Self-sampling for STIs in the rectum and throat as diagnostically accurate as sampling by a clinician

Self-sampling for sexually transmitted infections in the rectum and throat is just as accurate as testing performed by a healthcare worker, investigators from Leeds report in the online edition of Clinical Infectious Diseases. This is based on a study that involved women and gay men. They found that rectal and throat self-sampling was equal in accuracy to sampling by healthcare workers. Self-sampling also had cost-saving potential, especially when performed at home.

A large number of rectal infections were identified in individuals who did not report a history of anal sex. The investigators, led by Dr Janet Wilson of Leeds Teaching Hospitals, therefore recommend screening both women and gay men for rectal and throat infections irrespective of reported sexual activity. 

“We demonstrated good concordance between the clinician and self-taken samples with no difference in diagnostic accuracy,” write the authors. “As sexual history does not identify those with extragenital infections, our study strongly supports the existing evidence for universal sampling in both men who have sex with men and females undergoing chlamydia and gonorrhea testing.”

An accompanying editorial acknowledges the findings regarding the accuracy of self-sampling. However, it adopts a more sceptical stance concerning the health and economic benefits of universal screening of women and gay men for bacterial STIs in the rectum and throat. 

Chlamydia and gonorrhoea are common STIs and can infect not only the genitals but also the rectum and pharynx (throat). The majority of chlamydia and gonorrhoea infections among gay men are in the rectum and throat and are often asymptomatic.

Self-sampling for STIs, either in community settings or at home using mail-out kits, is becoming more common. Dr Wilson and her colleagues from Leeds Sexual Health, Leeds University and Imperial College London, however, were aware that the evidence for the accuracy of self-sampling is relatively weak.

They sought to address this important gap in knowledge by undertaking the first large prospective study comparing the diagnostic accuracy of healthcare worker-collected samples and self-sampling for the detection of chlamydia and gonorrhoea in the rectum and throat. The authors also compared the time needed for these sampling methods and calculated the possible economic benefits of self-sampling.

There were 1793 study participants, made up of 1284 women and 509 gay men, who received care at Leeds Sexual Health between 2015 and 2016. The women had a median age of 23. Just over a third (38%) had symptoms suggesting a genital STI. Almost all had a history of vaginal sex, 96% had given oral sex and 46% had received anal sex. The gay men had a median age of 29 and 13% had symptoms indicating a possible urethral STI. The vast majority (90%) had a history of receptive anal sex and almost all had a history of oral sex.

A healthcare worker took samples from the genitals, rectum, and throat. The participants then self-sampled from the same sites.

To help eliminate the risk of bias, laboratory staff analysing the samples did not know which were collected by healthcare workers and obtained via self-sampling. Rigorous testing was undertaken to help reduce the risk of false-positive results.

There were no marked differences in the diagnostic accuracy of swabs taken by healthcare workers and patients. For rectal gonorrhoea, detection rates were 93% and 98% for the clinician and self-collected samples respectively. The corresponding rates for rectal chlamydia were 96% and 97%. Sampling for throat infections also had comparable accuracy: clinician sampling accurately detected 93% of gonorrhoea and 92% of chlamydia infections, with the rates for self-sampling being 96% and 94%, respectively.

The investigators also found that large numbers of infections in the throat and rectum would have been missed if testing were guided by the sexual histories given to healthcare workers.

In women, 9% of gonorrhoea infections and 13% of chlamydia infections were in individuals who did not have a genital infection, while the prevalence of rectal chlamydia was higher than the prevalence of genital chlamydia (17% vs 16%).

Gay men were even more likely to have infections in the rectum or throat rather than the urethra. Two-thirds of gonorrhoea diagnoses and 71% of chlamydia cases were in men who did not have a penile infection.

“Pharyngeal gonorrhoea may be transmitted to the rectum in men who have sex with men using saliva as a lubricant for anal sex,” note the authors. “Interestingly, our pharyngeal and rectal gonorrhoea prevalence in men who have sex with men was identical at 6.7% whereas urethral prevalence was lower at 3.3%, supporting the oro-rectal transmission hypothesis.”

"Rectal and throat self-sampling was equal in accuracy to sampling by healthcare workers."

Self-sampling took slightly longer (an average of four minutes) than healthcare worker sampling (three minutes). But the demands on staff time meant that compared to self-sampling, it would cost £923 for gay men and £2054 for women to detect each additional gonorrhoea infection using healthcare worker sampling. The cost-saving would even be higher if self-sampling was undertaken at home.

Dr Wilson and her colleagues, therefore, believe that their findings have important implications for policymakers and STI services, showing that self-sampling for infections in the throat and rectum yields highly accurate results. Moreover, they believe their findings strongly support screening all gay men and women for STIs in the throat and rectum during sexual health check-ups.

An accompanying editorial by Dr Noah Kojima and Dr Jeffrey Klausner describes the study as “valuable” and praises its methodology.

But they were not convinced about the individual or public health benefits of universal three-site STI screening. They point out that chlamydia and gonorrhoea infections in both the throat and rectum can resolve on their own in the six months following infections and that there is “stigma” attached to having rectal STI tests. Drs Kojima and Klausner suggest that expanded screening could lead to throat infections being over-treated, increasing the risk of antimicrobial resistance, with significant costs to healthcare systems.

“While it is instinctual to find and treat infections, the various costs must be carefully considered to ensure the benefits outweigh the potential harms,” they conclude.

It remains to be seen if their ultra-cautious approach will have an impact on the recommendations for three-site STI screening in the British and US guidelines they quote.