MIC Testing
One-Sentence Definition
MIC testing determines the minimum inhibitory concentration — the lowest antimicrobial concentration that prevents visible growth — and converts it into a clinical category using breakpoints.
Simple Explanation
A number, not just “sensitive or resistant”. The number is what lets you reason about dose and site.
What the MIC is and is not
It is: an in-vitro measurement under standardized conditions, on a two-fold dilution scale (so 4 and 8 mg/L differ by one step, and one step is within normal method variation).
It is not: a measure of how sick the patient is, nor directly comparable between different antibiotics. An MIC of 1 mg/L is excellent for one drug and resistant for another — the breakpoint supplies the meaning.
Methods that yield an MIC
| Method | Notes |
|---|---|
| Broth Microdilution | Reference standard |
| Agar dilution | Reference for some organisms (e.g., Neisseria gonorrhoeae) |
| Gradient strip (E-test style) | Plastic strip with an antibiotic gradient; MIC read where the elliptical zone meets the strip. Convenient for single agents |
| Automated systems | Vitek/Phoenix/MicroScan — abbreviated dilution series plus algorithms |
Breakpoints
Set by EUCAST or CLSI from MIC distributions, pharmacokinetic/pharmacodynamic modelling, and clinical outcome data. They change — which is why a stored MIC value is more durable than a stored S/I/R category, and why laboratories must revalidate after breakpoint updates.
ECOFF (epidemiological cut-off) separates wild-type from non-wild-type populations and is used for surveillance, not therapy — a distinction that matters when comparing genomic and phenotypic data.
Clinical use
- Site-dependent interpretation: an MIC acceptable in urine may be unusable in CSF or bone
- PK/PD targets: time above MIC for β-lactams, AUC/MIC for vancomycin, peak/MIC for aminoglycosides (Antibiotics)
- Rising MICs within the susceptible range can signal emerging resistance
Computational relevance
MIC is the label that genomic and machine-learning models try to predict; errors are graded as very major/major, which is why Model Evaluation in Clinical Microbiology matters more than raw accuracy.
Related MOCs
Active Recall Questions
- Why can MICs not be compared across different antibiotics?
- What is an ECOFF and how does it differ from a clinical breakpoint?
- Why store the MIC rather than only the S/I/R result?