An erythropoietin (EPO) test is primarily used to help diagnose the cause of anemia. It is often performed following abnormal results on a complete blood count (CBC), such as low red blood cell (RBC) count, hemoglobin, and hematocrit levels. This test provides valuable insights into the underlying causes of anemia and helps determine if low erythropoietin (EPO) levels are contributing to or worsening the condition. EPO testing is crucial for assessing whether a deficiency in this hormone, which stimulates red blood cell production in the bone marrow, may be a factor in the development of anemia.