EVALUATION OF MYCOBACTERIA GROWTH INDICATOR TUBES FOR SUSCEPTIBILITY TESTING OF MYCOBACTERIUM-TUBERCULOSIS TO ISONIAZID AND RIFAMPIN

The reliability of mycobacteria growth indicator tubes (MGIT) for dete rmining the susceptibility of Mycobacterium tuberculosis to isoniazid (0.1 mu g/ml) and rifampin (2.0 mu g/ml) was evaluated by comparing MG IT results to those obtained by the radiametric BACTEC TB method. We t ested 29 isolates, many selected for resistance. The turnaround times were 3-8 days (median 6) for MGIT and 4-10 days (median 6) for BACTEC. Isoniazid results by both methods agreed for all isolates tested: 20 resistant and nine susceptible. Rifampin results agreed for 28 isolate s: 10 resistant and 18 susceptible. One isolate yielded discrepant res ults: resistant to rifampin by BACTEC, bur susceptible by Men. This is olate was also rifampin-resistant by the traditional Method of Proport ion and when tested by the MGIT method using 1.0 mu g/ml of rifampin. The MGIT system is a nonradiometric alternative to the BACTEC for rapi d susceptibility testing of M. tuberculosis to isoniazid and rifampin; however, additional testing is needed to determine the optimal concen tration of rifampin.