What does hematocrit (Hct) measure?

• A. The percent of blood volume occupied by red blood cells
• B. The rate of blood flow
• C. The white blood cell count
• D. The platelet count

Answer: (A)

Hematocrit measures the proportion of blood volume that is occupied by red blood cells. After the sample is spun, red cells settle to the bottom, plasma remains on top, and the height of the red cell layer is expressed as a percentage of the total blood volume. This reflects how much of the blood is made up of oxygen-carrying cells, which is why it’s linked to the blood’s capacity to carry oxygen. Clinically, a low hematocrit points to anemia or blood loss, while a high hematocrit can indicate dehydration or polycythemia. It relates to, but is not the same as, hemoglobin concentration (roughly, hematocrit ≈ hemoglobin × 3). It does not measure how fast blood flows, nor does it count white blood cells or platelets.