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are applied to the surface of the plate, with the lowest concentration towards the center. The antimicrobial drug diffuses into the medium, which results in an elliptical zone of growth inhibition around the strip. The MIC is read at the point of intersection of the ellipse with the MIC scale on the strip. Although the strips are expensive, Etests have the advantage of being adaptable to use with fastidious organisms and anaerobes if susceptibility testing of these organisms is deemed necessary. Disk diffusion involves application of commercially available drug-impregnated  lter paper disks to the surface of
an agar plate that has been inoculated to con uence with the organism of interest, and is also known as Kirby-Bauer antibiotic testing. Commercially available, mechanical disk-dispensing devices can be used to apply several disks simultaneously to the surface of the agar. The drug diffuses radially through the agar, the concentration of the drug decreasing logarithmically
as the distance from the disk increases. This results
in a circular zone of growth inhibition around the disk, the diameter of which is inversely proportional to the MIC. The zone diameters are interpreted on the basis of guidelines published by CLSI and the organisms are reported as susceptible, intermediate or resistant.
Once susceptibility testing has been performed, organisms are classi ed on the susceptibility panel
report as “susceptible” (S), “resistant” (R), and, in some cases, of “intermediate” (I) susceptibility. This refers
to a predicted in vivo situation, rather than in vitro susceptibility. The growth of “susceptible” isolates should be inhibited by concentrations of antimicrobial agent that are usually achievable in blood and tissues using normal dosage regimens. “Intermediate” isolates have MICs that approach usually attainable blood and tissue levels and for which response rates may be lower than those for susceptible isolates. This category implies clinical ef cacy in body sites where the drugs are normally concentrated (e.g., enro oxacin and amoxicillin in urine) or when a higher-than-normal dose of a drug can be used, and acts as a buffer zone in order to prevent technical factors from causing major discrepancies in interpretations. With the exception of UTIs, ‘intermediate’ should be interpreted as ‘resistant’. “Resistant” isolates should continue to grow
in the face of the usually achievable concentrations of the drug in blood and tissues.
In order to determine if an in vivo response is likely, the laboratory refers to breakpoints, or clinical cut-off MICs (or, for disk diffusion testing, cut-off zone diameters), which are established, published, and revised regularly by committees associated with standards agencies such as the CLSI. If the MIC determined in the microbiology laboratory is lower than the published breakpoint, then the organism is de ned as susceptible. The breakpoint is
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not reported to the clinician. Breakpoints are established on the basis of multiple factors, which include 1)
a knowledge of MIC distributions and resistance mechanisms for each organism-drug combination, 2) clinical response rates in humans and animal models,
3) how the drug is distributed and metabolized in the body (pharmacokinetics), and 4) whether the drug
is concentration-dependent or time-dependent as it relates to antibacterial effect (pharmacodynamics).
Zone diameter breakpoints for disk diffusion testing are determined by correlation with MIC values. For simplicity, breakpoints are established for bloodstream infections, and are based on a speci c dosage regime for the antimicrobial drug tested, which are selected by the standards agency involved. Because some antimicrobials are concentrated extensively in urine, some veterinary laboratories may report urine MIC panels, which provide breakpoints for lower urinary tract infections, which are higher than corresponding serum MIC breakpoints. These have been controversial because the possibility
of concurrent pyelonephritis cannot always be ruled out. Breakpoints are often re-evaluated when new mechanisms of resistance appear in bacteria or when new data are generated that improve understanding of the pharmacokinetics and pharmacodynamics of an antimicrobial drug.
The veterinary clinician should always remember that the list of drugs reported in the susceptibility panel is simply just a list of drugs tested. They are not suggestions
from the laboratory for patient care. The clinician should always ask the following questions, when faced with a susceptibility panel result:
1. Is this organism that was cultured likely to be the cause of disease? (i.e., should I treat this organism?)
2. Are any of the drugs shown as “susceptible” the appropriate drugs for treatment of the bacterial species cultured?
3. Assuming the drugs are active against the bacterial species isolated, are the drugs the right drugs for the patient in question?
4. Will they achieve adequate concentrations at the site of infection?
5. What route of administration is necessary and can the antimicrobials be administered by the route that is most appropriate for my patient?
6. Could adverse drug reactions occur in this patient with these antimicrobials?
7. Could drug interactions occur in this patient with these antimicrobials?
8. Is the antimicrobial drug currently being administered the most appropriate for the infection I am trying to treat?

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