Traditional versus reverse syphilis algorithms: A comparison at a large academic medical center

An increasing number of institutions are transitioning from the traditional syphilis testing algorithm (initial screening with nontreponemal tests) to the 'reverse' algorithm (initial screening with treponemal tests such as syphilis IgG). The aim of this study was to evaluate the switch in syphilis algorithm at an academic medical center with a population with low syphilis prevalence.

We performed a six-year retrospective study at the University of Iowa Hospitals and Clinics, an academic medical center, comparing the traditional algorithm (n=12,612) with the reverse algorithm (n=10,453). False positives were considered to be positive screens with negative confirmatory testing.

Using the traditional algorithm, 93 samples (0.7% of total) screened positive with RPR, with 40 of these samples having negative TP-PA testing (43% of positive screens, 0.3% of total). Using the reverse algorithm, 110 screened positive with syphilis IgG (1.1% of total), and 33 of these samples had both negative RPR and TP-PA (30% of positive screens, 0.3% of total). In both algorithms, higher RPR titers and syphilis IgG values were associated with increased probability of positive confirmation.

In this study at an academic medical center, the reverse algorithm had significantly more total positive screens than the traditional algorithm. Both algorithms produced equivalent rates of active infection. The quantitative difference in positives between the two algorithms are the category of patients who are syphilis IgG positive, RPR non-reactive, and TP-PA reactive. Specimens with higher RPR titers and syphilis IgG values are more likely to confirm positive.