The usefulness of a specific immunoglobulin G (IgG) avidity enzyme-linked immunosorbent assay (ELISA) based on recombinant GRA6 antigen for distinguishing between acute and chronic Toxoplasma infection was investigated. Two sets of serum samples obtained from pregnant women with acute, chronic, or no Toxoplasma infection collected in France and Iran were used. Among the French subjects, 19 of 20 (95%) women who experienced seroconversion during the past 4 months before sampling displayed low-avidity IgG antibodies against GRA6, while all 17 (100%) women with chronic infection had high-avidity antibodies. When the Euroimmun IgG avidity ELISA was used, 15 of 19 (78.9%) recently infected women had low-avidity antibodies, and 20 of 22 (90.9%) women with chronic infection displayed high-avidity antibodies. The results suggested better performance of the GRA6 avidity ELISA than the Euroimmun avidity ELISA for exclusion of a recent infection occurring less than 4 months previously. Similarly, all 35 Iranian women with acute Toxoplasma infection had low-avidity antibodies against GRA6, whereas all 34 women with chronic infection displayed IgG antibodies of high avidity, indicating the value of GRA6 avidity testing for ruling out a recent infection. Avidity tests based on lysed whole-cell Toxoplasma gondii antigen are currently used to exclude recently acquired infections; however, the use of recombinant antigen(s) might improve the diagnostic performance of avidity tests and facilitate the development of more standardized assays.Congenital toxoplasmosis may occur when maternal infection is acquired during pregnancy and results in severe fetopathy or miscarriage (28,38). While the rate of fetal infection with Toxoplasma gondii is extremely low in preconception infections, the transmission rate increases and the severity of fetal infection decreases as gestational age progresses (7, 18). Fortunately, prenatal treatment of the infection is effective for reducing both the incidence of clinical manifestations in infected newborns (8) and the maternal-fetal transmission rate (6,29,37). It is, therefore, essential to estimate the gestational age of the primary Toxoplasma infection as precisely as possible for the proper clinical management of pregnant women.Diagnosis of acute toxoplasmosis depends mainly on serological testing, as the infection is asymptomatic in 93% to 97% of pregnant women (10,27). In countries where a prenatal screening of T. gondii infection is performed, detection of seroconversion or a significant rise in the specific immunoglobulin G (IgG) titer establishes a recently acquired infection. In most countries, however, a single serum sample from a pregnant woman is available to determine whether the infection occurred during gestation. IgM antibodies have been traditionally known as the markers of acute infection; however, persistent specific IgM has been found for up to several years after primary infection (15,24,39). Measurement of specific IgG avidity was shown to be an effective confirmatory test to he...