Comparison of immunoradiometric and immunofluorometric assays for serum hCG

Earlier studies have demonstrated, that an extremely rapid and sensitive assay for human chorionic gonadotropin (hCG) can be obtained by utilizing time-resolved fluorometry in combination with an immunometric assay. In this assay 2 different monoclonal antibodies are used. One antibody, directed against the beta-subunit of hCG, is immobilized by coating onto a microtitre well whereas the other antibody is labelled with a europium chelate. This assay was compared with a similar assay, which differed only by using 125I instead of europium as a label. The results demonstrate, that the high sensitivity of the immunofluorometric assay was mainly the result of the high antibody concentration used since almost equal sensitivity was obtained using the same amounts of radiolabelled antibody. However, this implied the use of 10-100 times more radioactivity per tube than usually applied in immunoassays. For practical purposes the time-resolved immunofluorometric assay was superior because the measuring time was shorter and the reagents non-radioactive and stable.