The HCT blood test or hematocrit test is a test that measures the proportion of red blood cells in the blood, states Healthline. The measurement depends on the size and number of red blood cells, according to MedlinePlus.
The HCT blood test is done by putting a sample of the patient's blood in a test tube and placing it in a centrifuge, says Mayo Clinic. Anticoagulant is added to the blood to prevent it from clotting, states Healthline. The centrifuge spins the blood in the test tube very quickly. This motion separates the blood into plasma, red blood cells and other blood cells. This helps the physician determine the volume of red blood cells in the blood. Low hematocrit levels are a possible sign of leukemia, lymphoma and bone marrow diseases. High hematocrit levels are potential signs of congenital heart disease or lung diseases.