What is the measure of the percentage of red blood cells in the total volume of blood?

• A. Hematocrit
• B. Hemoglobin
• C. Erythrocyte Count
• D. Platelet Volume

Answer: (A)

Hematocrit measures the percentage of blood volume occupied by red blood cells. When blood is spun down, plasma is on top and red blood cells form the bottom layer; the height of that red-cell layer relative to the total height is the hematocrit. This value reflects how much of the blood is made up of the oxygen-carrying cellular component. It’s different from hemoglobin, which is the oxygen-carrying protein inside those cells; it’s different from erythrocyte count, which is the number of red cells per microliter rather than their proportion of total blood volume; and it’s not related to platelets. Normal ranges vary by sex and age, but roughly 38–46% in many women and 40–54% in many men. Abnormally low or high values point to anemia or polycythemia/dehydration, respectively.