is centrifuged onto a cell monolayer on a coverslip in a shell vial (centrifugation enhances adsorption). After 1-3 days, the coverslip is stained with fluorescent antibodies against early viral antigens, so infection is detected before CPE develops. Detection in 1-3 days vs 1-3 weeks for conventional culture. Lower sensitivity than PCR, so largely superseded.

Antigen Detection

Direct fluorescent antibody (DFA): patient specimen is stained with fluorescent antibodies against viral antigens, viewed on a fluorescent microscope.
Enzyme immunoassay (EIA/ELISA) / rapid antigen tests: antibodies detect viral antigens with colorimetric readout. Fast (15-60 min) but lower sensitivity than PCR, so negative results should be confirmed by PCR in high-yield cases.

Serology

Detects antibodies (IgM, IgG) or antigens in serum. Used when the virus is hard to culture/detect, when determining immune status, or for paired-serum diagnosis.

IgM = acute or recent infection (first antibody produced, peaks at 1-2 weeks, declines over months)
IgG = past infection, chronic infection, or vaccination (appears after IgM, persists long-term)
Seroconversion (negative to positive IgG) or a 4-fold rise in IgG between paired acute/convalescent sera (7-21 days apart) is the gold-standard evidence of acute infection. Both samples should be tested in the same run for comparability.
IgG avidity testing measures the strength of antibody binding. B cells undergo affinity maturation over time, so low avidity = recent primary infection (weak binding); high avidity = past infection (strong binding). Particularly useful for CMV and Toxoplasma in pregnancy - low avidity indicates recent primary infection with risk of congenital transmission.

CSF antibody interpretation:

IgM in CSF indicates intrathecal production (IgM is too large to passively cross BBB)
IgG in CSF can be from intrathecal production OR passive transfer across a damaged BBB. Calculate the CSF/serum IgG index to distinguish (elevated = intrathecal).

Viral Cytopathic Effect on Histology - Summary Table

Memorize this pattern-recognition table:

Virus	Nuclear inclusion	Cytoplasmic inclusion	Syncytia
HSV/VZV	+ (Cowdry A/B, Three M’s)	-	+
CMV	+ (owl eye, large basophilic with halo)	+ (granular basophilic)	-
Adenovirus	+ (smudge cells)	-	-
RSV	-	+	+
Measles	+	+	+ (Warthin-Finkeldey giant cells)
Rabies	-	+ (Negri bodies)	-
Parvovirus B19	+ (glassy, in lantern cells)	-	-
Molluscum	-	+ (Henderson-Patterson)	-
Smallpox	-	+ (Guarnieri bodies)	-
BK/JC	+ (ground-glass, decoy cells)	-	-

The Three M’s (Moulding, Margination, Multinucleation) plus Cowdry type A (eosinophilic intranuclear with halo) or type B (ground-glass) inclusions define HSV and VZV histologically. Clinical context distinguishes them. Measles is unique in having BOTH nuclear and cytoplasmic inclusions plus syncytia.

49.4 Prions

Prions are infectious proteins (proteinaceous infectious particles) - unique among infectious agents in having no nucleic acid, no DNA or RNA. They are composed entirely of misfolded protein: PrPSc (pathologic form) templates conversion of normal cellular PrPC into more PrPSc. This templated misfolding is how prions propagate.

Prion diseases cause spongiform encephalopathies - so named for the sponge-like vacuolar appearance of affected brain tissue on histology.

Human prion diseases:

Sporadic Creutzfeldt-Jakob disease (sCJD) - most common human prion disease
Familial CJD, Gerstmann-Straussler-Scheinker syndrome, fatal familial insomnia (inherited PRNP mutations)
Iatrogenic CJD (contaminated instruments, corneal grafts, cadaveric growth hormone)
Variant CJD (vCJD) - linked to consumption of BSE-contaminated beef (mad cow disease)
Kuru - formerly common in Papua New Guinea among the Fore people, transmitted by ritualistic endocannibalism (consumption of deceased relatives’ brains, especially by women and children at funerary rites). Presents with progressive cerebellar ataxia (“trembling”), also called “laughing sickness” for pathologic laughter bursts. Disappeared after cannibalism was abandoned. Provided the first evidence that spongiform encephalopathies are transmissible.

Animal prion diseases: BSE (mad cow), scrapie (sheep), chronic wasting disease (deer, elk).

Prions are extraordinarily resistant: not destroyed by standard autoclaving, formaldehyde, or routine disinfectants. Decontamination requires prolonged autoclaving at higher temperatures combined with sodium hydroxide or sodium hypochlorite. This is why suspected prion cases require specialized protocols in pathology laboratories and surgical suites.

49.5 Vaccines

Board questions often ask which vaccines are live (contraindicated in pregnancy and immunocompromised) vs inactivated/subunit. Know this table cold.

Vaccine	Type	Route
MMR	Live attenuated	SC
Varicella (chickenpox)	Live attenuated	SC
Zostavax (old shingles)	Live attenuated	SC
Shingrix (current shingles)	Recombinant subunit + adjuvant	IM
Rotavirus (RotaTeq, Rotarix)	Live attenuated	Oral
Sabin polio (OPV)	Live attenuated	Oral
Salk polio (IPV)	Killed	IM
Yellow fever (17D)	Live attenuated	SC
Influenza IIV	Killed	IM
Influenza LAIV (FluMist)	Live attenuated	Intranasal
Adenovirus (military)	Live (non-attenuated)	Oral
Hepatitis A	Killed	IM
Hepatitis B	Subunit (recombinant HBsAg)	IM
HPV (Gardasil 9)	Recombinant VLP (L1 protein)	IM
Rabies	Killed	IM
COVID-19 (Pfizer, Moderna)	mRNA	IM
COVID-19 (J&J)	Viral vector (adenovirus)	IM
Tetanus, Diphtheria	Toxoid	IM
Pertussis (acellular)	Subunit	IM
Hib	Conjugated polysaccharide	IM
Meningococcal ACWY	Conjugated polysaccharide	IM
Meningococcal B	Subunit (recombinant protein)	IM
PCV-13/15/20	Conjugated polysaccharide	IM
PPSV23	Polysaccharide (unconjugated)	IM/SC

Key teaching points:

Live vaccines are contraindicated in pregnancy and severe immunodeficiency (MMR, varicella, zostavax, yellow fever, rotavirus, OPV, LAIV). Check rubella and varicella IgG pre-pregnancy; vaccinate non-immune women postpartum. MMR is not contraindicated in HIV with CD4 ≥200.
Shingrix is preferred over Zostavax for adults >50 because it is not live and can be given to immunocompromised patients.
HPV vaccine uses L1 capsid virus-like particles (VLPs) - no viral DNA, cannot cause infection.
HBV vaccine contains only recombinant HBsAg. Vaccinated individuals are anti-HBs+ / anti-HBc- (distinguishes from natural immunity, which also has anti-HBc+).
Conjugation of capsular polysaccharides to a carrier protein (Hib, pneumococcal PCV, meningococcal ACWY) converts a T-independent antigen into a T-dependent one, enabling effective immune response in infants <2 years.
Meningococcal B uses recombinant proteins instead of capsular polysaccharide because the serogroup B capsule resembles human NCAM (autoimmunity risk).
Toxoid vaccines (tetanus, diphtheria) produce anti-toxin antibodies that neutralize the toxin but do NOT prevent colonization.
Adenovirus vaccine (types 4, 7) is live, oral, and used exclusively by the US military for recruit barracks.
J&J COVID vaccine was authorized via EUA February 2021; effectively withdrawn from US market in 2023 due to rare thrombosis with thrombocytopenia syndrome (TTS) risk relative to mRNA alternatives.

Chapter 50: Antimicrobial Susceptibility Testing and Resistance

Understanding antimicrobial resistance requires knowing both how we test for it (susceptibility testing) and the mechanisms by which bacteria become resistant. Both are essential for the clinical pathologist.

Quick Reference: Antibiotic Spectrum of Activity

Antibiotic Class	Gram-Positive	Gram-Negative	Anaerobes	Key Notes
Penicillin	Strep, Enterococcus	No	Some (oral)	Not Staph (penicillinase)
Ampicillin	Same + Listeria	H. flu, E. coli (some)	Better	HACEK organisms
Penicillinase-resistant PCN (nafcillin, oxacillin)	MSSA only	No	No	Not MRSA
Anti-pseudomonal PCN (piperacillin)	Yes	Yes including Pseudomonas	Yes	Usually with β-lactamase inhibitor
1st gen cephalosporin (cefazolin)	MSSA, Strep	E. coli, Klebsiella, Proteus	No	Surgical prophylaxis
2nd gen cephalosporin (cefuroxime)	Yes	Better GN coverage	Some	Variable
3rd gen cephalosporin (ceftriaxone)	Yes (not Enterococcus)	Excellent	No	Meningitis, gonorrhea
Anti-pseudomonal ceph (ceftazidime, cefepime)	Less GP	Pseudomonas	No	Neutropenic fever
Carbapenems (meropenem, imipenem)	Yes (not MRSA)	Yes including ESBL	Yes	Broadest β-lactam
Aztreonam	No	Yes including Pseudomonas	No	Safe in PCN allergy
Vancomycin	MRSA, Enterococcus	No	Some Clostridium	Not VRE
Linezolid	MRSA, VRE	No	Yes	Myelosuppression
Daptomycin	MRSA, VRE	No	No	Inactivated by surfactant (not for pneumonia)
Fluoroquinolones (cipro, levo, moxi)	Variable	Excellent	Moxi only	Cipro: best Pseudomonas
Aminoglycosides	Only synergy	Yes	No	Nephrotoxicity, ototoxicity
Macrolides (azithro)	Yes	H. flu, atypicals	No	Atypical pneumonia
Tetracyclines (doxy)	MRSA (some)	Yes	Yes	Tick-borne diseases
TMP-SMX	MRSA (CA-MRSA)	Yes	No	PCP prophylaxis
Metronidazole	No	No	Excellent	C. diff, abscesses
Clindamycin	Yes	No	Excellent	Inducible resistance in Staph

Key patterns to remember:

Enterococcus is intrinsically resistant to cephalosporins and aztreonam
Listeria is intrinsically resistant to cephalosporins (use ampicillin)
Atypical organisms (Mycoplasma, Chlamydia, Legionella) require intracellular-penetrating agents (macrolides, fluoroquinolones, tetracyclines)
Pseudomonas requires anti-pseudomonal agents (ceftazidime, cefepime, piperacillin-tazobactam, carbapenems, ciprofloxacin, aminoglycosides)

50.1 Susceptibility Testing Methods

The fundamental question in susceptibility testing is: will this antibiotic, at achievable concentrations in the patient, inhibit this organism? The answer comes from determining the minimum inhibitory concentration (MIC) - the lowest antibiotic concentration that prevents visible bacterial growth.

Broth microdilution is the reference method. Serial two-fold dilutions of antibiotic are prepared in broth in a microtiter plate. Bacteria are inoculated, and after incubation, the wells are examined for turbidity. The MIC is the lowest concentration with no visible growth. This method is precise but labor-intensive.

Disk diffusion (Kirby-Bauer) is simpler. An antibiotic-impregnated disk is placed on an agar plate inoculated with bacteria. Antibiotic diffuses out, creating a concentration gradient. Bacteria grow where the concentration is below the MIC, creating a zone of inhibition around the disk. Zone diameter correlates inversely with MIC - larger zones mean lower MICs (more susceptible).

E-test combines principles of both methods. A strip with a continuous antibiotic gradient is placed on an inoculated plate. The elliptical zone of inhibition intersects the strip at the MIC, which is read directly from the scale on the strip.

Interpretation requires comparing the MIC (or zone diameter) to established breakpoints. Breakpoints define “susceptible” (standard dosing should work), “intermediate” (higher dosing or specific sites may work), and “resistant” (unlikely to work). Breakpoints are set by organizations like CLSI and EUCAST based on pharmacokinetic/pharmacodynamic data and clinical outcomes.

50.2 Mechanisms of Resistance

Bacteria have evolved five main strategies to resist antibiotics. Understanding these mechanisms helps you predict cross-resistance and choose alternative agents.

Enzymatic inactivation: The bacteria produce enzymes that destroy or modify the antibiotic. Beta-lactamases hydrolyze the beta-lactam ring, inactivating penicillins and cephalosporins. Aminoglycoside-modifying enzymes add chemical groups to aminoglycosides, preventing ribosome binding.

Target modification: The bacteria alter the antibiotic’s target so it no longer binds. MRSA produces an altered penicillin-binding protein (PBP2a, encoded by mecA) that has low affinity for beta-lactams. Vancomycin-resistant enterococci alter the D-Ala-D-Ala terminus of peptidoglycan precursors to D-Ala-D-Lac, preventing vancomycin binding.

Decreased permeability: Bacteria can exclude antibiotics by reducing uptake. Gram-negative bacteria can lose or alter outer membrane porins, preventing antibiotic entry. This often contributes to carbapenem resistance.

Efflux pumps: Active transport systems pump antibiotics out of the cell faster than they can accumulate. Efflux pumps contribute to resistance to tetracyclines, fluoroquinolones, and multiple other drug classes.

Target bypass: The bacteria acquire a new pathway that circumvents the blocked target. Vancomycin resistance genes (vanA, vanB) encode enzymes that produce altered peptidoglycan precursors that vancomycin can’t bind.

50.3 Clinically Important Resistance Phenotypes

MRSA (Methicillin-resistant Staphylococcus aureus): The mecA gene encodes PBP2a, which has low affinity for all beta-lactams. MRSA is resistant to all penicillins, cephalosporins, and carbapenems - no beta-lactam is effective. Treatment options include vancomycin, daptomycin, linezolid, and sometimes TMP-SMX or doxycycline for non-severe infections.

ESBL (Extended-spectrum beta-lactamases): These enzymes hydrolyze penicillins AND extended-spectrum cephalosporins (ceftriaxone, ceftazidime, cefepime), but are inhibited by beta-lactamase inhibitors and don’t efficiently hydrolyze carbapenems. Carbapenems are the treatment of choice for serious ESBL infections.

AmpC beta-lactamases: Unlike ESBLs, AmpC enzymes are NOT inhibited by clavulanate. They’re chromosomally encoded and inducible in the “SPACE” organisms (Serratia, Pseudomonas, Acinetobacter, Citrobacter, Enterobacter). A dangerous scenario: the organism tests susceptible to a third-generation cephalosporin, but during treatment, AmpC is induced and the organism becomes resistant. Carbapenems or cefepime (stable to AmpC) are safer choices.

Carbapenemases: These are the most concerning beta-lactamases because they hydrolyze carbapenems - our “last-line” beta-lactams. KPC (Klebsiella pneumoniae carbapenemase) is most common in the US. NDM, VIM, and IMP are metallo-beta-lactamases, important because they’re NOT inhibited by avibactam (so ceftazidime-avibactam fails). Treatment options are severely limited - often polymyxins, tigecycline, or newer agents.

VRE (Vancomycin-resistant enterococci): vanA confers high-level resistance to both vancomycin and teicoplanin; vanB confers resistance to vancomycin only. Both are transferable on mobile genetic elements, raising the nightmare scenario of transfer to S. aureus (VRSA, which has occurred rarely). Treatment options include linezolid, daptomycin, and tigecycline.

This completes Part IV: Medical Microbiology.

PART V: LABORATORY MANAGEMENT AND INFORMATICS

Chapter 51: Laboratory Statistics and Diagnostic Test Evaluation

Understanding basic statistics is essential for interpreting test results, evaluating new methods, and understanding the limitations of diagnostic tests. This chapter covers the statistical concepts most relevant to clinical laboratory practice - from distributions and central tendency, through test performance metrics (sensitivity, specificity, PPV, NPV, ROC/AUC), to the applied infrastructure of method validation, reference intervals, QC (Levey-Jennings, Westgard), and proficiency testing.

Most of this chapter is board-testable at the medical director level. The math is straightforward; the subtle points are which metric applies to which question, what’s intrinsic to a test versus what floats with the population, and the regulatory floor (CLIA/CMS) for everything.

51.1 Distributions and Central Tendency

Before any performance metric makes sense, you have to know what your data look like. Whether an analyte is Gaussian or skewed decides whether you use mean or median, parametric or non-parametric statistics, and whether “mean ± 2 SD” is a valid reference interval at all.

Gaussian (Normal) Distribution

A Gaussian distribution is symmetric and bell-shaped, with most values clustered around the mean. Three properties define it:

Mean = median = mode (all three measures of central tendency are equal).
The 68-95-99.7 rule: 68% of values fall within ±1 SD, 95% within ±2 SD, and 99.7% within ±3 SD.
The curve is fully defined by its mean and SD - once you know those two numbers, the distribution is specified.

Many biological analytes are approximately Gaussian. Those that are not (enzyme activities, hormone levels, bilirubin, triglycerides, TSH, ferritin) typically show positive skew and require non-parametric handling.

Measures of Central Tendency: Mean, Median, Mode

The three measures of central tendency:

Mean: arithmetic average (sum of values divided by number of values). Sensitive to outliers. Reported with the standard deviation.
Median: middle value in an ordered dataset (or average of the two middle values if even). Resistant to outliers. Reported with the interquartile range (IQR).
Mode: most frequently occurring value. The only measure applicable to categorical (nominal) data.

Best measure by data type:

Data type	Best measure	Paired with
Continuous, Gaussian	Mean	SD
Continuous, skewed (unimodal)	Median	IQR
Categorical (nominal)	Mode	-

Example dataset: 5, 7, 7, 8, 10, 15, 21.

Mean = (5+7+7+8+10+15+21) / 7 = 10.4
Median = 8 (middle value)
Mode = 7 (appears twice)

The mean is pulled toward the 21 outlier; the median ignores it. This is why skewed data use the median.

Skewness: Which Way Does the Tail Point?

Skewness describes asymmetry. The tail tells the tale - the direction the tail points defines the skew.

Positive skew (right-skewed): tail extends to the right (toward higher values).

Mean > median > mode
Peak (mode) is on the left
Classic lab examples: triglycerides, bilirubin, TSH, ferritin. Values are bounded at zero on the low end but can be very high - so positive skew is the default for most clinical analytes.

Negative skew (left-skewed): tail extends to the left (toward lower values).

Mode > median > mean
Peak (mode) is on the right
Less common in lab data. Possible examples: serum albumin in a mostly-healthy population with a tail of low values in disease.

Gaussian (symmetric): mean = median = mode, perfect bell shape.

The key intuition: the mean is always pulled toward the tail, because every value in the dataset contributes to its calculation. The mode is anchored at the peak. The median sits between them.

Board question pattern: image of a distribution curve, asked to identify skew direction and relationship between mean/median/mode. Or given the relationship (e.g., mean > median > mode), asked to identify the distribution.

Handling skewed data: either use non-parametric methods (percentiles), or transform the data (e.g., log-transform positively skewed analytes to normalize them before applying parametric statistics).

Standard Deviation: Spread Around the Mean

Standard deviation (SD) describes the average distance a value is from the mean. It quantifies the spread or dispersion of the data, in the same units as the original measurement (mg/dL, mmol/L, etc.).

Small SD = values cluster tightly around the mean = precise, reproducible.
Large SD = values dispersed = imprecise.
Variance = SD² (same concept, squared units).

Sample SD formula (what labs actually use):

\[ SD = \sqrt{\frac{\sum (x_i - \bar{x})^2}{n - 1}} \]

The (n − 1) in the denominator is Bessel’s correction. In a sample, individual values are closer to the sample mean (x̄) than to the true population mean (μ), so using n would underestimate the true population SD. Dividing by n-1 corrects for that bias. Population SD uses n (no correction needed, because you have the whole population).

SD is the foundation of QC (Levey-Jennings charts), reference interval establishment, and method comparison statistics. You almost never calculate it by hand, but you need to understand what it is: the average squared deviation from the mean, square-rooted back to the original units.

The 68-95-99.7 Rule in Detail

For a Gaussian distribution:

Range	% of population	Relevance
Mean ± 1 SD	68%	Warning zone in QC (Levey-Jennings 1 SD lines)
Mean ± 2 SD	95%	Standard reference interval; QC action limit
Mean ± 3 SD	99.7%	Westgard 1:3s rejection limit

The 2 SD boundary is the main decision point in clinical chemistry. Reference intervals capture the central 95% of healthy values (mean ± 2 SD for Gaussian data). A QC value outside 2 SD triggers Westgard rule evaluation. A value beyond 3 SD is almost certainly a true error (probability by chance = 0.3%).

51.2 Precision, Accuracy, and Error Types

Precision (reproducibility): how close are repeated measurements to each other? Reflects absence of random error.

Accuracy: how close is the measured value to the true value? Reflects absence of systematic error (bias).

Analogy: precision is hitting the same spot repeatedly; accuracy is hitting the bullseye. You can be precise but inaccurate (tight cluster, wrong spot), or accurate but imprecise (centered on target, scattered widely).

Random Error → Imprecision

Lack of precision is caused by random error. Random error is unpredictable and varies in both magnitude and direction - sometimes high, sometimes low.

Sources of random error: pipetting variability, temperature fluctuations, electrical noise, bubble formation, imprecise timing.

Key properties:

Affects reproducibility - repeated measurements give different answers
Cannot be corrected after the fact
Can be reduced (not eliminated) by automation, replicates, and better technique
Detected by: widening SD, 1:3s Westgard rule, R:4s Westgard rule

Systematic Error → Inaccuracy

Lack of accuracy is caused by systematic error (bias). Systematic error is predictable and consistent - results are consistently too high or too low by a similar amount.

Sources of systematic error: miscalibration, reagent degradation, interfering substances, incorrect standard concentration, specimen matrix effects.

Key properties:

Shifts all results in one direction (shift) or gradually drifts (drift)
CAN be corrected - recalibrate, change reagents, apply correction factor
Detected by: 2:2s, 4:1s, 10:mean Westgard rules

Total Analytic Error

Total analytic error (TAE) = random error + systematic error.

\[ TAE = |bias| + 1.96 \times SD \]

TAE is the worst-case deviation from the true value for any single measurement (at 95% confidence). It is compared to the allowable total error (set by CLIA, biological variation data, or clinical requirements). If TAE exceeds allowable error, the method is unacceptable.

Coefficient of Variation: Normalizing Precision

Precision is quantified by the coefficient of variation (CV):

\[ CV = \frac{SD}{mean} \times 100\% \]

CV normalizes SD to the mean, expressing precision as a percentage. This allows comparison across analytes at very different concentrations.

Example: a glucose assay with mean 100 mg/dL and SD 3 mg/dL has CV = 3%. A potassium assay with mean 4.0 mEq/L and SD 0.12 mEq/L also has CV = 3%. Same relative precision despite very different absolute SDs.

Rule of thumb: desirable precision for most clinical analytes is CV < 10%. Most chemistry analytes achieve CV 1-5%. Immunoassays and molecular tests often run 5-15%. Point-of-care devices are typically higher than central lab analyzers. CLIA sets specific performance criteria for regulated analytes (e.g., glucose ±6 mg/dL or ±10%, whichever is greater).

Assessing Accuracy

Accuracy is assessed by:

Recovery experiments - test certified reference materials of known concentration
Method comparison - compare new method to a reference/gold-standard method (correlation coefficient, Bland-Altman)
Proficiency testing - compare results to peer group consensus (see 51.9)

Correlation coefficient (r) from method comparison gives a first-pass accuracy check. r > 0.975 generally acceptable. But correlation measures relationship, not agreement - two methods can have r = 1.0 and still have significant bias. Bland-Altman and Deming regression are better tools for agreement.

Monitoring Precision Over Time

Precision is monitored by regular testing of standardized (control) samples with known target values, plotted on Levey-Jennings charts (see 51.8). Types of precision:

Within-run (repeatability): variation within a single analytical run
Between-run (reproducibility): variation across different runs, days, or operators

Declining precision (widening SD, increasing CV) signals a need for maintenance, recalibration, or troubleshooting.

51.3 Test Performance Characteristics

Every diagnostic test has inherent performance characteristics that determine its clinical utility. Understanding these concepts allows you to interpret results appropriately and communicate their meaning to clinicians.

The 2×2 Table: Foundation of Test Evaluation

All test performance metrics derive from a simple 2×2 contingency table comparing test results to a reference (“gold”) standard:

	Disease Present (P)	Disease Absent (N)
Test Positive (PP)	True Positive (TP), hit	False Positive (FP), Type I error, false alarm
Test Negative (PN)	False Negative (FN), Type II error, miss	True Negative (TN), correct rejection

Row sums (predicted): $PP = TP + FP$, $PN = FN + TN$. Column sums (actual): $P = TP + FN$, $N = FP + TN$.

Full Confusion Matrix Reference: All Derived Metrics

The full family of metrics derived from the 2×2 table, organized by what they measure:

Column-based metrics (condition-given, intrinsic to the test)

Metric	Formula	What it answers
True Positive Rate (TPR), sensitivity, recall, hit rate, power	$TP / P = TP / (TP + FN)$	Given disease, probability test is positive
True Negative Rate (TNR), specificity, selectivity	$TN / N = TN / (TN + FP)$	Given no disease, probability test is negative
False Positive Rate (FPR), fall-out, Type I error rate	$FP / N = 1 - TNR$	Given no disease, probability of false alarm
False Negative Rate (FNR), miss rate, Type II error rate	$FN / P = 1 - TPR$	Given disease, probability of missing it

Row-based metrics (test-result-given, depend on prevalence)

Metric	Formula	What it answers
Positive Predictive Value (PPV), precision	$TP / (TP + FP) = TP / PP$	Given positive test, probability of disease
Negative Predictive Value (NPV)	$TN / (TN + FN) = TN / PN$	Given negative test, probability of no disease
False Discovery Rate (FDR)	$FP / (TP + FP) = 1 - PPV$	Given positive test, probability of no disease
False Omission Rate (FOR)	$FN / (TN + FN) = 1 - NPV$	Given negative test, probability of disease

Likelihood ratios (prevalence-independent, usable at bedside with pre-test odds)

Metric	Formula	Interpretation
Positive likelihood ratio (LR+)	$TPR / FPR$	Factor by which a positive test multiplies pre-test odds. LR+ > 10 = strong rule-in
Negative likelihood ratio (LR-)	$FNR / TNR = (1 - TPR) / TNR$	Factor by which a negative test multiplies pre-test odds. LR- < 0.1 = strong rule-out
Diagnostic Odds Ratio (DOR)	$LR+ / LR- = (TP \times TN) / (FP \times FN)$	Overall single-number summary; higher = better

Overall accuracy and single-number composite metrics

Metric	Formula	Notes
Accuracy (ACC)	$(TP + TN) / (P + N)$	Misleading in skewed prevalence
Balanced accuracy (BA)	$(TPR + TNR) / 2$	Robust to class imbalance
F1 score	$(2 \times PPV \times TPR) / (PPV + TPR) = 2 TP / (2 TP + FP + FN)$	Harmonic mean of PPV and TPR; ignores TN
Fowlkes-Mallows index (FM)	$\sqrt{PPV \times TPR}$	Geometric mean of PPV and TPR
Matthews Correlation Coefficient (MCC), phi	$\sqrt{TPR \times TNR \times PPV \times NPV} - \sqrt{FNR \times FPR \times FOR \times FDR}$	Most informative single metric; robust to class imbalance; ranges -1 to +1
Threat score (TS), Critical Success Index (CSI), Jaccard index	$TP / (TP + FN + FP)$	Ignores TN like F1
Informedness (Bookmaker, BM)	$TPR + TNR - 1 = J$	Youden’s J statistic; 0 = chance, 1 = perfect
Markedness (MK), deltaP	$PPV + NPV - 1$	Predictive-value version of informedness

Population-level quantities

Metric	Formula	Notes
Prevalence	$P / (P + N)$	Disease burden in tested population
Prevalence threshold (PT)	$(\sqrt{TPR \times FPR} - FPR) / (TPR - FPR)$	Prevalence below which PPV drops sharply

Core symmetries and relationships to hold in your head:

$TPR = 1 - FNR$ and $TNR = 1 - FPR$ (column sums)
$PPV = 1 - FDR$ and $NPV = 1 - FOR$ (row sums)
Accuracy is the weighted average of TPR and TNR, weighted by prevalence and (1 - prevalence)
LR+ rises when TPR is high and FPR is low; the test adds information only when $TPR \neq FPR$
MCC is the correlation coefficient between predicted and actual binary classifications

When to use which metric:

Screening: optimize for high TPR (sensitivity) and high NPV - don’t miss cases
Confirmation: optimize for high TNR (specificity) and high PPV - don’t false-alarm
Balanced populations: accuracy or F1 is fine
Imbalanced populations (rare disease): MCC, balanced accuracy, or informedness are more honest than accuracy or F1
Bedside clinical reasoning: likelihood ratios with pre-test probability (Fagan nomogram)

From this table, we calculate all the key metrics:

Sensitivity and Specificity

Sensitivity = TP / (TP + FN) = Proportion of diseased patients who test positive.

Answers: “If you have the disease, what’s the probability the test is positive?”
A sensitive test has few false negatives
High sensitivity is good for ruling OUT disease (mnemonic: SnNout - Sensitive test, Negative result, rules Out)
Example: HIV screening tests are designed for high sensitivity (>99.5%) to avoid missing infected individuals
Looks at the left column of the 2×2 table (disease-positive column)

Specificity = TN / (TN + FP) = Proportion of non-diseased patients who test negative.

Answers: “If you don’t have the disease, what’s the probability the test is negative?”
A specific test has few false positives
High specificity is good for ruling IN disease (mnemonic: SpPin - Specific test, Positive result, rules In)
Example: HIV confirmatory tests have high specificity to avoid false-positive diagnoses
Looks at the right column of the 2×2 table (disease-negative column)

Clinical vs. analytical sensitivity/specificity: the clinical terms above relate to TP/FP/TN/FN on disease detection. Analytical sensitivity/specificity refer to an instrument’s ability to detect small quantities of analyte (analytical sensitivity = lower limit of detection) or to discriminate the target analyte from interferents (analytical specificity = no cross-reactivity). They are different concepts even though the names overlap.

The tradeoff: Sensitivity and specificity are inversely related. Lowering a cutoff increases sensitivity but decreases specificity (more false positives); raising it does the opposite.

Critical property: sensitivity and specificity are NOT affected by prevalence. They are intrinsic properties of the test - determined by analytical characteristics and the chosen cutoff. Whether you test a high-risk or low-risk population, they stay the same.

Predictive Values: What Clinicians Really Want to Know

Sensitivity and specificity are intrinsic properties of the test. But clinicians ask a different question: “My patient tested positive - what’s the probability they actually have the disease?”

Positive Predictive Value (PPV) = TP / (TP + FP)

Answers: “Given a positive test, what’s the probability of disease?”
Looks at the top row of the 2×2 table (test-positive row)
Depends on disease prevalence in the population tested

Negative Predictive Value (NPV) = TN / (TN + FN)

Answers: “Given a negative test, what’s the probability of no disease?”
Looks at the bottom row of the 2×2 table (test-negative row)
Depends on prevalence

Board pearl: sensitivity and specificity are fixed; PPV and NPV float with prevalence.

As prevalence increases: PPV goes up, NPV goes down. As prevalence decreases: PPV goes down, NPV goes up.

How Prevalence Affects Predictive Value

Consider a test with 95% sensitivity and 95% specificity. What happens to PPV at different prevalences?

Prevalence	PPV	NPV
50%	95%	95%
10%	68%	99%
1%	16%	99.9%
0.1%	2%	99.99%

The lesson: In low-prevalence populations, even highly specific tests generate many false positives. A positive screening test in a low-risk patient is likely to be a false positive - this is why confirmatory testing is essential.

Clinical example: A patient with no risk factors for HIV has a positive screening test. Given the low pre-test probability (prevalence in this population ~0.1%), the PPV is only a few percent. The result must be confirmed before diagnosis.

Diagnostic Accuracy (Overall)

Diagnostic accuracy = (TP + TN) / (TP + TN + FP + FN) = total correct results / total results.

This is the test’s overall ability to distinguish diseased from non-diseased patients. Simple to calculate but misleading in populations with very skewed prevalence (a test that just calls everyone “negative” will look highly accurate when disease is rare). ROC/AUC analysis across all cutoffs is a more robust assessment of discriminating ability.

Note: this diagnostic accuracy is distinct from analytic accuracy, which is the ability of an instrument to correctly measure an analyte concentration.

Relative Risk

Relative risk (RR) compares the probability of disease in an exposed group vs. an unexposed group:

\[ RR = \frac{\text{incidence in exposed}}{\text{incidence in unexposed}} \]

From a 2×2 table with cells a (exposed + disease), b (exposed + no disease), c (unexposed + disease), d (unexposed + no disease):

\[ RR = \frac{a/(a+b)}{c/(c+d)} \]

RR = 1.0: no association
RR > 1.0: exposure increases risk
RR < 1.0: exposure decreases risk (protective)

RR is appropriate for cohort studies (follow exposed and unexposed groups forward). It cannot be directly calculated from case-control studies - use odds ratio instead. Example: RR of 2.5 for smoking and lung cancer means smokers have 2.5× the risk of non-smokers.

Likelihood Ratios

Likelihood ratios (LRs) express how much a test result changes the odds of disease. They combine sensitivity and specificity into a single metric and are independent of prevalence.

Positive Likelihood Ratio (LR+) = Sensitivity / (1 - Specificity)

How many times more likely is a positive result in someone with disease vs. without?
LR+ > 10 is very useful for ruling in disease
LR+ of 1 provides no diagnostic information

Negative Likelihood Ratio (LR-) = (1 - Sensitivity) / Specificity

How many times less likely is a negative result in someone with disease vs. without?
LR- < 0.1 is very useful for ruling out disease
LR- of 1 provides no diagnostic information

Using likelihood ratios with pre-test probability:

Post-test odds = Pre-test odds × Likelihood ratio

Where odds = probability / (1 - probability).

Or use a Fagan nomogram: draw a line from pre-test probability through LR to find post-test probability.

51.4 ROC Curves and AUC

A Receiver Operating Characteristic (ROC) curve plots sensitivity (y-axis) vs. 1 − specificity (x-axis) at all possible cutoff values. It visualizes the sensitivity/specificity tradeoff across every possible threshold, not just a single chosen one.

Reading an ROC Curve

The ROC curve lets you see how a test performs at every possible cutoff:

Lower the cutoff (e.g., point “a” on a classic ROC diagram): more positives detected → higher sensitivity, lower specificity. Fewer false negatives, more false positives.
Higher the cutoff (e.g., point “c”): fewer positives → lower sensitivity, higher specificity. More false negatives, fewer false positives.

Interpreting the shape:

Perfect test: curve passes through the upper-left corner (100% sensitivity AND 100% specificity achievable at some cutoff)
Useless test: diagonal line from lower-left to upper-right (no better than chance)
Better tests: curves closer to the upper-left corner

When multiple ROC curves are shown on the same plot, the curve closest to the upper-left corner (leftmost/uppermost) is the best test - it has higher sensitivity at every level of specificity.

Area Under the Curve (AUC)

AUC summarizes overall discriminating ability of the test. AUC ranges from 0.5 (no discrimination) to 1.0 (perfect discrimination). A test cannot have AUC < 0.5 - if it did, you’d simply invert its output to get an AUC > 0.5.

Interpretation of AUC: if you randomly select one diseased and one non-diseased patient, the AUC is the probability that the test correctly ranks the diseased patient higher.

AUC	Interpretation
0.5	No discrimination (coin flip)
0.7 - 0.8	Fair/acceptable
0.8 - 0.9	Good discriminating ability
> 0.9	Excellent/outstanding

AUC > 0.8 is the usual threshold for a clinically useful diagnostic test. Most clinical laboratory tests have AUCs between 0.7 and 0.95.

Board pearl: AUC is the single best summary metric for comparing two tests head-to-head. Higher AUC = better discriminating ability.

Clinical use: ROC curves help determine the optimal cutoff for a test, balancing sensitivity and specificity based on clinical priorities (e.g., for screening, prioritize sensitivity; for confirmation, prioritize specificity).

51.5 Reference Intervals

A reference interval is the central 95% of values in a healthy reference population - not a “normal” range in any absolute biological sense. It’s a statistical construct, and it depends entirely on how the population was defined.

CLIA Requirement

Every lab must either verify the manufacturer’s reference intervals or establish its own for each test. This applies to FDA-cleared methods (where verification suffices if the lab’s population matches the manufacturer’s), LDTs (must establish de novo), and any modified method. Reference intervals depend on the specific method, instrument, reagents, and patient population. A reference interval established on a Roche analyzer does not automatically apply to an Abbott analyzer for the same analyte.

How Reference Ranges Are Established

Reference intervals are created on healthy individuals representative of the lab’s patient population. “Healthy” is defined by exclusion criteria: no known disease, no medications affecting the analyte, fasting (if applicable), etc. Demographics (age, sex, ancestry/population group) matter - a reference interval established in a predominantly young European-ancestry population may not apply to an elderly population or other ancestry groups. Multiple reference intervals may be needed for a single analyte (male vs. female, pediatric vs. adult, pregnant vs. non-pregnant).

De novo (establishing new intervals): CLSI EP28-A3c recommends testing at least 120 healthy individuals. With 120 subjects, the 2.5th and 97.5th percentiles can be reliably estimated by non-parametric methods. The 120 minimum also allows partition by sex (60 males + 60 females) if needed.

Verification (adopting existing intervals): test 20 healthy individuals. At least 18 of 20 must fall within the proposed range for verification to pass. This corresponds to the 90% threshold - theoretically 95% should fall within, but small-sample leeway is given.

Non-parametric method (preferred by CLSI EP28-A3c): rank the data and use the 2.5th and 97.5th percentile values directly. Doesn’t assume any distribution.

Parametric method: mean ± 1.96 SD (approximately mean ± 2 SD). Only valid if data are normally distributed. Not preferred by CLSI.

What 95% Means in Practice

A reference interval captures 95% (more precisely 95.5% for ±2 SD) of healthy individuals. That leaves ~5% of healthy people with “abnormal” results by definition - 2.5% above the upper limit and 2.5% below the lower limit.

The 64% rule: if 20 independent tests are ordered on a healthy patient, the probability that at least one result is flagged abnormal is:

\[ P = 1 - (0.95)^{20} \approx 0.64 \]

So 64% chance of at least one “abnormal” value. This is why reflex confirmation and clinical correlation matter - the more tests you order, the more false flags you’ll see.

Reference Interval Methods Summary

Method	Sample Size	Use Case	Calculation
De novo (Non-parametric)	120 healthy subjects	Establish new reference interval	Rank data; use 2.5th - 97.5th percentile (CLSI EP28-A3c)
Verification (Transference)	20 healthy subjects	Adopt manufacturer/published range	Pass if ≥18 of 20 (90%) fall within proposed range
Parametric	≥120; normally distributed	Bell-curve data only	Mean ± 1.96 SD

Key Pearl: Non-parametric method is preferred by CLSI EP28-A3c for ALL reference interval studies - no assumption of normality required.

Reference Interval vs. Clinical Decision Limit

A reference interval is the central 95% of a healthy population - statistical.

A clinical decision limit is an outcome-based threshold set by expert bodies (e.g., ADA for HbA1c, ACC/AHA for lipids, CDC for lead). Clinical decision limits may be tighter or wider than the healthy reference interval - they’re defined by where clinical risk changes, not by where healthy people sit.

51.6 Method Comparison and Validation

Correlation and Regression

When validating a new method against an established one:

Correlation coefficient (r): measures strength of linear relationship (-1 to +1).

r > 0.95 (some say 0.975) generally acceptable
BUT correlation doesn’t assess bias - two methods can correlate perfectly but disagree systematically

Linear regression: y = mx + b

m (slope) should be close to 1.0
b (y-intercept) should be close to 0
Slope ≠ 1 indicates proportional bias
Intercept ≠ 0 indicates constant bias

Deming regression: better than ordinary least-squares regression when both methods have error (which is always true for laboratory methods).

Bland-Altman Analysis

Bland-Altman (difference) plots are superior to correlation for method comparison:

X-axis: Average of the two methods
Y-axis: Difference between the methods
Horizontal lines show mean difference (bias) and limits of agreement (±1.96 SD)

What it shows:

Systematic bias: mean difference not equal to zero
Proportional bias: difference changes with concentration (sloped pattern)
Outliers: points outside limits of agreement
Heteroscedasticity: spread changes with concentration

51.7 Written Procedures and Document Control

Every test in the lab must have written procedures addressing all aspects of the testing process. This is a CLIA requirement and one of the most commonly cited CAP inspection checklist items.

A complete written procedure covers: specimen requirements, patient preparation, reagent preparation, instrument setup, step-by-step analytical procedure, calibration, QC, result calculation and reporting, reference ranges, critical values, limitations, troubleshooting, and safety precautions. Procedures must be accessible at the bench (electronic or paper). Staff must acknowledge they have read and understand current procedures.

Medical Director Review

Written procedures must be reviewed by the medical director (or designee) at three trigger points:

Initially, before the procedure goes live
Each time a change is made
At least every 2 years (biennially), even if no changes

Each review must be documented (signature and date). The biennial review verifies that the procedure reflects current practice, reference ranges are current, QC requirements are met, safety information is accurate, and all referenced materials are available.

Accountability is non-delegable - the medical director owns the review - but the physical review may be delegated to a qualified designee.

Document Control

Written procedures must be maintained in a computerized document control program that manages version tracking, approval workflows, distribution, and archival of superseded versions. The goal: only the current, approved version is available at the bench; old versions are removed to prevent use of outdated procedures.

The system tracks who created, reviewed, approved, and revised each document. Examples: MasterControl, Qualio, MediaLab. Labs typically expose procedures through an intranet with a backup data source for system downtimes.

51.8 Statistical Quality Control (Traditional QC)

Quality control is the lab’s real-time mechanism for catching analytical errors before patient results go out. Traditional QC uses control materials with known target values, plotted on Levey-Jennings charts, and evaluated with Westgard rules.

Levey-Jennings Charts

A Levey-Jennings chart is the workhorse QC tool. Construction:

Run a control specimen ~20 times to establish the mean and SD.
Draw a horizontal line at the mean.
Add horizontal lines at ±1 SD, ±2 SD, and ±3 SD.
Plot each daily QC result as a point on the chart, with time/run number on the x-axis and control value on the y-axis.

The chart gives you a visual history of method performance - shifts, trends, and outliers all become obvious on a Levey-Jennings plot long before they’d be detected by looking at individual numbers.

Three Levels of QC

Daily QC typically uses 3 levels of control material:

Low QC: concentration near the lower clinical decision point (low end of the instrument’s testing range)
Mid-range QC: near the mean of the reference interval (normal range)
High QC: near the upper clinical decision point or pathological level

Each level gets its own Levey-Jennings chart. Using multiple levels catches errors that appear only at certain concentrations - proportional bias or nonlinearity at one end of the range that passes at the other.

CLIA QC Frequency

CLIA requires testing at least 2 levels of QC every 24 hours, or as many as recommended by the manufacturer (whichever is MORE frequent). If the manufacturer says every 8 hours or every run, that prevails. Some analytes (e.g., blood gases) require more than 2 levels. Accrediting organizations (CAP, Joint Commission) may set higher standards than CLIA. Waived tests are exempt from formal QC requirements but must follow manufacturer instructions.

Acceptance Criteria

A QC run is “in control” when all results fall within ±2 SD of the mean. Values outside 2 SD trigger Westgard rule evaluation - not automatic rejection, but a structured check for which type of error is occurring.

A single value between 2 SD and 3 SD is a warning (the 1:2s rule), not a rejection. A value beyond 3 SD is almost always rejected.

Westgard Rules

Westgard rules are applied to Levey-Jennings charts when QC data don’t all fall within 2 SD or show unusual patterns. Each rule detects a different type or severity of error:

Rule	Pattern	Error type	Action
1:2s	1 value between 2-3 SD	Warning only	Evaluate other rules
1:3s	1 value >3 SD from mean	Random error	Reject run
2:2s	2 consecutive values >2 SD on the same side	Systematic bias (or imprecision)	Reject run
R:4s	2 values in the same run differ by >4 SD	Random error	Reject run
4:1s	4 consecutive values >1 SD on the same side	Systematic bias	Investigate
10:mean	10 consecutive values on the same side of the mean	Systematic bias	Investigate / recalibrate

Why the probabilities matter:

1:3s: a value >3 SD by chance occurs only 0.3% of the time - so it’s almost certainly a true error.
4:1s: 4 consecutive values on the same side = (0.5)⁴ = 6.25% by chance - low enough to investigate.
10:mean: 10 in a row on the same side = (0.5)¹⁰ ≈ 0.098% - essentially impossible without a real shift.

Action flow:

1:3s or R:4s → suspect random error (pipetting, bubble, clot, intermittent malfunction). Reject the run. Do NOT report patient results from that run. Investigate cause, correct, re-run QC.
2:2s, 4:1s, or 10:mean → suspect systematic error (reagent, calibration, environmental). Reject or investigate depending on severity. Recalibrate if needed.

Drift vs. Shift

Systematic bias comes in two flavors:

Drift - slow, gradual movement of QC values away from the mean over time. Appears on Levey-Jennings as a progressive upward or downward trend. Causes: aging light source (spectrophotometer), electrode corrosion (ISE), gradual reagent deterioration, buildup of debris in sample pathway, gradual temperature change. Detected by 4:1s or 10:mean rules. Fix: preventive maintenance, recalibration, reagent replacement.

Shift - sudden, sustained change in QC values. Appears as an abrupt jump to a new level that persists. Causes: new reagent lot, sudden temperature/humidity change, component failure (replaced lamp, new electrode), air conditioning malfunction, calibrator preparation error. Detected by 2:2s or 10:mean rules. Fix: identify the sudden change and correct it.

Steps When QC Fails

When QC detects an abnormality, the escalation is:

First - superficially evaluate the test system (reagents, obvious issues: wrong control, expired reagent, bubble, clot, insufficient sample, wrong temperature). Repeat QC on a new aliquot. If it passes, QC handling/deterioration was the issue - patient results may be reported.
Second - thoroughly evaluate instrument and reagents. Repeat QC. If resolved, repeat patient results before reporting.
Third - repeat calibration, then repeat QC. If resolved, repeat patient testing before reporting.
Fourth - formal instrument maintenance. Instrument out of service. Shift to backup method.

No patient results are released from a run with an unresolved QC failure.

Commutable vs. Non-Commutable Control Materials

Commutable QC materials behave the same as real patient samples across different analytical methods. Results can be compared across platforms - useful for trueness assessment and reference method comparison.

Non-commutable materials have a modified matrix (stabilizers, preservatives) that makes them behave differently from patient samples. They’re fine for monitoring within-method precision (day-to-day reproducibility) but can’t validly compare accuracy across different methods. Matrix effects - where the non-analyte components of the material affect how the analyte measures - are the reason most lyophilized or spiked QC materials are non-commutable.

Example from CBC: a QC material for a hematology analyzer may not be blood at all, but a preparation whose scatter/fluorescence mimics blood closely enough to work on that instrument. It will give reproducible readings on that analyzer but may not run at all on a different manufacturer’s platform.

51.9 Individualized Quality Control Plans (IQCP)

Since 2005, CMS has allowed laboratories to use Individualized Quality Control Plans (IQCP) as an alternative to traditional QC for certain testing systems. IQCP lets a lab customize its QC strategy based on the specific risks of its setting rather than applying one-size-fits-all rules.

IQCP is particularly useful for: point-of-care testing, unit-use devices, tests with built-in electronic QC, and situations where traditional liquid QC is impractical. IQCP does NOT apply to waived tests or to tests where the manufacturer specifies particular QC requirements.

Three Components (QRQ Mnemonic)

An IQCP has three components:

Quality Control Plan - documents the standard operating procedure of the IQCP. Specifies which controls run, how often, at what levels, acceptance criteria, corrective actions, and documentation requirements. The QC plan is tailored to the risks identified in the risk assessment.
Risk Assessment - systematically evaluates potential failures and errors at each phase of testing: pre-analytical (specimen collection, transport, identification), analytical (instrument malfunction, reagent issues, environmental factors), and post-analytical (result reporting, interpretation). For each risk: likelihood, impact, existing mitigations, additional controls needed. The risk assessment is the core of the IQCP - it drives everything else.
Quality Assessment Plan - written policies for ongoing monitoring of IQCP effectiveness. What data the lab reviews (QC results, patient result patterns, complaints, errors), how often, what triggers a revision. This is the feedback loop that keeps the IQCP working.

