Introduction Many US women report same sex behaviour, yet data on risk factors and STIs in women who have sex with women (WSW), women who have sex with both men and women (WSB) and how these compare to women who have sex with men only (WSM) remain limited. Here we compared self-identified WSW, WSB and WSM attending two STI clinics in Baltimore, Maryland.

Methods This was a retrospective analysis using a database of first clinic visits 2005–2016. WSW and WSB were compared with an age-matched random sample of WSM. Proportions were compared using the χ² test. Acute STI (aSTI) was defined as gonorrhoea (Neisseria gonorrhoeae, GC), chlamydia (Chlamydia trachomatis, CT), trichomonas (Trichomonas vaginalis, TV) or early syphilis. Logistic regression was used to assess aSTI predictors. CT testing was not uniformly done, so a sensitivity analysis removing CT from the aSTI definition was conducted.

Results Visits from 1095 WSW, 1678 WSB and 2773 WSM were analysed. WSB had equal or higher test positivity for all STIs except urogenital chlamydia, had more sexual partners, were more likely to engage in transactional sex and were more likely to report drug use and binge drinking than WSM (p≤0.01). WSW had lower test positivity for urogenital GC and CT than WSM or WSB, but comparable test positivity for TV, higher reported binge drinking and comparable reported substance use as WSM. Younger age and cocaine use predicted STI diagnosis only in WSM.

Conclusions WSB in these clinics bear an equal or higher burden of most STIs, have more partners and report more substance use than WSM. WSW carry a lower, but still substantial burden of STIs, and many report substance use. Factors predicting STI diagnosis differ between WSW, WSB and WSM suggesting that tailored STI prevention and testing approaches are needed in these groups.