[Avidity of rubella IgG antibody in rubella virus infection].

The avidity assay of rubella IgG antibody with urea was evaluated to distinguish the early convalescent phase of primary rubella from the late one and reinfection. We examined 116 sera from 23 patients with primary infection collected 7 to 1,477 days after the rash appeared and 25 sera from 8 patients with rubella reinfection and 16 sera from 15 pregnant women without fetal infection having high hemagglutination inhibition (HI) antibody or weak positive rubella IgM antibody. The avidity index (AI) was calculated as the ratio of the optical density from the urea-washed well to that of the non-treated well, expressed as a percentage. In primary infection AI were lower than 30% within 30 days after the rash appeared, then gradually increased and reached a plateau of 60 to 80% three months later, whereas the AIs of patients after reinfection, with high HI antibody and weak positive IgM antibody were as high as 79.2 +/- 7.5%, 93.5 +/- 4.1% and 90.6 +/- 5.0%, respectively. These results indicate that the measurement of avidity in rubella IgG antibody is valuable in diagnosing recent primary rubella in pregnant women having a high HI antibody or IgM antibody.