Although likely still years from common usage, a new tuberculosis test that uses a simple tongue swab rather than requiring patients to produce sputum through coughing shows potential. Spotlight examines the workings of this test and explores with specialists where they envision its future role could lie.
Many TB examinations are performed using sputum specimens. However, one challenge lies in the fact that numerous individuals, especially young ones and those affected by HIV, find it difficult to produce the viscous mucosal discharge, often referred to as phlegm, from their respiratory system.
Although sputum-based tests will probably continue to be the norm for diagnosing tuberculosis for quite some time, scientists are diligently searching for easier options, particularly for kids and individuals who have HIV. A prominent contender is an old approach—a straightforward oral swab, sometimes referred to as a tongue swab—that has been available for many years.
How it works
Anyone who has undergone a Covid-19 test will likely recall using a swab for testing. In contrast, a tongue swab for detecting tuberculosis operates on a similar principle but is much less intrusive. This procedure requires simply swabbing the tongue instead of probing deeply into the nasal passages or reaching to the rear of the throat as done during a Covid-19 test.
“A tongue swab test for TB works by collecting bacteria from the surface of the tongue using a soft, flocked swab. The swab is gently rubbed over the back of the tongue for about 30 seconds, ensuring as much bacteria as possible is gathered without causing discomfort,” explains Anura David, a senior medical scientist at the Wits Diagnostic Innovation Hub.
The swab is subsequently dispatched to a lab, where they check for the existence of Mycobacterium tuberculosis (MTB), the bacterium responsible for causing TB.
David explains that the sample is evaluated with the diagnostics employed for sputum. "Nonetheless, our aim is to make tongue swab testing feasible directly in the field near the patient," she states. "This way, patients could receive their diagnosis during the same visit instead of having to come back another day for their results like they do now."
Using today’s technology, TB is simpler to identify in sputum samples compared to those obtained from the tongue. This is due to sputum originating directly from the lungs, usually the site of highest TB bacteria activity. Nonetheless, research into tongue swabs continues, and their exact accuracy remains undetermined.
As tongue swabs haven't been approved for diagnosing TB, David states that individuals receiving a positive outcome from a tongue swab test require further examination using a sputum sample.
"We're achieving notable advancements with tongue swabs for diagnosing tuberculosis, yet several crucial issues remain unresolved prior to their broader implementation," she states.
What is the usual method for diagnosing tuberculosis?
Dr Christopher Ealand, who works as a senior researcher and molecular biologist at the School of Pathology within the Faculty of Health Sciences at Wits University, characterizes sputum as "a sticky, viscous, dense, unpleasant kind of specimen" expelled via coughing into a collecting tube. This particular sample can undergo testing for tuberculosis through one of three techniques.
He notes that this process involves using a nucleic acid amplification test, similar to what’s employed in South Africa known as the Gene Xpert Ultra. This method checks for the presence of TB DNA and determines if the bacteria show resistance to rifampicin—a key drug in treating tuberculosis. Another approach is to place a sputum specimen inside a Mycobacteria Growth Indicator Tube (MGIT). If growth occurs within roughly 50 days, it indicates an active case of TB. Additionally, diagnosing TB may involve examining stained samples under a microscope; when illuminated properly, these bacteria appear vividly green.
A tuberculosis diagnosis method that doesn’t rely on sputum samples is through chest X-rays. These images can reveal lung harm brought about by active TB infection; however, they typically cannot distinguish between current illness and past damage resulting from previously healed infections. Therefore, for cases where X-rays indicate potential TB, further confirmation with sputum-based examinations is necessary.
Ealand highlighted that one issue with sputum-based tests is that not everybody manages to produce the necessary substance through coughing.
In cases where a child cannot expel sputum through coughing, an alternate method is performing a gastric lavage. This procedure entails placing a tube into the child’s throat to extract liquids from the stomach following a period of overnight fasting. The rationale behind this approach is that children affected by tuberculosis might have ingested sputum containing bacteria as it moved upwards from the lungs into the stomach.
An alternative approach suitable for both children and adults having difficulty producing phlegm is induction. It entails breathing in aerosolized salt water, usually prompting a coughing response.
There is definitely a necessity to enhance tuberculosis diagnostics," states David. She notes that the bacteria causing TB were discovered back in 1882, and for almost 130 years afterward, just four diagnostic methods existed. However, since 2010, according to David, considerable progress has been achieved in this field of TB testing, "especially through molecular assays which allow quicker and easier detection of TB.
Limitations of tongue swabs
A tongue swab could offer a promising and far more convenient option compared to obtaining a sputum sample or undergoing a chest X-ray, yet it comes with certain drawbacks.
For example, David states that tongue swabs yield better results for individuals carrying a high burden of TB bacteria. However, generally speaking, these swabs are not as sensitive as sputum when used for diagnosing tuberculosis.
An optimal test should have both high sensitivity and high specificity, which means it accurately detects those who are ill without falsely diagnosing people as being unwell when they're actually healthy," she explains. Sensitivity pertains to a test’s capability to properly identify the existence of an infection, whereas specificity relates to its capacity to appropriately confirm the non-existence thereof.
