Reevaluation of Clinical and Laboratory Standards Institute disk diffusion breakpoints for tetracyclines for testing Enterobacteriaceae

Abstract

We reevaluated Enterobacteriaceae disk diffusion breakpoints for the tetracyclines published in the Clinical and Laboratory Standards Institute (CLSI) document M100-S16, which were (susceptible/resistant) >or=19 mm/<or=14 mm for tetracycline, >or=16 mm/<or=12 mm for doxycycline, and >or=19 mm/<or=14 mm for minocycline. A collection of 504 recent clinical isolates of Enterobacteriaceae were tested against these tetracycline compounds by disk diffusion and broth microdilution methods according to CLSI guidelines. Regression line and scattergram plot analyses determined intermethod accuracy for current disk diffusion breakpoints and showed excellent r values of 0.91 to 0.95. However, error rates (minor/major [false-resistant]/very major [false-susceptible]) were 14.9/0.8/0.0% for tetracycline, 11.5/0.4/0.0% for doxycycline, and 30.6/0.7/0.0% for minocycline and only 4.4/0.0/0.0% for tetracycline, 5.6/0.0/0.2% for doxycycline, and 8.3/0.0/0.3% for minocycline when proposed breakpoints were modified to (susceptible/resistant) >or=15 mm/<or=11 mm for tetracycline, >or=14 mm/<or=10 mm for doxycycline, and >or=16 mm/<or=12 mm for minocycline. Listed modifications were recently approved by the CLSI (M100-S17).

FIG. 1.

Correlation between MIC results and disk diffusion inhibition zone diameters for minocycline. Solid lines indicate year 1982 to 2006 CLSI breakpoints (9), while broken lines show the proposed (CLSI, 2007) disk diffusion breakpoints (10).