Staphylococcal protein A in the serologic diagnosis of congenital rubella and toxoplasmosis.

Staphylococcal protein A (SPA) has been used to remove maternal IgG from cord or neonatal sera to make serologic testing for congenital infection more IgM specific. To evaluate the clinical usefulness of this procedure, sera sent for TORCH titers that had rubella hemagglutination inhibition (RHI) reciprocal titers greater than or equal to 8 or toxoplasmosis indirect fluorescent antibody (TFA) titers greater than or equal to 16 were adsorbed with SPA and retested. Nine of 109 sera were TFA positive by routine testing, and all but one from an infant with congenital toxoplasmosis became negative after SPA adsorption. A repeat TFA at 4 to 26 months of age was negative in all six children without evidence of toxoplasmosis who returned for follow-up. Fifty of 55 sera were RHI positive by routine testing, and all but one from an infant with congenital rubella became negative after SPA adsorption. The repeat routine RHI was negative in 32 of 35 children whose sera were initially negative after SPA adsorption. SPA adsorption greatly increased the specificity of routine RHI and TFA tests in the diagnosis of congenital rubella and toxoplasmosis.