Minimum inhibitory concentration

The lowest concentration of an antimicrobial agent that prevents visible microbial growth under specified laboratory conditions. It is measured using standardized dilution tests and does not necessarily indicate microbial killing.

Broth microdilution: A standardized susceptibility test that exposes microorganisms to serial antimicrobial dilutions in liquid growth medium. This is the common laboratory method for finding an MIC across a dilution series.
Clinical breakpoint: A threshold used to classify an organism as susceptible, intermediate, or resistant to an antimicrobial agent. Breakpoints translate an MIC into a clinical susceptibility category.
Minimum inhibitory concentration breakpoints: Drug- and organism-specific MIC thresholds used to categorize antimicrobial susceptibility. These thresholds are applied to measured MICs, but are distinct from the MIC measurement itself.
Disk diffusion test: A susceptibility test that estimates antimicrobial activity from growth-inhibition zones around drug-containing disks on agar. It produces zone diameters rather than a direct MIC measurement.
Agar dilution: A susceptibility test in which antimicrobial concentrations are incorporated into agar plates inoculated with microorganisms. It determines MIC by identifying the lowest drug-containing agar that prevents visible growth.
Antibiogram: A summary of antimicrobial susceptibility results for bacterial isolates from a defined population or period. Aggregated MIC-derived results help guide empirical therapy and track resistance locally.
Serial dilution: A stepwise procedure that produces a sequence of progressively lower concentrations from a starting solution. MIC testing uses serial antimicrobial dilutions to locate the lowest growth-preventing concentration.
Etest: A gradient diffusion method that estimates an antimicrobial MIC from an inhibition ellipse on agar. It yields an MIC estimate through a gradient strip instead of broth dilution wells.
Inoculum effect: A change in antimicrobial susceptibility measurements associated with the number of microorganisms tested. Larger test inocula can raise apparent MICs, making inoculum standardization essential.
Therapeutic drug monitoring: Measurement of drug concentrations in biological samples to optimize dosing and reduce toxicity. Measured exposure can be compared with MIC when tailoring doses for selected antimicrobials.

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