Inability of the standardized disk agar-diffusion test to measure susceptibility of the fluorescent group of pseudomonads to gentamicin

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.