Hematocrit is a measure of how much space (volume) the red blood
cells take up in the blood. A hematocrit check is often used to test for
anemia, which is a decrease in the amount of oxygen-carrying substance
(hemoglobin) found in red blood cells.
The hematocrit value may be given as a percentage of red blood
cells in a volume of blood. For example, a hematocrit of 33 means that 33% of
the blood's volume is composed of red cells. It may also be measured in
milliliters per deciliter (mL/dL) of blood.
E. Gregory Thompson, MD - Internal Medicine & Joseph O'Donnell, MD - Hematology, Oncology