Comparison of a veterinary breakpoint minimal inhibitory concentration system and a standardized disk agar diffusion procedure for antimicrobic susceptibility testing

Veterinary Diagnostic and Investigational Laboratory, College of Veterinary Medicine, University of Georgia, Tifton 31794.

The correlation between the antimicrobic standard disk agar diffusion procedure and a veterinary antimicrobic breakpoint minimal inhibitory concentration system was evaluated. Bacterial isolates representing 5 different genera were tested against 15 antimicrobics. There were 3,795 tests performed; 77.3% of the test results were in agreement, 22.6% were in disagreement. Thirty-one percent of the conflicting results were due to the organism being intermediate on the standard disk agar diffusion procedure. Results suggest that the breakpoint system needs more than 1 antimicrobic dilution per antimicrobic, or a change in some of the single dilutions in the breakpoint system would eliminate some of these discrepancies. Another possibility would be to utilize more than 1 panel design, i.e., 1 for gram-positive organisms, 1 for gram-negative organisms, and 1 for fastidious organisms.