Note: This is a brief description of some important concepts in the use of susceptibility data to guide therapy. This information is in no way intended to supersede clinical judgment, information provided in package inserts or textbooks, nor to replace a thorough review of the many fine books and information sources on antimicrobial use.

The efficacy of a particular antimicrobial drug in treating an infection depends on a variety of factors including those associated with the host, the drug, and the bacterium. The two factors that we will consider here are those associated with the drug and the "bug." Each antimicrobial drug has characteristics that should be considered as part of the drug selection process. These include: toxicity, mode of action (bacteriostatic vs. bacteriocidal), tissue distribution, plasma half-life, achievable plasma concentrations, spectrum, and cost among others. Much of this information is available on the package insert for the drug or a PDR.

The bacteria involved in the infectious process have individual characteristics of sensitivity/resistance to certain drugs, rapidity with which the bacteria acquire resistance to various classes of drugs, and site of persistence and growth (e.g. intracellular vs extracellular). The MSU VDL provides information regarding the susceptibility/resistance of individual bacterial isolates to various drugs and drug classes by providing either quantitative (MIC) and/or qualitative (S, I, R) antimicrobial susceptibility results.

Simply defined, the MIC is the lowest concentration of an antimicrobial agent that will prevent visible growth of the organism in an agar or broth dilution susceptibility test.

Another important variable in using MIC data is the "Breakpoint." The breakpoint is the MIC or zone diameter value use to categorize an organism S,I, or R. The VDL uses (when available) the breakpoints set by the Clinical Laboratory Standards Institute (CLSI). For some drugs these values have been established in various animal species but in many cases, the data must be extrapolated from human testing. These breakpoints are available free of charge.

We are frequently asked if the way to use MIC data to choose a drug is to use the drug with the "lowest number." The answer is an unequivocal NO! ! For more guidance on this topic, we strongly encourage clinicians to use the CLSI document VET09 – “Understanding Susceptibility Test Data as a Component of Antimicrobial Stewardship in Veterinary Settings”.