Hematology
Core concepts
Automated analyzers derive red cell indices from measured RBC count, hemoglobin, and hematocrit; understand MCV, MCH, MCHC, and RDW, plus the reticulocyte count and its correction for anemia. Classify anemia by MCV: microcytic (iron deficiency, thalassemia, anemia of chronic disease), normocytic, or macrocytic (B12/folate deficiency, alcohol, reticulocytosis), and confirm marrow response with the corrected or absolute reticulocyte count. Coagulation questions turn on PT/INR (extrinsic and common pathway: VII, X, V, II, fibrinogen), aPTT (intrinsic: VIII, IX, XI, XII), and the mixing study, where correction indicates factor deficiency and failure to correct suggests an inhibitor such as lupus anticoagulant or a specific factor antibody.
Traps
- Rule of three: RBC x 3 is about equal to Hb, and Hb x 3 is about equal to Hct. A break in the rule suggests spurious results or a hemoglobinopathy.
- Cold agglutinins falsely raise MCV and MCHC and lower the RBC count; platelet clumping in EDTA causes pseudothrombocytopenia, so recollect in citrate.
- RDW reflects anisocytosis; it is often normal in thalassemia trait despite a low MCV, while iron deficiency typically raises the RDW.
- Corrected reticulocyte percent = reticulocyte percent x (patient Hct / normal Hct); the absolute count is often more informative.
Worked example
RBC 4.50 x 10^12/L, Hb 13.5 g/dL, Hct 40.5%. The rule of three holds (4.5 x 3 = 13.5; 13.5 x 3 = 40.5). MCV = (Hct / RBC) x 10 = 90 fL, and MCH = (Hb / RBC) x 10 = 30 pg. These normocytic, normochromic indices point away from iron deficiency.