Understanding CLSI Breakpoints for Disk Diffusion and MIC Interpretation

Clinical and Laboratory Standards Institute (CLSI) breakpoints are the standardized interpretive criteria used to classify bacterial isolates as susceptible, intermediate, or resistant based on disk diffusion zone diameters or minimum inhibitory concentration (MIC) values. These breakpoints are essential for translating raw antimicrobial susceptibility testing (AST) results into clinically meaningful categories that guide treatment decisions. CLSI breakpoints are particularly useful for clinical microbiology laboratories, infection control programs, and antimicrobial stewardship initiatives because they provide evidence-based thresholds that correlate with pharmacokinetic/pharmacodynamic (PK/PD) data, clinical outcomes, and resistance mechanisms. This article explains how CLSI breakpoints are established, how to apply them correctly to disk diffusion and MIC results, and why using current breakpoints is critical for accurate susceptibility interpretation.

At a Glance

Scientific Principle of CLSI Breakpoints

CLSI breakpoints are not arbitrary thresholds; they are derived from multiple lines of evidence. The M100 document, updated annually, integrates data from:

• Pharmacokinetic/pharmacodynamic (PK/PD) analysis: Drug concentrations achieved in serum and tissues relative to MIC distributions
• Clinical outcome studies: Correlation between MIC values and treatment success or failure
• Microbiological data: MIC distributions for wild-type populations versus those with acquired resistance mechanisms
• Resistance mechanism characterization: Identification of genetic determinants that confer resistance

The breakpoint revision process involves expert committee review of new evidence. For example, the 2025 CLSI breakpoints for minocycline against Acinetobacter baumannii complex were revised based on data showing that previous breakpoints overestimated susceptibility. In a study of 276 clinical isolates, the updated breakpoints reduced the susceptibility rate from 73.9% to 46.4% and increased the resistance rate from 4.4% to 46.0%, with categorical agreement between disk diffusion and broth microdilution improving from 64.1% to 90.9% [1]. This revision corrected a systematic overestimation of minocycline activity that could have led to ineffective therapy.

Materials and Instrumentation Considerations

For Disk Diffusion (Kirby-Bauer Method)

• Mueller-Hinton agar (MHA): Standard medium; for fastidious organisms, use supplemented MHA (e.g., 5% sheep blood, or MHA with NAD for Haemophilus spp.)
• Antimicrobial disks: Commercially prepared with specified drug concentrations (e.g., 30 µg for cefiderocol, 20 µg for lefamulin)
• Inoculum: 0.5 McFarland standard suspension (approximately 1.5 × 10⁸ CFU/mL)
• Incubation: 35 ± 2°C, ambient air, 16-18 hours (or 20-24 hours for some organisms)
• Measuring device: Caliper or ruler with mm markings; automated zone readers improve precision

For Broth Microdilution (MIC Determination)

• Cation-adjusted Mueller-Hinton broth (CAMHB): Standard medium; may require supplements for fastidious organisms
• Microtiter plates: 96-well format with serial twofold dilutions of antimicrobial agents
• Inoculum: Final concentration approximately 5 × 10⁵ CFU/mL
• Incubation: 35 ± 2°C, ambient air, 16-20 hours
• Reading: Visual determination of the lowest concentration inhibiting visible growth

Instrumentation Choices

• Manual methods: Suitable for low-volume laboratories; require careful technique and standardized measurement
• Automated systems: VITEK, BD Phoenix, MicroScan provide MIC results but may use different methodologies; results should be interpreted using CLSI breakpoints
• Gradient diffusion strips (Etest): Provide quantitative MIC values; studies show good categorical agreement with broth microdilution for many agents, though systematic biases can occur. For lefamulin against S. aureus, gradient diffusion strips showed 100% categorical agreement but systematically underestimated MICs by 0.49 log₂ compared to broth microdilution [5]

Controls: The Foundation of Reliable Results

Quality Control Strains

Every batch of AST must include appropriate reference strains tested under identical conditions. Common QC strains include:

• Escherichia coli ATCC 25922 (for disk diffusion and MIC of many Gram-negative agents)
• Staphylococcus aureus ATCC 29213 (for MIC) or ATCC 25923 (for disk diffusion)
• Pseudomonas aeruginosa ATCC 27853
• Enterococcus faecalis ATCC 29212

QC results must fall within CLSI-established ranges. If QC results are out of range, patient results cannot be reported until the issue is resolved.

