Hematocrit, abbreviated Hct, is defined as the percentage of a person's blood occupied by red blood cells (RBCs). It is calculated as follows:
RBC Volume/ Total Blood Sample Volume x 100
The normal Hct value for adult females ranges from 35-45%. Hct in adult males ranges from 40-50%.
A hematocrit can be performed in a physician's office provided s/he has a tabletop centrifuge available. The patient's skin is pierced with a lancet, and a few drops of blood are collected in a thin glass tube by capillary action. To put it in perspective, the tube is slightly larger than a toothpick. The tube is then centrifuged at low speed for approximately 30 seconds. The tube is then placed against a device that acts as a ruler. The RBCs appear distinctly darker than the rest of the blood sample. If they occupy roughly half of the sample, the Hct is 50%. If they occupy one quarter of the sample volume, Hct is 25%. As stated above, normal Hct is > 35%.
There are numerous causes of a low hematocrit. Any disease state resulting in decreased red blood cell production or in which red cell destruction outpaces production of new red cells results in a low hematocrit. Iron deficiency anemia is a leading culprit along with deficiencies of folic acid or Vitamin B12. Bone marrow suppression, adverse drug reactions, and certain cancers like leukemia can also cause a low Hct.
The main diseases associated with an abnormally high Hct include iron overload states, especially hemochromatosis; the bone marrow condition polycythemia rubra vera; and cancers that release excessive amounts of the hormone erythropoietin, e.g. renal cell carcinoma. A high Hct is not usually life threatening but can increase the risk of blood clot formation.
Another noteworthy cause of a high Hct is the relatively recent practice of "blood doping" by athletes. Briefly, the athlete receives an injection of erythropoeitin 2 to 3 weeks prior to a sporting event to boost RBC production. S/he then donates a unit of blood which is fractionated into packed red cells. A few days prior to the competition, the athlete receives an autologous red cell transfusion, which increases Hct by 5% or so. Supposedly this technique enhances the amount of oxygen transported to skeletal muscle, resulting in a competitive advantage. Most sports organizations, including the IOC, have banned blood doping.