Hemolytic Anemia: A High-Yield USMLE Review

Hemolytic anemia is anemia caused by accelerated destruction of red blood cells that outpaces marrow production. It encompasses inherited defects (hereditary spherocytosis, G6PD deficiency, sickle cell disease), immune-mediated destruction (warm autoimmune hemolytic anemia), and microangiopathic processes (TTP, HUS, DIC). Recognizing the shared laboratory signature and then localizing the mechanism — and distinguishing intravascular from extravascular, immune from non-immune — is a recurring high-yield theme across Step 1 and Step 2 CK.

Presentation

• Anemia with fatigue and, depending on the cause, an acute drop from a known baseline hemoglobin
• Jaundice from indirect (unconjugated) hyperbilirubinemia
• Dark urine and back pain in acute intravascular hemolysis (as with oxidant-triggered G6PD hemolysis or a hemolytic transfusion reaction)
• Splenomegaly in extravascular hemolysis such as hereditary spherocytosis
• Pigment gallstones from chronic bilirubin excess (hereditary spherocytosis, sickle cell disease)
• Aplastic crisis with parvovirus B19 infection in chronic hemolytic states

Diagnosis

• Hemolysis panel: elevated reticulocyte count, elevated LDH, elevated indirect bilirubin, and low or undetectable haptoglobin
• Peripheral smear localizes the cause — spherocytes (hereditary spherocytosis or warm AIHA), bite cells and Heinz bodies (G6PD deficiency), sickle cells with target cells and Howell-Jolly bodies (sickle cell disease), or schistocytes/helmet cells (microangiopathic hemolytic anemia)
• Direct Coombs (direct antiglobulin) test: positive for IgG in warm autoimmune hemolytic anemia; negative in hereditary spherocytosis and other non-immune causes
• EMA binding test and osmotic fragility test support hereditary spherocytosis; elevated MCHC is a clue
• Hemoglobin electrophoresis for sickle cell disease (HbS predominant, HbA absent)
• In suspected microangiopathy: thrombocytopenia with schistocytes; ADAMTS13 activity <10% is diagnostic of TTP, whereas it is normal in HUS

Management

• Treat the underlying mechanism: corticosteroids are first-line for warm autoimmune hemolytic anemia, with rituximab as second-line and splenectomy for refractory cases
• Folate supplementation to meet increased demand from high RBC turnover (e.g., hereditary spherocytosis)
• Remove or avoid the trigger in G6PD deficiency (sulfa drugs, primaquine, dapsone, nitrofurantoin, fava beans); acute episodes are usually self-limited
• Splenectomy for severe hereditary spherocytosis (curative for the anemia) and cholecystectomy for symptomatic pigment stones
• Sickle cell disease: hydroxyurea to raise HbF, transfusion for acute chest syndrome/stroke/severe anemia, vaccination and penicillin prophylaxis against encapsulated organisms
• TTP is an emergency treated with plasma exchange (removes ultra-large vWF, replaces ADAMTS13) plus corticosteroids — do NOT transfuse platelets; typical HUS is supportive (plasma exchange not beneficial, avoid antibiotics)
• In warm AIHA requiring transfusion, the autoantibody makes all crossmatches appear incompatible — give the 'least incompatible' units after excluding underlying alloantibodies rather than withholding needed blood

High-yield

• Hemolysis signature: high reticulocytes, high LDH, high indirect bilirubin, low haptoglobin
• Bite cells + Heinz bodies after an oxidant exposure (sulfa, primaquine, dapsone, nitrofurantoin, fava beans) = G6PD deficiency, the most common enzymopathy worldwide; classic in males of Mediterranean, African, or Southeast Asian descent
• Positive direct Coombs for IgG + spherocytes + extravascular hemolysis = warm autoimmune hemolytic anemia; a recognized complication of CLL and SLE (Type II hypersensitivity)
• Schistocytes (helmet cells) + thrombocytopenia = microangiopathic hemolytic anemia (TTP, HUS, DIC, malignant hypertension, HELLP, mechanical valves, metastatic cancer)
• TTP: ADAMTS13 deficiency; HUS: Shiga toxin (E. coli O157:H7), ADAMTS13 normal, renal failure prominent
• Hereditary spherocytosis: spherocytes, elevated MCHC, negative Coombs, positive EMA binding — the most common inherited hemolytic anemia in Northern Europeans
• Sickle cell disease: Glu→Val at position 6 of β-globin; HbS polymerizes when deoxygenated causing chronic hemolysis and functional asplenia

Pitfalls

• Confusing warm AIHA with hereditary spherocytosis — both show spherocytes, but the Coombs test is positive in AIHA and negative in HS
• In warm AIHA, first-line therapy is corticosteroids; folic acid is supportive only, and plasmapheresis (used for TTP/cold agglutinin disease) is not the answer
• Do NOT transfuse platelets in TTP — it can worsen microvascular thrombosis ('adding fuel to the fire')
• Do NOT give antibiotics in typical (Shiga toxin) HUS — bacterial lysis releases more toxin and raises the risk of progression to HUS; plasma exchange (which helps TTP) is not beneficial here
• Never cancel a needed transfusion in warm AIHA just because the crossmatch is 'incompatible' — the autoantibody reacts with all donor cells; give least-incompatible blood after excluding alloantibodies
• Do not mistake schistocytes for a benign finding — in a thrombocytopenic patient they point to a microangiopathic emergency (e.g., distinguishing TTP from ITP)
• Basophilic stippling (lead poisoning, thalassemia) and hypersegmented neutrophils/oval macrocytes (megaloblastic anemia) are not features of hemolysis — don't conflate them