Hematology
Core concepts
Hematology questions test morphologic reasoning plus numeric interpretation. Know the complete blood count indices and their derivations (MCV, MCH, RDW), the reticulocyte index as a measure of marrow response, and the anemia classification by MCV and reticulocyte count. Understand iron studies (ferritin, TIBC, transferrin saturation) and the anemia of chronic disease pattern. Master the coagulation cascade in practical terms: PT/INR (extrinsic), aPTT (intrinsic), mixing studies to distinguish factor deficiency from inhibitor, and the lupus anticoagulant paradox (prolonged aPTT but thrombotic risk). Know platelet disorders, hemolysis markers (LDH, haptoglobin, indirect bilirubin, schistocytes), and the peripheral smear findings that define diagnoses (bite cells, blister cells, basophilic stippling, rouleaux).
Traps
Examiners exploit the reticulocyte count: a high reticulocyte percentage with a low absolute count can still indicate inadequate marrow response-always calculate the corrected count or index. Ferritin is an acute-phase reactant, so a "normal" ferritin does not exclude iron deficiency in inflammation. In mixing studies, correction at 37°C and 2 hours suggests a factor deficiency; failure to correct suggests an inhibitor. Do not confuse the prolonged aPTT of lupus anticoagulant with a bleeding diathesis. Watch for spurious thrombocytopenia from platelet clumping (EDTA-dependent).
Worked example / rule of thumb
Corrected reticulocyte count = reticulocyte % × (patient Hct ÷ normal Hct). If the corrected count is <2% in an anemic patient, the marrow response is inadequate-think nutritional deficiency, renal disease, or marrow failure rather than hemolysis or blood loss.