Erythropoietin (EPO) is a hormone primarily produced by the kidneys that stimulates the production of red blood cells in the bone marrow. The EPO test measures the level of this hormone in the blood. It is often used to evaluate the cause of anemia, especially in conditions where red blood cell production is impaired, such as chronic kidney disease, certain cancers, or bone marrow disorders. Abnormal EPO levels can indicate either an overproduction or insufficient production of red blood cells, helping to guide diagnosis and treatment decisions.