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The hematocrit (Ht or HCT) or packed cell volume (PCV) or erythrocyte volume fraction (EVF) is the proportion of blood volume that is occupied by red blood cells. It is normally about 48% for men and 38% for women.[1] 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" (British English: haematocrit) was coined in 1903. Its roots stem from the Greek words hema (Gr αίμα) - blood, and krites (Gr κριτής), judge - meaning to gauge or judge the blood.

Contents

Measurement methods

Packed cell volume diagram

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.[2] 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.[3] The hemoglobin level is the measure used by blood banks.

There have been cases where 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 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. Conversely, if saline or other fluids are being supplied, the blood sample would be diluted and the hematocrit will be artificially low.

Elevated hematocrit

In cases of dengue fever a high hematocrit is a danger sign of an increased risk of dengue shock syndrome.

Polycythemia vera (PV), a myeloproliferative disorder in which the bone marrow produces excessive numbers of red cells, is associated with elevated hematocrit.

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 which causes increased risk of blood clots resulting in strokes or heart attacks).

Steroid use can also increase the amount of RBC's and therefore impact the hematocrit.

If a patient is dehydrated, the hematocrit may be elevated. Repeat testing after adequate hydration therapy will usually result in a more reliable result.

Lowered hematocrit

Lowered hematocrit can imply significant 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.

Groups of individuals who are at risk for developing anemia include:

  • infants who may not have 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 who have an excessive 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.

See also

References

  1. ^ Purves, William K.; David Sadava, Gordon H. Orians, H. Craig Heller (2004). Life: The Science of Biology (7th ed.). Sunderland, Mass: Sinauer Associates. pp. 954. ISBN 0-7167-9856-5.  
  2. ^ "Hematocrit". Encyclopedia of Surgery: A Guide for Patients and Caregivers. http://www.surgeryencyclopedia.com/Fi-La/Hematocrit.html.  
  3. ^ "Hematocrit (HCT) or Packed Cell Volume (PCV)". http://www.thedoctorslounge.net/hematology/labs/hematocrit.htm.  
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