The minimum inhibitory concentration (MIC)

The MIC is among the standard infectious disease concepts which has thus far been left out of CICM SAQs. One can expect that at some stage the trainees will be called upon to define it, or to use MIC data to decide which antibiotic to use within a given scenario.

Definition

The results are usually reported as µg/mL.

How one arrives at this figure is described in detail in this 2001 article.

A lower MIC indicates that the antibiotic is more effective.

Advantages of the MIC as a measure of antimicrobial susceptibility

• Easily performed
• Frequently, an automated method is available
• Simplicity and automation of the test enhances reproducibility
• Rapid return of results

Disadvantages of the MIC

• Minor variations in methodology can result in large variations of the MIC.
  • For example, extended incubation will make the MIC appear higher
  • Lower inoculum concentrations will make the MIC appear lower
• Interlaboratory variation in technique makes comparison problematic
• MIC is inhibition of visible growth: the microorganisms weren't necessarily killed!
• MIC may not be related to in vivo efficacy, which is a complex parameter determined by numerous factors among which the MIC is only one. An antibiotic with a low MIC may have no effect if it does not penetrate into the infected tissue. An antibiotic with a high MIC will still be effective if it happens to be concentrated in the infected tissue (eg. gentamicin in urine).