Incidence of pelvic inflammatory disease in untreated women infected with Chlamydia trachomatis

Sir: Risser and Risser's interesting review shows that the evidence on the incidence of pelvic inflammatory disease (PID) in untreated chlamydia infected women is unclear. ¹ They highlight numerous methodological issues concerned with the ability to infer an estimation of the incidence of PID following chlamydia infection, including small sample size, short duration of follow-up period and lack of adequate knowledge of the natural history of chlamydia.

The POPI trial (Prevention of Pelvic Infection) is currently collecting data on the development of PID among 2524 female participants (mean age 21, range 16–27 years) recruited from 20 universities and further education colleges in 2004–2006. ² At baseline, participants completed questionnaires on demographic characteristics, sexual behaviour and history of sexually transmitted infections (STIs). They then provided self-taken vaginal swabs, which were stored in Aptima transport medium (Gen-Probe Inc.,). All participants were advised to get checked for STIs independently from the study.

Within two weeks of recruitment swabs were randomly allocated into two groups. Intervention swabs were tested for chlamydia and infected women referred for treatment. Control group specimens were frozen and tested 12 months later. To see if freezing reduced transcription-mediated amplification test sensitivity, we froze 10 chlamydia-positive specimens at –80°C for a year. All remained positive when retested after 342 to 389 days.

To date, an 86% (2161/2524) response rate for 12-month follow-up has been achieved, with questionnaires giving information on sexual behaviour, symptoms of possible PID and history of treatment within the follow-up period. These are verified by review of general practice and hospital records. In addition, 920 participants have returned repeat self-taken vaginal specimens for testing for chlamydia and other organisms 1–3 years after recruitment.

Although a relatively small study in terms of chlamydia positives, the results of our trial and prospective cohort study may contribute to Risser's findings on the incidence of PID. Risser notes that studies including asymptomatic women from settings other than sexual health clinics report a lower incidence of PID. ¹ Preliminary results indicate an overall chlamydiapositivity rate of 5.5% (139/2511). Information on the persistence of infection or spontaneous resolution in untreated controls, and on incidence of new infections may further provide insight into the natural history of chlamydia among women recruited outside health-care facilities. ¹

Ethical approval: Wandsworth Research Ethics Committee.