Hematology and Coagulation
Hematology and coagulation tests blood cell enumeration, morphology, and hemostasis. The exam tests analyzer principles, red cell index logic, anemia classification, smear findings, and the specimen handling that makes coagulation results valid.
Analyzer principles and indices
Impedance counters detect the resistance change as a cell passes an aperture; flow cytometers add light scatter and fluorescence for differentials and reticulocytes. Hemoglobin is measured, and indices are calculated: MCV = Hct/RBC x 10 (fL); MCH = Hgb/RBC x 10 (pg); MCHC = Hgb/Hct x 100. An MCHC above about 360 g/L (36 g/dL) is usually analytic - lipemia, in vivo hemolysis, cold agglutinin, or a very high WBC - and requires smear review.
Anemia classification
Microcytic (MCV under 80 fL): iron deficiency, thalassemia, sideroblastic anemia. Normocytic: acute blood loss, hemolysis, renal disease, marrow failure. Macrocytic: vitamin B12 or folate deficiency, liver disease, alcohol, hypothyroidism, myelodysplasia, drugs. The reticulocyte count separates decreased production from increased destruction or loss.
Coagulation
Use 3.2% citrate at a 9:1 ratio; the fill line is critical. Centrifuge at 1500 g for 15 minutes at room temperature to obtain platelet-poor plasma (under 10 x 10^9/L platelets). PT/INR monitors warfarin and is sensitive to factors VII, X, V, II, and fibrinogen; APTT monitors unfractionated heparin and detects intrinsic pathway factors VIII, IX, XI, and XII. In a mixing study, correction toward normal after a 1:1 mix with normal plasma indicates a factor deficiency; failure to correct indicates an inhibitor such as a factor antibody or lupus anticoagulant.
Worked example
APTT 58 seconds (reference 25-35) with a normal PT. The 1:1 mixing study corrects the APTT into the reference range, so a factor deficiency is present; the normal PT localizes it to the intrinsic pathway, and factor VIII assay confirms hemophilia A or von Willebrand disease. If the mix had failed to correct, a lupus anticoagulant or specific inhibitor would be pursued.
Traps that cost marks
- EDTA-induced platelet clumping causing pseudothrombocytopenia - redraw in citrate and examine the smear.
- Cold agglutinins producing a spuriously high MCV and MCHC.
- Underfilled citrate tubes prolonging PT and APTT.
- Assuming a normal platelet count from an analyzer without checking flags.
- Interpreting a prolonged APTT as a bleeding risk when a lupus anticoagulant is present.