Definition: Hematocrit is a blood test that tells us what proportion, by volume, of total blood cells are made up of red blood cells. Hematocrit is given as a percentage. For example, if 30% of total blood cell volume is taken up by red blood cells, the hematocrit is 30. Hematocrit is a part of a Complete Blood Count (CBC) test. Hematocrit is used to check for anemia. If low, it can indicate mild conditions, such as not getting enough iron in the diet, or serious conditions, such as abnormal bleeding due to colon cancer or other health problems.