Measure of the number of red blood cells in the blood. This number is actually a percentage of the blood volume that is composed of red blood cells. For example, a hematocrit of 50 percent would mean that within one milliliter of blood, red blood cells take up 50 percent or 0.5 milliliters. The hematocrit measurement is commonly used to identify anemia and determine when an infant needs a blood transfusion. This test may also be used to identify abnormal bleeding in an infant.