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Evaluation of serological markers for the immunodiagnosis of acute acquired toxoplasmosis

Department of Clinical Pathology, Faculty of Medicine and *Department of Parasitology, Insitute of Biology, State University of Campinas (Unicamp), Campinas, São Paulo, Brazil

Corresponding author: Dr C. L. Rossi.

Received 26 Nov. 1999; revised version accepted 2 June 2000.

Abstract

The detection of specific IgM antibodies has been the most frequently used serological marker for diagnosing recent toxoplasmosis. However, the persistence of specific IgM antibodies in some patients and the use of tests with a low specificity have complicated the interpretation of serological results when toxoplasmosis is suspected. The purpose of the present study was to determine the value of newer serological techniques in the diagnosis of acute acquired toxoplasmosis. Sixty-four sera, 31 from patients with Toxoplasma gondii infection and 33 from patients with latent infection, were tested. Anti-T. gondii IgA was measured by two antibody capture ELISA tests (Platelia^® Toxo IgA and ETI-TOXOK A) and an automated direct ELISA (IMx^® Toxo IgA); all three assays detected antibody levels compatible with a recent infection in sera from all 31 patients with acute toxoplasmosis. However, significant levels of IgA were also detected with high frequency by all three assays in sera from patients with latent infection. IgE antibodies detected by IgE immunosorbent agglutination assay (ISAGA) were present in 26 (84%) of 31 patients with acute toxoplasmosis and in sera from two subjects with latent infection taken >1 year after the beginning of the clinical symptoms of infection. Thirty (97%) of 31 patients with a recent T. gondii infection 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 evaluated by two methods. One method was based on the titration of each serum sample 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 dilution was used and the absorbances of the reactions in the presence and absence of urea were compared. The titration method was more sensitive for diagnosing recent primary infection; all 31 sera from patients with acute toxoplasmosis had avidity indices compatible with acute toxoplasmosis by the titration method, whereas with the single dilution method, sera from four patients 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 results obtained in the present study show that the current serological markers used for diagnosing acute acquired toxoplasmosis have significant limitations. The data suggest that determination of the avidity of T. gondii-specific IgG by the titration method in patients with detectable IgM antibodies defines most accurately the stage of infection by T. gondii.

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