A 42-year-old male with chronic nephritis with uremia has undergone hemodialysis for 3 months, with weekly hemofiltration, and subcutaneous injection of erythropoietin for 4 months. Apart from antihypertensive drugs, the patient takes no other medications. Uremic symptoms are controlled, but anemia is significant (Hb 6.5 g/dL). To investigate the cause of anemia, which test should be prioritized?