Hematology and Hemostasis
What to master
Start with the CBC and derived indices, then classify anemia by MCV. Microcytic: iron deficiency (low ferritin, high TIBC, low saturation, high RDW), thalassemia trait (normal or high ferritin, normal RDW, target cells, Mentzer index below 13), anemia of chronic disease (low TIBC, high ferritin). Normocytic: hemolysis (reticulocytosis, high LDH, low haptoglobin, high indirect bilirubin) versus production failure. Macrocytic: B12/folate deficiency versus alcohol, liver disease, or marrow failure. Use the corrected reticulocyte count to judge marrow response, and read the smear for morphology (schistocytes, spherocytes, bite cells, rouleaux, blasts).
Hemostasis separates primary from secondary. PT/INR reflects the extrinsic and common pathway, aPTT the intrinsic and common pathway. A mixing study that corrects indicates factor deficiency; failure to correct indicates an inhibitor such as a lupus anticoagulant or specific factor antibody, and incubated mixing studies reveal time- and temperature-dependent inhibitors. Know factor assay interpretation, heparin monitoring by aPTT or anti-Xa, DOAC effects, and D-dimer as an exclusion test.
Traps
Cold agglutinins falsely raise MCV and MCHC and lower RBC count; EDTA-induced platelet clumping causes pseudothrombocytopenia, so recheck in citrate or heparin. Lipemia and delayed smears distort morphology.
Worked example
MCV 68 fL and RBC 5.6 × 10^12/L: Mentzer index = 68 / 5.6 = 12.1, below 13, favoring thalassemia trait over iron deficiency. Confirm with iron studies and hemoglobin electrophoresis.