Reverse Screening Vs Traditional Screening Algorithm for Syphilis

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The reverse screening algorithm identified more patients with syphilis than the traditional screening algorithm, but also showed more false positives.

The reverse screening algorithm identified more patients with syphilis than the traditional screening algorithm, but also led to an increased likelihood of false positives, making retesting with a second treponemal test necessary, according to a study published in the Journal of the European Academy of Dermatology and Venereology.

Researchers compared the performance of the reverse screening algorithm with the traditional screening algorithm, thus serum screenings for syphilis were conducted in parallel with chemiluminescence immunoassay and toluidine red unheated serum testing.

Chemiluminescence immunoassay-reactive sera from reverse screening algorithms were confirmed with Treponema pallidum-particle agglutination assay. Traditional screening algorithms with toluidine red unheated serum testing which were reactive were further tested with Treponema pallidum-particle agglutination assay for confirmation.

Out of a total 110,663 serum samples, the reverse screening algorithm identified 2.0% (n=2259) with chemiluminescence immunoassay-reactive results, 16.7% (n=377) of which showed nonreactive Treponema pallidum-particle agglutination assay results. In addition, 61.6% (n=1392) were toluidine red unheated serum test-nonreactive, 25.1% (n=350) of which were Treponema pallidum-particle agglutination assay-nonreactive. Of the 350 Treponema pallidum-particle agglutination assay-nonreactive sera, 182 were tested again by a second chemiluminescence immunoassay, which showed 85.2% (n=155) as nonreactive, and 14.8% (n=27) as reactive.

Further testing of the 27 VITROS Treponema Pallidum assay-reactive sera with treponemal IgG western blot assays, which found 37% (n=10) as reactive, 22% (n=6) as nonreactive, and 41% (n=11) as indeterminate. Out of the 10 western blot-reactive sera, 2 seroconverted to Treponema pallidum-particle agglutination assay 1:80 approximately 1 year after follow-up. Of note, 3.1% (n=27) of the 867 sera samples which were chemiluminescence immunoassay-reactive and also subsequently toluidine red unheated serum test reactive, were Treponema pallidum-particle agglutination assay-nonreactive.

Study investigators concluded that “the present study revealed that the traditional screening algorithm is more suitable for laboratories with low-volume samples due to the lower false-reactive rate and its cost-effectiveness.”

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Zhuang YH, Liu H, Tang J, et al. Screening for syphilis with dual algorithms: analysis of discordant and concordant serology results in a population with a low prevalence of syphilis [published online September 17, 2018]. J Eur Acad Dermatol Venereol. doi: 10.1111/jdv.15251