What does a hematocrit test measure?

• A. The number of white blood cells per microliter of blood
• B. The concentration of platelets in the blood
• C. The oxygen saturation of hemoglobin
• D. The percentage of blood volume occupied by red blood cells

Answer: (D)

Hematocrit is the fraction of blood volume made up by red blood cells, usually expressed as a percentage. To measure it, blood is spun in a centrifuge so the plasma rises to the top and red blood cells pack at the bottom. The percentage is the height of the red cell layer divided by the total height of all components (times 100). This value helps show how much space red cells occupy in the blood, which is why it’s useful for spotting anemia (low hematocrit) or an unusually high red cell mass (polycythemia). Hydration also matters: dehydration can raise hematocrit by concentrating cells, while excess fluid can lower it. It’s distinct from other tests: the amount of white blood cells or platelets refers to cell counts, not how much of the blood volume is red cells, and oxygen saturation measures how much oxygen is bound to hemoglobin, not the proportion of red cells in blood.