Within the risk assessment, a lab may justify performing traditional QC less often than every 24 hours if mitigating controls (built-in electronic QC, short use intervals, low-risk operator pool) adequately cover the risk.

51.10 Proficiency Testing

Proficiency testing (PT) is the external check on the lab’s analytical performance - how the lab stacks up against peers or against reference methods using blinded unknown specimens. PT is how systematic inter-laboratory errors get caught.

CLIA Enrollment

CLIA requires enrollment in proficiency testing from a CMS-approved provider for all regulated analytes performed using non-waived methods. Major PT providers: CAP (College of American Pathologists), API (American Proficiency Institute), AAB (American Association of Bioanalysts). Failure to enroll in PT for regulated analytes - or intentional referral of PT samples - is a serious CLIA violation that can result in certificate revocation.

Survey Structure

PT programs send 3 surveys per year, each with 5 samples, per analyte (total 15 samples/year). Each survey is one “PT event.” Some samples are placed at decision-point concentrations (e.g., near the upper limit of normal) to specifically test correct classification.

How PT Samples Must Be Handled

PT samples must be treated as normal patient samples: same methods, instruments, personnel, and procedures as routine clinical specimens. This means:

No special handling
No repeat testing unless the SOP repeats all specimens
No consultation with other labs before submitting results
No sending to a reference lab

The purpose of PT is to evaluate routine performance, not best-case performance. Any special treatment constitutes manipulation.

If the lab normally refers the test to an outside lab, it CANNOT refer the PT sample out. PT must be performed in-house. If the lab doesn’t routinely perform the test in-house, it shouldn’t be enrolled in PT for that analyte. Referring PT samples (for testing OR just for “checking” results) is PT fraud - one of the most serious CLIA violations, with potential certificate revocation and criminal penalties.

Commutable vs. Non-Commutable PT Samples

Like QC materials, PT samples can be commutable or non-commutable, and this determines how they’re graded:

Commutable PT → graded against reference method mean/SD. Commutable samples behave identically across platforms, so all labs are compared against a single reference value. More rigorous, more expensive to produce.
Non-commutable PT → graded against peer group mean/SD. Peer groups are defined by manufacturer/method. Only labs on the same platform are compared. This accounts for matrix-dependent behavior but has a blind spot: systematic errors shared across an entire peer group go undetected.

Grading

Each PT sample is graded on its deviation from the mean:

Grade	Deviation
Acceptable	Within ±2 SD
Needs improvement	2-3 SD
Unacceptable	>3 SD

For some analytes (glucose ±6 mg/dL or ±10%, potassium ±0.5 mmol/L, etc.), CLIA uses fixed numeric criteria instead of SD-based grading.

A lab passes a survey when at least 4 of 5 (80%) specimens are acceptable. One unacceptable out of 5 is allowed per event. Failing a single event (≥2/5 unacceptable) triggers mandatory investigation and corrective action.

Failing 2 consecutive events, or 2 of 3 events, for the same analyte = unsuccessful PT performance. This can result in a directed plan of correction, sanctions, or loss of the CLIA certificate for that analyte.

Unacceptable Results: Mandatory Corrective Action

Any single unacceptable result (>3 SD) requires investigation and documented corrective action, even if the survey overall passed. The investigation should cover:

Review of QC data from the time the PT sample was tested
Verification of calibration
Reagent logs, maintenance logs
Check for concurrent patient result issues
Pre-analytical possibilities (mislabeling, wrong method, specimen handling)

Corrective action must be documented alongside the investigation. Common causes: reagent problems, instrument malfunction, calibration problems, climate/environmental problems.

Cembrowski Rules: What Type of Error Caused the PT Failure?

Cembrowski rules help classify PT failures as systematic or random error. They apply when at least 2 of 5 (40%) results are >1 SD from the mean. The pattern tells you the error type:

Cembrowski criterion	Error type	Interpretation
Average departure from mean for all 5 data points >1.5 SD	Systematic	All/most results biased same direction
≥1 result >3 SD from the mean	Random	Extreme outlier on a single measurement
Greatest difference between any 2 results >4 SD	Random	Wide spread between individual results

Systematic → investigation of method: recalibration, reagent lot check, standard verification, interferent investigation.

Random → investigation of specimen-level factors: pipetting, bubbles, clots, mislabeling, intermittent instrument problems.

Even surveys with 100% acceptable results can be evaluated with Cembrowski rules to flag subtle bias that doesn’t trigger the basic pass/fail criteria.

Alternative Proficiency Assessment

For assays without an available PT program (rare tests, LDTs, novel analytes), the lab must demonstrate proficiency through alternative means, at least twice per year. Options:

Split-sample comparison with a reference lab or peer lab that performs the assay
Interlaboratory exchange with peer labs
Split samples run against an established in-house methodology
Clinical correlation (patient chart review comparing results to clinical status)
Testing of commercially available characterized/assayed reference materials

The alternative assessment must be documented. The medical director must approve the alternative assessment plan.

51.11 Common Laboratory Calculations

Anion Gap

Anion Gap = Na - (Cl + HCO3)

Normal: 8-12 mEq/L (varies by method)

Elevated anion gap: Unmeasured anions present

Mnemonic: MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates)

Correction for albumin: for every 1 g/dL decrease in albumin below 4, add 2.5 to the expected anion gap (albumin is an unmeasured anion).

Osmolal Gap

Calculated osmolality = 2(Na) + Glucose/18 + BUN/2.8

Osmolal gap = Measured osmolality - Calculated osmolality

Normal: <10 mOsm/kg

Elevated osmolal gap: Unmeasured osmoles present (toxic alcohols - methanol, ethylene glycol, isopropanol)

Corrected Calcium

For hypoalbuminemia: Corrected Ca = Measured Ca + 0.8 × (4 - Albumin)

Or measure ionized calcium directly (preferred in critical illness).

LDL Cholesterol (Friedewald Equation)

LDL = Total Cholesterol - HDL - (Triglycerides / 5)

Not valid if: Triglycerides >400 mg/dL, non-fasting sample, or type III hyperlipoproteinemia.

Creatinine Clearance (Cockcroft-Gault)

CrCl = [(140 - age) × weight (kg)] / [72 × serum creatinine]

Multiply by 0.85 for women.

Note: this estimates creatinine clearance, not GFR. eGFR equations (CKD-EPI, MDRD) are now preferred for most purposes.

Reticulocyte Production Index

RPI = Reticulocyte % × (Patient Hct / Normal Hct) × (1 / Maturation factor)

Maturation factor: 1.0 (Hct 45%), 1.5 (Hct 35%), 2.0 (Hct 25%), 2.5 (Hct 15%).

RPI >2-3 indicates adequate marrow response to anemia; RPI <2 indicates inadequate response (hypoproliferative anemia).

Chapter 52: Quality Management Systems

Quality in the clinical laboratory is not just “did the analyzer give the right number.” It spans the whole path from the moment a clinician clicks an order in Epic to the moment a physician acts on a result. A quality management system (QMS) is the scaffolding that makes all of that consistent, documented, and improvable. This chapter covers the overarching QMS framework, the vocabulary you will be asked to distinguish on the exam (QA vs QC vs QI, verification vs validation, AMR vs CRR, analytical sensitivity vs clinical sensitivity), the statistics behind internal QC, the rules around proficiency testing, and the specific experiments required to bring a new instrument or assay into routine use. Personnel qualifications, training retention, LIS validation, and downtime procedures all live here too - they are medical-director responsibilities that show up on board exams.

52.1 The Quality Management Framework

Quality in the clinical laboratory encompasses everything from test ordering to result reporting and interpretation. A quality management system (QMS) provides the organizational structure, processes, and resources needed to achieve quality objectives.

A quality management system is defined as coordinated activities to direct and control an organization with regard to quality. It is the umbrella term that contains QA, QC, and QI as sub-activities.

Key components of a laboratory QMS:

Document control and records management
Personnel qualifications and competency assessment
Equipment management
Process control (including QC)
Nonconformance management and corrective action
Internal audits
Continuous quality improvement

The 12 Quality System Essentials (CLSI)

CLSI organizes laboratory QMS activities into 12 Quality System Essentials. You should at least recognize them:

Organization - roles, responsibilities, reporting structure
Personnel - job descriptions, qualifications, orientation, training, competency, continuing education
Equipment - acquisition, installation, calibration, validation, maintenance, repair
Purchasing and Inventory - vendor management, reagent/supply control, labeling, storage
Process Control - QC, specimen collection/handling/storage, method verification and validation
Information Management - confidentiality and accuracy of requisitions, results, reports
Documents and Records - creation, revision, director approval, distribution, retention
Occurrence Management - identification, documentation, investigation of nonconformances; root cause analysis; corrective action
Assessment - internal and external inspections
Process Improvement - quality metrics, ongoing improvement, patient safety
Customer Service - feedback and suggestions
Facilities and Safety - minimize risk to patients and staff

QA, QC, QI - How the Three Actually Differ

These terms get conflated, but the board will make you distinguish them.

Quality assurance (QA) is a proactive system of planned activities aimed at fulfilling quality requirements - preventing errors before they happen. It asks “are we doing things right?” QA includes:

Adherence to standard operating procedures
Internal and external audits
Proficiency testing participation
Monitoring quality metrics (turnaround times, specimen rejection rates, critical value notification compliance)

Quality control (QC) is a system that detects systematic or analytical errors that could cause release of erroneous patient test results. QC is concurrent - it runs alongside patient testing. It measures accuracy, precision, and reproducibility of test results over time. If QC fails, patient results from that run are held until the problem is fixed. QC detects problems, but it does not prevent them.

Quality improvement (QI) describes systems and processes aimed at raising product quality above the current level. QI is both retrospective (why did we have this problem) and prospective (can we do better than the current baseline). QI tools: root cause analysis, Pareto charts, fishbone diagrams, PDCA (Plan-Do-Check-Act), Six Sigma, Lean.

Continuous quality improvement (CQI) is a specific QI philosophy built on cyclical, never-ending improvement: identify, change, measure, refine, repeat - the PDCA (Deming) cycle. The point is that quality is never “good enough.”

Quick memory hook:

QA = doing it right (prevention)
QC = checking that it was right (detection)
QI = making it better (improvement)

52.2 Pre-analytical, Analytical, and Post-analytical Phases

Laboratory errors can occur at any phase of the testing process:

Pre-analytical phase (~60-70% of errors):

Test ordering (appropriate test selection)
Patient preparation
Specimen collection (identification, technique)
Specimen transport and handling
Specimen processing

Analytical phase (~10-15% of errors):

Instrument malfunction
Reagent problems
Calibration issues
QC failures

Post-analytical phase (~20-25% of errors):

Result transcription/entry
Result interpretation
Critical value notification
Report delivery

Total testing process recognizes that laboratory quality extends from the clinical question through to the clinical action taken on results.

52.3 Internal Quality Control

Quality control is the safety net that catches analytical errors before they harm patients. The concept is simple: if you analyze a sample with a known value and get the expected result, you have evidence that the test system is working properly. If the control result is wrong, something is wrong with the test system, and patient results from that run cannot be trusted.

The Logic of Control Materials

Control materials are samples with known analyte concentrations that behave like patient samples. When you run a control alongside patient samples, it goes through the exact same analytical process. If the instrument correctly measures the control, it’s presumably correctly measuring the patients.

Good control materials share the matrix (the fluid composition) of patient samples. A control dissolved in water might behave differently than one in a protein-rich serum matrix. Controls should cover the clinically important range - a normal level control and an abnormal level control ensure the test works across the decision-making range.

Many laboratories use two or three levels of controls, run at the beginning of each shift or each day of testing. This seems like a lot of effort, but consider the alternative: without QC, you might not discover a problem until clinicians notice that results don’t match clinical findings - by which time hundreds of erroneous results may have been reported.

Understanding Statistical Process Control

Laboratory QC applies statistical process control principles developed in manufacturing. The fundamental insight is that even when a test is working properly, results vary slightly from measurement to measurement due to random factors. This variation follows a predictable pattern - the normal (Gaussian) distribution.

When you establish QC ranges, you analyze the control material many times (at least 20 runs over 20 days) to determine the mean and standard deviation of that variation. In a normal distribution, about 68% of values fall within 1 SD of the mean, 95% within 2 SD, and 99.7% within 3 SD.

This means that even when everything is working correctly, you expect about 5% of control values to exceed 2 SD by chance alone. If you reject every run where a control exceeds 2 SD, you’ll reject many perfectly good runs - a problem called “false rejection.” But if you’re too permissive, you’ll miss true errors.

The Westgard Rules: Balancing Sensitivity and Specificity

The Westgard multi-rule QC system solves this problem by using multiple rules together. A single control exceeding 2 SD (the “1:2s rule”) is just a warning that triggers closer inspection - it’s not grounds for rejection because it could easily be chance. But patterns that are unlikely to occur by chance signal true problems.

The 1:3s rule rejects when a single control exceeds 3 SD. By chance, this should happen only 0.3% of the time. When it does happen, something is almost certainly wrong.

The 2:2s rule rejects when two consecutive controls exceed 2 SD in the same direction. While a single value exceeding 2 SD is common (5% chance), two consecutive values both exceeding 2 SD on the same side has only a 0.25% probability by chance. This pattern suggests a systematic error - the test has shifted away from the true value.

The R:4s rule (“range” rule) rejects when one control exceeds +2 SD and another exceeds -2 SD in the same run. This spread - a range of more than 4 SD - indicates excessive random error, such as a pipetting problem or an unstable reagent.

The 4:1s and 10x rules detect trends and shifts. Four consecutive values on the same side of the mean (even if within 1 SD) or ten consecutive values on the same side suggests the method is drifting. Catch it early before it becomes a larger problem.

Random Errors vs. Systematic Errors

Understanding error types helps you troubleshoot QC failures.

Random errors are unpredictable - a bubble in a pipette, electrical interference, a scratched cuvette. They increase imprecision (results scatter more widely) but don’t consistently push results in one direction. The R:4s rule catches large random errors.

Systematic errors consistently shift results in one direction - an incorrectly prepared calibrator, a deteriorating reagent, or a temperature drift. They cause bias (results are consistently too high or too low). The 2:2s, 4:1s, and 10x rules catch systematic errors.

When QC fails, think about which rules were violated to guide your investigation. A 1:3s violation could be random (try repeating the control), while a 2:2s violation suggests you need to look for systematic causes like reagent problems or calibration drift.

52.4 External Quality Assessment (Proficiency Testing)

Proficiency testing (PT) evaluates laboratory performance by comparing results on unknown samples to peer laboratories and/or reference values.

CLIA requirements:

Participation in approved PT program for each regulated analyte
Analysis must be performed by routine methods and personnel
Results cannot be shared with other laboratories during testing
Unsatisfactory performance requires root cause analysis and corrective action
Two consecutive or two of three consecutive failures = unsuccessful

Benefits of PT:

Identifies systematic bias
Provides peer comparison
Detects method-specific problems
Required for regulatory compliance

52.5 Verification vs. Validation of New Methods

Before an instrument or assay can be used for patient testing, the laboratory must prove it performs correctly. There are two flavors of this pre-go-live work, and the exam will ask you to tell them apart.

Verification = experiments to confirm that, within your laboratory, a test performs as the manufacturer claims.

Validation = experiments to prove both the clinical value of a test and its performance characteristics. Validation is more extensive than verification - it includes everything verification does plus greater statistical power, assessment of additional parameters, and establishment of clinical claims.

Memory aid: Validation = “Does it work?” (you are proving it for the first time). Verification = “Does it work HERE?” (the manufacturer already proved it works in general; you confirm it works in your hands).

When Verification Is Enough

Verification is the minimum requirement for an FDA-approved test used for its intended scope without modification. The manufacturer has already validated the assay during the FDA clearance process. Your job is just to confirm the manufacturer’s claims hold in your lab with your patient population.

When Validation Is Required

Validation is required when implementing:

A laboratory-developed test (LDT)
A non-FDA-cleared test
Any modification of an FDA-approved test (different specimen type, different cutoff, different instrument, any deviation from the package insert)

Validation is comprehensive: accuracy, precision, analytical sensitivity (LOD, LOQ), analytical specificity (interference, cross-reactivity), reportable range, reference intervals, and clinical validity all must be established.

CLIA Verification Elements

Per CLIA, instrument/method verification must include:

Precision (within-run and between-run reproducibility)
Accuracy (comparison to reference method or materials)
Reportable range (AMR and CRR)
Reference intervals (verify or establish)
Sensitivity (analytical and clinical) - required for LDTs
Specificity (analytical and clinical) - required for LDTs

Items 1-4 are required for all tests; items 1-6 for laboratory-developed tests. All data must be documented and reviewed by the medical director before the method is used for patient testing.

52.6 Calibration and Calibration Verification

These sound similar but do different jobs. Keep them straight.

Calibration is the process of adjusting an instrument to accurately read the value of a known control substance. Calibrators (standards) with precisely known concentrations are run through the instrument, and the instrument’s response curve is adjusted so that the readout matches the known values. Calibration establishes the mathematical relationship between the raw signal (absorbance, fluorescence, voltage) and analyte concentration.

Example: a sample has a known hemoglobin of 13.4 g/dL, but the instrument reads 12.0 g/dL. Calibration adjusts the instrument so the readout matches 13.4 g/dL.

Calibration verification is the step that comes after calibration. You test the instrument with several specimens of known concentration across the reportable range to make sure the calibration is still valid at every clinically relevant point, not just at the concentrations used for calibration. It confirms linearity and accuracy at multiple points.

Analogy: calibration is like zeroing a scale. Calibration verification is like testing the scale with a series of known weights.

Frequency Requirements

Calibration verification must be performed at least every 6 months. It must also be repeated when:

A new lot of reagents is introduced
Major preventive maintenance is performed
QC indicates a problem
The manufacturer recommends it

Calibration verification is not the same as recalibration. Verification checks that the existing calibration still holds. If verification passes, no action is needed. If it fails, the instrument must be recalibrated and re-verified.

52.7 Precision Verification

Precision is the reproducibility of a result. It is assessed two ways:

Within-run precision (repeatability) - variation when the same sample is run multiple times in a single analytical run (same batch, same operator, same reagents). Captures short-term random error.
Between-run precision (reproducibility) - variation when the same control is run across different runs, days, operators, or reagent lots. Captures longer-term variability.

Between-run precision is always greater than or equal to within-run precision because it adds sources of variability (calibration changes, environmental drift, operator differences).

Precision is expressed as standard deviation (SD) or coefficient of variation (CV = SD/mean × 100%).

Precision testing requires at least 20 samples (replicates). These may be distributed as 20 within-run replicates in a single run, or split across days (e.g., 2 replicates/day × 10 days). CLSI EP05-A3 provides the standard protocol. The measured SD and CV are compared to the manufacturer’s claims or clinical requirements to determine acceptability.

52.8 Accuracy Verification

Accuracy is how close the measured value is to the true value. There are two standard ways to verify it.

Recovery Experiment

Accuracy can be verified by a recovery experiment - certified reference materials (CRMs) of known quantity are analyzed and compared to the certified value.

Percent recovery = (measured / certified) × 100%. Acceptable recovery is typically 90-110% (within 10% of true value). CRMs are traceable to recognized reference standards (e.g., NIST). Recovery detects systematic bias but does not assess precision.

Method Comparison

Accuracy can also be verified by a method comparison experiment - results from the new instrument/method are compared to results from a previously validated method using real patient samples.

Method comparison requires at least 40 samples spanning the clinically relevant range, tested over multiple days (not all 40 in one run). CLSI EP09-A3 recommends running each sample on both methods within 2 hours to minimize specimen degradation.

Plot the results: old/reference method on the y-axis, new method on the x-axis. Analyze with regression:

Y-intercept indicates constant bias - a fixed offset across all concentrations (e.g., the new method always reads 3 mg/dL higher). Correctable by recalibration.
Slope indicates proportional bias - the difference between methods grows with concentration. Suggests a fundamental analytical difference.
Ideal: slope = 1.0, y-intercept = 0.

Additional tools: correlation coefficient (r), Bland-Altman plots (bias at each concentration). Method comparison is the most clinically meaningful accuracy verification because it uses real patient samples.

52.9 Analytical Specificity

Analytical specificity is the ability of an instrument to detect the target analyte despite interfering substances. It is distinct from clinical specificity (disease detection). Interferents include:

Hemoglobin (hemolysis)
Bilirubin (icterus)
Triglycerides (lipemia)
Drugs, cross-reactive substances, heterophile antibodies

Testing Analytical Specificity

Analytical specificity is tested using recovery experiments with defined interferents: add known amounts of a potential interferent (e.g., hemoglobin, bilirubin, intralipid) to samples with known analyte values, then run the test. Compare to unspiked controls. If the interferent shifts the result beyond acceptable limits, the assay has poor analytical specificity for that substance. CLSI EP07-A3 provides the protocol.

Indirect ISE and Pseudohyponatremia

The classic board pearl on analytical specificity is indirect ion-selective electrode (ISE) sodium measurement causing pseudohyponatremia in lipemia or hyperproteinemia.

The mechanism: indirect ISE first dilutes the specimen with a fixed ratio of diluent, then measures sodium in the diluted mixture. The final calculation assumes water occupies ~93% of plasma volume. When lipids or proteins occupy more than the usual 7%, the water fraction is lower, but the instrument still divides by the total volume. The math yields a falsely low sodium.

Fixes:

Direct ISE (undiluted) - not affected by lipemia/protein
Blood gas analysis (uses direct ISE)
Ultracentrifuge the specimen to remove lipids before indirect ISE

Board pearl: pseudohyponatremia = indirect ISE artifact; direct ISE gives the correct sodium.

Carryover

Carryover is an analytical specificity problem: analyte from one specimen contaminates the instrument and affects the next specimen’s result. The instrument is detecting analyte that was carried over from the previous sample rather than the current sample’s true value.

Key features:

Carryover is mostly a problem for analytes with a wide range of positive values - hCG, PSA, tumor markers. A specimen with 500,000 mIU/mL of hCG followed by one with 5 mIU/mL can produce a falsely elevated result from even trace contamination.
Carryover studies are performed by running serial specimens of high and low concentration - CLSI EP10-A3 protocol: run 3 high-concentration specimens (H1, H2, H3) followed by 3 low-concentration specimens (L1, L2, L3). If L1 is significantly higher than L2/L3, carryover is present. Percent carryover = (L1 - L3) / (H3 - L3) × 100%.
Acceptable carryover: <1.5%. Modern analyzers typically achieve well below 0.1%.
High carryover usually points to a dispensing/pipetting system problem - worn or cracked probes, inadequate wash volume or wash solution, air bubbles, worn tubing. The probe tip is the most common source.
Fixes: replace probe, increase wash volume, add detergent to wash solution, switch to disposable tips, or reorder specimens (low before high).

52.10 Analytical Sensitivity

Analytical sensitivity is the ability of an instrument to detect low levels of an analyte. It is distinct from clinical sensitivity (disease detection). Analytical sensitivity is quantified by two thresholds.

Limit of blank (LOB) / Limit of detection (LOD) = the lowest concentration that can be distinguished from the background (blank). LOB is the highest result expected from an analyte-free specimen. LOD = LOB + 1.645 × SD(low-concentration sample). A result above the LOD means the analyte is detectable, but the exact value may not be reliable.

Functional sensitivity / Limit of quantitation (LOQ) = the lowest concentration that can be reliably quantified with an acceptable coefficient of variation (typically CV ≤ 20%). LOQ is always greater than LOD. You can detect analyte below the LOQ, but you can’t say how much is there.

Example: high-sensitivity troponin may have an LOD of 1 ng/L but an LOQ of 6 ng/L. A result of 3 ng/L means troponin is detectable but the exact concentration is uncertain. Results between LOD and LOQ are often reported as “detected” without a number.

This matters clinically for analytes where small concentrations drive decisions: troponin, hCG, drug levels, tumor markers.

52.11 Reportable Range: AMR vs. CRR

The full reportable range has two tiers. The distinction is board-testable.

Analytical measuring range (AMR) = the range of concentrations an analyte can be measured without manipulation (no dilution, no concentration). Determined by linearity experiments. Below the AMR, results are unreliable (near LOQ/LOD). Above the AMR, the assay saturates (hook effect, plateau).

Clinical reportable range (CRR) = the range of quantitative results that can be reported, including samples that required dilution to fall within the AMR. CRR extends the upper limit of the AMR by the maximum validated dilution factor.

Example: AMR upper limit = 500 mg/dL, maximum validated dilution = 1:10. CRR upper limit = 5,000 mg/dL.

Rules:

CRR upper limit > AMR upper limit - because dilution extends the upper end.
CRR lower limit = AMR lower limit - there is no routine way to concentrate patient specimens to extend the lower limit. Results below the AMR lower limit are reported as “< [value].”

Linearity Experiments

AMR is determined by linearity experiments: samples of known concentration spanning the expected range (typically 5-7 levels from near-zero to above the claimed upper limit) are measured. Measured values are plotted against expected values. The AMR is the range over which the relationship stays linear (slope ≈ 1, no systematic deviation). Deviations at the extremes define the AMR boundaries. CLSI EP06-A is the standard.

52.12 Specimen Stability

Specimen stability is the length of time a stored specimen will continue to produce reliable results. Stability depends on:

Analyte characteristics (some are labile, others robust)
Specimen type (serum vs whole blood vs urine)
Storage temperature (room temp, 4°C, -20°C, -70°C)
Container type
Preservatives/anticoagulants

Common labile analytes: ammonia, blood gases, lactic acid, ACTH, renin, catecholamines.

Stability is determined by testing specimens at different intervals under defined storage conditions. Protocol: collect specimens, test immediately (baseline), then test aliquots at defined time points (1 h, 2 h, 4 h, 24 h, 48 h, etc.) under specified storage. Compare to baseline. The stability limit is the longest interval at which results stay within acceptable total error. Stability must be established or verified for each analyte / specimen type / storage condition combination.

52.13 Laboratory Information Systems and LIS Validation

Laboratory information systems (LIS) are the software systems that support laboratory operations and communicate patient data to the EMR. LIS functions include:

Order entry
Specimen tracking
Result entry / review / auto-verification
QC management
Billing
Instrument interfacing

The LIS is the central nervous system of the laboratory. It connects to analyzers via middleware, to the EMR via HL7/FHIR interfaces, and to billing systems.

Middleware refers to the software interfaces between instruments and the LIS. Middleware handles:

Auto-verification (rule-based release without manual review)
Reflex testing (automatically ordering confirmatory tests)
Delta checks (comparing to previous values)
Dilution management
Result routing

Examples: Data Innovations Instrument Manager, MYLA (bioMerieux).

LIS Validation During Instrument Implementation

When testing a new instrument, the LIS function must be tested to ensure values are correctly transcribed from the instrument into the EMR. Validation includes:

Correct patient/specimen linkage
Result accuracy at every transmission point (instrument → middleware → LIS → EMR)
Auto-verification rules
Flagging and critical value alerts
Units, reference ranges, interpretive comments

A single mapping error can cause systematic result misreporting across thousands of patients.

Downtime Procedures

Every laboratory must have downtime procedures - typically paper-based orders and manual result communication - so clinicians can order tests and blood products when the LIS is unavailable. Components:

Paper order requisitions
Manual specimen labeling
Manual result recording and communication
Alternative critical value reporting
Manual blood product issue

Staff must be trained, drills should happen regularly, and after LIS recovery all paper records must be reconciled into the system. Downtime procedures are checked by inspectors.

52.14 Personnel, Training, and Competency

Personnel management is a recurring medical-director responsibility. A handful of specific items are board-testable.

Required Documents

Job description - every laboratory must have one for each role defining qualifications, education, certification requirements, and duties. Verified during CAP inspection - inspectors check that actual staff qualifications match what the job description requires.
Organizational chart - demonstrates the reporting structure; who reports to whom; where the medical director sits. Must be kept current.
Personnel files - educational qualifications, licenses, training, continuing education, competency assessments, disciplinary actions. Files must be readily available to inspectors on the day of inspection, even if physically stored outside the laboratory.

Training Documentation Retention

Training documentation retention:

Non-blood bank staff training: 2 years
Blood bank staff training: 5 years

The longer blood bank retention reflects FDA’s heightened regulatory oversight of blood establishments (annual FDA inspections in addition to biennial CLIA/CAP).

Competency Assessment

Competency of laboratory personnel must be documented within 6 months of initial assessment, then annually thereafter. The CLIA-required competency assessment uses six methods, and all six must be used at least once per assessment period:

Direct observation of test performance
Monitoring of recording and reporting results
Review of QC records
Assessment of proficiency testing performance
Problem-solving exercises
Written or oral tests

Communiqué for New Assays or Changes

Whenever the laboratory introduces a new assay/instrument or makes a major change to an existing test (including discontinuation), the medical director must compose a communiqué informing clinical staff. It should include:

What is changing
When the change takes effect
Impact on ordering and result interpretation
New reference ranges (if applicable)
Who to contact with questions

Distribution should be documented.

52.15 Laboratory Test Panels and Billing

This section is narrow but shows up as a clinical pathology board pearl on medical directorship.

AMA-approved test panels (e.g., BMP, CMP, hepatic function panel, lipid panel) must be billed as a panel using the panel CPT code. The sum of reimbursements for the individual components is greater than the panel reimbursement. Billing the components separately to recover that difference is insurance fraud called “unbundling”. Example: billing separately for Na, K, Cl, CO2, BUN, creatinine, glucose, and calcium instead of CPT 80048 (BMP) is unbundling and violates the False Claims Act.

In-house custom panels (e.g., an “autoimmune hepatitis” Epic order that triggers ANA, anti-smooth muscle, anti-LKM, and anti-mitochondrial) may bill components separately as long as the panel is approved by medical staff and published annually. The annual publication requirement ensures transparency: ordering physicians know exactly which tests each custom panel includes. Custom panels must be clinically justified - grouping tests solely to increase billing is fraud.

52.16 Six Sigma in the Laboratory

Six Sigma is a data-driven quality improvement methodology that measures process performance in terms of defects per million opportunities.

Sigma metric: Number of standard deviations between the process mean and the nearest specification limit

Sigma calculation for laboratory tests: σ = (TEa - |Bias|) / CV

Where:

TEa = Total allowable error (from CLIA or biological variation)
Bias = Systematic error (difference from true value)
CV = Coefficient of variation (imprecision)

Interpretation:

6σ: World-class (3.4 defects per million)
5σ: Excellent
4σ: Good (industry standard for most labs)
3σ: Marginal (minimum acceptable)
<3σ: Unacceptable

Higher sigma allows less frequent QC; lower sigma requires more frequent QC monitoring.

52.17 Lean Methodology

Lean focuses on eliminating waste and improving efficiency.

The eight wastes (DOWNTIME):

Defects: Errors requiring rework
Overproduction: Doing more than needed
Waiting: Idle time
Non-utilized talent: Underusing staff skills
Transportation: Unnecessary movement of materials
Inventory: Excess stock
Motion: Unnecessary movement of people
Extra-processing: Doing more than required

5S methodology:

Sort: Remove unnecessary items
Set in order: Organize remaining items
Shine: Clean workspace
Standardize: Create consistent processes
Sustain: Maintain improvements

Value stream mapping: Analyzes process flow to identify waste and bottlenecks

Chapter 53: Regulations and Accreditation

Regulations are the scaffolding of clinical lab practice. Nothing you do at the bench is exempt from them, and the board exam tests them heavily. This chapter walks through the major federal frameworks - CLIA ’88, FDA device regulation, Medicare / Medicaid, OSHA, HIPAA - and the major accreditation bodies (CAP, TJC, AABB, COLA). It also covers billing law (Stark, Anti-Kickback, False Claims Act), record retention, personnel qualifications, and the pre-examination / post-examination variables that CLIA pushes labs to control.

The structure of the exam’s questions on this material is predictable. Which agency does what under CLIA? Who issues which certificate? How long do you retain which records? Which tube do you draw in what order? Those are the patterns, and that is how this chapter is organized.

53.1 CLIA ’88: Overview and Agencies

CLIA ’88 is the Clinical Laboratory Improvement Amendments of 1988, an amendment to the original Clinical Laboratory Act of 1967. It overhauled federal lab regulation in the wake of Pap smear errors that exposed inadequate QC across the industry. CLIA applies to all laboratories performing testing on materials derived from the human body for health assessment, diagnosis, or treatment - hospital labs, reference labs, physician offices, nursing homes, pharmacies, mobile units. If it is a specimen from a person and the result drives care, CLIA regulates it.

Exemptions from CLIA

A few categories sit outside CLIA:

Collection kits meant for specimen collection only (no testing performed on site) - the testing lab bears CLIA responsibility, not the collection site
Workplace (forensic) drug testing - regulated instead by SAMHSA under DHHS, following the Mandatory Guidelines for Federal Workplace Drug Testing Programs
Research testing - exempt only if results are NOT used for patient care. An IRB-approved study with blinded, non-returned results is CLIA exempt. A clinical trial where lab values determine dosing or eligibility requires CLIA compliance. Intended use of the result, not the physical location, governs.

Board trap: “forensic” vs. “clinical” drug testing are two different regulatory frameworks. Clinical drug testing (ordered by a physician for patient care) IS subject to CLIA.

State exemptions

A state may be granted CLIA exemption by CMS if its own laboratory regulatory program is equal to or more stringent than CLIA. Currently only New York and Washington have approved CLIA-exempt state programs. Labs in these states follow state regulations, not CLIA directly, but the standards are equivalent or stricter.

The three agencies

CLIA delegates authority to three federal agencies. This is the most commonly tested organizational fact in the whole chapter.

Agency	Role
CMS (Centers for Medicare & Medicaid Services)	Administrative / enforcement. Issues certificates, collects fees, conducts inspections, approves accrediting organizations, approves state exemptions, publishes CLIA regulations, approves PT programs, monitors PT performance
FDA (Food and Drug Administration)	Categorizes test complexity, reviews waiver requests, develops complexity guidance. Also regulates medical devices and biologics as a separate authority
CDC (Centers for Disease Control)	Scientific / technical. Develops technical standards and practice guidelines, conducts quality improvement studies, provides technical assistance, manages the CLIA advisory committee (CLIAC), develops educational resources

Memory aid: CMS = Certificates & Money & Surveillance; FDA = complexity & devices; CDC = science & standards.

A few sub-points the exam likes to test:

FDA categorizes test complexity and reviews waiver applications. But CMS issues the actual Certificate of Waiver to the lab. These are two separate steps by two separate agencies.
CMS approves private accrediting organizations (CAP, TJC, COLA) by granting them “deemed” status.
CDC manages CLIAC (the advisory committee). CLIAC recommendations are advisory only and require CMS rulemaking to take regulatory force.

53.2 Test Complexity Categorization

The FDA classifies tests based on the highest complexity testing a lab performs. The scoring system uses 7 criteria (knowledge, training, reagent prep, operational technique, calibration / QC / PT, troubleshooting, interpretation), each scored 1-3. Score ≤12 = moderate complexity; >12 = high complexity. Waived is a separate category below moderate.

Waived tests

Waived tests are simple, low-risk tests that untrained users can perform accurately (or that are FDA-cleared for home use). They have minimal CLIA requirements.

Personnel: at minimum a high school diploma; no specific training or certification required by CLIA, though state law may add requirements
The lab must possess a Certificate of Waiver (issued by CMS)
No PT requirement
No routine inspections (CMS may inspect on complaints)
Must follow manufacturer instructions - waived ≠ unregulated

Common waived tests: urine dipstick, urine hCG, fecal occult blood, rapid strep A, rapid influenza, glucose by FDA-approved POC devices, some HbA1c and PT/INR POC devices.

Board trap: adding a single non-waived test means the lab needs a higher-level certificate.

Non-waived: moderate and high complexity

Non-waived testing (moderate or high complexity) has the full CLIA framework:

Qualified laboratory director and personnel
Validation of lab tests, written procedures
Daily positive and negative controls when patient samples are run
Enrollment in a CMS-approved proficiency testing program
Record retention per CLIA
Biennial (every 2 years) external laboratory inspections, with an internal self-inspection in the intervening year

Moderate vs. high complexity differs primarily in personnel requirements and the amount of manual skill the test requires.

Moderate complexity = heavily automated. Instrument does most of the work, operator intervention is minimal. Examples: automated chemistry, automated hematology, automated coagulation.
High complexity = heavily manual, requires significant skill, judgment, and interpretation. Examples: manual differential, immunofluorescence, flow cytometry interpretation, cytogenetics, molecular, Pap smear screening. Most pathology-specific tests fall here.

53.3 CLIA Certificates

There are five CLIA certificate types:

Certificate	Issued by	Notes
Certificate of Waiver	CMS	Waived tests only
Certificate of PPM (Provider-Performed Microscopy)	CMS	Subcategory of moderate complexity
Certificate of Registration	CMS	Temporary; allows testing to begin while awaiting initial inspection
Certificate of Compliance	CMS	After CMS (or state) inspection
Certificate of Accreditation	CMS-deemed accrediting organization (CAP, TJC, COLA)	After deemed-organization inspection

All CLIA certificates are issued by CMS except the Certificate of Accreditation, which comes from the deemed private accrediting organization. This is a commonly tested distinction.

Certificates must be renewed every 2 years, regardless of type, volume, or complexity.

Starting a new lab

The sequence: (1) Apply to CMS for certification → (2) receive Certificate of Registration (temporary, allows testing to begin) → (3) undergo inspection → (4) receive Certificate of Compliance (from CMS) or Certificate of Accreditation (from a deemed organization). Testing cannot begin until the Certificate of Registration is in hand.

Five-certificate limit on the medical director

Under CLIA, a single medical director cannot hold more than 5 CLIA certificates simultaneously. Each certificate corresponds to a unique laboratory location. The rationale: meaningful oversight requires presence. Board pearl: if a question describes a director wanting to add a 6th lab, they would need to relinquish directorship of one existing lab first.

Given there are only 5 certificate types, the only way to hit this limit is to be director at more than one lab at a time.

Certificate of PPM (Provider-Performed Microscopy)

PPM is a niche carve-out within moderate complexity. To qualify as a PPM procedure, all three of the following must be true:

Performed by a qualified provider (physician MD/DO, dentist, or midlevel practitioner under physician supervision) - not a laboratory technician
Uses a microscope in brightfield or phase contrast only. Fluorescence, polarized light, darkfield, and EM do NOT qualify as PPM.
Specimen is labile - a delay in examination would compromise the specimen’s diagnostic integrity

Classic examples: wet mount for Trichomonas / clue cells, KOH prep for fungal elements, urinalysis microscopy, fern test for amniotic fluid, nasal smear for eosinophils, pinworm (Enterobius vermicularis) scotch tape prep.

The same physician doing a wet prep in clinic = PPM (lower QC burden). A medical technologist doing urine microscopy in the main lab = regular moderate complexity testing (full CLIA framework).

53.4 Personnel Qualifications

Medical director (laboratory director)

Under CLIA, the medical director must hold an MD, DO, DPM, or PhD (or equivalent). A board-certified pathologist (AP/CP or CP) automatically meets all requirements for any complexity level. Non-pathologist physicians and PhDs can direct labs but must document additional training / experience in the relevant disciplines.

The director must have a license in the state where the lab is located. Separate from CLIA, it is a state-level requirement. Multi-site labs need a qualified director for each site.

If the medical director is not a physician or PhD - for example a DPM running a limited-scope lab - a clinical consultant with a doctoral degree (MD, DO, or PhD) is required to provide clinical oversight.

The medical director’s non-delegable responsibilities include overall responsibility for lab operations, ensuring regulatory compliance, and accountability for results. CLIA specifies which responsibilities can be delegated and which must be overseen directly. Daily QC review, procedure manual updates (with MD approval), and competency assessments can be delegated to qualified supervisors. Delegation must be documented.

The medical director must also ensure that lab staff is adequate in number, training, and competency. This is not delegable - even if HR handles hiring, the medical director is responsible for verifying that qualifications and competency standards are met.

CAP is sometimes stricter than CLIA on director qualifications. CLIA allows some lesser-education arrangements; CAP typically does not.

Technical supervisor

Non-waived labs must have a technical supervisor responsible for technical oversight. Minimum education: bachelor’s degree in a chemical, physical, biological, or clinical laboratory science (plus relevant experience, especially for high complexity). Duties include selecting test methods, establishing / verifying performance specs, resolving technical problems, evaluating QC and PT, and ensuring corrective action.

Testing personnel

The individuals actually performing the tests. Minimum education: high school diploma or equivalent. For moderate complexity, documented training is required. For high complexity, an associate or bachelor’s degree in a relevant science is typically needed.

Testing personnel must undergo documented competency assessment initially, at 6 months, at 12 months, and annually thereafter. Competency assessment has 6 required elements: direct observation of test performance, monitoring of recording and reporting, review of QC and PT records, direct observation of instrument maintenance, written testing, and problem-solving scenarios. Missing the 6-point competency assessment in the first year is a classic CAP Phase II deficiency.

53.5 Accreditation Organizations

CMS grants “deemed” status to private accrediting organizations whose standards meet or exceed CLIA’s. The three most common:

CAP (College of American Pathologists) - the largest; most widely used for hospital labs. Detailed section-by-section checklists. Often exceeds CLIA standards.
TJC (The Joint Commission) - accredits entire hospitals; lab inspection may be part of a hospital-wide survey. TJC may inspect a lab if it’s part of a hospital-wide inspection.
COLA (Commission on Office Laboratory Accreditation) - primarily serves smaller physician office laboratories.

Other deemed organizations exist (AABB for blood banks, ASHI for HLA labs) but these three are the most commonly tested.

A lab can hold a Certificate of Accreditation from EITHER TJC or CAP - it does not need both. Many hospitals use CAP for the lab (the checklist is considered more comprehensive) and TJC for the rest of the hospital.

Inspection cycle

External inspection every 2 years; internal self-inspection in the intervening year. Every year there is an inspection, alternating external / internal. The internal inspection should use the same checklist as the external one, with documented corrective actions available at the next external visit.

CMS may also conduct unannounced validation inspections (random checks of accredited labs) or complaint investigations.

CAP Inspection Phases

CAP classifies checklist deficiencies by phase, and these come up on the exam.

Phase	Timeframe	Meaning
Phase 0	Not officially graded	New standards in development; educational / informational only
Phase I	Correct by next internal inspection (~1 year)	Important but not immediately threatening to patient safety
Phase II	Correct within 30 days	Directly threatens result accuracy or patient / staff safety

Phase II deficiencies require a Corrective Action Plan submitted within 30 days with documented evidence of correction. Persistent Phase II deficiencies can lead to loss of CAP accreditation and, consequently, loss of CLIA certification.

Common Phase II Deficiencies

Category	Example Phase II Deficiency
Proficiency Testing	Unacceptable PT performance without documented investigation
Validation / Verification	Using a test for patient care before completing full validation
Personnel	Lab Director not meeting CLIA / CAP education requirements
QC	Releasing patient results when QC was out of range
Safety	Improper storage of flammables; missing eyewash maintenance
Competency	Missing 6-point competency assessment in first year

The “Big Three” most commonly cited Phase II deficiencies:

Reagent / kit expiration - in-use expired reagents = automatic Phase II
Temperature log gaps - fridge / freezer gap with no corrective action documented = Phase II
Instrument comparability - twice-yearly correlation studies required for duplicate platforms

53.6 Proficiency Testing

CLIA requires enrollment in an approved PT program for all non-waived (moderate and high complexity) tests. PT is the external quality assessment mechanism: unknown samples are analyzed as if they were patient specimens, and results are graded against peer group or reference method values.

Key PT mechanics:

PT vendors must be approved by CMS. Major providers: CAP (largest), API (American Proficiency Institute), others.
Once a lab selects a PT vendor, it must remain with that vendor for at least 1 year. Prevents “vendor shopping” to avoid unfavorable scoring.
Typical PT cycle: 3 shipments per year, 5 samples per event, must score ≥80% on each event.
PT is not required for waived tests (voluntary enrollment is encouraged).
For analytes without available PT (rare tests, LDTs), the lab must demonstrate proficiency through alternative assessment - split-sample analysis, interlaboratory comparison, clinical correlation.

Intentionally referring PT samples to another lab (collusion) is a serious violation that can result in certificate revocation. The sample must be run as if it were a routine patient specimen in the lab that received it.

53.7 Record Retention

Retention requirements are a board-favorite because they are memorizable and exploitable for CLIA-vs-CAP trick questions.

Summary table

Material	CLIA requirement	CAP variant (if different)
Histology slides	10 years	-
Non-forensic autopsy slides	10 years	-
Forensic autopsy slides	Indefinitely	-
Cytology slides	5 years	FNA slides: 10 years
Blood and body fluid smears	7 days	-
Microbiology slides (Gram, AFB, KOH)	7 days	-
Flow cytometry plots	10 years	-
Tissue blocks	2 years	10 years
Wet tissue	Until report completed	2 weeks after report
Requisitions	2 years	-
Test reports (non-pathology)	2 years	-
Pathology reports	10 years	-
PT records, QM / QC records	2 years	-
Discontinued procedure records	2 years after discontinuation	-
Blood bank QC records	5 years	-
Blood bank donor / recipient records	10 years	-
Deferred donor records	Indefinitely	-

Mnemonic that actually works:

Most weird documents (requisition, discontinued procedures, PT, QM, QC, test reports) → 2 years
Blood bank paperwork → 5 years
Important clinical stuff (pathology reports, histology slides, flow plots, blood bank donor/recipient) → 10 years
Only two things retained indefinitely: forensic autopsy materials and deferred donor records

Two classic trick questions:

CLIA cytology = 5 years; CAP FNA = 10 years. A CAP-accredited lab must follow the more stringent standard.
CLIA blocks = 2 years; CAP blocks = 10 years. Since most hospital labs are CAP-accredited, the effective standard is 10 years.

General principle: when CLIA and CAP (or any other regulatory / accreditation body) differ, the lab must follow the most stringent standard among all bodies it is subject to.

53.8 Required Lab Documentation

Requisitions

CLIA requires a written or electronic requisition documenting the physician / clinician order for every test. Required fields: patient identification, name of authorized ordering provider, test(s) requested, specimen source, date of collection, relevant clinical information. Verbal orders must be documented within a defined timeframe. Requisitions are retained for 2 years.

Written procedures

All laboratories must maintain written (or electronic) procedures for every test performed. Procedure manuals must include: specimen requirements, step-by-step analytical procedure, reagent preparation, calibration, QC, result reporting, reference ranges, and limitations. Procedures must be reviewed by the medical director initially and with each change, and biennially thereafter. All staff must have access to current procedures at the bench. Discontinued procedures are retained for 2 years after discontinuation.

Laboratory reports (post-examination)

CLIA requires every laboratory report to contain:

Patient identification (at least 2 identifiers)
Date the result is reported (time is NOT required by CLIA, though best practice)
Specimen source
Identity of the test performed
Test result with units and reference range
Name and address of the performing laboratory

Specimen source matters because reference ranges vary by specimen type - CSF glucose is not plasma glucose; clean-catch urine culture is not catheterized urine culture.

53.9 FDA: Medical Devices, Biologics, and Tests

Under its separate authority (and cross-linked with CLIA), the FDA regulates medical devices and biologics. In the lab context this means diagnostic instruments, test kits, reagents, blood products, and in vitro diagnostic (IVD) devices.

Clearance vs. approval

All medical devices receive either clearance or approval from the FDA. These are NOT the same thing.

Pathway	Application	When used	Rigor
Clearance (510(k))	Premarket Notification	Device is substantially equivalent to an existing legally marketed predicate device	Lower - no clinical trials typically required, 3-6 months
Approval (PMA)	Premarket Approval	Novel, high-risk devices without a suitable predicate	Higher - clinical trial data required, 1-3 years

Most clinical laboratory analyzers, reagent kits, and POC devices reach market through 510(k) clearance. PMA is reserved for Class III devices and novel high-risk products. Example: HER2 / neu IHC kits used to determine trastuzumab eligibility often require PMA (companion diagnostics).

