Chen and colleagues present, in this issue of STI (p 318), some apparently paradoxical data about the recent epidemiology of chlamydial infection in New South Wales, Australia—hospital admissions for pelvic inflammatory disease declined in women and rates of epididymo-orchitis remained constant during a period when notifications of chlamydia increased substantially. Their findings differ from the well documented fall in chlamydia complication rates that accompanied reductions in chlamydia notifications in Sweden^1,2 and the United States.³

Clinically trained readers of the journal will easily identify possible explanations for this discrepancy: the proportion of women with pelvic inflammatory disease who are admitted to hospital could be decreasing; increased use of azithromycin might have improved compliance and prevented more complications, despite increasing incidence; while trends in notifications do not represent changes in incidence because only a minority of all infections are diagnosed and reported. The authors have recognised and discussed these possibilities.

Chen et al used readily available health service databases and obtained aggregated numbers of cases of chlamydial complications and of notifications in New South Wales from 1992 to 2001. It is essential for clinicians and other sexual health professionals to understand both the potential and the limitations of ecological data such as these, because they are often used to make or justify health policy and funding decisions. Often they are the only …