Antimicrobial Susceptibility Testing (AST)
How does NDSU VDL determine what to report?
Relevant antimicrobials are: (1) labeled/appropriate for use against a specific pathogen in a particular animal species; (2) have clinical breakpoints established by the Clinical and Laboratory Standards Institute (CLSI). The commercial test must also have the desired antimicrobial included on the plate with the right drug concentrations to determine a clinical breakpoint. Some platforms have not been updated according to CLSI guidelines to test appropriate minimum inhibitory concentration (MIC) values. In this case, even if the antimicrobial is on a test panel, a laboratory is unable to determine an interpretation because the test does not test a low/high enough MIC value. Of the antimicrobials that meet these criteria, priority is given to antimicrobials that are easily accessible, are first-generation agents than promote appropriate prescribing practices, and those that are appropriate for treatment in a particular body site.
Conversely, some drugs are on a testing panel for epidemiological purposes and are used by laboratories to monitor resistance trends and development. Others are critical to human health, so should be conserved as a treatment of last resort. In these instances, the VDL doesn’t routinely report these antimicrobial results.
Additionally, some organisms are inherently (intrinsically) resistant to certain antibiotics, so laboratories may choose to not report these drugs. As an example, Klebsiella pneumoniae is intrinsically resistant to ampicillin so this antimicrobial may not be tested/reported.
What is a breakpoint?
This is the minimal inhibitory concentration of an antimicrobial used to categorize an organism as susceptible (S), susceptible-dose dependent (SDD), intermediate (I), or resistant (R). These interpretive criteria are established based on microbiological characteristics, pharmacokinetic/pharmacodynamic (PK/PD) parameters and/or clinical outcome data.
What does Susceptible-Dose Dependent (SDD) mean?
SDD indicates that susceptibility of an organism depends on the dosage regimen used in the patient. To achieve levels of antimicrobial that are likely to be effective against a pathogen, it is necessary to use a dosage regimen that results in higher drug exposures than that achieved with the dose that was used to establish the susceptible breakpoint. The drug label should be consulted for recommended doses and adjustment for organ function, if possible. CLSI Standard Vet01S, Appendix E also has information regarding dosage regimens used to establish SDD breakpoints.
Why do some pathogens have AST performed, and others do not?
This is a complicated answer. Pathogens with unpredictable susceptibility patterns should always have AST performed. However, all testing platforms have limitations and may not have the capability of testing all relevant antimicrobials.
There are certain pathogens that have reliably predictable treatment options. Certain Streptococci (S. canis, S. equi, etc) are routinely susceptible to penicillin and other beta-lactams, which are key first-line treatment options. In these cases, it rarely necessitates testing additional antimicrobial treatment options.
Inference Rules
Some antimicrobials may predict the efficacy of a related drug or class. Common surrogates are listed in the table below:
Antimicrobial Agent | Predicts Susceptibility to which other agents? |
Penicillin | Ampicillin, Amoxicillin |
Ampicillin |
Amoxicillin |
Susceptibility predicts susceptibility to Amoxicillin-clavulanate and ampicillin-sulbactam. | |
Oxacillin (or cefoxitin-depending on Staphylococcus species) | Susceptibility predicts susceptibility to: amoxicillin-clavulanate, ampicillin-sulbactam, piperacillin-tazobactam, oral and parenteral cephems, carbapenems (ertapenem, imipenem, meropenem). |
Resistance predicts resistance to all available beta lactam agents, except ceftaroline. | |
Minocycline | Resistance predicts resistance to doxycycline and tetracycline |
Tetracycline | Susceptibility predicts susceptibility to minocycline and doxycycline, except in horses. |
Where can I find more information?
Clinical Laboratory Standards Institute: www.clsi.org/all-free-resources and select CLSI VET01S
- Common intrinsic resistance patterns: Appendix B
- Dosage regimens used to establish SDD breakpoints: Appendix E
- Antimicrobials that could be considered for testing by laboratories: Table 1
- Breakpoints for veterinary pathogens: Tables 2A-2J.
International Society for Companion Animal Infectious Diseases: www.iscaid.org/guidelines
- Antimicrobial guidelines for urinary, respiratory and dermatology infections
- Stewardship resources
- Infection control practices
We are passionate and actively involved in the CLSI standards development process and AST best practices nationally. We are deeply invested in the outcomes of your patients and public health. Please let us know how we can help you with your AST questions or needs by calling us at 701-231-8307.
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Updated 10-2024