Board pearl: cleared ≠ approved. The exam tests this.

Labs using FDA-cleared / approved devices for their intended use need only verify performance. Labs using devices off-label or with modifications must validate performance (a more extensive process).

Devices that don’t need FDA review

510(k) exempt devices - usually Class I. Must be labeled with a statement that the device is not FDA cleared / approved. Still subject to general controls (registration, listing, GMP). Exempt ≠ unregulated.
All medical devices marketed before 1976 - grandfathered in under the Medical Device Amendments of 1976.
Microscopes and microscope accessories - common pathology example of FDA-exempt devices. Class I, well-established safety.
Humanitarian Use Devices (HUDs) - devices for conditions affecting fewer than ~4,000 to 8,000 individuals per year in the US. Exempted from the full PMA process via Humanitarian Device Exemption (HDE), but the institution’s IRB must approve the device’s use.

Device classes

Class	Risk	Regulation	Examples
Class I	Lowest	General controls; majority are 510(k) exempt	Tongue depressors, microscopes, pipettes, most ASRs, manual cell counters, dental floss
Class II	Moderate	Special controls + 510(k)	Most automated analyzers; ASRs used in transfusion medicine
Class III	Highest	PMA required	Heart valves, HER2 / PD-L1 companion diagnostics, HIV screening assays, some molecular diagnostics

Class III devices are generally those used to diagnose fatal conditions or conditions affecting public health (HIV tests, TB tests, cancer companion diagnostics, blood donor infectious disease screens). The rationale: false negatives for these have catastrophic consequences.

Analyte-Specific Reagents (ASRs)

An ASR is an individual reagent (antibody, receptor protein, ligand, nucleic acid sequence, etc.) that binds to a specific analyte to produce a diagnostic result. ASRs are building blocks for laboratory-developed tests, sold as individual components, not as complete test systems.

Key classification rules:

Most ASRs are Class I medical devices (minimal regulation)
ASRs used in transfusion medicine are Class II. The exception reflects the immediately life-threatening nature of blood bank errors.

A reagent is NOT classified as an ASR if:

It is a component of a manufacturer’s complete test system but is not separately marketed (then the whole kit is regulated as an IVD)
It is labeled for in vitro diagnostic use (then it is a full IVD product, not an ASR)

The FDA’s 1997 ASR rule requires a specific disclaimer on all tests built from ASRs: “This test was developed and its performance characteristics determined by [laboratory name]. It has not been cleared or approved by the US Food and Drug Administration.” Institutions cannot make performance claims about ASR-based tests in marketing or advertising unless the test undergoes formal PMA. Performance data can still appear in peer-reviewed publications, validation records, and reports to ordering clinicians.

IHC ASR classification (pathology-specific)

IHC use	ASR class	Examples
Adjunctive (clarifies findings already evident on H&E)	Class I	Pankeratin, most general IHC
Prognostic / predictive	Class II	ER / PR, Ki-67
Therapeutic target (directly determines treatment eligibility)	Class III	HER2, PD-L1

This is a classic high-yield board topic.

Laboratory-Developed Tests (LDTs)

LDTs are tests designed, manufactured, and used within a single laboratory - not commercially distributed. Examples: many molecular diagnostic tests, specialized IHC panels, rare disease assays, pharmacogenomics.

LDTs are regulated by CMS through CLIA rather than FDA. The FDA has historically exercised “enforcement discretion” over LDTs, although the regulatory framework has been evolving and is politically contested. Because LDTs don’t go through FDA premarket review, the developing lab must perform full validation (not just verification): accuracy, precision, analytical sensitivity / specificity, reportable range, reference intervals, and clinical validity.

FDA inspection of blood facilities

Laboratories involved with blood products are inspected by the FDA, annually - more frequent than CLIA’s biennial cycle. Blood products are regulated as biologics under the Public Health Service Act. Blood banks must follow FDA’s current Good Manufacturing Practices (cGMPs). A hospital blood bank is often double-regulated: FDA for blood product operations, CAP / CMS for lab testing operations, and often AABB on top for voluntary accreditation.

Forensic drug testing

Laboratories performing forensic drug testing are certified under a separate DHHS registry (NLCP), not CLIA. These labs follow SAMHSA’s Mandatory Guidelines for Federal Workplace Drug Testing Programs: initial immunoassay screen, MS confirmation, chain of custody, Medical Review Officer (MRO) review.

53.10 Medicare and Medicaid

Medicare and Medicaid were established by the Social Security Act of 1965, signed by LBJ. Both are administered by CMS at the federal level.

Medicare

Medicare is a federal program, federally administered, and covers three groups:

Age ≥65
Permanently disabled (after 24 months of disability benefits)
End-stage renal disease (ESRD) patients (regardless of age) - because dialysis is prohibitively expensive

Medicare is an entitlement program - eligibility is not income-based.

Memory aid: you CARE for the elderly (MediCARE).

Medicare parts

Part	Covers	Lab relevance
A	Inpatient care: hospital stays, hospice, SNF, home health	Inpatient lab tests bundled into the DRG, not billed separately
B	Outpatient services, AND inpatient physician professional services (including pathologist interpretation)	Outpatient lab billed via the Clinical Laboratory Fee Schedule (CLFS); pathologist interpretation always billed here
C	Medicare Advantage (managed care)	-
D	Optional prescription drug coverage	Pharmacogenomics increasingly relevant but not directly billed here

Board pearl: inpatient lab = Part A (bundled in DRG); outpatient lab = Part B. But pathologist interpretation of an inpatient biopsy is Part B, not Part A, because it is a physician professional service. The technical / facility component is Part A; the professional component is Part B. This distinction is frequently tested.

DRG and the 3-day rule

Under Part A, hospitals are reimbursed a fixed sum per admission based on the patient’s Diagnosis Related Group (DRG). The DRG payment covers all facility costs: room, nursing, supplies, drugs, labs. If actual costs exceed the DRG payment, the hospital absorbs the loss. If costs are below, the hospital profits. This means inpatient test volume does not directly generate additional revenue.

The CMS 3-day rule (72-hour rule): laboratory tests performed during the 3 calendar days before admission are bundled into the DRG if related to the reason for admission. They cannot be separately billed under Part B. This prevents cost-shifting.

The full bundling window is 3 days before admission through 14 days after discharge (for tests performed on samples obtained during the hospital stay). Gaming is blocked on both ends.

CLIA certificate required for Medicare

A laboratory must hold an active CLIA certificate to receive Medicare reimbursement. No CLIA = no Medicare payment. Losing CLIA certification has immediate, severe financial consequences - this is one of CLIA’s most powerful enforcement mechanisms.

Claims processing

Medicare claims are processed by nongovernmental contractors referred to as “fiscal intermediaries” (Part A) and “carriers” (Part B). In practice these functions have been consolidated under Medicare Administrative Contractors (MACs), but the Part A / Part B split persists.

Advanced Beneficiary Notice (ABN)

Providers must give Medicare patients an ABN when ordering services that may not be covered by Medicare. The ABN explains: what services may not be covered, why Medicare may deny coverage, and the estimated cost. The patient then chooses whether to proceed (accepting financial responsibility) or decline. Without a signed ABN, the provider cannot bill the patient for denied services. ABNs are commonly used for screening tests lacking Medicare coverage or tests ordered outside established medical necessity criteria.

Medicaid

Medicaid is a federal-state program, administered by individual states, providing healthcare to low-income individuals of any age. Key distinctions from Medicare:

Means-tested (income-based)
All ages, not just elderly / disabled
Jointly funded federal + state
Each state sets its own eligibility criteria, benefits, and reimbursement rates within federal guidelines

Some patients qualify for both (“dual eligible”). Medicaid reimbursement for lab testing is typically lower than Medicare rates.

Memory aid: you AID the poor (MediCAID).

53.11 Billing Regulations

Direct vs. client billing

Direct billing = the laboratory bills the patient or insurance directly for lab services. Standard for reference labs. This is the only billing method allowed by Medicare.

Client billing = the laboratory bills the physician or physician’s office, who then bills the patient / insurer (often at a markup). Medicare does not allow client billing for Medicare patients. Some state Medicaid programs and private insurers do allow it.

The concern with client billing: physicians may seek the cheapest labs rather than the best-quality labs, because they pocket the markup. Some states that allow client billing have anti-markup laws requiring disclosure of the actual cost to the patient / insurer and capping the markup.

Exception to Medicare’s direct-billing rule: physician office labs (POLs) that perform their own testing can bill Medicare directly under their own CLIA certificate.

False Claims Act (FCA)

The False Claims Act prohibits submitting knowingly fraudulent claims to Medicare or Medicaid. “Knowingly” includes deliberate ignorance or reckless disregard for the truth - you cannot legally claim you didn’t know merely because you didn’t do due diligence.

In the laboratory context, FCA violations include: unbundling, billing for tests not performed, upcoding (using a higher-paying CPT code than appropriate), billing for medically unnecessary tests, and falsifying QC data.

Penalties:

Civil: treble damages (3× the government’s losses) plus per-claim penalties ($11,000-$23,000+ per false claim)
Criminal: imprisonment and criminal fines for knowing violations; exclusion from all federal healthcare programs
For lab directors and managers, criminal liability can attach to those who knowingly approve or participate in fraudulent billing

The FCA includes a whistleblower (qui tam) provision allowing a private individual (a “relator”) to file suit on behalf of the United States. The government decides whether to intervene.

A successful whistleblower receives a percentage of the recovery:

Government intervenes: 15-25% of the recovery
Government declines and whistleblower proceeds alone: 25-30% of the recovery

FCA recoveries routinely run into the hundreds of millions, so whistleblower rewards can be substantial. Many major laboratory fraud cases have been initiated by whistleblowers.

Stark Law (Physician Self-Referral Law)

The Stark Law prohibits physicians from referring Medicare / Medicaid patients to designated health services (DHS) - including clinical laboratory services - in which the physician or an immediate family member has a financial relationship (ownership / investment interest or compensation arrangement).

Key features:

Applies only to Medicare / Medicaid patients, not privately insured or self-pay
Strict liability statute - no intent to defraud is required. A prohibited referral, even if clinically appropriate, violates Stark. This is the key distinction from the Anti-Kickback Statute.
Originally enacted in 1992; expanded in 1995 to cover additional DHS including PT / OT, radiology, DME, home health, outpatient drugs, inpatient / outpatient hospital services

Stark has several exceptions. The most relevant for pathologists is the in-office ancillary services exception, which allows physicians to refer to and bill for lab testing performed within their own practice if: testing is performed in the same building where the physician practices, testing is supervised by the referring physician or a group member, and the service is billed by the physician or group. This allows physician office laboratories to exist within the Stark framework.

Anti-Kickback Statute (AKS)

The Anti-Kickback Statute makes it a criminal offense to knowingly offer or receive payment, gifts, or other remuneration as a reward for referring patients to any service payable by Medicare / Medicaid. Remuneration includes cash, housing, hotels, meals, and inflated medical directorship salaries.

Key differences from Stark:

AKS requires knowing and willful intent; Stark is strict liability
AKS applies to anyone; Stark applies to physicians
AKS covers all federal healthcare programs; Stark focuses on DHS referrals

Penalties: criminal felony, up to 10 years imprisonment, fines up to $100,000 per kickback plus 3× the remuneration value, and program exclusion.

Safe Harbor Regulations describe payment / business practices that may appear to violate the AKS but are legally protected if they meet all the specific criteria of the safe harbor. Examples: fair-market-value employment, space / equipment rental at fair market value, personal services contracts at fair market value, properly disclosed discounts. If arrangements fall outside a safe harbor (e.g., a medical director paid far above industry standard), AKS liability is still possible.

HIPAA

HIPAA (Health Insurance Portability and Accountability Act) creates national standards protecting personal health information (PHI) and forbids disclosure without patient consent.

Two main rules:

Privacy Rule: restricts who can access PHI and how it can be used / disclosed
Security Rule: mandates safeguards for electronic PHI (ePHI)

For labs: patient results, specimen labels, requisitions, and pathology reports all contain PHI.

Violations: civil fines $100-$50,000 per violation (up to $1.5M / year per category); criminal penalties up to 10 years imprisonment for intentional violations.

Minimum necessary standard: only access the PHI needed for your job function.

53.12 OSHA

OSHA (Occupational Safety and Health Administration) is a division of the Department of Labor - not DHHS or CMS. Jurisdiction is worker safety, not patient care or test quality.

Board pearl: CLIA = test quality and patient results; OSHA = worker safety and hazard protection. Different agencies, different purposes.

Major OSHA-regulated hazards in the lab:

Chemical (formalin, xylene, acids, solvents)
Bloodborne pathogens (HIV, HBV, HCV)
Physical (needlesticks, ergonomics, electrical, radiation)
Fire

Penalties: serious violations can exceed $15,000 per violation; willful violations can exceed $156,000 each.

Chemical Hygiene Plan

OSHA requires every lab to develop and implement a written Chemical Hygiene Plan (CHP) under the OSHA Laboratory Standard (29 CFR 1910.1450). Contents:

SOPs for chemical use
Criteria for PPE selection
Fume hood requirements (volatile / toxic chemicals must be handled in fume hoods)
Employee training
Medical surveillance for exposed workers
Emergency procedures
Post-exposure evaluation

The CHP is lab-specific.

OSHA requires every lab to designate a Chemical Hygiene Officer (CHO) responsible for implementing the CHP. CHO duties: chemical inventory, labeling and storage, fume hood monitoring, safety training, SDS (formerly MSDS) maintenance, spill / exposure investigation, record keeping.

Bloodborne Pathogen Standard (29 CFR 1910.1030)

Key requirements:

Written exposure control plan
Universal / standard precautions
Personal protective equipment
Hepatitis B vaccination offered free of charge to all employees with occupational exposure to blood or other potentially infectious materials (OPIM), within 10 working days of initial assignment. Employees may decline (must sign a declination form) but can change their mind later.
No vaccines exist for HCV or HIV, so OSHA does not mandate them
Post-exposure evaluation and follow-up protocols (source patient testing, prophylaxis)
Hazard communication (labels, SDS)

Techs working with blood in any section (chemistry, hematology, coag, blood bank) are covered.

Permissible Exposure Limits (PELs)

OSHA requires employers to ensure that employee chemical exposures remain at or below PELs, defined as 8-hour time-weighted average (TWA) concentrations. Examples for pathology labs:

Formaldehyde PEL = 0.75 ppm TWA (with a short-term limit of 2 ppm over 15 minutes)
Xylene PEL = 100 ppm TWA

If monitoring reveals exposures above PELs, the employer must implement engineering controls (fume hoods, ventilation), administrative controls (work practice changes), or respiratory protection.

Training and post-exposure

OSHA requires documented safety training at initial assignment, when new hazards are introduced, and annually for bloodborne pathogens. The employer must verify employees understand the content.

Post-exposure evaluation protocols must cover: immediate decontamination, medical evaluation by a qualified provider, documentation of exposure details (chemical or pathogen, concentration, duration, route), and follow-up monitoring. Bloodborne exposures trigger the additional BBP Standard requirements.

53.13 Employment Law: FLSA

The Fair Labor Standards Act (FLSA) classifies employees as exempt (no overtime required) or non-exempt (overtime pay required for >40 hrs / week).

Exempt categories (all must pass three tests: salary level, salary basis, duties test):

Professional Exemption (“learned professional”): advanced degree and specialized intellectual work (physicians, pathologists). Physicians are exempt from overtime under FLSA even when working >40 hours / week.
Executive Exemption: manages a department / enterprise, supervises ≥2 FTEs, has hiring / firing authority
Administrative Exemption: office work related to management / business operations with independent judgment

Salary Basis: fixed predetermined salary not reduced for quality / quantity of work variations. Always check current DOL thresholds - these change with administration.

53.14 Laboratory Budget and Cost Analysis

For full detail on budget categories and break-even analysis, see Chapter 54. The headlines for this chapter:

Labor (personnel) = the largest single budget item (~50-70% of total operating budget)
Among supply items, blood products are typically the largest non-labor expense in a hospital laboratory
Break-even volume = Fixed Costs / (Revenue per test − Variable cost per test)

Example: $6,000 monthly lease / ($40 reimbursement − $20 reagent cost) = 300 tests / month to break even.

53.15 Non-Examination Variables

“Non-examination” (non-analytical) variables are the things that happen before the test is run (pre-examination / pre-analytical) or after the result is produced (post-examination / post-analytical). Pre-analytical errors account for ~60-70% of all laboratory errors; post-analytical errors account for ~25-30%; analytical errors are the smallest fraction. That is where the exam’s high-yield content lives.

Patient identification

At least 2 patient identifiers must be on all patient samples (specimen containers, blood tubes, etc.). Acceptable identifiers: patient name, date of birth, medical record number, Social Security number, phone number, or other person-specific identifier. A hospital room number is NOT a valid patient identifier - rooms change, patients transfer. The two-identifier rule prevents specimen mix-ups, which are among the most dangerous lab errors. Blood bank has even more stringent identification requirements.

Two identifiers must also be used when reporting results.

Physiologic pre-analytical variables

Effects to know:

Serum gastrin is increased after a meal (G cells release gastrin in response to protein). For accurate gastrin measurement (e.g., Zollinger-Ellison workup), the patient should be fasting. Gastrin is also elevated by PPI use, which must be held before testing.
Serum bilirubin decreases after a meal (increased hepatic blood flow and clearance). Conversely, fasting increases unconjugated bilirubin - this is the basis for Gilbert syndrome’s fasting hyperbilirubinemia.
CK, LDH, and AST are all increased after exercise (physiologic muscle cell leakage). Elevation can persist 24-72 hours. Don’t diagnose rhabdo or MI on CK alone in someone who just ran a marathon; don’t call AST elevation liver disease without considering recent exercise.
Cigarette smoking increases CEA (up to 5-10 ng/mL in smokers vs. a typical non-smoker baseline).

Tourniquet effects

Prolonged tourniquet application (>1 minute) causes:

Increased potassium (tissue hypoxia triggers K+ release; venous stasis can cause hemolysis)
Increased calcium (hemoconcentration of protein-bound calcium)
Increased anaerobic metabolism (lactic acid accumulates, lowers pH)
Hemoconcentration - fluid shifts from intravascular to extravascular space under increased hydrostatic pressure. Total protein, albumin, cholesterol, iron, and cell counts all rise.

Release the tourniquet within 1 minute or loosen before filling tubes for affected analytes.

Tube top colors and additives

Memorize the table. This is a certainty on the exam.

Tube color	Additive	Use	Mechanism
Yellow (SPS)	Sodium polyanethol sulfonate	Microbiology blood cultures	Anticoagulant + anti-complement + anti-phagocytic + neutralizes aminoglycosides
Red	None	Serum chemistry, serology, drug levels	Blood clots naturally; supernatant = serum
Gold / SST	Clot activator + gel separator	Serum chemistry	Silica speeds clotting; gel barrier separates serum from clot
Blue	3.2% sodium citrate	Coagulation (PT / INR, PTT, fibrinogen, D-dimer, factor assays)	Chelates Ca²⁺; 9:1 blood:citrate ratio is critical
Light green	Lithium heparin (or sodium heparin)	Plasma chemistry (STAT)	Activates antithrombin III; plasma usable immediately (no clotting wait)
Lavender / purple	EDTA (K2 or K3)	Cell counts (CBC), blood smears, ESR, HbA1c, flow cytometry, molecular	Chelates Ca²⁺; preserves cell morphology
Pink	EDTA	Blood bank (type and screen, crossmatch, antibody panel, DAT)	Same EDTA as lavender but larger volume
Gray	Sodium fluoride + potassium oxalate	Glucose, lactate, blood alcohol	Fluoride inhibits enolase (antiglycolytic); oxalate anticoagulates

Additional nuances:

Yellow is confusing because there are two yellow tubes: SPS (microbiology cultures) and ACD (specialized blood bank / HLA / DNA studies). Board questions typically refer to the SPS version.
Red and gold both yield serum; light green, lavender, pink, gray, and blue yield plasma (or whole blood).
Plasma vs. serum: plasma retains coagulation factors (anticoagulated, fibrinogen intact); serum has higher levels of CLaMP (Calcium, LDH, Magnesium, Phosphate), released from platelets and RBCs during clotting. Serum potassium also runs higher than plasma potassium for the same reason, so plasma K is more accurate than serum K.
Light green cannot be used for lithium levels (the tube itself has lithium) and cannot be used for coagulation (heparin affects clotting times).
EDTA (lavender / pink) cannot be used for coagulation testing or ionized calcium.
EDTA-dependent pseudothrombocytopenia: EDTA can trigger anti-GPIIb/IIIa antibody-mediated platelet clumping, causing a spuriously low platelet count. If suspected, redraw in citrate or heparin.

Blue top: the 9:1 ratio

The citrate:blood ratio in a blue top must be exactly 9:1 (blood:citrate). Under-filling produces falsely prolonged clotting times because excess citrate chelates more calcium than intended. Over-filling dilutes the citrate and may allow partial clotting. Board trap: a short-draw blue tube invalidates the sample.

Order of draw

When multiple tubes are collected, follow the order of draw to prevent cross-contamination of additives:

Yellow (SPS / blood culture)
Blue (citrate / coag)
Red (no additive)
Gold / SST (serum separator)
Green (heparin)
Lavender / purple / pink (EDTA)
Gray (fluoride / oxalate)

Rationale: blue is drawn early to avoid EDTA contamination (EDTA chelates Ca²⁺ and would falsely prolong clotting times). EDTA tubes are drawn before gray to avoid oxalate contamination.

Memory aids: “Stop, Before Really Good People Get Grumpy” (SPS, Blue, Red, Gold, Green, Purple, Gray) or “Your Body Really Gets Purple/Pink and Gray.”

Specimen handling timing

Ideally, tests should be run within 2 hours of blood collection. After 2 hours, cellular metabolism in the tube drives analyte drift:

Glucose ↓ (glycolysis; ~5-7% per hour at room temperature without fluoride)
Potassium ↑ (cellular leakage)
LDH ↑ (cellular lysis)
Phosphate ↑
pH ↓ (lactic acid accumulation)

Separation of serum / plasma from cells (by centrifugation) within 2 hours mitigates most of these changes. Some analytes are more time-sensitive: ammonia, blood gases, lactic acid should be analyzed within 15-30 minutes.

Post-examination errors

Post-examination (post-analytical) errors include: transcription errors (copying results incorrectly), proofreading failures, incorrect interpretation by clinicians, delayed reporting, reporting to the wrong patient / provider, and incorrect follow-up actions. These are 25-30% of all laboratory errors.

Critical values

Critical values (panic values) are lab results indicating a potentially life-threatening condition requiring immediate clinical action. Examples: K⁺ >6.5 mEq/L, glucose <40 or >500 mg/dL, platelet count <20k, PTT >100 seconds.

CLIA requires labs to have a critical value notification policy. Results must be communicated directly to a responsible caregiver within a defined timeframe (typically 15-30 minutes). Documentation must include: the value, person notified, date / time, and read-back confirmation.

Chapter 54: Laboratory Operations

Laboratory operations is the administrative backbone that keeps the lab running - test utilization, turnaround time, critical value programs, reference lab management, and the financial machinery of billing, coding, and budgeting. The medical director is responsible for all of it. Board questions in this domain lean heavily on billing/coding vocabulary (ICD, HCPCS, CPT, RVU, professional vs. technical component) and on the accounting frameworks used for cost analysis (fixed vs. variable, direct vs. indirect, break-even, budget categories). This chapter covers the operational topics and then steps through the financial mechanics in depth.

54.1 Test Utilization Management

Appropriate test utilization ensures the right test is ordered for the right patient at the right time.

Overutilization problems:

Increased costs
Patient harm from unnecessary follow-up
Downstream imaging and procedures
Anxiety from false positives

Underutilization problems:

Missed diagnoses
Delayed treatment
Increased morbidity

Utilization management strategies:

Clinical decision support (alerts, order guidance)
Reflex and cascade testing protocols
Test menus designed for clinical relevance
Education and feedback to clinicians
Hard stops for inappropriate orders

54.2 Turnaround Time (TAT)

TAT is the interval from specimen collection (or order) to result availability.

Components:

Order to collection
Collection to receipt in lab
Receipt to testing
Testing to result validation
Result to clinician notification

Monitoring and improvement:

Define TAT targets based on clinical needs (STAT vs. routine)
Track TAT metrics (median, outliers)
Identify bottlenecks through workflow analysis
Implement process improvements

54.3 Critical Values

Critical values are results that require immediate communication because they represent life-threatening conditions.

Elements of a critical value program:

Defined list of critical values (specific to institution and patient population)
Notification procedures (who, how, when)
Time limits (typically 30-60 minutes)
Read-back verification
Documentation of notification (date, time, person notified, person calling)
Monitoring and escalation procedures

54.4 Reference Laboratory Management

Selecting a reference laboratory:

Test menu and methodology
Turnaround time
Quality indicators (accreditation, PT performance)
Cost
Specimen requirements
Customer service

Managing send-out testing:

Specimen preparation protocols
Tracking systems
Result verification and entry
Communication of critical values

54.5 Billing and Coding

Every laboratory claim submitted to a payer (Medicare, Medicaid, private insurance) requires two coding systems - one to describe why the test was ordered, and one to describe what was actually done. If either is missing, or if the two don’t match up, the claim gets denied.

ICD and HCPCS: The Two Required Codes

Every claim requires both an ICD code and an HCPCS code.

ICD (International Classification of Diseases) - describes the patient’s medical problem (the diagnosis or clinical reason for the test). The current version is ICD-10-CM. Examples: E11.9 (Type 2 diabetes without complications), D50.9 (iron deficiency anemia, unspecified), Z12.4 (screening for malignant neoplasm of cervix).
HCPCS (Healthcare Common Procedure Coding System) - describes the services rendered (what test or procedure was performed).

The pairing is how the payer decides if the test was medically necessary. The ICD code must support medical necessity for the HCPCS code - if the diagnosis doesn’t justify the test, Medicare will deny payment. This is called failing the medical necessity requirement. The ordering physician is responsible for supplying the ICD code, but lab staff have to make sure the requisition actually has one.

HCPCS Levels: Level I (CPT) and Level II

HCPCS is split into two levels:

Level I = CPT (Current Procedural Terminology) codes. Maintained by the AMA. These are 5-digit numeric codes. Laboratory testing uses CPT codes.
Level II codes are alphanumeric (a letter followed by 4 digits) and cover non-physician services, supplies, and durable medical equipment. Example: P9612 (catheterized urine collection).

Lab CPT codes live in the 80000 - 89999 range. Key series to recognize:

CPT range	Area
80047 - 80081	Organ/disease panels
81000 - 81099	Urinalysis
85004 - 85999	Hematology / coagulation
86000 - 86999	Immunology
87001 - 87999	Microbiology
88000 - 88399	Surgical pathology / cytopathology

Specific examples: 85025 (CBC with differential), 80053 (comprehensive metabolic panel), 88305 (surgical pathology, Level IV).

Professional vs. Technical Component

Anatomic pathology and some CP services (e.g., peripheral smear review) split into two billable components:

Professional component (PC, modifier -26) - the pathologist’s interpretation and diagnosis
Technical component (TC, modifier -TC) - tissue processing, staining, slide preparation, instrument time
Global (no modifier) - both together

For a Medicare patient, the pathologist’s interpretation is always billed under Medicare Part B - regardless of whether the patient is inpatient or outpatient. The technical component follows the setting: Part A for inpatients (bundled into the DRG) and Part B for outpatients.

Relative Value Units (RVUs)

Payment for each CPT code is based on Relative Value Units (RVUs). RVUs are meant to be a standardized measure: if procedure X generates twice the RVUs of procedure Y, it’s supposed to be worth twice as much. Each CPT code gets RVUs assigned in three categories:

Work RVU - physician time, skill, and judgment
Practice expense RVU - overhead, equipment, supplies
Malpractice RVU - liability insurance cost

Total RVU = work + practice expense + malpractice.

Payment = Total RVU $\times$ Geographic Practice Cost Index (GPCI) $\times$ Conversion Factor ($)

RVUs are the foundation of the Resource-Based Relative Value Scale (RBRVS) used by Medicare. Higher RVU means higher payment. The GPCI adjusts for regional cost differences. The conversion factor is a dollar amount set annually by CMS.

54.6 Cost Analysis and Financial Management

Two different lenses are used to classify laboratory costs - they overlap but they’re not the same thing. Board questions lean on knowing which category a given cost falls into under each system.

Two Cost Classification Systems

Lab costs can be viewed through either the fixed/variable lens or the direct/indirect lens. These systems overlap but are not identical:

A reagent is both variable (changes with volume) and direct (tied to a specific test).
A manager’s salary is both fixed (doesn’t change with volume) and indirect (not tied to a specific test).

You need to know both systems because the board will ask the same cost from both angles.

Fixed vs. Variable Costs

Fixed costs are unaffected by the number of tests performed.

Rent
Instruments / equipment (purchase or lease)
Equipment depreciation
Management salaries
Insurance premiums
IT infrastructure

Fixed costs create a floor - the lab has to cover this baseline whether it runs 100 or 10,000 tests. Per-test fixed cost decreases as volume increases because the fixed cost is spread over more tests. This is why high-volume labs have better economics.

Variable costs change proportionally with the number of tests performed.

Reagents
Disposable consumables (pipette tips, cuvettes, sample cups)
Technologist wages (more tests requires more hours, overtime, additional hires)
Shipping / transport for send-outs

A few classifications to memorize:

Cost	Fixed or Variable
Rent	Fixed
Instruments	Fixed
Managerial salaries	Fixed
Reagents	Variable
Technologist wages	Variable

The manager vs. technologist distinction is worth internalizing: managers are salaried and you don’t hire more of them when the test volume goes up. Technologists are paid hourly, and more tests means more overtime or more hires. For board purposes, technologist wages are variable.

Caveat: real labs have a core technologist staff that is essentially fixed, with overtime and per diem flexing as variable. Reagent rental agreements (the vendor provides the instrument “free” in exchange for a minimum reagent commitment) blur the line too - a chunk of fixed equipment cost gets shifted into variable reagent cost. For the exam, stick with the clean classifications.

Unit Cost

Unit cost is the total cost of performing one test:

Unit cost = (Fixed costs / number of tests) + Variable cost per test

Because the fixed cost component is divided by test volume, unit cost decreases as test volume increases. Two reasons:

Fixed costs are spread over more tests (the big one).
Economies of scale - higher-volume labs get bulk pricing on reagents, so variable cost per test can drop a bit too.

This is the economic argument for lab consolidation and for reference lab models. Spreading a fixed cost base over millions of tests drives unit cost down dramatically.

Direct vs. Indirect Costs

Direct costs are incurred by running a specific test. They can be attributed to that specific test or procedure.

Reagents for the assay
Calibrators and controls
Disposable supplies used during the test
Electricity to run the instrument during the test
Bench technologist labor allocated to running the test

Direct costs are relatively easy to track and assign, which makes them the basis for test-level cost accounting and pricing decisions.

Indirect costs are not incurred by running a specific test. These are overhead costs shared across the whole lab operation.

Rent for laboratory space
Utilities (lighting, heating, telephone)
Equipment depreciation
Proficiency testing fees
Administration, housekeeping, IT support
Quality management
Management salaries

Indirect costs get allocated to individual tests using various formulas (proportional to direct costs, test volume, or labor hours). That allocation is inherently imprecise - it’s one of the reasons “true” per-test cost is hard to pin down.

Break-Even Analysis

The break-even point is the number of tests the lab must perform so that total revenue = total cost (net income = $0). Below break-even the lab loses money. Above it, every additional test contributes profit equal to the contribution margin.

Number of tests to break even = Fixed costs / (Revenue per test - Variable cost per test)

The denominator - revenue per test minus variable cost per test - is the contribution margin per test. That’s the amount each test throws toward covering fixed costs. Below break-even, contribution margins are still eating into the fixed-cost hole. Above break-even, each unit of contribution margin becomes profit.

Worked example from the Quick Compendium:

Revenue per test: $3
Variable cost per test: $1
Fixed costs: $10,000 per month

Break-even = $10{,}000 / (3 - 1) = 5{,}000$ tests per month.

Another: fixed costs $100,000/year, revenue $50/test, variable $30/test. Contribution margin = $20. Break-even = $100{,}000 / 20 = 5{,}000$ tests/year.

Break-even analysis is the financial core of decisions about bringing a test in-house vs. sending it out, buying new equipment, or evaluating whether the current test menu is sustainable.

Allowances and Bad Debt

The lab almost never collects what it charges. Two concepts describe the shortfall:

Allowance = amount lab charges for a test - amount actually received. The difference between charge and actual reimbursement. Allowances exist because payers reimburse at contracted or set rates, not at the lab’s list price. Allowances can run 30-70% of charges depending on payer mix.
Bad debt = total charges that are never paid at all. Uninsured patients who can’t pay, denied claims that aren’t appealed, unpaid copays. Bad debt is a complete loss. It’s distinct from allowances (contractual adjustments) and from charity care (voluntary write-offs).

Revenue = Total charges - Allowances - Bad debt

This is the real money that comes in. When you plug “revenue per test” into the break-even formula, it has to be net revenue, not sticker price. Labs with unfavorable payer mix (lots of Medicaid or uninsured) see higher allowances and more bad debt, and need higher volumes to break even on the same fixed-cost base.

Example: Lab bills $1,000,000 in charges. Allowances are $400,000. Bad debt is $50,000. Actual revenue = $550,000 - and that $550K has to cover all fixed and variable costs before any profit.

Laboratory Budget: COPA

The laboratory budget has four main components, mnemonic COPA:

==Capital - “big ticket” items (instruments, major equipment, renovations, information systems) whose cost and return on investment are mapped over several years. Capital items are typically >$5,000 - $10,000 (institution-dependent). Capital purchases require formal justification: clinical need, expected test volume, ROI analysis, total cost of ownership (purchase + installation + maintenance + reagents + training), and lease vs. buy comparison. Capital requests compete with other hospital departments for limited funds.
Operating== - day-to-day expenses: reagents, reference lab (send-out) expenses, consumables, minor equipment, maintenance contracts, proficiency testing fees, subscriptions, depreciation. Operating budgets are prepared annually and monitored monthly. Variance analysis compares actual to budget; variances >5 - 10% need explanation. Operating budget management is a core medical director responsibility.
Personnel - salaries, benefits, overtime, temporary staff. Personnel is typically the largest budget component, 50 - 70% of total lab costs.
Allocation - the lab’s share of hospital-wide fixed costs: electricity, heating/cooling, administration, marketing, housekeeping, security, building depreciation, IT infrastructure. Allocation is determined by a hospital-level formula (square footage, FTEs, or revenue) and the lab has limited control over it. Allocation typically runs 15 - 25% of the lab’s total budget.

Strategies for Cost Reduction

Consolidation of testing (spread fixed costs over more volume)
Automation (lower variable labor per test)
Eliminating unnecessary tests (utilization management)
Negotiating contracts (reagent rental agreements, group purchasing)
Reducing waste
Productivity metrics: tests per FTE, cost per test, revenue vs. expense ratios

Chapter 55: Laboratory Information Systems

The Laboratory Information System (LIS) is the digital backbone of the modern laboratory. Every order, every specimen, every result, every billing transaction flows through this system. Understanding LIS concepts matters because the integrity of patient care depends on accurate, timely data flow.

This chapter starts at the level of transistors and bytes, works up through networks and standards, and ends at whole slide imaging and computational pathology. That breadth mirrors what the Clinical Pathology boards can ask - rudiments one minute, middleware auto-verification the next, FDA regulation of blood bank software the minute after.

Two terms worth distinguishing before we start. Information technology (IT) is the application of computer resources to process data. Informatics is the collection and analysis of data to generate knowledge. IT is the infrastructure. Informatics is the discipline that turns the outputs into clinical insight. The Data-Information-Knowledge-Wisdom (DIKW) hierarchy captures the progression: raw facts, organized data, interpreted information, applied decisions.

55.1 Computing Rudiments

Hardware: CPU, Memory, and the Motherboard

Every computer has a central processing unit (CPU), also called the microprocessor. The CPU receives and transmits information and is the part that actually computes. Physically, a CPU is a thin sheet of silicon (a “chip”) etched with billions of transistors and capacitors. Silicon is a semiconductor: it can be doped to conduct or insulate, making it ideal for transistor construction. Modern nodes etch features at 3-7 nm using photolithography. Moore’s Law held for decades, with transistor density roughly doubling every 2 years.

The transistors and capacitors encode information in binary. A transistor acts as a switch: it allows current to flow to a capacitor when the bit value is 1 (on), and blocks current when the value is 0 (off). A charged capacitor = 1, a discharged capacitor = 0. This on/off switching at billions of times per second is all computation.

Information units:

A bit (binary digit) is the smallest unit: 0 or 1. Term coined by Claude Shannon in 1948.
1 byte (B) = 8 bits (b). Case matters - network speeds are usually in Mb/s (megabits), file sizes in MB (megabytes). A 100 Mbps link transfers about 12.5 MB/s.
Storage can use base 10 (SI: 1 KB = 1,000 bytes, 1 MB = $10^6$ bytes, 1 GB = $10^9$ bytes) or base 2 (binary: 1 KiB = 1,024 bytes, 1 MiB = $2^{20}$ bytes, 1 GiB = $2^{30}$ bytes). Hard drive manufacturers use base 10 (looks bigger on the box); operating systems usually use base 2. That’s why a “500 GB” drive shows as ~465 GiB.

CPU performance depends on:

Word length (word size): the number of bits processed per clock cycle (e.g. 8-, 16-, 32-, 64-bit). Modern systems are 64-bit, which allows addressing up to 16 exabytes of memory vs 4 GB for 32-bit. The move from 32-bit to 64-bit was essential for handling large genomic datasets and whole slide images.
Clock speed: the frequency of electrical pulses from the internal oscillator, measured in Hertz (MHz or GHz). 1 MHz = $10^6$ cycles/s; 1 GHz = $10^9$ cycles/s. Modern CPUs run 3-5 GHz.
Core count, cache size, and pipeline depth also matter.

The motherboard is the main circuit board to which all hardware components attach - CPU, RAM, expansion cards, storage, BIOS chip. Hardware connects via slots (internal, e.g. PCIe) and ports (external, e.g. USB, Ethernet). Hardware interacts with the CPU through a chipset, a set of ICs that manage data flow. Modern chipsets integrate the memory controller directly into the CPU (the old “northbridge”) while the “southbridge” (now the Platform Controller Hub) handles I/O.

A bus is a circuit (set of wires/traces) connecting different parts of the motherboard. Types: data bus, address bus, control bus. The front side bus (FSB) connected the CPU to the chipset in older designs. Modern CPUs use direct connections (Intel QPI/UPI, AMD Infinity Fabric). PCIe is the modern high-speed bus standard.

The Universal Serial Bus (USB) is the external peripheral standard - keyboards, scanners, analyzers, flash drives. USB 2.0: 480 Mbps. USB 3.0: 5 Gbps. USB 3.1: 10 Gbps. USB4: 40 Gbps. USB-C is the modern connector.

Memory: RAM, ROM, and Storage

Computers have three types of memory:

RAM (Random Access Memory): short-term, volatile
ROM (Read-Only Memory): long-term, non-volatile, on motherboard
Storage media: long-term, non-volatile, large capacity (hard drives, SSDs, flash drives)

RAM. RAM is the computer’s short-term working memory holding active programs and data for fast CPU access. Physically, RAM lives on sticks called DIMMs (Dual Inline Memory Modules). Current standards are DDR4 and DDR5. Server-grade DIMMs include ECC (Error-Correcting Code) RAM which catches and corrects single-bit errors - important for LIS servers where silent corruption is unacceptable.

RAM encodes data the same way a CPU does - transistors route current to capacitors. But RAM capacitors are “leaky”: they discharge, so the charge must be refreshed thousands of times per second (that’s what makes it DRAM - Dynamic RAM). When power is lost, capacitors discharge completely and RAM resets. This is why RAM is volatile and why unsaved work is gone after a power failure. UPS (uninterruptible power supply) units matter for critical LIS infrastructure. SRAM (Static RAM) uses flip-flop circuits instead of capacitors, doesn’t need refreshing, and is used in CPU cache - faster but more expensive.

RAM access is in nanoseconds ($10^{-9}$ s); hard drive access is in milliseconds ($10^{-3}$ s) - ~1,000,000x slower. That gap is why RAM amount matters so much for responsiveness.

ROM. ROM is the long-term, non-volatile memory built into the motherboard. The canonical ROM contents is the BIOS (Basic Input/Output System), which boots the computer: runs POST (Power-On Self-Test), initializes hardware, and loads the OS from storage. Modern systems use UEFI (Unified Extensible Firmware Interface) instead of legacy BIOS - faster boot, Secure Boot.

True ROM is permanent: information is encoded at manufacturing using diodes - a diode present = 1 (current can flow), no connection = 0. Modern ROM variants (EEPROM, flash ROM) can be electrically rewritten, which is how BIOS/UEFI updates are applied.

Storage media. Almost all usable information lives in storage media.

Hard disk drive (HDD): rotating magnetic platters read/written by movable magnetic arms. 5,400-7,200+ RPM. Access ~5-10 ms, sequential read ~100-200 MB/s. Cheap per GB, mechanical, vibration-sensitive.
Solid-state drive (SSD): integrated circuits (NAND flash), no moving parts. Access ~0.1 ms, throughput 500-7,000 MB/s. Shock-resistant, lower power, more expensive per GB. SSDs and USB flash drives have a retention limitation - data can leak if the device is unpowered for months to years (electron leakage from floating-gate transistors). HDDs don’t have this issue. For long-term archival of critical lab data, this matters.
External (removable) storage: USB flash drives, external HDDs/SSDs, CDs/DVDs/Blu-rays, floppy discs, magnetic tape. Many hospitals restrict USB drives on LIS workstations because of HIPAA exposure. Magnetic tape is still relevant for large-scale archival - high capacity, low cost, long shelf life.
Cloud storage: physically housed on servers in data centers at multiple geographic locations. Data is replicated across servers for redundancy. Providers: AWS, Azure, GCP. Data is uploaded to the cloud via the internet. Security is ultimately the vendor’s responsibility; for HIPAA, the covered entity needs a Business Associate Agreement (BAA) with the provider and enforceable encryption/access controls.

Hardware is the most important predictor of a computer’s performance - software can only run as fast as hardware allows. The key determinants: CPU, RAM, storage type (SSD vs HDD), and GPU (for image analysis, AI).

Software: Operating Systems, Applications, and Programming

The operating system (OS) manages interactions between the CPU and other hardware components. It mediates between hardware and applications, managing memory, CPU time, storage, device drivers, file systems, and user permissions. Examples: Microsoft Windows, macOS, Linux, iOS, Android. Most LIS/EMR systems run on Windows Server. Linux dominates bioinformatics because core genomic tools (BWA, GATK, samtools) are designed for it.

An application is software that performs a specific function - word processor, browser, image viewer. In pathology: LIS, EMR, middleware, APIS, blood bank software, genomics pipelines, image analysis, statistical packages (R, Python). Applications run on top of the OS. The most relevant pathology applications are the EMR (hospital-wide) and the LIS (laboratory-specific). EMR is also called EHR (electronic health record). The EMR is the clinician’s interface for ordering tests and viewing results; the LIS manages lab workflow.

Programming languages. Software is written in code. A set of code lines that performs a task is a script. Scripting languages (Python, R, Perl, shell) are interpreted line-by-line at runtime; they’re slower but easier to write and modify - ideal for laboratory data analysis. Compiled languages (C, C++, Java) are translated in advance into machine code - faster at runtime. Compiling translates human-readable code into machine code, expressed as 0s and 1s, which the computer can execute.

Languages worth recognizing for boards:

BASIC (Kemeny and Kurtz, Dartmouth, 1964; popularized on Microsoft platforms) and Pascal (Niklaus Wirth, 1970; widely used in early Apple development) - early high-level languages widely used in personal computing.
R: open-source, built for data analysis, developed by statisticians Ross Ihaka and Robert Gentleman (hence the “R”). Strong in bioinformatics and QC analysis.
MUMPS (aka M): MGH Utility Multi-Programming System, developed at Massachusetts General Hospital in the 1960s for laboratory information. MUMPS evolved into the foundation of most modern EMRs - Epic’s Cache/IRIS is MUMPS-derived, and the VA’s VistA is written in MUMPS. MUMPS has built-in hierarchical database functionality, which is why it stuck in healthcare.
SQL (Structured Query Language): built to search databases, plus INSERT/UPDATE/DELETE. Essential for pulling data from LIS/EMR for QA, research, or regulatory reporting.

Software intellectual property. Copyrighted software requires a software licensing agreement between the end-user and the copyright owner that specifies terms of use (who can install it, how many copies, whether it can be modified). The “I agree” box counts.

Open-source (FOSS): license gives users the right to modify the source code. Source is public. Examples relevant to pathology: R, Python, OpenSlide, Linux, BWA/GATK/samtools. Free and transparent; validation burden falls on the user.
Proprietary (closed-source): source code is not public. Users can’t view, modify, or redistribute. Examples: Cerner, Sunquest, Epic, and most instrument software. Proprietary software usually carries a specific license called an EULA (end-user license agreement), which defines terms of use plus maintenance/support.
Public domain: no ownership, explicitly placed there through an active step (CC0 dedication or explicit declaration). Simply not copyrighting something is not enough. Example: BLAST for sequence alignment.
A dongle is a small hardware key (usually USB) that must be plugged in for certain software to run - a hardware licensing mechanism. Some specialty instrument and pathology applications still use these.

Networks

A network is a group of computers or devices connected to share information. In healthcare this is the whole stack - LIS, EMR, instruments, printers, workstations - and network reliability directly determines whether results can be reported.

Classes of networks:

Intranet: a private network, accessible only to authorized users in an organization. Hospital EMRs and LISs live on intranets.
Internet: a public network, the global one.
Local area network (LAN): a type of intranet covering a limited area - office, lab, campus. Typically 1-10 Gbps Ethernet.
Wireless LAN (WLAN): connects devices via radio transmitters (Wi-Fi) rather than wires. Standards: Wi-Fi 5 (802.11ac), Wi-Fi 6 (802.11ax). Healthcare WLANs use WPA2/WPA3 Enterprise encryption. Matters for mobile phlebotomy and point-of-care testing.
Wide area network (WAN): multiple interconnected LANs over a larger geographic area (city to city, hospital system with multiple campuses). The internet is the largest WAN.
Virtual Private Network (VPN): securely routes traffic through a public internet connection by encrypting data and masking the IP address. This is how pathologists sign out from home - the VPN tunnels the connection through the firewall into the hospital intranet. Essential for HIPAA-compliant remote access.

A firewall is the security boundary. Computers within an intranet access the internet via the firewall, and outside computers can access the intranet by tunneling through the firewall using an encrypted channel. The firewall is both hardware and software - a device physically interposed between the intranet and the internet that runs software to control information flow (filter packets by source/destination IP, port, protocol). Modern next-generation firewalls (NGFW) add deep packet inspection, application awareness, and integrated intrusion prevention.

Network components:

Servers and mainframes: computers that run shared programs, store shared files, and hold the network OS. The LIS runs on a server (or cluster) with redundant power, RAID, ECC RAM, and 24/7 uptime expectations. Downtime means manual reporting.
Client: an individual computer/device connected to the network. End users access via clients. In the lab: workstations, instrument computers, label printers, barcode scanners, mobile devices.
System architecture: the way a network is configured.
- Client-server architecture: clients share computing with the server. Both do processing - the client runs the application interface locally while the server handles the database. Pros: distributes load, works during brief network blips. Cons: more powerful (expensive) clients, complex software management.
- Thin client architecture: the server houses all/most computational effort. Clients are simple terminals for display and input. Pros: cheap clients, easy centralized updates, better security (no local data). Cons: fully network-dependent. Many modern web-based LIS deployments use thin clients.
Network interface card (NIC): handles data flow between a device and the network. Every NIC has a permanent MAC (Media Access Control) address assigned at manufacture. Modern NICs support Gigabit or 10-Gigabit Ethernet.
Device identifiers (each device on a network has all three):
- IP address (Internet Protocol) - logical, can be dynamic (assigned by DHCP) or static
- MAC address - hardware, permanent
- Port number - identifies a specific application/service (e.g. port 443 for HTTPS)
Routers: connect two or more networks via a modem (which translates between analog and digital). Routers use IP addresses to determine the best path between networks.
Hub: connects devices by passively passing information to all ports. Broadcasts every packet to every device. Obsolete - wasteful and insecure.
Switch: connects devices by selectively routing information to intended recipients only. Keeps a table of MAC addresses by port and sends traffic only where needed. Modern networks use managed switches, which add VLANs, QoS, and port security.

