Diagnostic utility of GeneXpert MTB/RIF assay versus conventional methods for diagnosis of pulmonary and extra-pulmonary tuberculosis

Background Tuberculosis (T.B) is one of the major infectious diseases in the developing countries. The diagnosis of extrapulmonary T.B (EPTB) remains problematic and emergence of resistant strains poses a significant threat. Improved diagnosis of tuberculosis is a global priority for proper control. The study aimed to assess the diagnostic accuracy of GeneXpert MTB/RIF assay for diagnosis of pulmonary TB (PTB) and EPTB and to evaluate the performance of GeneXpert system for demonstrating rifampicin resistance among the studied patients. Methods A total of 582 clinical samples (449 pulmonary; 430 sputum and 19 bronchoalveolar lavage (BAL) and 133 extra-pulmonary origins; 26 pleural fluid, 62 CSF, 19 ascetic fluid, 12 pus and 14 urine) were collected from patients under clinical and radiological assessment of either PTB or EPTB who were admitted to Menoufia Chest Hospital over a period of three years. Clinical samples were processed and investigated for detection of Mycobacterium tuberculosis (MTB) by both Xpert assay and the conventional methods including Ziehl-Neelsen (ZN)/acid-fast bacillus (AFB) smear microscopy and Lowenstein-Jensen (LJ) culture. Patients ' demographic, clinical characteristics and risk factors for acquiring rifampicin resistance were analyzed. Results The sensitivity, specificity, false- negative rate and total accuracy of AFB smear microscopy respectively were 72.1 %, 81.3 %, 27.9 and 78.8 % for PTB. However for EPTB, they were 63.2 %, 70.5 %, 36.8 and 68.4 % respectively in relation to LJ culture as the gold standard. GeneXpert MTB/RIF revealed better performance for PTB than EPTB. For PTB, it showed 90.2 % sensitivity, 86.9 % specificity, and 9.8 % false- negative rate. For EPTB, the assay showed a sensitivity of 81.6 %, specificity of 78.9 % and false- negative rate of 18.5 %. Multivariate regression analysis showed that presence of EPTB and contacts with known TB cases were independent risk factors for developing rifampicin resistance. Conclusions GeneXpert MTB/RIF assay is a rapid and highly sensitive technique for diagnosis of PTB or EPTB. Its simplicity and accuracy make this new method a very impressive tool for diagnosis of MTB and rifampicin resistance.