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'Polycythemia' is a condition in which there is a net increase in the total number of red blood cells in the body. The overproduction of red blood cells may be due to a primary process in the bone marrow (a so-called myeloproliferative syndrome), or it may be a reaction to chronically low oxygen levels or, rarely, a malignancy.

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Primary polycythemia (Polycythemia vera)

Secondary polycythemia

Relative polycythemia

Primary polycythemia (Polycythemia vera)

''Primary polycythemia'', often called ''polycythemia vera'' (PCV), polycythemia rubra vera (PRV), or erythremia, occurs when excess red blood cells are produced as a result of an abnormality of the bone marrow. Often, excess white blood cells and platelets are also produced. Polycythemia vera is classified as a myeloproliferative disease.

Secondary polycythemia

Secondary polycythemia is caused by either natural or artificial increases in the production of erythropoietin that result in an increased production of erythrocytes. In secondary polycythemia, there may be 6 to 8 million and occasionally 9 million erythrocytes per cubic millimeter of blood. A type of secondary polycythemia in which the production of erythropoietin increases appropriately is called physiologic polycythemia. Physiologic polycythemia occurs in individuals living at high altitudes, where oxygen availability is less than at sea level. Many athletes train at higher altitudes to take advantage of this effect — a legal form of blood doping. Actual polycythemia sufferers have been known to use their condition as an athletic advantage for greater stamina.
Other causes of secondary polycythemia include smoking, renal or liver tumors, or heart or lung diseases that result in hypoxia. Endocrine abnormalities, prominently including pheochromocytoma and adrenal adenoma with Cushing's syndrome are also secondary causes. Athletes and bodybuilders who abuse anabolic steroids or erythropoietin may develop secondary polycythemia.

Relative polycythemia

Relative polycythemia is an apparent rise of the erythrocyte level in the blood; however, the underlying cause is reduced blood plasma. Relative polycythemia is often caused by fluid loss eg. burns, dehydration and stress polycythemia.