secondary polycythemia
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Polycythemia (also known as polycythaemia) is a laboratory finding in which the
hematocrit The hematocrit () (Ht or HCT), also known by several other names, is the volume percentage (vol%) of red blood cells (RBCs) in blood, measured as part of a blood test. The measurement depends on the number and size of red blood cells. It is norm ...
(the
volume percent In chemistry and fluid mechanics, the volume fraction φ''i'' is defined as the volume of a constituent ''V'i'' divided by the volume of all constituents of the mixture ''V'' prior to mixing: :\phi_i = \frac Being dimensionless, its unit is ...
age 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 in the
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 cir ...
) and/or
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 (erythrocyt ...
concentration are increased in the blood. Polycythemia is sometimes called erythrocytosis, and there is significant overlap in the two findings, but the terms are not the same: polycythemia describes any increase in hematocrit and/or hemoglobin, while erythrocytosis describes an increase specifically in the number of red blood cells in the blood. Polycythemia has many causes. It can describe an increase in the number 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 ("absolute polycythemia") or to a decrease in the volume of plasma ("relative polycythemia"). Absolute polycythemia can be due to genetic mutations in the bone marrow ("primary polycythemia"), physiologic adaptations to one's environment, medications, and/or other health conditions. Laboratory studies such as serum erythropoeitin levels and genetic testing might be helpful to clarify the cause of polycythemia if the physical exam and patient history don't reveal a likely cause. Mild polycythemia on its own is often asymptomatic. Treatment for polycythemia varies, and typically involves treating its underlying cause. Treatment of primary polycythemia (see
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 healt ...
) could involve
phlebotomy Phlebotomy is the process of making a puncture in a vein, usually in the arm, with a cannula for the purpose of drawing blood. The procedure itself is known as a venipuncture, which is also used for intravenous therapy. A person who performs a p ...
, antiplatelet therapy to reduce risk of blood clots, and additional cytoreductive therapy to reduce the number of red blood cells produced in the bone marrow.


Definition

Polycythemia is defined as serum hematocrit (Hct) or hemoglobin (HgB) exceeding normal ranges expected for age and sex, typically Hct > 49% in healthy adult men and >48% in women, or HgB >16.5g/dL in men or >16.0g/dL in women. The definition is different for neonates and varies by age in children.


Differential diagnoses


Polycythemia in adults

Different diseases or conditions can cause polycythemia in adults. These processes are discussed in more detail in their respective sections below. Relative polycythemia is not a true increase in the number of red blood cells or hemoglobin in the blood, but rather an elevated laboratory finding caused by reduced blood plasma ( hypovolemia, cf.
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 ...
). Relative polycythemia is often caused by loss of
body fluid Body fluids, bodily fluids, or biofluids, sometimes body liquids, are liquids within the human body. In lean healthy adult men, the total body water is about 60% (60–67%) of the total body weight; it is usually slightly lower in women (52-55% ...
s, such as through burns, dehydration, and stress. A specific type of relative polycythemia is Gaisböck syndrome. In this syndrome, primarily occurring in
obese Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's we ...
men, hypertension causes a reduction in plasma volume, resulting in (amongst other changes) a relative increase in red blood cell count. If relative polycythemia is deemed unlikely because the patient has no other signs of hemoconcentration, and has sustained polycythemia without clear loss of body fluids, the patient likely has absolute or true polycythemia. Absolute polycythemia can be split into two categories: * Primary polycythemia is the overproduction of red blood cells due to a primary process in the bone marrow (a so-called
myeloproliferative disease 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 grow ...
). These can be familial or congenital, or acquired later in life. * Secondary polycythemia is the most common cause of polycythemia. It occurs in reaction to chronically low oxygen levels, medications, other genetic mutations that impact the body's ability to transport or detect oxygen, or, rarely because of certain cancers. * Alternatively, additional red blood cells may have been received through another process—for example, being over-transfused (either accidentally or, as
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 blo ...
, deliberately).


Polycythemia in neonates

Polycythemia in newborns is defined as hematocrit > 65%. Significant polycythemia can be associated with blood hyperviscosity, or thickening of the blood. Causes of neonatal polycythemia include: * Hypoxia: Poor oxygen delivery (hypoxia) in utero resulting in compensatory increased production of red blood cells ( erythropoeisis). Hypoxia can be either acute or chronic. Acute hypoxia can occur as a result of perinatal complications. Chronic fetal hypoxia is associated with maternal risk factors such as hypertension, diabetes and smoking. * Umbilical cord stripping: delayed cord clamping and the stripping of the umbilical cord towards the baby can cause the residual blood in the cord/placenta to enter fetal circulation, which can increase blood volume. * The recipient twin in a pregnancy undergoing
twin-to-twin transfusion syndrome Twin-to-twin transfusion syndrome (TTTS), also known as feto-fetal transfusion syndrome (FFTS), twin oligohydramnios-polyhydramnios sequence (TOPS) and stuck twin syndrome, is a complication of monochorionic multiple pregnancies (the most comm ...
can have polycythemia.