55.2 Data and Graphics Standards

Character Encoding

ASCII (American Standard Code for Information Interchange): a set of standard binary equivalents for common characters. ASCII proper uses 7 bits per character, covering 128 values ($2^7$) - the English alphabet (upper and lower case), digits 0-9, and a set of special characters. The 8th bit of a byte allows extended ASCII variants (ISO-8859, code pages) with 256 values ($2^8$). Example: “A” = 1000001, “a” = 1100001, “0” = 0110000. ASCII is English-centric and cannot represent non-Latin characters.

Unicode supports approximately 1.1 million code points ($$1,114,112) covering virtually all world languages, mathematical symbols, and emojis. The legacy UCS-2 / Basic Multilingual Plane simplification (“2 bytes / 65,536 values, $2^{16}$”) only covers a subset; modern Unicode is encoded via variable-width schemes - UTF-8 (1-4 bytes), UTF-16 (2 or 4 bytes), and UTF-32 (fixed 4 bytes). UTF-8 is the dominant encoding for the modern web and international healthcare communication - matters when patient names contain non-Latin characters.

Vector vs Raster Graphics

Vector graphics: images made of geometric shapes (points, lines, curves) defined on a Cartesian plane. Each shape is stored as a mathematical formula. Vector graphics appear equally sharp when massively enlarged because they’re recalculated from math at any scale. Used for diagrams, logos, charts. Vector file formats: PDF, SVG, EPS.

Raster graphics: images made of pixels, each with a specific color value. Also known as bitmaps. Raster graphics appear fuzzy when massively enlarged (pixelation) because the pixel count is fixed. This matters in pathology: a low-resolution raster image cannot be meaningfully enlarged to see cellular detail that wasn’t captured in the original scan, which is why whole slide imaging requires high initial resolution (20x or 40x objective). Raster file formats: JPEG, PNG, GIF, TIFF.

Resolution depends on pixel count in the array. “140 x 120” means 140 pixels horizontal by 120 pixels vertical = 16,800 total pixels. At 24-bit color depth (3 bytes/pixel), that’s ~49 KB uncompressed. Whole slide images at 40x can hit 100,000 x 100,000 pixels - about 10 gigapixels.

Image Compression

Image compression reduces file size, which matters because an uncompressed WSI at 40x is 1-3 GB. Compression algorithms group pixels where they are similar to each other - instead of storing every pixel individually, the algorithm encodes “this 100x100 region is the same shade of pink” as one instruction. Histology with large areas of similar staining compresses well; dense cellular detail does not.

JPEG: lossy compression, permanently sacrifices pixel information for smaller files. Higher compression, more quality loss. Fine for clinical photos; not recommended for diagnostic pathology archival because the quality can’t be recovered and blocky artifacts can obscure fine detail.
TIFF and PNG: lossless compression, allows retrieval of the original image. Achieves typically 2:1 to 3:1 on histology (vs 10:1+ for aggressive JPEG). TIFF is the gold standard for whole slide imaging and scientific publications.
One weakness of lossless compression is that files are larger - a TIFF can be 10-50x larger than a heavily compressed JPEG. This drives the storage cost of WSI archives.
GIF: lossy for images with >256 colors, lossless for images with $\leq$ 256 colors. Not used for medical imaging because of the 256-color cap.

55.3 Interfaces, Interoperability, and Nomenclature

Interfaces

Healthcare is plagued by disconnected information systems. The LIS must communicate with the EMR, with analyzers on the bench, with billing systems, and potentially with public health registries. This communication happens through interfaces - electronic connections that pass data between systems.

In computing, an interface is a shared boundary across which two or more components exchange information. In healthcare IT, that means EMR-LIS, LIS-instrument, LIS-reference lab, LIS-billing, LIS-public health. Interfaces translate between formats and protocols and are a major operational responsibility - an interface failure can silently stop result reporting.

The critical concept is that interfaces can fail. Messages can be lost, duplicated, or corrupted. A bidirectional interface (where data flows both ways) is more useful but more complex than a unidirectional interface. Interface validation - ensuring that what goes in one end comes out the other correctly - is essential when setting up new connections.

HL7 and FHIR

HL7 (Health Level Seven) is the standard messaging format that makes healthcare system communication possible. HL7 is a standardized set of standards aimed at interoperability between healthcare software. The “Level 7” refers to the application layer (Layer 7) of the OSI networking model. HL7 defines the format of messages exchanged between healthcare systems (orders, results, admissions, demographics).

HL7 messages are structured text strings that contain segments of information. When you order a test in the EMR, an ORM (Order Message) is sent to the LIS. When the result is ready, an ORU (Observation Result) message returns it to the EMR. When a patient is admitted, discharged, or transferred, ADT (Admit/Discharge/Transfer) messages update the patient’s location and status.

HL7 v2.x uses pipe-delimited text with segments (MSH for header, PID for patient ID, OBR for observation request, OBX for observation). HL7 v3 uses XML. HL7 has a limitation: it’s not ideal for sharing data between the EMR and nonmedical/web-based applications. That gap is filled by APIs.

API (Application Programming Interface): structured to allow EMRs to interact with non-medical/web-based applications. APIs define how software components communicate. RESTful APIs (HTTP requests) are the modern standard.

HL7 FHIR (Fast Healthcare Interoperability Resources): an API specification created to allow EMRs to interface with nonmedical/web-based applications. FHIR uses web standards (REST, JSON, OAuth), making it easier to build apps that interact with EMR data. FHIR is increasingly mandated by regulation (21st Century Cures Act) for patient data access. Pathology applications: mobile result viewers, clinical decision support, research database integration.

Clinical Document Architecture (CDA) of HL7 version 3 allows special symbols and punctuation to be transmitted in pathology reports without issue. The old pipe-delimited v2.x format used pipes, carets, tildes, and ampersands as delimiters, so if those appeared in the report text they’d corrupt parsing. Some departments worked around this by sending PDFs (which lost mineability). CDA uses XML, which handles special characters natively - matters for AP reports with measurements (e.g. 2.5 x 1.3 cm) and special characters.

Middleware (Interface Engines)

An interface engine (middleware) acts as a hub allowing several systems to interface with each other. Direct point-to-point interfaces scale poorly: n systems need n*(n-1)/2 interfaces. An interface engine collapses this to n interfaces into the engine. It also transforms messages, routes them, and handles errors. Examples: Rhapsody, Mirth Connect, Cloverleaf.

We’ll return to middleware in the LIS section below because the term has a second, specifically laboratory, meaning - the software sitting between analyzers and the LIS that runs auto-verification.

Standard Nomenclatures

LOINC (Logical Observation Identifiers, Names, and Codes): standard nomenclature for laboratory tests. Product of the Regenstrief Institute. Each LOINC code combines properties: analyte, property being measured, specimen type, and analytical method. LOINC is compatible with HL7 and is required under Meaningful Use for electronic reporting. Enables aggregation across institutions.
SNOMED (Standardized Nomenclature of Medicine, now SNOMED CT): standard nomenclature for clinical information - findings, diagnoses, procedures, observations. Multilingual. In AP, SNOMED morphology and topography codes encode diagnoses. SNOMED is the standard terminology for entering information into the EMR (physical exam findings, etc.); ICD classifies patients into problems/diseases.
ICD (International Classification of Diseases): published by the WHO. Currently ICD-10 (ICD-11 adopted 2022). US uses ICD-10-CM for diagnoses, ICD-10-PCS for procedures. ICD is used to categorize clinical findings/medical problems. Attached to lab test orders to justify medical necessity - without an appropriate ICD code, the test may not be reimbursed.
CPT (Current Procedural Terminology): standard nomenclature used to report services provided. Maintained by the American Medical Association, updated annually. Also known as HCPCS Level I. HCPCS Level II covers Medicare national alphanumeric codes for services/supplies/devices not in CPT. Pathology CPT ranges: 80000-89999 (lab), 88300-88399 (surgical path), 88104-88199 (cytopath), 81000-81599 (molecular path), 88321-88325 (consultations). CPT coding determines reimbursement, so it directly drives lab revenue.

55.4 Databases

A database holds large quantities of computerized information in a structured method for efficient interaction. Characteristics of a good database: secure, provides data definition (organization rules), easily accepts data entry and updates, provides audit trails, allows efficient searches, produces structured reports, and has backup/recovery.

Database Structures

Flat file database: comprised of single lines of data with no hierarchy. Like a spreadsheet or CSV. Same fields in each row, in the same order. Simple, but relationships between records must be inferred from the data - redundancy is inevitable for any non-trivial dataset. Good for simple exports or logs; bad for healthcare.
Hierarchical database: linear parent-child relationships, one parent to one or many children. Like a family tree or file system. MUMPS (basis of most EMR/LIS systems) uses a hierarchical model. Fast for predefined access paths; painful for navigating between branches or adding new relationships.
Network database: can include multiple parents in relation to multiple children (many-to-many). Extends hierarchical. More flexible but harder to implement and query. Common in the 70s-80s, now mostly replaced by relational.
Relational database: multiple interrelated tables, each with rows/columns. Tables linked by shared key fields (primary keys and foreign keys). Typically queried using SQL. The dominant modern model, including many LIS/EMR systems. Normalization prevents redundancy; SQL supports complex joins across tables.
Object database: stores data as objects. Objects are self-contained units holding data and methods. Best suited for handling packets of disparate data (images, text, sound). Conceptually fits pathology well because cases combine text reports, images, and structured data. In practice, relational databases with BLOB support or document/NoSQL databases have filled this niche.

Database Management and Migration

Databases are organized and accessed by database management system (DBMS) software. Examples: Oracle, Microsoft SQL Server, MySQL, PostgreSQL (relational); MongoDB (document); InterSystems Cache/IRIS (hierarchical, used by Epic). The DBMS is the layer between users/applications and the raw data files - ensures integrity, handles concurrent access, manages security and backups.

MUMPS is a database software with hierarchical design that has inherent (built-in) database management software. The programming language and the DBMS are fused, which is unusual but it’s why MUMPS-derived systems dominate healthcare - no separate DBMS needed, tight integration with the application layer.

Database migration: transfer of data from one database to another with a different DBMS. Required when an organization switches from one EMR or LIS to another (e.g. Cerner to Epic). Migration is one of the most complex and risky IT projects in healthcare. Drawbacks: high cost and imperfect fidelity. Costs often run into millions of dollars (software, consulting, training, downtime). Data fidelity issues can affect patient care - extensive validation is required pre-, during, and post-migration. Projects typically take 12-24+ months for large health systems.

EMR and LIS databases usually do not allow data deletion due to legal and ethical reasons; they allow data suppression/inactivation instead. Suppressed data is hidden from standard views but remains in the database and audit trail. If an error is found, the incorrect result is suppressed and a corrected one entered, with both retained. This preserves evidence for CLIA, HIPAA, and legal defense.

Data Normalization

Relational databases use data normalization to prevent inconsistencies between related tables. Normalization requires a gold standard table - one authoritative table per data element (the master test dictionary, the master provider list) that all other tables reference. If the value is updated in the gold standard table, it propagates everywhere. Without it, you get the same test recorded as “Hemoglobin,” “Hgb,” and “HGB” in different places, and no way to join them reliably.

Normalization is especially challenging when separate EMRs/LISs interface with each other, because no consensus informational table exists across the databases. Each system has its own master tables. Bridging them requires mapping (translation tables between terminologies). LOINC and SNOMED help by providing universal codes, but local implementation still varies.

55.5 The LIS as the Laboratory’s Central Nervous System

The LIS manages the complete lifecycle of laboratory testing. When a clinician orders a test, that order must reach the laboratory. When the specimen arrives, it must be logged, tracked, and matched to the order. When the analyzer produces a result, that result must be captured, validated, and reported back to the clinician. Every step must be documented, traceable, and secure.

The core functions reflect this lifecycle: order management (receiving and processing test requests), specimen tracking (knowing where every tube is at every moment), result management (capturing, reviewing, and reporting results), and quality management (monitoring QC, tracking errors, generating reports for regulatory compliance).

Health IT: Clinical and Operational Systems

Health information technology is made up of clinical and operational systems.

Operational systems: ADT (Admission/Discharge/Transfer), payroll, scheduling, billing, accounting. ADT sends patient demographic updates to the LIS. Billing receives CPT/ICD codes from the LIS for claims submission.
Clinical systems: EMR and department-specific workflow systems - radiology (PACS), pharmacy, lab (LIS), surgery, APIS for anatomic pathology.

Health IT systems interact through integrated or interfaced methods.

Integrated: a central database is shared by multiple systems, each transacting with it independently. Simpler but creates vendor lock-in.
Interfaced (more common): separate databases that communicate via an interface engine using the HL7 protocol. Best-of-breed selection possible but requires ongoing interface development and monitoring.
Stand-alone system: a system that does not interface with others at all. Results must be manually entered elsewhere. Some specialty instruments (early flow cytometers, molecular platforms) start this way. Stand-alone is error-prone (transcription errors) and regulatory/quality standards push toward electronic interfacing.

LIS Interfaces

Unidirectional interface: information passes from the LIS to the EMR only. Results out, but orders can’t flow back in.
Bidirectional interface: information flows back and forth between LIS and EMR. A bidirectional interface is essential for clinicians to place lab orders - this is what CPOE (Computerized Physician Order Entry) requires. Order flows EMR-to-LIS via HL7 ORM message; results flow back via HL7 ORU. Bidirectional is the modern standard.
LIS interfacing with instruments allows results to be automatically uploaded to the chart (EMR) without manual data entry. Instruments use various protocols - ASTM/LIS2, serial RS-232, TCP/IP. Usually, criteria exist for which results can auto-release vs which require second review (this is auto-verification).

While MUMPS databases usually cannot be queried using SQL, adapter software may allow this - ODBC drivers or SQL gateways (provided by InterSystems Cache/IRIS) translate SQL queries into MUMPS global references. Matters because most modern reporting/analytics tools (Crystal Reports, Tableau, Power BI, R) assume SQL access.

Patient and Specimen Identification

To bill for a lab test, it must be linked to an encounter (a specific visit/admission, characterized by location and timeframe). Without an encounter, the test can’t be properly billed - the encounter provides context (inpatient, outpatient, emergency), the responsible provider, and the justifying ICD codes.

Bedside labeling of test tubes is preferred to reduce errors. Pre-labeling tubes increases mislabeling risk. Wristband scanning plus bedside label printing ensures correct patient-specimen match. Joint Commission and CAP require two patient identifiers at the time of collection.

Barcodes:

1D barcodes: limited by interoperability issues and higher error rate. 1D encodes data linearly, so damage to any part can corrupt the entire code. Limited data capacity (~20-25 characters) and no built-in error correction.
2D barcodes encode data in both dimensions (squares, dots, hexagons). Types: QR, Data Matrix, PDF417. Much higher capacity (up to 7,000 characters), built-in error correction, and can be scanned from any angle. Data Matrix is common for specimen labels; QR is common for patient identifiers and resource links.
Radiofrequency identification (RFID) chips: comprised of a microprocessor chip, used increasingly instead of barcodes. Advantages: no line-of-sight needed (read through packaging), simultaneous reading of multiple tags, larger data capacity, read/write capability. In blood banking, RFID tags on units enable automated inventory and tracking. Adoption is limited by cost, lack of universal standards, and hacking risk.

Order Entry, Accession Numbers, and Container IDs

Once an order is placed in the LIS, an accession number is generated. This number uniquely identifies the specimen/case within the LIS and is the primary tracking identifier through testing. Accession numbers typically include a date component, sequential number, and possibly a department identifier. Orders can be placed manually by lab personnel or by clinicians (CPOE). Once an order lands in the LIS, the system may add it to a collection list, print a barcoded specimen label, or (in modern systems) trigger printing of the label at bedside only after a wristband is scanned.

For CP (lab) specimens, each separate part under the same accession number is assigned a container identification (CID) number. Example: accession 24-12345 with three tubes has CIDs 24-12345-001, -002, -003. CIDs track individual tubes through centrifugation, aliquoting, and routing.

For AP specimens, a CID number is not required; a suffix is affixed to the accession number for each different container. Example: a single surgical path case S-25-01234 with three parts becomes S-25-01234 A, B, C. Unlike CP, AP systems do allow a single accession number to apply to multiple specimen containers - reflects the clinical reality that multiple specimens from one surgery are related and reported together.

Manual entry of data from an instrument into an LIS is error-prone, with an average error rate of approximately once per 300 keystrokes - misplaced decimal points, transposed digits. For a lab processing thousands of results daily, this is multiple errors per shift. Electronic interfaces (LIS-instrument) eliminate these transcription errors; that’s the core ROI justification.

Results Review and Auto-Verification

Instrument results were traditionally considered preliminary until technologist verification, but this is now often done by the LIS or middleware - the “auto-verify” workflow.

The concept behind auto-verification is simple: most results are normal or unremarkably abnormal and don’t require a technologist’s interpretation. By defining rules that identify which results can be released automatically, you free technologists to focus on the results that need human judgment.

Auto-verification rules typically check: Is the result within the reportable range? Is QC acceptable? Are there instrument flags suggesting a problem (clot, hemolysis, bubble)? Does this result make sense given the patient’s previous results (delta check)? Are there conflicting results that need resolution?

When all rules pass, the result is released without human review. When any rule fails, the result is held for technologist review. The key is that auto-verification must be validated - you must prove that the rules correctly identify which results can be safely released - and continuously monitored.

LIS can be programmed to perform calculations in order to report a lab value. Examples: INR (from PT), eGFR (from creatinine, age, sex; current race-free equations should be used), LDL (Friedewald equation), corrected calcium (for albumin), anion gap. These reduce manual computation error but must be validated during LIS implementation.

LIS is usually programmed to identify critical values and alert the technologist, and to provide a mechanism to document notification of clinicians. Critical value policies are required by CAP, Joint Commission, and CLIA. The LIS tracks: the critical value, time identified, time of clinician notification, identity of notifier and recipient, and read-back confirmation. Failure to promptly communicate critical values is a patient safety risk and a frequent accreditation finding. The LIS is also often programmed for reflex testing (ordering follow-up or alternative tests based on initial results).

Result Transmission and Regulatory Responsibility

CLIA stipulates that the laboratory medical director is only responsible for verification of data accuracy transmitted from LIS to the first downstream interfaced system. “First downstream” is the key nuance: the lab director must validate that results appearing in the EMR match what the LIS sent. If the EMR then sends data to another system, that further step falls outside the lab director’s CLIA purview.

Dictionary (Maintenance) Tables

Dictionary (maintenance) tables are the terms that bind the function of an LIS. They define the operating parameters: test definitions (name, code, units, reference ranges, critical values), specimen requirements, result formats, auto-verification rules, reflex rules, report formats. Typical elements: registered patients, ordering physicians, sample types, container types, test names, laboratory departments, instrument names. Errors in dictionary tables (wrong reference range, wrong units) can cause widespread result misinterpretation.

Dictionary tables of an LIS may conflict with those of an EMR, which requires a process termed mapping to reconcile discrepancies. Mapping creates translation tables (LIS test code A -> EMR test code B, LIS unit “mg/dL” -> EMR unit “mg/dL”). Mapping errors can cause results to appear under the wrong test name, wrong patient, or in the wrong format. Mapping validation is critical at interface implementation and whenever tests are added or modified.

Worksheets and Audit Trails

LIS can generate worksheets that list all tests to be performed by a specific lab department, instrument, or technician. Worksheets organize the daily workload and help link tests to QC data. Electronic worksheets allow real-time specimen status tracking and can be filtered by urgency (STAT first), instrument, or test type.

LIS/EMR systems must contain audit trails: documentation of updates/alterations with electronic time stamps. Also called event logs or audit logs. Granularity ranges from user + timestamp + action to “clickstream data” that records mouse movement and every keystroke (even deleted ones). Audit trails are required by HIPAA and Meaningful Use legislation. Meaningful Use provisions are part of the HITECH Act (2009), which is what actually compelled hospital EMR adoption. Audit trail review should be routine - it’s how you catch unauthorized access and unusual activity.

Middleware (Laboratory Sense)

Between the analyzer and the LIS sits middleware - software that applies rules to results before they reach the LIS. This layer enables auto-verification, one of the most important efficiency tools in the modern laboratory.

Middleware is software that links instruments to the LIS and performs tasks like test calculations, reflex testing, and auto-verification. It directs the instrument and manages QC procedures. Middleware consolidates management of multiple instruments into a single platform. Examples: Data Innovations (Instrument Manager), Siemens CentraLink.

APIS (Anatomic Pathology Information System)

APIS is the computer system used to manage and streamline anatomic pathology workflows. Most labs use a separate APIS for surgical pathology, autopsy, and cytopathology; some labs build the APIS into the LIS as a distinct module. APISs are designed for AP’s unique workflow: free-text-heavy reports, graphical interfaces, word processing. Examples: CoPath, PowerPath, Epic Beaker AP.

Anatomic pathology reports differ from lab reports in that their output is usually not standardized. AP sign-out varies by pathologist (and by the same pathologist’s mood on different days). CAP reporting templates (synoptic cancer checklists) are the effort to approximate structured data within AP reports.

AP reports contain structured data, unstructured text, and unstructured images:

Structured data: synoptic reports (tumor size, margins, grade)
Unstructured text: header, diagnosis, comment, clinical history narrative, gross description
Unstructured images: gross photos, microscopic images, radiographs, clinical photos

This mix makes AP informatics harder than CP informatics. NLP and AI are being developed to extract structured data from unstructured AP text.

APIS is often not integrated with the EMR and therefore functions as a unidirectional interface - reports flow APIS to EMR but orders don’t flow back. Clinicians order AP tests through the EMR, but the order must be manually entered into the APIS (or a separate interface built). Bidirectional APIS-EMR interfaces are becoming more common.

Once an order is entered into an APIS, an accession number is generated. AP accession numbers typically include a specimen type prefix (S = surgical, C = cytology, A = autopsy), year, and sequential number (e.g. S25-12345). Accession number tracks the case through grossing, processing, staining, and sign-out.

APISs often have a built-in protocol for a specimen type that automatically assigns cassettes to print, slides to cut, and stains for each specimen. Example: a gallbladder protocol specifies 3 cassettes (margin, fundus, cystic duct), 1 H&E per block. Protocols standardize processing and can be customized per institution/indication.

Blood Bank Information Systems

Blood bank information systems are regulated by the FDA under the category of medical device. This is stricter than the regulation of standard LIS software. The software must go through FDA clearance/approval, follow GMP, and comply with 21 CFR Part 820 (Quality System Regulation). Any change (update, customization) must follow change control procedures. As a result, blood bank IT innovation lags AP/CP informatics because FDA validation (typically Class II 510(k) clearance for blood establishment computer software) is expensive.

A laboratory information system is validated using test patients - fictitious patient records used to verify system functionality without touching real data. Validation covers: order entry, result reporting, auto-verification rules, reflex testing, critical value alerts, billing, reference ranges, and interface function. Validation must be documented and repeated whenever significant changes are made (software updates, new tests, interface changes).

For blood bank systems specifically:

Validation must ensure the system correctly performs an electronic crossmatch under a variety of conditions - ABO compatible, ABO incompatible (should flag/block), patients with antibodies, patients with previous reactions, expired units, and special requirements (irradiated, CMV-negative). Electronic crossmatch replaces serologic crossmatch when the patient has no clinically significant antibodies and has two ABO/Rh type confirmations.
Must track blood collections with an electronic timestamp and identification of the phlebotomist. Traceability from donor to recipient supports look-back investigations if a donor later tests positive for a transfusion-transmissible infection. The system must also track component preparation, testing, storage, expiration, modification (irradiated/washed/etc), and transfusion records.
Must have a system that produces appropriate flags when discrepancies arise - ABO/Rh discrepancies between current and historical, unexpected antibodies, incompatible crossmatch, expired components, previous reactions, special requirements not met. Hard stops for critical incompatibilities; soft stops (overridable with documentation) for other issues.

Flow Cytometry Data in the LIS

In its simplest form, flow cytometry data is stored as numerical data in tables - each row is a cell, each column is a measured parameter (forward scatter, side scatter, fluorescence channels). A standard panel measures 6-12 parameters and a sample contains 10,000 to 1,000,000 events, so data volumes are substantial.

Flow cytometry data collected from optical sensors is stored in the Flow Cytometry Standard (FCS) format - the universal format defined by the International Society for Advancement of Cytometry (ISAC). FCS files contain a header, a text segment (parameters, reagents, settings), a data segment (event measurements), and an optional analysis segment. Current version: FCS 3.2.

Data collected about each cell is saved as a list mode file - every measurement for every cell is stored individually rather than as summary statistics. Preserves raw data and allows re-gating after acquisition. List mode files are large but essential for quality re-analysis and QA.

Flow cytometry software converts raw FCS data into a variety of plots: histograms (1-parameter), dot plots (2-parameter), contour plots, density plots, overlays. Gating identifies specific cell populations. Software: FlowJo, FCS Express, Kaluza, DiVa. Standardized strategies (EuroFlow, ICCS) improve consistency. Computational approaches (FlowSOM, viSNE) handle high-dimensional data.

55.6 Data Integrity and Security

Laboratory data is medical data, subject to HIPAA and other regulations. The principles that govern data integrity apply: every change must be traceable (audit trails), access must be limited to those with a need (role-based permissions), data must be protected from loss (backup and disaster recovery), and systems must be protected from attack (cybersecurity).

Cybersecurity has become increasingly critical as healthcare has become a target for ransomware and data theft. Laboratory systems, often running older software, can be vulnerable. Defense in depth - multiple layers of protection including firewalls, encryption, access controls, and staff training - is essential.

HIPAA and the Security Layers

The security of data transmitted from an LIS or EMR falls under the purview of HIPAA (law). HIPAA’s Security Rule requires administrative safeguards (security management, training), physical safeguards (facility access, workstation security), and technical safeguards (access control, audit controls, integrity controls, transmission security). Violations can run from $100 to $50,000+ per violation, up to $1.5 million per year per category.

The security layers in an LIS/EMR include:

Password protection to ensure only authorized users access the system
Firewall protection that restricts the ingress/egress of data packets
Firewall rules that restrict access to/from specific IP addresses
Digital certificates that verify the identity of a device/user, enabling encrypted connections

Malware

Malware is malicious software: viruses, computer worms, trojan horses, and spyware (plus ransomware, adware). Healthcare is a prime ransomware target because hospital systems are critical and organizations may pay to restore access.

Computer virus: code that is inserted into pre-existing software/files, altering functionality. Requires a host file; spreads when the infected file is executed or shared.
Computer worm: a standalone program that can alter a computer’s functionality and spread to other devices without a host file. Self-replicates across networks. WannaCry (2017) was a ransomware worm that hit multiple hospitals.
Trojan horse: malware disguised as legitimate software. Users install it thinking it’s useful; hidden code creates backdoors, steals credentials, logs keystrokes, or drops more malware. Often delivered via phishing.
Phishing: outside agents attempt to obtain information by tricking users to access a compromised email/website. Classic social engineering. Healthcare workers are prime targets. Prevention: awareness training, email filtering, multi-factor auth, verification of unexpected requests.

Authentication

LIS and EMR primarily use a password to authenticate users. Organizations typically enforce requirements for strong passwords, frequent changes, and a strict prohibition on password sharing. Passwords alone are increasingly insufficient.

Some organizations require two-factor authentication - especially for access from outside networks. 2FA combines something you know (password), something you have (phone, token, smart card), or something you are (biometric). Even if the password is compromised, the attacker still needs the second factor.

Firewalls (Security Sense)

A firewall is a device physically interposed between the intranet and internet. The firewall device contains software designed to control information flow. Rules specify allowed/denied source/destination IPs, ports (80 for HTTP, 443 for HTTPS, specific ports for HL7 traffic), protocols, and time-of-day restrictions. Modern “next-generation” firewalls add deep packet inspection, application awareness, and intrusion prevention. In a lab, firewall rules must permit legitimate traffic (instrument updates, reference lab interfaces, vendor remote support) while blocking everything else.

SSL/TLS Certificates

SSL (Secure Sockets Layer) certificate: a type of digital certificate provided to devices to ensure their identity when interacting with a server. SSL and its successor TLS (Transport Layer Security) encrypt data in transit. Servers supporting SSL certificates have “https” preceding their web addresses (not “http”). All healthcare web applications should use HTTPS for HIPAA compliance.

SSL certificates are issued by a certificate authority (CA) - a trusted third party that verifies identity. Major CAs: DigiCert, Let’s Encrypt, Comodo, GlobalSign. The browser trusts the CA, the CA vouches for the server, therefore the browser trusts the server. Organizations can also run internal CAs.

SSL certificates employ public and private keys to ensure a secure connection. Each certificate holder has a unique public key (shared openly) and a unique private key (kept secret). Data encrypted with the public key can only be decrypted with the private key. In practice: the client encrypts a session key with the server’s public key; the server decrypts it with its private key; then both sides use the shared session key for symmetric encryption of the actual traffic (faster than asymmetric for bulk data).

Encryption

Encryption uses software algorithms to disguise data such that a specific algorithm is required to decipher it. Types: symmetric (same key encrypts and decrypts - e.g. AES), asymmetric (public/private key pair - e.g. RSA). Encrypted data requires a specific key to be decrypted. Key management is critical - lose the key and the data is permanently inaccessible; expose the key and all the data is exposed.

Encryption protects data at rest (databases, laptops, backups) and data in transit (network traffic). HIPAA requires that affected individuals be notified if their data is lost/stolen - except if the data was appropriately encrypted (the encryption safe harbor). “Appropriately” means encrypted per NIST specifications. Practically: if a laptop with encrypted PHI is stolen, it’s not a reportable breach, because the data is unreadable without the key. This is the main incentive to encrypt everything that contains PHI - laptops, mobile devices, USB drives, databases.

User Behavioral Management

User behavioral management encompasses security on the user level: password management, de-identification of research data, avoiding exposure to malicious software, etc. Human behavior is usually the weakest link - tech can only help if users follow the procedures (lock workstations, don’t share credentials, don’t click suspicious links, report incidents).

55.7 Digital Pathology

Digital pathology: the analysis of pathology slides using a format displayed on a device monitor rather than through a traditional microscope. Encompasses static image capture, live video microscopy, and whole slide imaging. Several WSI systems have received FDA clearance for primary diagnosis (Philips IntelliSite, Leica Aperio AT2, Hamamatsu NanoZoomer).

Pixels, Coordinates, and Whole Slide Imaging

A pixel is the smallest unit of information that forms an image. Each pixel has a color value set by its bit depth (24-bit color = 16.7 million possible values). Pathology images contain millions to billions of pixels. A WSI at 40x of a standard glass slide is roughly 10 billion pixels (10 gigapixels).

On a monitor, each pixel has a specific coordinate (x, y) defining its location. Top-left is (0,0). In WSIs, coordinates enable precise localization, annotation, and measurement - digital annotation replaces physical slide markings.

Digital pathology may include dynamically viewing a slide scanned with a whole slide imager - a device that captures numerous high-resolution images of the slide and stitches them via onboard software into a single virtual slide. Scanning technologies: line, tile, or point. Scan at 20x or 40x objective. File sizes typically 0.5-3 GB per slide. WSI enables remote viewing, AI analysis, and permanent digital archival.

Modalities

Static digital pathology: viewing images (still photographs) of selected fields of a slide. What was captured is what you see - no navigation. Field selection bias is the main limitation. Good for quick consults, tumor boards, education.
Live/dynamic digital pathology: camera attached to the microscope, streaming real-time video to a remote viewer. The remote pathologist can drive the stage via robotic microscopy. Most useful for frozen sections.
Whole slide imaging (WSI): scan the whole slide, view it like a virtual slide. Highest fidelity, FDA-cleared platforms for primary diagnosis, enables AI analysis.

Telepathology

Telepathology: practicing pathology entirely using digital pathology. Usually means analyzing digital images transmitted to another location/device or posted to a web application accessed remotely. Requirements: adequate bandwidth, high-resolution monitors, validated digital pathology systems, and regulatory compliance (licensure in the state where the patient is located).

Uses of telepathology:

Primary diagnosis (digital-only practices) on FDA-cleared WSI platforms
Frozen section: real-time intraoperative consultation when a pathologist isn’t physically present (small hospitals, after-hours). Diagnostic concordance >95% with adequate image quality.
Touch preps (cyto adequacy)
Smears (hemepath, especially when the smear is prepared at a remote site)
Expert consultation with a separate facility - scan and share with a subspecialist anywhere. Turnaround in hours instead of days. Cheaper and safer than shipping glass.
Review of ancillary studies (e.g. IHC) performed at a remote site - view digital images of stains as soon as they’re done, without waiting for slides to be returned.

55.8 Computational Pathology

Computational pathology: a discipline within pathology that derives data from images for the purpose of AI and machine/deep learning. Transforms subjective visual assessments into objective, reproducible, quantitative measurements.

Applications:

Automated cell counting
Mitotic figure detection
Tumor grading
Biomarker quantification (e.g. IHC scoring for Ki-67, HER2, ER/PR, PD-L1)
Tumor microenvironment analysis
Outcome prediction

Deep learning for image classification uses deep convolutional neural networks (CNNs). CNNs learn hierarchical image features: early layers detect edges/textures, middle layers detect shapes/patterns, deep layers detect complex structures (cell types, tissue architecture). They are trained on thousands to millions of labeled images. For many specific tasks - metastasis detection in lymph nodes, prostate cancer grading - CNN performance approaches or matches expert pathologists. The bottleneck is the labeled data: computational pathology is as good as its annotated training sets.

Chapter 56: Patient Safety in the Laboratory

Most laboratory errors don’t occur during analysis - they occur before the specimen reaches the analyzer (pre-analytical) or after the result leaves it (post-analytical). Patient safety in the laboratory means understanding where errors happen and building systems to prevent them.

55.1 Specimen Identification: The Most Dangerous Error

A mislabeled specimen is perhaps the most dangerous error in laboratory medicine. If blood from Patient A is labeled as Patient B, the results will be attributed to the wrong patient. Patient A might receive treatment based on someone else’s values. Patient B might not receive treatment they need. In transfusion medicine, this error can be fatal - give Patient A blood matched to Patient B’s type and you may cause a fatal hemolytic reaction.

The solution is rigorous identification procedures at the point of collection. The standard requires two patient identifiers - typically name and date of birth, or name and medical record number. Identification must be confirmed with the patient directly (not from a wristband alone, which could be on the wrong patient). Labeling must occur at the bedside, immediately after collection, before the phlebotomist leaves the patient.

Technology helps. Barcoding systems that require scanning patient wristbands and specimen labels reduce transcription errors. Positive patient identification systems that require biometric or photographic confirmation add additional safety. But technology doesn’t replace the fundamental principle: verify identity before you draw, label before you leave.

55.2 Error Reporting and Root Cause Analysis

Errors will happen despite our best efforts. The question is whether we learn from them or repeat them. A culture of safety requires a non-punitive approach to error reporting - staff must feel safe reporting mistakes without fear of punishment, or errors will be hidden rather than addressed.

When errors occur, root cause analysis (RCA) identifies the underlying system failures. The goal is to ask “why” repeatedly until you reach a root cause that, if corrected, would prevent recurrence. A technologist entering a result incorrectly is not the root cause - you must ask why the error occurred. Was the interface confusing? Was the technologist distracted by a system that generates too many alerts? Was staffing inadequate?

Tools like the fishbone (Ishikawa) diagram organize potential causes into categories: equipment, process, people, materials, environment, management. The “5 Whys” technique involves asking “why” successively until you reach a root cause. The output of RCA is corrective action - a system change that addresses the root cause.

Failure mode and effects analysis (FMEA)

55.3 Hemolysis and Interference

Pre-analytical hemolysis is a major source of rejected specimens and spurious results.

Causes:

Traumatic venipuncture
Improper needle gauge
Mixing too vigorously
Expired tubes
Improper transport (temperature extremes, pneumatic tube trauma)

Detection: Hemolysis index on modern analyzers (spectrophotometric detection)

Management:

Reject specimens exceeding hemolysis threshold for affected tests
Report with appropriate comments if result provided
Investigate root causes if hemolysis rates are high

Chapter 57: Point-of-Care Testing

Point-of-care testing (POCT), also called bedside testing or near-patient testing, moves laboratory analysis from the central laboratory to the patient’s location. It trades some analytical precision for immediate results that can drive faster clinical decisions.

56.1 Why POCT Matters

The fundamental value of POCT is turnaround time. A glucose result available in 30 seconds at the bedside is more useful for titrating an insulin drip than a more precise result available in 30 minutes from the central lab. The clinical decision can be made immediately, while the patient is still in front of the clinician.

POCT is most valuable when rapid results change management and when waiting for central lab results would harm the patient. Classic applications include:

Glucose monitoring: Insulin dosing decisions can’t wait for central lab turnaround. Bedside glucometers provide immediate results, though with wider analytical variation than central lab instruments.

Blood gases and electrolytes: In critically ill patients, acid-base status and electrolyte abnormalities require immediate intervention. Devices like the i-STAT bring blood gas analysis to the bedside or the operating room.

Cardiac biomarkers: In chest pain evaluation, rapid troponin results accelerate the decision to treat or discharge. Point-of-care troponin assays are increasingly used in emergency departments.

Coagulation testing: INR monitoring in anticoagulation clinics and ACT (activated clotting time) monitoring during cardiac surgery require immediate results to guide therapy.

Infectious disease: Rapid strep tests, influenza tests, and COVID-19 antigen tests provide immediate answers that guide treatment and isolation decisions.

56.2 The Quality Challenge

POCT presents unique quality management challenges. When testing moves from the controlled central lab environment to clinical areas, many things can go wrong:

Operator variability: Central lab testing is performed by trained medical laboratory scientists. POCT is often performed by nurses, respiratory therapists, or physicians with variable training and experience. Competency assessment and ongoing training are essential.

Environmental factors: Central labs are climate-controlled. POCT devices may be exposed to temperature extremes, humidity, vibration, and other environmental stresses. Devices must be validated for the conditions where they’ll be used.

Quality control: Central labs run QC materials at defined intervals with rigorous documentation. POCT QC is often less structured. Establishing and enforcing QC schedules across dispersed testing sites is challenging.

Documentation: Every result needs to be associated with the correct patient. Without the structured workflow of the central lab, patient identification errors are more common. Interface with the electronic medical record is critical.

56.3 Regulatory Requirements

POCT is regulated just like central laboratory testing. Under CLIA (Clinical Laboratory Improvement Amendments), each POCT site is either included under the hospital’s main laboratory certificate or has its own certificate. The complexity of the test determines the regulatory requirements:

Waived tests: Simple tests approved by FDA for home use or with minimal risk of error. Examples include urine dipsticks, fecal occult blood tests, and simple glucose meters. Quality requirements are minimal, but the laboratory must still follow manufacturer instructions and document testing.

Moderate complexity tests: Most POCT falls here. Requires documented training, competency assessment, quality control, and proficiency testing. Examples include i-STAT analyzers and most rapid infectious disease tests.

High complexity tests: Rarely performed as POCT. Requires extensive quality systems similar to central laboratory.

56.4 Glucose Meter Limitations

Glucose meters deserve special attention because they’re ubiquitous, their results directly drive insulin dosing, and they have important limitations:

Hematocrit effects: Most glucose meters measure glucose in whole blood but report a “plasma equivalent” value. The conversion assumes normal hematocrit. In severely anemic patients, glucose readings may be falsely high; in polycythemic patients, falsely low.

Oxygen effects: Some glucose oxidase-based meters are affected by oxygen tension. In patients on supplemental oxygen or with severe hypoxia, results may be inaccurate.

Interfering substances: Various medications and conditions can interfere with specific glucose meter technologies. Maltose in some IV immunoglobulin products can cause falsely elevated readings with certain meters.

Critically ill patients: Multiple factors in critically ill patients (hypotension, edema, medications) can affect glucose meter accuracy. Many hospitals mandate central lab glucose confirmation for critically ill patients.

PART VI: MOLECULAR PATHOLOGY

Molecular pathology represents the application of molecular biology techniques to the diagnosis and management of disease. It has transformed medicine by enabling detection of genetic mutations, infectious agents, and molecular markers with unprecedented sensitivity and specificity. Understanding these techniques - how they work, their strengths, and their limitations - is essential for interpreting results and knowing when to order them.

Chapter 58: Molecular Biology Fundamentals

Before understanding molecular diagnostic tests, you must understand the molecules being tested. The central insight of molecular biology is that genetic information flows from DNA to RNA to protein, and disruptions at any level can cause disease.

58.1 Nucleic Acid Structure: The Information Molecules

DNA: The Archive

DNA (deoxyribonucleic acid) is the hereditary material - the stable repository of genetic information. Its structure explains its function.

The double helix: DNA consists of two strands wound around each other, held together by hydrogen bonds between complementary base pairs. This structure provides redundancy - damage to one strand can be repaired using the other as a template.

Chemical structure: Each strand is a polymer of nucleotides. The core of every nucleotide is a pentose sugar, either ribose (2’-OH) in RNA or deoxyribose (2’-H) in DNA. To complete the nucleotide, a phosphate group and a nitrogenous base are added to the sugar. A nucleoside is the same structure minus the phosphate - nucleotides are phosphorylated nucleosides.

Each nucleotide contains:

A pentose sugar (deoxyribose in DNA, ribose in RNA - the 2’-OH of ribose is what makes RNA susceptible to alkaline hydrolysis and explains most of RNA’s chemical instability)
A phosphate group (linking nucleotides together via phosphodiester bonds)
A nitrogenous base (carrying the genetic information)

Variations of the sugar matter for the lab. A dNTP lacks the 2’-OH; a ddNTP lacks both the 2’-OH and the 3’-OH. That missing 3’-OH on a ddNTP is the entire trick behind Sanger sequencing - once the polymerase incorporates a ddNTP, there’s no 3’-OH for the next phosphodiester bond, so the chain terminates. Each of the four ddNTPs carries a different fluorescent dye, which is how the capillary reader calls the terminal base.

The four bases and base pairing:

Purines (two rings): Adenine (A) and Guanine (G) - a 6-member ring fused to an imidazole
Pyrimidines (one ring): Cytosine (C), Uracil (U, RNA only), Thymine (T, DNA only)
A pairs with T (or U) via 2 hydrogen bonds
G pairs with C via 3 hydrogen bonds

Mnemonic for which are pyrimidines: CUT the PY (pie) - Cytosine, Uracil, Thymine are Pyrimidines. Larger purines always pair with smaller pyrimidines, which is what keeps helix width uniform at ~2 nm.

The G-C pairing is stronger than A-T pairing because of the extra hydrogen bond. This has practical consequences everywhere in the molecular lab. dsDNA melting temperature (Tm) is defined as the temperature at which 50% of the sequence is single-stranded, and it is higher for GC-rich sequences, lower for AT-rich. For short oligonucleotides, a useful rule of thumb is Tm ~ 2(A+T) + 4(G+C). PCR annealing is typically set ~5°C below primer Tm. GC-rich regions are harder to denature, harder to amplify, and prone to secondary structure that stalls polymerase.

Phosphodiester bond: Sequential nucleotides are joined by the phosphodiester bond, formed when the 5’ triphosphate of an incoming nucleotide is hydrolyzed against the 3’-OH of the growing chain. The process is called polymerization, and each step releases pyrophosphate (PPi) along with the newly extended oligonucleotide. Pyrophosphatase then hydrolyzes PPi, which pulls the reaction forward thermodynamically. Pyrosequencing exploits this released PPi: sulfurylase converts it to ATP, and luciferase turns the ATP into light. Nucleases cleave these same phosphodiester bonds - endonucleases cut within a chain, exonucleases need a free 3’ or 5’ end.

Directionality: DNA strands have chemical polarity. The 5’ end has a free phosphate; the 3’ end has a free hydroxyl. Polymerization is directional, 5’ to 3’, which is also the direction sequence is written and read by convention. This is why: the leading strand is synthesized continuously, the lagging strand requires Okazaki fragments, and PCR primers must be designed so their 3’ end points into the target.

Antiparallel strands: DNA is a double helical, antiparallel duplex. The coding / forward / sense strand runs 5’ to 3’; the template / antisense / noncoding strand runs 3’ to 5’. RNA polymerase reads the antisense strand 3’ to 5’ and produces an mRNA whose sequence matches the sense strand (with U for T). Mutations are always reported on the sense strand.

Helix geometry: The most common in vivo form is B-form DNA, which has 10 base pairs per helical turn and one major and one minor groove. Most sequence-specific DNA-binding proteins (transcription factors) contact the major groove. A-form (RNA-DNA hybrids, dsRNA) is more compact at ~11 bp/turn; Z-form is left-handed and associated with alternating purine-pyrimidine tracts.

The human genome: About 3 billion base pairs, organized into 23 pairs of chromosomes. Only ~1.5% encodes proteins (exons); the rest includes regulatory regions, introns, repetitive elements, and sequences of unknown function.

RNA: The Working Copy

RNA (ribonucleic acid) differs from DNA in three key ways:

Ribose instead of deoxyribose: The extra 2’ hydroxyl makes RNA less stable (susceptible to hydrolysis)
Uracil instead of thymine: U pairs with A instead of T
Usually single-stranded: Can fold into complex secondary structures

RNA is less stable than DNA because of the ubiquity of ribonucleases (RNases) in the environment. RNases are absurdly stable enzymes - they survive autoclaving, boiling, and most denaturants. That is the real reason the molecular lab treats RNA like it’s radioactive: DEPC-treated water, dedicated pipettes and benches, RNase-free consumables, snap freezing. RNA integrity is typically assessed by the 28S:18S rRNA ratio (should be ~2:1) or a RIN (RNA Integrity Number) >7.

Types of RNA:

Messenger RNA (mRNA): Carries the genetic code from DNA to ribosomes for protein synthesis. In eukaryotes, mature mRNA has a 5’ cap (protects from degradation), a 3’ poly-A tail (stabilizes the molecule), and no introns (removed by splicing). mRNA is only ~1-5% of total cellular RNA, so poly-A selection (oligo-dT beads) is standard for enriching mRNA before RNA-seq.

Transfer RNA (tRNA): Delivers specific amino acids to the ribosome for addition to a growing peptide chain. Each tRNA has an anticodon that recognizes a specific mRNA codon. There are 61 sense codons but only ~45 tRNAs - the gap is covered by wobble pairing at the third codon position. tRNAs exist in two main forms: aminoacylated (amino acid attached, “charged”) and nonacylated (uncharged). Aminoacyl-tRNA synthetases are the enzymes that charge tRNAs, one per amino acid, and they are largely responsible for the fidelity of the genetic code.

tRNA nomenclature is worth knowing for the exam. Using alanine: the nonacylated form is written tRNA-Ala or tRNA^Ala; the aminoacylated form is alanyl-tRNA, Ala-tRNA, or Ala-tRNA^Ala. Clinically, autoantibodies against aminoacyl-tRNA synthetases (anti-Jo-1 vs histidyl-tRNA synthetase) define antisynthetase syndrome (myositis, ILD, mechanic’s hands). Mitochondrial tRNA mutations cause MELAS (MT-TL1, tRNA-Leu) and MERRF (MT-TK, tRNA-Lys).

Ribosomal RNA (rRNA): Plays a structural and catalytic role within ribosomes during translation. rRNA is the most abundant RNA species (~80% of total cellular RNA). The 28S and 18S bands on a gel are what you look at to judge RNA quality - degraded RNA loses the sharp bands and smears. rRNA genes are highly conserved, which makes them excellent targets for identifying organisms - the 16S rRNA gene is the standard target for bacterial identification in clinical microbiology.

