Evaluation of the fully automated BACTEC MGIT 960 system for testing susceptibility of Mycobacterium tuberculosis to pyrazinamide, streptomycin, isoniazid, rifampin, and ethambutol and comparison with the radiometric BACTEC 460TB method

The performance of the fully automated BACTEC MGIT 960 (M960) system for the testing of Mycobacterium tuberculosis susceptibility to streptomycin (SM), isoniazid (INH), rifampin (RMP), ethambutol (EMB), and pyrazinamide (PZA) was evaluated with 100 clinical isolates and compared to that of the radiometric BACTEC 460TB (B460) system. The agar proportion method and the B460 system were used as reference methods to resolve the discordant results for SM, INH, RMP, and EMB (a combination known as SIRE) and PZA, respectively. The overall agreements were 96.3% for SIRE and 92% for PZA. For SIRE, a total of 26 discrepancies were found and were resolved in favor of the M960 system in 8 cases and in favor of the B460 system in 18 cases. The M960 system produced 8 very major errors (VME) and 10 major errors (ME), while the B460 system showed 4 VME and 4 ME. No statistically significant differences were found. Both systems exhibited excellent performance, but a higher number of VME was observed with the M960 system at the critical concentrations of EMB and SM. For PZA, a total of eight discrepancies were observed and were resolved in favor of the M960 system in one case and in favor of the B460 system in seven cases; no statistically significant differences were found. The M960 system showed four VME and three ME. The mean times to report overall PZA results and resistant results were 8.2 and 9.8 days, respectively, for the M960 system and 7.4 and 8.1 days, respectively, for the B460 system. Statistically significant differences were found. The mean times to report SIRE results were 8.3 days for the M960 system and 8.2 days for the B460 system. No statistically significant differences were found. Twelve strains tested for SIRE susceptibility and seven strains tested for PZA susceptibility had been reprocessed because of contamination. In conclusion, the M960 system can represent a valid alternative to the B460 for M. tuberculosis susceptibility testing; however, the frequent contamination of the tests needs to be improved.