A proposed fluorimetric determination of unsaturated iron-binding capacity

A completely new method for the analysis of uncombined iron-binding capacity (UIBC) in human serum is proposed. The method is based on the discovery by C. K. Luk that the fluorescence of terbium ion (Tb-3+) is enhanced enormously when it binds to an empty Fe-3+ binding site of transferrin. Tb-3+ cannot displace Fe-3+ from an occupied binding site on transferrin. The analytic procedure is to dilute the serum (in order to lower its ultraviolet absorbance), add a solution of Tb-3+, excite with ultraviolet radiation, and measure the intensity of the resulting green fluorescence. With proper calibration this intensity is a measure of UIBC. Analyses of 65 serum samples previously measured at a hospital laboratory show no obvious difficulty with the method. Its absolute accuracy has not been probed in detail.