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Lab approaches and strategies

Some points to consider when deciding whether or not to conduct antimicrobial susceptibility testing should include:

  • clinical relevance of the isolate
  • purity of the isolate
  • logical panel of antimicrobial agents to be tested (i.e., do not include antibiotics to which the isolate is known to have intrinsic resistance)
  • availability of test methodology, resources, and trained personnel
  • standardization of testing
  • valid interpretation of results
  • cost efficiency
  • effective means to communicate results and interpretation to end-users

Most often, interpretation is reduced to whether the isolate is classified as susceptible, intermediately susceptible, or resistant to a particular antibiotic..  It should, however, be remembered that these in vitro procedures are only approximations of in vivo conditions which can be very different depending on the nature of the drug, the nature of the host and the conditions surrounding the interaction between the antibiotic and the target pathogen.  One critical aspect is following standardized procedures that can generate reproducible results, i.e. quality control.  Aspects of quality control include:

  • standardized  bacterial inoculum size
  • culture conditions (growth medium, pH, cation concentration
  • blood and serum supplements and thymidine content)
  • incubation conditions (atmosphere, temperature, duration)
  • concentration of antimicrobials for testing. 


Because of the required culture time, antimicrobial susceptibility testing may take several days, which is not ideal particularly in critical clinical cases demanding urgency.   Often, practitioners may utilize locally established antibiograms as guideline for therapy.   An antibiogram is a compiled susceptibility report or table of commonly isolated organisms in a particular hospital, farm, or geographic area, which can serve as a useful guideline in therapy before actual culture and susceptibility data becomes available for reference.



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Organizations Publishing



United States: Clinical and Laboratory Standards institute (CLSI; formerly NCCLS)


European Union: European Committee of Antimicrobial Susceptibility Testing (EUCAST)


European Union: Office International des Èpizooties (OIE)


United Kingdom: British Society for Antimicrobial Chemotherapy


Germany: Deutsches Institut für Normung  


France: Société Française de Microbiologie


Sweden: Swedish Reference Group for antibiotics


Australia: CDS disk diffusion method