Iron-Binding Capacity

Total iron-binding capacity (TIBC) is a crucial laboratory test for diagnosing iron metabolism disorders and inflammatory diseases. Iron-binding capacity is the capacity at which transferrin binds with iron. Transferrin, previously known as siderophilin, is the principal plasma transport protein for ferric iron (Fe). Transferrin has a molecular weight of 79.6 kDa and comprises 5.5% carbohydrates. Transferrin is a single polypeptide chain with 2 N-linked oligosaccharides and 2 homologous domains, each with a Fe-binding site. Transferrin is synthesized mainly in the liver and circulates with a half-life of 8 to 10 days. Transferrin reversibly binds 2 ferric ions with high affinity at physiological pH but lower affinity at decreased pH; this permits iron release within intracellular compartments. After cellular delivery of iron through receptor-mediated endocytosis, apotransferrin is recycled back into circulation. A few clinical indications exist for directly measuring transferrin. However, the indirect laboratory assessment of transferrin concentration may be inferred by TIBC. TIBC may be calculated as total or unsaturated. Depleting bodily iron stores by any mechanism increases circulating levels of transferrin. At optimal health, only one-third of transferrin is saturated with iron, and serum transferrin has an extra binding capacity of 67%, the unsaturated iron-binding capacity (UIBC). TIBC is the total serum iron and UIBC. Percentage transferrin saturation is calculated by dividing serum iron by TIBC and multiplying the result by 100.