Calibration of disk diffusion antibiotic susceptibility testing: Species-related trovafloxacin interpretive zone breakpoints and selection of disk potency

International comparisons of antibiotic susceptibility require the use of c ommon minimum inhibitory concentration (MIC) Limits. Disk diffusion test re sults are not directly suitable for such comparisons, since different stand ards are often used and zone breakpoints issued might reflect different MIC limits. We have used single strain regression analysis (SRA) for the calib ration of the disk test, both according to species and individual laborator y, and for quality control of trovafloxacin disk diffusion tests in 5 labor atories in Sweden. Preliminary controls using histogram analysis including subtraction histograms of reference strains revealed marked differences bet ween different laboratories, SRA was performed on 4 reference strains, S. a ureus, E. faecalis, E. coli and P. aeruginosa, using disks containing 1, 3, 10, 30 and 100 mu g trovafloxacin. The results using SRA showed a differen ce between laboratories using Biodisk PDM medium, which produced smaller zo nes, and those using Oxoid IsoSensitest, Species-related regression lines f or laboratories using either medium were calculated and corresponding inter pretive zone breakpoints determined for MIC limits, Rational criteria for t he selection of a suitable disk content of an antibiotic were also defined and applied to trovafloxacin. The 10 pg disk selected by NCCLS (National Co mmittee for Clinical Laboratory Standards) proved optimal.