Modified protocol for drug susceptibility testing of MGIT cultures of Mycobacterium tuberculosis by the MGIT 960

•Agreement with the manufacturer's methodology was >90% for TB first line drugs.•Subculturing of M. tuberculosis isolates in MGIT as recommended can be excluded.•Modified protocol is more rapid and has lower cost than manufacturer's methodology.

A rapid detection of resistance in Mycobacterium tuberculosis is crucial for management and control of tuberculosis. This study evaluated a more rapid and cost-effective drug susceptibility testing (DST) protocol using primary isolates of M. tuberculosis in mycobacteria growth indicator tube (MGIT). Ninety-four M. tuberculosis isolates in MGIT were subjected to DST by the manufacturer's method, i.e., primary isolates were subcultured and DST was performed from positive cultures for a maximum of 5 days; and by our modified method, i.e., DST was performed directly from primary MGIT cultures positive for more than 5 days. Results were concordant for 76 (81%) isolates. Agreement between both methods was 92.0%, 98.9%, 97.7%, and 95.5% for streptomycin, isoniazid, rifampicin, and ethambutol, respectively. Six isolates failed to grow on the recommended method, including 3 resistant isolates. Not performing subculture of primary M. tuberculosis isolates yields reliable results, decreasing the turnaround time and the cost of the test.