NewApproachto CompetitiveLanthanideImmunoassay:Time-ResolvedFluoroimmunoassayof

This solid-phase competitive europium immunoassay of pro-gesterone in plasma relies on antigen labeling. With this new approach, time-resolved fluoroimmunoassay can attain sen-sitivity and precision similar to that of conventional radioim-munoassay. The europium-labeling involves coupling dieth-ylenetriaminepentaacetic acid (chelating agent for Eu3) to a progesterone-protein conjugate. The solid-phase antibody is immobilized inside polystyrene tubes in which plasma sam-ples (50 L) are assayed directly, without preliminary extrac-tion. After incubation in the presence of trichloroacetic acid, the tubes are washed and the fluorescence intensity of europium is measured by time-wavelength-resolved fluo-rometry, 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)