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Journal of Clinical Microbiology, January 2001, p. 183-190, Vol. 39, No. 1
Department of Pathology, University of Iowa
College of Medicine, Iowa City, Iowa 52242,1 and
Disciplina de Doenças Infecciosas e Parasitàrias,
Universidade Federal de São Paulo, São Paulo,
Brazil2
Received 29 June 2000/Returned for modification 4 September
2000/Accepted 16 October 2000
The emergence of infections caused by multidrug-resistant
Pseudomonas aeruginosa and Acinetobacter spp.
has necessitated the search for alternative parenteral agents such as
the polymyxins. The National Committee for Clinical Laboratory
Standards (NCCLS) documents do not currently provide interpretative
criteria for the testing of the polymyxins, colistin and polymyxin B. Therefore, an evaluation of the antimicrobial activity of colistin and
polymyxin B was initiated using 200 bloodstream infection pathogens
collected through the SENTRY Antimicrobial Surveillance Program. All
susceptibility tests were performed according to the NCCLS
recommendations. Polymyxin B and colistin displayed a nearly identical
spectrum of activity, exhibiting excellent potency against P. aeruginosa (MIC90, 2 µg/ml) and
Acinetobacter sp. (MIC90, 2 µg/ml). In
contrast, they showed limited activity against some other
nonfermentative bacilli such as Burkholderia cepacia
(MIC90,
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.1.183-190.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Contemporary Assessment of Antimicrobial
Susceptibility Testing Methods for Polymyxin B and Colistin: Review of
Available Interpretative Criteria and Quality Control
Guidelines
128 µg/ml). Excellent correlation was achieved
between broth microdilution and agar dilution tests (r = 0.96 to 0.98); 94.3% of the results were ±1 log2
dilution between the methods used for both compounds. At a resistance
breakpoint of
4 µg/ml for both agents, unacceptable
false-susceptible or very major errors were noted for colistin (5%)
and polymyxin B (6%). Modified zone criteria for colistin (
11 and
14 mm) and polymyxin B (
10 and
14 mm) were suggested, but some
degree of error persisted (
3.5%). It is recommended that all
susceptible disk diffusion results be confirmed by MIC tests using the
preferred reference NCCLS method. The quality control (QC) ranges
listed in the product package insert require an adjusted range by
approximately 3 mm for both NCCLS gram-negative quality control
strains. This evaluation of in vitro susceptibility test methods for
the polymyxin class drugs confirmed continued serious testing error
with the disk diffusion method, the possible need for breakpoint
adjustments, and the recalculation of disk diffusion QC ranges.
Clinical laboratories should exclusively use MIC methods to assist the
therapeutic application of colistin or polymyxin B until disk diffusion
test modifications are sanctioned and published by the NCCLS.
*
Corresponding author. Present address: 345 Beaver Kreek
Centre, Suite A, North Liberty, IA 52317. Phone: (319) 665-3370. Fax: (319) 665-3371.
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