Friday July 31 2009
This test may encourage more men to get tested for chlamydia
A urine test can diagnose male chlamydia within an hour, according to the BBC. The sexually transmitted infection, which often has no symptoms, can cause fertility problems and is estimated to be carried by 6.8% of young men in England. This new urine test reportedly enables same-day treatment after a one-hour wait for results.
The well-conducted study behind this story has demonstrated that the Chlamydia Rapid Test is highly accurate in confirming that a man is not infected. It also has a good, but slightly lower, level of accuracy in correctly identifying that a man did have the infection. The technique also avoids using urethral swabs to extract samples from inside the penis.
This quick, accurate test is an important step towards both reducing the growing problem of chlamydia in the young and providing rapid treatment for those affected. It is also hoped that men would be more likely to agree to this type of testing, as they currently get tested less often than women. Further testing in a larger set of samples will probably be needed before this test could be brought into current practice.
Where did the story come from?
Elpidio-Cesar Nadala and colleagues of Cambridge University, Barts and the London Charity, and other UK centres, carried out this research. The study was funded by the Wellcome Trust and the National Institute for Health Research’s Cambridge Biomedical Research Centre, and published in the British Medical Journal.
What kind of scientific study was this?
This was a diagnostic cohort study assessing the use of a rapid urine test to diagnose and screen for chlamydia infection in men.
The study recruited participants between March and November 2007 from a young people’s sexual health centre, and a genitourinary medicine (GUM) clinic in the UK. They enrolled men aged 16 or over, who had not taken antibiotics in the previous month, and were able to understand the written information forms used in the study. The researchers enrolled 1211 men, who were confidentially interviewed about their symptoms and relevant sexual history.
Each participant gave two urine samples, two hours apart, which were collected using two different methods. The first sample was collected using the new technique being trialled, The Chlamydia Rapid Test. The testing process involved using a centrifuge and adding various ‘reagent’ chemicals that reacted with substances in the urine to separate and identify them. This testing of the first sample was carried out on-site by trained clinic staff.
The second urine sample was divided into two portions. One was sent to the laboratory for testing, using the standard ‘polymerase chain reaction’ (PCR) method, which normally returns results in 7-10 days in a clinical setting. The other half of the sample was frozen and stored in case further testing was necessary.
The rapid urine test was compared to standard testing using four key measures:
- Sensitivity: accurately identifying a positive sample,
- Specificity: accurately identifying a negative sample,
- Positive predictive value: the proportion of people with a positive test result who are correctly diagnosed as positive, and
- Negative predictive value: the proportion of people with a negative test result who are correctly diagnosed as negative.
What were the results of the study?
Using standard PCR laboratory testing, chlamydia was detected in 4.4% of samples at the sexual health centre (20/454) and in 11.9% of samples at the GUM clinic (90/757). Compared with standard testing, the Chlamydia Rapid Test had:
- a sensitivity of 82.6% (90/109),
- a specificity of 98.5% (1085/1102),
- a positive predictive value of 84.1% (90/107) and
- a negative predictive value of 98.3% (1085/1104), respectively.
The researchers used a DNA test that estimated the number of bugs, (the organism load) in the samples. The organism load in samples that tested positive for chlamydia had a significant correlation with the results of the Chlamydia Rapid Test, i.e. the more bugs that were seen in the sample, the better the accuracy of the test’s results.
What interpretations did the researchers draw from these results?
The researchers conclude that using the new Chlamydia Rapid Test with samples of males’ first urination of the day would be an effective diagnostic tool for chlamydia infection in men. They say that the availability of test results within an hour would enable immediate treatment and contact tracing, which could potentially reduce the risks of persistent infection and onward transmission.
What does the NHS Knowledge Service make of this study?
This well-conducted study has tested the validity of a new rapid urine test for chlamydia in men. The test results demonstrate a high accuracy in correctly confirming that a man did not carry the infection, with both high test specificity and negative predictive value. The technique also had a good but slightly lower accuracy in identifying that a man did carry the infection, with test sensitivity and positive predictive values of 82-84%.
The development of this technique is an important step forward in efforts to slow the increasing spread of this harmful, but often undetected, infection among young people. The rapid urine test has the advantage of being a fairly quick and non-invasive test that avoids the 7-10 day waits for results and the use of urethral swabs, which are common in current practices.
The rapid availability of results of this new test offers would potentially enable prompt treatment and contact testing. Currently, women get tested more often than men, and it is hoped that this new method would make men more willing to go for testing.
Further testing in a larger number of samples is likely to be required before this test could be brought into current practice. If it were to be used for screening, it will be especially important to consider the staff, training and equipment required, and where the test would be made available.