Hematology — USMLE Step 3 Practice
Hematology on USMLE Step 3 covers anemias, coagulation disorders, leukemias, lymphomas, and thrombocytopenia. Peripheral blood smear findings and transfusion triggers are commonly tested clinical skills.
Frequently Asked Questions
How do you differentiate iron deficiency anemia from thalassemia on Step 3?
Both cause microcytic anemia. Iron deficiency: low serum iron, low ferritin, high TIBC, elevated RDW, high platelets. Beta-thalassemia trait: normal or high ferritin, normal TIBC, normal or high RBC count, normal RDW, and Mentzer index <13. Hemoglobin electrophoresis confirms thalassemia.
What are the diagnostic criteria for DIC on USMLE Step 3?
DIC: elevated PT/PTT, elevated D-dimer, low fibrinogen, thrombocytopenia, and schistocytes on peripheral smear. Common triggers: sepsis, obstetric emergencies (placental abruption, amniotic fluid embolism), malignancy (especially AML M3). Treat underlying cause; replace with FFP, cryoprecipitate, and platelets.
How do you manage heparin-induced thrombocytopenia (HIT)?
Stop all heparin products immediately (including LMWH and heparin flushes). Start a direct thrombin inhibitor (argatroban if hepatic function normal; bivalirudin also used). Do NOT give platelets — this can worsen thrombosis. Warfarin is contraindicated until platelet count >150,000 and after at least 5 days of DTI.
When is transfusion indicated on USMLE Step 3?
Packed RBCs: hemoglobin <7 g/dL in stable patients (or <8 in ACS/cardiovascular disease). Platelets: <10,000 prophylactically, or <50,000 before procedures. FFP: elevated PT/PTT with active bleeding or before procedures. Cryoprecipitate: fibrinogen <150 mg/dL with bleeding.
What is the presentation of thrombotic thrombocytopenic purpura (TTP)?
TTP pentad: fever, thrombocytopenia, microangiopathic hemolytic anemia (schistocytes), neurological symptoms (confusion, seizures), and renal dysfunction. Unlike HUS, neurological symptoms predominate. Treatment: plasmapheresis urgently. Do NOT transfuse platelets (worsens thrombosis). Corticosteroids and rituximab are adjuncts.
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