Comparison of the Xpert MTB/RIF and cobas TaqMan MTB assays for detection of mycobacterium tuberculosis in respiratory specimens

The Xpert MTB/RIF assay (Cepheid, Sunnyvale, CA) is a fully automated, cartridge-based real-time PCR assay designed to detect Mycobacterium tuberculosis and rifampin resistance within 2 h. The performance of the Xpert assay has been evaluated in various clinical settings. However, there are few data comparing the Xpert assay to the Cobas TaqMan MTB test (Roche Diagnostics, Basel, Switzerland), one of the most widely utilized molecular assays for M. tuberculosis detection. In this prospective study, 320 consecutive respiratory specimens were processed simultaneously using acid-fast bacillus (AFB) staining, mycobacterial cultures with both solid and liquid media, and the Cobas and Xpert assays. The Xpert assay was performed with direct respiratory specimens, while the Cobas assay was done with decontaminated concentrated specimens. Based on the culture asa reference method, the overall sensitivities of the Cobas and Xpert assays were 71.4% and 67.9%, respectively. When AFB smear results were taken into consideration, the sensitivities of the Cobas assay for smear-positive and -negative specimens were 87% and 54%, while those of the Xpert assay were 67% and 69%, respectively. The Cobas assay showed 100% specificity and 100% positive predictive value (PPV) regardless of smear results, while the Xpert assay showed 100% specificity and 100% PPV for smear-positive specimens but 98% specificity and 60% PPV for smear-negative specimens. In conclusion, the Xpert assay showed performance that was slightly inferior to that of the Cobas assay but seems useful for the rapid detection of M. tuberculosis, considering that it was performed without laborious and time-consuming decontamination and concentration procedures.