Time-resolved fluoroimmunoassay of steroid hormones

The use of europium chelates as labels in immunoassays and their sensitive quantitation based on time-resolved fluorescence is reviewed. The technique is applied on competitive solid-phase immunoassays for direct determination of progesterone and estradiol in serum samples. Both antigen- and antibody-labelled competitive assays are described. The nonisotopic label technology, which provides a very high specific activity, as well as the antibody-labelled competitive assays, present several advantages in the assay of haptens as e.g. steroids. As the optimal sensitivity of competitive methods is not limited by the specific activity of the label the steroid assays which employ europium chelates as labels do not show any marked increase in sensitivity as compared to that achieved by using 125I. The potential sensitivity provided by the high specific activity of the label is optimally utilized in noncompetitive immunometric assays.