Serum iron measures the amount of circulating iron that is bound to trans ferrin. Clinicians order this laboratory test when they are concerned about iron deficiency, which can cause anemia and other problems.Total iron-binding capacity The test measures the extent to which iron-binding sites in the serum can be saturated. Because the iron-binding sites in the serum are almost entirely dependent on circulating transferrin, this is really an indirect measurement of the amount of transferrin in the blood. Taken together with serum iron and percent transferrin saturation clinicians usually perform this test when they are concerned about anemia, iron deficiency or iron deficiency anemia. However, because the liver produces transferrin, liver function must be considered when performing this test. It can also be an indirect test of liver function, but is rarely used for this purpose.