Hematocrit (Blood Test): A hematocrit blood test measures the portion of red blood cells in the blood. Normal values of hematocrit in the blood vary according to age, sex, pregnancy, altitude where a person lives, and vary slightly between testing methods. Causes of low hematocrit levels in the blood may be due to bleeding disorders, destruction of red blood cells, cancer, anemia, and overhydration. High hematocrit levels may be caused by dehydration, smoking, pulmonary conditions, COPD, sleep apnea, and polycythemia vera.