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The hematocrit (Ht or HCT, British spelling haematocrit), also known as packed cell volume (PCV) or erythrocyte volume fraction (EVF), is the volume percentage (%) of red blood cells in blood. It is normally about 45% for men and 40% for women. It is considered an integral part of a person's complete blood count results, along with hemoglobin concentration, white blood cell count, and platelet count.
In mammals, hematocrit is independent of body size.
The term hematocrit (Greek αιματοκριτης) comes from the Greek words hema (Greek αιμα, meaning "blood") and criterion (Greek κριτηριον)[clarification needed]. It was coined by Magnus Blix at Uppsala in 1891 as haematokrit.  The word haematokrit was modeled after lactokrit which was used in dairy farming. (See also: Scudder & Self in NEJM Oct 30, 1941 225:18 p. 679 "Controlled Administration of Fluid in Surgery").
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. Because a tube is used, this can be calculated by measuring the lengths of the layers.
With modern lab equipment, the hematocrit is calculated by an automated analyzer and 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 hemoglobin concentration is the measure used by blood banks to assess the level of iron in the donor's blood.
There have been cases in which the blood for testing was inadvertently drawn distal 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 would 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. On the converse, if saline or other fluids are being supplied, the blood sample would be diluted and the hematocrit will be artificially low.
Chronic obstructive pulmonary disease (COPD) and other pulmonary conditions associated with hypoxia may elicit an increased production of red blood cells. This increase is mediated by the increased levels of erythropoietin by the kidneys in response to hypoxia.
Professional athletes' hematocrit levels are measured as part of tests for blood doping or erythropoietin (EPO) use; the level of hematocrit in a blood sample is compared with the long-term level for that athlete (to allow for individual variations in hematocrit level), and against an absolute permitted maximum (which is based on maximum expected levels within the population, and the hematocrit level that causes increased risk of blood clots resulting in strokes or heart attacks).
If a patient is dehydrated, the hematocrit may be elevated.
Capillary leak syndrome also leads to abnormally high hematocrit counts, because of the episodic leakage of plasma out of the circulatory system.
Sleep Apnea has been known to cause elevated hematocrit levels.
Lowered hematocrit can simply signify hemorrhage.
The mean corpuscular volume (MCV) and the red cell distribution width (RDW) can be quite helpful in evaluating a lower-than-normal hematocrit, because it can help the clinician determine whether blood loss is chronic or acute. The MCV is the size of the red cells and the RDW is a relative measure of the variation in size of the red cell population. A low hematocrit with a low MCV with a high RDW suggests a chronic iron-deficient erythropoiesis, but a normal RDW suggests a blood loss that is more acute, such as a hemorrhage. One unit of packed red blood cells will elevate the hematocrit by about 3%.
Groups of individuals at risk for developing anemia include:
- infants without adequate iron intake
- children going through a rapid growth spurt, during which the iron available cannot keep up with the demands for a growing red cell mass
- women in childbearing years with a greater need for iron because of blood loss during menstruation
- pregnant women, in whom the growing fetus creates a high demand for iron
- patients with chronic kidney disease, as their kidneys no longer secrete sufficient levels of the hormone erythropoietin, which stimulates red blood cell production by the bone marrow
- Purves, William K.; David Sadava, Gordon H. Orians, H. Craig Heller (2004). Life: The Science of Biology (7th ed.). Sunderland, Mass: Sinauer Associates. p. 954. ISBN 0-7167-9856-5.
- SG Hedin in Skandanavia Arch.f Physiolgie 2:134-140,1891 "The Haematokrit: a New Apparatus for the Investigation of Blood."
- Medicine and the Reign of Technology, Stanley Joel Reiser, Cambridge University Press, Feb 27, 1981, p. 133
- "Hematocrit". Encyclopedia of Surgery: A Guide for Patients and Caregivers.
- "Hematocrit (HCT) or Packed Cell Volume (PCV)".