ELISA for specific anti-toxoplasma IgM antibodies: aspects related to serum interference.
Two different methods were used to prepare solid-phase antigen (Ag) from soluble extracts of tachyzoites of Toxoplasma gondii: (A) physical adsorption on polystyrene beads; and (B) formaldehyde fixation of Ag previously dried in microtitration wells. In both cases a horseradish peroxidase conjugate with anti-IgM IgG was used as tracer. The assay scheme consisted of sequential incubations of diluted serum samples and tracer solution (1 or 2 h, 37 degrees C), colour development in the presence of substrate (10 min at room temperature), addition of H2SO4, and absorbance reading at 492 nm. In procedure A no cut-off value for positives could be determined owing to a large overlap between positive and negative sera. The extent of overlap directly correlated with the total IgM content of samples. With negative sera similar values were obtained with sensitized and untreated beads: thus a correction could be made by directly subtracting absorbance values determined in parallel runs with uncoated beads. Results with negative sera correlated with total IgM concentration in procedure B also, but much less variability of blank values allowed negative and positive sera to be effectively discriminated. A series of reference positive and negative sera was correctly classified by both procedures A and B. However, the latter appeared preferable, as not requiring blank correction.
D000888:Antibodies, Anti-Idiotypic / Q000032:analysis*; D000918:Antibody Specificity; D004797:Enzyme-Linked Immunosorbent Assay; D007124:Immunoenzyme Techniques; D007075:Immunoglobulin M / Q000276:immunology*; D014122:Toxoplasma / Q000276:immunology*
D000888:Antibodies, Anti-Idiotypic; D007075:Immunoglobulin M