Diagnostic accuracy of the Cobas MTB and Cobas MTB/RIF-INH assays on sputum and the Cobas MTB assay on tongue swabs for Mycobacterium tuberculosis complex detection in symptomatic adults in South Africa

Publication date: Jun 30, 2025

Accurate and rapid detection of Mycobacterium tuberculosis complex (MTBC) and drug resistance is essential for effective tuberculosis (TB) management, particularly in high-burden settings. The Cobas MTB and Cobas MTB/RIF-INH assays are moderate-complexity nucleic acid amplification tests that detect MTBC and resistance to rifampicin (RIF) and isoniazid (INH). This study evaluated clinical diagnostic performance of the Cobas assays on sputum, using liquid culture as the reference standard and Xpert MTB/RIF Ultra (Xpert Ultra) for comparison. Diagnostic accuracy of the Cobas MTB assay on tongue swabs (TS) was also assessed. In a study population (n=354) with 56% HIV prevalence, the overall sensitivity and specificity of Cobas MTB on sputum was 93.8% (95% CI: 84.8-98.3) and 100% (95% CI: 98.7-100) compared with culture. The assay showed almost perfect agreement with Xpert Ultra ({kappa} = 0.904). Among HIV-positive participants, sensitivity was 88.2% (95% CI: 97.8-100). RIF resistance profiling by Cobas MTB/RIF-INH was fully concordant with culture and Xpert Ultra. Three INH-resistant cases were missed, likely due to genotypic-phenotypic discordance. Although specimen numbers were small, TS demonstrated better diagnostic accuracy when using a diluted (66%) Microbial Inactivation Solution. The Cobas MTB and MTB/RIF-INH assays demonstrated high diagnostic accuracy compared to culture and Xpert Ultra on sputum. Findings support TS as an alternative specimen type for MTBC detection using an optimized protocol. These findings underscore the potential of the Cobas assays as reliable alternatives for TB and resistance diagnostics, particularly in settings where rapid, accurate detection of MTBC and RIF or INH resistance is crucial.

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