Clinical and Laboratory Standards Institute VET09—Understanding Susceptibility Test Data as a Component of Antimicrobial Stewardship in Veterinary Settings discusses antimicrobial susceptibility testing (AST), which provides important, clinically relevant information to the submitting veterinarian, provided the veterinarian understands how the testing is performed and how the results can be interpreted. CLSI VET09 includes background information on laboratory processes, including how AST is performed, why AST may not be performed, and how AST results are assessed by the laboratory. It also describes the reasons for varying degrees of confidence in applying breakpoints for interpreting AST results, which will inform veterinarians as they make decisions about the use of antimicrobial agents to treat bacterial disease in animals.

CLSI VET09 reviews some of the factors that affect antimicrobial drug selection in animals, including principles of antimicrobial pharmacology, the effect of bacterial species identification on AST results interpretation, and the veterinarian’s role in ensuring AST results are accurate and useful. The example AST reports include callout boxes, highlighting important facets of AST reports to help the veterinarian apply the data.

CLSI VET09 has separate chapters for animal species–specific guidance on interpreting AST data for dogs, cats, horses, cattle, pigs, fish, and poultry. It also includes a chapter on guidance and limitations on interpreting AST data for animal species for which there are no approved breakpoints, including sheep, goats, donkeys and mules, camelids, and exotic animals. Uniquely, CLSI VET09 provides evidence-based appraisals of confidence in the AST data reported by laboratories. This guidance can also be used by laboratories to select appropriate breakpoints for assigning interpretive categories and to help client veterinarians interpret their data. CLSI VET09 includes information on the most recent veterinary-specific breakpoints published in CLSI VET01S1 Tables 2A to 2M.

CLSI VET09 is designed to facilitate common understanding among veterinarians, microbiologists, and laboratorians of how antimicrobial susceptibility testing (AST) is performed and how the results can be interpreted and applied to clinical decision-making. The intended audience includes veterinarians, microbiologists, laboratorians conducting in vitro testing, veterinary educators, pharmacists, and students.

CLSI VET09 provides information that may be used in conjunction with other efforts and activities to support antimicrobial stewardship, such as those advocated by professional associations3 or regulatory agencies.

CLSI VET09 provides an overview of factors affecting antimicrobial agent selection, including pharmacokinetics (PK) and pharmacokinetics/pharmacodynamics (PK/PD), breakpoint development, effects of disease and patient factors on antimicrobial agent efficacy, characteristics of bacteria relevant to AST, regulatory and legal aspects, and the public health significance of antimicrobial agent selection. Example AST reports are included, in which common questions are answered and misconceptions are clarified. CLSI VET09 also provides animal species-specific guidance on applying AST results; reasonable extrapolations that can be made across bacterial species, infection sites, or antimicrobial agents; and the expected degree of confidence in these extrapolations. Also included are excerpts from other CLSI documents relevant to interpreting AST reports.

Based on available expertise and perceived need, CLSI VET09 covers dogs, cats, horses, cattle, pigs, fish, and poultry. These animal species represent all the veterinary breakpoints that have been approved to date by the CLSI Subcommittee on Veterinary Antimicrobial Susceptibility Testing (VAST). CLSI VET09 also covers interpretation of AST results for animal species for which there are no approved breakpoints, eg, sheep, goats, donkeys and mules, and exotic animal species.

Clinical decision-making about antimicrobial agents is complicated, and AST data account for only part of the information needed to successfully manage bacterial disease in animals. CLSI VET09 does not include guidance on diagnosis or treatment of bacterial disease in animals, prescribing information, or dosing recommendations. Dosage regimens used to set breakpoints are included only to aid in AST results interpretation. CLSI VET09 is not intended to produce standards for antimicrobial agent prescribing, to affect or supersede current regulatory restrictions on antimicrobial agent use, or to endorse any particular product for antimicrobial use. Additional factors veterinarians consider when prescribing antimicrobial agents include disease factors (eg, severity and duration), patient factors (eg, potential for adverse effects or selection for antimicrobial resistance), and legal restrictions on drug use, as well as their own experience. However, these factors are not discussed in depth in CLSI VET09.