Hematology and Coagulation
Hematology and Coagulation questions test CBC interpretation, anemia classification, white cell and platelet disorders, and coagulation test selection. The exam expects you to link numeric results to morphology, sample integrity, and clinical meaning.
Core rules
- CBC parameters include Hb, MCV, MCH, RDW, WBC with differential, and platelet count. Anemia is classified by MCV: microcytic, normocytic, or macrocytic.
- Iron deficiency shows low ferritin, low MCV, and high RDW; thalassemia trait often shows low MCV with normal or high RBC count and normal ferritin.
- PT monitors extrinsic and common pathway; aPTT monitors intrinsic and common pathway. Mixing studies distinguish factor deficiency from an inhibitor.
- INR = (patient PT / mean normal PT)^ISI. A higher ISI means a stronger reagent sensitivity to warfarin effect.
- Platelet clumping in EDTA can cause pseudothrombocytopenia; examine the smear and repeat in citrate.
Worked example
A patient on warfarin has PT 18 s, mean normal PT 12 s, and reagent ISI 1.0. INR = (18/12)^1.0 = 1.5. If the ISI were 1.2, INR = (1.5)^1.2 ≈ 1.63. The reasoning: INR standardizes PT across thromboplastins; the ISI exponent corrects for reagent responsiveness. Report the INR, not just seconds, for warfarin monitoring.
Traps
Underfilled citrate tubes give falsely prolonged PT/aPTT because excess citrate binds calcium. Heparin contamination also prolongs aPTT. Cold agglutinins can falsely raise MCV and RBC count. Lipemia and icterus affect hemoglobin methods. Do not interpret a prolonged aPTT without checking sample quality and mixing-study results.