EPO is a hormone produced in the kidneys that regulates red cell production. The levels vary inversely with hematocrit. This assay is useful as an aid in distinguishing between primary and secondary Polycythemia. It also differentiates between appropriate secondary Polycythemia (High altitude, pulmonary disease, tobacco use) and inappropriate secondary Polycythemia (tumors). The assay helps to identify candidates for EPO replacement therapy (Chronic renal failure). The assay evaluates patients undergoing EPO replacement therapy who demonstrate inadequate hematopoeitic response.