P BITTEL¹, N JEANNET¹, D MAYOR¹, F M SUTER-RINIKER<sup>1

1</sup>Clinical Microbiology, Institute for Infectious Diseases, University of Bern, Friedbühlstrasse 51, CH-3001 Bern

Aim of this study was to review and compare serologic results of different treponemal specific tests (Architect Syphilis TP and SERODIA-TPPA) in order to determine diagnostic performance with a special focus on T. pallidum reinfections. Architect Syphilis TP, a chemiluminescent microparticle immunoassay, is widely used as a primary screening test due to its high degree of automation and high sensitivity. We reviewed samples from patients received between 2005 and 2015 (n=905) with parallel Architect and TPPA results. Upon detection of Architect and TPPA discordant results (Architect positive and TPPA negative) we analyzed the temporal characteristics of the agreement between Architect and TPPA. Additionally, we analyzed 32 samples from HIV positive patients with a T. pallidum reinfection, which is defined as at least four-fold increase in TPPA titers. Thereby we assessed whether the (qualitative) Architect Syphilis TP assay can be used to detect T. pallidum reinfections. Architect and TP•PA results plotted against each other showed that TPPA titers from 10’240 up to 327’680 can no longer be grouped into statistically significantly different groups according to the respective values obtained by Architect when performing multiple comparison one-way ANOVA. In HIV and syphilis co-infected patients, TPPA titers are likely to remain elevated despite appropriate antimicrobial treatment. In the T. pallidum reinfection dataset, ten out of 32 HIV co-infected patients had a long- term residual TPPA titer of ≥10’240 prior to reinfection. Reinfection with T. pallidum in these patients is accordingly characterized by a non-significant increase of the corresponding Architect result. Additionally, a total of 21 patients had positive Architect and negative TPPA results combined with antecedent positive TPPA results. We conclude that the Architect Syphilis TP as a screening tool for the detection of T. pallidum reinfections should be used with caution, particularly when HIV co-infected patient samples are analyzed. However, Architect Syphilis TP has a higher sensitivity when compared to the SERODIA-TPPA especially for the detection of residual antibody titers against T. pallidum.