EPO and sickle cell disease: Second, the lack of relevant markers for etiological diagnosis and classification of anemia (e.g., serum ferritin, folate, vitamin B12, and erythropoietin) prevents us from more thoroughly exploring the relationships between RDW, subtypes of anemia (e.g., vitamin B12 or folate deficiency anemia, iron deficiency anemia, sickle cell anemia, and aplastic anemia), and physical function in older adults.