Erythropoietin (EPO) is a hormone produced primarily by the kidneys that stimulates red blood cell production in the bone marrow. This test measures EPO levels in the blood to evaluate erythropoiesis and diagnose various blood disorders. EPO production is regulated by tissue oxygen levels, with increased production occurring in response to hypoxia, anemia, or low oxygen states. Elevated EPO levels can indicate conditions such as anemia, high-altitude exposure, or certain lung diseases, while low levels may suggest kidney disease or polycythemia vera. The test is particularly valuable for distinguishing between different types of anemia, evaluating kidney function, and monitoring response to recombinant EPO therapy in patients with chronic kidney disease or cancer-related anemia. EPO measurements help guide treatment decisions and assess the appropriateness of EPO-stimulating agent therapy.