Clinical Guide
How to Interpret Antibiotic Susceptibility Test Results
A practical guide to reading MIC values and understanding S, I, and R classifications using CLSI and EUCAST standards.
What Is Antibiotic Susceptibility Testing?
Antibiotic susceptibility testing (AST) determines how effective specific antibiotics are against a particular bacterial isolate. Laboratories report results as either qualitative categories — Susceptible (S), Intermediate (I), and Resistant (R) — or as a quantitative Minimum Inhibitory Concentration (MIC) value. Understanding both forms is essential for selecting appropriate antimicrobial therapy.
Understanding MIC Values
The Minimum Inhibitory Concentration (MIC) is the lowest concentration of an antibiotic that prevents visible growth of a microorganism after overnight incubation. MIC is expressed in μg/mL and provides a precise measure of bacterial susceptibility.
A lower MIC generally indicates greater susceptibility, because less drug is needed to inhibit growth. However, MIC values cannot be compared across different antibiotics directly — each drug has its own pharmacokinetic profile and breakpoint thresholds.
S, I, and R Interpretations
Breakpoint committees — primarily CLSI (Clinical and Laboratory Standards Institute) in the United States and EUCAST (European Committee on Antimicrobial Susceptibility Testing) in Europe — publish standard tables that map MIC values to clinical categories:
| Category | Abbreviation | Clinical Meaning |
|---|---|---|
| Susceptible | S | The isolate is inhibited by concentrations achievable with standard dosing. The antibiotic is likely effective. |
| Intermediate | I | The isolate is inhibited at higher or more frequent doses, or the infection is at a site where drug concentrates (e.g., urine). May also indicate a buffer zone to prevent reporting small technical errors as resistant. |
| Resistant | R | The isolate is not inhibited by concentrations achievable with standard dosing, or clinical efficacy has not been reliably demonstrated. |
CLSI vs. EUCAST Breakpoints
CLSI and EUCAST are the two major breakpoint-setting organizations. Both review pharmacokinetic, pharmacodynamic, and clinical outcome data to define susceptibility thresholds, but their methodologies and breakpoint values can differ for certain organism–drug combinations.
- CLSI breakpoints are widely used in the United States and many other regions. CLSI documents (e.g., M100) are updated annually.
- EUCAST breakpoints are adopted throughout much of Europe and an increasing number of countries globally. EUCAST tables are revised regularly and often incorporate epidemiological cutoff values (ECOFFs) to distinguish wild-type from non-wild-type populations.
When interpreting a report, always confirm which guideline the laboratory used. A result labeled "S" under CLSI may occasionally differ under EUCAST for the same MIC, especially for agents with narrow therapeutic indices.
Practical Tips for Clinicians
- Do not compare MIC values across different antibiotic classes — a lower MIC for one drug does not guarantee superior clinical outcome compared to another.
- Consider the infection site. Some antibiotics achieve much higher concentrations in urine, bile, or CSF than in serum, which can affect interpretation.
- Review the full antibiogram rather than relying on a single agent. Combination therapy may be indicated for polymicrobial infections or resistant organisms.
- Check for additional comments or notes on the laboratory report, such as inducible resistance mechanisms (e.g., AmpC β-lactamases) or intrinsic resistance.
What About BMQ?
The Breakpoint to Minimum Inhibitory Concentration Quotient (BMQ) is a simple ratio that helps compare how far an isolate's MIC sits from the susceptible breakpoint (S breakpoint ÷ MIC). A higher BMQ indicates greater susceptibility margin. It should be considered primarily for sensitive antimicrobials as a supplementary decision-making tool, not as a replacement for standard interpretive categories.
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Enter an organism, antibiotic, and MIC value to get instant S/I/R and BMQ interpretations using both CLSI and EUCAST breakpoints.