There are two types of screening programs – proactive and opportunistic. Proactive screening uses population registers to invite people to be screened at regular intervals, while opportunistic screening targets people attending health services for unrelated reasons.

The value of opportunistic chlamydia screening is being called into question.

Dr Nicola Low, an epidemiologist at the University of Berne in Switzerland argues that claims about screening are not supported by rigorous research or practice.

And she shows how uncritical acceptance of the effectiveness of chlamydia screening in Sweden and the United States led to the funding of the National Chlamydia Screening Programme in England, before the balance of benefits and harms was understood.

Chlamydia trachomatis is a common, curable, easily diagnosed, sexually transmitted infection that usually causes no symptoms. It can, however, cause devastating complications, including infertility, ectopic pregnancy, neonatal infection, and facilitation of HIV transmission.

Chlamydia screening of selected groups is currently recommended in a range of health care settings in Sweden, the United States and Canada. In England, a programme offering opportunistic chlamydia screening to all sexually active women and men under 25 years is due to be implemented by 2008. Yet no randomised controlled trial has shown that this type of screening programme reduces long term illness.

Furthermore, most studies showing that chlamydia screening is cost effective do not satisfy accepted quality criteria for economic evaluations, says the author. They also tend to overestimate the cost effectiveness of chlamydia screening. Introducing a chlamydia screening program is therefore likely to be an expensive intervention.

In Sweden, decreases in rates of chlamydia and its complications occurred at the same time as both widespread chlamydia testing and national HIV prevention efforts were introduced. This trend was widely attributed to opportunistic chlamydia screening. This has led to uncritical acceptance of the effectiveness of chlamydia screening, which still persists, despite increasing rates of diagnosed chlamydia since 1995, writes Low. In the US, screening has also been credited with decreases in rates of infection.

Belief in the success of opportunistic screening persists, despite an absence of evidence of effectiveness and increasing rates of chlamydia in countries that are assumed to have such programmes. Unsubstantiated belief also seems to have allowed the requirements of the National Screening Committee and the experience of other UK screening programs to be over-ridden, she adds.

She believes that an agreed definition of a screening programme is needed, and that the same standards should be applied to all diseases for which screening is in place, or is being considered. Countries implementing or contemplating national chlamydia screening should conduct research to determine if such screening programs do more good than harm at reasonable cost, she concludes.

"Despite multiple campaigns in the media, the diagnosis of sexually transmitted infections continues to increase," write two senior doctors in an accompanying editorial. "Most people who are affected are unlikely to seek sexual health testing and may only be assessed via a proactive approach rather than the opportunistic screening programme currently offered."

Written from a news release by British Medical Journal.