MicroRNA (miRNA): Small (~22 nucleotides) regulatory RNAs transcribed by RNA polymerase II that regulate mRNA levels and protein expression by binding complementary sequences in the 3’ UTR of target mRNAs, causing translational repression or mRNA degradation. They regulate ~60% of human genes. miRNAs are remarkably stable in FFPE tissue, and miRNA expression profiling can help classify tumors of unknown primary (e.g., miR-122 for HCC).

The term nuclear RNA refers specifically to the sum of miRNA and mRNA - distinguishing it from cytoplasmic species (tRNA, rRNA).

Enzymes That Act on Nucleic Acids

Several enzyme families show up repeatedly in both biology and the molecular lab:

Polymerase: adds single nucleotides to a growing chain via phosphodiester bonds. DNA polymerase makes DNA; RNA polymerase makes RNA. Board-relevant lab polymerases: Taq polymerase (from Thermus aquaticus, heat-stable, used in PCR), reverse transcriptase (converts RNA to cDNA for RT-PCR), and proofreading polymerases like Pfu (lower error rate than Taq).
RNA Polymerase I makes rRNA. RNA Polymerase II makes mRNA and miRNA. RNA Polymerase III makes tRNA (and 5S rRNA). Mnemonic: I ribosomal, II messenger, III transfer. Alpha-amanitin (Amanita phalloides death cap toxin) preferentially inhibits Pol II > Pol III > Pol I, which is why Amanita poisoning wipes out the liver - hepatocytes have enormous transcriptional demand.
Ligase: catalyzes phosphodiester bond formation between ends of adjacent nucleotide chains. Essential for sealing Okazaki fragments on the lagging strand, joining insert-to-vector in cloning, and the ligase chain reaction (LCR). T4 DNA ligase is the workhorse enzyme for joining blunt or cohesive ends in molecular biology.
Nuclease: catalyzes cleavage of phosphodiester bonds. DNase cleaves DNA; RNase cleaves RNA. In the lab, DNase I is used to remove contaminating genomic DNA from RNA preps before RT-PCR; therapeutically, DNase I is marketed as dornase alfa (Pulmozyme) to degrade extracellular DNA in CF mucus.
Endonucleases cut within a chain; exonucleases require a free end (3’ or 5’). A specialized type of endonuclease that recognizes and cleaves palindromic sequences is a restriction endonuclease. These are bacterial defense enzymes. Named by organism of origin (EcoRI from E. coli, HindIII from Haemophilus influenzae), they produce either sticky ends (5’ or 3’ overhangs) or blunt ends. They are the foundation of RFLP (restriction fragment length polymorphism) analysis and of molecular cloning. Bacteria protect their own genomes from their own restriction enzymes via methylation.

The Mitochondrial Genome

Mitochondria carry their own genome: a circular, double-stranded DNA with 37 genes that encode 13 oxidative phosphorylation complex subunits, 22 tRNAs, and 2 rRNAs. mtDNA has no introns, no histones, limited repair machinery, and a 10-20x higher mutation rate than nuclear DNA. Each cell contains hundreds to thousands of mitochondria. mtDNA’s stability in challenging specimens (compared to nuclear DNA) makes it valuable in forensic identification of ancient remains or hair shafts without roots.

Replication of the mitochondrial genome happens independently of the nuclear genome and independently of the cell cycle. During cell division, mitochondria distribute randomly to daughter cells, so mitochondrial copy number can vary cell to cell and tissue to tissue. Tissues with high energy demand (brain, muscle, heart) carry more mitochondria and are preferentially affected by mitochondrial mutations.

Here’s a gotcha: most mitochondrial proteins are encoded in the nucleus, not in mtDNA. The mitochondrial proteome is ~1,500 proteins; mtDNA encodes only 37 genes. That is why some “mitochondrial” diseases follow Mendelian inheritance (nuclear gene mutations) rather than maternal inheritance - MNGIE is caused by nuclear TYMP mutations, for instance.

Heteroplasmy vs homoplasmy: Heteroplasmy occurs when a mutation arises in a single mitochondrion and is then passed unevenly to daughter cells as mitochondria divide, so a cell ends up with a mixture of mutant and wild-type mtDNA. Disease manifests once the proportion of mutant mtDNA crosses a threshold (typically 60-90% depending on the mutation) - this threshold effect explains variable expressivity and age-dependent penetrance in mitochondrial disease. Homoplasmy means all mitochondria in a cell are genetically identical (all mutant or all wild-type). Homoplasmic pathogenic mutations tend to cause milder phenotypes (e.g., Leber hereditary optic neuropathy, aminoglycoside-induced deafness from m.1555A>G) because they have to be compatible with survival to reproductive age.

Maternal inheritance: mtDNA is almost exclusively maternal in the embryo, because (1) the egg contributes ~100,000 mitochondria while sperm contribute ~100, and (2) paternal mitochondria are actively tagged with ubiquitin and destroyed after fertilization. Mitochondrial disorders are therefore inherited from the mother’s side and affect male and female offspring equally. An affected mother passes the mutation to all her children, but only daughters transmit it further - no male-to-offspring transmission. On a pedigree, affected individuals in every generation through the maternal line = mitochondrial inheritance. Key distinction from X-linked recessive: X-linked recessive mostly affects males; mitochondrial affects both sexes equally.

58.2 The Central Dogma: Information Flow

The central dogma of molecular biology describes how genetic information flows:

DNA → DNA (Replication): The genome is copied before cell division

DNA → RNA (Transcription): Genes are copied into RNA

RNA → Protein (Translation): The genetic code is translated into amino acid sequence

Understanding each step is essential because molecular tests target different steps.

Replication: Copying the Genome

Before a cell divides, it must copy its entire genome. This process is remarkably accurate - only about 1 error per billion nucleotides copied.

Key features:

Semi-conservative: Each daughter double helix contains one original strand and one newly synthesized strand
DNA polymerase synthesizes 5’ to 3’: The enzyme adds nucleotides to the 3’ end of a growing strand
Requires a primer: DNA polymerase cannot start synthesis de novo; it needs a short RNA primer
Proofreading: DNA polymerase has 3’ to 5’ exonuclease activity to correct errors

Clinical relevance: Many chemotherapy drugs target replication (antimetabolites, topoisomerase inhibitors). Understanding replication also explains how mutations occur - replication errors that escape proofreading.

Transcription: Making RNA from DNA

Gene expression begins with transcription - copying a gene into RNA.

The process:

RNA polymerase binds to the promoter region upstream of the gene
The enzyme unwinds DNA and reads the template strand 3’ to 5’
RNA is synthesized 5’ to 3’, complementary to the template (identical to the coding strand, except U replaces T)
Transcription continues until a termination signal

In eukaryotes, the primary transcript (pre-mRNA) must be processed:

5’ capping: A modified guanine is added to protect from degradation
3’ polyadenylation: A poly-A tail (100-250 adenines) is added
Splicing: Introns are removed, and exons are joined together

Alternative splicing allows one gene to produce multiple proteins by including different combinations of exons. This explains why humans have ~20,000 genes but many more proteins.

Translation: Making Protein from mRNA

The ribosome reads mRNA in triplet codons, each specifying an amino acid.

The genetic code:

64 codons (4³ = 64 combinations of 3 nucleotides)
61 sense codons (specify amino acids)
3 stop codons (UAA, UAG, UGA - terminate translation)
AUG is the start codon (also codes for methionine)

Degeneracy: The code is degenerate - most amino acids are specified by multiple codons (e.g., leucine has 6 codons). This provides some protection against mutations - a change in the third position of a codon often doesn’t change the amino acid (wobble position).

Gene Structure

A gene is not just a string of coding sequence. It contains a promoter sequence that regulates transcription initiation (TATA box, CpG islands, enhancers) plus a coding sequence comprising exons and introns. Exons are the translated sequences that end up in the mature mRNA; introns remain untranslated and are spliced out. After transcription the pre-mRNA is (1) 5’ capped, (2) 3’ polyadenylated, and (3) spliced to remove introns.

Promoter CpG island methylation is a major epigenetic mechanism of gene silencing. This is how sporadic MLH1 is silenced in the methylator (CIMP) pathway of colorectal cancer, and how MGMT is silenced in glioblastoma (with downstream implications for temozolomide response).

58.3 Signaling Pathways: How Cells Decide What to Do

Most molecular pathology boards questions on “pathway X is activated in cancer Y” come down to three core pathways: MAPK, JAK-STAT, and PI3K-AKT-mTOR. These pathways are also the drug target list for a huge fraction of modern oncology.

Signaling pathways typically begin at the cell surface with a soluble ligand binding the extracellular portion of a cell surface receptor. Exceptions exist - steroid hormone receptors (ER, PR) are intracellular because the ligand is lipophilic and crosses the membrane directly. Receptor location determines therapy: trastuzumab targets the extracellular domain of HER2, whereas tamoxifen blocks the intracellular estrogen receptor. Ligand binding triggers modification of the intracellular portion of the receptor (autophosphorylation, conformational change), which recruits cytoplasmic signaling proteins.

MAPK (RAS-RAF-MEK-ERK) Pathway

The MAPK pathway is activated in HER2-amplified breast cancer, a subset of NSCLCs, colorectal carcinoma, and BRAF- and KIT-mutated tumors. It starts when a ligand binds a transmembrane receptor tyrosine kinase (RTK). EGFR, HER2 (ErbB2/Neu), and KIT are canonical RTKs. HER2 is unusual - it has no known ligand and is activated by heterodimerization with other HER family members.

Mechanistically: ligand binding causes dimerization and autophosphorylation of the RTK’s intracellular kinase domain. The activated RTK then activates RAS (rat sarcoma virus protein) via adapter proteins (GRB2/SOS) that promote GDP-to-GTP exchange. Activated RAS then triggers sequential activation of RAF, then MEK, then ERK, and ERK finally drives transcription of genes for cellular growth and division.

Mutations anywhere along this axis can be oncogenic, and importantly these activating mutations are mutually exclusive within a tumor - EGFR, HER2, HER3, KIT, RAS, RAF, MEK, and ERK all converge on the same output, so one is enough. That clinical principle matters: KRAS-mutated colorectal tumors are resistant to anti-EGFR therapy (cetuximab/panitumumab) because the pathway is already activated downstream of the EGFR block.

Key single mutations to have in memory:

BRAF V600E: melanoma (~50%), papillary thyroid (~45%), hairy cell leukemia (~100%), a subset of colorectal cancers. Vemurafenib and dabrafenib specifically target V600E-mutated BRAF.
KRAS codon 12/13/61: locks RAS in the GTP-bound (active) state by impairing GTPase activity. KRAS G12 mutations occur in >90% of pancreatic ductal adenocarcinoma.
EGFR exon 19 deletions and L858R: lung adenocarcinoma.
HER2 amplification: breast (~15-20%), gastric (~20%).
KIT exon 11: GIST (most imatinib-responsive).

JAK-STAT Pathway

JAK (Janus kinase) is an intracellular cytoplasmic kinase that transduces cytokine-mediated signals through the JAK-STAT pathway. The JAK family includes JAK1, JAK2, JAK3, and TYK2. Cytokine receptors have no intrinsic kinase activity - they depend entirely on associated JAKs.

STAT (Signal Transducer and Activator of Transcription) is a latent transcription factor that JAK activates by phosphorylation.

The cascade: cytokine binding drives receptor dimerization, which brings two JAK molecules together. The JAKs trans-phosphorylate each other (activation), then phosphorylate tyrosine residues on the receptor to create docking sites for STATs. STATs dock, are phosphorylated by JAK, dimerize, and translocate to the nucleus to regulate gene transcription.

Clinically: the JAK2 V617F mutation is the hallmark of myeloproliferative neoplasms - present in ~97% of polycythemia vera, ~55% of essential thrombocythemia, and ~65% of primary myelofibrosis. Ruxolitinib is a JAK1/JAK2 inhibitor used in myelofibrosis. STAT3 activation shows up in large granular lymphocytic leukemia and anaplastic large cell lymphoma.

PI3K-AKT-mTOR Pathway

PI3K catalyzes conversion of phosphatidylinositol bisphosphate (PIP2) to phosphatidylinositol triphosphate (PIP3) on the inner leaflet of the cell membrane. PTEN catalyzes the reverse reaction - PIP3 back to PIP2. PI3K and PTEN are therefore mutually antagonistic. Tip that sticks: “PI3K makes it, PTEN takes it.”

PI3K can be activated by either GPCRs or RTKs, so it runs in parallel with MAPK. Cross-talk between pathways is why single-agent targeted therapy often fails - tumors bypass blockade through the alternative pathway.

Once PIP3 is made, it recruits AKT (protein kinase B) to the membrane, where PDK1 phosphorylates and activates it. AKT promotes cell division and survival by inactivating BAD (pro-apoptotic), activating mTOR (protein synthesis and cell growth), and inactivating GSK3-beta (which normally degrades cyclin D1). AKT is promoted by mTOR and inhibited by TSC1 and TSC2.

TSC1 (hamartin) and TSC2 (tuberin) are tumor suppressors that restrain mTOR. Germline TSC1/TSC2 mutations cause tuberous sclerosis complex (cortical tubers, cardiac rhabdomyomas, renal angiomyolipomas, LAM). Rapamycin analogs (everolimus, temsirolimus) inhibit mTOR and are used in RCC and select neuroendocrine tumors.

PIK3CA (the PI3K catalytic subunit) is one of the most commonly mutated oncogenes across cancer: ~35% of breast, ~50% of endometrial, ~15% of colorectal. PTEN loss (tumor suppressor) shows up in sporadic glioblastoma, endometrial carcinoma, prostate cancer, and germline in Cowden syndrome (hamartomas with increased breast/thyroid/endometrial cancer risk). PIK3CA gain-of-function and PTEN loss-of-function mutations tend to be mutually exclusive in a tumor.

58.4 DNA Repair and the Cell Cycle

DNA Repair

Replication errors happen. DNA polymerase has 3’ to 5’ proofreading exonuclease activity, but misincorporations and mutagen-induced damage still escape. Two repair pathways dominate boards content:

Mismatch repair (MMR): corrects single-base mismatches and insertion/deletion loops after replication. MMR proteins act as heterodimers: MLH1/PMS2 and MSH2/MSH6. Loss of MLH1 obligates loss of PMS2; loss of MSH2 obligates loss of MSH6. Germline MMR defects in MLH1, MSH2, MSH6, PMS2 cause hereditary nonpolyposis colorectal cancer (HNPCC) / Lynch syndrome. Amsterdam criteria (3-2-1): 3 affected relatives, 2 successive generations, 1 diagnosed <50. MMR deficiency produces microsatellite instability (MSI) and confers susceptibility to checkpoint inhibitors.
Homologous recombination repair (HRR): repairs double-strand breaks using the sister chromatid as template. Germline BRCA1 (17q21) and BRCA2 (13q12.3) mutations impair HRR and cause hereditary breast and ovarian cancer. BRCA1-associated breast cancers tend to be high-grade, triple-negative, medullary-like with pushing margins and lymphocytic infiltrate. BRCA2 tumors lack distinctive morphology. PARP inhibitors (olaparib) exploit synthetic lethality in HRR-deficient tumors.

The Cell Cycle

The cell cycle has four phases: M (mitosis), G1 (gap 1), S (DNA synthesis), G2 (gap 2). M phase is when cell division and cytokinesis happen. Interphase consists of G1, S, and G2. Terminally differentiated, non-dividing cells are in G0 (neurons, cardiac myocytes, mature skeletal muscle).

Cyclin-CDK complexes drive progression: Cyclin D-CDK4/6 in G1, Cyclin E-CDK2 at G1/S, Cyclin A-CDK2 in S, Cyclin B-CDK1 at G2/M. Key checkpoints: G1/S (restriction point, Rb and p53 guarded), G2/M (DNA damage). CDK4/6 inhibitors (palbociclib, ribociclib) are used in ER+ breast cancer.

Lab-relevant points: conventional karyotyping requires metaphase cells, so colchicine/colcemid arrests cells in metaphase by disrupting microtubules. FISH works on interphase cells, which is why FISH can be done on FFPE sections, cytology preps, and blood smears without cell culture - a huge advantage over conventional cytogenetics. G0 cells are resistant to cell cycle-dependent chemotherapy and can also cause conventional cytogenetics to fail when sampled tissues have low proliferative activity.

58.5 Mutations: When the Code Changes

Mutations are changes in the DNA sequence. They can be inherited (germline, present in every cell) or acquired (somatic, present only in certain cells like a tumor).

Mutation vs Polymorphism

The textbook cutoff: a mutation is a genetic change present in <1% of the population. A polymorphism is a change present in >1% of the population and generally NOT deleterious (blood type, HLA alleles, SNPs). In practice, pathogenicity is determined by functional consequence, not by frequency - plenty of clinically significant variants are common, and plenty of rare variants are benign. Single nucleotide polymorphisms (SNPs) are the most common type of polymorphism; there are roughly 10 million SNPs in the human genome, and SNP arrays are the backbone of GWAS, pharmacogenomics, and chromosomal microarray analysis.

Loss of Function, Gain of Function, Dominant Negative

Mutations can be loss of function or gain of function. Loss-of-function is typical for tumor suppressors (needs both alleles inactivated - Knudson’s two-hit hypothesis: TP53, RB1, APC, MLH1). Gain-of-function is typical for oncogenes (single mutated allele is enough: RAS, RET in MEN2, BRAF V600E).

A special and high-yield category is the dominant negative mutation, in which the mutant protein actively inhibits the function of the wild-type protein (or “overcomes” its function), rather than simply being inactive. This is worse than haploinsufficiency because one mutant allele disables the products of both. Classic example: mutant p53 forms tetramers with wild-type p53, inactivating the whole tetramer. Other examples: mutant collagen chains in osteogenesis imperfecta, mutant keratin in epidermolysis bullosa simplex.

Germline vs Somatic

Germline mutations arise in undifferentiated (germ-line) cells, are present in every cell of the offspring, and can be passed to future generations. Germline testing uses blood or buccal DNA. Classic examples: BRCA1/2, MLH1/MSH2, APC (FAP), RB1, TP53 (Li-Fraumeni). Germline variants are reported using the ACMG/AMP 5-tier classification: pathogenic, likely pathogenic, VUS, likely benign, benign.

Somatic mutations arise in differentiated cells, are present only in the affected tissue (usually a tumor), and are unlikely to be passed to offspring. Somatic testing uses tumor tissue. Examples: BRAF V600E in melanoma, KRAS G12D in pancreas, EGFR L858R in lung. Somatic variants are reported with a focus on actionability (therapeutic, diagnostic, prognostic). Paired tumor-normal sequencing helps distinguish the two.

Penetrance and Expressivity

Penetrance is the proportion of patients with a mutation who manifest the phenotype. Complete penetrance: every carrier is eventually affected (Huntington disease approaches 100% by age 70). Incomplete/reduced penetrance: not all carriers are affected (BRCA1 has ~70% lifetime breast cancer risk, not 100%). Penetrance can be age-dependent.

Expressivity is the severity and range of manifestations among affected individuals. Variable expressivity means the same mutation causes different severity in different people - NF1 is the textbook example (same mutation can produce anything from a few café-au-lait spots to plexiform neurofibromas). Expressivity is modified by other genes, epigenetics, and environment. Distinction: penetrance = whether affected; expressivity = how severely affected.

Autosomal recessive disorders generally do not vary much in penetrance or expressivity, because affected individuals are homozygous (both alleles nonfunctional, no residual activity to create variability). Autosomal dominant disorders show more variable expressivity because carriers retain one normal allele.

Mode of Inheritance Rules of Thumb

Structural proteins, receptor proteins, and transmembrane proteins: typically autosomal dominant. A 50% reduction in protein or a dominant-negative effect is enough to cause disease. Examples: type I collagen (OI), fibrillin-1 (Marfan).
Enzymes: typically autosomal recessive. Enzymatic pathways usually have excess capacity, so 50% enzyme activity is sufficient. Disease requires both alleles knocked out. Examples: PKU, Gaucher, CF.
X-linked recessive: expressed almost exclusively in male offspring (hemizygous, only one X). Females are typically carriers. Key rules: no male-to-male transmission; all daughters of affected males are obligate carriers; carrier mothers have a 50% chance of affected sons. Classic examples: hemophilia A (F8), Duchenne (dystrophin), G6PD deficiency, Bruton agammaglobulinemia (BTK). Occasional females manifest due to skewed X-inactivation, Turner (45,X), or homozygosity.

Types of Mutations

There are five main categories of gene mutations: point mutations, insertions, deletions, inversions, and translocations. Detection method varies by type: point mutations by sequencing, translocations by FISH or karyotype, copy number changes by array or read depth.

Point Mutations (Single Nucleotide Variants)

A change in a single base pair. Transitions are purine-to-purine or pyrimidine-to-pyrimidine; transversions are purine-to-pyrimidine (or vice versa).

Silent (synonymous) mutations change the nucleotide but do NOT change the amino acid (codon degeneracy). Most are truly silent, but some are pathogenic by disrupting exonic splice enhancers/silencers, altering mRNA stability, or shifting codon usage bias. This is why in silico prediction tools matter for variant interpretation.

Example: CGA → CGG (both code for arginine)

Missense mutations change one amino acid to another. Effect depends on how similar the new amino acid is and how important the position is.

Conservative: New amino acid has similar properties (may have little effect)
Non-conservative: New amino acid has different properties (often pathogenic)
Example: HbS mutation (sickle cell) - GAG → GTG changes glutamic acid to valine
Activating examples: BRAF V600E, KRAS G12D. Loss-of-function: HFE C282Y (hemochromatosis).

Nonsense mutations change an amino acid codon to a premature stop codon, truncating translation and usually producing a non-functional protein. If the premature stop is >50-55 nucleotides upstream of the last exon-exon junction, the mRNA is degraded by nonsense-mediated decay (NMD), so the truncated protein is never made. This is a common mechanism of tumor suppressor inactivation (APC truncating mutations in FAP). One practical tip: if IHC shows complete loss of protein, consider a truncating mutation with NMD.

Example: CGA (arginine) → TGA (stop)

Insertions and Deletions (Indels)

Nucleotides added to or removed from the sequence.

Frameshift mutations: When the number of nucleotides inserted or deleted is NOT a multiple of 3, the reading frame shifts. Every codon downstream is wrong, usually leading to a premature stop codon and a truncated non-functional protein. Almost always loss-of-function. Most commonly caused by insertions and deletions (indels). Classic example: BRCA1/BRCA2 frameshift mutations in hereditary breast/ovarian cancer.

In-frame mutations: When 3 (or a multiple of 3) nucleotides are added or deleted, the reading frame is preserved. One or more amino acids are added or removed. May or may not be pathogenic depending on the location.

Example: The ΔF508 mutation in CFTR (cystic fibrosis) is an in-frame deletion of 3 nucleotides, removing a single phenylalanine - but this is pathogenic because the phenylalanine is essential for protein folding.
Another in-frame example: EGFR exon 19 deletions in lung adenocarcinoma are activating (the reading frame is preserved but the resulting kinase is constitutively active).

Splice Site Mutations

Mutations at intron-exon boundaries disrupt normal splicing. Splice site mutations can be caused by point mutations at the canonical GT at the 5’ end of the intron (splice donor) or AG at the 3’ end of the intron (splice acceptor). Consequences include:

Exon skipping (exon is left out)
Intron retention (intron is included)
Cryptic splice site activation (nearby sequence is used instead)

The result is usually an abnormal mRNA that produces a nonfunctional protein. Splice site mutations account for ~15% of all disease-causing mutations and are generally classified as likely pathogenic when they hit the canonical GT…AG dinucleotides.

Large-Scale Mutations

Deletions and duplications: Loss or gain of large DNA segments (exons, entire genes, or chromosomal regions). Detected by techniques that measure copy number (MLPA, arrays, read depth in NGS).

Translocations: Two chromosomes exchange segments. Can create oncogenic fusion genes (BCR-ABL1 in CML, PML-RARA in APL).

Inversions: A chromosomal segment is reversed. May disrupt genes at the breakpoints.

58.6 Nomenclature: Speaking the Language

Standardized nomenclature (HGVS - Human Genome Variation Society) ensures mutations are described unambiguously.

Sequence type prefixes:

1. = genomic (chromosomal position)
1. = coding DNA (position relative to translation start)
1. = protein (amino acid position)
1. = RNA

Common notations:

c.123A>G: Substitution of A to G at position 123 of the coding sequence
c.123_124delAT: Deletion of AT at positions 123-124
c.123_124insGGG: Insertion of GGG between positions 123 and 124
c.123+1G>A: Substitution in the first nucleotide of an intron (+1 is first intronic position after the exon)
p.Glu123Val or p.E123V: Glutamic acid to valine at protein position 123
p.Glu123*: Glutamic acid to stop codon (nonsense)
p.Glu123fs: Frameshift starting at position 123

Understanding this nomenclature is essential for interpreting molecular pathology reports.

Cytogenetic Nomenclature

Chromosome nomenclature is orthogonal to HGVS but just as important, because a huge fraction of board questions land on translocations and karyotype reports.

The p arm is the shorter arm (petit); the q arm is the longer arm, in reference to the centromere.
Acrocentric chromosomes (13, 14, 15, 21, 22) have very short p arms that contain rRNA genes - these are the chromosomes that participate in Robertsonian translocations.
Normal male karyotype: 46,XY. Normal female: 46,XX. Trisomy 21: 47,XX,+21.

The format for a band designation is [chromosome][arm][region][band][sub-band]. Regions are numbered outward from the centromere. Example: 9p21.3 = chromosome 9, short arm, region 2, band 1, sub-band 3 - this is the CDKN2A/p16 locus that is deleted across many tumor types. Another worth knowing: 18q21.3 = sub-band 3 of band 1 in region 2 on the long arm of chromosome 18 - the BCL2 locus. The t(14;18)(q32;q21) juxtaposes BCL2 with the IGH locus and is the hallmark of follicular lymphoma.

Reading a cytogenetics report:

The karyogram is the actual image of metaphase chromosomes arranged by size and banding.
The karyotype is the interpretation (the ISCN-formatted string like 46,XY,t(9;22)(q34;q11.2)).
Conventional karyotyping uses G-banding of metaphase-arrested cells. Resolution is ~5-10 Mb, which is why submicroscopic changes (microdeletions, CNVs) need FISH, array, or read-depth NGS.

58.7 Pre-Analytical Considerations

The cleanest way to ruin a molecular test is upstream of the molecular lab. A few pre-analytical traps worth memorizing:

Heparin inhibits PCR because it mimics DNA and binds Taq polymerase. EDTA (purple top) is the preferred anticoagulant for molecular studies. If a heparinized sample has to be used, heparinase treatment or dilution can help. Other known PCR inhibitors: hemoglobin, melanin, collagen, humic acid, and IgG. Internal amplification controls should catch these.
Mercury-based fixatives (B5, Zenker) cross-link proteins to DNA and interfere with nucleic acid extraction. Historically used in hematopathology for nuclear detail; largely replaced by zinc formalin.
Acidified decalcification solvents (formic acid, HCl) destroy DNA via depurination. If molecular testing is anticipated on bone or calcified tissue, use EDTA-based decalcification instead. Submitting an acid-decalcified marrow biopsy for FISH or NGS is a classic way to get degraded, uninterpretable results.
Average DNA fragment size from formalin-fixed tissue is ~300-400 bp, plus formalin introduces C>T deamination artifacts. This is why FFPE-based NGS assays use short amplicons (typically 150-250 bp). Optimal fixation is 6-48 hours in 10% NBF. Fresh or snap-frozen tissue yields >10 kb fragments and is preferred for WGS or complex assays.
RNA preparation requires high-quality preserved tissue (fresh/snap-frozen), RNase inhibitors (e.g., RNasin), and an RNase-free environment. RNA integrity is scored by RIN (1-10; >7 is good) and by the 28S:18S ratio on a bioanalyzer. Fusion transcript detection (e.g., BCR-ABL1 by RT-PCR) and gene expression profiling both depend on intact RNA.
DNA preparation usually includes RNase treatment to remove contaminating RNA, and should NOT use OCT-embedded frozen tissue - OCT compound (polyvinyl alcohol, PEG) inhibits PCR and interferes with extraction. Trim away OCT before extraction.

Extraction methods:

Liquid-phase nucleic acid isolation (alcohol precipitation): ethanol or isopropanol plus monovalent cations (sodium acetate) precipitates nucleic acids out of aqueous solution. Pellet, wash, resuspend. Cheap and simple; labor-intensive.
Solid-phase extraction: nucleic acids adsorb to a silica/selective matrix under high-salt, low-pH conditions and are eluted under low-salt, high-pH conditions. Basis of spin columns (Qiagen) and magnetic bead systems (MagNA Pure). Dominant method in automated high-throughput labs.

Quantification: Nucleic acids absorb UV at 260 nm. A260 of 1.0 = ~50 ug/mL dsDNA or ~40 ug/mL RNA. Purity is assessed by the A260/A280 ratio: >1.8 for pure DNA, ~2.0 for pure RNA. A low ratio indicates protein contamination (proteins absorb at 280 nm). A260/A230 should also be >1.8 (lower indicates organic solvent or salt contamination). NanoDrop is standard but cannot distinguish intact from degraded nucleic acids; fluorometric methods (Qubit) measure only intact dsDNA.

58.8 Techniques Based on Hybridization

Restriction Enzymes and RFLP

Restriction enzymes are bacterial endonucleases that recognize and cleave specific palindromic DNA sequences, producing 5’ overhangs, 3’ overhangs, or blunt ends. Typical recognition sequences are 4-8 bp. Bacteria protect their own DNA by methylation.

Restriction fragment length polymorphism (RFLP) exploits this: a mutation that creates or destroys a recognition site changes the size of the restriction fragments produced. Workflow: digest DNA with the enzyme, run on a gel, transfer to membrane (Southern blot), hybridize with a labeled probe. RFLP was historically used for sickle cell (MstII), paternity testing, and early genetic mapping. Largely replaced by PCR-based methods, but the concept is still tested.

Methylation alters restriction endonuclease cleavage sites. Methylation-sensitive enzymes (HpaII) cut only unmethylated sites; methylation-insensitive isoschizomers (MspI) cut regardless. Comparing patterns reveals methylation status - the principle behind COBRA and methylation-specific MLPA, used clinically to detect promoter methylation of MLH1 in CRC.

Gel Electrophoresis

Gel electrophoresis separates DNA by size - smaller fragments migrate faster through agarose or polyacrylamide. Ethidium bromide intercalates DNA and fluoresces under UV (SYBR Safe and GelRed are less mutagenic alternatives). Agarose gels handle 100 bp to 25 kb; polyacrylamide gels resolve single-nucleotide differences and are used for Sanger sequencing.

Denaturing gradient gel electrophoresis (DGGE) separates DNA fragments of the same size but different sequences. As dsDNA migrates through an increasing denaturant gradient, it partially melts at sequence-dependent positions, retarding migration. Mostly replaced by HRM and direct sequencing now, but occasionally asked.

Blotting

Mnemonic: SNoW DRoP - Southern = DNA, Northern = RNA, Western = Protein. Southern was Ed Southern’s last name; Northern and Western are directional puns.

Southern blot: DNA separated by gel, transferred to nitrocellulose/nylon membrane, hybridized with labeled probe.
Northern blot: same principle for RNA.
Western blot: proteins separated by SDS-PAGE, transferred to membrane, detected with antibodies. Still used for HIV confirmation in some algorithms.

FISH

Fluorescence in situ hybridization (FISH) uses fluorescently labeled DNA probes that hybridize to complementary chromosomal sequences. The key advantage over conventional karyotyping: FISH works on unmanipulated interphase cells, so no cell culture or metaphase arrest is required. It can be run on FFPE sections, cytology preps, and blood smears. Resolution is ~100-200 kb (much better than karyotype), though it is targeted - only detects what the probe is designed for.

Three main probe types:

Chromosome/centromere enumeration probes (CEP) - target alpha-satellite repeat sequences at the centromere to enumerate chromosome copies. Used for trisomies (prenatal), monosomies (Turner), polysomies. The UroVysion test uses CEP3, CEP7, CEP17 plus a 9p21 (p16) locus probe to detect bladder cancer in urine cytology.
Locus-specific probes - detect specific gene fusions or deletions. Dual-color dual-fusion design: two differently colored probes flank the breakpoint of a translocation; fusion signals indicate the translocation (e.g., BCR-ABL1 in CML). Single-color probes detect amplification (HER2 with CEP17 as internal control). Key HER2 amplification threshold: HER2/CEP17 ratio >=2.0 or average HER2 signals >=6.0 per cell.
Whole chromosome painting probes - label entire chromosomes. Useful for complex rearrangements.

Break-apart probes handle promiscuous genes involved in multiple translocation partners. Differently colored probes flank both sides of a gene. Normal: signals are adjacent or fused (yellow). Rearranged: signals separate into distinct colors. A single break-apart probe detects rearrangement of that gene regardless of the partner - ideal for MLL/KMT2A on 11q23 (>80 partners) and ALK in NSCLC.

CGH, Arrays, and Spectral Karyotyping

Comparative genomic hybridization (CGH) gives pan-genomic copy number information - losses and gains in chromosomes and subchromosomal regions. Principle: tumor DNA (green) and normal reference DNA (red) are co-hybridized to normal metaphase chromosomes. Gains appear green, losses red, balanced regions yellow.

CGH clinical use: genetic characterization of developmentally delayed, dysmorphic, or autistic children with no recognized karyotypic abnormalities. It is a first-tier test for unexplained DD/ID/ASD per ACMG. Also used in tumor genomics and in products of conception when culture fails.

CGH cannot detect balanced translocations, inversions, point mutations, or low-level mosaicism - any change that doesn’t alter copy number is invisible to it.

Array CGH (aCGH) replaces metaphase chromosomes with thousands of DNA probes on a microarray - resolution improves to ~50-100 kb (from 5-10 Mb for metaphase CGH). Also called chromosomal microarray analysis (CMA). SNP arrays add genotyping to copy number detection, which is how you pick up copy-neutral loss of heterozygosity (uniparental disomy), consanguinity, and regions of homozygosity.

Spectral karyotype imaging (SKI) uses whole chromosome painting probes with different fluorochrome combinations to label each chromosome a unique color. Makes complex karyotypes (common in solid tumors) tractable when G-banding is ambiguous. M-FISH is a similar technique. Limitation: doesn’t detect small intrachromosomal rearrangements or precise breakpoints.

Allele-Specific Oligonucleotide (ASO) Probes

ASO probes use two short oligonucleotides - one complementary to the normal allele, one to the mutant allele - under stringent hybridization conditions so each binds only its perfect match. Used to genotype known mutations (sickle cell HbS, Factor V Leiden). Reverse dot blot arrays multiple ASO probes on a strip and hybridizes patient DNA to the whole panel - the principle behind many commercial CF panels and some HLA typing kits.

DNA Microarrays and Gene Expression Profiling

DNA microarrays immobilize thousands of probes on a small solid surface, allowing simultaneous detection of products from thousands of genes. Applications:

Gene expression profiling: total RNA is converted to labeled cDNA and hybridized to the array. Oncotype DX (21 genes, breast) and MammaPrint (70 genes, breast) are clinical examples. Expression profiling defined the molecular subtypes of breast cancer (Luminal A/B, HER2-enriched, Basal-like).
SNP genotyping arrays: GWAS, pharmacogenomics.
Chromosomal microarray (CMA): copy number analysis, as above.

58.9 Amplification: PCR and Its Cousins

PCR Basics

The basic principle of polymerase chain reaction (PCR) is cyclic polymerization of DNA copies - exponential amplification of a specific target through repeated thermal cycling. Each cycle theoretically doubles the target, so n cycles yields 2^n copies (30 cycles ~ 1 billion). That sensitivity is also the weakness: contamination control with separate pre- and post-PCR areas and strict negative controls is non-negotiable.

Six essential PCR reagents:

DNA template of interest
Forward and reverse (complementary) primers that define the amplicon boundaries
dNTPs as building blocks
Heat-stable polymerase (Taq from Thermus aquaticus is standard)
Magnesium (MgCl2), a required cofactor for polymerase activity
Buffer to maintain optimal pH

Too much Mg2+ drives non-specific amplification; too little kills yield. Primer Tm mismatch causes preferential amplification of one primer.

The three steps of PCR (Taq-based):

Denaturation at 94-95°C - separates dsDNA strands
Annealing at ~50-65°C (typically Tm - 5°C) - primers bind complementary sequences
Extension at 72°C - Taq synthesizes new strand

Typical protocol: 25-35 cycles, bookended by an initial denaturation (95°C, 5 min) and a final extension (72°C, 10 min). Hot-start PCR uses a modified Taq that is inactive until the first denaturation, reducing non-specific amplification during setup.

Each cycle amplifies the DNA ~2-fold. Efficiency drops in later cycles as reagents deplete and products start competing with primers.

Quantitative / Real-Time PCR

In real-time PCR, fluorescence is measured every cycle. The crossing threshold (Ct, also Cq) is the cycle number at which fluorescence exceeds background - it marks the point of exponential amplification. Lower Ct = more starting template. Each Ct difference of ~3.3 ~ a 10-fold difference in starting template (because 2^3.3 ~ 10).

Clinical uses of Ct: viral load quantification (HIV, HCV, CMV), gene expression analysis (normalized to housekeeping genes), and minimal residual disease monitoring (BCR-ABL1 in CML). Standard curves with known template concentrations give absolute copy numbers.

Methylation-Specific PCR

Methylation-specific PCR (MSP) detects promoter methylation. Treat DNA with sodium bisulfite (metabisulfite), which converts unmethylated cytosine to uracil (read as T after PCR) while methylated cytosine is protected. Two primer sets are then used: one for the originally methylated sequence (C preserved) and one for the unmethylated sequence (C→T). MSP is how MLH1 promoter methylation (CIMP pathway) and MGMT promoter methylation (predicts temozolomide response in glioblastoma) are detected.

Melting Curve Analysis

The melting point of a PCR product (50% single-stranded) can be used to distinguish the targeted product from other products by length, GC content, and sequence mismatch. After PCR, the product is gradually heated while fluorescence is monitored. Each species gives a characteristic Tm peak. High-resolution melting (HRM) can detect single nucleotide variants from subtle shifts in the melting curve - handy as a rapid screening step before confirmatory sequencing.

Isothermal Amplification

Transcription-mediated amplification (TMA) and nucleic acid sequence-based amplification (NASBA) are isothermal (no thermocycler) methods that amplify RNA targets. They use reverse transcriptase and RNA polymerase to exponentially amplify RNA through an RNA-DNA hybrid intermediate. Primary clinical use: detection of M. tuberculosis, Chlamydia/Gonorrhea NAAT, qualitative HIV/HCV RNA (blood donor screening). Bonus sensitivity comes from targeting rRNA, which is present in thousands of copies per cell.

Signal Amplification

Instead of amplifying the target, signal amplification amplifies the detection signal. Less susceptible to contamination but less sensitive than target amplification.

Branched DNA (bDNA) assay: target binds capture probes on a plate; signal probes bind and are branched for amplification. Used for HCV and HIV viral load in some platforms.
Hybrid capture uses a series of RNA probes directed against specific DNA sequences, and captures the RNA:DNA hybrid on a plate with specific antibodies (sandwich technique). Detection is via alkaline phosphatase-conjugated secondary antibodies and chemiluminescent substrate. Best-known application: Digene HC2 HPV test for high-risk HPV in cervical samples.

58.10 Sequencing

Sanger Sequencing

Sanger sequencing (dideoxy chain termination) historically involves four separate reactions, one each for A, T, C, G, to determine DNA sequence. Modern automated Sanger combines all four into a single reaction by labeling each ddNTP with a different fluorescent dye.

The method: DNA is amplified in the presence of an overabundance of dNTPs and a small proportion of ddNTPs. When a ddNTP incorporates, it terminates elongation (no 3’-OH for the next phosphodiester bond). This generates fragments of every possible length, each ending with a fluorescent ddNTP. Capillary electrophoresis separates by size; a laser reads the terminal fluorophore. Read length: 500-1000 bp.

Sanger’s sensitivity floor is about 15-20% variant allele frequency (VAF), which means it cannot reliably detect low-level mosaicism or subclonal tumor mutations. That’s why Sanger is still the gold standard for germline confirmation, but NGS is preferred for somatic tumor profiling where VAFs can be far below 10%.

Pyrosequencing

Pyrosequencing adds a single dNTP at a time and measures light produced by the released pyrophosphate. Incorporation releases PPi → sulfurylase converts PPi to ATP → luciferase burns ATP to produce light, proportional to the number of nucleotides incorporated. Unincorporated dNTPs are degraded by apyrase between cycles.

Pyrosequencing is quantitative (measures allele ratios well) and fast for short reads. Clinical targets: BRAF V600E, KRAS codons 12/13, EGFR mutations, methylation quantification.

Next-Generation Sequencing (NGS)

Next-generation sequencing uses short reads (75-400 bp), massively parallel sequencing, and compilation of fragments into a genomic sequence via computational assembly.

NGS has three workflow stages: library construction, sequencing, and analysis.

Library construction involves: (1) fragmenting patient DNA (sonication or enzymatic), (2) enriching regions of interest (by hybrid capture - probes bind targets in solution - or by amplicon-based multiplex PCR), (3) ligating adapter segments to both 5’ and 3’ ends (required for flow cell binding and sample barcoding), and (4) immobilizing the product on solid support. Target enrichment is the key scoping decision: whole exome (~20,000 genes) vs a targeted panel (50-500 genes). Panels have higher coverage depth and lower cost than exome or whole genome.

Sequencing occurs in a flow cell that contains the immobilized library, via a Sanger-type “sequencing by synthesis” reaction. In Illumina chemistry: library fragments bind complementary oligos on the flow cell surface; bridge amplification generates clonal clusters; each cycle adds one fluorescent reversible-terminator base, images the cluster, then cleaves the terminator and fluorophore for the next cycle. Each cluster produces a read. Coverage depth is the number of reads covering a given position - clinical NGS for somatic variants typically targets >500x.

NGS analysis detects: single nucleotide variants (SNVs), small insertions/deletions (indels), copy number alterations (CNAs), and structural variations (SV) like fusions and large rearrangements. The rate-limiting step is bioinformatics - alignment to reference, variant calling, annotation, and interpretation against ClinVar, COSMIC, and population databases (gnomAD).

58.11 Clonality Assessment, STRs, and Identity Testing

Clonality assessment in lymphoid neoplasms most commonly employs PCR. PCR amplification of immunoglobulin (B-cell) or T-cell receptor gene rearrangements demonstrates a dominant clone. On capillary electrophoresis, a monoclonal population produces a dominant peak; a polyclonal reactive process produces a Gaussian distribution of peaks. Useful when morphology and immunophenotype are equivocal. False negatives: somatic hypermutation in follicular lymphoma can prevent primers from binding; primer design has inherent coverage limits.

Short Tandem Repeats (STRs)

Short tandem repeats (STRs), also called microsatellites, are regions of DNA composed of a series of repeating dinucleotides or trinucleotides (2-6 bp repeat units, e.g., CACACACA). STRs are inherited as codominant alleles and their length varies between individuals, making them an effective genetic fingerprint for identification.

STR genotyping uses multiplex fluorescent PCR and capillary electrophoresis. Each STR locus shows one peak (homozygous) or two peaks (heterozygous). Power of discrimination scales with locus count - the CODIS forensic database uses 20 STR loci, giving a random match probability of ~1 in 10^28.

Clinical uses of STRs:

Engraftment/chimerism monitoring after stem cell transplant - quantify donor vs recipient DNA at informative loci. Full donor chimerism = >95% donor. Declining donor chimerism can flag impending relapse or graft rejection. Lineage-specific chimerism (sorted T-cells vs myeloid cells) can detect relapse earlier than whole-blood chimerism.
Forensic identification and paternity testing.
Specimen identity verification (detecting mix-ups by comparing tumor vs germline STR profiles).
Maternal cell contamination assessment in prenatal samples.

STRs can also be pathogenic when they expand. Trinucleotide repeat disorders and their expansion type:

Huntington disease: CAG repeats in HTT (normal <=26, mutant >=40)
Fragile X: CGG in FMR1
Myotonic dystrophy type 1: CTG in DMPK
Friedreich ataxia: GAA in FXN
Spinocerebellar ataxias: various CAG expansions

These disorders show anticipation - earlier onset and more severe phenotype in successive generations - because repeats expand further during meiosis.

58.12 Pharmacogenomics

Pharmacogenomics studies the individual’s response to a drug (pharmacokinetics, what the body does to the drug) or the drug’s effect on an individual (pharmacodynamics, what the drug does to the body). CPIC (Clinical Pharmacogenetics Implementation Consortium) publishes evidence-based guidelines.

CYP450

Genetic polymorphism of CYP (cytochrome P450) can alter metabolism of up to ~3/4 of medications. The main phase I drug-metabolizing enzymes in the liver. Clinically relevant isoforms: CYP2D6, CYP2C19, CYP2C9, CYP3A4.

Nomenclature: ==CYP2D6*1== = CYP family 2, subfamily D, isoenzyme 6, wild-type allele (*1). Star alleles (*1, *2, etc.) define haplotypes; *1 is the reference. The diplotype (maternal + paternal alleles) determines metabolizer status. CYP2D6 is unusually complex because it has >100 known alleles plus a common gene deletion (*5) and duplication (CYP2D6xN).

Metabolizer categories, all corresponding to different allele combinations:

Poor metabolizer (PM) - no functional alleles
Intermediate metabolizer (IM) - reduced function
Extensive / normal metabolizer (EM / NM) - typical activity
Ultra-rapid metabolizer (UM) - increased activity (e.g., CYP2D6 gene duplication)

The clinical impact depends on whether the drug is an active compound or a prodrug:

Poor metabolizer of an active drug → accumulation, toxicity (e.g., TCAs on CYP2D6 PM → QT prolongation, sedation, anticholinergic toxicity)
Poor metabolizer of a prodrug → failed activation, therapeutic failure (e.g., clopidogrel on CYP2C19 PM → no antiplatelet effect, FDA boxed warning)
Ultra-rapid metabolizer of a prodrug → excess active metabolite → toxicity (CYP2D6 UM + codeine → fatal morphine toxicity, notably in breastfed neonates)

Drug-enzyme pairs worth knowing:

CYP2D6: TCAs (nortriptyline, amitriptyline), SSRIs (fluoxetine, paroxetine), opioids (codeine, tramadol), tamoxifen, metoprolol. Metabolizes ~25% of all drugs.
CYP2C9: warfarin (S-enantiomer) and phenytoin, NSAIDs, sulfonylureas. CYP2C92 and 3 are reduced-activity alleles; PM patients need lower warfarin doses to avoid bleeding.
CYP2C19: omeprazole, clopidogrel, phenytoin, diazepam, voriconazole. *2 and *3 are loss-of-function; *17 is gain-of-function (ultra-rapid metabolism).

Warfarin Pharmacogenomics (VKORC1)

Warfarin targets vitamin K epoxide reductase (VKORC1) and blocks regeneration of reduced vitamin K, which is needed for gamma-carboxylation of factors II, VII, IX, X, protein C, and protein S. VKORC1 is located on 16p11.2.

VKORC1 polymorphisms affect warfarin sensitivity more than CYP2C9 variants. The -1639G>A promoter variant reduces VKORC1 expression and increases warfarin sensitivity - prevalent in populations with higher VKORC1 -1639A allele frequency, especially East Asian ancestry groups, which partly explains lower warfarin dose requirements in those populations. Haplotype summary:

H1 and H2 haplotypes (Group A, contain -1639A) → enhanced sensitivity to warfarin, lower dose needed
H7, H8, H9 haplotypes (Group B, contain -1639G) → warfarin resistance, higher dose needed

Rare VKORC1 coding mutations can cause true warfarin resistance requiring extreme doses (>15 mg/day) or alternative anticoagulants - these hit the warfarin binding site on the protein, distinct from the common promoter variants that just affect expression level. Combined VKORC1 + CYP2C9 genotyping explains up to ~40% of warfarin dose variability, and the FDA warfarin label includes pharmacogenomic dosing recommendations.

