C-201. Evaluation of the Use of Mitchison 7H11 Agar Plates in Conjunction with the BACTEC MGIT 960 Broth System for Recovery of Mycobacteria in a Public Health Laboratory

J. L. Tans-Kersten, D. M. Warshauer;
Wisconsin State Lab of Hygiene, Madison, WI.

Background: Current guidelines recommend the use of both a liquid and a solid medium for the isolation of mycobacteria from clinical specimens. We conducted a four-month trial using Mitchison 7H11 selective agar plates (7H11) in conjunction with the BACTEC MGIT 960 automated broth system (MGIT) to determine the impact of using 7H11 on the recovery rate of Mycobacterium species in clinical specimens, time to detection (TTD), and contamination rates. We also looked at the cost-effectiveness of using 7H11. Methods: 594 specimens were processed using the standard N-acetyl-L-cysteine (NALC)-NaOH decontamination method, concentrated by centrifugation, and each inoculated into a MGIT broth tube and a 7H11 plate. Additionally, 188 specimens from sterile and/or surgical sites were concentrated (without decontamination) and inoculated into the two media types. Results: A total of 114 Mycobacterium isolates were detected from 782 clinical specimens, including 32 M. tuberculosis complex (MTBC) isolates and 82 non-tuberculous mycobacteria. The overall recovery rate for Mycobacterium species was higher in the MGIT (95.6%, 109/114) than the 7H11 plates (47.3%, 54/114). Recovery rates for MTBC were 100% (32/32) for the MGIT and 56% (18/32) for 7H11 plates. While there were 60 Mycobacterium species (including 14 MTBC) isolated in the MGIT only, there were only 4 Mycobacterium isolates (3 M. avium complex and 1 M. gordonae) that were isolated solely on the 7H11 plates. The average TTD was shorter for the MGIT (18 days) than the 7H11 plates (26 days); isolates detected on both media types were never detected first on the 7H11 plates. Moreover, the average TTD for MTBC was 18 days for the MGIT and 24 days for the 7H11 plates. Contamination rates for the MGIT were higher for the four month period (9.0%, 71/782) than for the 7H11 media (2%, 16/782). Conclusion: Although contamination rates were lower, 7H11 did not increase the recovery rate of mycobacteria from clinical specimens nor decrease TDD when used in combination with the MGIT system. Our lab would save $1,532 in a four month period or approximately $4500 per year by omitting 7H11 plates from our routine protocol.