Pathophysiology

The pathophysiology of polycythemia varies based on its cause. The production of red blood cells (or erythropoeisis) in the body is regulated by erythropoietin, which is a protein produced by the kidneys in response to poor oxygen delivery. As a result, more erythropoeitin is produced to encourage red blood cell production and increase oxygen-carrying capacity. This results in secondary polycythemia, which can be an appropriate response to hypoxic conditions such as chronic smoking, obstructive sleep apnea, and high altitude. Furthermore, certain genetic conditions can impair the body's accurate detection of oxygen levels in the serum, which leads to excess erythropoeitin production even without hypoxia or impaired oxygen delivery to tissues. Alternatively, certain types of cancers, most notably renal cell carcinoma, and medications such as testosterone use can cause inappropriate erythropoeitin production that stimulates red cell production despite adequate oxygen delivery. Primary polycythemia, on the other hand, is caused by genetic mutations or defects of the red cell progenitors within the bone marrow, leading to overgrowth and hyperproliferation of red blood cells regardless of erythropoeitin levels. Increased hematocrit and red cell mass with polycythemia increases the viscosity of blood, leading to impaired blood flow and contributing to an increased risk of clotting (thrombosis).


Evaluation


History and physical exam

The first step to evaluate new polycythemia in any individual is to conduct a detailed history and physical exam. Patients should be asked about smoking history, altitude, medication use, personal bleeding and clotting history, symptoms of
sleep apnea Sleep apnea, also spelled sleep apnoea, is a sleep disorder in which pauses in breathing or periods of shallow breathing during sleep occur more often than normal. Each pause can last for a few seconds to a few minutes and they happen many tim ...
(snoring, apneic episodes), and any family history of hematologic conditions or polycythemia. A thorough cardiopulmonary exam including auscultation of the heart and lungs can help evaluate for cardiac shunting or chronic pulmonary disease. An abdominal exam can assess for
splenomegaly Splenomegaly is an enlargement of the spleen. The spleen usually lies in the left upper quadrant (LUQ) of the human abdomen. Splenomegaly is one of the four cardinal signs of ''hypersplenism'' which include: some reduction in number of circulatin ...
, which can be seen in polycythemia vera. Examination of digits for
erythromelalgia Erythromelalgia or Mitchell's disease (after Silas Weir Mitchell (physician), Silas Weir Mitchell) is a rare vascular peripheral pain disorder in which blood vessels, usually in the lower extremities or hands, are episodically blocked (frequently ...
, clubbing or cyanosis can help assess for chronic hypoxia.


Laboratory evaluation

Polycythemia is often initially identified on a
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 information about the cells in a person's blood. The CBC indicates the counts of white blood cells, red blood cells and pl ...
(CBC). The CBC is often repeated to evalute for persistent polycythemia. If an etiology of polycythemia is unclear from history or physical, additional laboratory evaluation might include: * Blood smear to evaluate cell morphology * Iron panel to evaluate for concurrent
iron deficiency Iron deficiency, or sideropenia, is the state in which a body lacks enough iron to supply its needs. Iron is present in all cells in the human body and has several vital functions, such as carrying oxygen to the tissues from the lungs as a key ...
*
JAK2 Janus kinase 2 (commonly called JAK2) is a non-receptor tyrosine kinase. It is a member of the Janus kinase family and has been implicated in signaling by members of the type II cytokine receptor family (e.g. interferon receptors), the GM-CSF rece ...
mutation testing * Serum erythropoeitin (EPO) levels * Oxygen saturation (usually via
pulse oximetry Pulse oximetry is a noninvasive method for monitoring a person's oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy (within 4% accuracy in 95% of cases) of the more accurate (and invasive) reading o ...
or
blood gas A blood gas test or blood gas analysis tests blood to measure blood gas tension values, it also measures blood pH, and the level and base excess of bicarbonate. The source of the blood is reflected in the name of each test; arterial blood gase ...
tests) or oxygen dissociation tests


