Estimating the Value of Novel Tests for Active Syphilis in Zimbabwe: How Much Overtreatment Can Be Avoided?

Syphilis diagnosis in resource-limited settings relies on syndromic management and treponemal-based antibody screening tests that cannot distinguish active infection from past-treated infection, both of which can lead to overtreatment. Point-of-care (POC) tests that detect active infection could reduce unnecessary treatment.

We developed an agent-based model of co-transmitting syphilis and human immunodeficiency virus, calibrated to Zimbabwe using population-based survey data (ZIMPHIA) and UNAIDS estimates. We modeled five scenarios involving two hypothetical new POC diagnostic products introduced from 2027: (1) a test detecting Treponema pallidum DNA in genital ulcer specimens, and (2) a non-treponemal test identifying active infection following a treponemal-positive result, applied across antenatal care, key population, and human immunodeficiency virus care settings. We also conducted a threshold-based cost analysis to assess whether reductions in unnecessary treatment alone could be cost-saving.

Under the current standard of care, approximately 112,000 adult syphilis treatments are administered annually across all modeled detection and screening use cases, of which approximately 80% are unnecessary (i.e., administered to individuals who do not have an active syphilis infection). Combining both diagnostics reduces overtreatment to 24% (a 3.4-fold reduction). Across use cases, each test avoids approximately 0.4 to 0.8 unnecessary treatments, implying cost savings whenever the test price is less than 40% to 80% of the treatment cost.

Even under the conservative assumption that current syndromic management and screening practices achieve high syphilis treatment rates among care-seeking individuals, introducing POC diagnostics for active syphilis could substantially reduce overtreatment while maintaining treatment of individuals with active infection. Reductions in unnecessary treatment alone may be cost-saving at low test prices, even without accounting for downstream health outcomes. This analysis does not model the impact of improved diagnostics on transmission dynamics, congenital syphilis burden, or other health outcomes; studies that quantify these additional benefits are needed to inform a full economic evaluation.