Growth detection failures by the nonradiometric BactecMGIT 960 mycobacterial culture system

Mycobacterial growth in liquid culture can go undetected by automated, nonradiometric growth detection systems. In our labo-ratory, instrument-negative tubes from the Bactec MGIT 960 system are inspected visually for clumps suggestive of mycobacte-rial growth, which (if present) are examined by acid-fast smear analysis. A 3-year review demonstrated that1 % of instrument-negative MGIT cultures containedmycobacterial growth and that 10 % of all cultures yielding mycobacteria were instrument negative. Isolates from instrument-negative MGIT cultures included both tuberculous and nontuberculous mycobacteria. Mycobacteria remain an important cause of infectious dis-eases in developing and developed countries alike. Although amplified molecular methods are making inroads as diagnostic tools, culture-based methods are still heavily relied upon. Owing to the challenges of cultivating mycobacteria, use of at least two culture media (one liquid, one solid) is recommended to maxi-mize recovery of clinical isolates (4, 10). Until recently, liquid broth mycobacterial culture involved a radiometric growth detectionmethod.Many clinical laboratories, including our own, have adopted nonradiometric detection methods, which offer several quality and safety advantages (1, 5, 8