Evaluation of mycobacteria growth indicator tubes for susceptibility testing of Mycobacterium tuberculosis to isoniazid and rifampin

The reliability of mycobacteria growth indicator tubes (M git) for determining the susceptibility of Mycobacterium tuberculosis to isoniazid (0.1 μg/ml) and rifampin (2.0 μg/ml) was evaluated by comparing M git results to those obtained by the radiometric B actec TB method. We tested 29 isolates, many selected for resistance. The turnaround times were 3–8 days (median 6) for M git and 4–10 days (median 6) for B actec. Isoniazid results by both methods agreed for all isolates tested: 20 resistant and nine susceptible. Rifampin results agreed for 28 isolates: 10 resistant and 18 susceptible. One isolate yielded discrepant results: resistant to rifampin by B actec, but susceptible by M git. This isolate was also rifampin-resistant by the traditional Method of Proportion and when tested by the M git method using 1.0 μg/ml of rifampin. The M git system is a nonradiometric alternative to the B actec for rapid susceptibility testing of M. tuberculosis to isoniazid and rifampin; however, additional testing is needed to determine the optimal concentration of rifampin.