Overview
AST interpretation is the laboratory process of assigning standardized susceptibility categories from valid zone diameters or MIC values using the institution’s adopted breakpoint tables and reporting rules, not selecting antimicrobials or doses.
Classification
| Category family | Lab meaning (general) | Hedge |
|---|---|---|
| Susceptible (S) | Phenotype within susceptible cutoff | Definition is edition/standard-specific |
| Intermediate (I) | Between S and R per adopted defs | Do not invent clinical severity ranks |
| Resistant (R) | At/beyond resistant cutoff | Confirm measurement + table row |
| Other labels | Per adopted standard/reporting policy | Do not assume CLSI≡EUCAST names |
Morphologic Features
Interpretation assumes a pure culture and congruent identification. Morphology/purity failures void category claims even if numbers look complete.
Laboratory Characteristics
- Controlled breakpoint tables (standard + edition)
- LIS/instrument category mapping congruence
- Selective/cascade reporting rules when used
- Approved interpretive comments only
Reference Intervals
Breakpoints are not universal reference intervals. They are committee cutoffs under a named standard and edition. Verify whether the laboratory adopts CLSI, EUCAST, or another authorized source before applying any numeric table.
Clinical and Laboratory Significance
Correct category assignment makes phenotypic AST reportable as a standardized laboratory claim. Wrong table, wrong edition, or hybrid standards destroy comparability.
Differential Considerations
When a category seems surprising, differentiate true phenotype, measurement error, mixed culture, ID mismatch, wrong breakpoint row, and CLSI/EUCAST/edition mismatch.
Comparison Tables
Interpretation decision cues
| Scenario | Correct action | Incorrect action |
|---|---|---|
| Edge MIC | Apply exact adopted cutoff | Round to preferred category |
| No breakpoint | SOP path (suppress/MIC/refer) | Invent related-drug cutoff |
| Two standards disagree | Verify adoption; hold if unclear | Average or pick stricter ad hoc |
| “Best drug?” call | Provide ID + categories/MICs | Recommend regimen/dose |
Classification Frameworks
Not applicable.
Laboratory Notes
- Strongly hedge CLSI vs EUCAST, verify institutional tables
- Never present breakpoints as universal dogma
- No treatment or dosing language
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
Common questions about using this Interpretation foundational topics Medical Laboratory Library reference in Medical Laboratory Science education.
What is the Interpretation laboratory reference?
Interpretation is a Foundational Topics entry in the LabPedia Medical Laboratory Library, written for Medical Laboratory Science reference lookup.
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Can reference details vary between laboratories?
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