Additional testing

* Sleep studies if high suspicion for sleep apnea * Abdominal imaging, such as ultrasound * Erythropoietin receptor or von Hippel–Lindau (VHL) genetic testing, if high suspicion for familial erythrocytosis * Hemoglobin (globin-gene) sequencing or
high-performance liquid chromatography High-performance liquid chromatography (HPLC), formerly referred to as high-pressure liquid chromatography, is a technique in analytical chemistry used to separate, identify, and quantify each component in a mixture. It relies on pumps to pa ...
to evaluate for high-affinity hemoglobin variants *
Bone marrow biopsy Bone marrow examination refers to the pathologic analysis of samples of bone marrow obtained by bone marrow biopsy (often called trephine biopsy) and bone marrow aspiration. Bone marrow examination is used in the diagnosis of a number of conditio ...
might be considered in specific cases


Absolute polycythemia


Primary polycythemia

Primary polycythemias are
myeloproliferative disease 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 grow ...
s affecting red blood cell precursors in the bone marrow.
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 healt ...
(PCV) (a.k.a. polycythemia rubra vera (PRV)) occurs when excess red blood cells are produced as a result of an abnormality of the bone marrow. Often, excess white blood cells 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 i ...
s are also produced. A hallmark of polycythemia vera is an elevated hematocrit, with Hct > 55% seen in 83% of cases. A somatic (non-hereditary) mutation (V617F) in the ''
JAK2 Janus kinase 2 (commonly called JAK2) is a non-receptor tyrosine kinase. It is a member of the Janus kinase family and has been implicated in signaling by members of the type II cytokine receptor family (e.g. interferon receptors), the GM-CSF rece ...
'' gene, also present in other myeloproliferative disorders, is found in 95% of cases. Symptoms include headaches and
vertigo Vertigo is a condition where a person has the sensation of movement or of surrounding objects moving when they are not. Often it feels like a spinning or swaying movement. This may be associated with nausea, vomiting, sweating, or difficulties w ...
, and signs on physical examination include an abnormally enlarged spleen and/or
liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...
. In some cases, affected individuals may have associated conditions including high blood pressure or formation of blood clots. Transformation to acute
leukemia Leukemia ( also spelled leukaemia and pronounced ) is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or ...
is rare.
Phlebotomy Phlebotomy is the process of making a puncture in a vein, usually in the arm, with a cannula for the purpose of drawing blood. The procedure itself is known as a venipuncture, which is also used for intravenous therapy. A person who performs a p ...
is the mainstay of treatment. Primary familial polycythemia, also known as primary familial and congenital polycythemia (PFCP), exists as a benign hereditary condition, in contrast with the myeloproliferative changes associated with acquired PCV. In many families, PFCP is due to an autosomal dominant mutation in the ''EPOR'' erythropoietin receptor gene. PFCP can cause an increase of up to 50% in the oxygen-carrying capacity of the blood;
skier Skiing is the use of skis to glide on snow. Variations of purpose include basic transport, a recreational activity, or a competitive winter sport. Many types of competitive skiing events are recognized by the International Olympic Committee (IO ...
Eero Mäntyranta had PFCP, which is speculated to have given him an advantage in endurance events.


