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Thrombocytosis (or thrombocythemia) is the presence of high platelet counts in the blood, and can be either primary (also termed essential and caused by a myeloproliferative disease) or reactive (also termed secondary). Although often symptomless (particularly when it is a secondary reaction), it can predispose to thrombosis in some patients. Thrombocytosis can be contrasted with thrombocytopenia, a loss of platelets in the blood. In a healthy individual, a normal platelet count ranges from 150,000 and 450,000 per mm³ (or microlitre) (150–450 x 109/L). These limits, however, are determined by the 2.5th lower and upper percentile, and a deviation does not necessary imply any form of disease. Nevertheless, counts over 750,000 (and especially over a million) are considered serious enough to warrant investigation and intervention. ==Signs and symptoms== High platelet levels do not necessarily signal any clinical problems, and are picked up on a routine full blood count. However, it is important that a full medical history be elicited to ensure that the increased platelet count is not due to a secondary process. Often, it occurs in tandem with an inflammatory disease, as the principal stimulants of platelet production (e.g. thrombopoietin) are elevated in these clinical states as part of the acute phase reaction. High platelet counts can occur in patients with polycythemia vera (high red blood cell counts), and is an additional risk factor for complications. A very small segment of patients report symptoms of erythromelalgia, a burning sensation and redness of the extremities that resolves with cooling and/or aspirin use. Scientific literature sometimes excludes thrombocytosis from the scope of thrombophilia by definition,〔 ()〕 but practically, by the definition of thrombophilia as an increased predisposition to thrombosis, thrombocytosis (especially primary thrombocytosis) is a potential cause of thrombophilia. Conversely, secondary thrombocytosis very rarely causes thrombotic complications. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Thrombocytosis」の詳細全文を読む スポンサード リンク
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