Immunoglobulin G avidity in the serodiagnosis of congenital rubella syndrome.

The avidity of specific IgG was investigated in three infants with serologically verified congenital rubella infection. Two sera were taken from each infant: the first soon after birth and the second at the age of 23 to 31 months. Avidity of specific IgG was measured by a protein-denaturing enzyme immunoassay using urea as the elution factor, and avidity then determined by the end-point ratio (derived from antibody titration) and the avidity index methods. Rubella-specific IgM was present in the first sera of all patients, but not in the second sera. However, low avidity of specific IgG persisted in two children until age 23 to 31 months, as determined by the end-point ratio method. These data are in agreement with the findings of previous studies of avidity in congenital rubella, and show the usefulness of the protein-denaturing IgG-avidity assays employing the end-point ratio method for serological diagnosis of congenital rubella even after disappearance of specific IgM.