Polycythaemia 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 haematocrit level. 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. Source: Wikipedia