Evaluation of different confirmatory algorithms using seven treponemal tests on Architect Syphilis TP-positive/RPR-negative sera

The Architect Syphilis TP is considered to be a suitable screening test due to its high sensitivity and full automation. According to the International Union against Sexually Transmitted Infections (IUSTI) 2014 guidelines, however, positive screening tests need confirmation with Treponema pallidum particle agglutination (TP.PA). Among Architect-positive results, samples with a negative non-treponemal test present the major diagnostic challenge. In this multicenter study, we investigated if other, preferable less labor-intensive treponemal tests could replace TP.PA. A total of 178 rapid plasma reagin (RPR)-negative sera with an Architect value between 1 and 15 S/CO were prospectively selected in three centers. These sera were analyzed with TP.PA and six alternative treponemal tests: three immunoblots and three tests on random-access analyzers. The diagnostic performance of the treponemal tests differed substantially, with the overall agreement between the six alternative tests ranging from 44.6 to 82.0 %. Based on TP.PA as the gold standard, the INNO-LIA IgG blot, the BioPlex 2200 IgG, and the Syphilis TPA showed a high sensitivity, while the EUROLINE-WB IgG blot, recomLine Treponema IgG blot, and the Chorus Syphilis screen showed a high specificity. However, an Architect cut-off of 5.6 S/CO can serve as an alternative for these confirmatory treponemal tests in case of an RPR-negative result. Treponemal tests show poor agreement in this challenging group of Architect-positive/RPR-negative sera. The most optimal algorithm is obtained by assigning sera with an Architect value >5.6 S/CO as true-positives and sera with a value between 1 and 5.6 S/CO as undetermined, requiring further testing with TP.PA.