Susceptibility testing is indicated for any organism that contributes to an infectious process warranting antimicrobial chemotherapy. If the susceptibility of a bacterial pathogen to antimicrobial agents cannot be predicted based on the identity of the organism alone, in vitro antimicrobial susceptibility testing of the organism isolated from the disease processes in animals is indicated. A variety of laboratory techniques can be used to measure the in vitro susceptibility of bacteria to antimicrobial agents. Clinical and Laboratory Standards Institute document VET06— Methods for Antimicrobial Susceptibility Testing of Infrequently Isolated or Fastidious Bacteria Isolated From Animals describes the standard disk diffusion method, as well as standard broth dilution (macrodilution and microdilution) and agar dilution techniques for infrequently isolated or fastidious bacteria from animals. It also includes procedures designed to standardize test performance. The performance, applications, and limitations of the current CLSI-recommended methods are described. The tabular information in this document presents test conditions, QC recommendations, agents to consider for primary testing, and breakpoints. In an increasing number of compounds for which veterinary-specific breakpoints are not available, human breakpoints are used. As more veterinary-specific information becomes available, these changes will be incorporated into future revisions of this document.

CLSI documents M02,5 M07,6 VET01,1 and M1007 describe standardized methods and breakpoints for antimicrobial susceptibility testing (AST) of common aerobic bacteria, including some fastidious organisms. However, there are a number of less frequently encountered or fastidious veterinary bacteria that are not covered in those CLSI documents. Some are organisms that may cause serious infections in companion and livestock animals. This document provides recommendations to microbiology laboratories for how and when to determine the susceptibility of these diverse organisms. VET06 also includes some fastidious or unusual organisms potentially associated with bioterrorism.

This document provides veterinary diagnostic laboratories with currently recommended antimicrobial agent disk and dilution susceptibility test methods for bacteria isolated from animals, criteria for QC testing, and breakpoints. The breakpoints are intended only to support therapeutic label claims for animal antimicrobial agent use and do not apply to label claims for disease prevention or performance enhancement. Additionally, the document provides a brief overview of the various antimicrobial classes and mechanisms of resistance to them, including specific tests for antimicrobial resistance.

This document does not cover commercial susceptibility testing devices.