Erythropoietin (EPO) is a hormone produced in the kidney (and liver) cells that controls red blood cell production. It also is a cytokine for red-blood-cell precursors in bone marrow. As a specialty test, EPO can assist in differentiation between various forms of anemia. Further, the hormone concentration can inform if an insufficiency of EPO may be causing and/or exacerbating the anemia and is often checked after abnormal CBC test results. The test may also be used in patients with chronic kidney disease in regular intervals to evaluate the kidneys' continued ability to produce sufficient erythropoietin, which can be supplemented as needed.