Improving tuberculosis diagnosis: evaluation of tongue swabbing as a complementary method to sputum analysis in Guinea

Background
Pulmonary tuberculosis (PTB) diagnosis primarily relies on sputum examination, which can be challenging for patients unable to produce sputum. Minimally invasive oral sampling methods, such as tongue swabs (TS), have been proposed as alternatives. We assessed the effectiveness of TS for PTB diagnosis using molecular tests.

Methods
In a prospective study at two TB clinics, 99 confirmed smear-positive PTB patients provided 11 TS and one additional sputum sample (SS) for GeneXpert-Ultra (Ultra) testing at the National Reference Laboratory for Mycobacteriology. Testing was performed on a single TS (1TS), three pooled TS (3TS), and the additional SS. Additionally, we retrospectively analyzed TS from 120 participants with TB symptoms using an in-house IS6110-qPCR at the Institute of Tropical Medicine, alongside two SSs tested by fluorescence microscopy, GeneXpert-MTB/RIF (Xpert) and Ultra in Guinea.

Results
In the prospective study, among 99 smear-positive patients, Ultra detected Mycobacterium tuberculosis (MTB) in 86.9% (1TS) and 91.9% (3TS), compared to 96.9% in SS. The highest positivity grade for sputum was “MTB detected high” (n = 31) with no “Trace” results, while for any TS the maximum grading was “Medium”. TS positivity grades were mostly “MTB Low”, and was not impacted by the sample swab number tested. In the retrospective study, positivity rates for sputum were 35% on microscopy, 44% on Xpert, 38.6% on Ultra, and 38.3% on in-house swab-IS6110-qPCR, with strong agreement between sputum-Xpert/Ultra and swab-IS6110-qPCR (k = 0.91 Xpert, k = 0.86 Ultra).

Results
TS demonstrated its ability to detect MTB on molecular tests, providing a minimally invasive complement for PTB diagnosis. Further studies in sputum scarce patients are needed.