Hematology
Hematology
This domain tests complete blood count interpretation, red cell indices, reticulocyte Response, and coagulation testing - translating numbers into a mechanism.
Core rules
- Indices: MCV = Hct/RBC × 10; MCH = Hb/RBC; MCHC = Hb/Hct × 100. A high MCHC (>36 g/dL) suggests spherocytosis, cold agglutinin, or lipemia.
- The corrected reticulocyte count = retic % × (patient Hct/45); the absolute reticulocyte count = retic % × RBC. A high index indicates a hyperproliferative marrow; a low index in anemia indicates hypoproliferation.
- Mentzer index = MCV/RBC; below 13 favors thalassemia trait, above 13 favors iron deficiency.
- PT/INR monitors warfarin (extrinsic, factor VII); aPTT monitors heparin (intrinsic, VIII/IX/XI/XII).
- A mixing study that corrects immediately suggests a factor deficiency; failure to correct suggests an inhibitor such as a lupus anticoagulant or a specific factor inhibitor.
Worked example
Hgb 9.0 g/dL, Hct 30%, RBC 4.6 × 10^12/L, retic 1.5%. MCV = 30/4.6 × 10 = 65 fL (microcytic). Corrected retic = 1.5 × (30/45) = 1.0% - an inadequate marrow response. Mentzer index = 65/4.6 ≈ 14, favoring iron deficiency. The low corrected retic in a microcytic anemia with a high RDW supports iron deficiency over thalassemia trait, where the RDW is usually normal.
Traps that cost marks
- Reading an uncorrected reticulocyte percentage in an anemic patient.
- Missing that a normal MCV can mask a dimorphic population; check the RDW and smear.
- Interpreting a prolonged aPTT without a mixing study.
- Reporting a high MCHC without checking for agglutination or lipemia.