All Tests

Erythropoietin (EPO) is a hormone produced primarily by the kidneys. It plays a key role in the production of red blood cells (RBCs), which carry oxygen from the lungs to the rest of the body. This test measures the amount of erythropoietin in the blood.

Erythropoietin is produced and released into the blood by the kidneys in response to low blood oxygen levels (hypoxemia). EPO is carried to the bone marrow, where it stimulates production of red blood cells. The hormone is active for a short period of time and then eliminated from the body in the urine.

The amount of erythropoietin released depends upon how low the oxygen level is and the ability of the kidneys to produce erythropoietin. Increased production and release of erythropoietin continues to occur until oxygen levels in the blood rise to normal or near normal concentrations, then production falls. The body uses this dynamic feedback system to help maintain sufficient oxygen levels and a relatively stable number of RBCs in the blood.

However, if a person's kidneys are damaged and do not produce sufficient erythropoietin, then too few RBCs are produced and the person typically becomes anemic. Similarly, if a person's bone marrow is unable to respond to the stimulation from EPO, then the person may become anemic. This can occur with some bone marrow disorders or with chronic diseases, such as rheumatoid arthritis. (Read Anemia of Chronic Diseases to learn more.)

Individuals who have conditions that affect the amount of oxygen they breathe in, such as lung diseases, may produce more EPO to try to compensate for the low oxygen level. People who live at high altitudes may also have higher levels of EPO and so do chronic tobacco smokers.

If too much erythropoietin is produced, as occurs with some benign or malignant kidney tumors and with a variety of other cancers, too many RBCs may be produced (polycythemia or erythrocytosis). This can lead to an increase in the blood's thickness (viscosity) and sometimes to high blood pressure (hypertension), blood clots (thrombosis), heart attack, or stroke. Rarely, polycythemia is caused by a bone marrow disorder called polycythemia vera, not by increased erythropoietin.