Adaptation of a commercial rubella-specific IgG kit to assess specific IgG avidity

Abstract

A commercially available antiglobulin enzyme-linked immunosorbent assay (ELISA) kit for the detection of rubella-specific IgG has been adapted to assess the relative avidity of the specific IgG in sera. A protein denaturant, diethylamine, was incorporated into the buffer used to wash the wells of the microtitre plates after the incubation of serum on the solid-phase antigen, and an avidity index was calculated. It was thus possible to distinguish cases of primary rubella from cases of rubella in the remote past or rubella reinfection on the basis of the presence of low-avidity specific IgG in the former. The results were compared with those obtained with two in-house enzyme-linked immunosorbent assays for IgG avidity: the diethylamine shift value method and the avidity index method. The latter method was used both with diethylamine and urea as the denaturant. The results with the adapted kit were similar to those obtained with the in-house diethylamine shift-value method and the avidity index method with diethylamine, for sera from cases of primary rubella up to two months after onset of illness, sera from reinfections, and sera from cases of rubella or rubella immunization in the remote past. The kit did not detect low-avidity specific IgG in as many sera from patients recently immunized with rubella vaccine as the diethylamine shift-value method but gave results similar to the in-house avidity index/diethylamine method. The in-house avidity index method, with urea as denaturant, was less sensitive for detecting low-avidity specific IgG than any of the other methods, including the adapted kit.