Susceptibility Testing in Animals - How Breakpoints Are Derived and Interpretation of Susceptibility Data

SUSCEPTIBILITY TESTS Antibacterial susceptibility testing is not necessary for every case in veterinary medicine because the majority of cases can be treated empirically. But performed properly, antibiotic susceptibility results are helpful to select proper antibacterial therapy for patients with refractory infections, seriously ill animals, or when resistant strains of bacteria are suspected. When MIC data is available, the test also is helpful to monitor trends in antibiotic activity. In order for those results to be reliable, appropriate test methods and quality control procedures should be performed by the laboratory. The laboratory should adhere to published standards (CLSI 2007). Factors such as depth of agar, size of inoculum, and time of incubation can affect the test result if these are not performed according to a standard. The standards are available from the Clinical and Laboratory Standards Institute – CLSI (http://www.clsi.org/) (formerly NCCLS – the National Committee for Clinical Laboratory Standards). Not all laboratories in the United States use CLSI standards. It is a voluntary program. However, if a laboratory does not adhere to a public standard such as CLSI, breakpoints may vary and interpretation may be inconsistent from laboratory to laboratory, or among different regions of the country.