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The hematocrit () (Ht or HCT), also known by several other names, is the volume percentage (vol%) of
red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
s (RBCs) in
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...
, measured as part of a
blood test A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a hypodermic needle, or via fingerprick. Multiple tests for specific blood components, such as a glucose test or a cholester ...
. The measurement depends on the number and size of red blood cells. It is normally 40.7–50.3% for males and 36.1–44.3% for females. It is a part of a person's
complete blood count A complete blood count (CBC), also known as a full blood count (FBC), is a set of medical laboratory tests that provide cytometry, information about the cells in a person's blood. The CBC indicates the counts of white blood cells, red blood cell ...
results, along with
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
concentration,
white blood cell White blood cells, also called leukocytes or leucocytes, are the cell (biology), cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and de ...
count and
platelet Platelets, also called thrombocytes (from Greek θρόμβος, "clot" and κύτος, "cell"), are a component of blood whose function (along with the coagulation factors) is to react to bleeding from blood vessel injury by clumping, thereby ini ...
count. Because the purpose of red blood cells is to transfer oxygen from the lungs to body tissues, a blood sample's hematocrit—the red blood cell volume percentage—can become a point of reference of its capability of delivering oxygen. Hematocrit levels that are too high or too low can indicate a blood disorder,
dehydration In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
, or other medical conditions. An abnormally low hematocrit may suggest
anemia Anemia or anaemia (British English) is a blood disorder in which the blood has a reduced ability to carry oxygen due to a lower than normal number of red blood cells, or a reduction in the amount of hemoglobin. When anemia comes on slowly, th ...
, a decrease in the total amount of red blood cells, while an abnormally high hematocrit is called
polycythemia Polycythemia (also known as polycythaemia) is a laboratory finding in which the hematocrit (the volume percentage of red blood cells in the blood) and/or hemoglobin concentration are increased in the blood. Polycythemia is sometimes called erythr ...
. Both are potentially life-threatening disorders.


Names

There are other names for the hematocrit, such as ''packed cell volume'' (PCV), ''volume of packed red cells'' (VPRC), or ''erythrocyte volume fraction'' (EVF). The term ''hematocrit'' (or ''haematocrit'' in
British English British English (BrE, en-GB, or BE) is, according to Lexico, Oxford Dictionaries, "English language, English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the English language in ...
) comes from the
Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...
words (, "blood") and (, "judge"), and hematocrit means "to separate blood". It was coined in 1891 by Swedish physiologist
Magnus Blix Magnus Gustaf Blix (25 December 1849 – 14 February 1904) was a Swedish physiologist born in the parish Säbrå, presently located in Härnösand Municipality. He is the grandfather of UN weapons inspector Hans Blix. During his career he was a ...
as ''haematokrit'', modeled after ''lactokrit''.


Measurement methods

With modern lab equipment, the hematocrit can be calculated by an
automated analyzer An automated analyser is a medical laboratory instrument designed to measure different chemicals and other characteristics in a number of biological samples quickly, with minimal human assistance. These measured properties of blood and other fluids ...
or directly measured, depending on the analyzer manufacturer. Calculated hematocrit is determined by multiplying the red cell count by the
mean cell volume The mean corpuscular volume, or mean cell volume (MCV), is a measure of the average volume of a red blood corpuscle (or red blood cell). The measure is obtained by multiplying a volume of blood by the proportion of blood that is cellular (the hema ...
. The hematocrit is slightly more accurate, as the PCV includes small amounts of
blood plasma Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the intra ...
trapped between the red cells. An estimated hematocrit as a percentage may be derived by tripling the
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
concentration in g/ dL and dropping the units. The packed cell volume (PCV) can be determined by centrifuging
EDTA Ethylenediaminetetraacetic acid (EDTA) is an aminopolycarboxylic acid with the formula H2N(CH2CO2H)2sub>2. This white, water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-soluble complexes eve ...
-treated or
heparin Heparin, also known as unfractionated heparin (UFH), is a medication and naturally occurring glycosaminoglycan. Since heparins depend on the activity of antithrombin, they are considered anticoagulants. Specifically it is also used in the treatm ...
ized blood in a
capillary tube Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...
(also known as a microhematocrit tube) at 10,000 
RPM Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
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. Another way of measuring hematocrit levels is by optical methods such as
spectrophotometry Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as spec ...
. Through differential spectrophotometry, the differences in optical densities of a blood sample flowing through small-bore glass tubes at
isosbestic In spectroscopy, an isosbestic point is a specific wavelength, wavenumber or frequency at which the total absorbance of a sample does not change during a chemical reaction or a physical change of the sample. The word derives from two Greek words ...
wavelengths for deoxyhemoglobin and oxyhemoglobin and the product of the luminal diameter and hematocrit create a linear relationship that is used to measure hematocrit levels. There are some risks and side effects that accompany the tests of hematocrit because blood is being extracted from subjects. Subjects may experience a more than normal amount of hemorrhaging,
hematoma A hematoma, also spelled haematoma, or blood suffusion is a localized bleeding outside of blood vessels, due to either disease or trauma including injury or surgery and may involve blood continuing to seep from broken capillary, capillaries. A he ...
, fainting, and possibly infection. While known hematocrit levels are used in detecting conditions, it may fail at times due to hematocrit being the measure of concentration of red blood cells through volume in a blood sample. It does not account for the mass of the red blood cells, and thus the changes in mass can alter a hematocrit level or go undetected while affecting a subject's condition. Additionally, 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.


