Clinical Evaluation of a Syphilis Treponemal Immunoassay for Use in United States Patient Populations

Abstract Background The clinical performance of the VITROS® Immunodiagnostic Products Syphilis Assay was evaluated by comparison with composite results obtained with widely used lipoidal antigen (nontreponemal) and T. pallidum (treponemal) tests. Methods Serum samples were tested from patients presenting for syphilis screening, and in relevant subpopulations including pregnant women, people living with HIV, known serologically positive for syphilis, and medically diagnosed with syphilis. Samples originated from 1710 and 113 patients in the United States and South America, respectively. Results were also compared for VITROS vs the Roche Elecsys® Syphilis immunoassay alone. Results Positive percentage agreement was ≥98.81% for VITROS and the comparator composite results within and across populations, with a 95% Wilson Score confidence interval of 98.01%–99.94% across the entire intended use population. Negative agreement was ≥90.63%, with 95% Wilson Score confidence interval of 96.93%–98.67% for the entire population. Method comparison between VITROS and Elecsys assays found 99.09% total agreement and a Cohen’s kappa coefficient of 0.97. In separate analyses, nonreactivity was observed for VITROS in 197 of 201 (98%) apparently healthy individuals, and positive reactivity was observed in 151/151 (100%) serum samples preselected from patients with medically diagnosed syphilis, indicating high clinical sensitivity of the VITROS Syphilis assay. In addition, specimens preselected as serologically positive showed 100% reactivity with VITROS. Conclusions These findings support strong clinical performance of the VITROS Syphilis assay for aiding in diagnosis of syphilis, and excellent concordance with the well-established Elecsys Syphilis test.