Polycythemia (also known as polycythaemia or polyglobulia) is a disease state in which the hematocrit (the volume percentage of red blood cells in the blood) and/or hemoglobin concentration are elevated in peripheral blood.
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 mass (due to an erythrocytosis or not), whereas erythrocytosis only refers to a documented increase of red cell count.
The emergency treatment of polycythemia (e.g., in hyperviscosity or thrombosis) is by phlebotomy (removal of blood from the circulation). Depending on the underlying cause, phlebotomy may also be used on a regular basis to reduce the hematocrit. Myelosuppressive medications such as hydroxyurea are sometimes used for long-term management of polycythemia.