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Phlebotomy (removal of blood) should not be done frequently, but may be recommended when erythrocytosis causes hyperviscosity (extreme thickness) of the blood. The volume of the blood removed is replaced with intravenous (IV) saline solution to thin the blood. Phlebotomy is generally done only when symptoms are severe and/or the hematocrit (the percentage of blood that is made up of red blood cells) becomes extremely elevated.