Comparative study of gene Xpert MTB/RIF, smear microscopy and TB MGIT culture in diagnosis of tuberculosis in India

Abstract

Background: India contributes to 25.5% of the global new TB case detection. The Gene Xpert assay is a rapid diagnostic test with sensitivity of 98% in smear positive cases and 70% in smear-negative cases. Limited Indian data is available as this test is fairly recent to our country. Aims and Objectives: To determine sensitivity and specificity of the Gene Xpert system for detection of M. Tuberculosis bacteria and rifampicin(RIF) resistance in both pulmonary and extra pulmonary specimens and to compare it with smear and TB culture (gold standard). Methods: Cross-sectional study enrolled 100 patients of suspected Pulmonary (63) and Extrapulmonary (37) TB cases. Samples were collected in sterile containers and processed for smear, Gene Xpert, TB MGIT 960 culture and DST. Results: Sensitivity and specificity of Xpert was 96.9% and 87% respectively in pulmonary whereas 77.8 and 84.2% respectively in extra pulmonary specimens. Xpert helped diagnose 40% and 58.8% of smear negative pulmonary and extrapulmonary cases respectively. Sensitivity and specificity of Xpert in detecting RIF resistance was 100%. Amongst the cases detected as resistant by Xpert, the DST classified drug resistant cases into MDR (14%), Pre-XDR (18%- all fluoroquinolone resistant) and XDR (2%) cases. Primary drug resistance was seen in 34.9%. Conclusion: Gene Xpert is useful for rapid detection of TB and identification of RIF resistance especially in a high prevelance country like India. The results are superior to smear microscopy and comparable to culture with shorter turn-around time. However test results must always be confirmed by culture and DST due to rising incidence of pre-XDR and XDR cases.