Effect of testing for IgG avidity in the diagnosis of Toxoplasma gondii infection in pregnant women: Experience in a US reference laboratory

The usefulness of testing for IgG avidity in association with Toxoplasma go ndii was evaluated in a US reference laboratory. European investigators hav e reported that high-avidity IgG toxoplasma antibodies exclude acute infect ion in the preceding 3 months. In this US study, 125 serum samples taken fr om 125 pregnant women in the first trimester were chosen retrospectively, b ecause either the IgM or differential agglutination (AC/HS) test in the Tox oplasma serologic profile suggested or was equivocal for a recently acquire d infection. Of 93 (74.4%) serum samples with either positive or equivocal results in the IgM ELISA, 52 (55.9%) had high-avidity antibodies, which sug gests that the infection probably was acquired before gestation. Of 87 (69. 6%) serum samples with an acute or equivocal result in the AC/HS test, 35 ( 40.2%) had high-avidity antibodies. Forty women were given spiramycin, to p revent congenital transmission, and 7 (17.5%) had high-avidity antibodies. These findings highlight the value of testing a single serum sample obtaine d in the first trimester of pregnancy for IgG avidity.