Immunoradiometric determination of thyroglobulin in serum samples by time calibration transfer

Abstract

A limitation of isotopic assays is decay of the radioactive signal and the need for a calibration curve in each run. Thus, calibration transfer has been proposed for immunoradiometric analysis (IRMA). The present study was undertaken to establish the main methodological issues applied to the determination of thyroglobulin (Tg). Serum samples were analyzed at a different experimental time than Tg standards. Tg concentrations were then determined using a transferred calibration curve estimated from a calibration model based on a three-parameter antigen-antibody interaction model and a decay kinetic constant. A kinetic constant for decay of the radiolabeled antibody signal was calculated (-0.018+/- 0.002 day(-1)). This strategy avoided full recalibration, despite degradation of the radiolabeled immunoreagents or experimental variations. The calibration transfer was validated using addition, dilution and recovery protocols. Repeated analyses of serum samples were performed during the life of the commercial immunoreagents, and concentrations were determined using daily and transferred calibration curves. Results were statistically comparable, but significant reductions in reagent consumption, analysis time, radioactive waste and staff time were obtained. Transferred curves can be used to analyze samples by IRMA without simultaneous measurement of standards, and provide reliable concentrations. This principle has been demonstrated for Tg determination, but the key issues to extend to other IRMA methods have been established.