Recommendations vary on the best combination of tests to use for the diagnosis of subclinical congenital toxoplasmosis at birth. The diagnostic accuracy of IgM and IgA tests was assessed in the context of routine clinical practice on 233 newborns with congenital toxoplasmosis and 661 healthy controls. IgM/IgA sensibility and specificity were compared in cord and postnatal samples. Both tests were considerably more specific in neonatal blood (IgM: 98%; IgA: 100%) than in cordblood (IgM: 85%; IgA: 88%). Sensitivity for IgM and IgA was not significantly different in neonatal blood (61% and 60%, respectively) and cord blood (67% and 54%, respectively). Combining IgM and IgA increased the overall sensitivity to 73% without any significant loss in specificity (98%). The influence of the date of maternal infection on the sensitivity and negative predictive value was also clearly demonstrated.
Conclusion: Because of their relatively low cost compared to more sophisticated methods, IgM and IgA tests should remain the main method for the routine diagnosis of congenital toxoplasmosis although follow up is essential to identify the Ca. 25% of infected children who are missed at birth on the basis of these tests.