The minimum inhibitory concentration (MIC) is the lowest concentration (mcg/mL) of an antibacterial agent which, under controlled in vitro conditions, prevents visible growth of an organism. The MIC is graded in 1 of 3 ways: sensitive, intermediate, or resistant. Here’s an example of the MICs for a urine culture of ESBL E.coli:
A few pearls to keep in mind:
- MIC grading differs for individual drugs (i.e., the MIC for one drug cannot be compared to that of another drug.)
- ex: Using the above example, the MICs for amoxicillin and aztreonam are the same number but the former is intermediate while the latter is resistant.
- The drug with the lowest MIC is not necessarily the best drug for the patient.
- ex: If this example was drawn in clinic for a patient with symptoms of acute cystitis, would you be sending them to the hospital for meropenem?
- A drug marked “sensitive” is not necessarily a good choice for the patient.
- ex: If this patient has symptoms most consistent with acute epididymo-orchitis, starting nitrofurantoin would not be indicated because of its poor efficacy outside the urine.
- Don’t dismiss a drug with intermediate susceptibility. “Intermediate” implies a high likelihood of therapeutic success if 2 criteria are fulfilled:
- 1) the drug is given at high/max dose, and
- 2) the drug is adequately concentrated at the site of infection.