Other Key Pharmacogenomic Targets

N-acetyltransferase 2 (NAT2) acetylates isoniazid (INH), hydralazine, procainamide, sulfonamides, and dapsone. Slow acetylators have increased risk of INH-induced hepatotoxicity and peripheral neuropathy (the reason for pyridoxine supplementation). Slow phenotype is more common in European- and African-ancestry populations (~50%) than in East Asian populations (~10%).
Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activates thiopurines (6-mercaptopurine, 6-thioguanine, azathioprine), converting prodrug to active thioguanine nucleotides that incorporate into DNA and cause cytotoxicity. HGPRT deficiency is Lesch-Nyhan syndrome (gout, self-mutilation, neurologic dysfunction).
Thiopurine methyltransferase (TPMT) inactivates thiopurines, and polymorphisms can cause reduced therapeutic effect or increased toxicity. TPMT-deficient patients (~0.3%) accumulate toxic TGN metabolites → severe, potentially fatal myelosuppression. TPMT heterozygotes (~10%) have intermediate activity and need dose reduction. CPIC and FDA recommend TPMT genotyping before thiopurine therapy. TPMT*2, *3A, *3B, *3C are the common loss-of-function alleles. NUDT15 polymorphisms are another important driver of thiopurine toxicity, especially in East Asian ancestry populations.
Methylenetetrahydrofolate reductase (MTHFR) is the target pathway for methotrexate toxicity. Methotrexate inhibits dihydrofolate reductase (DHFR), depleting the folate pool. MTHFR converts 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate; reduced MTHFR activity compounds folate depletion from MTX. MTHFR C677T and A1298C polymorphisms are associated with increased methotrexate toxicity and increased homocysteine levels. Homozygous C677T (TT) has ~30% residual activity; homozygous A1298C (CC) has ~60%. Important counter-point: MTHFR variants are NOT a significant risk factor for venous thromboembolism, despite historical testing, and ACMG explicitly recommends against MTHFR testing for thrombophilia.

Chapter 59: Molecular Techniques

The power of molecular diagnostics lies in its techniques - methods that can detect single copies of DNA, identify specific mutations, and characterize entire genomes. Understanding how these techniques work allows you to appreciate their capabilities and limitations.

58.1 Nucleic Acid Extraction: Getting the Material

Every molecular test begins with extracting nucleic acid from the clinical specimen. The quality of the extraction determines the quality of the test.

Specimen Types and Their Challenges

Blood: The most common source for constitutional (germline) testing. EDTA is the preferred anticoagulant (heparin inhibits PCR). DNA is extracted from white blood cells (RBCs lack nuclei). For RNA testing, cells must be processed quickly or stabilized (PAXgene tubes) because RNA degrades rapidly.

Fresh/frozen tissue: Provides high-quality DNA and RNA. Best for research and molecular profiling.

Formalin-fixed, paraffin-embedded (FFPE) tissue: The standard for surgical pathology, but problematic for molecular testing. Formalin causes:

Cross-linking between DNA strands and between DNA and proteins
Fragmentation (average fragment size 200-300 bp)
Cytosine deamination (creates artifactual C→T changes)

Modern molecular tests are increasingly designed to work with degraded FFPE DNA, but results should be interpreted cautiously.

Body fluids: CSF, pleural fluid, urine. Cell-free DNA (cfDNA) in plasma is now used for liquid biopsy.

Swabs: Nasopharyngeal, oropharyngeal, cervical. Primary source for infectious disease testing.

Extraction Methods

The goal is to separate nucleic acids from proteins, lipids, and other cellular components.

Organic extraction (phenol-chloroform): The traditional method. Proteins partition into the organic phase; DNA stays in the aqueous phase. Labor-intensive and uses hazardous chemicals, but produces very pure DNA. Now largely replaced by other methods.

Solid-phase extraction (silica columns): The workhorse of modern extraction. Principle: In the presence of chaotropic salts (guanidinium), DNA binds to silica while proteins and other contaminants wash through. DNA is then eluted in low-salt buffer. Available in spin column or automated formats.

Magnetic bead-based extraction: Similar principle to silica columns, but nucleic acid binds to magnetic beads that are captured by a magnet. Highly automatable and scalable.

Quality Assessment

Purity: Spectrophotometry measures absorbance at 260 nm (nucleic acid) and 280 nm (protein). A260/A280 ratio of ~1.8 for DNA and ~2.0 for RNA indicates good purity. Low ratios suggest protein contamination; high ratios suggest RNA contamination of DNA.

Quantity: Spectrophotometry (less accurate) or fluorometry using DNA-binding dyes (PicoGreen, Qubit). Fluorometry is more accurate, especially for low concentrations.

Integrity: For DNA - visualize on gel or automated electrophoresis (high molecular weight band indicates intact DNA). For RNA - calculate RNA Integrity Number (RIN) based on the ratio of 28S to 18S rRNA peaks. RIN of 7+ is generally acceptable for sensitive applications.

58.2 Polymerase Chain Reaction (PCR): The Foundation

PCR is the single most important technique in molecular diagnostics. It allows selective amplification of a specific DNA sequence, generating millions of copies from a single template molecule.

The Principle: Exponential Amplification

PCR exploits the ability of DNA polymerase to synthesize a complementary strand. By using two primers that flank the region of interest and cycling through temperatures that denature DNA, anneal primers, and extend, you create an exponential chain reaction.

The components:

Template DNA: The sample containing the sequence to be amplified
Primers: Short oligonucleotides (~18-25 bp) that define the boundaries of amplification. One primer binds each strand, flanking the target.
Thermostable DNA polymerase: Usually Taq (from Thermus aquaticus). Survives the denaturation temperature.
dNTPs: The four deoxynucleotides (dATP, dTTP, dGTP, dCTP) - building blocks for new DNA
Buffer: Provides optimal pH and ion concentrations; includes Mg2+ (essential cofactor for polymerase)

The three-step cycle:

Denaturation (~94-98°C): Heat separates the double-stranded DNA into single strands
Annealing (~50-65°C): Temperature is lowered to allow primers to bind (anneal) to their complementary sequences on the template. The annealing temperature depends on primer composition (higher G-C content → higher Tm → higher annealing temperature).
Extension (~72°C): The optimal temperature for Taq polymerase. The enzyme extends from the 3’ end of each primer, synthesizing the complementary strand.

Exponential amplification: After each cycle, the number of target molecules doubles. After n cycles: 2^n copies. After 30 cycles: 2^30 ≈ 1 billion copies. This exponential amplification is both the power and the danger of PCR - contamination is readily amplified.

Reverse Transcription PCR (RT-PCR)

Standard PCR amplifies DNA. To amplify RNA (for gene expression analysis or RNA virus detection), you first convert RNA to complementary DNA (cDNA) using reverse transcriptase.

The process:

Reverse transcriptase synthesizes cDNA from an RNA template
The cDNA is then amplified by standard PCR

Priming options:

Oligo(dT): Hybridizes to the poly-A tail; converts all mRNA
Random hexamers: Prime throughout; converts all RNA (including non-polyadenylated)
Gene-specific primers: Convert only the target gene

Applications: HIV viral load (HIV RNA genome), SARS-CoV-2 detection (RNA virus), gene expression studies.

Real-Time PCR (Quantitative PCR, qPCR)

Conventional PCR is qualitative - you see a band or you don’t. Real-time PCR monitors amplification as it happens, enabling quantification.

The concept: A fluorescent signal increases as DNA is amplified. By measuring when fluorescence crosses a threshold, you can determine how much template was present initially.

Ct (Cycle threshold): The cycle number at which fluorescence exceeds the threshold. Lower Ct = more starting template (reached threshold sooner). The relationship is logarithmic - each 3.32-cycle difference in Ct represents a 10-fold difference in starting material.

Detection chemistries:

SYBR Green: An intercalating dye that fluoresces when bound to double-stranded DNA. Simple and inexpensive, but non-specific - it detects ANY double-stranded DNA, including primer dimers. Requires melt curve analysis to confirm specificity.

TaqMan probes: A probe with a fluorophore at one end and a quencher at the other. When intact, the quencher suppresses fluorescence. During PCR, Taq polymerase’s 5’→3’ exonuclease activity cleaves the probe, separating fluorophore from quencher and generating signal. Highly specific - signal is generated only if the probe hybridizes to the correct sequence.

Molecular beacons: Hairpin-shaped probes that fluoresce only when bound to target. The hairpin brings fluorophore and quencher together; hybridization opens the hairpin.

Quantification approaches:

Absolute quantification: Compare Ct to a standard curve of known concentrations. Used for viral loads.
Relative quantification: Compare target gene expression to a reference gene (housekeeping gene). Used for gene expression studies.

Digital PCR

The newest evolution of PCR. Instead of measuring fluorescence in real-time, digital PCR partitions the sample into thousands of individual reactions (droplets or chambers). Each partition either has template (positive) or doesn’t (negative). By counting positive and negative partitions, you calculate the absolute number of template molecules using Poisson statistics.

Advantages:

Absolute quantification without a standard curve
More precise than qPCR
Can detect rare variants (1 mutant in 10,000 wild-type)

Applications: Detecting minimal residual disease, monitoring transplant rejection (donor cfDNA), liquid biopsy.

58.3 DNA Sequencing: Reading the Code

Sequencing determines the actual nucleotide sequence of DNA. It’s the definitive way to identify mutations.

Sanger Sequencing: The Gold Standard

Developed by Frederick Sanger in 1977, this method remains important for confirming variants and sequencing small targets.

The principle (chain-termination method):

DNA synthesis requires a 3’ hydroxyl group to form the phosphodiester bond. Dideoxynucleotides (ddNTPs) lack the 3’ hydroxyl - when incorporated, they terminate chain elongation.

The process:

Set up a reaction with template DNA, primer, DNA polymerase, normal dNTPs, AND a small amount of ddNTPs (each labeled with a different fluorescent color)
During synthesis, each time a ddNTP is incorporated instead of a dNTP, the chain terminates at that position
The result is a collection of fragments of all possible lengths, each terminated at a specific base
Capillary electrophoresis separates fragments by size
A detector reads the fluorescent color of each fragment as it passes, generating a sequence chromatogram

Reading a chromatogram: The tracing shows peaks of different colors, each representing a base. Clean, evenly spaced peaks indicate good sequence quality. Overlapping peaks suggest heterozygosity (germline) or mixed populations (somatic).

Limitations:

One target at a time (low throughput)
Cannot reliably detect variants below ~15-20% allele frequency
Limited read length (~700-900 bp maximum)

Current role: Confirming variants detected by NGS, sequencing single genes, low-throughput applications.

Next-Generation Sequencing (NGS)

NGS (also called massively parallel sequencing) revolutionized genomics by sequencing millions of DNA fragments simultaneously. This enables gene panels, whole exome sequencing, and whole genome sequencing.

The general workflow:

Library preparation: DNA is fragmented (enzymatically or mechanically) to ~150-500 bp. Adapters (short known sequences) are ligated to the fragment ends. Unique identifiers (barcodes/indices) can be added to pool multiple samples.
Clonal amplification: Each fragment is amplified to create a cluster of identical copies. This generates sufficient signal for detection. Methods include:

Bridge amplification (Illumina): Fragments bind to a flow cell surface and amplify in clusters
Emulsion PCR (Ion Torrent): Fragments are amplified on beads in water-oil emulsion droplets

Sequencing: Bases are identified as they’re incorporated:

Illumina (sequencing by synthesis): Fluorescently labeled nucleotides are added one at a time; a camera captures which nucleotide was incorporated at each cluster
Ion Torrent: Nucleotides are flowed in sequence; incorporation releases H+, causing a pH change detected by a semiconductor chip

Data analysis: Bioinformatics pipelines align reads to a reference genome and identify variants. This is complex and requires significant computational resources.

Key concepts:

Read length: How many bases are sequenced per fragment. Short-read platforms (Illumina, Ion Torrent) produce 75-300 bp reads. Long-read platforms (PacBio, Nanopore) produce 10,000+ bp reads. Longer reads help span repetitive regions and detect structural variants.

Depth of coverage: The number of independent reads covering each base position. Higher depth = higher confidence in variant calls = better detection of low-frequency variants.

30x: Minimum for germline testing
500-1000x: Needed for somatic variants at low allele frequencies
Thousands-fold: Needed for liquid biopsy detecting rare circulating tumor DNA

Paired-end sequencing: Sequencing from both ends of a fragment. Helps with alignment and detecting structural variants.

Applications by scope:

Gene panels: Targeted sequencing of selected genes (tens to hundreds). Cost-effective when the clinical question is focused (e.g., hereditary cancer panel, inherited cardiomyopathy panel).
Whole exome sequencing (WES): All protein-coding regions (~1.5% of genome, ~20,000 genes). Useful for undiagnosed genetic diseases.
Whole genome sequencing (WGS): The entire genome. Most comprehensive but most expensive. Detects non-coding variants and structural variants.

NGS Platform Comparison: Understanding the Technologies

Different NGS platforms have distinct characteristics that affect their clinical applications. Understanding these differences helps you interpret results and select appropriate testing.

Illumina (Sequencing by Synthesis with Reversible Terminators)

Illumina dominates the clinical sequencing market because of its high accuracy and throughput. The technology uses fluorescently labeled nucleotides with reversible terminators - after each incorporation event, the fluorescent label is detected and then cleaved, allowing the next nucleotide to be added.

Key characteristics:

Read length: 75-300 bp (short reads)
Error profile: Low overall error rate (~0.1-0.5%); errors are predominantly substitutions, randomly distributed
Throughput: Very high (up to terabases per run on high-end instruments)
Strengths: Excellent for variant detection, gene expression, most clinical applications
Limitations: Short reads struggle with repetitive regions and structural variants; GC-rich regions may have reduced coverage

The short read length means that repetitive regions longer than the read length cannot be spanned, making assembly difficult in these areas. However, for most clinical applications - detecting SNVs, small indels, and known fusion genes - Illumina’s accuracy makes it the platform of choice.

Ion Torrent (Semiconductor Sequencing)

Ion Torrent uses a fundamentally different detection approach: instead of fluorescence, it detects the hydrogen ion released when a nucleotide is incorporated. A semiconductor chip measures the pH change.

Key characteristics:

Read length: 200-400 bp
Error profile: Higher error rate than Illumina (~1%); predominantly indel errors, especially in homopolymer regions (stretches of the same nucleotide, like AAAA or GGGG)
Throughput: Moderate
Strengths: Faster run times, lower instrument cost, good for targeted panels
Limitations: Homopolymer errors are problematic; a stretch of 6 adenines might be called as 5 or 7

The homopolymer issue arises because the system measures the magnitude of pH change to determine how many identical nucleotides were incorporated in a row. Distinguishing 5 from 6 incorporations is inherently imprecise. This makes Ion Torrent less suitable for applications where indel accuracy is critical, but it works well for SNV detection in targeted panels.

Pacific Biosciences (PacBio) - Single Molecule Real-Time (SMRT) Sequencing

PacBio represents a different paradigm: single-molecule sequencing without amplification. A polymerase is fixed at the bottom of a tiny well (zero-mode waveguide), and fluorescently labeled nucleotides are watched in real-time as they’re incorporated.

Key characteristics:

Read length: Very long - 10,000 to 100,000+ bp; HiFi mode produces highly accurate 10-20 kb reads
Error profile: Raw reads have high error rate (~10-15%), but errors are random. Circular consensus sequencing (CCS/HiFi) corrects errors by reading the same molecule multiple times, achieving >99.9% accuracy
Throughput: Lower than short-read platforms
Strengths: Resolves repetitive regions, detects structural variants, phases haplotypes, detects methylation directly
Limitations: Higher cost per base, lower throughput

PacBio’s long reads are invaluable for certain applications: assembling genomes de novo, resolving complex structural variants, spanning trinucleotide repeat expansions, and detecting methylation (which alters polymerase kinetics). The ability to read through repetitive regions that confound short-read technologies makes PacBio essential for characterizing certain mutations.

Oxford Nanopore

Nanopore sequencing threads DNA through a protein pore embedded in a membrane. As each nucleotide passes through, it causes a characteristic disruption in ionic current, which is decoded into sequence.

Key characteristics:

Read length: Extremely long - potentially hundreds of kilobases; limited only by DNA fragment length
Error profile: Higher error rate (~5-10% with current chemistry), predominantly indels; rapidly improving
Throughput: Variable depending on device
Strengths: Portability (MinION device is USB-sized), real-time sequencing, ultra-long reads, direct RNA sequencing, methylation detection
Limitations: Higher error rate limits some applications; basecalling is computationally intensive

Nanopore’s portability has enabled sequencing in the field - Ebola outbreak response, International Space Station, and point-of-care settings. The ability to sequence in real-time means results can be obtained within minutes of starting. Direct RNA sequencing (without reverse transcription) preserves RNA modifications that would otherwise be lost.

Choosing the Right Platform

The choice depends on the clinical question:

Application	Preferred Platform	Rationale
Variant detection (SNVs, small indels)	Illumina	Highest accuracy
Targeted cancer panels	Illumina or Ion Torrent	Good accuracy, established workflows
Structural variant detection	PacBio or Nanopore	Long reads span breakpoints
Trinucleotide repeat sizing	PacBio	Reads through entire repeat
Methylation analysis	PacBio or Nanopore	Direct detection without bisulfite
Rapid pathogen identification	Nanopore	Real-time results, portability
Whole genome sequencing	Illumina (routine) or PacBio HiFi (comprehensive)	Cost vs. completeness tradeoff

58.4 Bioinformatics: From Raw Data to Clinical Results

The raw data from sequencing instruments is meaningless without bioinformatics analysis. Understanding the data flow helps you appreciate where errors can occur and what the limitations of testing are.

File Formats: The Language of Sequencing Data

FASTQ: The starting point. FASTQ files contain the raw sequence reads and their quality scores.

Each read has four lines:

Sequence identifier (information about the instrument, run, and position)
The nucleotide sequence (A, C, G, T, or N for uncertain)
A separator line (just a “+”)
Quality scores (ASCII-encoded Phred scores)

Quality scores indicate confidence in each base call. A Phred score of 20 means 1% error probability (99% accuracy); 30 means 0.1% error (99.9% accuracy); 40 means 0.01% error (99.99% accuracy). Most clinical applications require Q30 or higher.

SAM/BAM: After reads are aligned to a reference genome, the results are stored in SAM (Sequence Alignment/Map) format. BAM is the compressed binary version.

SAM files contain:

Where each read aligned on the reference genome
The mapping quality (confidence in the alignment)
The CIGAR string (describes how the read aligns - matches, insertions, deletions, soft clips)
Information about paired-end reads

The CIGAR string is particularly important for understanding indels. “50M2I48M” means 50 matches, a 2-base insertion, then 48 more matches.

VCF (Variant Call Format): The final product for clinical interpretation. VCF files list variants found in the sample compared to the reference.

Each variant entry includes:

Chromosome and position
Reference allele and alternate allele(s)
Quality score
Filter status (PASS or reason for filtering)
Additional annotations (read depth, allele frequency, functional impact)

Understanding VCF is essential because this is what variant interpretation software uses. A variant might be filtered out due to low quality, low depth, or strand bias - information captured in the VCF.

BED (Browser Extensible Data): Defines genomic regions. Used to specify which regions were targeted by the assay (important for understanding what was and wasn’t tested).

The Analysis Pipeline

Alignment (Mapping): Reads are aligned to a reference genome to determine where they originated. This step requires sophisticated algorithms because:

Reads may contain errors
The patient’s genome differs from the reference
Repetitive regions can align to multiple locations

Reads that align to multiple locations equally well (multi-mapping reads) are often excluded from analysis, which can cause false-negative results in duplicated regions.

Variant Calling: Algorithms identify positions where the sample differs from the reference.

For germline variants, heterozygous variants should have approximately 50% alternate allele reads, homozygous variants approximately 100%. Significant deviation suggests artifact, mosaicism, or copy number variation.

For somatic variants, the expected allele frequency depends on tumor purity and copy number. A mutation present in 25% of reads might represent a heterozygous mutation in a tumor that’s 50% pure, or a subclonal mutation.

Annotation: Variants are annotated with information needed for interpretation:

Gene and transcript affected
Consequence (missense, nonsense, frameshift, splice site)
Population frequency (gnomAD)
Previous classifications (ClinVar)
In silico predictions (SIFT, PolyPhen, CADD)
COSMIC frequency for somatic variants

Quality Metrics to Understand

When reviewing molecular results, several quality metrics matter:

Depth of coverage: How many reads cover each position. Higher depth = more confidence. Clinical germline testing typically requires 20-30x; somatic testing often requires 500-1000x to detect low-frequency variants.

Uniformity of coverage: Are all targeted regions covered equally? Poor uniformity means some regions may have insufficient coverage for confident variant calling.

On-target rate: What percentage of reads mapped to the targeted regions? Low on-target rates waste sequencing capacity.

Duplicate rate: PCR duplicates (reads that are copies of the same original molecule) don’t provide independent information. Very high duplicate rates suggest starting DNA was limited.

Mapping rate: What percentage of reads could be aligned? Low mapping rates may indicate contamination or severe DNA degradation.

58.5 Hybridization Techniques

Hybridization exploits the complementary base pairing of nucleic acids. A labeled probe with a known sequence will bind (hybridize) to its complementary target sequence.

Southern Blot (DNA)

Detects specific DNA sequences and can assess gene rearrangements, large deletions, or copy number.

The process:

Genomic DNA is digested with restriction enzymes (cut at specific sequences)
Fragments are separated by gel electrophoresis (by size)
DNA is transferred (blotted) to a membrane
The membrane is incubated with a labeled probe
Probe binds to complementary sequences
Detection (autoradiography or chemiluminescence) reveals bands

Applications: Historically used for diagnosing fragile X syndrome (CGG repeat expansion) and detecting clonal immunoglobulin gene rearrangements in lymphomas. Largely replaced by PCR-based methods and FISH, but still used for specific applications.

Northern blot is the same technique applied to RNA (expression analysis). Largely replaced by RT-PCR and RNA-seq.

Fluorescence In Situ Hybridization (FISH)

FISH uses fluorescently labeled probes to visualize specific DNA sequences directly in cells or tissue sections.

The process:

Cells are fixed and permeabilized
DNA is denatured (separated into single strands)
Fluorescent probes are added and hybridize to their targets
Excess probe is washed away
Visualization by fluorescence microscopy

Probe types:

Centromere probes: Hybridize to repetitive sequences at the centromere; count chromosome number (aneuploidy)
Locus-specific probes: Hybridize to a specific gene region; detect deletions, amplifications, translocations
Whole chromosome paints: Label an entire chromosome; detect complex rearrangements

Applications:

Detecting HER2 amplification in breast cancer (guides therapy)
Identifying the BCR-ABL1 fusion in CML (Philadelphia chromosome)
Detecting ALK rearrangements in lung cancer
Prenatal diagnosis of trisomy 21

Advantages: Works on single cells, preserves tissue architecture, can analyze nondividing cells (interphase FISH).

Limitations: Can only detect what you probe for (not a discovery tool), limited number of probes per sample.

Detect

Applications: Large deletions/duplications, trinucleotide repeat expansions

Northern Blot

Same principle for RNA detection.

Fluorescence In Situ Hybridization (FISH)

Detects specific DNA sequences in intact cells or tissue sections.

Principle:

Fluorescent-labeled DNA probes hybridize to target sequences
Visualize under fluorescence microscope
Can detect chromosome number, translocations, deletions, amplifications

Probe types:

Centromere probes: Detect aneuploidy
Locus-specific probes: Detect gene deletions, amplifications, translocations
Whole chromosome paints: Identify chromosomal rearrangements

Applications:

Cancer cytogenetics (BCR-ABL, HER2 amplification)
Prenatal diagnosis (aneuploidy)
Microdeletion syndromes

Microarrays

Principle: Thousands of probes fixed on solid surface; labeled sample hybridizes to complementary probes

Types:

SNP arrays: Genotyping millions of variants; also detects copy number
CGH (Comparative Genomic Hybridization) arrays: Copy number analysis
Expression arrays: Gene expression profiling

58.6 Cytogenetics

Karyotyping

Visualization of metaphase chromosomes arranged by size and banding pattern.

Process:

Culture cells (need dividing cells)
Arrest in metaphase (colchicine)
Hypotonic treatment, fixation, spreading
Banding (G-banding most common)
Analysis and interpretation

Nomenclature (ISCN):

46,XX = normal female
47,XY,+21 = male with trisomy 21
46,XX,t(9;22)(q34;q11) = female with Philadelphia chromosome translocation

Resolution: ~5-10 Mb (cannot detect smaller abnormalities)

Applications: Constitutional cytogenetics, hematologic malignancies

Chapter 60: Clinical Applications of Molecular Diagnostics

Molecular diagnostics have transformed clinical medicine across multiple disciplines. Understanding when and how to use these tests - and their limitations - is essential for optimal patient care.

60.1 Infectious Disease Molecular Diagnostics

The Revolution in Microbiology

Traditional microbiology relies on culture - growing organisms to identify and characterize them. Culture remains the gold standard for many infections because it provides a living isolate for susceptibility testing. But culture has significant limitations:

Time: Many organisms take days to grow; some (mycobacteria) take weeks
Fastidious organisms: Some pathogens won’t grow on routine media or require special conditions
Unculturable organisms: Some pathogens cannot be cultured at all (Treponema pallidum, Pneumocystis)
Prior antibiotics: Treatment may sterilize cultures before collection
Sensitivity: Low organism loads may not grow

Molecular methods address many of these limitations by detecting pathogen nucleic acids directly.

Quantitative Viral Loads

Viral load testing measures the amount of viral nucleic acid in blood, providing information that serology and culture cannot.

HIV RNA viral load: The cornerstone of HIV management.

Diagnosis: Detects acute HIV before antibodies develop (during the “window period”)
Monitoring: Tracks response to antiretroviral therapy. Goal is undetectable viral load (<20-50 copies/mL depending on assay)
Resistance: Persistent viremia on treatment suggests resistance; genotyping guides drug selection
Transmission risk: Undetectable = untransmittable (U=U)

HCV RNA viral load:

Diagnosis: Confirms active infection (antibody alone may indicate resolved infection)
Treatment monitoring: Tests at baseline, during treatment (to assess response), and post-treatment (SVR = sustained virologic response = cure)
Treatment decision: Baseline viral load may influence treatment duration in some regimens

CMV DNA viral load: Critical for transplant patients

Post-transplant patients are immunosuppressed and at risk for CMV reactivation
Preemptive therapy: Monitor viral load regularly; treat when it exceeds a threshold (prevents symptomatic disease)
Treatment monitoring: Viral load should decrease with antiviral therapy; persistent viremia suggests resistance

EBV DNA viral load: Monitors risk of post-transplant lymphoproliferative disorder (PTLD)

Rising EBV DNA in transplant patients may herald PTLD, a serious malignancy
Guides reduction of immunosuppression and other interventions

BK virus DNA viral load: Monitors kidney transplant recipients

BK virus can cause nephropathy in the transplanted kidney
Rising viral load triggers reduction of immunosuppression before clinical nephropathy develops

Syndromic Panels

Multiplex PCR panels can detect many pathogens simultaneously from a single specimen. This “syndromic” approach matches clinical presentation (respiratory illness, diarrhea, meningitis) rather than testing for one pathogen at a time.

Respiratory panels detect 15-20+ pathogens from a nasopharyngeal swab:

Influenza A/B, RSV, SARS-CoV-2, parainfluenza, rhinovirus, adenovirus, human metapneumovirus, coronaviruses
Bacteria: Bordetella pertussis, Mycoplasma pneumoniae, Chlamydophila pneumoniae

These panels provide rapid results (1-2 hours) that guide isolation precautions and treatment decisions. However, detecting a pathogen doesn’t always mean it’s causing current illness - many respiratory viruses can persist or be carried asymptomatically.

Gastrointestinal panels detect common causes of infectious diarrhea:

Bacteria: Salmonella, Shigella, Campylobacter, Shiga toxin-producing E. coli, C. difficile, Yersinia, Vibrio
Viruses: Norovirus, rotavirus, adenovirus
Parasites: Giardia, Cryptosporidium, Entamoeba histolytica

Important consideration: Positive results don’t always indicate the cause of current illness. C. difficile colonization is common; detection alone doesn’t prove disease. Interpretation requires clinical correlation.

Meningitis/encephalitis panels test CSF for multiple pathogens:

Bacteria: S. pneumoniae, N. meningitidis, H. influenzae, E. coli, L. monocytogenes, S. agalactiae
Viruses: HSV-1/2, VZV, enterovirus, CMV, HHV-6, parechovirus
Cryptococcus neoformans

This allows rapid identification in a medical emergency. However, empiric antibiotics should not be withheld pending results.

Antimicrobial Resistance Detection

Molecular tests can detect resistance genes, providing results faster than phenotypic susceptibility testing.

MRSA screening: Detection of mecA (or mecC) gene confirms methicillin resistance in staphylococci. Used for screening before surgery or for infection control.

Carbapenemase genes: Detecting KPC, NDM, VIM, IMP, OXA-48 identifies carbapenem-resistant organisms and guides antibiotic selection and infection control.

TB drug resistance: The Xpert MTB/RIF assay simultaneously detects M. tuberculosis and rifampin resistance (rpoB mutations) in about 2 hours - critical for rapid identification of MDR-TB in endemic areas.

HIV genotypic resistance testing: Sequencing of reverse transcriptase, protease, and integrase genes identifies mutations associated with drug resistance, guiding antiretroviral selection.

Molecular Assay Quality Controls

Control	Purpose
No Template Control (NTC)	Contains all reagents but no DNA template; detects master mix/reagent contamination
Internal Control (IC)	Non-target DNA/RNA spiked into each specimen; monitors extraction efficiency and PCR inhibition
Positive Control	Known positive specimen; confirms assay primers/probes function correctly
Negative Extraction Control	Reagent blank processed through entire extraction procedure; detects carryover contamination

Immunoglobulin Gene Rearrangement and CDR3 Diversity

Immunoglobulin diversity is generated through V(D)J recombination:

Heavy chain CDR3 (most diverse region): Formed by V + D + J gene segments plus junctional diversity (P-nucleotides + N-nucleotides added by TdT). This is why heavy chain CDR3 is the primary determinant of antigen specificity and the principal target for B-cell clonality assessment.
Light chain CDR3: Uses only V + J segments (no D segment, minimal TdT contribution) → significantly less diverse.

DNA Methylation: Methylation is the addition of a methyl group to the 5-carbon of cytosine, forming 5-methylcytosine, occurring almost exclusively at CpG dinucleotides. Methylation of promoter CpG islands → epigenetic gene silencing. Spontaneous deamination of 5-methylcytosine → thymine creates C>T transitions, the most common somatic point mutation in cancer.

60.2 Oncology Molecular Diagnostics

Cancer is fundamentally a genetic disease - caused by mutations that drive uncontrolled proliferation. Understanding the molecular alterations in a patient’s tumor increasingly guides treatment selection. This paradigm of “precision oncology” matches targeted therapies to specific molecular targets.

The Concept of Companion Diagnostics

A companion diagnostic is a test that must be performed before administering a specific therapy. The test identifies patients whose tumors harbor the molecular alteration targeted by the drug. Patients without the alteration won’t benefit (and may be harmed by toxicity without benefit).

EGFR mutations in lung adenocarcinoma

The epidermal growth factor receptor (EGFR) is a tyrosine kinase that drives proliferation when activated. Certain mutations in EGFR (most commonly deletions in exon 19 and the L858R point mutation in exon 21) cause constitutive activation, driving tumor growth.

Tyrosine kinase inhibitors (TKIs) block this signaling:

First-generation: Erlotinib, gefitinib
Third-generation: Osimertinib (also effective against T790M resistance mutation)

Testing is standard of care for all non-squamous lung cancers. EGFR mutations are more common in never-smokers, women, and patients with East Asian ancestry, but testing should not be limited by demographics.

ALK and ROS1 rearrangements in lung cancer

Chromosomal rearrangements can create oncogenic fusion genes. ALK rearrangements (most commonly EML4-ALK) occur in ~5% of lung adenocarcinomas. ROS1 rearrangements are less common (~1-2%).

Both create constitutively active kinases that can be targeted with specific inhibitors (crizotinib, alectinib, lorlatinib for ALK; crizotinib, entrectinib for ROS1).

Detection methods include FISH (visualizes the rearrangement), IHC (detects overexpressed fusion protein), and NGS (identifies the specific fusion partners).

KRAS mutations and EGFR inhibitor resistance

KRAS is downstream of EGFR in the signaling pathway. If KRAS is mutated and constitutively active, blocking EGFR upstream has no effect - the signal continues through KRAS.

In colorectal cancer, KRAS (and NRAS) mutations predict resistance to anti-EGFR antibodies (cetuximab, panitumumab). Testing for RAS mutations is required before using these drugs.

KRAS was long considered “undruggable,” but sotorasib and adagrasib now target the specific KRAS G12C mutation, common in lung cancer.

BRAF V600E mutations

BRAF is another kinase in the MAPK pathway. The V600E mutation (valine to glutamate at position 600) causes constitutive activation.

In melanoma, BRAF V600 mutations are present in ~50% of cases. BRAF inhibitors (vemurafenib, dabrafenib) combined with MEK inhibitors (trametinib, cobimetinib) produce dramatic responses.

In colorectal cancer, BRAF V600E indicates poor prognosis and different treatment approach (BRAF inhibition alone is ineffective; requires combination with EGFR inhibitor and MEK inhibitor).

BCR-ABL1 in chronic myeloid leukemia

The Philadelphia chromosome - t(9;22) - creates the BCR-ABL1 fusion gene, producing a constitutively active tyrosine kinase that drives CML.

Imatinib (Gleevec), the first successful targeted cancer therapy, inhibits BCR-ABL1. Quantitative PCR monitoring of BCR-ABL1 transcript levels guides treatment:

Major molecular response (MMR): BCR-ABL1 ≤0.1% (3-log reduction)
Deep molecular response: BCR-ABL1 ≤0.01%

Rising BCR-ABL1 levels suggest resistance; mutation analysis identifies the mechanism (point mutations in the ABL kinase domain) and guides selection of alternative TKIs.

Tumor Mutational Burden and Microsatellite Instability

These markers predict response to immune checkpoint inhibitors (anti-PD-1, anti-PD-L1, anti-CTLA-4) - drugs that unleash the immune system against tumors.

Tumor Mutational Burden (TMB): Tumors with many mutations produce many neoantigens - abnormal proteins that the immune system can recognize as foreign. High TMB correlates with response to immunotherapy because there are more targets for immune attack.

TMB is measured by NGS (counting somatic mutations per megabase of coding DNA). Cut-offs vary, but ≥10 mutations/Mb is often considered high TMB.

Microsatellite Instability (MSI): Microsatellites are short repetitive DNA sequences prone to errors during replication. Normally, the mismatch repair (MMR) system corrects these errors. When MMR is defective, errors accumulate, causing MSI.

MSI-high (MSI-H) tumors have thousands of mutations due to MMR deficiency, producing many neoantigens. Pembrolizumab (anti-PD-1) is FDA-approved for any MSI-H solid tumor, regardless of tissue of origin - the first tissue-agnostic cancer drug approval.

MSI testing also screens for Lynch syndrome (hereditary MMR deficiency), which increases risk of colorectal, endometrial, and other cancers.

Testing methods:

PCR-based: Compare microsatellite lengths in tumor vs. normal tissue
IHC: Stain for the four MMR proteins (MLH1, MSH2, MSH6, PMS2). Loss of staining indicates MMR deficiency.
NGS: Can detect MSI computationally from sequencing data

Mutational Signatures: The Fingerprints of Mutagenesis

Every mutagen leaves a characteristic pattern of mutations - a “signature” that reflects its mechanism of action. Analyzing these patterns provides insight into what caused a cancer’s mutations, which has both biological and clinical implications.

The concept: Different mutational processes cause different types of base changes in different sequence contexts. By cataloging these patterns across thousands of tumors, researchers have identified over 60 distinct mutational signatures (designated SBS1, SBS2, etc., for single base substitutions).

Clinically important signatures:

SBS1 (Age-related): C>T mutations at CpG dinucleotides. Results from spontaneous deamination of 5-methylcytosine, which occurs constantly. The number of SBS1 mutations correlates with age at diagnosis. Present in virtually all cancers - it’s the “clock” signature.

SBS4 (Tobacco smoking): Predominantly C>A transversions with a transcriptional strand bias. Caused by tobacco carcinogens like benzo[a]pyrene forming DNA adducts. Strongly associated with lung cancer in smokers. Finding SBS4 in a lung tumor provides molecular evidence of tobacco-related carcinogenesis.

SBS7a/b (Ultraviolet light): C>T mutations at dipyrimidine sequences (CC, CT, TC, TT). UV light causes cyclobutane pyrimidine dimers; if not repaired, these lead to C>T transitions. Dominant signature in melanoma and other skin cancers. Nearly pathognomonic for UV exposure.

SBS2 and SBS13 (APOBEC): C>T and C>G mutations at TpC dinucleotides (where the mutated C is preceded by T). APOBEC enzymes are cytidine deaminases normally involved in antiviral defense; when aberrantly expressed, they deaminate cytosines in single-stranded DNA. Common in breast, bladder, cervical, head/neck cancers. Associated with kataegis - localized hypermutation.

SBS10a/b (POLE proofreading deficiency): C>A and C>T mutations in specific trinucleotide contexts. Mutations in the POLE exonuclease domain disable proofreading during DNA replication. Results in extremely high mutation burden (>100 mutations/Mb). POLE-mutant tumors often respond well to immunotherapy due to high neoantigen load.

SBS6 and SBS15 (Mismatch repair deficiency): Small indels at microsatellites plus C>T at non-CpG sites. Reflects accumulation of replication errors when MMR is deficient. Correlates with MSI-high status.

SBS11 (Temozolomide): C>T mutations, particularly at CpC and CpT contexts. Temozolomide is an alkylating agent used for glioblastoma. Finding SBS11 in a recurrent tumor indicates temozolomide-induced mutagenesis - the treatment contributed to tumor evolution.

Clinical applications of mutational signatures:

Signature analysis can reveal:

Whether a tumor arose from a known environmental exposure (smoking, UV)
Whether MMR or POLE defects are present (immunotherapy implications)
Whether treatment itself contributed to tumor evolution
Potential hereditary predisposition (certain signatures suggest germline defects in DNA repair)

Clonal Hematopoiesis of Indeterminate Potential (CHIP)

CHIP represents a paradigm shift in how we understand the relationship between somatic mutations and disease. It refers to the presence of cancer-associated somatic mutations in blood cells of individuals without hematologic malignancy.

The discovery: When researchers began sequencing tumor DNA from blood samples, they noticed something unexpected - many patients had somatic mutations in genes like DNMT3A, TET2, and ASXL1 in their blood, even when their blood counts were normal and they had no evidence of leukemia or myelodysplasia. These mutations increased dramatically with age: rare before 40, but present in 10-20% of people over 70.

What CHIP represents: A single hematopoietic stem cell acquires a mutation that gives it a proliferative advantage. Over years, this clone expands and comes to represent a substantial fraction of blood cells. The mutations are real somatic mutations in cancer-associated genes, but they haven’t (yet) caused overt malignancy.

The most common CHIP mutations:

DNMT3A: The most frequently mutated gene in CHIP. DNMT3A encodes a DNA methyltransferase involved in epigenetic regulation. Mutations are typically loss-of-function. The R882 hotspot mutation is particularly common. DNMT3A mutations alone rarely progress to malignancy - they create a “pre-leukemic” state that may persist for decades.

TET2: Encodes an enzyme that oxidizes 5-methylcytosine, initiating DNA demethylation. TET2 and DNMT3A have opposing effects on methylation, yet mutations in both promote clonal expansion. TET2 mutations are associated with increased inflammation.

ASXL1: Involved in chromatin remodeling. ASXL1 mutations are more associated with myelodysplastic phenotypes and may carry higher risk of progression.

Other genes: TP53, JAK2, SF3B1, SRSF2, and others. TP53 mutations in CHIP may have higher malignant potential.

Clinical implications of CHIP:

Cancer risk: CHIP mutations increase the risk of developing hematologic malignancy by approximately 10-fold. However, the absolute risk remains low - about 0.5-1% per year. Most people with CHIP will never develop leukemia.

Cardiovascular risk: Surprisingly, CHIP is strongly associated with cardiovascular disease. TET2-mutant macrophages show increased inflammation; DNMT3A and JAK2 mutations also increase cardiovascular risk. This association is independent of traditional risk factors and may explain some “unexplained” cardiovascular events.

Confounding factor in liquid biopsy: This is critically important for cancer testing. When analyzing cell-free DNA from blood (liquid biopsy), mutations from CHIP can be detected and misinterpreted as tumor-derived. A VAF of 2% for a DNMT3A mutation in plasma might be from CHIP, not from a solid tumor. Paired analysis with white blood cell DNA can distinguish CHIP from true circulating tumor DNA.

Therapy-related considerations: Patients with pre-existing CHIP who receive cytotoxic chemotherapy have increased risk of therapy-related myeloid neoplasms. The chemotherapy may select for the resistant clone.

How to interpret CHIP in clinical reports: If NGS testing (especially from blood) reveals mutations in DNMT3A, TET2, ASXL1, or similar genes at variant allele frequencies suggesting clonal origin, consider:

This may represent CHIP, not cancer
The patient has increased (but still low absolute) risk of hematologic malignancy
Hematology referral may be appropriate for monitoring
For liquid biopsy interpretation, these mutations should be excluded as tumor-derived unless confirmed

FLT3 Mutations in Acute Myeloid Leukemia

FLT3 (fms-like tyrosine kinase 3) is a receptor tyrosine kinase expressed on hematopoietic progenitor cells. FLT3 mutations are among the most common genetic alterations in AML and have major prognostic and therapeutic implications.

FLT3-ITD (Internal Tandem Duplication)

The FLT3-ITD mutation is a duplication of a segment within the juxtamembrane domain (exons 14-15). The duplicated segment varies in size (from 3 to over 400 base pairs) but is always in-frame, preserving the reading frame.

Mechanism: The juxtamembrane domain normally acts as an autoinhibitory region, keeping the kinase inactive in the absence of ligand. The ITD disrupts this autoinhibition, causing constitutive kinase activation. The receptor signals continuously, driving proliferation.

Molecular characteristics:

Variable size (affects detection method choice)
Variable insertion site (all within juxtamembrane domain)
Head-to-tail orientation (the duplicated segment is inserted in the same direction as the original)
Often accompanied by loss of wild-type allele or additional ITD alleles

Detection methods:

PCR with capillary electrophoresis: Amplify across the region; ITDs appear as larger fragments. Sensitive to about 1-5% mutant allele.
NGS: Can detect ITDs but alignment is challenging for large insertions; specialized algorithms required
Fragment analysis: Traditional method; still widely used

Clinical significance:

Present in ~25% of AML
Associated with poor prognosis (higher relapse rate, shorter overall survival)
Allelic ratio matters: Higher mutant-to-wild-type ratio confers worse prognosis
Targetable with FLT3 inhibitors (midostaurin, gilteritinib)

FLT3-TKD (Tyrosine Kinase Domain Mutations)

Point mutations in the tyrosine kinase domain, most commonly D835 and I836, also activate FLT3 but by a different mechanism.

Mechanism: These mutations occur in the activation loop of the kinase domain, stabilizing the active conformation. Like ITD, they cause constitutive activation.

Clinical significance:

Present in ~7% of AML
Prognostic significance less clear than ITD (may be less adverse)
Also targetable with FLT3 inhibitors, though response may differ

Therapeutic implications: FLT3 inhibitors have changed AML management:

Midostaurin (first-generation): Added to chemotherapy for newly diagnosed FLT3-mutant AML
Gilteritinib (second-generation): More selective; used for relapsed/refractory FLT3-mutant AML
Resistance mutations can emerge, particularly in the TKD, requiring ongoing monitoring

Testing recommendations: All AML patients should be tested for FLT3 mutations at diagnosis (and at relapse). Results should be available within 24-48 hours if possible, given the impact on treatment decisions. Both ITD and TKD mutations should be assessed.

Liquid Biopsy

Traditional tumor testing requires tissue - obtained by biopsy or surgery. Liquid biopsy analyzes tumor-derived material circulating in blood.

Circulating tumor DNA (ctDNA): Tumor cells release DNA fragments into the bloodstream (through necrosis, apoptosis, or active secretion). This cell-free DNA can be captured and analyzed.

Applications:

Tumor genotyping when tissue is unavailable: Especially useful in lung cancer, where repeat biopsies are difficult
Monitoring treatment response: ctDNA levels reflect tumor burden; decrease indicates response
Detecting resistance mutations: Rising ctDNA or new mutations signal disease progression
Minimal residual disease (MRD): Detecting ctDNA after apparently complete treatment identifies patients at high relapse risk

Challenges: ctDNA is a tiny fraction of total cell-free DNA, especially with low tumor burden or after treatment. Highly sensitive methods (often requiring tumor-specific panels) are needed.

Circulating tumor cells (CTCs): Intact tumor cells circulating in blood. Rarer than ctDNA and technically challenging to capture, but provide living cells for additional characterization.

60.3 Inherited Disease Testing

Molecular diagnostics play multiple roles in inherited disease: confirming clinical diagnoses, identifying carriers, enabling prenatal diagnosis, and increasingly, predicting disease before symptoms appear.

Testing Strategies

Single-gene testing: When clinical findings point to a specific disorder, test that gene. Most cost-effective when the diagnosis is clear.

Gene panels: When multiple genes cause similar phenotypes (e.g., hereditary cardiomyopathy, inherited cancer syndromes), test all relevant genes simultaneously.

Exome/genome sequencing: When the clinical presentation is nonspecific or previous testing was negative, comprehensive sequencing may identify unexpected diagnoses.

Classic Single-Gene Disorders

Cystic Fibrosis (CFTR gene)

CF is caused by mutations in CFTR on 7q31.2, which encodes a chloride channel (ABC transporter family) in epithelial cells. Autosomal recessive. Over 2,000 mutations have been identified and are grouped into six functional classes:

Class I: no protein synthesis (nonsense / premature stop codons, e.g., G542X, W1282X)
Class II: protein made but misfolded and degraded in ER, never reaching the membrane - F508del is class II
Class III: gating defect (e.g., G551D - responsive to ivacaftor)
Class IV: reduced conductance
Class V: reduced protein quantity
Class VI: reduced protein stability at the membrane

Organ involvement follows from chloride channel failure at epithelial surfaces: lungs (thick mucus, bronchiectasis, Pseudomonas colonization), pancreas (exocrine insufficiency), GI (meconium ileus), sweat glands (elevated chloride >60 mEq/L on sweat test), and vas deferens (congenital bilateral absence of the vas deferens - a common cause of obstructive azoospermia in otherwise asymptomatic men).

Common mutations you should recognize by name:

F508del - deletion of phenylalanine at position 508 - the single most common CF mutation worldwide (~70% of CF alleles in Northern Europeans). A class II trafficking defect. Elexacaftor-tezacaftor-ivacaftor (Trikafta) restores F508del-CFTR function and transformed CF care for any patient carrying at least one F508del allele.
G542X - second most common globally (~2-5% of alleles). Class I nonsense mutation - nothing to potentiate, since no protein is made. Read-through agents (ataluren) have been investigated.
W1282X - accounts for ~60% of CF alleles in individuals with Ashkenazi Jewish ancestry. Also a class I nonsense mutation.

Testing approaches:

Screening panels: ACMG recommends a 23-mutation panel, detecting ~90% of carriers in Northern European populations. Carrier frequencies vary by ancestry/population group: approximately ~1/25 in Northern European and Ashkenazi Jewish populations, ~1/46 in Hispanic/Latino populations, and ~1/65 in African American populations.
Expanded panels detect more mutations and improve sensitivity in populations underrepresented by older targeted panels.
Full gene sequencing detects nearly all mutations but may yield variants of uncertain significance.
Deletion/duplication analysis (MLPA) is needed when one mutation is found but the patient has classic CF.

Sickle Cell Disease and Hemoglobinopathies

Sickle cell disease results from homozygosity for HbS (or compound heterozygosity with another β-globin variant like HbC or β-thalassemia).

Testing:

Newborn screening by hemoglobin electrophoresis or HPLC identifies affected infants
Molecular testing confirms the genotype and distinguishes HbSS from Sβ-thalassemia

Huntington Disease (HTT gene)

HD is caused by expansion of a CAG trinucleotide repeat in HTT on 4p16.3. Normal alleles have <27 repeats; ≥40 CAG repeats = full penetrance. Intermediate ranges carry reduced penetrance: 27-35 (usually asymptomatic but expansion-prone in offspring) and 36-39 (some will develop HD, some won’t).

