Evaluating Diagnostic Utility of geneXpert Ultra (Mycobacterium tuberculosis/Rifampicin), Microscopy and Liquid Culture to Isolate Mycobacterium tuberculosis and Nontuberculous Mycobacteria among Pulmonary Tuberculosis Suspects

Abstract

CONTEXT: Nontuberculous mycobacteria (NTM) are posing major public health problems and have recently gained importance as they are being isolated from patients with pulmonary disorders. Most of the time, NTM are misinterpreted as tuberculosis (TB) and this misdiagnosis leads to treatment failure. Hence, this study aimed to identify NTMs isolated from suspected cases of pulmonary TB cases and was aimed to distinguish Mycobacterium TB (MTB) from NTM infections. AIMS: Our study aimed to compare microscopy grading of respiratory samples with the Ct values of GeneXpert Ultra (MTB/rifampicin [RIF]), to assess the diagnostic accuracy of microscopy, GeneXpert ultra (MTB/RIF), liquid culture, to compare GeneXpert ultra (MTB/RIF) results with microscopy with culture and clinical implications, importance of performing a rapid test for pulmonary TB and pulmonary NTM infections in suspected cases of adults. SETTINGS AND DESIGN: The study was conducted in the Mycobacteriology laboratory, Department of Microbiology, in Kasturba Hospital, Manipal. The study was a prospective, cross-sectional study, single center. SUBJECTS AND METHODS: A total of 452 respiratory samples obtained from April to July 2023 were included in the study. Smearpositive pulmonary TB cases were identified and subjected to amplification by GeneXpert ultra (MTB/RIF) and 84 samples were GeneXpert ultra (MTB/RIF) positive and RIF sensitive. Liquid culture by mycobacteria growth indicator tube (MGIT) 960 was performed and for positive tubes, TB Ag MPT64 was performed. The results were compared and analyzed with patients’ demographic information. Clinical and radiological data from the requisition forms were assessed. STATISTICAL ANALYSIS USED: IBM SPSS statistics software version 22 was used. Correlations between GeneXpert ultra (MTB/RIF) Ct values and smear status were calculated by polychoric correlation. Extended McNemar’s test was used to find the association between the variables. RESULTS: GeneXpert ultra (MTB/RIF) yielded a positivity rate of 22.2% compared to smear microscopy 17.2%. Liquid culture yielded a positivity rate of 20.3%. Ct value and smear grading yielded a positive correlation (ρ = 0.8681; P < 0.05). Undetected cases of TB by GeneXpert ultra (MTB/RIF) were identified as positive under liquid culture by MGIT 960. TB Ag MPT64 was performed and yielded NTM and speciated as Mycobacterium abscessus complex by line probe assay. CONCLUSIONS: Our study states the need for identification and differentiation of MTB from NTM.