The hematocrit, also known as packed cell volume or erythrocyte volume fraction, 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.
Etymology The term hematocrit comes from the Ancient
Greek words haima and kritēs. It was coined by Magnus Blix at Uppsala in 1891 as haematokrit,
modeled after lactokrit, which was used in dairy farming.
Measurement methods
The packed cell volume can be determined by centrifuging heparinized blood in a capillary
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.
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.
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. On the converse, if saline or other fluids are being supplied, the blood sample
would be diluted and the hematocrit will be artificially low.
Elevated In cases of dengue fever, a high hematocrit
is a danger sign of an increased risk of dengue shock syndrome.
Polycythemia vera, a myeloproliferative disorder in which the bone marrow produces excessive
numbers of red cells, is associated with elevated hematocrit.
Chronic obstructive pulmonary disease 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 use; the level of hematocrit in a blood sample is compared with the long-term
level for that athlete, and against an absolute permitted maximum.
Anabolic androgenic steroid use can also increase the amount of RBCs and, therefore, impact
the hematocrit, in particular the compounds boldenone and oxymetholone.
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
The mean corpuscular volume and the red cell distribution width can be quite helpful in
evaluating a lower-than-normal hematocrit, because they can help the clinician determine
whether blood loss is chronic or acute, although acute blood loss typically does not manifest
as a change in hematocrit, since hematocrit is simply a measure of how much of the blood
volume is made up of red blood cells. 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 anemia resulting in
abnormal hemoglobin synthesis during erythropoiesis. 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
menstruating woman, which have 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 whose kidneys no longer secrete sufficient levels
of the hormone erythropoietin that promotes RBC proliferation. Erythropoietin prevents
the death of cells in the erythrocyte cell line in the bone marrow. Therefore, erythropoietin
allows those cells to continue to mature, exit the bone marrow and become RBCs.
See also Anemia
Blood doping Blood tests
Hematology References
External links Hematocrit - Lab Tests Online