Evaluation of the bactec MGIT 960 TB system for recovery and identification of Mycobacterium tuberculosis   complex in a high volume tertiary care centre

Aim: To evaluate the performance of an automated BACTEC MGIT 960, a non-radioactive, non-invasive liquid culture system for cultivation of M. tuberculosis complex in terms of recovery rate and time.
Materials and Methods: From March 2005 to December 2007, 14,597 specimens were processed using the MGIT 960 system and the results were compared with conventional L.J medium. We standardised r-nitro benzoic acid (PNBA) assay on MGIT 960 TB system for identification of M. tuberculosis complex and evaluated its usefulness by comparing the results with an in-house molecular assay and sequencing.
Results and Discussion: Of the total 6143 (42%) isolates positive for M. tuberculosis complex, 6015 (41%) were positive by MGIT 960 TB system. In contrast, 3526 (24%) M. tuberculosis complex isolates grew on the conventional L.J medium. The mean turn around time for mycobacterial growth in smear-positive specimens was nine days for MGIT 960, and 38 days for L.J. medium whereas in smear negative specimens it was 16 days by MGIT vs. 48 days by L.J.
Conclusion: MGIT 960 system with PNBA assay for identification of M. tuberculosis complex is a rapid and useful method in laboratories processing a large number of specimens.