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polycythemia
Polycythemia (also known as polycythaemia or polyglobulia) is a disease state in which the proportion of blood volume that is occupied by red blood cells increases. Blood volume proportions can be measured as hematocrit level. A hematocrit of >55% is seen in polycythemia.
It can be due to an increase in the number of red blood cells ("absolute polycythemia") or to a decrease in the volume of plasma ("relative polycythemia"). Polycythemia is sometimes called erythrocytosis, but the terms are not synonymous because polycythemia refers to any increase in red blood cells, whereas erythrocytosis only refers to a documented increase of red cell mass.
The emergency treatment of polycythemia (e.g., in hyperviscosity or thrombosis) is by venesection, the removal of blood from the circulation. Depending on the underlying cause, venesection may also be used on a regular basis to reduce the red blood cell count. Cytostatics (busulfan, hydroxyurea) are sometimes used if venesection is ineffective or contraindicated.
==Absolute polycythemia==
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. Alternatively, additional red blood cells may have been received through another process—for example, being over-transfused (either accidentally or, as blood doping, deliberately) or being the recipient twin in a pregnancy, undergoing twin-to-twin transfusion syndrome.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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