Antimicrobial susceptibility testing: Part 3 – Selection of antimicrobials

Ðurđa Slavić

Animal Health Laboratory, University of Guelph, Guelph, ON.

AHL Newsletter 2022;26(2):8.

When performing antimicrobial susceptibility testing (AST), all diagnostic labs in North America are expected to follow the standards and guidelines established by Clinical Laboratory Standard Institute (CLSI).  The goal of the institute is to promote accurate and reproducible AST results, as well as appropriate reporting and interpretation by standardizing all aspects of AST.  A subcommittee on veterinary antimicrobial susceptibility testing (VAST) establishes guidelines for veterinary laboratories that include: testing methods, bacterial density, media and drug types, the drug dilutions, incubation times, QC requirements, and - most importantly - interpretative criteria. Previously, AHL published information about antimicrobial susceptibility test methods and selection of bacterial organisms for testing.  In this third article of this series, selection of antimicrobials is discussed.  The full document is available on AHL website as LabNote 64:

Selection of antimicrobials:

  1. Ideally, only one representative of the drug category which has similar efficacy as the other drugs in the same group needs to be tested.  Table 1 indicates which drugs are typically tested on behalf of the entire group, and which drugs have to be tested individually.
  2. Only non-proprietary or generic drug names are used.
  3. Preferably, drugs should be approved for use in specific animal species and for specific disease processes.

Table 1. Drugs on AST panel that predict susceptibility to other antimicrobial agents

Drugs Predicts susceptibility to


Amoxicillin-clavulanate and amoxicillin-sulbactam (only if ampicillin is S*)

Cephalexin 1st generation cephalosporins
Cephalothin 1st generation cephalosporins
Clindamycin Lincomycin
Gentamicin All aminoglycosides, except streptomycin
Tetracycline Doxycycline, minocycline, and oxytetracycline (only if tetracycline is S*)
Trimethoprim-sulfamethoxazole Trimethopri-sulfadiazine
Sulfisoxazole Sulfonamides
Erythromycin Azithromycin and clarithromycin (only if erythromycin is S*)

Reprinted with permission from Clinical and Laboratory Standards Institute from: CLSI. Understanding Susceptibility Test Data as a Component of Antimicrobial Stewardship in Veterinary Settings. 1st ed. CLSI report VET09. Wayne, PA: Clinical and Laboratory Standards Institute; 2019.

*S=susceptible. If results for these drugs are R (resistant), then susceptibility to the other specified drugs cannot be predicted and they have to be tested individually.   AHL