Diagnostic performance of Xpert MTB/RIF Ultra assay for tuberculosis in stool specimens among adult presumptive TB patients in a generalized HIV epidemic setting
In a prospective study of adult presumptive TB patients at the Thailand-Myanmar border, the Xpert MTB/RIF Ultra assay performed on stool specimens demonstrated high sensitivity (90.32%) and specificity (98.78%) comparable to sputum testing, supporting its use as a viable alternative diagnostic method when respiratory samples cannot be obtained.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Tuberculosis is an ancient disease that still claims millions of lives every year, primarily by attacking the lungs. For decades, the standard way to find it has been to ask a patient to cough up mucus from their lungs, known as sputum, and examine it under a microscope or test it for genetic material. This method works well for many, but it fails for those who cannot cough up enough mucus, such as the very young, the elderly, or people who are too sick to produce a sample. In these difficult cases, doctors have often had to rely on invasive procedures to get the necessary fluid, which can be risky and uncomfortable. Because the bacteria that cause tuberculosis can be swallowed along with mucus, scientists have long wondered if a sample of stool could serve as a reliable, non-invasive alternative to find the infection.
A team of researchers working along the border of Thailand and Myanmar decided to test this idea in a real-world setting where tuberculosis and HIV are common. They focused on a population of adult migrants who often face barriers to healthcare and are at high risk for the disease. The researchers gathered stool samples from adults who were suspected of having tuberculosis but could not provide a good sputum sample. They used a modern, rapid molecular test called Xpert MTB/RIF Ultra, which looks for the genetic signature of the tuberculosis bacteria, to analyze the stool. They compared these results against the gold standard: a culture test performed on sputum samples, which grows the bacteria in a lab to confirm their presence. The goal was to see if the stool test could catch the disease as accurately as the traditional sputum test in this specific group of people.
The study involved 113 adult participants, most of whom were migrants living in the border region. Among them, 31 were confirmed to have tuberculosis based on the sputum culture results. When the researchers tested the stool samples with the rapid molecular tool, they found that it correctly identified the disease in 90 percent of the confirmed cases. This means that if a person had tuberculosis, the stool test was highly likely to show a positive result. The test was also very good at ruling out the disease in people who did not have it, correctly identifying negative cases about 99 percent of the time. When the researchers compared the stool results directly to the sputum results, they found no statistically significant difference in performance. The stool test was just as effective as the sputum test for these adults, with only a tiny, insignificant drop in sensitivity that did not matter in practice.
The researchers also looked at an older, simpler method called smear microscopy, which involves staining a sample and looking for the bacteria under a microscope. While this method worked reasonably well on sputum, it performed much worse on stool, missing more than half of the confirmed cases. This suggests that while the rapid molecular test is a powerful tool for stool samples, the older microscope method is not reliable enough for this purpose. The study confirmed that the rapid test on stool could detect the bacteria even when the bacterial load was low, although it did not find any cases where the bacteria were present in such tiny amounts that the test could only give a "trace" result.
The findings suggest that for adults who cannot produce a sputum sample, collecting a stool sample and testing it with the rapid molecular tool is a viable and accurate alternative. This is particularly important for vulnerable populations, such as migrants and people living with HIV, who might otherwise go undiagnosed. The study did not claim that stool testing should completely replace sputum testing, as sputum remains the primary method when it can be obtained. However, the results provide strong evidence that stool testing can fill a critical gap in diagnosis when respiratory samples are difficult to collect, offering a safer and easier path to confirming tuberculosis and starting life-saving treatment.
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