[Evaluation of a syphilis testing algorithm using a treponemal test for screening]

Traditional testing algorithm for syphilis is initial screening with nontreponemal tests, then retesting positive samples for confirmation using a specific treponemal test. Commercial treponemal tests those are more sensitive than nontreponemal tests and suitable for automation are now commercially available to screen syphilis. Although the treponemal tests are offered as the first choice for syphilis screening by some of the guidelines, to date there is no consensus on recommendations in guidelines for followup testing. As some of the guidelines recommend to perform a nontreponemal test for persons with a positive treponemal screening test, the others recommend to confirm the result by another treponemal test. In this study, a testing algorithm using treponemal chemiluminescence microparticle enzyme immunoassay (CMIA; Architect Syphilis TP; Abbott Japan Co, Japan) for primary screening were retrospectively evaluated by reviewing laboratory data. A total of 12.195 serum samples obtained from 10.878 patients (7104 of them were female) who were screened by means of syphilis, between January 2007-February 2010 period have been included to the study. According to this algorithm, no further test was performed for CMIA negative samples. Samples positive by CMIA were retested by RPR (Rapid Plasma Reagin; Omega Diagnostics, UK) test. The test results of both CMIA and RPR positive samples were reported, while positive CMIA results were confirmed by TPHA (Treponema pallidum hemagglutination; Omega Diagnostics, UK) if any discrepancy in the results were identified. Screen test revealed positive results in 1.1% (120/10.878) of the patients and 2% (206/12.195) of the samples. In this study, quantitative values (sample/cut-off absorbance ratio; s/co) of positive CMIA samples were also compared with TPHA and RPR test results. It was observed that while 19.1% of CMIA positive samples with s/co ratios less than 12 were confirmed by TPHA, the confirmation rate was 100% with s/co ratios above 12. Additionally, RPR positive samples with a titer of ≥ 1/32 yielded CMIA s/co ratios above 21. As low s/co levels detected with CMIA may lead to false-positive results for syphilis, it was concluded that CMIA positive results should be confirmed by another treponemal test before RPR testing. A new syphilis testing algorithm in accordance with the results of this review and recommendations in new guidelines were established in the light of this study.