Hematology
CBC and smear logic
The CBC combines impedance or optical counting with clinical correlation. Understand RBC indices (MCV, MCH, MCHC), RDW, reticulocyte count, and the WBC differential. Anemia classification starts with MCV: microcytic (iron deficiency, thalassemia), normocytic (acute blood loss, renal disease, hemolysis), macrocytic (B12/folate, alcohol, liver disease). The peripheral smear explains flags: schistocytes, spherocytes, teardrops, blasts, toxic granulation, and rouleaux. Platelet clumping from EDTA is a classic pseudothrombocytopenia; examine the smear and recollect in citrate if needed. Cold agglutinins can falsely lower RBC and raise MCV; warm the sample. Nucleated RBCs falsely elevate the WBC count, so correct it. Coagulation questions test PT/INR (extrinsic/common) and aPTT (intrinsic/common), mixing studies, and factor interpretation. A mixing study that corrects suggests factor deficiency; failure to correct suggests an inhibitor. DIC shows prolonged PT/aPTT, low fibrinogen, elevated D-dimer, and thrombocytopenia.
Traps and worked example
Traps: giant platelets counted as WBCs, lipemia interference, clotted samples, and reporting a critical platelet count without smear review. Worked correction: corrected WBC = automated WBC × 100/(100 + nRBCs per 100 WBCs). If the analyzer reports 12.0 × 10^9/L and there are 20 nRBCs per 100 WBCs, corrected WBC = 12.0 × 100/120 = 10.0 × 10^9/L. Rule of thumb: verify every unexpected low platelet count on the smear before calling it critical.