Seroconversion in syphilis screening without positive confirmatory tests points at early infection.

Introduction: The chemiluminescence immunoassay (CLIA) is a widely used screening test for syphilis. A CLIA seroconversion in the absence of a positive line immunoassay (LIA) or rapid plasma reagin (RPR) could indicate either an early incubating syphilis or a false positive result. We aimed to evaluate the diagnostic value of such seroconversions.

Methods: We retrospectively analysed data of clients visiting the Centre for Sexual Health Amsterdam between July 2013 and August 2021 with a positive CLIA and a negative RPR and negative or indeterminate LIA (at time T_o), and a preceding visit (T_-1) with a negative CLIA <6 months of T_o ('unconfirmed CLIA seroconversion'). If available, data of follow-up visits (T₁) <2 months of T_o were also included. A syphilis diagnosis was confirmed if darkfield microscopy or PCR for Treponema pallidum was positive at T₀ or T₁, or if RPR and/or LIA were positive at T₁.

Results: We included data of 107 clients with unconfirmed CLIA seroconversion. The value of CLIA seroconversion could not be established in 13 (12.1%) clients. In the remaining 94 clients, the unconfirmed CLIA seroconversion was confirmed as early syphilis in 72 (76.6%) clients and probable syphilis in 6 (6.4%) clients. In 16 (17.0%) clients, the unconfirmed CLIA seroconversion was regarded as a false positive reaction of whom 4 (5.3%) clients had a seroreversion of the CLIA at T₁.

Conclusion: The majority of unconfirmed CLIA seroconversions represented early syphilis infections. Therefore, additional T. pallidum PCR, a follow-up consultation or early treatment is recommended.