Molecular Stool Test Shows Potential for Diagnosing TB in Adults with HIV

Tuberculosis (TB), caused by the bacterium Mycobacterium tuberculosis, led to 1.25 million deaths in 2023, with 13% of those occurring in people living with HIV. The current primary diagnostic method for TB relies on sputum samples, which are collected by deep coughing and expectoration of lung secretions. The World Health Organization (WHO) recommends molecular sputum tests for individuals living with HIV, in addition to urine antigen detection (TB-LAM). However, access to these tests remains limited in many regions, and they are not suitable for all individuals with HIV. Specifically, people with HIV often have difficulty producing sputum; more than half of those in advanced stages of the disease are unable to do so. Moreover, the bacteria concentration in the sputum is frequently too low to detect. To overcome this challenge, researchers have proposed using a molecular test, which is currently employed on respiratory samples, for stool samples instead to test for TB.

Cepheid’s (Sunnyvale, CA, USA) Xpert MTB/Ultra molecular diagnostic test, which has thus far only been recommended for children, could also be used to diagnose tuberculosis in HIV-positive adults. This was the central conclusion of the Stool4TB Alliance study, led by the Barcelona Institute for Global Health (ISGlobal, Barcelona, Spain). Published in The Lancet Microbe, the results could potentially lead to a paradigm shift in diagnosing TB among this population. The test is currently recommended for children who have difficulty producing sputum. The study, conducted between December 2021 and August 2024 in medical centers across three African countries, included 677 HIV-positive patients aged 15 and older with suspected TB.

Participants provided sputum, urine, stool, and blood samples. The results revealed that the stool test had a sensitivity of 23.7% and a specificity of 94.0% when compared to the reference standard. For patients with CD4 counts below 200 cells/μl, sensitivity increased to 45.5%. CD4 lymphocytes are immune cells that are damaged by HIV infection, making individuals with CD4 counts below 200 more susceptible to serious opportunistic infections, including tuberculosis. Additionally, the Stool Ultra test identified cases of TB that were missed by other methods, such as TB-LAM, Ultra sputum tests, or bacterial cultures.

“People living with HIV are at higher risk of developing pulmonary tuberculosis, but diagnosis in these cases is particularly challenging due to the low sensitivity of conventional tests,” explains George William Kasule, a PhD student at ISGlobal and the University of Barcelona, and the first author of the study. “The variety of samples obtained in this study allowed us to compare the sensitivity and specificity of the Stool Ultra test with a microbiological reference standard consisting of three WHO-recommended tests: TB-LAM in urine, liquid culture, and Xpert Ultra from sputum.”

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