Erythropoietin (EPO) is a glycoprotein hormone secreted by the kidneys which acts on bone marrow precursors of the erythrocyte lineage to stimulate red blood cell production (erythropoiesis). It is released into the blood stream in response to low oxygen levels. EPO also plays an important role in the brain’s response to neuronal injury and is involved in the wound healing process. This test is primarily ordered to help differentiate between different types of anemia, and to determine whether the amount of erythropoietin being produced is appropriate for the level of anemia present. INCREASED erythropoietin production is associated with some benign or malignant kidney tumors, polycythemia which might lead to hypertension, lung disease, and chronic heart failure. DECREASED erythropoietin levels are seen in patients with renal insufficiency, anemic patients with cancer, as well as those with rheumatoid arthritis, HIV infection, ulcerative colitis, and sickle cell anemia.