Novel interpretation of molecular diagnosis of congenital toxoplasmosis according to gestational age at the time of maternal infection

Abstract

From a prospective cohort of 344 women who seroconverted for toxoplasmosis during pregnancy, 344 amniotic fluid, 264 placenta, and 216 cord blood samples were tested for diagnosis of congenital toxoplasmosis using the same PCR assay. The sensitivity and negative predictive value of the PCR assay using amniotic fluid were 86.3% and 97.2%, respectively, and both specificity and positive predictive value were 100%. Using placenta and cord blood, sensitivities were 79.5% and 21.2%, and specificities were 92% and 100%, respectively. In addition, the calculation of pretest and posttest probabilities and the use of logistic regression allowed us to obtain curves that give a dynamic interpretation of the risk of congenital toxoplasmosis according to gestational age at maternal infection, as represented by the three sample types (amniotic fluid, placenta, and cord blood). Two examples are cited here: for a maternal infection at 25 weeks of amenorrhea, a negative result of prenatal diagnosis allowed estimation of the probability of congenital toxoplasmosis at 5% instead of an a priori (pretest) risk estimate of 33%. For an infection at 10 weeks of amenorrhea associated with a pretest congenital toxoplasmosis risk of 7%, a positive PCR result using placenta at birth yields a risk increase to 43%, while a negative result damps down the risk to 0.02%. Thus, with a molecular diagnosis performing at a high level, and in spite of the persistence of false negatives, posttest risk curves using both negative and positive results prove highly informative, allowing a better assessment of the actual risk of congenital toxoplasmosis and finally an improved decision guide to treatment.

Fig 1

Description and outcome of the cases of Toxoplasma gondii maternal infection analyzed in this study shown as a flow diagram. (Top boxes) Samples and biological tests used in this study. Below are shown the descriptions of cases. PND, prenatal diagnosis; AF, amniotic fluid.

Fig 2

Assessment of the actual risk of congenital toxoplasmosis using posttest predictive values of amniotic fluid, placenta, and/or cord blood PCR results. Determination of the final diagnosis at the end of child follow-up allowed the calculation, in our cohort, of the prevalence of congenital toxoplasmosis (or pretest risk) according to gestational age at maternal infection, all along the course of pregnancy (bold curve). We then calculated the same risk but differentiating, for each case, positive and negative PCR results (posttest probabilities of being infected). Solid triangles, posttest risk curves when the PCR test was positive (PPV); solid diamonds, posttest risk curves when the PCR test was negative (1 − NPV). These curves are represented on three graphs for amniotic fluid (A), placenta (B), and cord blood (C). Results from all of the cases in this study are shown on the logistic regression curves.