Friday November 30 2007
Chlamydia bacteria: a silent infection
A “while you wait” test for chlamydia has been developed for women, reported The Sun. The chlamydia bacteria, the most common sexually transmitted infection in the UK, is detected by “self-collected vaginal swabs, which are much simpler to obtain than cervical swabs required by existing tests”, the newspaper said. BBC News reports that the test is “similar in appearance to a home pregnancy test”, and can reduce the diagnosis time from as much as two weeks to as little as 10 minutes.
The story is based on a diagnostic study in the UK which evaluated how well the new test can identify women who have chlamydia. A rapid diagnosis of chlamydia would be hugely beneficial for patients and could promote early access to treatment. However, the current UK chlamydia screening programme, which began in 2002, is successful and far-reaching and also screens for infection in young men – a high-risk group. This study did not test the accuracy of the rapid test for men.
Where did the story come from?
Dr Lourdes Mahilum-Tapal and colleagues from the University of Cambridge and medical centres in London and Birmingham carried out this study. The study was funded by grants from the Wellcome Trust and the NIHR Cambridge Biomedical Research Centre. It was published in the peer-reviewed British Medical Journal.
What kind of scientific study was this?
This was a diagnostic study to see how accurately the Chlamydia Rapid Test could identify women who had the sexually transmitted infection. The researchers invited women over 16, who were attending a sexual health centre (in Birmingham) and two genitourinary medicine (GUM) clinics in London, to participate in the study. In all, 1,349 women participated. Each woman provided two vaginal swabs and a urine sample. In the sexual health centre, both vaginal swabs were collected by the women themselves, while in the GUM clinics, clinicians collected one of the two vaginal samples while the women themselves collected the second.
One of the vaginal swabs was tested for chlamydia infection using the rapid test and a turine specimen was sent to a laboratory to test for chlamydia in the usual way. The researchers then compared the proportion of women identified as having chlamydia using the rapid test with those identified through the standard testing methods. Through this, they could determine how accurate the new test was. Women were also given a questionnaire to see how they felt about the rapid test.
What were the results of the study?
The study found that using the self-collected vaginal swabs, the rapid test could identify 82% of women who had chlamydia. The rapid test was also correctly negative (i.e. confirmed a person wasn’t infected when they weren’t) 99% of the time. For the clinician-collected swabs, the test was still very “sensitive”, and 78% of infected women were correctly identified. The clinician-collected swabs also correctly excluded 99% of women who weren’t infected. There was no statistically significant difference in the results from self-collected and clinician-collected samples.
Overall, the researchers report that compared with the usual way of diagnosing chlamydia, the rapid test correctly identifies infected women 84% of the time and correctly excludes those who aren’t infected 99% of the time. In the questionnaire, 75% of women said they were happy to wait between 30 minutes and two hours for their test results, and preferred to provide vaginal swabs over urine samples.
What interpretations did the researchers draw from these results?
The researchers concluded that the test was accurate and has many benefits over the traditional methods used to diagnose chlamydia. They say that it could be a “valuable addition” to screening programmes because results are available quickly and the test could be used in in mobile clinics or at home. The researchers also highlight the value of such a test for women in developing countries, particularly in high-risk groups such as sex workers.
What does the NHS Knowledge Service make of this study?
This diagnostic study provides early results suggesting that rapid testing has a place in the diagnosis of chlamydia.
- There are other rapid tests for chlamydia, but this one – the Chlamydia Rapid Test – is developed for testing vaginal swabs. Most other tests are not licensed for use in this way.
- There are high rates of chlamydia seen in young men too. This diagnostic test has been developed for women and as such, it could not “replace” current screening approaches, which also seek to identify infection in men.
- One weakness that the researchers raise is that the rapid test results were not compared with endocervical swabs. This was because such swabs were only collected at the GUM clinics and not at the sexual health centre.
The implications of using rapid tests are far reaching and they will have an impact in settings where resources for screening are limited. In addition, screening programmes could be extended by using a mobile or outreach approach to target populations that are usually harder to reach. The study group has acknowledged that further evaluation will provide important information on the usability of the test to diagnose chlamydia.
Sir Muir Gray adds...
Simplify, simplify, simplify is a very important principle for tests used for screening; this looks like an important step, a simpler test with little change in accuracy.