Hematology
What this domain tests
Hematology covers automated blood counting technology, smear correlation, anemia classification, and coagulation testing. Questions often give you a set of indices plus a smear finding and ask what is real, what is artifact, and what to do next.
Analyzer technology and control
- Impedance counting (Coulter principle) sizes and counts cells by voltage change; optical scatter adds internal complexity; VCS (volume, conductivity, scatter) generates the five-part differential; reticulocytes are counted with fluorescent nucleic acid dyes.
- Calibrate with the manufacturer's whole-blood calibrator; run at least two levels of QC every 24 hours of patient testing under CLIA.
- Smear review is triggered by flags: blasts, immature granulocytes, nucleated RBCs, platelet clumps, and any delta-check failure.
Anemia reasoning
- Start with MCV and RDW, then the reticulocyte count corrected for anemia (retic % × patient Hct ÷ normal Hct).
- Microcytic: iron deficiency (low ferritin, high TIBC), thalassemia (normal ferritin, often elevated RBC count and target cells), anemia of chronic disease (low TIBC, normal or high ferritin).
- An MCHC above 36 g/dL is a red flag for cold agglutinin, lipemia, or spherocytosis - not true hyperchromia.
Coagulation
- INR = (patient PT ÷ mean normal PT)^ISI; a clot-based PT/INR is unreliable in lupus anticoagulant and in patients on direct oral anticoagulants.
- Mixing study: correction in a 1:1 mix indicates factor deficiency; failure to correct indicates an inhibitor (lupus anticoagulant, specific factor inhibitor).
- Heparin contamination prolongs the aPTT and thrombin time.
Worked example
Hemoglobin 13.5 g/dL, RBC 2.1 × 10^12/L, MCV 121 fL, MCHC 38 g/dL, platelets 180 × 10^9/L; the smear shows agglutination. Reasoning: a low RBC count with high MCV and an MCHC above 36 is the classic cold agglutinin pattern - the analyzer counted clumps as single large cells. Warm the EDTA tube at 37 °C for 30 minutes, mix, and rerun; report the corrected indices and note the interference. Do not report the initial MCHC as spherocytosis.
Traps that cost marks
- Pseudothrombocytopenia from EDTA-dependent platelet clumping; rerun in citrate.
- Reporting a cold-agglutinin hemoglobin and MCHC without warming the sample.
- Ignoring lipemic interference with hemoglobin measurement.
- Using an uncorrected reticulocyte percentage to judge marrow response.