Rapid detection of hepatitis B surface antigen by double antibody radioimmunoassay

HBsAg and anti-HBs can be detected by double-antibody radioimmunoassay in a reliable routine overnight assay of sensitivity approaching that of solid-phase immunoradiometric assay. The test requires careful standardisation and regular preparation of I125-HBsAg of appropriate quality, but otherwise uses only small quantities of commercially available reagents. In this paper, the reaction kinetics of the first incubation (test sample + limiting amount of rabbit anti-HBs), second incubation (reaction mixture + I125 -HBsAg) and precipitation reaction (reaction mixture + antirabbit globulin) were examined, to determine the minimum time required for the assay compatible with adequate sensitivity and user convenience; in addition, in order to reduce the time of radioactive counting, the maximum amount of I125-HBsAg per test compatible with a satisfactory assay was examined. It was concluded that, by terminating each reaction before equilibrium was reached, results could be obtained in 2--3 hours with a detection limit similar to the standard overnight procedure. To allow routine examination of large numbers of samples, the possibilities of complete automation of this liquid-phase type of assay were discussed.