Setting Breakpoints

Breakpoints for phenotypic antimicrobial susceptibility testing have been determined by breakpoint committees and as part of regulatory processes for the approval of new drugs since the 1970s. Knowledge of dosage, modes of administration, target infections and clinical outcome when wild type organisms of defined species are treated, pharmacokinetics and pharmacodynamics of the agent, MIC distributions and ECOFFs (epidemiological cut-off values), resistance mechanisms, zone diameter distributions, are all used in the breakpoint-setting process. There is no mathematical algorithm in which all of these can be fitted to obtain a breakpoint. Instead it is a scientific process where the importance and influence of the various components are weighed against each other.