Levels

Hematocrit can vary from the determining factors of the number of red blood cells. These factors can be from the age and sex of the subject. Typically, a higher hematocrit level signifies the blood sample's ability to transport oxygen, which has led to reports that an "optimal hematocrit level" may exist. Optimal hematocrit levels have been studied through combinations of assays on blood sample's hematocrit itself, viscosity, and hemoglobin level. Hematocrit levels also serve as an indicator of health conditions. Thus, tests on hematocrit levels are often carried out in the process of diagnosis of such conditions, and may be conducted prior to surgery. Additionally, the health conditions associated with certain hematocrit levels are the same as ones associated with certain hemoglobin levels. As blood flows from the arterioles into the capillaries, a change in pressure occurs. In order to maintain pressure, the capillaries branch off to a web of vessels that carry blood into the venules. Through this process blood undergoes micro-circulation. In micro-circulation, the
Fåhræus effect The Fåhræus effect is the decrease in average concentration of red blood cells in human blood as the diameter of the glass tube in which it is flowing decreases. In other words, in blood vessels with diameters less than 500 micrometers, the he ...
will take place, resulting in a large change in hematocrit. As blood flows through the arterioles, red cells will act a feed hematocrit (Hf), while in the capillaries, a tube hematocrit (Ht) occurs. In tube hematocrit, plasma fills most of the vessel while the red cells travel through in somewhat of a single file line. From this stage, blood will enter the venules increasing in hematocrit, in other words the discharge hematocrit (Hd).In large vessels with low hematocrit, viscosity dramatically drops and red cells take in a lot of energy. While in smaller vessels at the micro-circulation scale, viscosity is very high. With the increase in shear stress at the wall, a lot of energy is used to move cells.


Shear rate relations

Relationships between hematocrit, viscosity, and shear rate are important factors to put into consideration. Since blood is non-Newtonian, the viscosity of the blood is in relation to the hematocrit, and as a function of shear rate. This is important when it comes to determining shear force, since a lower hematocrit level indicates that there is a need for more force to push the red blood cells through the system. This is because shear rate is defined as the rate to which adjacent layers of fluid move in respect to each other. Plasma is a more viscous material than typically red blood cells, since they are able to adjust their size to the radius of a tube; the shear rate is purely dependent on the amount of red blood cells being forced in a vessel.


