Reverse Syphilis Screening Algorithm in a High-Volume Laboratory: CLIA–RPR–TPHA Concordance and Determinants of RPR Reactivity

Objective: Syphilis remains a major global health concern, with an estimated 8 million new infections annually. The reverse syphilis screening algorithm, which starts with a treponemal assay followed by a non-treponemal test, has gained wide adoption in diagnostic laboratories. Despite its advantages, the interpretation of discordant results and the factors influencing RPR reactivity in real-world practice are not fully understood. This study aimed to evaluate the diagnostic concordance between CLIA, RPR, and TPHA assays and to identify demographic and clinical factors associated with RPR reactivity in a high-volume tertiary laboratory.Methods: A retrospective analysis was performed on 109,390 serum samples tested between January 2021 and January 2023. CLIA-reactive samples underwent RPR testing and, when indicated, TPHA confirmation. Logistic regression analysis was used to determine predictors of RPR positivity, and receiver operating characteristic (ROC) analysis evaluated the performance of CLIA signal-to-cutoff (S/CO) ratios in predicting TPHA positivity.Results: Among 995 (0.91%) CLIA-reactive samples, 278 (29.4%) were RPR-positive. TPHA testing confirmed 76 (64.4%) of 118 RPR-negative samples as true positives. Younger age and outpatient status were independent predictors of RPR reactivity (adjusted OR for 18–45 years: 3.39, 95% CI: 2.10–5.48, p