Diagnostic accuracy of the BD MAXTM MDR-TB assay on sputum and tongue swabs for Mycobacterium tuberculosis complex detection in adults under investigation for TB in South Africa

BackgroundDespite advances in molecular diagnostics, only 48% of newly diagnosed tuberculosis (TB) cases were confirmed using nucleic acid amplification tests (NAATs) in 2023. The BD MAXTM MDR-TB (MAX MDR-TB) assay, a moderate complexity NAAT, detects Mycobacterium tuberculosis complex (MTBC) and resistance to rifampicin (RIF) and isoniazid (INH), but data on clinical performance is limited. This study assessed assay performance on raw sputum, NALC/NaOH decontaminated sputum, and tongue swab (TS) specimens. MethodsThis evaluation assessed the MAX MDR-TB assay for MTBC detection and RIF and INH resistance profiling on sputum, using liquid culture as the reference standard. Additionally, diagnostic accuracy for MTBC detection in TS specimens was evaluated under different transport and processing conditions. ResultsAssay sensitivity was similar for sputum pellet (87%) and raw sputum (89%), with one additional case detected using raw sputum. Two false RIF-resistant results were observed. INH resistance was missed in two cases. Although specimen numbers were small, TS demonstrated better diagnostic accuracy when using a diluted (66%) STR buffer. A total of 15/55 (27%) were classified as "MTB Low POS." ConclusionThese findings suggest that MAX MDR-TB assay performance is comparable between sputum pellet and raw sputum. While TS showed promise, further validation in larger studies is warranted. The high rate of "MTB Low POS" results across specimen types underscores the importance of assay optimisation to reduce the burden of repeat testing and improve diagnostic reliability. Future research should enhance sensitivity and integration into diagnostic algorithms to improve patient outcomes.