Mechanism: the expanded CAG codes for a polyglutamine tract in huntingtin - toxic gain-of-function with selective loss of striatal medium spiny neurons (caudate and putamen atrophy). Classic triad: chorea, dementia, psychiatric disturbance.

Anticipation is the board-favorite feature: earlier onset and/or more severe disease in successive generations. This is because the repeat expands further during meiosis - paternal transmission is far more expansion-prone (spermatogenesis involves many more cell divisions than oogenesis). Juvenile HD (onset <20 years, rigidity instead of chorea, >60 repeats) almost always comes through the father.

This is the paradigmatic example of predictive genetic testing - DNA can tell you with near certainty whether you will develop HD decades before symptoms. Pre-symptomatic testing requires structured genetic counseling per Huntington Disease Society of America guidelines.

Trinucleotide Repeat Disorders

Multiple diseases are caused by unstable repeats that expand across generations. The mechanism depends on where in the gene the repeat sits:

CAG in coding exon (HD, spinocerebellar ataxias) - polyglutamine tract - toxic protein aggregation
CGG in 5’UTR (Fragile X, FMR1) - methylation-mediated gene silencing
CTG in 3’UTR (myotonic dystrophy type 1, DMPK) or CCTG in intron 1 (myotonic dystrophy type 2, CNBP) - toxic RNA that sequesters splicing factors

Testing requires specialized methods (PCR with repeat-primed PCR, triplet-primed PCR, or Southern blot) because standard PCR cannot reliably amplify large repeats.

Renal Genetic Disease

Alport Syndrome (Type IV Collagen)

Alport syndrome is inherited nephritic syndrome caused by mutations in type IV collagen. Type IV collagen is the major structural component of the glomerular basement membrane (GBM), lens capsule, and cochlear basement membrane, which explains the classic triad.

Genetics:

80% X-linked recessive, COL4A5 on Xq22.3
15% autosomal recessive, COL4A3 / COL4A4 on chromosome 2
~5% autosomal dominant

Classic triad: glomerulonephritis + sensorineural hearing loss + ocular lesions. Mnemonic: “can’t see, can’t pee, can’t hear a bee.” Hearing loss is bilateral, high-frequency, progressive. Ocular findings include anterior lenticonus (pathognomonic), dot-fleck retinopathy, and corneal erosions.

Affected males (X-linked) progress to ESRD by age 30-40. Carrier females have asymptomatic microscopic hematuria from random X-inactivation producing a mosaic GBM. About 15% of carriers develop proteinuria; rare carriers progress to ESRD (usually with extremely skewed X-inactivation).

Diagnosis: absence of alpha-5 chain of type IV collagen on IHC or IF of skin or kidney biopsy. In X-linked males, alpha-5(IV) staining is completely absent from GBM, Bowman capsule, and distal tubular basement membranes. Carrier females show mosaic (patchy) staining. EM shows the characteristic basket-weave or lamellated GBM. Genetic testing (COL4A5 sequencing) is increasingly used as a primary diagnostic tool.

Heterozygous COL4A3 / COL4A4 mutations cause thin basement membrane disease (benign familial hematuria), a milder variant with uniformly thin GBM and isolated microscopic hematuria.

Congenital and Familial Nephrotic Syndromes

Congenital nephrotic syndrome = nephrotic syndrome detected before 3 months of age (sometimes in utero). Most cases are genetic and therefore steroid-resistant - the defect is structural, not immune-mediated. Elevated maternal serum AFP in pregnancy is a clue (fetal proteinuria, amniotic fluid AFP, maternal serum AFP).

Syndrome	Gene (Locus)	Protein	Key feature
Finnish type	NPHS1 (19q13.1)	Nephrin	Most common congenital NS; slit diaphragm pore; microcystic proximal tubules
SRNS (podocin)	NPHS2 (1q25)	Podocin	Most common genetic cause of childhood steroid-resistant FSGS; AR
Denys-Drash / Frasier	WT1 (11p13)	Zinc-finger TF	Diffuse mesangial sclerosis vs FSGS; male pseudohermaphroditism; Wilms (DD) or gonadoblastoma (Frasier) risk
Pierson	LAMB2 (3p21)	Beta-2 laminin	Microcoria (pathognomonic) + congenital NS
Nail-Patella	LMX1B (9q34.1)	TF regulating COL4A3	Nail dysplasia, absent patella, iliac horns, nephropathy
Familial FSGS (AD)	ACTN4, TRPC6	Alpha-actinin-4; calcium channel	Adult-onset proteinuria / FSGS

WT1 genotype-phenotype: exon mutations cause Denys-Drash (diffuse mesangial sclerosis + Wilms risk); intron 9 splice site mutations cause Frasier (FSGS + gonadoblastoma risk). Both autosomal dominant.

Clinical pearl for transplant: genetic FSGS does not recur after transplant, because the defect is in the native podocytes, not in a circulating permeability factor. Immune-mediated FSGS often does recur.

Renal Fanconi Syndrome

Proximal tubular dysfunction with glycosuria (with normal blood glucose), amino aciduria, phosphaturia, hypokalemia, and bicarbonate wasting (type 2 RTA). Causes:

Inherited: cystinosis (most common in children, CTNS gene), galactosemia, hereditary fructose intolerance, Wilson disease, tyrosinemia, Lowe syndrome
Acquired: multiple myeloma (light chain tubulopathy), tenofovir, heavy metals, expired tetracycline

Dent disease: X-linked recessive CLCN5 mutations (Xp11.22). ClC-5 is a proximal tubule endosomal chloride channel needed for megalin/cubilin-mediated protein reabsorption. Presents with low-molecular-weight proteinuria (beta-2 microglobulin, retinol-binding protein), hypercalciuria, nephrocalcinosis, nephrolithiasis, and progressive renal failure. Dent type 2 involves OCRL1 (also mutated in Lowe syndrome).

Polycystic and Cystic Kidney Disease

Feature	ARPKD	ADPKD
Gene	PKHD1 (6p12), 95%	PKD1 (16p13.3, 85%) / PKD2 (4q22, 15%)
Protein	Fibrocystin	Polycystin-1 / polycystin-2
Cyst origin	Collecting duct, radial	All nephron segments
Gross morphology	Reniform shape preserved	Massively enlarged, architecture destroyed
Age of onset	In utero / infancy	Adulthood (30s-50s)
Associated	Congenital hepatic fibrosis	Liver cysts, berry aneurysms, MVP

PKD1 is more severe and earlier onset than PKD2. Polycystin-1 and polycystin-2 form a complex in primary cilia that senses tubular flow - mutations cause aberrant proliferation and cystogenesis. ADPKD is the most common hereditary renal disease (1:400-1000). Tolvaptan (vasopressin V2 receptor antagonist) slows cyst growth and progression.

ARPKD presents prenatally with bilateral enlarged echogenic kidneys, oligohydramnios, and Potter sequence. PKHD1 also causes the associated biliary fibrocystic diseases (congenital hepatic fibrosis, Caroli disease, and Caroli syndrome), so PKHD1 patients may present with portal hypertension and variceal bleeding.

Glomerulocystic kidney disease is genotypically related to ADPKD (often an early PKD1/PKD2 manifestation) but radiographically resembles ARPKD, with cystic dilation of Bowman capsule plus renal dysplasia. Also seen in tuberous sclerosis.

Multicystic dysplastic kidney (MCDK) is sporadic / non-inherited. Non-communicating cysts of varying sizes with no functional parenchyma, usually unilateral, with primitive ducts and cartilage histologically. Most common cystic renal disease diagnosed prenatally; typically involutes over time.

Nephronophthisis is an autosomal recessive medullary cystic disease (NPHP1-8, most commonly NPHP1 on 2q12-13). Nephrocystins are ciliary proteins. Most common genetic cause of ESRD in children and young adults. Associated syndromes: Senior-Loken (retinitis pigmentosa), Joubert (cerebellar vermis hypoplasia - “molar tooth sign”), Bardet-Biedl.

Bilateral renal agenesis and posterior urethral valves are sporadic, not inherited. Bilateral agenesis produces Potter sequence (oligohydramnios, pulmonary hypoplasia, limb deformities, flattened facies); pulmonary hypoplasia is what kills. Posterior urethral valves are the most common cause of congenital bladder outlet obstruction in males.

Prune belly (Eagle-Barrett) syndrome triad: (1) deficient abdominal wall muscles (wrinkled, “prune-like”), (2) urinary tract dilation (megacystis, hydroureter, hydronephrosis), (3) bilateral cryptorchidism. Almost exclusively males. Most cases sporadic; rare autosomal recessive cases involve CHRM3 mutations on 1q43 (muscarinic acetylcholine receptor M3, smooth muscle contraction).

Cardiac Genetic Disease

Channelopathies

Disease	Gene	Mechanism / Trigger
Brugada syndrome	SCN5A (3p22), 25%	Loss-of-function sodium channel; fever, sodium channel blockers unmask
LQTS1	KCNQ1 (11p15.5)	Loss-of-function IKs; exercise / swimming
LQTS2	KCNH2/HERG (7q35-36)	Loss-of-function IKr; auditory / emotional triggers
LQTS3	SCN5A (3p22)	Gain-of-function sodium channel; sleep / rest triggered
LQTS7 (Andersen-Tawil)	KCNJ2	Kir2.1, shared skeletal muscle channel - periodic paralysis + LQT + dysmorphism

SCN5A loss-of-function = Brugada; SCN5A gain-of-function = LQTS3. Same gene, opposite mutations, different diseases - board classic.

Brugada ECG: coved ST elevation in V1-V3 with RBBB pattern. Risk of sudden cardiac death from VF. Sodium channel blockers (flecainide, procainamide) are used as diagnostic challenge tests. Treatment: ICD.

LQTS1 is the most common form (~40-55% of LQTS). Homozygous KCNQ1 mutations cause Jervell and Lange-Nielsen syndrome (LQTS + congenital sensorineural deafness, autosomal recessive). Beta-blockers work well in LQTS1, less so in LQTS2, and mexiletine (sodium channel blocker) helps in LQTS3. Most drug-induced QT prolongation blocks IKr (the KCNH2/HERG channel).

Andersen-Tawil triad: periodic paralysis, prolonged QT with prominent U waves, and dysmorphic features (low-set ears, hypertelorism, small mandible, clinodactyly). The dual cardiac / skeletal phenotype reflects the shared channel.

Long QT penetrance is higher in women (longer baseline QTc; testosterone shortens QT). Diagnostic thresholds are sex-specific: QTc >470 ms in males, >480 ms in females.

Arrhythmogenic right ventricular dysplasia (ARVD / Uhl anomaly): progressive fibrofatty replacement of RV myocardium. Desmosomal protein genes predominate - PKP2 (plakophilin-2) is most common, also DSP (desmoplakin), DSG2, DSC2, JUP. RYR2 (cardiac ryanodine receptor) is also implicated. Autosomal dominant, variable penetrance. Sudden death in young athletes. Diagnosis: MRI plus Task Force criteria.

Cardiomyopathies

Hypertrophic cardiomyopathy (HCM) is autosomal dominant, caused by sarcomeric protein mutations. Most common genes: MYH7 (beta-myosin heavy chain, 14q) and MYBPC3 (myosin binding protein C, 11p). The R403Q mutation in MYH7 was the first HCM mutation identified and carries severe phenotype and high sudden death risk. Pathology: asymmetric septal hypertrophy, myocyte disarray, interstitial fibrosis. Most common cause of sudden cardiac death in young athletes.

Genotype-phenotype correlation in HCM is imperfect - the same mutation can cause different severity within families. Genetic testing finds a causative mutation in ~60% of index cases. Cascade screening of first-degree relatives is standard; genotype-positive / phenotype-negative relatives need long-term surveillance.

Dilated cardiomyopathy has both X-linked and autosomal dominant forms:

X-linked DCM = dystrophin (DMD) gene mutations (cardiac-specific promoter / 5’ regions)
Autosomal dominant: TTN (titin) is the most common overall (up to 25% of familial DCM), MYH7, and LMNA (lamin A/C). LMNA mutations carry a high risk of sudden death even before significant heart failure - early ICD consideration.

Restrictive cardiomyopathy is mostly sporadic - amyloidosis (most common), sarcoidosis, hemochromatosis, Fabry, Gaucher, eosinophilic endomyocarditis, carcinoid. Rare familial cases: TNNI3 (troponin I), DES (desmin), TTR (transthyretin).

Familial cardiac amyloidosis is caused by transthyretin (TTR) mutations, autosomal dominant. TTR V30M is the most common globally (~30-45% of hereditary ATTR). TTR V122I is carried by ~3-4% of African American individuals and causes late-onset cardiac amyloidosis. Wild-type TTR (senile / age-related) amyloidosis affects elderly men. Treatment: tafamidis (TTR tetramer stabilizer), patisiran (siRNA), inotersen (antisense oligonucleotide). Congo red shows apple-green birefringence.

Dystrophinopathies (Cardiac and Skeletal Involvement)

DMD on Xp21.2 is the largest known human gene (2.4 Mb, 79 exons). X-linked. The reading frame rule predicts severity ~90% of the time:

Out-of-frame (frameshift / nonsense) = Duchenne - near-absent dystrophin - wheelchair by 12, death by 30 from cardiac / respiratory failure
In-frame = Becker - partially functional dystrophin - ambulatory into 20s-30s, later cardiomyopathy
Cardiac-specific promoter / 5’ region mutations = isolated X-linked DCM

CK is massively elevated in Duchenne (10,000-50,000 U/L). Diagnostic workflow: MLPA for deletions / duplications first (catches ~70%), then sequencing for point mutations. Muscle biopsy is reserved for equivocal genetic results.

Congenital Heart Syndromes and Structural Defects

Syndrome	Gene / Lesion	Classic features
Holt-Oram	TBX5 (12q24.1)	Heart-hand: radial ray defects + ASD (secundum) / VSD / conduction abnormalities. 100% have skeletal anomaly (at least carpal bone abnormality on X-ray)
DiGeorge / velocardiofacial	22q11.2 microdeletion (TBX1 critical)	CATCH-22: Cardiac (conotruncal: TOF, interrupted aortic arch, truncus), Abnormal facies, Thymic aplasia, Cleft palate, Hypocalcemia
Noonan	PTPN11 (12q24, 50%)	“Male Turner” phenotype, pulmonary stenosis (most common cardiac defect). RASopathy (also RAF1, KRAS, SOS1, BRAF)
Alagille	JAG1 (20p12), Notch ligand	Pentad: bile duct paucity, peripheral pulmonary stenosis, butterfly vertebrae, posterior embryotoxon, characteristic facies
Williams	7q11.23 microdeletion (ELN)	Supravalvar aortic stenosis (elastin haploinsufficiency), elfin facies, hypercalcemia, friendly personality
NKX2-5	NKX2-5 (5q35)	ASD + progressive AV block
GATA4	GATA4 (8p23)	Ostium secundum ASD; interacts with TBX5 and NKX2-5

TBX5 + NKX2-5 + GATA4 form a transcriptional complex for septation - disease-causing mutations often hit the interactions between these proteins.

Structural heart defects are common in chromosomal disorders:

Down syndrome (trisomy 21): CHD in >50%. AVSD (complete atrioventricular canal) is most characteristic, also VSD, ASD, TOF
Turner syndrome (45,X): CHD in up to 50%. Bicuspid aortic valve is most common (~15-30%), coarctation of aorta is most serious, aortic root dilation / dissection

Osler-Weber-Rendu syndrome (hereditary hemorrhagic telangiectasia, HHT): autosomal dominant. ENG (endoglin, 9q34) = HHT1; ACVRL1 (ALK1, 12q13) = HHT2. Both encode TGF-beta pathway receptors. Features: mucocutaneous telangiectasias (lips, tongue, fingers), epistaxis (most common symptom), pulmonary AVMs (paradoxical emboli, brain abscess risk - HHT1 more), hepatic AVMs (HHT2 more), GI telangiectasias.

Endocrine Genetic Disease

Congenital Adrenal Hyperplasia (CAH)

Most common cause (>90%): 21-hydroxylase deficiency (CYP21A2 / CYP21 on 6p21.3). Large deletions of CYP21 are the most frequent mutation, followed by gene conversions between the active gene and the CYP21P pseudogene. 21-hydroxylase converts progesterone to deoxycorticosterone and 17-OHP to 11-deoxycortisol in the cortisol and aldosterone pathways. Deficiency shunts precursors toward androgens:

Classic salt-wasting: aldosterone and cortisol deficiency + virilization
Classic simple virilizing: cortisol deficiency + virilization
Non-classic: late-onset hyperandrogenism (PCOS-like)

17-OHP is the screening biomarker on newborn screen.

Second most common (5-8%): 11-beta-hydroxylase deficiency (CYP11B1 on 8q24). Converts 11-deoxycortisol to cortisol. Distinguishing feature: hypertension, because deoxycorticosterone accumulates and has mineralocorticoid activity. Compare with 21-hydroxylase, which causes salt-wasting from aldosterone deficiency.

Gonadal Development and Hypogonadism

Androgen insensitivity syndrome (AIS): X-linked recessive, AR gene on Xq12. >95% of complete AIS (CAIS) cases have identifiable AR mutations, usually missense in the ligand-binding domain; in partial AIS (PAIS), AR mutations are found in ~50%.

Labs: elevated testosterone (because LH is unsuppressed), elevated estradiol (testosterone aromatized), elevated AMH. SRY is present. Mullerian structures are absent because AMH is normal.

46,XY CAIS phenotype: female external genitalia, blind-ending vagina, absent uterus / tubes, intra-abdominal or inguinal testes, and normal female breast development at puberty (testosterone aromatized to estrogen). Gonadoblastoma risk in undescended testes - gonadectomy recommended after puberty.

Steroid sulfatase deficiency: 90% from STS deletion on Xp22.3, causing X-linked ichthyosis. Steroid sulfatase converts sulfated DHEA to DHEA and cholesterol sulfate to cholesterol. Deficiency causes placental estrogen deficiency (failure of labor induction), ichthyosis (cholesterol sulfate accumulates in stratum corneum), and corneal opacities. STS is contiguous with KAL1 on Xp22.3, so large deletions cause Kallmann + ichthyosis as a contiguous gene deletion syndrome.

Kallmann syndrome: hypogonadotropic hypogonadism + anosmia/hyposmia. KAL1 mutations on Xp22.3 in ~15% of cases (X-linked). KAL1 encodes anosmin-1, essential for GnRH neuron migration from the olfactory placode to the hypothalamus. Other forms: FGFR1 (AD), PROKR2, PROK2. Treatment: pulsatile GnRH or gonadotropin replacement.

Growth Hormone Deficiency

Inheritance pattern determines the gene:

Autosomal dominant GH deficiency: mutations in the GH1 gene (17q23) itself - often splice-site mutations that produce a dominant-negative 17.5 kDa form disrupting normal 22 kDa GH secretion
Autosomal recessive GH deficiency: GHRHR (GH releasing hormone receptor) - upstream signaling failure, more severe phenotype
X-linked GH deficiency: BTK (Bruton tyrosine kinase) - same gene as X-linked agammaglobulinemia. Combined immunodeficiency + GH deficiency. Classic board question.

G-Protein / GNAS Disorders

GNAS1 (20q13.3) encodes the alpha subunit of the stimulatory G-protein (Gs-alpha). Loss or gain of function causes very different diseases:

Inactivating mutations = Albright hereditary osteodystrophy / pseudohypoparathyroidism type 1A (autosomal dominant, with imprinting). Short stature, round facies, brachydactyly (short 4th / 5th metacarpals), obesity, subcutaneous calcifications, intellectual disability, and end-organ resistance to PTH, TSH, and gonadotropins. Hypocalcemia + hyperphosphatemia despite high PTH.
Gain-of-function mutations = McCune-Albright syndrome - constitutively active Gs-alpha. Somatic mosaic; germline mutations are lethal. Triad: polyostotic fibrous dysplasia, precocious puberty, cafe-au-lait macules with irregular “coast of Maine” borders (contrast with NF1’s smooth “coast of California” borders). Also possible: hyperthyroidism, Cushing, GH excess. Mosaic distribution explains the segmental / asymmetric pattern.

Thyroid

Congenital hypothyroidism genes fall into two groups:

Thyroid dysgenesis / agenesis: PAX8 and TSHR, also NKX2-1 (TTF-1), FOXE1
Thyroid dyshormonogenesis (goitrous hypothyroidism): DUOX2, TPO, TG, SLC5A5, SLC26A4. Pendred syndrome: SLC26A4 - goiter + sensorineural deafness with Mondini malformation of cochlea.

Detected by elevated TSH on newborn screening.

Diabetes

Type 1 DM is strongly HLA-associated: HLA-DR3 and HLA-DR4. DR3/DR4 heterozygosity confers the highest risk (~1:15 lifetime vs ~1:300 in general population). HLA-DQ also contributes (DQ2 with DR3, DQ8 with DR4). HLA-DR2 (DQ6) is protective. Non-HLA: INS VNTR, CTLA4, PTPN22. Autoantibodies: anti-GAD65, anti-IA2, anti-insulin, anti-ZnT8.

MODY (maturity-onset diabetes of the young) is autosomal dominant monogenic diabetes, typically diagnosed before age 25. Major subtypes:

MODY1 (HNF4A) - sulfonylurea-responsive
MODY2 (GCK / glucokinase) - mild stable fasting hyperglycemia, reset glucose thermostat, usually no treatment needed
MODY3 (HNF1A) - most common MODY, sulfonylurea-responsive
MODY4 (IPF1 / PDX1)
MODY5 (HNF1B) - renal cysts + diabetes

Therapeutic importance: HNF1A-MODY mimics type 1 but has no autoantibodies and responds to sulfonylureas - don’t put these patients on insulin if you can avoid it.

GI and Hepatobiliary Genetic Disease

Hirschsprung Disease

Loss-of-function mutations in RET (10q11.2), GDNF (5p13), and EDNRB (13q22) cause Hirschsprung disease (aganglionic megacolon) by disrupting neural crest cell migration to distal bowel. Autosomal dominant with reduced penetrance. Neonatal failure to pass meconium, abdominal distension. Diagnosis: rectal suction biopsy showing absent ganglion cells + hypertrophied nerve trunks. Note: RET gain-of-function mutations cause MEN2 - same gene, opposite effect, different disease.

Microvillus Inclusion Disease

Autosomal recessive, MYO5B gene (myosin Vb, apical recycling endosome trafficking). Intractable secretory diarrhea in neonates. Pathognomonic EM finding: apical microvillus inclusions (intracellular vesicles lined by microvilli) within enterocytes. PAS-positive, CD10-positive inclusions on IHC. STX3 is another causative gene. Fatal without TPN and eventual intestinal transplantation.

Esophageal atresia (with / without TE fistula), pyloric stenosis, intestinal atresia, omphalocele, and gastroschisis are all multifactorial / sporadic - no single causative gene. They may occur within syndromes (VACTERL for EA-TEF, trisomy 18 for omphalocele) but isolated cases are multifactorial. Pyloric stenosis has the strongest genetic component (5x more common in males, polygenic with threshold model).

Iron Overload

Hereditary hemochromatosis: HFE gene on 6p22, autosomal recessive. HFE interacts with transferrin receptor to regulate iron absorption via hepcidin signaling. Iron deposits in hepatocytes (NOT Kupffer cells - that pattern suggests transfusional hemosiderosis). Clinical: cirrhosis, HCC, “bronze diabetes,” cardiomyopathy, arthropathy, hypogonadism.

C282Y homozygosity is the most common clinically significant genotype (~85-90%). C282Y / H63D compound heterozygosity: ~5%. H63D homozygosity rarely causes significant iron overload. Penetrance of C282Y homozygosity is incomplete: ~28% of males, ~1% of females develop clinical disease. Workup: elevated ferritin, transferrin saturation >45%, then HFE genotyping. Hepatic iron concentration on biopsy confirms tissue iron loading.

Juvenile hemochromatosis: HAMP (hepcidin, 19q13) or HFE2 / HJV (hemojuvelin, 1q21). Hepcidin is the master regulator of iron homeostasis - blocks ferroportin to prevent iron export. Loss of hepcidin = unregulated iron absorption. Presents in the 2nd-3rd decade with cardiomyopathy and hypogonadism earlier and more severely than classical HFE disease.

Other hemochromatosis genes:

TFR2 (7q22, transferrin receptor 2) - autosomal recessive, similar to classical HFE but rarer
SLC40A1 (ferroportin) - autosomal dominant “ferroportin disease” - unique: iron accumulates in macrophages / Kupffer cells, and phlebotomy can worsen anemia

Copper Handling

ATP7A (Xq21.1) = Absorption (intestine) = Menkes. ATP7B (13q14) = Biliary excretion (liver) = Wilson. Mnemonic: A = Absorption, B = Biliary.

Wilson disease (ATP7B): autosomal recessive. Failure to load copper onto ceruloplasmin or excrete it into bile. Copper accumulates in liver (hepatitis, cirrhosis), brain (lenticular nucleus - movement disorders), cornea (Kayser-Fleischer rings, pathognomonic Descemet membrane deposits), and kidneys (Fanconi). Can present as hemolytic anemia (acute). Diagnosis: low ceruloplasmin, elevated urine copper (>100 mcg/day), elevated hepatic copper (>250 mcg/g dry weight). Treatment: penicillamine, zinc (blocks absorption), liver transplant (curative).

Menkes kinky hair syndrome (ATP7A): X-linked recessive. Copper can’t cross the intestinal enterocyte into blood - systemic copper deficiency. Copper-dependent enzymes fail: lysyl oxidase (tortuous vessels, bladder diverticula), tyrosinase (hypopigmentation), cytochrome c oxidase (neurodegeneration), superoxide dismutase. Progressive neurodegeneration, connective tissue abnormalities, kinky / steely hair (pili torti). Usually fatal by age 3. Low serum copper and ceruloplasmin.

Alpha-1 Antitrypsin Deficiency

SERPINA1 / PI gene on 14q32. AAT is a serine protease inhibitor (serpin) that inhibits neutrophil elastase. PiMM = normal; PiZZ (Glu342Lys) = severe deficiency. The Z allele produces a misfolded protein that polymerizes in hepatocyte ER.

Lung disease = loss-of-function (uninhibited neutrophil elastase) - panacinar emphysema, lower lobes, younger age
Liver disease = gain-of-function (toxic ER accumulation) - neonatal hepatitis, cirrhosis, HCC

PAS-positive, diastase-resistant globules in periportal hepatocytes are pathognomonic. PiSZ causes mild deficiency; PiMZ carriers have intermediate levels and usually no disease.

Inherited Hyperbilirubinemias

Disorder	Gene	Defect	Bilirubin	Clinical
Gilbert	UGT1A1 5’ TATA box (extra TA - (TA)7 / UGT1A1*28)	Reduced expression (~30% normal)	Unconjugated, mild	Most common; ~5-10% population; episodic jaundice with fasting / stress. Predicts irinotecan toxicity
Crigler-Najjar type I	UGT1A1 coding sequence	Complete absence	Unconjugated, >20 mg/dL	Autosomal recessive; no response to phenobarbital; kernicterus risk; needs phototherapy / liver transplant
Crigler-Najjar type II	UGT1A1 coding sequence	Partial deficiency	Unconjugated, 6-20 mg/dL	Responds to phenobarbital
Dubin-Johnson	MRP2 / ABCC2 (10q24)	Canalicular transporter	Conjugated (direct)	Liver grossly black; benign; reversed coproporphyrin ratio (>80% isomer I)
Rotor	SLCO1B1 / 1B3	Basolateral OATP	Conjugated	No black liver; both coproporphyrin isomers elevated

Pharmacogenomic point: UGT1A1*28 homozygosity predicts irinotecan toxicity because SN-38 (active metabolite) cannot be glucuronidated, leading to severe diarrhea and neutropenia.

Neurologic and Psychiatric Genetic Disease

Alzheimer Disease

Late-onset (sporadic): APOE on 19q13.2 has three alleles (E2, E3, E4). E4 is the strongest genetic risk factor. E4 homozygosity increases risk 12-15 fold; E4 heterozygosity 3-4 fold. E2 is protective. ApoE4 clears amyloid-beta less efficiently. APOE genotyping is not recommended for clinical diagnosis - incomplete penetrance, no disease-modifying treatment.

Early-onset familial (ages 30-50, autosomal dominant, high penetrance): PSEN1 (14q24, most common), PSEN2 (1q42), and APP (21q21). All increase production of amyloid-beta 42 (more aggregation-prone). Down syndrome patients develop Alzheimer pathology by age 40-50 from lifelong APP gene dose effect (extra copy on chromosome 21).

Frontotemporal Dementia

MAPT on 17q21 encodes microtubule-associated protein tau. Mutations cause frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Pick disease: circumscribed frontal / temporal atrophy (“knife-edge gyri”), Pick bodies (round, argyrophilic, tau-positive cytoplasmic inclusions). Other FTD genes: GRN (progranulin), C9orf72 (hexanucleotide repeat expansion - most common genetic cause of both FTD and ALS).

Parkinson Disease

Familial PD genes: PARK1-8. Key players:

PARK1 / 4 (SNCA, alpha-synuclein) - AD, Lewy body formation
PARK2 (Parkin) - AR, early onset
PARK6 (PINK1), PARK7 (DJ-1) - AR
PARK8 (LRRK2) - AD, most common genetic cause of PD, also ~1% of sporadic cases. LRRK2 G2019S is the most common PD-causing mutation worldwide

Lewy bodies (eosinophilic cytoplasmic inclusions of aggregated alpha-synuclein) are the pathologic hallmark.

CADASIL

==NOTCH3 (19p13.2) mutations == cause CADASIL (Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy). Autosomal dominant. Most common hereditary cause of stroke and vascular dementia in adults. Pathology: granular osmiophilic material (GOM) deposits in small vessel walls on EM (pathognomonic), PAS-positive thickened arteriolar walls. MRI: diffuse white matter lesions, especially in temporal poles and external capsules (distinguishing feature). Onset 30s-60s with migraine, TIAs / strokes, dementia.

Neurodegeneration with Brain Iron Accumulation

Pantothenate kinase-associated neurodegeneration (PKAN, formerly Hallervorden-Spatz - name deprecated due to Nazi-era associations): PANK2 on 20p13, autosomal recessive. Coenzyme A synthesis defect. Iron accumulates in the globus pallidus, creating the eye of the tiger sign on T2 MRI (central bright spot within dark globus pallidus). Childhood onset - progressive dystonia, spasticity, cognitive decline. Part of the broader NBIA group.

Prion Disease

PRNP gene on 20p13 encodes cellular prion protein (PrPc). Mutations cause the protein to misfold into PrPSc (scrapie form, protease-resistant, self-propagating, neurotoxic). All familial prion diseases are autosomal dominant:

Familial CJD (most common): E200K
Gerstmann-Straussler-Scheinker: P102L (ataxia-dominant)
Fatal familial insomnia: D178N with methionine at codon 129 on the same allele

Spongiform encephalopathy on neuropathology.

Peripheral Nerve

Charcot-Marie-Tooth type 1A (CMT1A): PMP22 duplication on 17p12, autosomal dominant. Most common inherited peripheral neuropathy. Demyelinating: slow NCV, distal muscle weakness / atrophy (stork legs, champagne bottle legs), pes cavus, loss of DTRs. Nerve biopsy: onion bulb formation (repeated demyelination / remyelination). PMP22 deletion (reciprocal of the CMT1A duplication) causes hereditary neuropathy with liability to pressure palsies (HNPP).

Familial dysautonomia (Riley-Day syndrome): IKBKAP / ELP1 on 9q31, autosomal recessive. seen predominantly in individuals with Ashkenazi Jewish ancestry (carrier frequency ~1/30). Autonomic dysfunction (labile BP, temperature instability), alacrima (absent tearing), absent fungiform papillae on tongue, decreased pain / temperature sensation, absent DTRs, episodic vomiting crises.

Motor Neuron Disease

Spinal muscular atrophy (SMA): autosomal recessive, SMN1 on 5q13 (homozygous deletion / mutation in ~95%). Lower motor neuron disease. Severity types I-IV by age of onset.

SMN2 is a nearly identical paralog. A C-to-T change in exon 7 causes ~90% of SMN2 transcripts to skip exon 7, producing truncated, unstable protein. Only ~10% of SMN2 mRNA makes full-length functional SMN. SMN2 copy number is the primary modifier of SMA severity - more SMN2 copies = more residual full-length SMN = milder phenotype. SMA I: 1-2 copies; SMA IV: 4+.

Therapies all target this biology:

Nusinersen (antisense oligonucleotide) and risdiplam (small molecule) modify SMN2 splicing to include exon 7
Onasemnogene abeparvovec (AAV9 gene therapy) replaces SMN1

Muscle and Muscular Dystrophies

Dystrophinopathies were covered under “Cardiomyopathies” above. Summary:

DMD (Xp21) mutations - Duchenne (out-of-frame), Becker (in-frame), X-linked DCM

Myotonic Dystrophies

Myotonic dystrophy type 1 (DM1): CTG trinucleotide repeat expansion in the 3’ UTR of DMPK on 19q13.3. Normal 5-34 repeats; premutation 35-49; affected 50-2000+. Autosomal dominant with anticipation (maternal transmission more likely to expand to congenital form). Most common adult muscular dystrophy.

Clinical: myotonia (delayed relaxation), distal muscle weakness, cataracts, cardiac conduction defects, frontal balding, insulin resistance, testicular atrophy. Mechanism: the expanded mRNA forms toxic nuclear foci that sequester splicing factors (RNA gain-of-function).

Myotonic dystrophy type 2 (DM2): CCTG tetranucleotide repeat expansion in intron 1 of CNBP / ZNF9 on 3q21. Similar phenotype but generally milder, with proximal rather than distal weakness, and less cardiac / cognitive involvement. No congenital form.

Malignant Hyperthermia

RYR1 on 19q13.2 encodes the skeletal muscle ryanodine receptor (calcium release channel in sarcoplasmic reticulum). Gain-of-function mutations cause uncontrolled calcium release in response to volatile anesthetics (halothane, sevoflurane, desflurane) or succinylcholine. Result: sustained muscle contraction, hypermetabolism, hyperthermia (>40C), rhabdomyolysis, hyperkalemia, metabolic acidosis. Autosomal dominant.

Emergency treatment: dantrolene - directly blocks RYR1 calcium channel. Caffeine-halothane contracture test is the gold standard diagnostic.

Hearing Loss

Nonsyndromic hearing loss: GJB2 (connexin 26) on 13q12 is the most common cause. Autosomal recessive. Connexin 26 is a gap junction protein essential for potassium recycling in the cochlea. GJB2 accounts for ~50% of prelingual nonsyndromic hearing loss and ~20% of all genetic hearing loss. 35delG is the most common pathogenic allele in individuals of European ancestry (~70%); 167delT in individuals with Ashkenazi Jewish ancestry. GJB2 testing is first-tier for congenital sensorineural hearing loss. GJB6 (connexin 30) deletions are second most common.

Syndromic hearing loss you should know:

Jervell and Lange-Nielsen: homozygous KCNQ1 - LQTS + congenital sensorineural deafness (AR)
Pendred: SLC26A4 - goitrous hypothyroidism + SNHL with Mondini malformation
Alport: COL4A5 / COL4A3 / COL4A4 - nephritis + SNHL + ocular

Mitochondrial Disease

Mitochondrial DNA (mtDNA) disease has distinct features: maternal inheritance (sperm mitochondria are destroyed after fertilization), heteroplasmy (cells contain mixtures of normal and mutant mtDNA, with variable ratios), and a threshold effect (phenotype appears only when the mutant fraction exceeds a tissue-specific threshold).

Large mtDNA Deletions (Kearns-Sayre and Pearson)

Large-scale mtDNA deletions (most commonly the 4,977 bp “common deletion”) cause both Kearns-Sayre and Pearson syndromes. These are usually sporadic (de novo deletions) rather than maternally inherited.

Pearson syndrome: sideroblastic anemia with ring sideroblasts, vacuolated marrow precursors, pancreatic exocrine insufficiency, lactic acidosis. Presents in infancy. Survivors may evolve into Kearns-Sayre.
Kearns-Sayre: progressive external ophthalmoplegia (onset <20), retinitis pigmentosa, cardiac conduction defects, plus elevated CSF protein, cerebellar ataxia.

mtDNA Point Mutations (MELAS, MERRF)

Disease	Gene	Mutation	tRNA affected	Key feature
MELAS	MT-TL1	m.3243A>G (~80%)	tRNA-Leu	Mitochondrial Encephalomyopathy, Lactic Acidosis, Stroke-like episodes (cortical, not vascular territories). Ragged red fibers on Gomori trichrome
MERRF	MT-TK	m.8344A>G (~80%)	tRNA-Lys	Myoclonic Epilepsy with Ragged Red Fibers, ataxia, myopathy, symmetric cervical lipomatosis

Ragged red fibers reflect subsarcolemmal mitochondrial proliferation - strongly SDH-reactive (complex II, entirely nuclear-encoded) but COX-negative (complex IV, partially mtDNA-encoded).

The same m.3243A>G mutation can also cause MIDD (maternally inherited diabetes and deafness) - phenotypic heterogeneity from variable heteroplasmy.

Nuclear DNA Mutations Causing Mitochondrial Disease

Not all “mitochondrial” disease is mtDNA disease. Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is caused by nuclear TYMP on 22q13.3 (thymidine phosphorylase, autosomal recessive). Thymidine phosphorylase deficiency leads to toxic nucleoside accumulation that impairs mtDNA replication. GI dysmotility dominates (cachexia), plus peripheral neuropathy, ophthalmoparesis, leukoencephalopathy. Elevated plasma thymidine is diagnostic.

Shwachman-Diamond Syndrome

SBDS gene on 7q11, autosomal recessive. Ribosome biogenesis and mitotic spindle function. Second most common inherited cause of exocrine pancreatic insufficiency (after CF). Features: pancreatic lipomatosis, neutropenia, metaphyseal chondrodysplasia, 10-30% lifetime risk of MDS / AML. Distinguished from CF by normal sweat chloride and from Pearson by absence of ring sideroblasts.

Chromosomal Microarray (CMA) and Microdeletion Syndromes

CMA is now the first-tier test for:

Unexplained intellectual disability/developmental delay
Autism spectrum disorder
Multiple congenital anomalies

How it works: Patient DNA is compared to reference DNA, identifying regions where the patient has more or fewer copies than expected. Detects deletions and duplications (copy number variants, CNVs) across the entire genome.

Advantages over karyotype:

Much higher resolution (~50-100 kb vs. 5-10 Mb)
Detects microdeletion/microduplication syndromes that are invisible on karyotype
Diagnostic yield 15-20% in intellectual disability (vs. ~3% for karyotype)

Limitations:

Cannot detect balanced rearrangements (translocations, inversions) - no copy number change
Cannot reliably detect low-level mosaicism (<15-20%)
May detect variants of uncertain significance
Does not replace karyotype when balanced rearrangement is suspected (family history of multiple miscarriages)

Microdeletion Syndromes (Board Grid)

Syndrome	Region	Key features
Cri du chat	5p15.2	High-pitched cat-like cry (laryngeal hypoplasia), microcephaly, round face, intellectual disability
Wolf-Hirschhorn	4p (4p16.3 critical; WHSC1 / WHSC2)	“Greek warrior helmet” facies, severe ID, seizures, cardiac / renal defects
Williams	7q11.23 (includes ELN)	Supravalvar aortic stenosis, elfin facies, hypercalcemia, friendly personality, stellate iris
Miller-Dieker	17p13.3 (LIS1 / PAFAH1B1 + YWHAE)	Classic type I lissencephaly (agyria), severe ID, seizures, characteristic facies
Smith-Magenis	17p11.2 (RAI1)	ID, self-injurious behavior (self-hugging, pulling nails), inverted circadian rhythm
Prader-Willi	15q11.2 paternal	Infantile hypotonia then hyperphagia / obesity, ID, hypogonadism
Angelman	15q11.2 maternal (UBE3A)	Severe ID, absent speech, seizures, ataxic gait, frequent laughter / happy affect
DiGeorge / VCFS	22q11.2 (TBX1 critical)	CATCH-22: conotruncal cardiac, abnormal facies, thymic aplasia, cleft palate, hypocalcemia
Kallmann (X-linked)	Xp22.3 (KAL1, contiguous with STS and SHOX)	Hypogonadotropic hypogonadism + anosmia

Prader-Willi and Angelman are the paradigmatic imprinting disorders. Same chromosomal region (15q11.2), opposite parental origin, opposite syndromes:

PWS: paternal 15q11.2 deletion (70%) or maternal UPD (25%). The PWS region genes are imprinted - only paternal copy expressed.
Angelman: maternal 15q11.2 deletion (70%) or paternal UPD (5%). UBE3A is imprinted - only maternal copy expressed in brain.

Diagnosis is by DNA methylation analysis, which detects all molecular classes (deletion, UPD, imprinting defect, UBE3A mutation for Angelman). FISH alone misses UPD and imprinting defects.

Note on CHARGE syndrome: despite the Anki card text, CHARGE is actually caused by CHD7 mutations (8q12.2), not 22q11.2 deletion. 22q11.2 causes DiGeorge / VCFS. CHARGE: Coloboma, Heart defects, Atresia choanae, Retardation of growth, Genital anomalies, Ear anomalies. Autosomal dominant, mostly de novo. CHD7 is a chromodomain helicase DNA-binding protein involved in chromatin remodeling.

60.4 Pharmacogenomics

Pharmacogenomics applies genetic testing to optimize drug therapy - selecting the right drug at the right dose for each patient based on their genetic makeup. While still emerging, several pharmacogenomic tests have clear clinical utility.

Drug Metabolism: CYP450 Enzymes

The cytochrome P450 enzymes metabolize most drugs. Genetic variants can cause increased or decreased enzyme activity, affecting drug levels and response.

CYP2D6: One of the most polymorphic drug-metabolizing enzymes

Metabolizer phenotypes:

Poor metabolizers (PM): No functional CYP2D6; drug accumulates (toxicity) or prodrug isn’t activated (lack of efficacy)
Intermediate metabolizers (IM): Reduced activity
Normal (extensive) metabolizers (EM): Normal activity
Ultrarapid metabolizers (UM): Multiple copies of active genes; drug cleared too quickly (lack of efficacy) or prodrug over-activated (toxicity)

Clinical examples:

Codeine: Prodrug activated by CYP2D6 to morphine. PMs get no pain relief. UMs may get fatal respiratory depression (deaths reported in children).
Tamoxifen: Prodrug activated to endoxifen. PMs have lower endoxifen levels and potentially reduced breast cancer treatment efficacy.
Antidepressants: Many metabolized by CYP2D6. PMs may need lower doses to avoid toxicity.

CYP2C19: Important for clopidogrel and some psychiatric drugs

Clopidogrel is a prodrug that requires CYP2C19 activation. PMs and IMs have reduced antiplatelet effect, increasing risk of cardiovascular events (especially with coronary stents). Guidelines recommend alternative antiplatelet agents in PMs undergoing percutaneous coronary intervention.

Thiopurine Methyltransferase (TPMT) and Thiopurines

TPMT inactivates thiopurines (azathioprine, 6-mercaptopurine, thioguanine). Patients with low TPMT activity accumulate active metabolites, causing severe, potentially fatal myelosuppression.

Testing before initiating thiopurines is standard of care:

Normal activity: Standard doses
Intermediate activity: Reduce dose by ~50%
Low/absent activity: Use alternative drug or drastically reduced dose

NUDT15 is another enzyme affecting thiopurine toxicity, particularly important in East Asian ancestry populations.

Dihydropyrimidine Dehydrogenase (DPD) and Fluoropyrimidines

DPD (encoded by DPYD) is the rate-limiting enzyme for degradation of 5-fluorouracil and capecitabine. DPD deficiency causes severe, sometimes fatal toxicity (mucositis, diarrhea, myelosuppression, hand-foot syndrome).

Partial DPD deficiency affects ~3-5% of the population. Complete deficiency is rare but nearly uniformly fatal with standard doses. Pre-treatment testing is increasingly recommended, particularly in Europe.

HLA-Associated Drug Hypersensitivity

Certain HLA alleles are strongly associated with severe drug reactions, likely because the drug or its metabolites bind to specific HLA molecules and trigger T-cell responses.

**HLA-B*57:01 and abacavir hypersensitivity**: Abacavir (an HIV medication) causes a hypersensitivity reaction in ~5-8% of patients, which can be fatal on re-challenge. HLA-B57:01 is present in virtually all cases. Testing before initiation is standard of care; abacavir should not be prescribed to HLA-B57:01-positive patients.

**HLA-B*15:02 and carbamazepine/phenytoin SJS/TEN**: In certain East and Southeast Asian ancestry groups, including Han Chinese, Thai, and Malaysian populations, HLA-B*15:02 strongly predicts Stevens-Johnson syndrome and toxic epidermal necrolysis from carbamazepine. Testing is recommended in at-risk populations before initiating these medications.

**HLA-A*31:01 and carbamazepine**: Associated with carbamazepine hypersensitivity in patients with European or Japanese ancestry.

Warfarin Pharmacogenomics

Warfarin dosing is affected by variants in two genes:

CYP2C9 metabolizes the potent S-enantiomer of warfarin (3-5x more potent than R-enantiomer)
CYP2C9 2 and 3 alleles = reduced metabolism, drug accumulates, increased bleeding risk
VKORC1 variants affect sensitivity to warfarin’s target (vitamin K epoxide reductase); some alleles require lower doses to achieve the same anticoagulant effect
CYP2D6 has NO role in warfarin metabolism (common board trap)

FDA-approved pharmacogenomic dosing algorithms incorporate CYP2C9 and VKORC1 genotype along with clinical variables (age, weight, interacting medications). Clinical trials have shown modest benefit in reducing time to stable dosing, though widespread implementation remains debated given the availability of direct oral anticoagulants that don’t require monitoring.

60.5 Identity Testing

STR (Short Tandem Repeat) analysis: Standard for identity testing

Applications:

Parentage testing
Forensic identification
Bone marrow engraftment monitoring

Principle: STRs are highly polymorphic repeating units; combination of multiple STR loci creates unique “DNA fingerprint”

60.6 HLA Typing

HLA molecules present antigens to T cells; matching is crucial for transplantation.

Class I: HLA-A, B, C (on all nucleated cells) Class II: HLA-DR, DQ, DP (on antigen-presenting cells)

Testing methods:

Serology: Historical; low resolution
Molecular: PCR-based; various resolution levels
Low resolution: Broad allele groups
High resolution: Specific alleles (needed for stem cell transplant)

Applications:

Solid organ transplant matching
Stem cell donor selection
Disease associations (HLA-B27 and spondyloarthropathies)

Chapter 61: Variant Interpretation and Reporting

Variant interpretation is where the science of sequencing meets clinical medicine. A sequence change means nothing until it is interpreted in the context of human disease. This chapter covers the systematic frameworks used to classify variants - different systems for germline (inherited) and somatic (tumor) variants - and the tools that inform these classifications.

The core tension to understand: classification is probabilistic, not binary. You are never proving a variant causes disease. You are weighing evidence until the probability crosses a threshold. Two labs with the same data can legitimately land on different classifications, and a variant called VUS today may become Pathogenic in 5 years as evidence accumulates. This is why classification systems are evidence-weighted, not algorithmic.

61.1 Germline Variant Classification: The ACMG/AMP Framework

The American College of Medical Genetics and Genomics (ACMG) and the Association for Molecular Pathology (AMP) published joint guidelines in 2015 that standardized germline variant classification. This framework provides a systematic, evidence-based approach to determining whether a variant causes disease.

The Five-Tier Classification System

Variants are classified into five categories based on the weight of evidence:

Pathogenic: The variant causes disease. There is strong evidence supporting a causal role. Clinical action is warranted based on this finding.

Likely Pathogenic: The evidence strongly favors pathogenicity (>90% certainty that the variant causes disease). For clinical purposes, likely pathogenic variants are generally treated the same as pathogenic variants - they guide clinical management.

