Which measure determines the average size of an erythrocyte?

• A. Mean corpuscular hemoglobin
• B. Mean corpuscular volume
• C. Mean corpuscular hemoglobin concentration
• D. Packed cell volume

Answer: (B)

Mean corpuscular volume directly measures the average size of red blood cells. It’s reported in femtoliters and is calculated from the hematocrit and red blood cell count (or measured by an analyzer). Because it reflects cell size, it’s the key index used to classify anemia as microcytic, normocytic, or macrocytic. A low MCV indicates smaller-than-normal cells, as seen in iron deficiency anemia; a high MCV indicates larger-than-normal cells, as seen with B12 or folate deficiencies. The other indices describe hemoglobin content rather than size: mean corpuscular hemoglobin is the average amount of hemoglobin per cell, mean corpuscular hemoglobin concentration is the concentration of hemoglobin within a given volume of packed red cells, and packed cell volume (hematocrit) is the proportion of blood that is red cells, not their size.