Step 1 topics → Hematology and Oncology → G6PD Deficiency

G6PD Deficiency

USMLE Step 1 trap: Misses that G6PD assay during acute hemolysis can be falsely normal due to selective destruction of older deficient cells. G6PD assay performed during an acute episode may be falsely normal because older G6PD-deficient RBCs have already been destroyed, leaving younger RBCs with relatively higher enzyme activity.

G6PD deficiency is the most common enzymatic disorder of red blood cells, affecting hundreds of millions of people worldwide — and USMLE Step 1 loves it. The core concept: G6PD is the rate-limiting enzyme of the HMP shunt (pentose phosphate pathway), which is the RBC's only way to generate NADPH. NADPH keeps glutathione reduced, which neutralizes reactive oxygen species. The most testable misconception: Heinz bodies have nothing to do with iron — they are precipitates of oxidized, denatured hemoglobin, not iron deposits (which belong to sideroblastic anemia). Without enough G6PD activity, oxidative stress denatures hemoglobin into Heinz bodies, which get plucked out by splenic macrophages, leaving behind bite cells on the peripheral smear. The result is episodic, self-limited hemolytic anemia triggered by oxidative stress — not chronic baseline anemia in most patients.

The exam tests this from three directions: mechanism (why does oxidative stress cause hemolysis specifically in G6PD deficiency?), triggers (which drugs, foods, and infections provoke episodes and why some variants are worse than others), and laboratory interpretation (what the smear shows and when the G6PD assay can fool you). Passage-based questions will often give you a clinical scenario — a patient who just started primaquine or dapsone, or ate fava beans — and ask you to explain the smear or predict the lab result. Don't just memorize the trigger list; understand why each one generates oxidative stress.

The trickiest parts of G6PD for USMLE Step 1 are the assay timing pitfall (testing during an acute episode gives a falsely normal result), the nature of Heinz bodies (students routinely confuse these with iron), and the nuance around female carriers (not universally protected). Get those three right and you'll handle most G6PD questions correctly.

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Common misconceptions

Common mistake
Wrong: G6PD enzyme assay should be performed during an acute hemolytic episode to confirm the diagnosis.
Right: G6PD assay performed during an acute episode may be falsely normal because older G6PD-deficient RBCs have already been destroyed, leaving younger RBCs with relatively higher enzyme activity.
During an acute hemolytic episode, the oldest and most G6PD-deficient RBCs are selectively destroyed first, leaving behind a population of younger reticulocytes that have relatively higher G6PD enzyme activity. If you run the assay at that moment, the remaining cells look almost normal, and you miss the diagnosis. The correct approach is to wait 2-3 months after the acute episode — once the RBC population normalizes — before testing enzyme activity.
Common mistake
Wrong: Heinz bodies in G6PD deficiency are composed of iron deposits.
Right: Heinz bodies are precipitates of oxidized, denatured hemoglobin that form when G6PD-deficient RBCs cannot regenerate NADPH to neutralize oxidative stress.
Heinz bodies have nothing to do with iron. They are precipitates of oxidized, denatured hemoglobin — specifically globin chains that clump together when the RBC can no longer regenerate NADPH to keep glutathione in its reduced (active) form. Think of it this way: NADPH → reduced glutathione → neutralizes H2O2 → hemoglobin stays soluble. Break that chain at NADPH and hemoglobin oxidizes and precipitates. Iron deposits are a feature of sideroblastic anemia and hemochromatosis, not G6PD.
Common mistake
Wrong: Female carriers of G6PD deficiency are always asymptomatic.
Right: Female carriers can be symptomatic if lyonization results in a majority of RBCs expressing the deficient X chromosome, though males are more severely affected.
G6PD deficiency is X-linked, so males (hemizygous) are either fully affected or unaffected. Female carriers have two X chromosomes, but X-inactivation (lyonization) is random — if a female happens to inactivate the normal X chromosome in the majority of her RBCs, most of her RBCs will express the deficient enzyme and she can have clinically significant hemolysis. This is why you cannot simply rule out G6PD disease in a female based on carrier status alone.
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What the exam tests

  1. Explain the HMP shunt's role in RBC oxidative defense — why G6PD deficiency specifically makes RBCs vulnerable to oxidative stress, and what Heinz bodies and bite cells represent on the peripheral smear.
  2. Identify which triggers (primaquine, dapsone, fava beans, infections, sulfonamides) precipitate hemolytic episodes in G6PD deficiency, and understand why more severe enzyme variants (like those in the Mediterranean population) cause hemolysis even without strong triggers.
  3. Interpret peripheral smear findings (Heinz bodies, bite cells) and recognize the critical pitfall that a G6PD enzyme assay performed during an acute hemolytic episode may return a falsely normal result.

Can you avoid these mistakes?

A patient with known G6PD deficiency is started on dapsone for dermatitis herpetiformis. Three days later he presents with fatigue and jaundice. His peripheral smear shows bite cells. You send a G6PD enzyme assay and it comes back normal. What is the most likely explanation for this result?
Why are RBCs uniquely dependent on the HMP shunt for oxidative protection, while most other cells are not? What does this tell you about why G6PD deficiency causes hemolysis but not damage to other tissues?
A 35-year-old woman of Mediterranean descent is found to have mild normocytic anemia after eating fava beans at a family gathering. Her brother has documented G6PD deficiency. Can you diagnose or exclude G6PD deficiency in her based on the fact that she is female? Explain the mechanism behind your answer.
On a peripheral smear from a patient with G6PD deficiency during an acute episode, you see Heinz bodies and bite cells. Describe what each of these findings represents at the molecular and cellular level — what caused each one to form?

Related topics

Hemolytic Anemia — Intravascular vs Extravascular Hematopoiesis and Lineage Differentiation RBC Physiology and Lifespan WBC Differential and Function Platelet Physiology and Primary Hemostasis

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