Evaluation of serological markers for the immunodiagnosis of acute acquired toxoplasmosis

The detection of specific IgM antibodies has been the most frequently used serological marker for diagnosing recent toxoplasmosis. However, the persis tence of specific IgM antibodies in some patients and the use of tests with a low specificity have complicated the interpretation of serological resul ts when toxoplasmosis is suspected. The purpose of the present study was to determine the value of newer serological techniques in the diagnosis of ac ute acquired toxoplasmosis. Sixty-four sera, 31 from patients with Toxoplas ma gondii infection and 33 from patients with latent infection, were tested . Anti-T. gondii IgA was measured by two antibody capture ELISA tests (Plat elia(R) Toro IgA and ETI-TOXOK A) and an automated direct ELISA (IMx(R) Tor o IgA); all three assays detected antibody levels compatible with a recent infection in sera from all 31 patients with acute toxoplasmosis. However, s ignificant levels of IgA were also detected with high frequency by all thre e assays in sera from patients with latent infection. IgE antibodies detect ed by IgE immunosorbent agglutination assay (ISAGA) were present in 26 (84% ) of 31 patients with acute toxoplasmosis and in sera from two subjects wit h latent infection taken >1 year after the beginning of the clinical sympto ms of infection. Thirty (97%) of 31 patients with a recent T. gondii infect ion and 15 (45%) of 33 subjects with latent infection had an AC/HS pattern compatible with acute toxoplasmosis. The avidity of T. gondii IgG was evalu ated by two methods. One method was based on the titration of each serum sa mple and calculation of the titres, in the absence and presence of urea, in relation to a defined cut-off value. In the other method, a single serum d ilution was used and the absorbances of the reactions in the presence and a bsence of urea were compared. The titration method was more sensitive for d iagnosing recent primary infection; all 31 sera from patients with acute to xoplasmosis had avidity indices compatible with acute toxoplasmosis by the titration method, whereas with the single dilution method, sera from four p atients had equivocal results. In the 33 individuals with latent infection, similar results were obtained with the two avidity methods; only one serum sample had a non-compatible avidity value with the titration method. The r esults obtained in the present study show that the current serological mark ers used for diagnosing acute acquired toxoplasmosis have significant limit ations. The data suggest that determination of the avidity of T. gondii-spe cific IgG by the titration method in patients with detectable IgM antibodie s defines most accurately the stage of infection by T. gondii.