Hematocrit 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 (αἷμα, "blood") and
kritēs (κριτής, "judge"). It was coined by Magnus Blix at Uppsala in 1891 as haematokrit,
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".)
Measurement methods 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.
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 (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 that causes increased risk of blood
clots resulting in strokes or heart attacks). Anabolic androgenic steroid (AAS) 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 (MCV) and the
red cell distribution width (RDW) 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.