New approach to competitive lanthanide immunoassay: time-resolved fluoroimmunoassay of progesterone with labeled analyte.

This solid-phase competitive europium immunoassay of progesterone in plasma relies on antigen labeling. With this new approach, time-resolved fluoroimmunoassay can attain sensitivity and precision similar to that of conventional radioimmunoassay. The europium-labeling involves coupling diethylenetriaminepentaacetic acid (chelating agent for Eu3+) to a progesterone-protein conjugate. The solid-phase antibody is immobilized inside polystyrene tubes in which plasma samples (50 microL) are assayed directly, without preliminary extraction. After incubation in the presence of trichloroacetic acid, the tubes are washed and the fluorescence intensity of europium is measured by time-wavelength-resolved fluorometry, with a nitrogen laser as the pulsed excitation source. Progesterone values obtained by this procedure agreed well with those obtained by radioimmunoassay (r = 0.98, n = 97). The detection limit was equivalent to that of most RIAs (0.2 micrograms/L).