Hunting for Efficiency: Number Needed to Interview to Find New Syphilis and HIV Cases Among Partners of People Diagnosed with Syphilis, North Carolina, 2015

Background: Increasing syphilis morbidity requires maximizing efficiency in case  investigations. We attempted to identify case information that can be used to improve case finding efficiencies for both syphilis and HIV.

Methods: We reviewed North Carolina 2015 syphilis records. The proportion (prevalence) of patient interviews  yielding at least one new syphilis/HIV case was compared by clinical, demographic, and  behavioral factors using prevalence difference (PD) and 95% confidence intervals (CI). The number of original patients needed to interview (NNTI=inverse of prevalence) and total interviews conducted to find one new syphilis/HIV case were calculated.

Results: We reviewed interview records for 1,646 early syphilis patients and 2,247 contacts. Overall, 220 patient interviews yielded ≥1 new syphilis case (NNTI=6.6, 248 total cases) and 35 yielded ≥1 new HIV case (NNTI=47, 38 total cases). Prevalence of newly-identified syphilis cases did not differ by the patient’s stage (primary versus secondary=0.21%, 95% CI 4.8-5.2; primary versus early latent=4.5%, 95% CI 0.4-9.4). Interviews of women (PD=6.8%, 95% CI 0.6-13.0), patients <30 years old (PD=4.5%, 95% CI 1.1-8.0), and patients with titers >1:16 (PD=3.9%, 95% CI 0.01-7.64) were more likely to yield new syphilis cases. Interviews of men reporting sex with men (PD versus other men=2.4%, 95% CI 0.9-3.9) and patients meeting sex partners online in the past year (PD=2.7%, 95% CI 1.0-4.5) yielded more new HIV cases.  Interviewing only cases in these groups reduces NNTIs to 5.7 (syphilis; missing 101 new cases) and 43 (HIV; missing 10 new cases). Interviewing only cases in these groups and their named contacts reduces total (case and contact) interviews from 3,893 to 1,752. 

Conclusions: Prioritizing early syphilis investigation by stage, demographic, and risk factors provides small increases in efficiency. Discussion of other potential prioritization factors is valuable. NNTI does not include contact interviews and therefore may not be the optimal measure of efficiency for syphilis follow-up.