The relevance of IgA and IgE assays, IgG avidity and western blotting in the diagnosis of Toxoplasma infection
To improve diagnosis of Toxoplasma gondii in difficult patient groups, IgA- and IgE-immunosorbent agglutination assays (ISAGA), IgG avidity and Western blotting were developed and assessed. In the ISAGA, rabbit anti-human IgA or anti-human IgE (for the IgA-ISAGA and IgE-ISAGA respectively) was adsorbed onto microtitre plates; formaldehyde fixed tachyzoites were used to identify specific antibody. Both ISAGAs were specific; only 1/482 (0.2%) and 1/513 (0.19%) false positive results were recorded for the IgA and IgE-ISAGA respectively. Both were produced early in infection and were detected for up to 11 (IgA) and 10 (IgA) months. In the avidity ELISA, the performance of excretory/secretory, surface, cytoplasmic and mixed antigen was similar when tested with sera from patients with known duration of infection. Thirteen pregnant women were tested. Specific IgA was detected in all and so was not useful to time infection but specific IgE was absent in 5 women and may therefore have been useful to exclude recent infection. Specific IgE however, was not detected in one woman who seroconverted; detection of IgE may indicate severity of infection. High IgG avidity was measured in 4 patients and could have been useful in conjunction with IgE to exclude recent infection. In immunocompromised patients, IgA and IgE were detected with similar frequency to IgM, but their presence in some IgM negative patients makes them a useful addition to the repertoire of testing. High avidity in immunocompromised patients was not of use to improve diagnosis. Using Western blotting it was possible to differentiate between acute (< 4 months) and recent (4-10 months) infection. This test was less useful for timing infection of infection in pregnancy.