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Test Catalog

Test ID: ZMMLS    
Antimicrobial Susceptibility, Aerobic Bacteria, MIC

Useful For Suggests clinical disorders or settings where the test may be helpful

Determining the in vitro susceptibility of aerobic bacteria involved in human infections

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Results are reported as minimum inhibitory concentration (MIC) in mcg/mL and as susceptible, susceptible dose dependent, intermediate, or resistant according to the Clinical and Laboratory Standards Institute (CLSI) guidelines.

Interpretation Provides information to assist in interpretation of the test results

A “susceptible“ category result and a low minimum inhibitory concentration value indicate in vitro susceptibility of the organism to the antimicrobial tested.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

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.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

Cockerill FR: Conventional and genetic laboratory tests used to guide antimicrobial therapy. Mayo Clin Proc 1998;73:1007-1021

Special Instructions and Forms Describes specimen collection and preparation information, test algorithms, and other information pertinent to test. Also includes pertinent information and consent forms to be used when requesting a particular test