Hematology: CBC Interpretation and Morphology
Core concepts
Know the CBC components: RBC, hemoglobin, hematocrit, MCV, MCH, MCHC, RDW, WBC with differential, and platelets. Understand the anemia classification by MCV: microcytic (iron deficiency, thalassemia, anemia of chronic disease), normocytic (acute blood loss, hemolysis, renal disease), and macrocytic (B12/folate deficiency, alcohol, hypothyroidism). Know the reticulocyte count and its correction as the key discriminator between decreased production and increased destruction/loss.
Morphology and special findings
Know classic morphology: microcytes and pencil cells in iron deficiency, target cells in thalassemia and liver disease, schistocytes in microangiopathic hemolysis (TTP, HUS, DIC), hypersegmented neutrophils in megaloblastic anemia, and spherocytes in hereditary spherocytosis or autoimmune hemolysis. Understand the WBC differential shifts: neutrophilia with left shift in bacterial infection, lymphocytosis in viral illness, eosinophilia in allergy/parasites, and the significance of blasts.
Traps
Questions test the difference between MCV and RDW (thalassemia has normal RDW; iron deficiency has elevated RDW). Beware assuming a low hemoglobin always means iron deficiency, and remember that a normal MCV can hide a mixed anemia.
Worked example
MCV 70 fL with elevated RDW and low ferritin → iron deficiency. MCV 70 fL with normal RDW and normal ferritin → suspect thalassemia trait. Rule of thumb: RDW helps separate the microcytic anemias.