What primarily determines the hematocrit value?

• A. Red blood cell mass
• B. Plasma volume
• C. White blood cell count
• D. Platelet count

Answer: (A)

Hematocrit is the fraction of blood that is made up of red blood cells. The main thing that sets this fraction is the red blood cell mass relative to total blood volume (hematocrit equals RBC volume divided by total blood volume). If you have more RBCs, the hematocrit goes up; if you have fewer RBCs, it goes down. Plasma volume can shift the value up or down by dilution or concentration, but the primary determinant is how many red blood cells are present. White blood cells and platelets contribute only a tiny share of the volume and don’t determine the hematocrit.