Iron needs of the body are met by dietary intake averaging 1520 mg/day. Only ferrous iron can be absorbed. Transferrin is the plasma iron transport protein. Normally, it is only 25-30% saturated with iron. Transferrin bound iron is largely derived from hemoglobin breakdown.
Serum iron and transferrin saturation are commonly used to screen for iron deficiency and iron overload disorders. The degree of transferrin saturation is a very sensitive indicator of functional iron depletion. Iron concentration and percent saturation decrease with iron deficiency. Transferrin saturation is usually less than 16% in iron deficiency. A threshold of 20% is often used for patients with inflammation. When screening for iron deficiency, serum ferritin should also be ordered because it is a more sensitive and reliable test for iron deficiency.
Transferrin saturation is a better screening test for hereditary hemochromatosis than ferritin. In hereditary hemochromatosis, serum iron is usually above 150 ug/dL and transferrin saturation exceeds 60%.
- Increased serum iron may be associated with hemolysis, transfusion, hemochromatosis, hemosiderosis, and hepatitis.
- Decreased serum iron may be associated with chronic hemorrhage, dietary deficiency, anemia of chronic disease ,and uremia.
- Increased total iron binding capacity (or transferrin) may be associated with iron deficiency, hepatitis, pregnancy and the administration of estrogen.
- Decreased total iron binding capacity may be associated with pernicious anemia, hemochromatosis, uremia, acute and chronic inflammation and protein malnutrition.
Causes of an elevated transferrin saturation and a normal ferritin level include:
- Iron supplements or iron-rich meal prior to blood draw
- Hereditary hemochromatosis being treated with phlebotomy
- Hereditary hemochromatosis and menstruation
- Very low TIBC due to severe liver disease (ferritin may be increased)
- Acute hemolysis
Serum iron, and therefore iron saturation exhibit biological variation due to diurnal variation and post-prandial effects. Levels can vary by 25-30% from day to day.
Reference range for serum iron is 50-150 mg/dL in males and 25-145 ug/dL in females. Reference range for transferrin is 200-360 mg/dL.
Percent saturation is calculated with the following formula: Iron/transferrin x 1.2. The reference range for percent saturation is 15-50%.
Specimen requirement is one SST tube of blood
References
Markowitz H, Fairbanks VF: Transferrin assay and total iron binding capacity. Mayo Clin Proc 1983;58:827-828
Szoke D, Panteghini M: Diagnostic value of transferrin. Clin Chim Acta 2012 Aug 16;413(15-16):1184-1189