Reversing the sequence of screening tests has improved case identification but — in low-prevalence populations — has increased the likelihood of false-positive results.

The diagnosis of syphilis has never been straightforward. For the last several decades, the standard procedure has involved a nontreponemal (NT) screening assay followed by a confirmatory treponemal test (TT). The recent introduction of automated treponemal screening (ATS) tests has provided increased efficiency for high-volume laboratories; it has also led some laboratories to reverse the order of testing, such that an ATS assay is followed by a confirmatory NT assay. Two research groups have now investigated the effects of this reversal.

The CDC evaluated reverse-sequence screening in low- and high-prevalence populations (3 and 2 clinical sites, respectively). Among a total of 140,176 samples tested between 2006 and 2010, 4834 (3.4%) had a positive ATS result, with 2743 (56.7%) having a negative NT result. Of these 2743 samples, 866 (31.6%) had a negative confirmatory TT assay, suggesting that 17.9% of the positive ATS results were false positives. The percentage of samples with positive ATS results was higher at the high-prevalence clinical sites than at the low-prevalence ones; the percentage of samples with positive ATS results but negative NT results was higher at the low-prevalence sites.

In a retrospective, population-based study, Mishra and colleagues examined laboratory-based diagnoses of syphilis at the Public Health Laboratory of Toronto between 1998 and 2008. After the testing sequence was reversed, in August 2005, the monthly rate of confirmed positive results more than quadrupled, from 3.2 to 13.5 per 100,000 population. During the ATS-NT sequence period, 23,237 (2.2%) of 1,037,025 samples tested had a positive ATS result, with 69.6% of these having a negative NT result and 11.7% appearing — after confirmatory testing — to be falsely positive.

Comment: Together, these reports indicate that testing using the ATS-NT sequence identifies many individuals with syphilis who would have been missed using the traditional NT-TT sequence, but at the cost of additional false-positive results — not an insignificant concern, given the social ramifications of the diagnosis and the necessity to report positive results to public health authorities. The CDC continues to recommend the traditional testing sequence but notes that if the ATS-NT sequence is used, sera with ATS-NT discordant results should be further tested with Treponema pallidum particle agglutination. Patients with positive results on this last assay are considered to have past or present syphilis; those with negative results are unlikely to have syphilis.

— Richard T. Ellison III, MD

Published in Journal Watch Infectious Diseases March 2, 2011