Variant of Uncertain Significance (VUS): There is insufficient evidence to classify the variant as pathogenic or benign. This is the most challenging category clinically. VUS results should not be used for clinical decision-making, but patients should be informed because classification may change as evidence accumulates.

Likely Benign: The evidence strongly favors a benign interpretation (>90% certainty the variant does not cause disease). Generally not reported on clinical reports.

Benign: The variant does not cause disease. Common polymorphisms fall into this category. Not reported on clinical reports.

One practical point the guidelines are explicit about: Pathogenic and Likely Pathogenic should be managed identically in the clinic. The label “likely” exists to document uncertainty honestly, not to soften the clinical recommendation. A patient with a Likely Pathogenic BRCA1 variant gets the same surveillance offered to a patient with a Pathogenic BRCA1 variant. Same thing on the benign side: Benign and Likely Benign both mean “do not act on this.”

The Evidence Framework: Criteria for Classification

The ACMG/AMP guidelines define specific criteria, each weighted by strength of evidence. Criteria supporting pathogenicity are labeled P (pathogenic); those supporting benign interpretation are labeled B.

Pathogenic Criteria (Evidence FOR disease causation):

PVS1 (Pathogenic Very Strong): Null variant in a gene where loss of function (LOF) is a known mechanism of disease.

This is the strongest single piece of evidence for pathogenicity. Null variants include:

Nonsense mutations (create premature stop codon)
Frameshift mutations (shift reading frame, usually causing premature stop)
Canonical splice site mutations (±1 or ±2 positions)
Initiation codon mutations
Single or multi-exon deletions

Critical caveats: PVS1 applies only when:

LOF is an established disease mechanism for that gene (not all genes cause disease through LOF)
The variant is not in the last exon or last 50 bp of the penultimate exon (where truncation may not trigger nonsense-mediated decay)
There are no alternative transcripts that would rescue the mutation

Examples of genes where LOF is the canonical disease mechanism:

APC (familial adenomatous polyposis): APC is a tumor suppressor; both alleles must be inactivated (Knudson two-hit). Truncating mutations are present in ~80% of sporadic CRCs and essentially all FAP patients. Non-truncating mutations produce attenuated FAP or Gardner syndrome. The fact that the vast majority of known pathogenic APC variants are truncating is why you can apply PVS1 to a novel APC nonsense or frameshift variant with confidence.
BRCA1 / BRCA2: caretaker tumor suppressors involved in homologous recombination repair. Autosomal dominant with high penetrance. Truncating mutations throughout both genes are pathogenic. Founder truncating mutations in individuals with Ashkenazi Jewish ancestry: BRCA1 185delAG, BRCA1 5382insC, BRCA2 6174delT.
MMR genes (MLH1, MSH2, MSH6, PMS2): Lynch syndrome. LOF is the mechanism. Truncating variants meet PVS1.
CDH1: hereditary diffuse gastric cancer. LOF of E-cadherin disrupts cell adhesion.
VHL: autosomal dominant, 3p25-26. VHL is a tumor suppressor whose product is part of the E3 ubiquitin ligase that degrades HIF. Truncating variants meet PVS1.
TP53, RB1, PTEN, STK11, FLCN, FH, NF1, NF2, TSC1/TSC2, APC, SMAD4, BMPR1A: all classic LOF tumor suppressors.

Where PVS1 does NOT apply cleanly:

Genes that cause disease through gain of function (activating mutations). Examples: RET in MEN2, MET in hereditary papillary RCC, KIT in familial GIST, PRSS1 in hereditary pancreatitis (gain-of-function causes premature trypsinogen activation). A truncating variant in RET would actually be loss-of-function and is not expected to cause MEN2 - it might cause Hirschsprung disease (LOF RET phenotype) but not MEN2. The mechanism matters.
Genes with dominant-negative mechanisms where truncation may rescue rather than cause disease.
Last exon / last 50 bp of penultimate exon: nonsense-mediated decay (NMD) typically rescues these transcripts, so the truncated protein may actually be expressed. Depending on the gene, this may or may not be pathogenic.

PS1 (Pathogenic Strong): Same amino acid change as a previously established pathogenic variant.

If p.Arg248Trp is established pathogenic, and you find p.Arg248Trp in a new patient, that’s strong evidence. However, if you find a different nucleotide change causing the same amino acid change (e.g., different codon for the same substitution), be cautious about splice effects.

Classic example of a recurrent amino acid substitution: BRAF V600E. Valine to glutamic acid at codon 600 causes constitutive kinase activation. It shows up in ~45% of papillary thyroid carcinoma, ~50% of cutaneous melanoma, most hairy cell leukemia, a subset of CRC (methylator pathway), ~5% of lung adenocarcinoma, and many CNS tumors (papillary craniopharyngioma, pleomorphic xanthoastrocytoma, ganglioglioma, epithelioid GBM). Note BRAF V600E is almost always somatic, not germline - so PS1 applies in somatic classification, and in germline analysis of melanoma or thyroid predisposition it would be context-specific.

PS2 (Pathogenic Strong): De novo (confirmed) in a patient with disease and no family history.

A new mutation arising in the patient, not inherited from either parent, is strong evidence when the phenotype matches. Requires confirmation of parentage (both parents tested and confirmed as biological parents).

PS3 (Pathogenic Strong): Well-established functional studies show a deleterious effect.

Functional evidence from validated assays showing the variant disrupts protein function. The strength depends on how well the assay models the disease mechanism. Not all published “functional studies” qualify - the assay must be validated and appropriate.

PS4 (Pathogenic Strong): Prevalence in affected individuals is significantly increased compared to controls.

Statistical association with disease in case-control studies. Typically requires odds ratio >5 with confidence interval not including 1.

PM1 (Pathogenic Moderate): Located in a mutational hotspot or well-established functional domain without benign variation.

Variants in critical protein domains (e.g., active site, DNA-binding domain) are more likely to be damaging. This requires that the domain is truly critical and that benign variants don’t occur there.

Concrete hotspots you should know:

APC mutation cluster region (MCR): codons 1286-1513 in exon 15. This region contains the beta-catenin binding/downregulation domains. Germline APC variants between codons 1250-1464 cause classic FAP with >100 polyps; mutations outside this region tend to cause attenuated FAP.
KIT exon 11 (juxtamembrane domain, autoinhibitory switch): hotspot for activating mutations in GIST. Exon 11 mutations have the best imatinib response (~80%).
KRAS codons 12 and 13: the classic KRAS hotspot. Present in ~95% of pancreatic ductal adenocarcinomas (G12D, G12V, G12R most common). KRAS G12C is the targetable KRAS mutation in NSCLC (sotorasib, adagrasib).
BRAF codon 600: V600E hotspot discussed above.
EGFR exon 19 deletions and L858R in exon 21: sensitizing mutations in lung adenocarcinoma (respond to EGFR TKIs).
RET codon 634 (MEN2A) and M918T (MEN2B): activating germline mutations. M918T in MEN2B is nearly universal and is the most aggressive of the RET mutations, driving prophylactic thyroidectomy by 6 months of age.
IDH1 R132 and IDH2 R140/R172: gliomas, AML.
TP53 DNA-binding domain: residues 175, 245, 248, 249, 273, 282 - the classic p53 hotspots.

PM2 (Pathogenic Moderate): Absent from controls (or extremely low frequency if recessive) in population databases.

A rare variant is more likely to be pathogenic than a common one. “Absent” typically means not present in gnomAD or present at frequency well below disease prevalence. Note: ClinGen has recommended downgrading PM2 to supporting level because many rare variants are benign. Every individual carries ~10,000 variants that are rare; most are not pathogenic.

PM3 (Pathogenic Moderate): For recessive disorders, detected in trans with a pathogenic variant.

If a patient has a known pathogenic variant on one chromosome and a candidate variant on the other, and they have the recessive disease, that’s evidence the candidate variant is pathogenic. Requires confirmation of phase (trans, not cis).

PM4 (Pathogenic Moderate): Protein length changes due to in-frame deletions/insertions or stop-loss variants.

In-frame changes that alter protein length in a non-repeat region suggest functional impact. Less definitive than frameshift (which is PVS1).

PM5 (Pathogenic Moderate): Novel missense at a residue where a different pathogenic missense has been seen.

If p.Arg248Trp is pathogenic, and you find p.Arg248Gln (different amino acid change at the same position), that’s moderate evidence. The position is clearly important.

PM6 (Pathogenic Moderate): Assumed de novo without confirmation of paternity/maternity.

Like PS2 but without formal parentage confirmation. Weaker evidence.

PP1 (Pathogenic Supporting): Co-segregation with disease in multiple affected family members.

The variant tracks with disease in a family. Strength can be upgraded based on the number of meioses (more informative segregations = stronger evidence). This is why cascade testing of first-degree relatives is central to Lynch syndrome and hereditary breast/ovarian cancer evaluation: it both generates segregation evidence and identifies at-risk relatives who need surveillance.

PP2 (Pathogenic Supporting): Missense in a gene with low rate of benign missense variation where missense is a common mechanism of disease.

Some genes are highly intolerant of missense variation (e.g., genes with Z-score >3 for missense constraint in gnomAD). These tend to be dosage-sensitive genes or genes encoding proteins with tightly constrained structure. Example: missense variants in RET (a gain-of-function oncogene where specific missenses cause MEN2) are weighed differently than missense in PTEN (a tumor suppressor where truncating is more common).

PP3 (Pathogenic Supporting): Multiple computational (in silico) predictions support a deleterious effect.

Algorithms predict the variant is damaging. See section on in silico tools below. This is supporting evidence only - computational predictions alone cannot classify a variant.

PP4 (Pathogenic Supporting): Patient’s phenotype or family history is highly specific for a disease with a single genetic etiology.

A classic presentation of a rare monogenic disease increases the prior probability that any variant found in the causative gene is pathogenic. Examples of phenotypes that raise prior probability:

Hundreds to thousands of adenomatous polyps carpeting the colon by adolescence - points to APC/FAP. With this clinical picture, a novel APC variant gets a PP4 bump.
Diffuse (signet ring cell) gastric adenocarcinoma diagnosed at a young age (median ~38) - points to CDH1. Lobular breast cancer in the same family strengthens the case.
Multiple atypical nevi (>50) + melanoma + pancreatic cancer in the family - points to CDKN2A (familial atypical mole-melanoma, FAMMM).
Cutaneous leiomyomas + uterine leiomyomas + aggressive type 2 papillary RCC - points to FH (HLRCC). The uterine leiomyomas in HLRCC are atypical with prominent red/orange nucleoli.
Renal mass + spontaneous lower-lobe pneumothorax + facial fibrofolliculomas - points to FLCN (Birt-Hogg-Dube).
Bilateral, multifocal clear cell RCC + CNS/retinal hemangioblastoma + pheochromocytoma - points to VHL.
Medullary thyroid carcinoma + pheochromocytoma + mucosal neuromas + marfanoid habitus - points to RET/MEN2B.
Mucocutaneous pigmentation + hamartomatous intestinal polyps - points to STK11/Peutz-Jeghers.

PP5 (Pathogenic Supporting): Reputable source reports variant as pathogenic.

ClinVar submissions, literature reports. ClinGen has recommended removing PP5/BP6 from active use because they risk circular reasoning.

Benign Criteria (Evidence AGAINST disease causation):

BA1 (Benign Stand-Alone): Allele frequency >5% in any general population database.

A common variant is almost certainly benign for rare Mendelian disease. This single criterion is sufficient to classify as benign. The logic is arithmetic: if a Mendelian disease has a prevalence of 1 in 10,000 and a variant is present in 5% of the general population, the variant cannot be the cause of that disease in any meaningful sense.

BS1 (Benign Strong): Allele frequency greater than expected for the disorder.

Frequency is higher than compatible with disease prevalence and penetrance. For a dominant disease with prevalence 1/10,000 and full penetrance, a variant frequency of 1% would be incompatible with causation.

BS2 (Benign Strong): Observed in a healthy adult individual for a fully penetrant dominant condition or in the homozygous state for a recessive condition.

If the variant is found in healthy individuals who should have disease (based on the inheritance pattern and penetrance), it’s likely benign.

BS3 (Benign Strong): Well-established functional studies show no deleterious effect.

The inverse of PS3. Validated functional assays show normal protein function.

BS4 (Benign Strong): Lack of segregation in affected family members.

The variant doesn’t track with disease in a family (affected individuals don’t have it, or unaffected individuals do).

BP1 (Benign Supporting): Missense variant in a gene where only truncating variants cause disease.

Some genes only cause disease through LOF (truncating variants). Missense variants in these genes are less likely to be pathogenic. The practical question: does the variant type match the known disease mechanism for the gene?

In APC, pathogenic variants are overwhelmingly truncating. A novel missense in APC is less likely to be pathogenic - BP1 may apply.
In RET (MEN2), the opposite is true: specific missense mutations cause MEN2. A truncating RET variant wouldn’t cause MEN2 (it might cause Hirschsprung).
In CDH1, truncating variants are the most common pathogenic type, though some missense variants disrupting E-cadherin function are also pathogenic.
Know the gene’s mechanism before weighing variant type.

BP2 (Benign Supporting): Observed in trans with a pathogenic variant for a dominant disorder, or in cis with a pathogenic variant.

For dominant conditions: if a patient has a known pathogenic variant causing their disease, a second variant in the same gene (in trans) is less likely to also be pathogenic - one pathogenic variant is sufficient.

BP3 (Benign Supporting): In-frame insertion/deletion in a repetitive region without known function.

Changes in non-functional repetitive sequences are less likely to be damaging.

BP4 (Benign Supporting): Multiple computational predictions suggest no impact.

The inverse of PP3. In silico tools predict the variant is benign/tolerated.

BP5 (Benign Supporting): Found in a case with an alternate molecular basis for disease.

If the patient’s disease is explained by a different variant, an additional VUS is less likely to be pathogenic.

BP6 (Benign Supporting): Reputable source reports variant as benign.

BP7 (Benign Supporting): Synonymous variant with no predicted splice impact and not highly conserved.

Combining Criteria: Rules for Classification

The ACMG/AMP guidelines provide rules for combining evidence:

Pathogenic:

1 Very Strong (PVS1) AND ≥1 Strong (PS1-PS4), OR
1 Very Strong AND ≥2 Moderate (PM1-PM6), OR
1 Very Strong AND 1 Moderate AND 1 Supporting (PP1-PP5), OR
1 Very Strong AND ≥2 Supporting, OR
2 Strong, OR
1 Strong AND ≥3 Moderate, OR
1 Strong AND 2 Moderate AND ≥2 Supporting, OR
1 Strong AND 1 Moderate AND ≥4 Supporting

Likely Pathogenic:

1 Very Strong AND 1 Moderate, OR
1 Strong AND 1-2 Moderate, OR
1 Strong AND ≥2 Supporting, OR
≥3 Moderate, OR
2 Moderate AND ≥2 Supporting, OR
1 Moderate AND ≥4 Supporting

Benign:

1 Stand-Alone (BA1), OR
≥2 Strong (BS1-BS4)

Likely Benign:

1 Strong AND 1 Supporting, OR
≥2 Supporting

VUS: Does not meet criteria for pathogenic/likely pathogenic or benign/likely benign

61.2 Somatic Variant Classification: The AMP/ASCO/CAP Tier System

Somatic (tumor) variants require a different classification framework than germline variants. The question isn’t “does this cause an inherited disease?” but rather “what is the clinical significance of this mutation in this cancer?”

The Association for Molecular Pathology (AMP), American Society of Clinical Oncology (ASCO), and College of American Pathologists (CAP) published guidelines in 2017 establishing a four-tier system for classifying somatic variants based on their clinical actionability.

The Four Tiers

Tier I: Variants of Strong Clinical Significance

These variants have established clinical utility - they inform diagnosis, prognosis, or treatment selection based on strong evidence.

Level A Evidence: Variant predicts response or resistance to FDA-approved therapies, or is included in professional guidelines for this tumor type.

Examples worth knowing in depth:

EGFR exon 19 deletions or L858R in lung adenocarcinoma: predicts response to EGFR TKIs. First-line is osimertinib (3rd generation, also active against the T790M resistance mutation that emerges on earlier TKIs). Exon 20 insertions are resistant to standard TKIs and require amivantamab or mobocertinib.
BRAF V600E in melanoma: predicts response to BRAF + MEK inhibitor combinations (dabrafenib + trametinib, or encorafenib + binimetinib). Never give BRAF monotherapy - paradoxical MAPK activation in BRAF wild-type cells leads to secondary cutaneous SCC. Always combine with MEK inhibitor.
BCR-ABL1 in CML: diagnostic AND predicts response to TKIs (imatinib, dasatinib, nilotinib, bosutinib, ponatinib).
KRAS mutations in colorectal cancer: predicts RESISTANCE to anti-EGFR antibodies (cetuximab, panitumumab). Extended RAS testing is mandatory before cetuximab (KRAS exons 2-4, NRAS exons 2-4). BRAF V600E in CRC also predicts poor anti-EGFR response.
HER2 amplification in breast / gastric / GEJ adenocarcinoma: predicts trastuzumab response. 15-20% of breast cancers, ~20% of gastric, ~30% of GEJ. IHC 3+ or FISH-positive (ratio >=2.0).
KIT exon 11 mutations in GIST: best imatinib response (~80%). Exon 9 needs higher dose.
ALK rearrangements in NSCLC: ~2-5%, usually EML4-ALK from inversion on chromosome 2. First-line alectinib or lorlatinib.
MSI-H / dMMR (tissue-agnostic): predicts response to anti-PD1 (pembrolizumab, nivolumab). Approved regardless of tumor histology - the first true tumor-agnostic cancer approval.
NTRK fusions (tissue-agnostic): larotrectinib, entrectinib.
KRAS G12C in NSCLC: sotorasib, adagrasib. First approved direct KRAS inhibitors.

Level B Evidence: Variant predicts response to therapy based on well-powered studies with expert consensus, but not yet FDA-approved for this indication.

Examples:

BRAF V600E in colorectal cancer (off-label use of BRAF inhibitors in combination)
NTRK fusions in tumors other than the FDA-approved indications

Tier II: Variants of Potential Clinical Significance

These variants may be clinically significant but have less definitive evidence.

Level C Evidence: FDA-approved therapy for a different tumor type, or investigational therapies with clinical trials available.

Example: A rare kinase fusion identified in a tumor type where it hasn’t been specifically studied, but TKI therapy is available in clinical trials.

Level D Evidence: Preclinical data (in vitro or animal models) or limited case reports suggesting potential significance.

Example: A novel EGFR mutation with in vitro sensitivity data but no clinical experience.

Tier III: Variants of Unknown Significance

The somatic equivalent of VUS. The variant is detected but there is insufficient evidence to determine clinical relevance.

Characteristics:

Not observed at significant frequency in tumor databases (COSMIC)
No published evidence of association with cancer behavior or therapy response
May be a passenger mutation (present in the tumor but not driving its behavior)

Most detected variants fall into this category. They are reported but should not drive clinical decisions.

Tier IV: Benign or Likely Benign Variants

Variants with evidence suggesting they are not oncogenic - likely germline polymorphisms or passenger mutations with no functional consequence.

Characteristics:

Observed at significant allele frequency in general population (gnomAD)
No evidence of association with cancer
Common polymorphisms

These are typically not reported, or reported only to document that testing was performed at that position.

Key Differences from Germline Classification

Context matters: The same variant can have different significance in different tumor types. BRAF V600E is Tier I in melanoma but has different implications in colorectal cancer. Specifically:

BRAF V600E in melanoma: Tier I, predicts response to BRAF + MEK inhibitors.
BRAF V600E in metastatic CRC: predicts poor prognosis AND poor response to anti-EGFR monotherapy. But there IS a regimen for BRAF V600E metastatic CRC: encorafenib + cetuximab (off-label in some settings, on-label in others). Also strongly predicts methylator pathway / sporadic MSI-H, and effectively rules out Lynch syndrome (>99% NPV when BRAF V600E is present in a tumor with MLH1/PMS2 loss on IHC).
BRAF V600E in NSCLC: Tier I (dabrafenib + trametinib approved).
BRAF V600E in hairy cell leukemia: defining mutation.
BRAF V600E in thyroid: prognostic (aggressive features) and predictive (dabrafenib + trametinib in radioiodine-refractory disease).

Same variant. Five different tumor types. Five different interpretations and management pathways.

Actionability is central: The somatic tier system focuses on clinical utility - does this variant change what we do for the patient?

Passenger vs. driver: Many somatic mutations are passengers (bystanders that don’t contribute to cancer behavior). The challenge is distinguishing them from drivers. In pancreatic cancer, by the time a tumor is detectable, it has accumulated ~63 genetic alterations. Most are passengers. KRAS (~95%), TP53 (~70%), CDKN2A (~90%), and SMAD4 (~50%) are the known drivers; the rest are mostly noise.

Allele frequency interpretation differs: In germline testing, we expect 50% (heterozygous) or 100% (homozygous). In tumors, variant allele frequency reflects tumor purity, copy number, and clonal architecture. A 5% VAF might represent a subclonal mutation or a mutation in a low-purity sample. A VAF near 100% in a tumor with LOH suggests the second allele was lost (Knudson’s second hit). Always check tumor purity on the report before interpreting VAF.

Mutual exclusivity of driver mutations: an important concept for variant interpretation. In most solid tumors, driver mutations in the same pathway are mutually exclusive because once the pathway is activated, a second hit on the same pathway confers no further fitness advantage:

In NSCLC adenocarcinoma: EGFR, KRAS, ALK, BRAF, ROS1 are mutually exclusive drivers. Finding one essentially rules out the others.
In CRC: BRAF and KRAS are mutually exclusive. BRAF V600E mutation is the hallmark of the methylator/CIMP pathway; KRAS is the hallmark of the APC/CIN pathway.
In melanoma: BRAF V600E and NRAS are mutually exclusive.
In thyroid: BRAF V600E, RAS mutations, and RET rearrangements are mutually exclusive.
Non-mutual-exclusivity is a clue that one of the variants may be a passenger or subclonal.

61.3 Hereditary Cancer Syndromes - Variant Interpretation in Context

Board variant-interpretation questions are almost always framed around hereditary cancer syndromes. The key skill is matching a clinical presentation to the right gene, choosing the right test, and then interpreting a specific variant in the context of the patient’s phenotype. This section walks through the high-yield syndromes organized by where you’d encounter them clinically.

Lynch Syndrome (HNPCC) - The Canonical Case Study

Lynch syndrome = germline mutation in an MMR gene (MLH1, MSH2, MSH6, PMS2) or EPCAM deletion that silences MSH2 by promoter methylation. Autosomal dominant. This is responsible for ~5% of all colorectal cancers and is the most common inherited CRC syndrome.

Why it matters for variant interpretation: Lynch exemplifies the whole workflow - phenotype recognition, tumor screening, variant detection, classification, and cascade testing.

MMR protein biology you need for interpretation:

MMR proteins correct base-base mismatches and insertion-deletion loops missed by polymerase proofreading during replication.
They work as heterodimers: MSH2/MSH6 (MutS-alpha) recognizes mismatches; MLH1/PMS2 (MutL-alpha) coordinates repair. MSH2/MSH3 (MutS-beta) handles insertion/deletion loops.
MLH1 is the obligate partner for PMS2: when MLH1 is lost, PMS2 is unstable and also degrades. The reverse is not true - PMS2 can be lost while MLH1 remains.
MSH2 is the obligate partner for MSH6: MSH2 loss causes MSH6 degradation. MSH6 can be lost alone.

This biology drives IHC interpretation patterns:

IHC pattern	Interpretation
Loss of MLH1 + PMS2	MLH1 mutation OR sporadic MLH1 promoter methylation - test BRAF V600E and/or MLH1 methylation to distinguish
Loss of MSH2 + MSH6	MSH2 germline mutation (or EPCAM deletion) - proceed to germline testing
Isolated PMS2 loss (MLH1 intact)	PMS2 germline mutation - lowest penetrance of the four Lynch genes
Isolated MSH6 loss (MSH2 intact)	MSH6 germline mutation - higher endometrial cancer risk than CRC

The BRAF / MLH1 methylation step is critical: MLH1 promoter methylation and BRAF V600E are mutually exclusive with germline MLH1 mutations. If a tumor has MLH1/PMS2 loss on IHC AND BRAF V600E, it’s sporadic methylator pathway, not Lynch. If BRAF is wild-type and no MLH1 methylation is seen, suspicion for Lynch is high and the patient should proceed to germline MLH1 sequencing. This is the most cost-effective first step after IHC.

Histology clues that should trigger MMR workup even without universal screening:

Right-sided colon tumor
Mucinous or medullary features
Tumor-infiltrating lymphocytes (TILs)
Crohn-like peritumoral lymphoid reaction
Pushing (rather than infiltrative) tumor margins

Variant interpretation in MMR genes:

Truncating variants (nonsense, frameshift, canonical splice): PVS1, almost always pathogenic given established LOF mechanism.
Missense variants: trickier. Functional assays may be needed. Some classic missense pathogenic variants exist (e.g., MLH1 p.Val384Asp) but many MMR missense variants end up as VUS.
~5% of Lynch patients have large genomic rearrangements (deletions) not detected by sequencing - MLPA or equivalent is needed. If Lynch is clinically suspected but sequencing is negative, add deletion/duplication analysis.
Discordance between IHC and MSI testing is possible: missense mutations that produce non-functional but antigenically intact protein can give MSI-H with normal IHC. Concordance is ~95%.

Penetrance and cancer risk by gene:

MLH1 and MSH2: highest penetrance. Lifetime CRC risk 40-80%. Endometrial cancer risk 40-60% (females).
MSH6: lower CRC risk, higher endometrial cancer risk.
PMS2: lowest penetrance (~15-20% lifetime CRC risk).
Extracolonic cancers: endometrial, ovarian, urinary tract, gastric, small bowel, hepatobiliary, sebaceous (Muir-Torre syndrome = Lynch with prominent sebaceous neoplasms and keratoacanthomas), CNS (MMR-associated Turcot syndrome with glioblastoma).

Board pearl for Turcot syndrome: APC-associated Turcot has medulloblastoma; MMR-associated Turcot has glioblastoma multiforme. Different brain tumor, different gene.

Cascade testing: Once a specific pathogenic MMR variant is identified in a proband, relatives are offered targeted testing for that specific variant, not full gene sequencing. Relatives who test positive need intensive surveillance (colonoscopy every 1-2 years starting at age 20-25 or 5 years before the earliest CRC in the family, endometrial surveillance). If the family mutation is unknown, at-risk relatives need full panel sequencing.

FAP and APC Variants

Familial adenomatous polyposis: germline APC mutation, autosomal dominant. By age 20, at least one adenoma; by 40, over 100 adenomatous polyps. Without colectomy, nearly 100% CRC risk by age 50.

APC variant interpretation specifics:

Most APC mutations are truncating (nonsense or frameshift). PVS1 applies broadly.
Genotype-phenotype correlation: mutations between codons 1250-1464 cause classic FAP; mutations outside this region tend to cause attenuated FAP (<100 polyps, later onset).
Extracolonic manifestations (all from the same APC locus): desmoid tumors (the leading cause of death after prophylactic colectomy), duodenal/ampullary adenomas, hepatoblastoma (childhood), thyroid cancer (papillary, cribriform-morular variant), CHRPE (congenital hypertrophy of retinal pigment epithelium).
Gardner syndrome = FAP + epidermoid cysts + osteomas (mandibular/maxillary) + desmoids + dental anomalies + CHRPE. Same APC mutations - no separate gene. Considered part of the FAP phenotypic spectrum.
APC-associated Turcot = FAP + medulloblastoma.

MUTYH is the flip side: a base-excision repair gene causing autosomal recessive polyposis. Biallelic MUTYH mutations cause MAP (MUTYH-associated polyposis) - a milder polyposis phenotype that looks like attenuated FAP but inherits as autosomal recessive. Board trap: polyposis with a family pedigree that doesn’t fit autosomal dominant - think MUTYH.

Hereditary Breast and Ovarian Cancer - BRCA1 and BRCA2

BRCA1 (17q21) and BRCA2 (13q12.3): autosomal dominant, high penetrance, caretaker tumor suppressors involved in homologous recombination repair of DNA double-strand breaks. Without BRCA, cells rely on error-prone NHEJ, leading to genomic instability.

Penetrance estimates (know these):

Lifetime breast cancer risk: BRCA1 ~65-72%, BRCA2 ~45-55%.
Lifetime ovarian cancer risk: BRCA1 ~39-44%, BRCA2 ~11-17%.
Male breast cancer in BRCA2: ~6-7% lifetime risk. Lower in BRCA1.
BRCA2 also strongly associated with pancreatic and prostate cancer.

BRCA1-associated breast cancer histology (distinctive, though not specific):

High nuclear grade, invasive ductal (no special type)
Pushing margins, lymphocytic infiltrate, geographic necrosis
Triple negative (ER-/PR-/HER2-), basal-like molecular subtype (CK5/6+, EGFR+)
Medullary-like features

BRCA2-associated breast cancer: no distinctive histology. Tends to be ER-positive, more like sporadic breast cancer morphologically.

Ashkenazi Jewish founder mutations: 25% of women with Ashkenazi Jewish ancestry and breast cancer harbor BRCA mutations. Three founder mutations account for ~90%:

BRCA1 185delAG
BRCA1 5382insC
BRCA2 6174delT

In patients with Ashkenazi Jewish ancestry, targeted founder testing is often the first step, much cheaper than full sequencing.

Variant interpretation in BRCA:

Both genes are large with thousands of variants described - VUS rates are substantial, especially for BRCA2.
Truncating variants dominate the pathogenic class. PVS1 applies.
Missense variants that disrupt the BRCA1 BRCT domain or BRCA2 DNA-binding domain can be pathogenic but functional data is often needed.
ENIGMA (Evidence-based Network for the Interpretation of Germline Mutant Alleles) is an expert panel in ClinVar that provides high-quality BRCA1/2 classifications.
PARP inhibitors (olaparib, niraparib, rucaparib, talazoparib) cause synthetic lethality in BRCA-deficient cells: they can’t repair DNA by either HR (BRCA deficient) or base excision repair (PARP inhibited) - cell death. Pathogenic BRCA variants make patients candidates for PARP inhibitor therapy in ovarian, breast, prostate, and pancreatic cancers.

Other breast cancer predisposition genes you should know: TP53 (Li-Fraumeni), PTEN (Cowden), CDH1 (hereditary diffuse gastric + lobular breast), STK11 (Peutz-Jeghers), PALB2 (emerging as the third major BRCA-like gene), ATM, CHEK2 (moderate risk). Multi-gene panel testing is standard for hereditary breast cancer evaluation now - testing only BRCA1/2 misses ~50% of actionable mutations.

Other High-Yield Cancer Predisposition Syndromes

Peutz-Jeghers (STK11/LKB1 on 19p13.3): autosomal dominant, mucocutaneous melanotic macules (lips, buccal mucosa, digits - buccal ones persist; others fade after puberty) + hamartomatous GI polyps with characteristic arborizing smooth muscle pattern. STK11 is a serine/threonine kinase that activates AMPK, inhibiting mTOR. Loss -> constitutive mTOR activation. Lifetime cancer risk ~85%. >100-fold increased risk of pancreatic adenocarcinoma. Unique tumors: sex cord tumor with annular tubules (SCTAT) of ovary, adenoma malignum (minimal deviation adenocarcinoma) of cervix, calcifying Sertoli cell tumor of testis.

Juvenile polyposis syndrome: autosomal dominant. SMAD4 (18q21) or BMPR1A (10q23) mutations. SMAD4 mutations can also cause hereditary hemorrhagic telangiectasia (HHT-JPS overlap). Moderately increased CRC risk.

Hereditary diffuse gastric cancer (CDH1 on 16q22): autosomal dominant. Diffuse signet ring cell gastric adenocarcinoma at young age (median ~38). Lifetime gastric cancer risk 70-80% in men, 56-83% in women. Also lobular breast cancer in women. Prophylactic total gastrectomy recommended for mutation carriers. CDH1 encodes E-cadherin; loss disrupts cell-cell adhesion (explains discohesive signet ring morphology).

Li-Fraumeni (TP53): autosomal dominant. Broad cancer spectrum: sarcoma, breast, brain, adrenocortical, leukemia. Classic criteria: proband with sarcoma < 45 + first-degree relative with any cancer < 45 + second affected relative. TP53 is the most frequently mutated gene in human cancer overall, but germline mutations are rare.

Cowden syndrome (PTEN): autosomal dominant. Hamartomas, macrocephaly, increased risk of breast, thyroid (follicular), endometrial, colon, and renal cancers. Trichilemmomas on face.

VHL (3p25-26): autosomal dominant. Clear cell RCC (bilateral, multifocal), hemangioblastomas (CNS, retina), pheochromocytoma, pancreatic cysts/neuroendocrine tumors, endolymphatic sac tumors. VHL encodes a component of the E3 ubiquitin ligase that degrades HIF - loss leads to HIF accumulation and VEGF/PDGF/EPO/TGF-alpha upregulation. This same gene is somatically inactivated in ~80-90% of sporadic clear cell RCC.

Other RCC syndromes:

Birt-Hogg-Dube (FLCN, 17p11.2): fibrofolliculomas + spontaneous lower-lobe pneumothorax + chromophobe/oncocytic RCC
Hereditary papillary RCC (MET, 7q31): Type 1 papillary RCC, gain-of-function MET mutations
HLRCC (FH, 1q43): cutaneous + uterine leiomyomas + aggressive type 2 papillary RCC. FH encodes fumarate hydratase; fumarate accumulation inhibits prolyl hydroxylase, stabilizing HIF (similar downstream effect to VHL loss).
Familial clear cell RCC: constitutional 3p translocations without other VHL features.

MEN2 (RET, 10q11.2): autosomal dominant, gain-of-function germline mutations. Three phenotypes:

MEN2A: medullary thyroid carcinoma + pheochromocytoma + parathyroid hyperplasia. Most common mutation: codon 634.
MEN2B: medullary thyroid carcinoma + pheochromocytoma + mucosal neuromas + marfanoid habitus. Mutation: M918T, nearly universal.
FMTC: medullary thyroid carcinoma only.
Testing drives timing of prophylactic thyroidectomy: MEN2B by age 6 months; MEN2A by age 5. Specific codon matters.

FAMMM (CDKN2A/p16, 9p21): autosomal dominant. Multiple atypical nevi (>50) + melanoma + 50-fold increased pancreatic adenocarcinoma risk. CDKN2A encodes p16 (CDK4/6 inhibitor) and p14ARF (p53 stabilizer). CDKN2A is also the most commonly deleted tumor suppressor in human cancers overall.

Hereditary pancreatitis (PRSS1 or SPINK1): 50-fold increased pancreatic adenocarcinoma risk. PRSS1 gain-of-function mutations cause premature trypsinogen activation in the pancreas. R122H prevents trypsin autolysis.

BAP1 tumor predisposition syndrome: uveal melanoma + mesothelioma + RCC + cutaneous melanocytic tumors. BAP1 (3p21) is also prognostically important in uveal melanoma - BAP1 loss by IHC predicts metastasis (liver).

61.4 In Silico Prediction Tools

Computational (in silico) tools predict the functional impact of variants based on sequence conservation, protein structure, and other features. These predictions provide supporting evidence but are never sufficient alone to classify a variant.

Missense Prediction Tools

SIFT (Sorting Intolerant From Tolerant): Predicts whether an amino acid substitution affects protein function based on sequence homology.

Score range: 0-1
Score <0.05: Predicted deleterious (“damaging”)
Score ≥0.05: Predicted tolerated
Principle: If an amino acid position is conserved across evolution, substitutions are likely damaging

PolyPhen-2 (Polymorphism Phenotyping v2): Predicts impact using sequence and structure-based features.

Score range: 0-1
0.85: Probably damaging
0.15-0.85: Possibly damaging
<0.15: Benign
Principle: Combines conservation with structural features (if known) like proximity to active sites

CADD (Combined Annotation Dependent Depletion): Integrates multiple annotations into a single deleteriousness score.

Score: Phred-scaled (higher = more deleterious)
Score ≥20: Top 1% most deleterious (often used as threshold)
Score ≥30: Top 0.1%
Principle: Machine learning model trained to distinguish known pathogenic variants from simulated de novo variants

REVEL (Rare Exome Variant Ensemble Learner): Ensemble method combining 13 individual tools, optimized for rare variants.

Score range: 0-1
ClinGen-recommended thresholds for use with ACMG criteria:
Score >0.644: Supporting pathogenic (PP3)
Score >0.773: Moderate pathogenic
Score >0.932: Strong pathogenic
Score <0.290: Supporting benign (BP4)
Score <0.183: Moderate benign
Score <0.016: Strong benign
Principle: Ensemble methods reduce the weaknesses of individual tools

MutationTaster: Predicts disease-causing potential using multiple features including conservation, splice site changes, and protein features.

Splicing Prediction Tools

SpliceAI: Deep learning model that predicts splice alterations from sequence alone.

Outputs probability scores (0-1) for:
Donor gain: Creation of new donor site
Donor loss: Disruption of existing donor site
Acceptor gain: Creation of new acceptor site
Acceptor loss: Disruption of existing acceptor site
Score >0.5: High probability of splice effect
Score >0.8: Very high confidence
Principle: Neural network trained on RNA-seq data to recognize splice consequences

MaxEntScan: Calculates the strength of splice sites using maximum entropy modeling.

Compares wild-type and mutant splice site scores
Score decrease suggests weakening of splice site
Principle: Models the consensus sequences at splice junctions

Conservation Scores

PhyloP: Measures conservation at individual nucleotides based on evolutionary rate.

Positive scores: Conservation (slower evolution than expected)
Negative scores: Acceleration (faster evolution than expected)
Score >2: Strong conservation
Principle: Conserved positions are under purifying selection because they’re functionally important

PhastCons: Predicts probability that a nucleotide belongs to a conserved element.

Score range: 0-1
Score >0.9: High probability of conservation
Principle: Uses hidden Markov model to identify conserved regions

GERP (Genomic Evolutionary Rate Profiling): Measures “rejected substitutions” - the difference between expected and observed mutations.

Higher scores = more conservation
Score >2: Often used as threshold for conservation

Best Practices for Using In Silico Tools

Use multiple tools: No single tool is reliable enough alone. Concordance among multiple tools increases confidence.

Understand the limitations:

Tools are trained on known variants, so novel variant types may not be well-predicted
Performance varies by gene and variant type
Missense tools don’t assess splice effects; splice tools don’t assess protein function

Context matters: A variant predicted “benign” by tools but occurring at a known critical residue should be viewed with suspicion. A variant predicted “damaging” but common in the population is likely benign regardless of prediction.

Supporting evidence only: In silico predictions are supporting (PP3/BP4) level evidence in ACMG/AMP framework - they cannot drive classification alone.

61.5 Key Databases for Variant Interpretation

gnomAD (Genome Aggregation Database): The primary resource for population allele frequencies.

Contains >140,000 exomes and >76,000 genomes
Provides allele frequencies by genetic ancestry/population labels used by the database (African/African American, European, East Asian, South Asian, Latino/Admixed American, etc.)
Critical for applying BA1, BS1, PM2 criteria
Also provides constraint metrics (pLI, Z-scores) indicating how intolerant a gene is to various mutation types

ClinVar: NIH database of variants and their clinical significance.

Aggregates interpretations from clinical laboratories
Shows submitter, assertion criteria, and review status
Gold star system indicates level of review:
1 star: Single submitter
2 stars: Multiple submitters with no conflicts
3 stars: Expert panel review
4 stars: Practice guideline
Critical for determining prior classifications

COSMIC (Catalogue of Somatic Mutations in Cancer): Database of somatic mutations.

Records mutations found in tumors with tumor type annotation
Useful for distinguishing oncogenic drivers from passengers
Provides frequency data for somatic variants
A variant observed hundreds of times in a specific tumor type (e.g., BRAF V600E in melanoma) has a strong prior toward being a driver. A variant seen once in an unrelated tumor is likely a passenger.
COSMIC Cancer Gene Census is a curated list of genes with established roles in cancer - the starting point for deciding whether a variant is in a plausible oncogene or tumor suppressor.

OMIM (Online Mendelian Inheritance in Man): Database of genes and genetic disorders.

Describes gene-disease relationships
Specifies inheritance patterns
Links to literature
Essential for understanding what phenotypes are associated with variants in a gene

ClinGen: Clinical Genome Resource.

Provides expert-curated gene-disease validity assessments
Publishes variant-specific classifications from Expert Panels
Develops and refines variant interpretation guidelines
Dosage sensitivity maps

61.6 Molecular Pathology Report Interpretation

A molecular pathology report should contain all information needed for clinical decision-making while being interpretable by non-geneticist clinicians.

Essential elements:

Patient and specimen information: Demographics, specimen type, collection date, tumor percentage (for oncology).

Test methodology: What technology was used (NGS, Sanger, FISH)? What genes/regions were tested? What is the reportable range? What is the analytical sensitivity (limit of detection)?

Results: Variants detected with complete HGVS nomenclature at both DNA and protein levels. Classification (pathogenic, VUS, etc.). Variant allele frequency for somatic testing.

Interpretation: What do the results mean for this patient? This should translate molecular findings into clinical implications - does this affect diagnosis, prognosis, treatment selection, or testing of family members?

Limitations: What could the test miss? Regions with low coverage, types of variants not detected by the methodology (e.g., large deletions may be missed by NGS), variants of uncertain significance.

Recommendations: Should family members be tested? Is genetic counseling indicated? Are there clinical trial options (for oncology)?

Understanding limitations is crucial: A negative result doesn’t mean no mutations exist - it means none were detected within the test’s technical limitations and in the regions examined. A VUS doesn’t mean the patient doesn’t have a pathogenic variant - it means we can’t yet determine whether this particular variant is disease-causing.

61.7 Biomarker Testing for Immune Checkpoint Inhibitors

Immunotherapy is reshaping oncology, and the biomarkers that predict response are now a core part of variant interpretation and reporting. The reader should understand both the tests and the conceptual framework tying them together.

The underlying biology: Immune checkpoints are inhibitory pathways that prevent autoimmunity by checking T-cell activation against self-antigens.

CTLA-4 (on T-cells): competes with CD28 for B7 ligand on APCs, inhibiting T-cell activation in lymph nodes. Early checkpoint.
PD-1 (on T-cells): binds PD-L1 on tumor cells, inhibiting T-cell effector function in the tumor microenvironment. Late checkpoint.

Tumors exploit these to evade immune destruction. Blocking the checkpoints unleashes the pre-existing antitumor immune response.

Checkpoint inhibitor agents (memorize the categories):

Agent	Target	Notable indications
Ipilimumab	CTLA-4	Melanoma (first ICI approved)
Pembrolizumab	PD-1	Tissue-agnostic for MSI-H/dMMR, NSCLC, many others
Nivolumab	PD-1	Melanoma, NSCLC, RCC, HL, many others
Atezolizumab	PD-L1	NSCLC, bladder, TNBC, SCLC
Avelumab	PD-L1	Merkel cell carcinoma, urothelial
Durvalumab	PD-L1	NSCLC (post-chemoradiation), SCLC

CTLA-4 inhibition affects early T-cell priming (broader effect, more autoimmune toxicity). PD-1/PD-L1 inhibition affects effector function at the tumor (more targeted). Combination ipilimumab + nivolumab has higher response rates but also higher toxicity.

Biomarkers that predict response (know all four):

Microsatellite instability (MSI): by PCR. MSI-H tumors respond to anti-PD1. MSI panel: 5 markers, 2+ unstable = MSI-H; 1 = MSI-L; 0 = MSS. The Bethesda panel uses BAT25, BAT26 (mononucleotide) plus D2S123, D5S346, D17S250 (dinucleotide).
Mismatch repair deficiency (dMMR): by IHC for MLH1, MSH2, MSH6, PMS2. Equivalent to MSI-H. ~95% concordance with MSI PCR.
Tumor mutation burden (TMB): high TMB (>10 mutations/Mb) predicts immunotherapy response across tumor types. TMB and MSI are correlated (MSI-H tumors have high TMB) but not identical - some MSS tumors have high TMB through other mechanisms (POLE mutations, UV signature in melanoma, smoking signature in lung cancer).
PD-L1 expression by IHC: used in NSCLC (TPS score - tumor proportion score), urothelial (CPS - combined positive score), gastric, TNBC. Different tumor types use different scoring systems and different thresholds.

Why MMR deficient tumors respond to immunotherapy: MMR deficiency creates a hypermutated phenotype (10-100x more mutations than MMR-proficient tumors). Frameshift mutations produce novel peptides (neo-antigens) not seen during immune development. The immune system recognizes these as foreign, generating a robust response (explaining the dense TIL infiltrate characteristic of MSI-H tumors). Tumors evade this by upregulating checkpoints. Blocking the checkpoints unleashes the pre-existing response.

Tumor-agnostic approvals (a genuinely new thing in oncology):

Pembrolizumab for MSI-H/dMMR solid tumors: first tissue-agnostic cancer approval. Biomarker-defined, not histology-defined.
Pembrolizumab for TMB-high tumors (>=10 mut/Mb): tissue-agnostic.
Larotrectinib / entrectinib for NTRK fusion tumors: tissue-agnostic.
Selpercatinib for RET-altered tumors (thyroid, NSCLC): tissue-agnostic in certain settings.

This is the foundation of precision oncology - molecular features, not tumor histology, drive treatment.

61.8 Common Reporting Scenarios and Pitfalls

A few specific situations come up often enough on boards and in practice that they deserve dedicated treatment.

Germline vs somatic distinction in tumor-only testing: When tumor-only NGS is performed (no paired germline), some variants detected in the tumor might actually be germline. Key flags:

Variant at ~50% VAF in a high-purity tumor - could be heterozygous germline, could be somatic at 100% in half the tumor. Check tumor purity.
Variant in a classic cancer predisposition gene (BRCA1, BRCA2, TP53, MLH1, MSH2, MSH6, PMS2, APC, CDKN2A, PTEN, VHL, RET, STK11, CDH1, SDHx, etc.) - germline is possible and clinically important.
When a germline implication is suspected, the patient should be offered germline confirmation testing. Labs increasingly flag high-VAF variants in hereditary cancer genes for this purpose.

The VUS conversation: VUS is the most common source of confusion for non-geneticist clinicians. Things to emphasize on a report:

VUS = not enough data, not “maybe pathogenic.” The most likely outcome is reclassification to benign as more data accumulates.
Do not use VUS results to guide clinical management. Don’t change cancer surveillance, don’t offer prophylactic surgery, don’t test relatives based on a VUS.
Reclassification happens regularly - labs should reanalyze and reissue reports as evidence accumulates. Some institutions have policies to recontact patients when classifications change.
Do NOT use PP5 (reputable source reports pathogenic) and BP6 (reputable source reports benign) as primary evidence anymore - ClinGen has moved away from these due to circular reasoning risk.

Classification changes over time: A variant classified as Likely Pathogenic in 2015 may be Pathogenic or VUS today. Always check the date of the classification and whether the lab has updated its interpretation. ClinVar’s gold star system helps: 1 star = single submitter (weak), 2 stars = multiple submitters no conflicts, 3 stars = expert panel, 4 stars = practice guideline.

When labs disagree on ClinVar: Conflicts happen. When you see a variant with “Conflicting interpretations of pathogenicity” on ClinVar, look at submitter quality, assertion criteria, and dates. An expert panel (3-star) classification generally supersedes individual submitter classifications, even recent ones.

Inheritance pattern matters for variant interpretation: The same variant in the same gene can have different implications depending on the inheritance pattern. RET loss-of-function causes Hirschsprung disease (autosomal dominant, low penetrance). RET gain-of-function (specific missenses) causes MEN2 (autosomal dominant, high penetrance). Different mechanisms, different diseases, different variant types.

Secondary findings (ACMG SF v3.2): When doing exome or genome sequencing, the ACMG recommends reporting pathogenic / likely pathogenic variants in a specific list of medically actionable genes regardless of the original indication. Patients should be consented for secondary findings in advance and can opt out. Current list includes BRCA1/2, Lynch genes, TP53, PTEN, STK11, CDH1, VHL, RET, MEN1, and the cardiac / connective tissue / lipid genes. This is about finding things that are clinically actionable if detected incidentally.