Susceptibility testing methods, resistance and breakpoints: what do these terms really mean?

The Clinical and Laboratory Standards Institute and the European Committee on Antimicrobial Susceptibility Testing can be considered the major international contributors to antimicrobial susceptibility testing. In this review, the author considers the differences between the respective organisations, examines the terminology used in antimicrobial susceptibility testing and argues for an urgent need to harmonise these definitions. While this may seem somewhat surprising, the terminology used to define resistance does differ. In this context, attention is given to the trend for 'resistance' to be defined by the epidemiological cut-off value, rather than by the long-established clinical breakpoint. The author goes on to discuss susceptibility testing methodologies and present an approach to setting clinical breakpoints.