Background Rapid treponemal tests performed at point-of-care (POC) are being used to scale up syphilis (Tp) screening in high-prevalence resource-limited settings. However, their advantage over offsite rapid plasma reagin (RPR) may be offset by variable sensitivity when used as POC, and the potential for resurgence in infectious cases due to enhanced treatment of latent Tp and loss of acquired immunity. We compare the change in prevalence of infectious Tp using different screening methods within a community of female sex workers (FSWs) in Karnataka, India.
Methods We developed a deterministic model of heterosexual Tp transmission between FSWs and their clients. Test sensitivities were estimated against a reference laboratory RPR with a confirmatory test. Main outcomes included predicted changes in FSW and client infectious Tp prevalence at 30 years, and post-intervention rebound in infectious Tp.
Results Reductions in infectious Tp prevalence achieved by screening were highest in areas with low initial treponemal test positivity, and were relatively insensitive to variations in parameter assumptions. At least once a year screening with a 100% sensitive POC test reduced infectious prevalence to an equal or greater extent as compared to a 100% sensitive offsite RPR test. A paradoxical rebound in infectious prevalence was observed after start of POC or RPR screening in approximately half of scenarios but resurgence above pre-intervention levels was rare (<5% of scenarios). Rebound was most likely in high-prevalence regions (treponemal test positivity 35–45%), where it manifested after 3.5 to 8.6 years, depending on assumptions on duration of immunity and sex-work turnover, and could be prevented with screening every 3 months. The ability to detect resurgence depended on frequency of monitoring and size of the surveyed population. POC screening of variable sensitivity was equally or more effective at reducing Tp prevalence than offsite RPR (sensitivity 100%) when follow-up was less than 50% (POC sensitivity 70%) and 70% (POC sensitivity 95%).
Conclusions Rapid treponemal POC tests have the potential to be as or more useful than offsite RPR tests within a high-risk population, when offsite RPR is unfeasible or return visits are low, irrespective of the sensitivity of available POC tests. In high-prevalence settings, sufficient coverage and screening frequency must be maintained to avert an increase in infectious syphilis prevalence.