Assessing the Utility of GeneXpert MTB/Rif Assay in a Tertiary Care Centre in Southern India with Established Microscopy and Liquid Culture Facilities.

Background: To evaluate the diagnostic accuracy of the Xpert MTB/RIF assay for the detection of M. tuberculosis in pulmonary and extra pulmonary specimens and to compare it with conventional techniques.

Methods: A prospective study was conducted with the introduction of GeneXpert in a tertiary care hospital which relied on microscopy and culture for diagnosis of tuberculosis. All patients for whom geneXpert was ordered by the physician were included in the study. Samples which did not have accompanying microscopy or MGIT culture requests were excluded from the analysis of the results. Sensitivity and specificity of GeneXpert was calculated using liquid culture as the reference test.

Results: Xpert MTB/RIF assay was performed on 742 samples of which 116 were positive for Mycobacterium tuberculosis. Rifampicin resistance was seen in 6 samples. The pulmonary samples showed a positivity rate of 16.8% while 17.1% of the extrapulmonary samples were positive by GeneXpert. A comparative analysis of microscopy, liquid culture and GeneXpert could be done for 88 samples. Of the 88 geneXpert positive samples, 42 were positive by smear microscopy and MGIT culture while 46 showed discordant results. Of these, 18 samples were positive by geneXpert but showed no growth in MGIT culture. 15 of these patients had undergone anti-tuberculous treatment (ATT) within the past 12 months. The sensitivity of geneXpert was 89.7% and specificity was 95.1% when compared to liquid culture as a gold standard. Sensitivity for extrapulmonary samples was 85.7% and specificity was 98.05%.

Conclusion: To conclude, though GeneXpert detects tuberculosis within the shortest possible time, it still suffers from intermediate level sensitivity, which makes culture facilities relevant even in settings that offer an Xpert/Rif assay.