Factors Associated with Missed Detection of Mycobacterium tuberculosis by Automated BACTEC MGIT 960 System

Despite the demonstration of excellent performance, mycobacterial growth in BACTEC MGIT 960 can go undetected. The aim of this study was to investigate the prevalence of "false-negative" culture sample in Beijing and the potential factors associated with the detection failures by MGIT 960. Of the 577 sputum samples tested, 141 (24.4%) were culture-positive for mycobacteria, of which 133 (94.3%) were automatically determined by MGIT 960 system and 8 (5.7%) were positive for visual growth (false negative by MGIT). Statistical analysis showed that positive grade of specimen had no influence on the false-negative rate by MGIT 960 system (χ² = 2.207, P = 0.820). In addition, the mean time to detection (TTD) was 241.4 (range: 224-261) hours for false-negative group and 186.8 (range: 173-199) hours for positive group. The difference in TTD between false-negative and positive groups was statistically significant (P < 0.01). In conclusion, our data demonstrate that the automatic MGIT missed a small portion of bacteriological mycobacterial patients. In addition, the poor growth rate rather than the low grade of AFB smear is associated with the detection failure by MGIT. Our findings highlight the notion that manual inspection for all instrument-negative MGIT tubes will bring about considerable benefit to patients and clinicians.