Soaring UK Rates of Chlamydia Infection Partly Caused by More Sensitive Tests

More sensitive testing methods might account for some of the sharp increase in reported Chlamydia trachomatis infections, suggests research in the journal Sexually Transmitted Infections. Chlamydia is the most common sexually transmitted infection in the developed world, and is thought to cost the United Kingdom 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 percent of women and just over 7 percent of men tested positive. But almost 10 percent of women and just over 11 percent of men tested positive, using NAAT.

The increase equates to a 62 percent jump in positive results for women and a 56 percent 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.

Reference: What is the impact of change in diagnostic test method on surveillance data trends in Chlamydia trachomatis infection? Sex Transm Infect 2006; 82: 24-30