Chlamydia is the most common sexually transmitted infection in the developed world, and is thought to cost the UK alone around 50 million a year.

The findings are based on 11 years of monitoring test results for the detection of Chlamydia at one Scottish health board.

More than 81,000 test results were obtained between 1992 and 2003. But after 1998, tests were switched from cultures to the more sensitive nucleic acid amplification testing, or NAAT as it is known.

Using the culture tests, just over 6 per cent of women and just over 7 per cent of men tested positive. But almost 10 per cent of women and just over 11 per cent of men tested positive, using NAAT.

The increase equates to a 62 per cent jump in positive results for women and a 56 per cent jump in positive results for men, say the authors.

Further analysis of the results showed that the increases were strongly linked to the type of test used, irrespective of the year of test, sex of the patient, or young age, a known risk factor for the infection.

The authors explain that the way national Chlamydia infection rates are calculated means that switching to a more sensitive test would give the impression of an increase in prevalence even if there were none.

"Part of the recently seen increase in Chlamydia trachomatis prevalence in the United Kingdom is likely to be directly and/or indirectly caused by the gradual shift of laboratories to the more sensitive NAAT diagnostics," they add.

And they suggest that as more laboratories switch to NAAT, their rates of positive test results will also rise. Nationally collected data should therefore record type of test used to avoid any "artificial" increases, they say.

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Contact: Emma Dickinson
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BMJ Specialty Journals
1-Feb-2006