Hematocrit

The hematocrit (/hɪˈmætəkrɪt/) (Ht or HCT), also known by several other names, is a blood test that measures the volume percentage (vol%) of red blood cells (RBC) in blood. The measurement depends on the number and size of red bloods cells. It is normally 40.7% to 50.3% for men and 36.1% to 44.3% for women. 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. The hematocrit (/hɪˈmætəkrɪt/) (Ht or HCT), also known by several other names, is a blood test that measures the volume percentage (vol%) of red blood cells (RBC) in blood. The measurement depends on the number and size of red bloods cells. It is normally 40.7% to 50.3% for men and 36.1% to 44.3% for women. 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 is calculated by an automated analyzer and is not directly measured. It 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 heparinized blood in a capillary tube (also known as a microhematocrit tube) at 10,000 RPM for five minutes. This separates the blood into layers. The volume of packed red blood cells divided by the total volume of the blood sample gives the PCV. Since a tube is used, this can be calculated by measuring the lengths of the layers. Another way of measuring hematocrit levels is by optical methods such as spectrophotometry. Through differential spectrophotometry, the differences in optical densities of a blood sample flowing through small-bore glass tubes at isosbestic wavelengths for deoxyhemoglobin and oxyhemoglobin and the product of the luminal diameter and hematocrit create a linear relationship that is used to measure hematocrit levels. There are some risks and side effects that accompany the tests of hematocrit because blood is being extracted from subjects. Subjects may experience a more than normal amount of hemorrhaging, hematoma, fainting, and possibly infection. While known hematocrit levels are used in detecting conditions, it may fail at times due to hematocrit being the measure of concentration of red blood cells through volume in a blood sample. It does not account for the mass of the red blood cells, and thus the changes in mass can alter a hematocrit level or go undetected while affecting a subject's condition. Additionally, there have been cases in which the blood for testing was inadvertently drawn proximal to an intravenous line that was infusing packed red cells or fluids. In these situations, the hemoglobin level in the blood sample will not be the true level for the patient because the sample will contain a large amount of the infused material rather than what is diluted into the circulating whole blood. That is, if packed red cells are being supplied, the sample will contain a large amount of those cells and the hematocrit will be artificially very high. Hematocrit can vary from the determining factors of the number of red blood cells. These factors can be from the age and sex of the subject. Typically, a higher hematocrit level signifies the blood sample's ability to transport oxygen, which has led to reports that an 'optimal hematocrit level' may exist. Optimal hematocrit levels have been studied through combinations of assays on blood sample's hematocrit itself, viscosity, and hemoglobin level. Hematocrit levels also serve as an indicator of health conditions. Thus, tests on hematocrit levels are often carried out in the process of diagnosis of such conditions, and may be conducted prior to surgery. Additionally, the health conditions associated with certain hematocrit levels are the same as ones associated with certain hemoglobin levels.