Research is still in progress and there is controversy over which tests of iron status should be used in patients receiving erythropoietin. Many such tests exist, all of which assess different parts of the iron metabolic pathway, and no one single test can be used to give a global picture of iron status. Thus, the serum ferritin gives an approximate indication of iron stores, the transferrin saturation assesses how much iron is circulating in plasma relative to the total iron-binding capacity (transferrin), and the percentage of hypochromic red cells has been advocated as the best means of assessing how much iron is being incorporated into the red cell. Iron stores are said to be normal or near-normal when serum ferritin maintained above 100 µg/L, the transferrin saturation above 20%, and the proportion of hypochromic red cells below 10%.