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The World Health Organization estimates that every year 40% of people who fall ill with TB globally are not diagnosed. There is therefore an urgent need for faster, safe and more convenient TB tests. 

The gold standard for testing still requires people to cough up sputum, which some people and children, in particular, struggle with. It also poses an infection risk to healthcare workers collecting samples.

One alternative, an oral swab, which entails scraping the tongue, is the subject of studies in South Africa and elsewhere, and if it turns out to be accurate enough, it may revolutionise TB testing. Not only does it promise to be quick and easy to collect, but it might also open the door to people collecting their own samples for testing – which is not possible with current tests.

Professor Grant Theron, head of the Clinical Mycobacteriology and Epidemiology research group at Stellenbosch University’s Molecular Biology and Human Genetics Unit, says the test involves a swab with a tip (flock, foam or cotton) designed to scrape the upper surface of the tongue. Theron and his team use flocked swabs, whose fibres of variable lengths absorb and release samples better.

“Scientific evidence suggests that Mycobacterium tuberculosis (Mtb) DNA is deposited on the oral epithelium during active TB disease,” he says. 

Dr Angelique Luabeya, chief research officer at the South African Tuberculosis Vaccine Initiative, says healthcare workers gently brush the tongue with a sterile swab for about 10 seconds (seven to eight times) to collect cells and saliva. The head of each swab is then put in a tube containing buffer.

“The procedure is non-invasive and does not put the subject at risk for physical injury,” says Luabeya. 

As Luabeya and colleagues reported previously , the tongue is a better site for sample collection, with higher positivity for TB DNA, than the cheek or gums. “One explanation might be that the tongue has more biomaterial than other sites in the mouth,” she says.

“How these samples are transported is a crucial process,” says Theron. “The swabs need to be transported under the correct conditions and temperature to ensure the DNA is preserved and intact.” Samples have to be stored at -20 0 C or -80 0 C (depending on how long they have to be stored). After collection, samples are typically taken in special cooler boxes to laboratories.

The samples are then analysed with a brand of polymerase chain reaction test called GeneXpert MTB/RIF Ultra, which is already widely used in South Africa to test for TB in sputum samples. GeneXpert machines have also been used for some SARS-CoV-2 testing.

“At this stage of research, we are batching the samples before performing the PCR testing,” says Luabeya. “We would like to have the oral swab analysis methods as point-of-care testing in clinics. The aim is to have results available within a few hours.”

In the 2019 study by Luabeya and colleagues, two tongue swabs were found to have a sensitivity of 92.8% compared with sputum-based GeneXpert testing – results were less impressive when only one tongue swab was done. Doing two tongue swabs, however, also detected some cases of TB missed by sputum-based testing so that the overall yield from the two methods was similar.

The several further research projects under way in South Africa and elsewhere are, among other things, exploring the use of oral swabs in children and adolescents, and people living with HIV. Research is also being done on how oral swabs can be rolled out in diverse settings and how they might be used as a point-of-care test (where analysis is done at the healthcare facility rather than in a lab). Details on how to optimally take, store and analyse samples at scale are also being gathered.

Luabeya says healthcare workers find it easier and less time-consuming to collect oral swabs compared with sputum, especially in patients who struggle to provide sputum. One potential advantage of tongue swabs is that people can do the swabbing themselves, thereby reducing the potential exposure of healthcare workers even further.

“We have trained patients and provided videos to demonstrate how to collect the sample. They find it easy. Healthcare workers were also trained and could perform the sample collection without difficulty,” says Luabeya.

Theron echoes this: “As tongue swab collection is a non-invasive procedure, it is easy to collect and poses no to minimal risks to patients. Anyone can have it done. This is especially useful in children, people living with HIV and those in the early stages of the disease who are diagnostically more challenging and often cannot produce sputum.

“Self-collection of the tongue swabs is already under way in our clinical studies. The patients collect the swabs themselves under the supervision of a research nurse or healthcare worker. The nurse or healthcare workers will demonstrate and explain to the patient how the collection is done prior to sampling. Minimal training is required, and a video is also made available on the collection procedure. Overall, the patients have been receptive to the procedure and no problems have been anticipated.”

“The results from the tongue swab studies are very encouraging,” says Professor Keertan Dheda, a general physician, pulmonologist and critical care specialist who heads the Division of Pulmonology at Groote Schuur Hospital and the University of Cape Town.

“In individuals who could produce sputum, swabbing the tongue had almost the same diagnostic yield as detecting DNA in the sputum using the conventional PCR test that is being used throughout South Africa (Gene Xpert). Patients with TB may not always be able to produce good-quality sputum samples [and] often instructions on how to produce sputum are not made clear to patients. Using a simple swab on the tongue would be much more convenient for patients,” he says.

Other advantages, Dheda says, would include creating a safer environment for healthcare workers, who already have high rates of TB infection.

“However, although these preliminary studies are encouraging, we need larger studies in different settings to confirm these findings, and also to see whether the tongue-swabbing method would work on an automated detection platform. The major unanswered question is whether the yield of the tongue-swabbing method would be as good as conventional methods in people who have smear-negative TB (low concentration of TB bugs in the sputum) and in individuals who are sputum-scarce (unable to produce adequate quantities of sputum).” 

He also points out that tongue swabs will specifically have to be studied in children and in people living with HIV, with whom sputum sampling is challenging. It will also be important to establish whether one could determine drug resistance from samples taken with tongue swabs, as is the case with sputum samples. DM/MC

* This article was published by Spotlight – health journalism in the public interest.

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