ANTIMICROBIAL SUSCEPTIBILITY TESTING IN SWEDEN - II - SPECIES-RELATEDZONE DIAMETER BREAKPOINTS TO AVOID INTERPRETIVE ERRORS AND GUARD AGAINST UNRECOGNIZED EVOLUTION OF RESISTANCE

The Swedish Reference Group for Antibiotics appointed a subcommittee o n methodology (SRGA-M) in 1987 to investigate ways of defining interpr etive breakpoints for antimicrobial susceptibility testing. The minimu m inhibitory concentration (MIC) breakpoints for susceptibility catego ries are mainly based on pharmacological properties of the antibiotic, and they are, with few exceptions, valid for all species. However, fo r several species the MIC breakpoints have failed to distinguish strai ns with reduced susceptibility from normal susceptible strains. Disk d iffusion is the routine method for susceptibility testing in Sweden. S tudies of distribution of MICs and zone diameters for clinically impor tant bacterial species have resulted in an emphasis on resistance rath er than on susceptibility. The SRGA-M chose to place the zone diameter breakpoints close to the native (often susceptible) population of eac h species or group of related species. Such species-related zone diame ter breakpoints used for susceptibility categories no longer correspon d to the pharmacological MIC breakpoints, but divide each species into the fully susceptible (native) population and into those isolates/pop ulations that have acquired a resistance mechanism, resulting in high- or low-grade resistance. By this method the risk of reports of false s usceptibility is minimized and early detection of the emergence of ant ibiotic resistance is ensured.