Overview
AST quality control is the scheduled testing of defined control organisms with expected zone or MIC ranges to confirm that disk diffusion, dilution, and automated susceptibility methods remain in control before patient interpretive categories are released.
Classification
| Event | Immediate action | Before resume |
|---|---|---|
| QC out of range | Hold affected patient AST | Correct; document; re-QC |
| Suspect disk/card lot | Quarantine lot per SOP | Confirm replacement lot |
| Incubator fault | Assess impacted runs | Restore monitored conditions |
| Repeated fails | Escalate supervisor/QA | Root-cause per policy |
Morphologic Features
QC plates and panels should show expected control-strain morphology and purity. Contaminated QC cultures invalidate the control run.
Laboratory Characteristics
- Designated QC strain inventory and passage rules
- Worksheets or electronic QC capture
- Lot numbers for media, disks, cards
- Corrective-action and supervisory review trails
Reference Intervals
Acceptable QC ranges are method- and edition-specific. Verify against the laboratory’s adopted CLSI or EUCAST source and controlled SOP, do not treat remembered training numbers or another laboratory’s chart as universal.
Clinical and Laboratory Significance
In-control AST supports trustworthy S/I/R (or institution-equivalent) claims. Out-of-control episodes reduce claim strength until corrected.
Differential Considerations
When QC fails, differentiate inoculum error, expired/wrong disks, medium problems, incubator issues, reader technique, wrong control strain, and incorrect range table (wrong standard/edition).
Comparison Tables
QC decision cues
| Response | Appropriate? | Why |
|---|---|---|
| Hold patient AST; troubleshoot | Yes | Protects category validity |
| Release anyway; fix QC tomorrow | No | Removes method warrant |
| Use remembered ranges | No | Edition/SOP may differ |
| Suggest drugs while waiting | No | Outside laboratory AST scope |
Classification Frameworks
Not applicable.
Laboratory Notes
- Hedge QC frequency, strains, and ranges as institutional
- Tie ranges to adopted CLSI or EUCAST edition
- No treatment or dosing language during holds
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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