Total iron-binding capacity calculated from serum transferrin concentration or serum iron concentration and unsaturated iron-binding capacity.

Abstract

Total iron-binding capacity (TIBC) indicates the maximum amount of iron needed to saturate plasma or serum transferrin (TRF), which is the primary iron-transport protein (1). Theoretically, 1 mol of TRF [average molecular mass, 79 570 Da (2)] can bind 2 mol of iron (55.8 Da) at two high-affinity binding sites for ferric iron (3). Therefore, TIBC correlates well with TRF concentration, and the theoretical ratio of TIBC (in μmol/L) to TRF (in g/L) is 25.1: TIBC (μmol/L) = 25.1 × TRF (g/L) (4)(5). Measurements of TIBC, serum iron, and the percentage of iron saturation of TRF are useful for the clinical diagnosis of iron-deficiency anemia and chronic inflammatory disorders (6)(7) and as screening tests for other clinical conditions (8). TIBC is routinely determined (9)(10)(11)(12) by saturation of TRF with an excess predetermined amount of iron, removal of the unbound iron, and measurement of the iron that is dissociated from TRF. For removal of the unbound iron, magnesium carbonate (9), ion-exchange resin (10), alumina columns (11), or magnetic particles(12) are used. Most direct TIBC measurement methods require manual procedures that involve centrifugation or pretreatment of serum samples. As an alternative to direct measurement methods, TIBC values are also calculated from the sum of serum iron and unsaturated iron-binding capacity (UIBC), both of which are determined by colorimetric methods (calculation method). We developed a direct and fully automated TIBC (DTIBC) assay for use with an automated multipurpose analyzer (13)(14). A fully automated TIBC measurement method is also commercially available (15). In our previous study (14), TIBC values obtained by DTIBC assay correlated strongly with serum TRF concentrations ( r = 0.984; n = 59), and the slope of the regression line was consistent with the theoretical TIBC/TRF ratio. We …