Polycythemia (or polycythaemia)'s a condition in which there's an increase in hematocrit. It can be due to an increase in the mass of red blood cells ("absolute polycythemia") or to a decrease in the volume of plasma ("relative polycythemia"). 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. Primary polycythem… (More on Polycythemia)