Although likely several years from widespread use, tuberculosis testing, which relies on simple tongue swabs rather than having to cough a person sput, shows promise. Spotlight stocks how tests work and asks experts what their future role is.
Most tuberculosis (TB) tests are performed on sput samples. The difficulty with this is that many people, especially children and people who live with HIV, have a hard time coughing thick mucus called thickness from the lungs.
Sputum-based testing could remain the standard for TB diagnosis for some time, but researchers are looking for a more convenient alternative, especially for children and people who live with HIV. One of the main candidates is a simple oral swab, also known as a tongue swab, which has been around for many years.
How it works
Using swabs to test illnesses must be fresh in the memory of those who have received the Covid-19 test. Tuberculosis tongue swabs work in a similar way, but are far less invasive than the Covid-19 test, as they only apply the tongue, not the deep nose passage or the back of the throat.
“Tuberculous tongue swab test works by using soft, herd swabs to collect bacteria from the surface of the tongue. The swab is slowly rubbed behind the tongue for about 30 seconds, ensuring that as many bacteria as possible are collected without causing discomfurt.”
“The swabs are then sent to the laboratory to test for the presence of Mycobacterium tuberculosis (MTB), the bacteria that cause tuberculosis.”
David says the specimens are being tested using the same diagnostic methods used for sput. “But we want the tongue swabs to be able to ultimately test patients in a field close to the patient, so patients can diagnose simultaneously (as they do now) without having to go back to another day for results,” she says.
With current technology, tuberculosis is easier to detect in sput samples than in tongues. That’s because the sput comes directly from the lungs. This is usually the most active place where Mycobacterium tuberculosis is. That said, tongue swabs are still being studied, and it is not yet clear how accurate they are.
Because tongue swabs are not approved as a tuberculosis diagnostic tool, David explains that anyone who gets positive results from tongue swabs should test again with a sput sample.
“We have made great strides with the use of tongue swabs for TB diagnosis, but there are still important questions to address before implementing them widely,” she says.
How is TB normally diagnosed?
Christopher Ealand, a senior researcher and molecular biologist in the Department of Pathology at Wits University’s Department of Health Sciences, describes someone as a “viscous, thick, messy, ugly type of sample” in which someone coughs into a collection vial. This sample can be tested for TB using one of three methods:
He explains that it can be performed via nucleic acid amplification tests. The one used in South Africa is called the gene Xpert Ultra. This can determine whether TB DNA is present and whether the bacteria are resistant to Rifampicin, one of the first drugs of tuberculosis. Alternatively, sputum samples can be placed in mycobacterial growth indicator tubes (MGITs), meaning that tuberculosis is present if the sample grows over approximately 50 days. Tuberculosis can also be diagnosed by examining the sample under a microscope after being treated with special staining. When present, bacteria become brighter with bright green.
One Sputum-independent TB diagnosis is chest x-ray. X-rays can indicate lung damage caused by tuberculosis disease, but they usually cannot tell from the x-ray whether there is an ongoing disease or if there is damage from a disease that has already been resolved. For this reason, follow-up of x-rays suggesting tuberculosis should be followed up with a sputum-based test.
As Ealand emphasizes, the problem with the Sputum-based test is that not everyone can cough things.
For children who cannot cough sput, one option is stomach lavage. This involves inserting a tube into the child’s throat to collect liquid from the stomach after the night of fasting. This is because if a child with tuberculosis swallows a sput that comes out of the lungs, they can have bacteria in their stomachs.
Another option for both children and adults who struggle to create Sputum is induction. This usually involves inhaling a sprayed saline solution that causes a person to cough.
“It’s clear that we need to improve our TB diagnosis,” David says. The bacteria that cause tuberculosis were first isolated in 1882, she said, and for nearly 130 years only four diagnostic tests were available. Since 2010, David says there have been significant advances in tuberculosis diagnosis. “Especially with molecular tests that allow for faster, more accessible TB diagnosis.”
Major Killers | Tuberculosis can be cured, but it can be difficult to diagnose and treat. This week’s newsletter focuses on TB bacteria from centuries ago: https://t.co/soezxvdipj
Limiting tongue swabs
Using tongue swabs may offer potential and more convenient sput x-rays and chest x-ray alternatives, but not without limitations.
For example, David said that testing people with high loads of TB bacteria has improved tongue swab performance. Overall, however, when it comes to the diagnosis of TB, swabs are less sensitive than sput.