Research findings from tongue swab studies indicate inconsistent accuracy in identifying individuals with tuberculosis, according to Dr. Ryan Dinkele, a research officer and epidemiologist at the University of Cape Town’s School of Public Health, who spoke to Spotlight.
A 2024 systematic review A study published in The Lancet Global Health journal examined 20 research papers—15 focusing on adults and 5 on children. The findings indicated that the sensitivity of oral swab tests for adults varied between 36% and 91%, whereas for children, this range was narrower, spanning from 5% to 42%. However, the specificity of these tests showed less variation, with the majority of studies noting a rate above 90%.
Dinkele suggests this difference might stem from various elements such as the individual collecting the sample, the kind of swab utilized, or the proficiency level of the lab technician in isolating DNA. Additionally, a tongue swab generally includes a smaller quantity of TB bacteria compared to a sputum sample; thus, an insufficient number of these bacteria can complicate the process of DNA extraction.
Commenting In The Lancet regarding the systematic review, the authors observe: "Oral swabs show potential, yet their reported sensitivity falls short of the WHO threshold for diagnostic equipment. Hopefully, this sensitivity can be enhanced by refining sample collection processes, such as self-collection, along with improving testing techniques."
David states that additional studies are necessary to refine the process of sample gathering, enhance the precision of diagnostics, and maintain uniformity among various groups and environments.
Ealand contends that because there isn’t a uniform approach to employing tongue swabs for testing tuberculosis, the initial move ought to be establishing a standardized procedure. Such guidelines would cover aspects like which buffering solution is utilized, at what intervals specimens are obtained, the kind of swab employed, and how long after collection they must be processed before analysis begins. Implementing these standards could simplify result comparisons across various labs and research projects.
Moreover, Ealand suggests that another method to enhance the sensitivity of the test might involve pooling results. Rather than gathering only one swab sample, multiple samples—such as three or four—could be taken and examined collectively. Doing so would probably boost the bacterial concentration, thereby simplifying detection.
In which part of the assessment could this test be placed?
Although widespread implementation of tongue swab TB tests appears unlikely in the near future, and it seems unfeasible that they would ever supplant sputum-based testing, experts concur that this method holds promise for specific scenarios.
Dinkele remains doubtful about the effectiveness of tongue swabs as a TB diagnosis at present, yet he suggests they might serve well as a screening instrument. Alternatively, they could be used as an additional test for individuals when other diagnostic methods prove ineffective.
Ealand concurs, suggesting that this method might serve as an effective means of assessing TB prevalence within families. He elaborates that each member of a household would offer a tongue swipe specimen, which would subsequently be examined collectively as one combined sample. A negative outcome would indicate the absence of TB in that home with confidence. However, should TB be identified, further verification could be conducted through additional sputum sampling.
He notes that it might also prove helpful in identifying TB in individuals with subclinical (asymptomatic) TB — those who carry the disease and can transmit it but do not exhibit any signs of illness.
A portion of the argument for broader implementation as a screening tool involves the increased ease associated with using a tongue swab. This enhanced convenience might facilitate more extensive and regular testing. While these discussions can become rather intricate, an underlying concept suggests that the expanded frequency of tests made possible through tongue swabs could compensate somewhat for their reduced accuracy to some degree.
One approach under consideration involves the possibility of individuals testing themselves for tuberculosis through the use of tongue swabs.
One recently published study The research, in which David participated as an author, examined the precision of self-administered tongue swab tests in diagnosing TB. Conducted at the Hillbrow Community Health Centre in Johannesburg, this study involved 399 individuals who were undergoing investigation for TB. These participants were requested to collect their own tongue swabs, provide sputum samples, and offer urine samples. The self-collected tongue swabs underwent testing via the Gene Xpert MTB/RIF Ultra assay in a lab setting. This approach was contrastingly evaluated against the analysis of patients' sputum specimens through liquid culture—the alternative term used here refers back to what has been previously mentioned about the MIGIT test.
In general, the sputum test conducted using liquid culture showed greater sensitivity than the tongue swabs, with percentages of 95% compared to 78%. This indicates that the tongue swabs were less effective at identifying TB in individuals carrying the bacterium when contrasted with the sputum test. Nonetheless, a 78% sensitivity rate for the tongue swabs remains relatively high considering findings from other similar research involving tongue swabs.
The study also revealed that the effectiveness of self-administered tongue swabs matched up with those collected by medical professionals in previous research. According to David, this suggests that self-collected tongue swabs can be considered equally dependable as those obtained by healthcare providers.
Following the survey conducted afterward, every participant expressed satisfaction with the swab method used. Seventy-four percent indicated a preference for using a tongue swab instead of providing a urine or sputum sample for TB testing. Notably, fifty-two percent favored having a healthcare professional carry out the procedure, whereas forty-eight percent opted for performing the swab themselves.
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