Antimicrobial susceptibility testing in veterinary medicine: performance, interpretation of results, best practices and pitfalls

Abstract The performance of antimicrobial susceptibility testing (AST) of bacteria and the interpretation of AST results for bacteria isolated from animals are complex tasks which must be performed using standard published methodology and overseen by experts in clinical microbiology and in consultation with clinical pharmacologists. Otherwise, AST has significant potential for errors and mistakes. In this review, we provide guidance on how to correctly perform AST of bacteria isolated from animals and interpret the AST results. Particular emphasis is placed on the various approved or published methodologies for the different bacteria as well as the application of interpretive criteria, including clinical breakpoints and epidemiological cut-off values (ECVs/ECOFFs). Application of approved interpretive criteria and definitions of susceptible, susceptible dose-dependent, nonsusceptible, intermediate, and resistant for clinical breakpoints as well as wild-type and non-wildtype for ECVs, are explained and the difficulties resulting from the lack of approved clinical breakpoints for other bacteria, indications, and animal species is discussed. The requirement of quality controls in any AST approach is also emphasized. In addition, important parameters, often used in monitoring and surveillance studies, such as MIC 50 , MIC 90 , and testing range, are explained and criteria for the classification of bacteria as multidrug-resistant, extensively drug-resistant or pandrug-resistant are provided. Common mistakes are presented and the means to avoid them are described. To provide the most accurate AST, one must strictly adhere to approved standards or validated methodologies, like those of the Clinical and Laboratory Standards Institute or other internationally accepted AST documents and the detailed information provided therein.