Define hematocrit.

• A. The percentage of red blood cells within a sample of whole blood
• B. The percentage of white blood cells within a sample of whole blood
• C. The volume of plasma
• D. The number of platelets per microliter

Answer: (A)

Hematocrit is the proportion of blood that is made up of red blood cells, usually expressed as a percentage of the total blood volume. It’s measured by spinning a blood sample so the packed red cells form a bottom layer; the hematocrit equals the volume of that red-cell portion divided by the total volume, times 100. This value reflects the oxygen-carrying capacity of blood because red blood cells carry hemoglobin.

The other components mentioned are different measurements: the percentage of white blood cells would indicate leukocyte proportion; the volume of plasma is the liquid portion of blood, not the cellular component; and the number of platelets per microliter is a platelet count.