Rubella-specific IgG subclass avidity ELISA and its role in the differentiation between primary rubella and rubella reinfection.

An antiglobulin enzyme-linked immunosorbent assay for rubella-specific IgG1 and IgG3 was adapted to measure antibody avidity by incorporating a mild protein denaturant, diethylamine (DEA), into the serum diluent. Sera were tested at varying dilutions, both with and without DEA, if they contained sufficient specific IgG1 or IgG3. The optical density (OD) was measured and curves were plotted. The highest OD (V) was noted and halved (V/2). The distance between the OD curves at V/2 was measured as the DEA shift value. Sera were examined from people whose sera contained rubella-specific antibodies as a consequence of infection or vaccination in the distant past (24 sera), recent primary rubella (66 sera), symptomatic reinfection (11 sera) or asymptomatic reinfection (64 sera). For specific IgG1 the DEA shift value was less than 0.6 for cases of rubella in the distant past, compared with greater than 0.8 for the first month after primary infection. The maximum DEA shift value for the sera from cases of reinfection was 0.65. No serum from cases of rubella in the distant past contained sufficient specific IgG3 to estimate avidity. The sera collected within 1 month of onset of primary rubella gave DEA shift values greater than 0.7 compared with sera from reinfections, which gave DEA shift values less than 0.6, except for two sera from a case of symptomatic reinfection. Thus the assessment of specific IgG subclass avidity is of value in differentiating serologically primary rubella from reinfection.