Hematology
CBC and indices
Analyzers use impedance and light scatter; MCV = Hct/RBC × 10, MCH = Hb/RBC × 10, MCHC = Hb/Hct. Anemia is classified morphologically: microcytic (iron deficiency, thalassemia, anemia of chronic disease, sideroblastic), normocytic (hemolysis, renal disease, marrow failure), macrocytic (B12/folate, reticulocytosis, liver disease, MDS).
Reticulocyte response
Corrected retic % = retic % × (patient Hct / normal Hct); the production index additionally divides by a maturation factor (~2). A high corrected count with anemia indicates blood loss or hemolysis; a low count indicates underproduction.
Iron studies and hemoglobinopathies
Iron deficiency: low ferritin, high TIBC, low saturation. Anemia of chronic disease: low TIBC, normal-to-high ferritin. Beta-thalassemia trait shows elevated HbA2 (4–6%); alpha trait is usually normal on electrophoresis.
Coagulation
PT/INR reflects the extrinsic pathway (VII, X, V, II, fibrinogen); aPTT reflects the intrinsic pathway (XII, XI, IX, VIII). Mixing studies: correction indicates factor deficiency; no correction indicates an inhibitor (lupus anticoagulant, factor VIII inhibitor); time-dependent correction suggests a slow-acting inhibitor. TTP shows schistocytes, thrombocytopenia, and normal PT/aPTT with low ADAMTS13; DIC prolongs both.
Traps
- Normal MCV masking combined iron and B12 deficiency.
- Reticulocyte percentage not corrected for anemia.
- Mild factor deficiencies can give normal screening tests.
Example: Hct 27%, retic 6% → corrected = 6 × (27/45) = 3.6%.