Modification of the Colorimetric Assay for Serum Unsaturated Iron-binding Capacity

We reported recently that total iron-binding capacity (TIBC) values calculated from serum iron and unsaturated iron-binding capacity (UIBC) values were significantly lower than those obtained by a direct and fully automated TIBC assay (1)(2). We also reported that slopes of regression lines for calculated TIBC values plotted against serum transferrin (TRF) were ∼7% lower than the theoretical ratio of TIBC to TRF (TIBC/TRF = 25.1 μmol/g). We found that this could be attributed to underestimation of UIBC values by colorimetric methods. One possible reason for underestimation of UIBC values was insufficient saturation of TRF. We modified the assay conditions of a colorimetric method for UIBC measurement to improve the correspondence between TIBC values converted from TRF and those calculated from serum iron and UIBC.Both serum iron and UIBC were determined by colorimetric methods (Wako Pure Chemical Industries) with a Hitachi Model 7070 automated analyzer. Serum TRF concentrations were determined by a nephelometric assay on a Behring Nephelometer II analyzer (Dade Behring). UIBC values were determined by four modified methods (Table 1⇓ ). UIBCA was …