Secondary polycythemia

Secondary polycythemia is caused by either natural or artificial increases in the production of erythropoietin, hence an increased production of erythrocytes. Secondary polycythemia in which the production of erythropoietin increases appropriately is called physiologic polycythemia. Conditions which may result in physiologic polycythemia include: * Altitude related – Polycythemia can be a normal adaptation to living at high altitudes (see
altitude sickness Altitude sickness, the mildest form being acute mountain sickness (AMS), is the harmful effect of high altitude, caused by rapid exposure to low amounts of oxygen at high elevation. People can respond to high altitude in different ways. Sympt ...
). Many athletes train at high altitude to take advantage of this effect, which can be considered a legal form of blood doping, although the efficacy of this strategy is unclear. * Hypoxic disease-associated – for example, in cyanotic heart disease where blood oxygen levels are reduced significantly; in hypoxic lung disease such as
COPD 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 ...
; in chronic obstructive
sleep apnea Sleep apnea, also spelled sleep apnoea, is a sleep disorder in which pauses in breathing or periods of shallow breathing during sleep occur more often than normal. Each pause can last for a few seconds to a few minutes and they happen many tim ...
; conditions that reduce blood flow to the kidney e.g. renal artery stenosis. Chronic
carbon monoxide poisoning Carbon monoxide poisoning typically occurs from breathing in carbon monoxide (CO) at excessive levels. Symptoms are often described as " flu-like" and commonly include headache, dizziness, weakness, vomiting, chest pain, and confusion. Large ...
(which can be present in heavy smokers) and rarely methemoglobinemia can also impair oxygen delivery. * Genetic – Heritable causes of secondary polycythemia include abnormalities in hemoglobin oxygen release, which results in a greater inherent affinity for oxygen than normal adult hemoglobin and reduces oxygen delivery to tissues. Conditions where the secondary polycythemia is not caused by physiologic adaptation, and occurs irrespective of body needs include: * Neoplasms –
Renal cell carcinoma Renal cell carcinoma (RCC) is a kidney cancer that originates in the lining of the Proximal tubule, proximal convoluted tubule, a part of the very small tubes in the kidney that transport primary urine. RCC is the most common type of kidney cance ...
,
liver tumor Liver tumors (also known as hepatic tumors) are abnormal growth of liver cells on or in the liver. Several distinct types of tumors can develop in the liver because the liver is made up of various cell types. Liver tumors can be classified as beni ...
s,
Von Hippel–Lindau disease Von Hippel–Lindau disease (VHL), also known as Von Hippel–Lindau syndrome, is a rare genetic disorder with multisystem involvement. It is characterized by visceral cysts and benign tumors with potential for subsequent malignant transformation ...
, and endocrine abnormalities including
pheochromocytoma Pheochromocytoma (PHEO or PCC) is a rare tumor of the adrenal medulla composed of chromaffin cells, also known as pheochromocytes. When a tumor composed of the same cells as a pheochromocytoma develops outside the adrenal gland, it is referred t ...
and adrenal adenoma with
Cushing's syndrome Cushing's syndrome is a collection of signs and symptoms due to prolonged exposure to glucocorticoids such as cortisol. Signs and symptoms may include high blood pressure, abdominal obesity but with thin arms and legs, reddish stretch marks, a ...
. * Anabolic steroid use - people whose testosterone levels are high, including athletes who abuse steroids, people on testosterone replacement for
hypogonadism Hypogonadism means diminished functional activity of the gonads—the testes or the ovaries—that may result in diminished production of sex hormones. Low androgen (e.g., testosterone) levels are referred to as hypoandrogenism and low estroge ...
or
transgender A transgender (often abbreviated as trans) person is someone whose gender identity or gender expression does not correspond with their sex assigned at birth. Many transgender people experience dysphoria, which they seek to alleviate through ...
hormone replacement therapy Hormone replacement therapy (HRT), also known as menopausal hormone therapy or postmenopausal hormone therapy, is a form of hormone therapy used to treat symptoms associated with female menopause. These symptoms can include hot flashes, vaginal ...
. * Blood doping - Athletes who take erythropoietin-stimulating agents or receive blood transfusions to increase their red blood cell mass. * Post-transplant erythrocytosis - About 10-15% of patients after renal transplantation are found to have polycythemia at 24 months after transplantation, which can be associated with increased thrombotic (clotting) risk.


Altered oxygen sensing

Rare inherited mutations in three genes which all result in increased stability of
hypoxia-inducible factors Hypoxia-inducible factors (HIFs) are transcription factors that respond to decreases in available oxygen in the cellular environment, or hypoxia. They are only present in parahoxozoan animals. Discovery The HIF transcriptional complex w ...
, leading to increased erythropoietin production, have been shown to cause secondary polycythemia: * Chuvash erythrocytosis or Chuvash polycythemia is an autosomal recessive form of erythrocytosis endemic in patients from the Chuvash Republic in Russia. Chuvash erythrocytosis is associated with homozygosity for a C598T mutation in the von Hippel–Lindau gene ('' VHL''), which is needed for the destruction of
hypoxia-inducible factors Hypoxia-inducible factors (HIFs) are transcription factors that respond to decreases in available oxygen in the cellular environment, or hypoxia. They are only present in parahoxozoan animals. Discovery The HIF transcriptional complex w ...
in the presence of oxygen. Clusters of patients with Chuvash erythrocytosis have been found in other populations, such as on the Italian island of
Ischia Ischia ( , , ) is a volcanic island in the Tyrrhenian Sea. It lies at the northern end of the Gulf of Naples, about from Naples. It is the largest of the Phlegrean Islands. Roughly trapezoidal in shape, it measures approximately east to west ...
, located in the Bay of Naples. Patients with Chuvash erythrocytosis experience a significantly elevated risk of events. * PHD2 erythrocytosis:
Heterozygosity Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
for loss-of-function mutations of the '' PHD2'' gene are associated with autosomal dominant erythrocytosis and increased hypoxia-inducible factors activity. * HIF2α erythrocytosis: Gain-of-function mutations in'' HIF2α ''are associated with autosomal dominant erythrocytosis and pulmonary hypertension.


