Diagnostic performance of GeneXpert MTB/RIF in detecting MTB in smear-negative presumptive TB patients

Tuberculosis (TB) is a leading cause of morbidity and mortality worldwide. Control of TB is lingering by the lack of diagnostic tests that are simple, rapid, yet accurate. Thus, smear-negative pulmonary TB often misses the diagnosis. The study evaluated the performance of GeneXpert MTB/RIF assay for the detection of Mycobacterium tuberculosis (MTB).The study was carried out from June to December 2016 in Nepal Tuberculosis Center, Bhaktapur, Nepal. A total of 173 sputum samples were collected and processed by microscopy [Auramine-O staining and Ziehl-Neelsen (ZN) staining], followed by GeneXpert MTB/RIF assay and culture in Lowenstein-Jensen (LJ) medium.Of 173 sputum samples, 162 (93.6%) were smear-negative. Of 162 smear-negative sputum samples, 35 (21.6%) were confirmed to have MTB by culture, and 31 (19.1%) by GeneXpert MTB/RIF assay. Of 31 GeneXpert-positive samples, 25 (80.6%) were susceptible, 4 (12.9%) were resistant, and 2 (6.45%) were intermediate to rifampicin. The sensitivity, specificity, positive predictive value, and negative predictive value of GeneXpert MTB/RIF assay for smear-negative sputum samples were 74.3%, 96.6%, 86.7%, and 92%, respectively. The GeneXpert MTB/RIF has a substantial diagnostic agreement of 90.91% with culture (Cohen's Kappa coefficient = 0.73).The diagnostic performance of GeneXpert MTB/RIF assay was almost on par with culture, and thus can be relied upon for MTB detection in smear-negative sputum samples.