The
hematocrit or
packed cell volume (
PCV) or
erythrocyte volume fraction (EVF) is the percentage (%) of the concentration of red blood cells in
bloodBlood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
. It is normally about 45% for men and 40% for women. It is considered an integral part of a person's
complete blood countA complete blood count , also known as full blood count or full blood exam or blood panel, is a test panel requested by a doctor or other medical professional that gives information about the cells in a patient's blood...
results, along with
hemoglobinHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
concentration,
white blood cellWhite blood cells, or leukocytes , are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...
count, and
plateletPlatelets, or thrombocytes , are small,irregularly shaped clear cell fragments , 2–3 µm in diameter, which are derived from fragmentation of precursor megakaryocytes. The average lifespan of a platelet is normally just 5 to 9 days...
count.
In
mammalMammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, hematocrit is independent of body size.
The term "hematocrit" comes from the German/Swedish "haematokrit" coined by Blix at Upsala in 1891 as referenced by SG Hedin in Skandanavia Arch.f Physiolgie 2:134-140,1891 in an article entitled "The Haematokrit: a New Apparatus for the Investigation of Blood." The "Haematokrit" was modeled after the "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
centrifugingA centrifuge is a piece of equipment, generally driven by an electric motor , that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis...
heparinHeparin , also known as unfractionated heparin, a highly sulfated glycosaminoglycan, is widely used as an injectable anticoagulant, and has the highest negative charge density of any known biological molecule...
ized 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 cellRed blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...
s 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 volumeThe mean corpuscular volume, or "mean cell volume" , is a measure of the average red blood cell size that is reported as part of a standard complete blood count....
. The hematocrit is slightly more accurate as the PCV includes small amounts of
blood plasmaBlood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...
trapped between the red cells. An estimated hematocrit as a percentage may be derived by tripling the
hemoglobinHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
concentration in
gThe gram is a metric system unit of mass....
/dL and dropping the units. The hemoglobin level is the measure used by
blood bankA blood bank is a cache or bank of blood or blood components, gathered as a result of blood donation, stored and preserved for later use in blood transfusion. The term "blood bank" typically refers to a division of a hospital laboratory where the storage of blood product occurs and where proper...
s to get a picture of the donors hydration level.
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 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.
Elevated
In cases of
dengue feverDengue fever , also known as breakbone fever, is an infectious tropical disease caused by the dengue virus. Symptoms include fever, headache, muscle and joint pains, and a characteristic skin rash that is similar to measles...
, a high hematocrit is a danger sign of an increased risk of dengue shock syndrome.
Polycythemia veraPolycythemia vera is a blood disorder in which the bone marrow makes too many red blood cells. It may also result in the overproduction of white blood cells and platelets. Most of the health concerns associated with polycythemia vera are caused by the blood being thicker as a result of the...
(PV), a myeloproliferative disorder in which the bone marrow produces excessive numbers of red cells, is associated with elevated hematocrit.
Chronic obstructive pulmonary diseaseChronic obstructive pulmonary disease , also known as chronic obstructive lung disease , chronic obstructive airway disease , chronic airflow limitation and chronic obstructive respiratory disease , is the co-occurrence of chronic bronchitis and emphysema, a pair of commonly co-existing diseases...
(COPD) and other pulmonary conditions associated with
hypoxiaHypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
may elicit an increased production of red blood cells. This increase is mediated by the increased levels of
erythropoietinErythropoietin, or its alternatives erythropoetin or erthropoyetin or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production...
by the kidneys in response to hypoxia.
Professional athletes' hematocrit levels are measured as part of tests for
blood dopingBlood doping is the practice of boosting the number of red blood cells in the bloodstream in order to enhance athletic performance. Because such blood cells carry oxygen from the lungs to the muscles, a higher concentration in the blood can improve an athlete’s aerobic capacity and...
or
ErythropoietinErythropoietin, or its alternatives erythropoetin or erthropoyetin or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production...
(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
SteroidA steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...
(AAS) use can also increase the amount of RBCs and, therefore, impact the hematocrit, in particular the compounds
boldenoneBoldenone , also known under the trade names Equipoise, Ganabol, Equigan and Ultragan, is an anabolic steroid developed for veterinary use, mostly for treatment of horses...
and
oxymetholoneOxymetholone is a synthetic anabolic steroid developed in 1960 by Zoltan 'Anadrol Z' F. Its primary clinical applications include treatment of osteoporosis and anaemia, as well as stimulating muscle growth in undernourished or underdeveloped patients. The drug was approved for human use by the FDA...
.
If a patient is dehydrated, the hematocrit may be elevated.
Capillary leak syndromeCapillary leak syndrome is a rare medical conditioncharacterized by self-reversing episodes during which the endothelial cells which line the capillaries are thought to separate for a few days, allowing for a leakage of fluid from the circulatory system to the interstitial space, resulting in a...
also leads to abnormally high hematocrit counts, because of the episodic leakage of plasma out of the circulatory system.
Lowered
Lowered hematocrit can imply significant hemorrhage.
The
mean corpuscular volumeThe mean corpuscular volume, or "mean cell volume" , is a measure of the average red blood cell size that is reported as part of a standard complete blood count....
(MCV) and the
red cell distribution widthThe red blood cell distribution width is a measure of the variation of red blood cell width that is reported as part of a standard complete blood count. Usually red blood cells are a standard size of about 6–8 μm. Certain disorders, however, cause a significant variation in cell size. Higher...
(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
erythropoiesisErythropoiesis is the process by which red blood cells are produced. It is stimulated by decreased O2 in circulation, which is detected by the kidneys, which then secrete the hormone erythropoietin...
, 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 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
Erythropoietin, or its alternatives erythropoetin or erthropoyetin or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production...
, which stimulates red blood cell production by the bone marrow