Background: It is necessary to have an easy and quickly test to distinguish "false positive" rubella IgM results and residual antibodies from the antibodies produced in the primary infection, in pregnant women. The avidity of IgG antibodies test seems to differentiate between primary rubella infection and past infections, reinfections or postvaccination, showing its utility in the diagnosis of primary infection in other infectious diseases.
Method: For 30 months, 178 sera from 157 patients with clinical and/or epidemiological rubella suspicion or with a positive rubella IgM result as result of an accidental serological finding, were remitted to our laboratory for a serological follow up. We distinguished 3 patient groups: outbreak group, 112; pregnant women, 36, and newborn 11. Rubella IgM antibodies by indirect EIA previous the rheumatoid factor absorption; IgG antibodies of low avidity by indirect EIA previous treatment of serum with 6 M urea, were detected in the sera. It considered a positive result, a rubella avidity index (AI) < 50%.
Results: In the epidemic outbreak group, 90.2% of the patients were not vaccinated. 80% of cases occurred in young men between 14 an 20 years old. From 109 patients (97.3%) with rubella IgG antibodies, 92 (84.4%) showed AI-IgG lower than 50%. In this group, the mean rate of AI-IgG rubella was 29.0%. In the pregnant women group, except for two of them, rubella IgM antibodies were an accidental finding in a serological pregnancy screening. Thirty patients (83.8%) showed AI-IgG rubella > 50%. The two pregnant women who had evidence of clinical and epidemiological rubella showed AI-IgG rubella of 37.4% and 20.9%. Another four pregnant women showed AI-IgG rubella close to cut-off (44.7-49.0%). The mean AI-IgG rubella in this group was 71.8%. The mean AI-IgG Rubella between the epidemic outbreak group and the pregnant women group, 29.0 and 71.8% respectively, was statistical significance (p < 0.001).
Conclusions: The avidity IgG test is simple and quickly, and it allow to exclude most of positive results because of residual IgM antibodies and false reactive.