The erythropoietin test quantifies the level of the erythropoietin (EPO) hormone in the blood. The hormone instructs bone marrow stem cells to produce more red blood cells. The kidneys cells produce EPO. When blood oxygen levels are low these cells produce more EPO. The primary use of an erythropoietin (EPO) test is to identify the root cause of anaemia. It can be used to find potential erythropoietin replacement therapy patients (e.g. people with chronic kidney disease). It may also be used to assess a bone marrow problem or to assist in determining the origin of an excess of red blood cells.