Hematology and Coagulation
Core concepts
Build the complete blood count from first principles: red cell indices (MCV, MCH, RDW) classify anaemia as microcytic, normocytic or macrocytic, and the reticulocyte count separates hypoproliferative from hyperproliferative states. Know the iron studies pattern (low ferritin and high TIBC in iron deficiency; high ferritin and low TIBC in anaemia of chronic disease) and the haemoglobinopathy screen (HbA2 elevation in beta-thalassaemia trait). Leucocyte morphology and the left shift, plus the causes of eosinophilia and lymphocytosis, recur in questions.
Coagulation requires a pathway map: PT/INR reflects the extrinsic and common pathway; APTT the intrinsic and common pathway; thrombin time and fibrinogen the final common step. Understand heparin (anti-Xa, APTT monitoring), warfarin (factors II, VII, IX, X and proteins C and S), and DOAC effects on routine tests. Know the diagnostic logic of von Willebrand disease (low vWF antigen, low activity, normal or low factor VIII) and the mixing study for a prolonged APTT.
Traps in questions
A normal APTT does not exclude a factor deficiency (factor XIII and mild deficiencies are missed). A mixing study that fails to correct indicates an inhibitor, not deficiency. Distinguish DIC (prolonged PT and APTT, low fibrinogen, raised D-dimer, thrombocytopenia, schistocytes) from TTP (normal coagulation screen, severe thrombocytopenia, MAHA). Remember that EDTA samples cause pseudothrombocytopenia and pseudoneutrophilia.
Rule of thumb
For any prolonged clotting time, ask: deficient factor, inhibitor, or consumptive process? Perform a mixing study; corrects = deficiency, does not correct = inhibitor.