Anemias may be characterized morphologically as microcytic, normocytic, or macrocytic based on which parameter?

• A. Hematocrit
• B. MCV
• C. RDW
• D. Hemoglobin

Answer: (B)

Red blood cell size is the clue to classifying anemia morphologies, and the parameter that captures that size is the mean corpuscular volume. The MCV tells you, on average, how large the red cells are. When the MCV is below the normal range (roughly under 80 fL), the anemia is microcytic; when it’s normal (about 80–100 fL), it’s normocytic; and when it’s above the normal range (over 100 fL), it’s macrocytic. This makes MCV the best feature for distinguishing these morphologies. Hematocrit and hemoglobin describe how much red cell mass or oxygen-carrying capacity there is, not the size of the cells. RDW measures how much variation there is in cell size, which helps with diagnosis but isn’t the primary classifier of microcytosis or macrocytosis. For example, iron deficiency or thalassemia tends to be microcytic, B12 or folate deficiency macrocytic, and many etiologies are normocytic.