Pre-treatment syphilis titers: Distribution and evaluation of their use to distinguish early from late latent syphilis and to prioritize contact investigations

Background: Treatment, contact investigation, and reporting decisions for syphilis cases are based on the stage of disease. Because of limitations of current staging protocols, the rapid plasma reagin (RPR) titer has been proposed as an alternative priority marker for contact investigation. Methods: We describe the RPR titers and stages for 10,021 syphilis cases reported between 1997 and 1999 in Columbia, South Carolina; Houston, Texas; and Jackson, Mississippi. We constructed receiver operating characteristic curves (ROC curves) to compare titer and stage. We calculated the number of infected contacts to evaluate the use of titer to prioritize contact investigation. Results: RPR titers differed by stage, with 67% of primary, 95% of secondary, 78% of early latent, and 41% of late latent and unknown duration having titers >1:8; however, there was considerable overlap in titer distributions. The ROC curve based on titer values demonstrated good agreement between titer and latent stage. Prioritization by titer (>1:8) of latent cases would result in a similar number of cases interviewed and contacts located as stage prioritization, although different cases are prioritized. Conclusion: Titer distributions meaningfully but imperfectly distinguish populations with different stages. Recent analyses and anecdotal reports indicate the difficulty and inconsistency of staging latent syphilis. Over time, titer could provide a more objective and reliable historical record of syphilis trends. Titer may be a useful alternative or adjunct to stage in prioritizing latent syphilis cases for investigation.