Step 1 topics → Hematology and Oncology → Paroxysmal Nocturnal Hemoglobinuria (PNH)

Paroxysmal Nocturnal Hemoglobinuria (PNH)

USMLE Step 1 trap: Misclassifies PNH as macrocytic rather than normocytic hemolytic anemia. PNH itself causes normocytic hemolytic anemia; macrocytosis would suggest a concurrent marrow failure state rather than PNH per se.

Paroxysmal Nocturnal Hemoglobinuria (PNH) is an acquired clonal disorder caused by a somatic mutation in the PIG-A gene, and USMLE Step 1 tests it primarily through the most exploitable misconception in hematology: students think PNH RBCs are lacking complement, but they're being destroyed BY complement. Without functional GPI anchors, red blood cells can't express CD55 (DAF) or CD59 — the two complement regulatory proteins that normally prevent terminal complement complex formation on RBC surfaces — so complement runs unchecked and lyses RBCs from the inside out. The result is chronic complement-mediated intravascular hemolysis, and getting the direction of that mechanism right is what separates correct answers from traps. USMLE Step 1 tests this primarily through mechanism questions (why are RBCs destroyed?) and clinical presentation questions (what's the classic triad, and where does thrombosis occur?). You'll often see a vignette with dark morning urine and need to connect that to a hemolytic process, not something like rhabdomyolysis.

The trickiest part of PNH is the layered pathophysiology. Students routinely confuse 'lacking GPI-anchored complement inhibitors' with 'lacking complement entirely' — these are opposites. The RBCs are destroyed BY complement, because they can't inhibit it. The other common trap is diagnosis: older tests like the sucrose lysis (sugar water) test and Ham test (acidified serum) are still mentioned in resources, but flow cytometry showing absent CD55/CD59 is the current gold standard. USMLE Step 1 will test whether you know this distinction. Similarly, students associate PNH with aplastic anemia and assume the anemia must be macrocytic — but PNH itself produces normocytic hemolytic anemia; macrocytosis would point to a superimposed marrow failure state.

Clinically, PNH sits at the intersection of hematology and vascular medicine. The thrombosis in PNH is not your typical DVT pattern — it characteristically hits unusual venous sites: hepatic veins (Budd-Chiari), portal veins, cerebral venous sinuses. This is a high-yield distinguishing feature that appears in clinical vignettes. Knowing the full picture — mechanism, triad, unusual thrombosis sites, flow cytometry diagnosis, and eculizumab therapy — is what separates a complete PNH answer from a partial one on USMLE Step 1.

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Common misconceptions

Common mistake
Wrong: PNH causes macrocytic anemia because it is associated with aplastic anemia.
Right: PNH itself causes normocytic hemolytic anemia; macrocytosis would suggest a concurrent marrow failure state rather than PNH per se.
PNH is a hemolytic anemia, and hemolytic anemias are normocytic — RBCs are being destroyed prematurely, not improperly synthesized. The association with aplastic anemia misleads students into expecting macrocytosis, but marrow failure causes pancytopenia, not the PNH hemolysis itself. If you see macrocytosis in a PNH vignette, it signals a concurrent marrow problem, not a feature of PNH.
Common mistake
Wrong: PNH RBCs lack complement entirely.
Right: PNH RBCs lack GPI-anchored complement regulatory proteins CD55 and CD59, making them susceptible to complement-mediated lysis rather than lacking complement itself.
PNH RBCs have normal amounts of complement — they're being destroyed by it. What's missing are the GPI-anchored proteins CD55 and CD59, which normally inhibit complement activation on the RBC surface. Without these brakes, the terminal complement complex (MAC) assembles on RBCs and lyses them from the inside out. Complement is the weapon; PNH removes the shield.
Common mistake
Wrong: The sucrose lysis (sugar water) test is the gold standard for diagnosing PNH.
Right: Flow cytometry demonstrating absence of CD55 and CD59 on RBCs is the gold standard for PNH diagnosis; sucrose lysis is an older, less specific screening test.
Sucrose lysis and Ham tests were used historically because they activate complement in vitro and show abnormal RBC lysis — but they're nonspecific and largely obsolete. Flow cytometry directly detects the absence of CD55 and CD59 on the RBC surface with high sensitivity and specificity, making it the gold standard. If a Step 1 question asks for the best or confirmatory test, flow cytometry is the answer.
Common mistake
Gap: Misses that PNH thrombosis characteristically involves unusual venous sites such as hepatic and cerebral veins
PNH classically causes thrombosis in unusual sites including hepatic veins (Budd-Chiari syndrome), portal veins, and cerebral sinuses, which is a key distinguishing clinical feature.
PNH-associated thrombosis doesn't follow typical DVT/PE patterns. Complement activation on platelet surfaces leads to hypercoagulability, but the thrombosis preferentially affects unusual venous sites: hepatic veins causing Budd-Chiari syndrome, portal veins causing portal hypertension, and cerebral venous sinuses causing headache and neurologic symptoms. When you see Budd-Chiari in a young patient with hemolytic anemia and cytopenias, PNH should be at the top of your differential.
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What the exam tests

  1. Mechanism: Understand how a PIG-A mutation leads to loss of GPI-anchored proteins CD55 and CD59, and why this makes RBCs susceptible to complement-mediated intravascular hemolysis.
  2. Presentation: Recognize the classic PNH triad of intravascular hemolysis, cytopenias (due to associated marrow failure), and thrombosis — especially at unusual venous sites like hepatic veins (Budd-Chiari) and cerebral sinuses.
  3. Diagnosis: Identify flow cytometry demonstrating absent CD55/CD59 as the gold standard diagnostic test, and know when older tests like sucrose lysis and Ham test appear in a question stem.
  4. Management: Know that eculizumab (a terminal complement inhibitor) is targeted therapy, that meningococcal vaccination is required before starting it, and that allogeneic stem cell transplant is the only definitive cure.

Can you avoid these mistakes?

A 32-year-old woman presents with fatigue, episodic dark morning urine, and abdominal pain. Labs show elevated LDH, low haptoglobin, hemoglobinuria, and a peripheral smear with no schistocytes. What is the underlying molecular defect, and why does it cause hemolysis?
A patient with known PNH develops sudden-onset right upper quadrant pain and ascites. Doppler ultrasound shows hepatic vein thrombosis. What syndrome is this, and why are PNH patients at risk for thrombosis at this site specifically?
You suspect PNH in a patient. Which test confirms the diagnosis — the sucrose lysis test, the Ham test, or flow cytometry — and what does the correct test actually measure?
A patient with aplastic anemia is found to have a PNH clone on flow cytometry. What type of anemia would you expect from PNH itself, and how would this differ from the anemia caused by bone marrow failure?

Related topics

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