“The ideal test has both high sensitivity and high specificity. This means correctly identifying sick people while avoiding misdiagnosis of healthy people,” she says. (Sensitivity refers to the ability to correctly indicate the presence of a bug, while specificity refers to the ability to correctly indicate the absence of a test.)
Studies on tongue swabs show mixed results on how accurately they can identify people, Dr. Ryan Dinkele, the research director and epidemiologist at the University of Cape Town’s Faculty of Public Health.
A 2024 systematic review published in the Lancet Global Health Journal examined 20 studies (15 adults involved and five children involved). The sensitivity of oral swabs was found to range from 36% to 91% in adults and from 5% to 42% in children. Although the specificity of the tests did not change much, most studies reported more than 90% of specificity.
This variation states that Dinkelle could be caused by several factors, such as who collected the sample, the type of swab used, or how good the laboratory person is at extracting DNA. Another factor is that tongue swabs usually contain less M. tuberculosis than sput samples, and if the sample contains too few bacteria, it can be difficult to extract DNA.
Commenting in the Lancet of Systematic Reviews, the author states: “Oral swabs are promising, but the reported sensitivity is below the cutoff of the diagnostic tool WHO (World Health Organization).
David says further research is needed to optimize sample collection, improve diagnostic accuracy, and ensure consistency across different populations and settings.
Ealand argues that the first step is to create a standardized protocol, as there is no standard method for using tongue swabs to test TB. This includes the use of buffer solution when the sample is collected, the type of swab used, and the time between collection and processing. This allows for easy comparison of results between different laboratories and studies.
Furthermore, a potential improvement in test sensitivity is to pool the results. Instead of collecting only one swab sample, you can collect three or four, and test them together. This can increase the burden on bacteria and make detection easier.
Where will this test fit?
While tongue swab TB tests are unlikely to be implemented at the population level anytime soon, it seems unlikely that they will replace Sputum-based tests, experts agree that they may be possible in certain settings.
Dinkelle is skeptical of the usefulness of tongue swabs as a diagnosis of tuberculosis in current conditions, but he says there may be a place as a screening tool. Or as a supplementary test for individuals whose other diagnostic methods are not working.
Ealand agrees, saying it could be a useful tool for screening TB households. He explains that anyone in the household can provide a tongue swab sample, which can be tested together as a pooled sample. If the result is negative, it can be safely assumed that the household does not have tuberculosis. However, if TB is detected, you can collect a sput sample for verification tests.
He added that if you have tuberculosis and may be infectious but not show symptoms, it could also help detect tuberculosis in people with asymptomatic (asymptomatic) tuberculosis.
Sign up for the AllAfrica newsletter for free
Get the latest African news
success!
Almost finished…
You need to check your email address.
Follow the instructions in the email you sent to complete the process.
error!
There was a problem processing the submission. Please try again later.
TBAC calls for increased testing for #ENDTB, better care, and stronger accountability. In this detailed spotlight article, South Africa’s advances on TB rely on more people to diagnose more quickly: https://t.co/q7uicktbgz https://t.co/uo79gtb1fs
Part of such a wide range of applications, such as screening tests, is that the more convenient tongue swabs can allow for more broad and frequent testing. The discussion here has become very technical, but one idea is that the broader and more frequent testing made possible by tongue swabs can compensate for some lower sensitivity.
Along such lines, one path pursued is the possibility of TB self-testing using tongue swabs.
A recently published study in which David was the author examined the accuracy of self-collected tongue swab tests in the detection of tuberculosis. In this study, conducted at the Hillbrow Community Health Centre in Johannesburg, 399 people surveyed for tuberculosis were asked to take their tongue swabs. They were also asked for sp and urine samples. Self-collected tongue swabs were tested in the laboratory using the gene Xpert MTB/RIF ultra test. This was compared to sput specimens of people tested using liquid cultures, another name for the aforementioned MIGIT test.
Overall, sput in the liquid culture test was more sensitive than tongue swabs, 95% vs. 78%. Tongue swabs were not as good at detecting tuberculosis in people who had bacteria, like sput tests, but their sensitivity of 78% is higher compared to other tongue swab studies.
Researchers also found that the performance of self-collected tongue swabs was comparable to that of tongue swabs collected by healthcare workers in other studies. David says this means that self-collected tongue swabs are as reliable as healthcare workers have collected tongue swabs.
In a subsequent study, all study participants said they were satisfied with the swab, with 74% saying they preferred to use a tongue swab over urine or sput samples for the tuberculosis test. Interestingly, 52% of participants said that healthcare workers prefer to do swabing, while 48% preferred to do it themselves.
