Maturation of rubella IgG avidity over time after acute rubella infection

As the incidence of rubella has diminished, the proportion of unspecific rubella IgM reactivity among all samples with rubella IgM reactivity has increased. It is important to distinguish IgM reactivity caused by primary infection from that caused by reinfection or persistence, especially in pregnant women, as termination of pregnancy should be considered when primary rubella is diagnosed during the first trimester.

To elucidate the changes over time of the avidity of rubella IgG antibodies after acute rubella infection.

Serial samples, 84, were collected from 15 patients up to 4-5 months after acute rubella infection. Rubella specific IgG avidity was tested by the eluting principle adding 35 mM diethylamine to the washing buffer of a commercially available rubella IgG ELISA. As controls, 137 samples from women with remote rubella and 94 samples from patients with a rubelliform rash, were tested.

The avidity index increased steadily in all patients during the observation time. A low avidity index (< 40%) was seen up to 6 weeks after onset of rash. A high avidity index (> 60%) was not observed until 13 weeks after infection and only in four of the 15 patients during the observation time.

An increase of rubella IgG antibody avidity was seen during the whole observation time but was most pronounced during the first 3 months after onset of rash. Measurement of rubella IgG avidity is a good supplemental test for cases with rubella IgM reactivity to confirm or exclude a recent rubella infection.