Evaluation of a New Competitive Immunoassay (BioElisa Syphilis) for Screening for Treponema pallidum Antibodies at Various Stages of Syphilis

The BioElisa Syphilis, a new competitive enzyme immunoassay (EIA) for Treponema pallidum whole antigen that uses specific human immunoglobulin G (IgG) antibodies as the competitor, was evaluated for potential use in screening for syphilis at various stages. The results obtained by this competitive EIA were compared with those obtained by the fluorescent treponemal antibody absorption (FTA-abs) test and the T. pallidum hemagglutination assay (TPHA). Serum samples from 434 patients with positive TPHA and FTA-abs test results, including patients with primary, latent, secondary, and tertiary syphilis and neurosyphilis, were investigated. Two samples tested negative by competitive EIA but were weakly reactive by the TPHA and the FTA-abs test. Sixteen serum samples from patients with clinically documented active syphilis, including several patients infected with human immunodeficiency virus, tested positive by the competitive EIA. There was a direct inverse correlation between EIA indices and titers in the TPHA and the FTA-abs test for all samples that tested positive. Specificity was assessed by testing 358 serum samples which tested negative for syphilis by TPHA and the FTA-abs test, including 100 serum samples from patients with documented infectious or autoimmune diseases. Only two serum samples gave a weakly positive EIA result. Thus, competitive EIA had a sensitivity of 99.5% and a specificity of 99.4% relative to the results of the FTA-abs test and TPHA. Our evaluation shows that BioElisa Syphilis is a sensitive, specific, and simple assay for screening for syphilis.