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Appendix 1. Interpretation of antimicrobial susceptibility test results Phenotypic determination of antimicrobial susceptibility of a bacterial isolate is important to ensure appropriate therapy for infections in animals and humans and to produce monitoring data on the occurrence of acquired resistance among bacteria in different reservoirs. Semi- quantitative methods for determining the MIC of an antimicrobial agent for a given bacterial isolate are currently the gold standard for antimicrobial susceptibility testing. A MIC is defined as the lowest drug concentration that visibly inhibits bacterial growth. In routine antimicrobial susceptibility testing, MICs are usually determined by serial two-fold drug dilutions, thus implying that the actual MIC of an isolate is only approximated. Indeed, the true MIC of an isolate growing at 1 mgL, but not at 2 mgL thus, the recorded MIC is 2 mgL, lies somewhere between 1 mgL and 1.99 mgL. MIC values are interpreted using defined criteria to categorize bacterial isolates as susceptible or resistant, which is essential both for guiding appropriate clinical treatment and for comparing results from different monitoring programmes over time. However, interpretive criteria may differ among laboratories and countries, and also based on the purpose of the MIC determination. For example, MIC breakpoints appropriate for predicting clinical efficacy might differ from those used for surveillance purposes. An isolate might acquire reduced susceptibility to a given antimicrobial but still have a sufficiently low MIC to allow successful therapy. It is therefore important to differentiate between interpretative criteria used for clinical purposes clinical breakpoints and those used for monitoring epidemiological cut-off values [ECOFFs], as illustrated in the Figure A1.1 below. Fig.A1.1 MIC distribution for a hypothetical organism-antimicrobial combination. S, susceptible; I, intermediate; R, resistant 6 8