Immunoassay of triiodothyronine in serum by time-resolved fluorometric measurement of europium-chelate complexes in solution.

We describe the development of a competitive immunoassay for triiodothyronine (T3) in serum. The assay combines immobilized antigens in microtitration wells with a biotinylated monoclonal anti-T3 antibody and a streptavidin-based universal detection reagent labeled with the Eu3+ chelator 4,7-bis(chloro-sulfophenyl)-1,10 phenanthroline-2,9-dicarboxylic acid (BCPDA). In the assay, T3 released from binding proteins by thimerosal competes with immobilized antigen for binding to a limited amount of antibody. The bound biotinylated antibody is identified by a subsequent reaction with the detection reagent, and fluorescence of the final complex is then quantified in solution after it has been dissociated from the solid support by the addition of a detergent solution. Evaluation of the method demonstrated good overall precision and appropriate detection limit (0.2 nmol/L) and dynamic range. Analytical recovery averaged 99.9%, and results of dilution experiments were in agreement with the expected values. Measurements by the present method correlated well with those by a commonly used radioimmunoassay.