Contemporary assessment of antimicrobial susceptibility testing methods for polymyxin B and colistin: review of available interpretative criteria and quality control guidelines

Abstract

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 (MIC(90), 2 microg/ml) and Acinetobacter sp. (MIC(90), 2 microg/ml). In contrast, they showed limited activity against some other nonfermentative bacilli such as Burkholderia cepacia (MIC(90), >/=128 microg/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 log(2) dilution between the methods used for both compounds. At a resistance breakpoint of >/=4 microg/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.

FIG. 1

Scattergram results for colistin comparing broth microdilution MICs with the reference agar dilution method against 35 recent clinical isolates selected to possess a wide range of MIC values. The diagonal line represents complete agreement, and the numbers represent the occurrences observed at each point. The broken lines represent ±1-log₂ MIC agreement limits between test results. Horizontal and vertical broken lines indicate the resistant MIC breakpoints (≥4 μg/ml).

FIG. 2

Scattergram comparing broth microdilution MICs and 10-μg disk zone diameters for colistin tested against 200 contemporary clinical isolates. The solid lines represent the breakpoint values contained in reference , while the broken lines show the proposed new breakpoint values for colistin.

FIG. 3

Scattergram comparing broth microdilution MICs and 300-U disk zone diameters for polymyxin B tested against 200 contemporary clinical isolates. The solid lines represent the breakpoint values contained in reference , while the broken lines show the proposed new breakpoint values for colistin (see Fig. 2).