Evaluation of BACTEC micro MGIT system for isolation of Mycobacteria in a tertiary care setting

Background: Tuberculosis (TB) is a significant infectious disease in many parts of the world, which is of great concern. Prompt detection, isolation, identification and susceptibility testing of Mycobacterium tuberculosis from clinical specimens is essential for appropriate management of patients with tuberculosis. This study aims to compare the sensitivity of BACTEC MGIT method in detection of Mycobacterium tuberculosis among various clinical samples and to detect the drug resistance pattern to 1st line drugs among Mycobacterium tuberculosis isolates. Materials and Methods: A three year cross sectional study was done among 300 patients in the department of Microbiology, KIMS, Bangalore. Study group included cases with clinical or radiological evidence suggestive of tuberculosis. Specimens were subjected to direct microscopy by Ziehl-Neelsen staining and fluorescent staining. Culture was done by semi automated BACTEC MGIT system. Rapid antigen detection by SD TBAg MPT64 kit was performed to confirm the positive isolates. Isolates confirmed as Mycobacterium tuberculosis were subjected to drug susceptibility testing using MGIT method for 1st line drugs ie Streptomycin, Isoniazid, Rifampicin and Ethambutol. Results: The sensitivity of direct microscopy was 12.33% by Ziehl-Neelsen staining and 14.33% by fluorescent staining .Overall culture positivity was 18.6% (56). 55 isolates were Mycobacterium tuberculosis and one was nontuberculous mycobacterium. The mean detection time was 20.75 days by MGIT method. A sensitivity of 96.3% for streptomycin, 89% for isoniazid, 98.1% for rifampicin and 96.3% for ethambutol was noticed. Discussion and Conclusion: This study highlights the importance of culturing the suspected tuberculosis cases prior to empirical therapy. Newer automated culture methods aids in earlier detection of cases and drug susceptibility testing of isolates and helps in selection of appropriate treatment for tuberculosis.