Streptavidin-biotin based time-resolved immunofluorometric assay for direct measurement of high concentrations of human chorionic gonadotropin (hCG).

Quantitation of human chorionic gonadotropin (hCG) in maternal serum is widely used for screening of fetal trisomy-21 (Down's syndrome). When using immunometric assay, the sample usually has to be prediluted in order to quantitate the high hCG concentrations occurring during weeks 10-17 of pregnancy. Utilizing a streptavidin-biotin system we have developed an immunofluorometric assay (IFMA) for the quantitation of hCG in pregnancy serum that does not require predilution of the sample. The sample is added to a streptavidin coated microtiter strip well together with a biotinylated 'capture' antibody. About 1-2% of the capture antibody is biotinylated whereas most of the antibodies are unbiotinylated and thus unable to bind to the solid phase, while still binding antigen. This displaces the assay range from 0.5-5000 IU/l of the standard assay to 20-250,000 IU/l. The method shows good correlation with the standard procedure (r = 0.96). By eliminating a dilution step the assay procedure is both simplified and reinforced. The principle of this method should be readily applicable to other antigens and detection methods.