AskDefine | Define hematocrit

Dictionary Definition

hematocrit

Noun

1 the ratio of the volume occupied by packed red blood cells to the volume of the whole blood as measured by a hematocrit [syn: haematocrit, packed cell volume]
2 a measuring instrument to determine (usually by centrifugation) the relative amounts of corpuscles and plasma in the blood [syn: haematocrit]

User Contributed Dictionary

English

Noun

(Abbreviated as: Ht or HCT)
  1. The percentage (by volume) of packed red blood cells in a centrifuged sample of blood
  2. A centrifuge used to analyze the relative amount of red blood cells and plasma in blood

Translations

Extensive Definition

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 46% for men and 38% 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 was coined in 1903. Its roots stem from the Greek words hemat - blood, and krites, judge - meaning to gauge or judge the blood.

Method of measuring

The packed cell volume can be determined by centrifuging heparinized blood in a capillary tube (also known as a microhematocrit tube) at 10,000 RPM for five minutes.http://www.surgeryencyclopedia.com/Fi-La/Hematocrit.html 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 multiplying the hemoglobin concentration in g/dL times three and dropping the units. http://www.thedoctorslounge.net/hematology/labs/hematocrit.htm. The hemoglobin level is the measure used by blood banks.
There have been cases where the blood for testing was inadvertently drawn from the same arm with the intravenous line running in a transfusion of packed red cells. In this sample, the hemoglobin measurement will be high because it is measuring the fluid being transfused (that is, mostly red cells) rather than the diluted serum. In this case, the hematocrit measurement will be artificially very high.
Conversely, if blood for hematology testing is drawn from a site proximal to that of an intravenous line infusing fluids into a patient, the blood sample will be diluted by those fluids and the hematocrit will be artificially low.

Elevated hematocrit

In cases of dengue fever, where the full blood count is done daily, a high hematocrit is a danger sign of an increased risk of dengue shock syndrome.
Polycythemia vera (PV) is associated with elevated hematocrit. PV is a myeloproliferative disorder in which the bone marrow produces excessive numbers of red cells, and reflects excessive numbers of RBC precursors in the bone marrow, as well as some abnormal forms. This condition is called erythroid hyperplasia. Excessive production of both RBCs and WBCs is called bilineage hyperplasia, and if there are excessive numbers of platelets also, trilineage hyperplasia. If PV is present, it is not uncommon to see the serum uric acid level elevated, reflecting an increase in the rate of cell turnover, reflecting increased pyridine metabolism.
Another myeloproliferative disorder is called essential thrombocythemia, in which primarily the platelets are elevated in number.
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).
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 (for example, in an ectopic pregnancy.)
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 normal RDW suggests a chronic iron-deficient erythropoiesis, but a high 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.

Notes

References

hematocrit in Catalan: Hematòcrit
hematocrit in Czech: Hematokrit
hematocrit in Danish: Hæmatokritværdi
hematocrit in German: Hämatokrit
hematocrit in Spanish: Hematocrito
hematocrit in French: Hématocrite
hematocrit in Italian: Ematocrito
hematocrit in Hebrew: המטוקריט
hematocrit in Lithuanian: Hematokritas
hematocrit in Dutch: Hematocriet
hematocrit in Japanese: ヘマトクリット値
hematocrit in Low German: Hämatokrit
hematocrit in Polish: Hematokryt
hematocrit in Portuguese: Hematócrito
hematocrit in Russian: Гематокрит
hematocrit in Slovak: Hematokrit
hematocrit in Slovenian: Hematokrit
hematocrit in Finnish: Hematokriitti
hematocrit in Swedish: Hematokrit
hematocrit in Turkish: Hematokrit
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