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Journal of Clinical Microbiology, May 2006, p. 1921, Vol. 44, No. 5
0095-1137/06/$08.00+0     doi:10.1128/JCM.44.5.1921.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

LETTER TO THE EDITOR

Verification of Antimicrobial Susceptibility Testing of Mycobacterium tuberculosis


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LETTER
 
We recently completed verification studies comparing the MGIT 960 to the BACTEC 460 for antimicrobial susceptibility testing (AST) of Mycobacterium tuberculosis complex. A review of the literature revealed that most authors tested an overall large number of drug-susceptible isolates but only a small and arbitrary number of resistant isolates. The median numbers of resistant isolates of each drug used in published evaluations including streptomycin, isoniazid, rifampin, ethambutol, and pyrazinamide were 14 (range, 2 to 26), 25 (range, 17 to 77), 13 (range, 0 to 23), 12 (range, 4 to 15), and 11 (range, 6 to 13), respectively.

We were unable to find definitive recommendations (how many resistant strains should be tested) in CLSI (Clinical and Laboratory Standards Institute [formerly NCCLS]) guidelines, FDA recommendation and requirement documents, Clinical Laboratory Improvement Amendment regulations, local and national accrediting agency checklists, or published articles. In 1993, Jorgenson (2) recommended that ≥35 isolates be used for antibacterial susceptibility testing and that no more than 1 in 33 isolates should repeatedly test false susceptible (very major error rate of ≤3.0%); Elder et al. (1) endorsed the recommendation. A single study (3) used approximately 35 isolates resistant to each drug to verify the MGIT 960. Our own verification used at least 35 isolates resistant to each drug (except for ethambutol [18]) and showed a very major error rate of 1.8% and a 98.7% correlation between methods. It is our feeling that the use of 35 resistant isolates to test each antimicrobial is both optimal and achievable in the clinical laboratory.

We endorse the MGIT 960 for AST and specifically advocate that ≥35 isolates resistant to each drug be used for the verification of AST for mycobacteriology. Using this number would allow for the detection of a low but significant very major error rate. It would be difficult for a clinical laboratory to acquire and test for a larger number of isolates resistant to each drug. We encourage CLSI to endorse the recommendation to ensure that adequate numbers of resistant isolates be used to verify AST assays for M. tuberculosis and other mycobacteria and to endorse an acceptable very major error rate of ≤3.0% to ensure that test performance provides clinically valid information to clinicians. We also advocate that those agencies that accredit clinical laboratories and recommend that manufacturer's data or published reports can be used as verification data allow this only if an adequate number of organisms was used in the documentation. It is our hope that the information presented here will allow laboratories to better and more accurately verify AST comparisons for M. tuberculosis and other mycobacteria as newer methods are developed.


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REFERENCES
 
    1
  1. Elder, B., J. Hansen, J. Kellogg, R. Zabransky, and F. Marsik. 1997. Verification and validation of procedures in the clinical microbiology laboratory. Cumitech 31. ASM Press, Washington, D.C.
    2. 2
  2. Jorgensen, J. H. 1993. Selection criteria for an antimicrobial susceptibility testing system. J. Clin. Microbiol. 31:2841-2844.[Free Full Text]
    3. 3
  3. Scarparo, C., P. Ricordi, G. Ruggiero, and P. Piccoli. 2004. Evaluation of the fully automated BACTEC MGIT 960 system for testing susceptibility of Mycobacterium tuberculosis to pyrazinamide, streptomycin, isoniazid, rifampin, and ethambutol and comparison with the radiometric BACTEC 460TB method. J. Clin. Microbiol. 42:1109-1114.[Abstract/Free Full Text]
Leslie Hall
Kelly A. Doerr
W. Scott Harmsen
Nancy L. Wengenack
Glenn D. Roberts*
Mayo Clinic
200 First St. S.W.
Rochester, MN 55905

* Phone: (507) 284-3704Fax: (507) 284-4272E-mail: Roberts.glenn{at}mayo.edu

Journal of Clinical Microbiology, May 2006, p. 1921, Vol. 44, No. 5
0095-1137/06/$08.00+0     doi:10.1128/JCM.44.5.1921.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.




This Article
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