O06.3 Developing a symptom-based risk score for infectious syphilis among men who have sex with men

Abstract

Background Syphilis incidence is rising among men who have sex with men (MSM). An online tool based on a risk score identifying men with higher likelihood of infectious syphilis could motivate MSM to seek help. We aimed to develop a symptoms-based risk score for infectious syphilis. Methods We included data from all consultations by MSM attending the Amsterdam STI clinic, in 2018–2019. Infectious syphilis (i.e. primary, secondary or early latent syphilis) was diagnosed according to the clinic’s routine protocol. The associations between symptoms and infectious syphilis were expressed as odds ratios (OR), with 95% confidence intervals (CI). Based on multivariable logistic regression models we created several risk scores. We assessed the area under the curve (AUC) and cutoff based on the Youden index. We estimated which percentage of men should be tested based on a positive risk score and which percentage of infectious syphilis cases would then be missed. Results 21,646 consultations with 11,594 unique persons were conducted. The median age was 34 years (interquartile range 27–45), and 14% were HIV-positive (93% on antiretroviral treatment). 538 cases of infectious syphilis were diagnosed. Associations with textbook syphilis symptoms or signs were strong and highly significant, e.g. OR for a painless penile ulcer was 35.0 (CI 24.9–49.2) and OR for a non-itching rash 57.8 (CI 36.8–90.9). None of the individual symptoms or signs had an AUC >0.55, and the AUC of risk scores varied from 0.68 to 0.69; weighting for size of coefficient did not affect AUC. Using cutoffs based on Youden index, syphilis screening would be recommended in 6% of MSM, and 59% of infectious syphilis cases would be missed. Conclusion Symptom-based risk-scores for infectious syphilis perform poorly and cannot be recommended to select MSM for syphilis screening. All MSM with relevant sexual exposure should be regularly tested for syphilis.