Hematology
Hematology items test the complete blood count from generation to interpretation: how automated analyzers derive indices, how to classify an anemia, how to recognize a spurious result on the smear, and how to interpret white cell and platelet abnormalities.
CBC math and indices
- MCV = (Hct / RBC) x 10; MCH = (Hgb / RBC) x 10; MCHC = (Hgb / Hct) x 100. Microcytic is under 80 fL, normocytic 80-100 fL, macrocytic above 100 fL.
- Corrected reticulocyte count = retic % x (patient Hct / 45). Absolute retic count = retic % x RBC count.
- An MCHC above about 36 g/dL is suspicious rather than real and suggests cold agglutinin, lipemia, icterus, or hemoglobin C.
Anemia classification
- Microcytic: iron deficiency (low ferritin, high TIBC, low saturation, high RDW), thalassemia trait (normal iron studies, narrow RDW, elevated HbA2 or HbF), anemia of chronic disease (low TIBC, normal or high ferritin), sideroblastic anemia.
- Normocytic: acute blood loss, hemolysis (high LDH, high indirect bilirubin, low haptoglobin, high retic count), chronic kidney disease, aplastic anemia.
- Macrocytic: B12 or folate deficiency with oval macrocytes and hypersegmented neutrophils, liver disease and alcohol, drugs, myelodysplasia.
- Hemoglobinopathies are confirmed by alkaline hemoglobin electrophoresis; sickle cell also gives a positive solubility test.
White cells and platelets
- Left shift, toxic granulation and Dohle bodies indicate acute inflammation or infection; report the absolute neutrophil count.
- Chronic myeloid leukemia shows marked leukocytosis with basophilia and a low leukocyte alkaline phosphatase score.
- EDTA-induced platelet clumping causes pseudothrombocytopenia; the smear shows clumps, and the analyzer may count clumps as white cells, falsely raising the WBC. Recollect in citrate.
- Cold agglutinins lower the RBC count and raise MCV and MCHC because clumps are counted as single large cells.
Worked example
Hemoglobin 10.2 g/dL, hematocrit 31%, RBC 5.6 x 10^12/L, MCV 65 fL, RDW 13%, ferritin 90 ng/mL. Mentzer index = 65 / 5.6 = 11.6, below 13. Reasoning: the high RBC count for the degree of anemia, narrow RDW and normal ferritin all favor thalassemia trait over iron deficiency, which typically gives a low RBC count and RDW above 15%. Confirm with hemoglobin electrophoresis showing HbA2 at or above 3.5%. Give no iron.
Traps that cost marks
- Ordering iron for a Mentzer index below 13 without electrophoresis.
- Repeating a platelet count in the same EDTA tube when clumping is seen.
- Reading an MCHC above 36 as true hyperchromia.
- Judging morphology from a delay-drawn smear with artifactual crenation.