The hematocrit (HCT) test is a blood test that measures the percentage of your blood that is made up of red blood cells, the cells that carry oxygen from your lungs to the rest of your body. The result is a percentage — for example, a hematocrit of 42 means about 42% of your blood is red blood cells. It is often measured as part of a complete blood count (CBC).
According to MedlinePlus, a hematocrit test is used to help check general health and to help detect or monitor conditions such as anemia (too few red blood cells) or polycythemia (too many red blood cells). It may also be ordered when symptoms such as fatigue, shortness of breath, dizziness, or headache are present. It is one piece of a broader clinical picture, not a standalone diagnosis.
General reference values from the MedlinePlus Medical Encyclopedia are approximately: Male 37% to 48%; Female 34% to 43%; newborns 45% to 61%; infants 32% to 42%. These are general figures only. MedlinePlus notes that "normal value ranges vary slightly among different labs," that some labs use different measurements or test different samples, and that normal hematocrit also varies with your sex, age, whether you smoke, and the altitude where you live. The reference range printed on your own lab report — and your provider's interpretation of it — is what applies to you. A result outside the range does not by itself mean you have a condition needing treatment.
A higher-than-usual hematocrit is a general finding with several possible explanations, not a diagnosis by itself. Per MedlinePlus, it may be seen when the body is making too many red blood cells (for example with certain lung, heart, or bone marrow conditions such as polycythemia vera), or when blood plasma volume is low, which can be seen with dehydration. Other associations noted include cigarette smoking, low blood-oxygen levels, congenital heart disease, right-sided heart failure, a kidney tumor, scarring of the lungs (pulmonary fibrosis), and living at high altitude. This is not a complete list, and only a clinician can determine what a given result means.
A lower-than-usual hematocrit is also a general finding with many possible causes and is not a diagnosis. MedlinePlus notes it may be seen with bleeding (recent or ongoing), anemia, nutritional deficiencies (such as iron, folate, vitamin B12, vitamin B6, or copper), malnutrition, chronic illness, chronic kidney disease, bone marrow problems, red blood cell destruction, certain cancers such as leukemia or lymphoma, or pregnancy. This is not a complete list. A hematocrit value should always be interpreted by your health care provider in the context of your medical history, symptoms, and other test results. This is general information, not medical advice.
Reference ranges vary by laboratory, age, sex, and method — the range on your own report is what applies to you. A single value out of range doesn’t confirm any condition; your clinician interprets it alongside your symptoms, history, and other results. This page is general information, not medical advice.
General reference, not medical advice, and not a substitute for your clinician. Lab reference ranges and interpretation depend on the laboratory and on your individual situation — discuss your results with a licensed healthcare professional.