A solid-phase fluoroimmunoassay for the quantitation of antibodies to Toxoplasma gondii.

A solid-phase fluoroimmunoassay (FIAX¿) for the detection and quantitation of human antibodies to Toxoplasma gondii is described. The method is a modification of the procedure of Walls and Barnhart [1] which differs from the earlier test in that it uses a dual rather than single surface StiQTM Sampler, having both an antigen and a control surface. Each sampler is carried through four steps: reaction with diluted serum, a buffer wash, reaction with FITC-labeled goat anti-human IgG, and a final buffer wash. The fluorescence of each surface is measured using a dedicated surface-reading fluorometer. Serum titers are interpolated from a standard curve. The antibody titers of 233 sera were determined by both the indirect immunofluorescent antibody (IFA) and the FIAX¿ assays. The titers determined by the two tests agreed within one four-fold dilution for 226 (97%) of the sera. For replicates (n = 21) of the sera with low, mid and high antibody titers the coefficients of variation were 13.6, 10.0, and 12.3% respectively.