Hematology and Coagulation
CBC interpretation starts with the indices
Classify anemia by MCV, then use the reticulocyte count to separate underproduction from destruction or loss. Correct the retic for anemia: corrected retic % = retic % × (patient Hct ÷ normal Hct); the reticulocyte production index further divides by maturation time. Iron deficiency shows low ferritin, high RDW, and high TIBC; anemia of chronic disease shows normal-to-high ferritin, low TIBC, and normal RDW. Thalassemia trait is microcytic with a normal RDW and a Mentzer index (MCV ÷ RBC) below 13, whereas iron deficiency usually exceeds 13.
Megaloblastic anemia brings hypersegmented neutrophils, elevated LDH and indirect bilirubin, and a low retic. Hemolysis brings low haptoglobin, high LDH, and a positive DAT when immune.
Smear vocabulary
The exam expects you to link morphology to disease: schistocytes (microangiopathy, DIC, TTP, mechanical valve), bite cells and Heinz bodies (G6PD), target cells (hemoglobinopathy, liver disease), basophilic stippling (lead, thalassemia), Howell-Jolly bodies (asplenia), and rouleaux (myeloma, inflammation).
Coagulation logic
Prolonged aPTT with normal PT points to the intrinsic pathway; the mixing study is the pivot. Correction means factor deficiency; failure to correct means an inhibitor, either a specific factor inhibitor (time- and temperature-dependent) or a lupus anticoagulant (immediate, phospholipid-dependent). Watch for heparin contamination and underfilled citrate tubes.
Traps
EDTA-induced platelet clumping causes spurious thrombocytopenia - check the smear and redraw in citrate. Never interpret a mixing study result without the incubated (2-hour) arm.
Worked example: aPTT 58 s, PT normal, mixing study corrects immediately and after incubation → factor VIII, IX, XI, or XII deficiency; proceed to factor assays.