Elevated

Generally at both sea levels and high altitudes, hematocrit levels rise as children mature. These health-related causes and impacts of elevated hematocrit levels have been reported: * Fall in
blood plasma Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the intra ...
levels *
Dehydration In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
* Administering of
testosterone Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of Male reproductive system, male reproductive tissues such as testes and prostate, as well as promoting secondar ...
supplement therapy * In cases of
dengue fever Dengue fever is a mosquito-borne tropical disease caused by the dengue virus. Symptoms typically begin three to fourteen days after infection. These may include a high fever, headache, vomiting, muscle and joint pains, and a characterist ...
, a high hematocrit is a danger sign of an increased risk of dengue shock syndrome. Hemoconcentration can be detected by an escalation of over 20% in hematocrit levels that will come before shock. For early detection of dengue hemorrhagic fever, it is suggested that hematocrit levels be kept under observations at a minimum of every 24 hours; 3–4 hours is suggested in suspected dengue shock syndrome or critical cases of dengue hemorrhagic fever. *
Polycythemia vera Polycythemia vera is an uncommon myeloproliferative neoplasm (a type of chronic leukemia) 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 ...
(PV), a
myeloproliferative disorder Myeloproliferative neoplasms (MPNs) are a group of rare blood cancers in which excess red blood cells, white blood cells or platelets are produced in the bone marrow. ''Myelo'' refers to the bone marrow, ''proliferative'' describes the rapid growt ...
in which the bone marrow produces excessive numbers of red cells, is associated with elevated hematocrit. *
Chronic obstructive pulmonary disease Chronic obstructive pulmonary disease (COPD) is a type of progressive lung disease characterized by long-term respiratory symptoms and airflow limitation. The main symptoms include shortness of breath and a cough, which may or may not produce ...
(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 Erythropoietin (; EPO), also known as erythropoetin, haematopoietin, or haemopoietin, is a glycoprotein cytokine secreted mainly by the kidneys in response to cellular hypoxia; it stimulates red blood cell production (erythropoiesis) in the bo ...
by the kidneys in response to hypoxia. * Professional athletes' hematocrit levels are measured as part of tests for
blood doping Blood doping is a form of doping in which the number of red blood cells in the bloodstream is boosted in order to enhance athletic performance. Because such blood cells carry oxygen from the lungs to the muscles, a higher concentration in the bl ...
or
erythropoietin Erythropoietin (; EPO), also known as erythropoetin, haematopoietin, or haemopoietin, is a glycoprotein cytokine secreted mainly by the kidneys in response to cellular hypoxia; it stimulates red blood cell production (erythropoiesis) in the bo ...
(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 A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and a ...
(AAS) use can also increase the amount of RBCs and, therefore, impact the hematocrit, in particular the compounds
boldenone Boldenone (developmental code name RU-18761), is a naturally occurring anabolic–androgenic steroid (AAS) and the 1(2)- dehydrogenated analogue of testosterone. Boldenone itself has never been marketed; as a pharmaceutical drug, it is used as ...
and
oxymetholone Oxymetholone, sold under the brand names Anadrol and Anapolon among others, is an androgen and anabolic steroid (AAS) medication which is used primarily in the treatment of anemia. It is also used to treat osteoporosis, HIV/AIDS wasting syndrom ...
. *
Capillary leak syndrome Capillary leak syndrome, or vascular leak syndrome, is characterized by the escape of blood plasma through capillary walls, from the blood circulatory system to surrounding tissues, muscle compartments, organs or body cavities. It is a phenomenon ...
also leads to abnormally high hematocrit counts, because of the episodic leakage of plasma out of the circulatory system. * At higher altitudes, there is a lower oxygen supply in the air and thus hematocrit levels may increase over time. Hematocrit levels were also reported to be correlated with social factors that influence subjects. In the 1966–80 Health Examination Survey, there was a small rise in mean hematocrit levels in female and male adolescents that reflected a rise in annual family income. Additionally, a higher education in a parent has been put into account for a rise in mean hematocrit levels of the child.


Lowered

Lowered hematocrit levels also pose health impacts. These causes and impacts have been reported: * A low hematocrit level is a sign of a low red blood cell count. One way to increase the ability of oxygen transport in red blood cells is through blood transfusion, which is carried out typically when the red blood cell count is low. Prior to the blood transfusion, hematocrit levels are measured to help ensure the transfusion is necessary and safe. * A low hematocrit with a low
mean corpuscular volume The mean corpuscular volume, or mean cell volume (MCV), is a measure of the average volume of a red blood corpuscle (or red blood cell). The measure is obtained by multiplying a volume of blood by the proportion of blood that is cellular (the hema ...
(MCV) with a high red cell distribution width (RDW) suggests a chronic iron-deficient anemia resulting in abnormal hemoglobin synthesis during
erythropoiesis Erythropoiesis (from Greek 'erythro' meaning "red" and 'poiesis' "to make") is the process which produces red blood cells (erythrocytes), which is the development from erythropoietic stem cell to mature red blood cell. It is stimulated by decrea ...
. The MCV and the 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. * Decreased hematocrit levels could indicate life-threatening diseases such as leukemia. When the bone marrow no longer produces normal red blood cells, hematocrit levels deviate from normal as well and thus can possibly be used in detecting
acute myeloid leukemia Acute myeloid leukemia (AML) is a cancer of the myeloid line of blood cells, characterized by the rapid growth of abnormal cells that build up in the bone marrow and blood and interfere with normal blood cell production. Symptoms may includ ...
. It can also be related to other conditions, such as malnutrition, water intoxication, anemia, and bleeding. * Pregnancy may lead to women having additional fluid in blood. This could potentially lead to a small drop in hematocrit levels.


See also

*
Blood test A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a hypodermic needle, or via fingerprick. Multiple tests for specific blood components, such as a glucose test or a cholester ...
s *
Hematology Hematology ( always spelled haematology in British English) is the branch of medicine concerned with the study of the cause, prognosis, treatment, and prevention of diseases related to blood. It involves treating diseases that affect the produc ...
*
Reference ranges for blood tests Reference ranges (reference intervals) for blood tests are sets of values used by a health professional to interpret a set of medical test results from blood samples. Reference ranges for blood tests are studied within the field of clinical chem ...


References


External links


Hematocrit
– Lab Tests Online {{Authority control Blood tests Mathematics in medicine