Background Non-Chlamydial Non-Gonococcal Urethritis (NCNGU) is as prevalent as Chlamydial urethritis in UK men. Mycoplasma genitalium likely explains 10–46% of cases but there is no UK-licensed molecular test. In women, M genitalium is associated with pelvic inflammatory disease (PID), infertility and ectopic pregnancy. New UK guidelines do not recommend urethral smear microscopy in asymptomatic men, so asymptomatic NCNGU cannot be diagnosed, whereas previously affected men and their sexual partners were treated with antibiotics. The impact of this policy on reproductive health sequelae in women is unknown.

Methods We used a compartmental mathematical model to synthesise evidence on the natural history of M genitalium, diagnosis and treatment levels. Uncertainty was accounted-for using Latin hypercube sampling, with rejection sampling to fit to data. The model was stratified by sex, incorporates heterogeneous sexual behaviour, and was parameterised by behaviour data from key UK surveys (NATSAL2, PATSI, MSTIC); STI surveillance data; and natural history data from the literature. It incorporates symptomatic and asymptomatic infection; PID; care-seeking due to symptoms, partner notification, and routine screening; and treatment failure. We fitted to national STI surveillance (KC60) data, (allowing for uncertainty in the amount of NCNGU caused by M genitalium), representing current practice, and examined (i) increasing microscopy in asymptomatic men by making it routine in all genito-urinary medicine clinics and (ii) decreasing rates by discontinuing it in the 20% of clinics currently still using it.

Results Microscopy in asymptomatic men reduces the burden of infection in women by reducing prevalence in men thus reducing transmission to women; and treating women via partner notification from diagnosed men. The magnitude of these effects depends upon natural history in men and women, about which there is uncertainty. We determine ranges of parameter values in which routine microscopy would be effective in reducing M genitalium-related sequelae in women.

Conclusions Further empirical work is required to determine if natural history parameters fall in the ranges that would make routine microscopy in asymptomatic men effective. Particularly important are the prevalence of M genitalium; proportion of infections that are symptomatic in men and women; duration of untreated infection; and incidence of PID, infertility and ectopic pregnancy attributable to M genitalium.