ERYTHROPOIETIN - Erythropoietin is a protein hormone which is produced by specialized cells in the kidneys. These cells are sensitive to low arterial oxygen concentration and will release erythropoietin when oxygen is low. Erythropoietin stimulates the bone marrow to produce more red blood cells (to increase the oxygen caring capacity of the blood). The measurement of this hormone in the bloodstream can indicate bone marrow disorders or kidney disease. Normal levels of erythropoietin are 0 to 19 mU/ml (milliunits per milliliter). Elevated levels can be seen in polycythemia vera. Lower than normal values are seen in chronic renal failure.