Objective: The emerging reverse sequence on syphilis screening program generates special discordant results, characterized with the appearance of both positive treponemal test and negative nontreponemal test at the same time. The aim of this study was to analyze the characteristics of the discordant results among low syphilis prevalence population in China, to provide evidence for improving the process of reverse sequence syphilis screening program.
Methods: Laboratory data was retrospectively analyzed, under reverse sequence screening algorithm selecting ELISA as the initial screening test for syphilis. All the screening reactive samples were tested by TPPA for confirmation and by quantitative TRUST for the reactivity of syphilis.
Results: 666 out of a total of 21 049 serum samples were reactive under the screening program. Among the 666 reactive samples, 169 were reactive to TRUST. One in the 169 samples was confirmed negative on TPPA, and the faulse positive rate on ELISA was 0.6% (1/169). In those 666 reactive samples, 497 were nonreactive to TRUST. 74 in the 497 samples were confirmed negative to TPPA, with faulse positive rate of ELISA as 14.9% (74/497). In the group of 591 TPPA confirmed positive samples, the TRUST negative rate was found 71.6% (423/591), significantly higher than the TRUST positive rate(chi-square test, χ(2) = 110.025, P = 0.000).
Conclusion: Among the results from reverse sequence syphilis screening program, majority of the samples which showed positive treponemal antibody, would have negative nontreponemal antibody. We therefore recommended a more reasonable reverse sequence syphilis algorithm to be used. Faulse positivity could be eliminated if TPPA was performed on all screening reactive samples by ELISA a first and then followed by quantitative TRUST on samples that were TPPA confirmed as positive.