Main outcome measure Incidence of clinical pelvic inflammatory disease over 12 months.
Results Baseline prevalence of chlamydia was 5.4% (68/1254) in screened women and 5.9% (75/1265) in controls.
Intervention Participants completed a questionnaire and provided self taken vaginal swabs, with follow-up after one year.
Samples were randomly allocated to immediate testing and treatment for chlamydial infection, or storage and analysis after a year (deferred screening controls).
No trials of chlamydia screening have taken place in a British population.
The national chlamydia screening programme was progressively rolled out across England from 2003 to 2008.
In the POPI (prevention of pelvic infection) trial we investigated whether screening young sexually active female students for chlamydial infection and treating those found to be infected reduced the incidence of pelvic inflammatory disease in the subsequent 12 months.
For the primary analysis we estimated the relative risk of developing pelvic inflammatory disease in the 12 months after recruitment to the screened group compared with the deferred screening control group.
In secondary analyses we examined the proportion of control women with untreated chlamydial infection who developed pelvic inflammatory disease within 12 months.
Investigators were blind during recruitment and follow-up except PO when she referred women with chlamydial infection for treatment.
Categorisation of pelvic inflammatory disease status was also blind.13Assuming a 2% incidence of pelvic inflammatory disease in the control group, we needed a sample of 4122 women to detect a relative risk of 0.48 with 80% power and 5% significance.