How should minimum inhibitory concentrations (MICs) reported by the microbiology lab be interpreted?

MIC is the minimum inhibitory concentration of an antibiotic (AB) that inhibits an organism invitro, By itself, MIC does not take into account the pharmacokinetics (PKs) or pharmacodynamics (PDs) of an AB which ultimately determine its susceptibility cut-offs.

“Susceptible” implies that the isolate is inhibited by the usually achievable concentrations of the AB when recommended dosage is followed for the specific site of infection. “Intermediate” suggests that the MIC approaches usually attainable blood and tissue levels but at less- than-desirable expected clinical response rates, and generally should be avoided when an alternative AB exists. “Resistant” refers to isolates that are not inhibited by the usually achievable AB concentrations. In the absence of PK/PD data, comparing MICs can be misleading (i.e. lower MIC does not necessarily suggest a more potent AB).

Short answer: you can generally skip the MIC data and make antibiotic selection based on susceptibility report alone. Please also see a related pearl (type “MIC” in search) on selected circumstances involving quinolones and vancomycin when MICs should be taken into account despite report of “susceptibility”.

Turnridge J, Paterson DL. Setting and revising antibacterial susceptibility breakpoints. Clin Microbiol Rev 2007;20:391-408.

How should minimum inhibitory concentrations (MICs) reported by the microbiology lab be interpreted?

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s