Differential agglutination test for diagnosis of recently acquired infection with Toxoplasma gondii

We evaluated the recently described differential agglutination test (HS/AC test) to differentiate recently acquired toxoplasma infections from those acquired in the more distant past in sera obtained from 38 patients with carefully defined symptomatic and asymptomatic infections. AC antigens detect acute-phase-specific immunoglobulin G (IgG) antibodies against Toxoplasma gondii tachyzoites that are formed only during the acute stage of infection in humans. The HS/AC test correctly identified recently acquired infections in patients with toxoplasmic lymphadenopathy or asymptomatic infections (including infections in 7 women who seroconverted during gestation) in 31 of 33 patients. We also studied 15 individuals who had been infected for at least 2 years. In that group, only 13% had an acute pattern in the HS/AC test. However, the wide range in times from infection (from 2 to 14 years) did not allow for an estimate of when the pattern in the HS/AC test changed from acute to not acute. These results reveal that in the appropriate clinical situation, when both IgG and IgM tests are positive and a question still remains about the acuteness of infection, the HS/AC test may be useful for differentiating between toxoplasma infections acquired recently and those acquired in the more distant past.