This test measures the amount of erythropoietin in the blood. Erythropoietin is a hormone produced primarily by the kidneys. It is produced and released into the bloodstream in response to low blood oxygen levels (hypoxemia). Erythropoietin is carried to the bone marrow, where it works to stimulate precursor cells to become red blood cells (RBCs). RBCs contain hemoglobin, a protein that carries oxygen throughout the body. Normal RBCs have a lifespan of about 120 days and are usually similar in size and shape.