Rubella-specific IgG1 avidity: a comparison of methods.

Two methods of determining the avidity of specific IgG1 were compared with sera from different categories of rubella infection. Both methods were based on an antiglobulin enzyme-linked immunosorbent assay. In one method the absorbances were compared with and without diethylamine (DEA) in the serum diluent over a range of serum dilutions and the difference between the dilution curves measured (DEA-shift). In the other, the absorbances at a single serum dilution were compared with and without urea in the wash fluid used after the antigen/serum incubation (avidity index). Various concentrations of DEA were also assessed in the avidity-index method, as this method is simpler to perform. The DEA-shift method was shown to be more sensitive for diagnosing recent primary rubella or immunization by demonstrating specific IgG1 of low avidity. The avidity-index method, however, was more specific when sera from cases of reinfection or non-specific rubella IgM reactivity were tested. 35 mM DEA was found to be the optimal concentration of DEA when DEA was substituted for urea in the avidity index method.