title: Inability of the standardized disk agar-diffusion test to measure susceptibility of the fluorescent group of pseudomonads to gentamicin.
abstract: Four hundred thirteen clinical isolates of fluorescent pseudomonads consisting of 286 Pseudomonas aeruginosa and 127 organisms of the Pseudomonas fluorescent group were tested for susceptibilities to gentamicin by the broth-dilution method and by the standardized disk agar-diffusion test, using Mueller-Hinton agar from three sources. Different regression lines and significantly different mean inhibition zone diameters were found for the three Mueller-Hinton agars. Single zone-diameter breakpoints of either 13 or 16 mm for the standardized disk agar-diffusion test produced unacceptable rates of false-sensitive and false-resistant interpretations. When error rates were limited by using zone-diameter breakpoints determined by the method of error rate-bound analysis, the percentage of indeterminate interpretations became so large as to make the standardized disk agar-diffusion test impractical. It is concluded that the standardized disk agar-diffusion test should not be used for testing susceptibility of the fluorescent pseudomonads to gentamicin, and presumably other aminoglycosides.
mesh_terms: D000362:Agar; D005839:Gentamicins / Q000494:pharmacology*; D008826:Microbial Sensitivity Tests / Q000592:standards*; D011549:Pseudomonas / Q000187:drug effects*; D011550:Pseudomonas aeruginosa / Q000187:drug effects; D011551:Pseudomonas fluorescens / Q000187:drug effects
chemical_list: D005839:Gentamicins; D000362:Agar