Establishing MIC breakpoints and the interpretation of in vitro susceptibility tests

The purpose of undertaking susceptibility testing, by whatever method, is to attempt to integrate the susceptibility of a population of potential pathogens with the pharmacokinetics of the antimicrobial and, whenever possible, to review this relationship in the light of clinical experience following therapy in clinical trials. Breakpoints are discriminatory antimicrobial concentrations used in the interpretation of results of susceptibility testing to define isolates as susceptible, intermediate or resistant. Clinical, pharmacological and microbiological considerations are important in setting breakpoints, and the ideal mix of these factors is unknown. Different countries have different approaches to this problem but, by and large, these approaches have much in common. This paper attempts to summarize the philosophy of the British Society for Antimicrobial Chemotherapy (BSAC) Working Party in its approach to setting breakpoints and to update the activities of the Working Party since it initially published breakpoints, approximately 10 years ago. The formula outlined by the BSAC Working Party in 1991 has been used to set the breakpoints presented here. The Working Party accepts that in the light of new knowledge, there is a need to reassess how breakpoints are defined, and this paper also summarizes the future activities of the Working Party.