Evaluation of the BACTEC MGIT 960 in comparison with BACTEC 460 TB for detection and recovery of mycobacteria from clinical specimens

Abstract

The BACTEC MGIT system (M960), a fully automated, non radiometric instrument, designed for rapid detection of acid fast bacilli (AFB) from clinical specimens, was compared with the radiometric BACTEC 460 system (B460) and Löwenstein-Jensen (L-J) solid medium. A total of 1,093 respiratory and extrapulmonary specimens were decontaminated by the NALC-NaOH standard method, and randomly inoculated into the media. A total of 122 mycobacteria were recovered, including 47 Mycobacterium tuberculosis complex (MTB) isolates and 75 nontuberculous mycobacteria (NTM) isolates. Overall recovery rates were 59% for M960 system ( p < 0.0001), 58.2% for L-J. medium ( p < 0.0001) and 82% for Bactec 460 system, whereas rates for MTB alone were 91.5, 76.6 ( p = 0.007), and 95.7%, respectively. The combination of M960 or B460 systems with L-J medium showed the same recovery rates for MTB strains (97.9%), whereas NTM rates were 68% ( p < 0.0001) and 93.3%, respectively. Mean time to detection of smear-positive MTB, smear-negative MTB, and NTM were 12.2, 13.4, and 23.3 days, respectively, with the M960, 11.7, 21.3, and 24.8 days with the B460 and 20.4, 28.7, and 28.4 days with L-J medium. The M960 system showed a contamination rate of 9.8%, while B460 and L-J medium showed contamination rates of 4.3 and 3.8% respectively. In conclusion, the M960 system appeared to be accurate and rapid for the recovery of MTB, but reduced recovery of NTM and a high number of contaminated cultures deserve further study in order to assess if this system can represent a valuable alternative to the radiometric system.