Determination of hormones by time-resolved fluoroimmunoassay

Immunoassays based on europium labels and time-resolved fluorescence as the detection method, have been developed. The specific activity of the label is several orders of magnitude higher than that of radioactive labels. Consequently, the technique provides great potential, especially in the determination of analytes which require high sensitivity. Both competitive and immunometric assays which use labelled antibodies have been worked out. In competitive assays the antigen is immobilized on a solid phase with a protein carrier. The antigen in the standard or sample then competes with the labelled antibody in solution. Separation is done simply by washing the wells in the microtitre strip where the assays are performed. Model systems are described for the measurement of testosterone and cortisol. Immunometric assays of human thyrotropin (hTSH) and luteotropin (LH) were performed with monoclonal antibodies, by either a one-step (hTSH) or two-step (LH) incubation procedure. These assays, which exploit the specific activity of the label, give a very high sensitivity and good reproducibility. The standard curves are linear and the dynamic range is at least 1000-fold. Because of the properties of the europium label and the simple assay design, the immunoassays based on time-resolved fluorescence are expected to gain wide application both in research and in routine determinations.