Haematocrit is a measure of the number of red blood cells in the blood. The value is expressed as a percentage or fraction of cells in blood. 45 percent for men and 40 percent for women are normal levels. The job of the red blood cells is to transport oxygen from the lungs to the cells in the body.
An abnormally low haematocrit level is known as anaemia, while an abnormally high level is known as polycythaemia. Anaemia means that the body is not being oxygenated correctly, while polycythaemia increases the risk of stroke and cardiovascular disease. Haematocrit levels among professional sportsmen and women are measured as part of the tests for blood doping or doping with erythropoietin (EPO). The level of haematocrit in a blood sample is compared with the long-term level for the sportsman or woman (so as to take into account individual variations in haematocrit level) and against an absolute permitted maximum (which is based on the maximum anticipated levels in the population, and levels which result in increased risk of blood clots leading to stroke or heart attack).
High levels are seen in cases of polycythaemia.
Low levels are seen in cases of anaemia. Reduced haematocrit with a normal number of erythrocytes can be seen in pregnancy, following splenectomy and in cases of macroglobulinaemia. This is due to an increase in plasma volume.