The low avidity of immunoglobulin G has been reported to be a useful marker of recent infection with Toxoplasma. Several investigators, however, have published discrepant result on the maturation of avidity over time. The aim of this study was to analyse persistent low avidity of immunoglobulin G in immunocompetent individuals and in pregnant women and how it could interfere in the flowchart of antenatal diagnosis of toxoplasmosis in the latter group. An international literature search was conducted together with a retrospective study of a hospital database. Eleven publications that met the inclusion criteria reported delayed maturation of avidity at a frequency ranging from 0 to 66.6% of the patients. Examination of those publications demonstrated an important heterogeneity in the type of assay used, the calculation of avidity, the cutoff above which avidity was considered to be elevated, and the delay since infection after which indices are expected to be high. In the hospital database, persistent low avidity was found even after a median follow-up period of 6 years. Different factors could interfere with maturation of avidity, such as variations between individuals, the assay system used, and, possibly, the treatment administered. The results of this study clearly demonstrate that, in a pregnant woman, an acute infection cannot be reliably diagnosed solely on the basis of low avidity of immunoglobulin G. Further investigations and standardization of assays are urgently needed. Estimation of the time of infection remains difficult, especially in cases in which the samples are drawn late in pregnancy; the final estimate must be based on several tests repeated at intervals of weeks.

The objective of the present study was to investigate the maturation of immunoglobulin G (IgG) avidity after Toxoplasma gondii seroconversion during pregnancy and the factors that affect IgG avidity over time. The study used 309 serum samples from 117 women and a multiple linear mixed regression analysis to show the patterns of variation of IgG avidity throughout gestation. The IgG avidity ratios and the patterns of their evolution with time were quite diverse among the women and were statistically heterogeneous (P ‫؍‬ 0.011); however, the trend was toward a statistically significant increase (P < 0.0001). On average, a 1.0167-fold increase was observed for each additional gestational week after the putative date of infection. At 12 weeks after putative infection (the expected IgG avidity maturation time), the mean avidity ratio was 16.6% (95% confidence interval, 15.4 to 17.9%). At all times, the avidity ratio remained significantly heterogeneous among the women (P < 0.05); for 95% of them, that ratio ranged from 7.8 to 35.3% at 12 weeks after putative infection. Maternal age at the putative time of infection did not influence the maturation of IgG avidity. However, on average, a 1.009-fold decrease (P ‫؍‬ 0.03) in that avidity was observed for each additional week of gestational age before infection and a 1.03-fold increase (P ‫؍‬ 0.0003) was observed for each additional week of delay to the onset of spiramycin treatment. The rate of increase in the avidity ratio was lower if infection occurred late in pregnancy and higher if the delay to treatment was long. This information cannot allow accurate determination of the delay since the time of infection. The present results provide support for interpretation of the assay and caution against overinterpretation.Acute Toxoplasma gondii infection can have serious consequences on fetal development, including abortion, neurological and ocular fetal lesions, and subclinical infections (26). Moreover, some toxoplasma-linked ocular lesions appear or relapse during childhood or adolescence (25,33). This is why, in France, a monthly serological follow-up during pregnancy has been mandatory for seronegative women since 1992 (32).The risk of mother-to-child transmission of Toxoplasma gondii was shown to be inversely proportional to the stage of pregnancy at which maternal infection occurs (9, 17). Thus, dating of maternal toxoplasma infection during pregnancy is of great importance because it allows determination of the fetal risk. Today, various serological markers may be used to estimate the time that has elapsed since Toxoplasma seroconversion, including measurement of immunoglobulin G (IgG) avidity (16). Although it is well recognized that a high IgG avidity ratio can rule out acute infection, a low IgG avidity ratio is insufficient to infer a recent infection (21). Moreover, data on the evolution of the IgG avidity ratio over time after Toxoplasma infection are lacking. Although the heterogeneity of that evolution has already been reported (13), studies on the effect of ...