Evaluation of automated BACTEC MGIT 960 system for testing susceptibility of Mycobacterium tuberculosis to four major antituberculous drugs: comparison with the radiometric BACTEC 460TB method and the agar plate method of proportion.

We evaluated the performance of BACTEC MGIT 960 for automated testing of the susceptibility of 133 strains of Mycobacterium tuberculosis to streptomycin, isoniazid, rifampin, and ethambutol. The BACTEC MGIT 960 results were compared with those obtained with the radiometric BACTEC 460TB system, and when there was disagreement, the method of proportion on agar plates was used as a reference method. Strains resistant to the critical concentration of streptomycin, isoniazid, or ethambutol were also tested with a second, higher concentration. The overall agreement between the two systems was 96.7%, and the 18 discrepancies were resolved in favor of BACTEC 460TB in 11 cases and in favor of BACTEC MGIT 960 in 7, a difference which was not statistically significant. Apart from the assay's low specificity for ethambutol, which was low for the radiometric assay as well, good sensitivity and specificity values characterized BACTEC MGIT 960. The average time required for completion of the test was 2.5 days shorter with BACTEC 460TB. In conclusion, BACTEC MGIT 960 appears to be a suitable replacement for the radiometric method of antimicrobial susceptibility testing of M. tuberculosis. The problem of frequent contamination of BACTEC MGIT 960 tests needs to be quickly resolved; in fact, 14 strains had to be reprocessed because of contamination.