Overview
Disk diffusion AST places antimicrobial-impregnated disks on inoculated agar and measures inhibition zone diameters after standardized incubation for comparison with adopted breakpoints.
Classification
| Control point | Target | If wrong |
|---|---|---|
| Inoculum | Specified density | Zones falsely small or large |
| Media | Approved type/depth | Invalid diffusion/growth |
| Disks | Potency + spacing | Overlap or weak zones |
| QC | Institutional ranges | Stop patient release |
Morphologic Features
Describe lawn quality and zone edges. Inner colonies or haze: follow SOP reading rules and purity checks.
Laboratory Characteristics
- Swab within allowed time after inoculum prep
- Apply disks promptly; invert plates
- Incubate atmosphere/time per method
- Caliper or template measurement as required
- Document QC lot and results
Reference Intervals
Zone interpretive breakpoints and QC expected ranges are institutional, verify current CLSI or EUCAST adoption and edition. Do not use unverified textbook cutoffs for live reporting.
Clinical and Laboratory Significance
Valid diameters yield reportable categories. Invalid plates must be repeated; do not estimate categories from poor lawns or merged zones.
Differential Considerations
Differentiate true reduced zones (resistance) from heavy inoculum, degraded disks, wrong medium, mixed culture, and measurement error.
Comparison Tables
Disk diffusion stop-release cues
| Plate finding | Action | Avoid |
|---|---|---|
| Patchy lawn | Repeat inoculum/swab | Reading categories |
| Overlapping zones | Replate with spacing/fewer disks | Guessing diameters |
| QC not done | Run/document QC | Releasing patients |
Classification Frameworks
Not applicable.
Laboratory Notes
- No treatment or dosing on phone calls
- Hedge breakpoints and QC ranges as institutional
- Inoculum, media, reading, QC are non-negotiable
References
Authoritative textbooks, guidelines, and reviews supporting this reference entry. Verify reference intervals, critical limits, and reflex criteria against institutional protocols and current guideline editions.
Textbooks
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2020.
- Mahon CR, Lehman DC, Manuselis G. Textbook of Diagnostic Microbiology. 6th ed. Elsevier; 2018.
- Carroll KC, Hobden JA, Miller S, et al. Jawetz, Melnick, & Adelberg's Medical Microbiology. 28th ed. McGraw-Hill; 2019.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
CLSI and Professional Guidelines
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. CLSI document M100 (verify current edition and institutional adoption).
- Centers for Disease Control and Prevention / NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL). Current edition (verify institutional adoption).
Frequently Asked Questions
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