Inter- and Intralaboratory Variability in Antibiotic Susceptibility Tests with Pseudomonas aeruginosa and Enterobacteriaceae

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Abstract
A 2-phase collaborative study was designed for definition of the extent to which results of disk diffusion tests can be reproduced. Two currently recommended techniques, the Kirby-Bauer method and the agar overlay method, were used. Of special concern were tests of P. aeruginosa with moderate susceptibility to carbenicillin; gentamicin, tobramycin, polymyxin B and colistin were also tested with P. aeruginosa and with Klebsiella pneumoniae, Escherichia coli and Proteus spp. The agar overlay method tended to give zones that were somewhat smaller than those given by the Kirby-Bauer method. Both disk methods demonstrated an excellent degree of reproducibility, being at least as reproducible as the standard agar dilution technique for measurement of minimal inhibitory concentrations. Slight changes in carbenicillin disk potency affected the zones of inhibition around P. aeruginosa more than those around susceptible E. coli. A standard strain of P. aeruginosa way be advantageous for quality control of carbenicillin disk tests.

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