The erythropoietin test measures the amount of a hormone called erythropoietin (EPO) in blood. The hormone tells stem cells in the bone marrow to make more red blood cells. EPO is made by cells in the kidney. These cells release more EPO when blood oxygen level is low. An erythropoietin (EPO) test is used primarily to help diagnose the cause of anemia. It can help identify candidates for erythropoietin replacement therapy (e.g., people with chronic kidney disease). Sometimes it is used to help diagnose the cause of too many red blood cells or as part of an evaluation of a bone marrow disorder.