Objectives: To determine the accuracy of postnatal screening for toxoplasma-specific immunoglobulin (Ig) M and IgA.
Setting: Ten centres in three European countries.
Methods: We compared results of the first postnatal IgM or IgA test in infants with infected mothers identified by prenatal screening with the reference standard for congenital infection status of specific IgG status at one year of age.
Results: In all, 170 infected and 822 uninfected infants were analysed. Overall, IgM or IgA testing detected only 52-55% of infected infants. Sensitivity was highest between one and two weeks after birth and declined thereafter. Specificity was highest from four weeks after birth. For IgM, but not IgA, sensitivity was statistically significantly lower if the mother seroconverted in the first and second trimesters of pregnancy (29% and 34%, respectively) than the third (71%). Prenatal treatment with pyrimethamine-sulphonamide did not significantly reduce IgM or IgA sensitivity. Sensitivity was lowest for the immunofluorescence (IF) IgM test (10%) and the enzyme-linked immunosorbent assay (ELISA) IgM test (29%), but similar for the immunosorbent agglutination assay (ISAGA) IgM (54%), ISAGA IgA (58%) and ELISA IgA (52%) tests. Specificity was significantly lower for the ISAGA IgA test (91%) than for the ISAGA IgM (96%), IF IgM (100%), and ELISA IgA tests (98%).
Conclusions: Poor performance of IgM and IgA tests in the newborn, particularly if the mother seroconverted in early pregnancy, casts doubt on the value of neonatal screening in industrialized countries where the risk of clinical manifestations during childhood is low. More accurate diagnostic tests are needed for newborns identified by prenatal screening.