Objective provide information on the accuracy of tests for congenital toxoplasmosis performed prenatally or postnatally.
Methods 767 children born from a mother who acquired acute maternal Toxoplasma infection in pregnancy. Results of PCR analysis of amniotic fluid and of the detection at birth of anti Toxoplasma IgM (ISAGA, Biomérieux, France) and IgA (SFRI, France) were analyzed. Sensitivity, specificity and positive and negative likelihood ratios (LR) were estimated.
Results Congenital toxoplasmosis was diagnosed in 22% of children. Detection of IgM and IgA in peripheral blood was more sensitive (69%; 95% CI: 61 to 76) and specific (99.7%; 95% CI: 99 to 100) than in cord blood. Positive LR decreased with each week of maternal infection but remained high in the third trimester, ruling infection in with high confidence (up to 99.5% in late maternal infections) if IgM and/or IgA are detected. A previous positive PCR result on amniotic fluid increased the likelihood of congenital infection up to 99.7%. Negative LR for IgA and IGA detection were less informative. Knowledge of the PCR result on amniotic fluid was most useful in this setting, shifting probability of infection to “likely” (>75%) if PCR is positive or “unlikely” (<20%) if negative.
Conclusions Prenatal and neonatal tests for congenital toxoplasmosis have very high positive likelihood ratios. However, because their negative likelihood ratios were less satisfactory, newborns should be followed up until one year of age to exclude congenital toxoplasmosis.