Direct serum total iron-binding capacity assay suitable for automated analyzers.

BACKGROUND Present methods for measuring serum total iron-binding capacity (TIBC) involve manipulation of samples or performance of two assays on each sample. We developed a direct automated assay (DTIBC) for TIBC. METHODS We added to serum a saturating amount of iron bound to an excess of chelating dye at a low pH, recorded a blank reading that represented the sum of the saturating amount of iron plus the serum iron, and then added a strong neutral pH buffer. The decrease in absorbance (as transferrin extracts iron from the iron-dye complex) is directly proportional to the TIBC. TIBC values for 125 patients were determined by DTIBC, alumina column TIBC (AC), magnetic particle TIBC (MTIBC), and the UIBC method (UIBC) on Roche COBAS FARA and Mira chemistry analyzers. In a separate study, TIBC values for 128 patients were determined on an Olympus AU400 by the DTIBC and the MTIBC methods. RESULTS Methods comparisons on the COBAS analyzers yielded the following results: DTIBC = 1.05(MTIBC) - 1.0 micromol/L (r = 0.987; S(y/x) = 2.6 micromol/L); DTIBC = 1.07(AC) - 1.0 micromol/L (r = 0.982; S(y/x) = 3.0 micromol/L); and DTIBC = 1.14(UIBC) + 3.4 micromol/L (r = 0.982; S(y/x) = 3.0 micromol/L). A similar correlation study using the Olympus AU400 yielded DTIBC = 1.00(MTIBC) - 0.1 micromol/L (r = 0.983; S(y/x) = 2.7 micromol/L). The assay was linear from 12.5 to 125 micromol/L (70-700 microg/dL) TIBC on the COBAS FARA. Within- and between-run imprecision (CV) was <or=4.8% at two concentrations. Plasma samples were unsuitable for the method. No interference was seen with common interferants other than ascorbate, deferoxamine, and ferrous sulfate, and only at concentrations well above normal. CONCLUSION The new DTIBC assay is suitable for routine use in clinical laboratories and may improve the quality of iron metabolism studies.