Hematocrit (HCT) is the red blood cell (RBC) volume concentration compared to the whole blood volume. Viscosity (n) increases directly, but not linearly with increases in HCT. For example: an HCT increase from 40-50% increases n by 10%. But an HCT increase from 50-60% increases n by 40%.

Since resistance (R) is directly proportional to viscosity, a greater perfusion pressure is needed to maintain flow when viscosity increases.

Clinical indications:

Normal HCT – 37-52%

Polycythemia vera- HCT increase to 65-70%.

Anemia- HCT decrease.