Abstract Background The Center for Disease Control and Prevention’s (CDC’s) 2022 sexually transmitted infection (STI) surveillance report highlighted a concerning 78.9% increase in syphilis cases from 2018 to 2022, suggesting a syphilis epidemic in the U.S.1 In response, the clinical laboratory plays a crucial role in providing accurate and timely results for diagnosis and management. The CDC recommends two serological tests for syphilis: a treponemal and a non-treponemal test. Laboratories may use either the traditional or reverse algorithm for testing.2 In the traditional approach, a non-treponemal test (e.g., Rapid Plasma Reagin or VDRL) is conducted first, with reactive samples confirmed using a treponemal test. This method is widely used due to its low cost and simplicity. Alternatively, the reverse algorithm starts with a treponemal immunoassay followed by a non-treponemal test for confirmation. If results disagree, a second treponemal test is performed. The reverse algorithm offers benefits such as automation and more objective results compared to non-treponemal tests, which require manual interpretation. The VITROS Syphilis Assay is a treponemal test designed for automated screening of total, IgG and IgM antibodies to Treponema pallidum in human serum and plasma. This test can aid in syphilis diagnosis when used alongside non-treponemal tests and clinical findings. The goal of this study was to assess the concordance between the VITROS Syphilis Assay and the Roche Elecsys Syphilis Assay, an electrochemiluminescence immunoassay for detecting antibodies to T. pallidum. Both tests serve as tools for diagnosing syphilis and potentially improving diagnostic accuracy. Methods Samples were prospectively collected from the intended use population which included subjects with signs or symptoms of suspected syphilis infection or who are at high risk for exposure to syphilis. In addition, samples from HIV-positive patients, pregnant women and known-positive samples sourced from vendors were utilized in this analysis. The positive percent agreement (PPA), negative percent agreement (NPA), total percent agreement (TPA) and Cohen’s Kappa values with two-tailed 95% confidence intervals (CI) were determined by comparing the VITROS Syphilis Assay results interpretation to the Roche Elecsys Syphilis Assay interpretation. Concordance metrics were determined for the prospectively collected intended use population within and across subgroups (routine syphilis, pregnant and HIV positive) as well as for vendor-sourced positive samples. Analyses were performed after applying each syphilis assay’s interpretation. Results For the combined prospectively collected and vendor-sourced intended use population, after applying both syphilis assays’ cutoff to the assay results, the Cohen’s Kappa values were greater than 0.80 indicating near-perfect agreement between the VITROS Syphilis Assay results and the Roche Elecsys Syphilis Assay results for all subjects within and across the subgroups. Conclusion The VITROS Immunodiagnostic Products Syphilis Assay demonstrated near-perfect concordance with the Roche Elecsys Assay when evaluating the intended use population as well as subgroups including pregnant women and HIV-positive patients. This data suggests that both the VITROS and Roche Syphilis Assays would provide equivalent results and trigger the same next step when utilizing a reverse algorithm.
Mahmoud et al. (Wed,) studied this question.