Unknown Duration or Late Syphilis Cases with High Titers - North Carolina, 2015 & 2023

While recent national increases in unknown duration or late syphilis (UDLS) have outpaced diagnoses of earlier stages of syphilis, causes and contributors remain unknown. We evaluated changes in the proportion of UDLS cases with high nontreponemal test titers in one jurisdiction to identify if UDLS cases, which often have lower titers compared with early syphilis cases, might increasingly be early syphilitic infections.

We identified UDLS cases with high titers (defined as rapid plasma reagin titers ≥1:32) reported to North Carolina during years 2015 and 2023. We evaluated changes in case counts and in the proportion of UDLS cases with high titers overall, by demographics, clinical factors, and incarceration history.

When comparing years 2015 and 2023, UDLS case counts increased (628 vs. 2,498). The proportion of cases with high titers remained relatively stable between both years (51.3% vs. 50.2%), but increased among women (29.1% vs. 46.1%), women pregnant at the time of syphilis diagnosis (30.4% vs. 43.2%), men who have sex with women only (MSWO; 45.0% vs. 57.4%), and people who were recently incarcerated (41.7% vs. 52.9%). The proportion of cases with high titers decreased among men who have sex with men (66.4% vs. 58.1%), and men with HIV (68.0% vs 55.4%).

The proportion of cases with high titers increased among women, MSWO, and people who were recently incarcerated, suggesting that an increasing proportion of UDLS cases in these populations might be early syphilitic infections. Future initiatives could identify methods of improving case staging to facilitate syphilis prevention and control.