[Evaluation of Xpert MTB/RIF results for the detection of Mycobacterium tuberculosis in clinical samples]

Abstract

Tuberculosis is still a major global health problem. Nowadays nucleic acid amplification tests which are recommended by the World Health Organization (WHO) become popular methods for the rapid detection of Mycobacterium tuberculosis complex (MTC). Recently introduced commercial Xpert MTB/RIF (Cepheid, USA) system is also a molecular method based on real-time polymerase chain reaction for simultaneous detection of both MTC and rifampicin resistance in the clinical sample. The sample processing, nucleic acid extraction, amplification and detection of known mutations related to rifampicin resistance are performed in a single cartridge in this integrated system and the results are obtained in two hours. The aim of this study was to evaluate the performance of Xpert MTB/RIF system for the detection of M.tuberculosis in pre-processed clinical samples by comparing the results obtained by Bactec 460TB 12B (BD Diagnostic, USA), Löwenstein-Jensen (LJ) culture and direct microscopy of smears stained with Ziehl- Neelsen (ZN). A total of 85 clinical specimens (50 sputum, 25 bronchoalveolar lavage, five thorasynthesis fluid and five urine samples) obtained from tuberculosis-suspected patients were included to the study. All specimens were decontaminated and this decontaminated suspension was used in the diagnostic methods, except for Xpert MTB/RIF process. Twenty-five (29%) of the samples yielded positive result with Bactec 460TB, 25 (29%) were found positive with Xpert MTB/RIF, 15 (18%) were found positive with LJ and 11 (13%) were found positive with ZN staining method. High consistency was detected between the results of Bactec 460TB and Xpert MTB/RIF when Bactec 460TB was considered as the gold standard method (r= 0.943; p= 0.000). One specimen yielded false positive result with Xpert MTB/RIF when compared to the reference method. The sensitivity, specificity, positive and negative predictive values of Xpert MTB/RIF test were then estimated as 96%, 98%, 96% and 98%, respectively. No resistance were detected for the tested isolates. This study suggested that the sensitivity of Xpert MTB/RIF system in direct detection of M.tuberculosis in smear positive and smear negative samples was consistent with the reference methods. Moreover, the MTB/RIF test provided sensitive detection of tuberculosis in less than two hours.