An analysis of error rates for disc agar-diffusion testing of Pseudomonas aeruginosa versus aminoglycosides

Five hundred thirty-five recent clinical isolates of Pseudomonas aeruginosa were tested in parallel by the standardized disc agar-diffusion test and a micro-broth dilution test to evaluate the error rates associated with the use of both fixed indeterminate-zone breakpoints and floating indeterminate-zone breakpoints for assessing susceptibility to amikacin, gentamicin, and tobramycin. Error rate-bound analysis showed that unacceptably high rates of error were associated with the use of all fixed breakpoints. Error rates were improved when the floating indeterminate-zone principle was used, but rates still remained unacceptably high. These findings indicate the disc agar-diffusion test should not be used for testing the susceptibility of P. aeruginosa to the aminoglycosides.