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Detecting antimicrobial resistance

Historically, veterinary practitioners prescribed antibiotics based on expected mode of action, spectrum of activity and clinical experience.  With the emergence and spread of antimicrobial resistance, treatment of bacterial infections has become increasingly difficult and is no longer as straightforward as it was many years prior.  Practitioners now need to consider that the particular pathogen they wish to treat may be resistant to some or all of the available antibiotics, thus making antimicrobial susceptibility testing a standard procedure.

Antimicrobial susceptibility testing methods are in vitro procedures used to detect antimicrobial resistance in individual bacterial isolates.  Because these laboratory detection methods can determine resistance or susceptibility of an isolate against an array of possible therapeutic candidates, antimicrobial susceptibility testing  results can be a useful clinical guideline in selecting the best antibiotic treatment option for each particular patient.  These same methods can also be used for monitoring the emergence and spread of resistant microorganisms in the population. 

Clinical Breakpoints are threshold values established for each pathogen-antibiotic (i.e., bug-drug) combination indicating at what level of antibiotic the isolate should be considered to be sensitive, intermediate or resistant. The interpretative criteria for these are based on extensive studies that correlate laboratory resistance data with serum achievable levels for each antimicrobial agent and a history of successful and unsuccessful therapeutic outcomes.  Although veterinary laboratories originally based interpretations on standards established using human pathogens, it became apparent by the early 1980s that such an approach did not reliably predict clinical outcomes when applied to veterinary practrice.  Subsequently, groups within organizations that set standards were created for the purpose of developing veterinary-specific standards28.

Standard conditions for these assays have been established based on extensive batteries of laboratory testing.   Guidelines and recommendations for these are continuously updated by certain organizations worldwide, such as CLSI, EUCAST, OIE, BSAC, SFM, SRGA and CDS (see box, right).  Of these, those which specify antimicrobial testing methods and interpretative criteria for veterinary pathogens are: the CLSI in the USA, OIE in EU and CDS-AST in Australia26, 28



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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