Traditional versus reverse algorithm for diagnosis of syphilis - An Indian perspective from a national referral laboratory for sexually transmitted infections.

Background: Syphilis diagnosis can be confirmed by two testing algorithms. In the traditional algorithm, a nontreponemal test is used as a screening test to rule out syphilis which, if reactive, is confirmed by a treponemal test. In the reverse algorithm, a treponemal test is used as a screening test, which, if positive, is confirmed by a nontreponemal test. If the nontreponemal is negative, a second treponemal assay of a different principle is performed to resolve the discordant result. The aim of this study was to compare the traditional and reverse algorithms for the diagnosis of syphilis in a National Referral Apex Centre for Sexually Transmitted Infections.

Materials and methods: A retrospective study was done on samples received from male and female sexually transmitted infection clinics for syphilis testing. All samples were tested simultaneously using the Venereal Disease Research Laboratory (VDRL), Treponema pallidum hemagglutination assay (TPHA), and fluorescent treponemal antibody absorption (FTA-Abs) test. The results of syphilis serological testing were interpreted using the traditional and reverse algorithms. The traditional algorithm used VDRL as the screening test and TPHA as the confirmatory test. In the reverse algorithm, TPHA was the screening test, and reactive results were confirmed with VDRL. Discordant samples were confirmed with the FTA-Abs test.

Results: The reverse algorithm showed a significantly higher seroprevalence of syphilis than the traditional algorithm (62.4% vs. 40.8%, P < 0.0001). The traditional algorithm had a high missed serodiagnosis rate (34.6%) and showed a higher false-positive rate of the screening test (6.8%) as compared to the reverse algorithm (0.6%).

Conclusions: This study supports the use of reverse algorithms for diagnosing syphilis in high-prevalence populations.