Evaluation of Antimicrobial Activity
Evaluation of antimicrobial activity explains how the laboratory measures inhibition, interprets susceptibility, and connects in vitro results to possible therapy.
Core idea
This is the bridge between basic antimicrobial concepts and AST reports. It helps students understand zones, MICs, breakpoints, and quality control.
Principle
Antimicrobial activity is measured under standardized conditions, then interpreted with organism-specific rules and clinical breakpoints.
AST Language
| Term | Meaning | Student note |
|---|---|---|
| Zone diameter | Measured inhibition around a disk | Used in disk diffusion interpretation |
| MIC | Lowest tested concentration that inhibits visible growth | Reported by dilution, gradient, or automated methods |
| Breakpoint | Rule that converts a measurement into an interpretation | Depends on organism, drug, site, and standards |
| QC | Control testing that checks method performance | Bad QC means patient results may be unreliable |
Basic workflow
- Use a pure isolate that is clinically meaningful.
- Prepare the correct inoculum.
- Expose the organism to antimicrobial concentrations or disks.
- Measure inhibition using the approved method.
- Interpret using current organism-drug breakpoints and QC rules.
How to read it
- A larger zone usually means more inhibition in disk diffusion.
- A lower MIC usually means less drug is needed to inhibit growth in vitro.
- Susceptible, intermediate, resistant, and related categories are interpretive, not just measurements.
High-yield trapCommon mistakes
- Equating MIC numbers across unrelated drugs without context.
- Forgetting that breakpoint rules differ by organism and drug.
- Ignoring QC and method requirements.
ShortcutStudent shortcut
Zone or MIC is the measurement; the category is the interpretation.