Contemporary Assessment of Antimicrobial Susceptibility Testing Methods for Polymyxin B and Colistin: Review of Available Interpretative Criteria and Quality Control Guidelines

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Abstract
The emergence of infections caused by multidrug-resistantPseudomonas 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) andAcinetobacter sp. (MIC90, 2 μg/ml). In contrast, they showed limited activity against some other nonfermentative bacilli such as Burkholderia cepacia(MIC90, ≥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 log2dilution 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.

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