Combined Chlamydia and Neisseria gonorrhoea NAATs assays
This short article has been written to give the background to changes to the laboratory tests used to detect chlamydia and Neisseria gonorrhoeae in recent years. As well as improving the diagnosis of these infections, these have wider patient benefit, in that sampling is simpler and less invasive.
Technological advancements in molecular Nucleic Acid Amplification Tests (‘NAATs’) make it possible and inexpensive to test simultaneously for Neisseria gonorrhoeae (gonococcus or GC) alongside chlamydia in a single assay. The chlamydia/GC NAATs assays are much more sensitive than traditional culture or immunological assays and as a result can be used on a wider range of sample types. Uptake of chlamydia/GC NAATs assays throughout England has been widespread and in August 2014 updated guidance on their use was published by Public Health England (PHE) (see: https://www.gov.uk/government/publications/guidance-for-the-detection-of-gonorrhoea-in-england).
Chlamydia and gonococcal infection can be diagnosed in women by testing first-catch urine or collecting swab specimens from the endocervix or vagina. Self-collected vaginal swab specimens (but not vulval swabs) are equivalent in sensitivity and specificity to those collected by a clinician. Diagnosis of chlamydia and gonococcal urethral infection in men can be made by testing a urethral swab or first-catch urine specimen. Optimal urogenital specimen types for chlamydia screening using NAATs include first-catch urine (men) and vaginal swabs (women).
Rectal and oropharyngeal infection can be diagnosed by testing samples from these sites, although this is outside the licenced use of the assay. Nevertheless, NAATs have been demonstrated to have improved sensitivity and specificity compared with culture.
This all sounds brilliant and patients prefer to provide easily available specimens that they can take themselves. The number of samples needed to exclude both infections is reduced. However, there are some caveats.
The widespread use of the combined chlamydia/GC NAATs for chlamydia screening may lead to unintended positive gonococcal results. Gonococcal disease remains relatively uncommon and therefore use of the combined NAATs assay will mean that many people are tested for an infection that they are unlikely to have. The risk of this can be mitigated by re-testing reactive gonococcal samples using a second assay.
Secondly, it is still necessary to have a culture isolate of Neisseria gonorrhoeae to be able to perform antibiotic susceptibility testing and further characterisation of the organism. This is becoming more-and-more important as antibiotic resistance increases. The NAATs test is unable to provide this information, as it produces no bacterial isolate to test. Therefore, patients with confirmed positive gonococcal results should have fresh swabs taken for culture before antibiotic treatment is given.
Further information on this and other sexually transmitted diseases can be found on the British Association for Sexual Health and HIV (BASHH) website at http://www.bashh.org.
We hope that short articles of this sort are useful. If so, it would be possible to cover other diagnostic issues in future newsletters and therefore feedback would be gratefully received.
Dr Nick Brown
Consultant Medical Microbiologist
Public Health England