Xpert MTB/RIF assay for diagnosis of pulmonary tuberculosis in sputum specimens in remote health care facility

Background: Xpert MTB/RIF assay is considered as a great advance over conventional smear and culture in the diagnosis of TB and MDR-TB by simultaneously detecting M. tuberculosis and rifampicin resistance bacilli. However, very little information regarding the performance characteristics of Xpert MTB/RIF assay is available in Ethiopia. Therefore, the purpose of this study was to evaluate the performance of Xpert MTB/RIF assay compared to conventional sputum smear and culture methods for the diagnosis of pulmonary tuberculosis in remote health care facility. Methods: A paired expectorated sputum samples were obtained from 227 consecutively recruited patients with signs and symptoms suggestive of tuberculosis at Karamara hospital during December 2013 to May 2014. One of the sputum specimen was tested directly by Ziehl-Neelsen staining and Xpert MTB/RIF assay without NALC-NaOH decontamination. The other of pair of sputa specimen was cultured for isolation of TB bacilli by conventional methods. Diagnostic performance of Xpert MTB/RIF assay and AFB smear microscopy were calculated against culture as the gold standard. Results: Overall 25.5 % (58/227) samples were positive for Mycobacterium tuberculosis complex (MTBC) by MGIT and/or LJ media of which 36.2 % (21/58) and 65.5 % (35/58) were positive by AFB smear microscopy and Xpert MTB/RIF respectively. The sensitivity, specificity, as well as the positive and negative predictive value of Xpert MTB/ RIF assay were 65.5 % (95 % CI: 53.3-77.7 %), 96.3 % (95 % CI: 93.4-99.2 %), 86.4 % (95 % CI: 76.2-96.5 %), and 88.6 % (95 % CI: 83.9-93.3 %) respectively. Eighteen of 58 (31 %) cases that were smear microscopy negative, were positive by Xpert MTB/RIF assay. Conclusions: Although Xpert MTB/RIF assay demonstrated high sensitivity in detecting MTBC in sputum specimens compared with conventional AFB smear microscopy, it demonstrated suboptimal sensitivity in smear negative patients compared to conventional culture.