A hematocrit is used to determine the:

• A. level of oxygen in the blood.
• B. packed cell volume.
• C. red blood cell count.
• D. morphology of the red blood cells.

Answer: (B)

The main concept being tested is what a hematocrit actually measures: the proportion of blood volume taken up by red blood cells. This is determined by centrifuging a small blood sample in a hematocrit tube so the red cells form a packed layer at the bottom while the plasma is above. The height of the packed red-cell layer relative to the total column is expressed as a percentage, known as the packed cell volume. This value helps identify conditions like anemia (low hematocrit), polycythemia (high hematocrit), or dehydration that concentrates the red cells. It does not directly measure how much oxygen is in the blood, nor does it count red blood cells or assess their shape; those require different tests. Therefore, a hematocrit is used to determine the packed cell volume.