Symptoms

Polycythemia is often asymptomatic; patients may not experience any notable symptoms until their red cell count is very high. For patients with significant elevations in hemoglobin or hematocrit (often from polycythemia vera), some non-specific symptoms include: * A ruddy (red) complexion, or plethora * Headache, transient blurry vision (
amaurosis fugax Amaurosis fugax (Greek ''amaurosis'' meaning ''darkening'', ''dark'', or ''obscure'', Latin '' fugax'' meaning ''fleeting'') is a painless temporary loss of vision in one or both eyes. Signs and symptoms The experience of amaurosis fugax is clas ...
), other signs of a
transient ischemic attack A transient ischemic attack (TIA), commonly known as a mini-stroke, is a minor stroke whose noticeable symptoms usually end in less than an hour. TIA causes the same symptoms associated with strokes, such as weakness or numbness on one side of ...
(TIA) or stroke * Dizziness, fatigue * Unusual bleeding, nosebleeds * Pain in abdomen from enlarged spleen in polycythemia vera * Pain in hands and feet (
erythromelalgia Erythromelalgia or Mitchell's disease (after Silas Weir Mitchell (physician), Silas Weir Mitchell) is a rare vascular peripheral pain disorder in which blood vessels, usually in the lower extremities or hands, are episodically blocked (frequently ...
) * Itchiness, especially after a hot shower ( aquagenic pruritis) * Numbness or tingling in different body parts


Epidemiology

The prevalence of primary polycythemia (polycythemia vera) was estimated to be approximately 44-57 per 100 000 individuals in the United States. Secondary polycythemia is considered to be more common, but its exact prevalence is unknown. In one study using the
NHANES The National Health and Nutrition Examination Survey (NHANES) is a survey research program conducted by the National Center for Health Statistics (NCHS) to assess the health and nutritional status of adults and children in the United States, and t ...
dataset, the prevalence of unexplained erythrocytosis is 35.1 per 100,000, and was higher among males and among individuals between ages 50-59 and 60-69.


Management

The management of polycythemia varies based on its etiology: * See
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 healt ...
for management of primary polycythemia, which involves reducing thrombotic risk, symptom amelioration and monitoring for further hematologic complications. Treatment can include phlebotomy, aspirin, and myelosuppressive or cytoreductive medications based on risk stratification. * For secondary polycythemia, management involves addressing the underlying etiology of increased erythropoeitin production, such as smoking cessation, CPAP for sleep apnea, or removing any EPO-producing tumours. Phlebotomy is not typically recommended for patients with physiologic polycythemia, who rely on additional red cell mass for necessary oxygen delivery, unless the patient is clearly symptomatic and experiences relief from phlebotomy. It is unclear if patients with secondary polycythemia are at elevated thrombotic risk, but aspirin can be considered for patients at elevated cardiovascular risk or for patients with Chuvash polycythemia . The first-line treatment for post-transplant erythrocytosis specificially is angiotensin-converting enzyme (ACE) inhibitors or
angiotensin receptor blockers Angiotensin II receptor blockers (ARBs), formally angiotensin II receptor type 1 (AT1) antagonists, also known as angiotensin receptor blockers, angiotensin II receptor antagonists, or AT1 receptor antagonists, are a group of pharmaceuticals tha ...
.


Relation to Athletic Performance

Polycythemia is theorized to increased performance in endurance sports due to the blood being able to store more oxygen. This idea has led to the illegal use of blood doping and transfusions among professional athletes, as well as use of altitude training or elevation training masks to simulate a low-oxygen environment. However, the benefits of altitude training for athletes to improve sea-level performance are not universally accepted, with one reason being athletes at altitude might exert less power during training.


See also

*
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, t ...
, a decrease in red blood cell count * Cytopenia, a decrease in blood cell count *
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 ...
, another cause of
hemoconcentration The hematocrit () (Ht or HCT), also known by several other names, is the volume percentage (vol%) of red blood cells (RBCs) in blood, measured as part of a blood test. The measurement depends on the number and size of red blood cells. It is norm ...


References


External links

{{Pediatric conditions originating in the perinatal period Red blood cell disorders Haemorrhagic and haematological disorders of fetus and newborn Hematopathology