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Antimicrobial susceptibility testing (AST) determines the minimum inhibitory concentration (MIC) (of a series of increasing concentrations) of antimicrobial incorporated in agar plates, which inhibits the growth of bacteria inoculated on the surface of the agar.
Prior studies have determined a “breakpoint“ or MIC value for each antimicrobial, above which the bacterium being tested is considered resistant to that agent. The most important factor contributing to the determination of the “breakpoint“ is probably the expected serum concentration of antimicrobial achieved after giving the usual dosage. The “category“ result (“susceptible“ or “resistant“) provided along with the MIC is determined by comparing the MIC result with the “breakpoint.“
AST should be performed on pure culture isolates of pathogenic (or potentially pathogenic in special situations) bacteria grown from specimens that have been appropriately collected so as not to confuse clinically significant isolates with normal flora.
Determining the in vitro susceptibility of aerobic bacteria involved in human infections
A “susceptible“ category result and a low minimum inhibitory concentration value indicate in vitro susceptibility of the organism to the antimicrobial tested.
In vitro susceptibility does not guarantee clinical response. Therefore, the decision to treat with a particular agent should not be based solely on the antimicrobial susceptibility testing result.
Results are reported as minimum inhibitory concentration (MIC) in mcg/mL and as susceptible, susceptible-dose dependent, intermediate, resistant, or nonsusceptible according to the Clinical and Laboratory Standards Institute (CLSI) guidelines.
The "susceptible" category implies that isolates are inhibited by the usually achievable concentrations of antimicrobial agent when the dosage recommended to treat the site of infection is used.
Susceptible-Dose Dependent (D)
The "susceptible-dose dependent" category implies that susceptibility of an isolate is dependent on the dosing regimen that is used in the patient. In order to achieve levels that are likely to be clinically effective against isolates for which the susceptibility testing results are in the D category, it is necessary to use a dosing regimen (ie, higher doses, more frequent doses, or both) that results in higher drug exposure than the dose that was used to establish the susceptible breakpoint. Consideration should be given to the maximum approved dosage regimen, because higher exposure gives the highest probability of adequate coverage of a D isolate. The drug label should be consulted for recommended doses and adjustment for organ function. The D category may be assigned when doses well above those used to calculate the susceptible breakpoint are approved and used clinically.
The "intermediate" category includes isolates with antimicrobial agent minimum inhibitory concentrations (MIC) that approach usually attainable blood and tissue levels, and for which response rates may be lower than for susceptible isolates. The intermediate category implies clinical efficacy in body sites where the drugs are physiologically concentrated or when a higher than normal dosage of a drug can be used. This category also includes a buffer zone, which should prevent small, uncontrolled, technical factors from causing major discrepancies in interpretations, especially for drugs with narrow pharmacotoxicity margins.
The "resistant" category implies that the isolates are not inhibited by the usually achievable concentrations of the agent with normal dosage schedules and/or that demonstrate MICs that fall in the range where specific microbial resistance mechanisms are likely, and clinical efficacy of the agent against the isolate has not been reliably shown in treatment studies.
The "nonsusceptible" category is used for isolates for which only a susceptible interpretive criterion has been designated because of the absence or rare occurrence of resistant strains. Isolates for which the antimicrobial agent MICs are above the value indicated for the susceptible breakpoint is reported as nonsusceptible.
Note: An isolate that is interpreted as nonsusceptible does not necessarily mean that the isolate has a resistance mechanism. It is possible that isolates with MICs above the susceptible breakpoint that lack resistance mechanisms may be encountered within the wild-type distribution subsequent to the time the susceptible-only breakpoint was set.
(CLSI. Performance Standards for Antimicrobial Susceptibility Testing; Twenty-Fifth Informational Supplement. CLSI document M100-S25. Wayne, PA, Clinical and Laboratory Standards Institute, 2015)
1. Jorgensen JH, Ferraro MJ: Antimicrobial susceptibility testing; A review of general principles and contemporary practices. Clin Infect Dis 2009 Dec 1;49(11):1749-1755
2. CLSI. Performance Standards for Antimicrobial Susceptibility Testing; 25th Informational Supplement. CLSI document M100-S25. Wayne, PA, Clinical and Laboratory Standards Institute, 2015