What is a hematocrit blood test?
The hematocrit blood test determines the percentage of red blood cells (RBC's) in the blood. Blood is composed mainly of red blood cells and white blood cells suspended in an almost clear fluid called serum. The hematocrit test indicates the percentage of blood by volume that is composed of red blood cells. The condition called "anemia" results from having too few red blood cells. Anemia causes a variety of symptoms. The hematocrit is a basic test that can tell a physician a lot about a person's health.
How is the hematocrit measured?
In most labs, the hematocrit is measured by a machine that automatically determines a variety of blood tests referred to as the blood count (CBC). The complete blood count is a numerical listing of the hematocrit, as well as the hemoglobin concentration, and the three blood cell lines produced by the bone marrow (the red blood cells, the white blood cells, and the platelets).
Another simple method is termed the spun hematocrit or "spun crit." A small amount of blood (about 0.05 to 0.1ml) is placed in a thin capillary tube, the tube is sealed with wax or clay, and then placed in a centrifuge to be spun. The red cells collect at the bottom and form a red column and are separated from the straw-colored serum column by a small area composed of white blood cells. The height of the total blood in the capillary tube (red cells, white cells and serum equals 100%). The height of the red cell column divided by the height of the total fluid in the capillary tube equals the hematocrit (percentage of RBC's in the total blood volume). This test can be performed in a few minutes.
What is a normal hematocrit?
Normal values for the hematocrit test vary according to age, sex, pregnancy, altitude where people live, and even vary slightly between various testing methods. The following are reported ranges of normal hematocrit levels:
- Newborns: 55%-68%
- One (1) week of age: 47%-65%
- One (1) month of age: 37%-49%
- Three (3) months of age: 30%-36%
- One (1) year of age: 29%-41%
- Ten (10) years of age: 36%-40%
- Adult males: 42%-54%
- Adult women: 38%-46%
- Adult pregnant women: about 30% - 34% lower limits and 46% upper limits
- High Altitude residents: about 45% - 61% in males; 41% - 56% in females (These levels gradually average higher as the altitude where people live increases. This is a result of the increased demand for the oxygen-carrying capacity of red blood cells at higher altitudes where there is decreased oxygen concentration in the atmosphere.)
These values may vary from authorities in the field by as much as 7%. Consequently, it is best to have a doctor explain the significance of an individual's level of hematocrit if it is not normal.
What does a low hematocrit mean?
A low hematocrit means the percentage of red blood cells is below the lower limits of normal (see above) for that person's age, sex, or specific condition (for example, pregnancy or high-altitude living). Another term for low hematocrit is anemia. Causes of low hematocrit, or anemia, include:
- Bleeding (ulcers, trauma, colon cancer, internal bleeding)
- Destruction of red blood cells (sickle cell anemia, enlarged spleen)
- Decreased production of red blood cells (bone marrow supression, cancer, drugs)
- Nutritional problems (low iron, B 12, folate and malnutrition)
- Overhydration (polydypsia, intravenous overhydration)
What does a high hematocrit mean?
A high hematocrit means the percentage of red blood cells in a person's blood is above the upper limits of normal (see above) for that person's age, sex, or specific condition (for example, pregnancy or high altitude living). Causes of a high hematocrit include:
- Dehydration (heat exhaustion, no available source of fluids)
- Low availability of oxygen (smoking, high altitude, pulmonary fibrosis)
- Genetic (congenital heart diseases)
- Erythrocytosis (over-production of red blood cells by the bone marrow or polycythemia vera)
- Cor pulmonale (COPD, chronic sleep apnea, pulmonary embolisms)
How is a low or high hematocrit treated?
The treatment of high or low hematocrit depends on the underlying cause(s), the hematocrit level, and the overall health status of the individual. Most people are not treated with medications or procedures if the hematocrit is only slightly above or below the normal levels. Some patients with very low hematocrits may require intravenous iron, transfusions or medications to stimulate the production of red cells by the bone marrow. Some patients with very high hematocrits due to diseases, such as polycythemia rubra vera, may require blood letting (blood removal).
The patient's doctor will decide when medication or procedures are necessary for each particular individual. In general, abnormal hematocrit values are monitored by doctors with routine blood testing.
Medically reviewed by John A. Daller, MD
American Board of Surgery with subspecialty certification in surgical critical care
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FDA.gov. Blood Products Advisory Committee. Topic II: Iron Status in Blood Donors.
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