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Concept
Hématocrite
Résumé
The hematocrit (hɪˈmætəkrɪt) (Ht or HCT), also known by several other names, is the volume percentage (vol%) of red blood cells (RBCs) in blood, measured as part of a blood test. The measurement depends on the number and size of red blood cells. It is normally 40.7–50.3% for males and 36.1–44.3% for females. It is a part of a person's complete blood count results, along with hemoglobin concentration, white blood cell count and platelet count.
Because the purpose of red blood cells is to transfer oxygen from the lungs to body tissues, a blood sample's hematocrit—the red blood cell volume percentage—can become a point of reference of its capability of delivering oxygen. Hematocrit levels that are too high or too low can indicate a blood disorder, dehydration, or other medical conditions. An abnormally low hematocrit may suggest anemia, a decrease in the total amount of red blood cells, while an abnormally high hematocrit is called polycythemia. Both are potentially life-threatening disorders.
There are other names for the hematocrit, such as packed cell volume (PCV), volume of packed red cells (VPRC), or erythrocyte volume fraction (EVF). The term hematocrit (or haematocrit in British English) comes from the Ancient Greek words haima (αἷμα, "blood") and kritēs (κριτής, "judge"), and hematocrit means "to separate blood". It was coined in 1891 by Swedish physiologist Magnus Blix as haematokrit, modeled after lactokrit.
With modern lab equipment, the hematocrit can be calculated by an automated analyzer or directly measured, depending on the analyzer manufacturer. Calculated hematocrit is determined by multiplying the red cell count by the mean cell volume. The hematocrit is slightly more accurate, as the PCV includes small amounts of blood plasma trapped between the red cells. An estimated hematocrit as a percentage may be derived by tripling the hemoglobin concentration in g/dL and dropping the units.
The packed cell volume (PCV) can be determined by centrifuging EDTA-treated or heparinized blood in a capillary tube (also known as a microhematocrit tube) at 10,000 RPM for five minutes.
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