Internal Controls

• Inoculum verification: Periodically confirm 0.5 McFarland turbidity using a spectrophotometer
• Medium quality: Verify pH (7.2-7.4 for MHA), depth (4 mm for disk diffusion), and absence of inhibitory substances
• Disk potency: Store disks properly (desiccated, -20°C for long-term, 2-8°C for short-term); never use expired disks

Conceptual Workflow for Applying CLSI Breakpoints

Step 1: Perform AST According to Standardized Protocols

Follow CLSI M02 (disk diffusion) or M07 (broth microdilution) for methodology. Key parameters include:

• Inoculum preparation: Direct colony suspension method preferred; adjust to 0.5 McFarland
• Inoculation: Within 15 minutes of suspension preparation; swab evenly across entire plate
• Disk application: Within 15 minutes of inoculation; press firmly for complete contact
• Incubation: Do not stack plates more than 5 high; ensure adequate oxygenation

Step 2: Measure Results Accurately

• Zone diameters: Measure to nearest mm using reflected light; read at the point of complete inhibition (ignore faint growth or tiny colonies at edge)
• MIC values: Read at the lowest concentration showing no visible growth; for trailing endpoints, refer to specific CLSI guidance

Step 3: Identify the Correct CLSI Breakpoint Table

CLSI M100 is organized by organism group and antimicrobial agent. Critical considerations:

• Organism identification must be accurate: Breakpoints differ by species (e.g., Enterobacterales vs. Pseudomonas aeruginosa vs. Acinetobacter spp.)
• Use the most current edition: Outdated breakpoints cause significant misclassification. A study of 9,279 bacterial isolates found that using outdated CLSI breakpoints led to overestimation of susceptibility for multiple organism-drug combinations, including gentamicin against Enterobacterales (significant shift in susceptibility distribution, χ² = 95.27) and piperacillin-tazobactam against P. aeruginosa (χ² = 6.62, p = 0.0366) [2]
• Check for species-specific breakpoints: Some agents have different breakpoints for different organisms (e.g., cefiderocol breakpoints differ between Enterobacterales, P. aeruginosa, and Acinetobacter spp.)

Step 4: Apply the Breakpoint

Compare the measured zone diameter or MIC value to the CLSI breakpoint table:

• Susceptible (S): Zone diameter ≥ breakpoint value; MIC ≤ breakpoint value
• Intermediate (I): Zone diameter or MIC falls within the intermediate range
• Resistant (R): Zone diameter ≤ breakpoint value; MIC ≥ breakpoint value

For example, using CLSI M100-S34 breakpoints for lefamulin against S. aureus: susceptible = MIC ≤ 0.25 µg/mL or zone diameter ≥ 23 mm [5]

Step 5: Document and Report

Record the interpretive category along with the raw measurement. Note the CLSI edition used for interpretation.

Quality Checks and Verification

Categorical Agreement Assessment

When comparing AST methods, categorical agreement (CA) measures how often two methods classify isolates into the same interpretive category. Acceptable CA is typically ≥ 90%. Error rates are classified as:

• Very major error (VME): False susceptible (method calls S when reference method calls R)
• Major error (ME): False resistant (method calls R when reference method calls S)
• Minor error: One method calls I while the other calls S or R

For cefiderocol testing against Gram-negative bacilli, categorical agreement between broth microdilution and disk diffusion using CLSI breakpoints ranged from 90.0% to 100% across species for carbapenem-resistant isolates [4]. This high agreement supports the use of disk diffusion as a reliable alternative when broth microdilution is not available.

Verification of New Breakpoints

When CLSI updates breakpoints, laboratories must:

1. Obtain the current M100 edition
2. Update laboratory information systems (LIS) with new breakpoint tables
3. Retrain staff on changes
4. Assess impact on historical susceptibility rates
5. Verify that QC ranges remain appropriate

Result Interpretation: Common Scenarios

Scenario 1: Zone Diameter Falls Exactly at Breakpoint

When a zone diameter equals the susceptible breakpoint, the isolate is classified as susceptible. When it equals the resistant breakpoint, it is classified as resistant. The intermediate range provides a buffer zone to account for technical variability.

Scenario 2: Discrepancy Between Disk Diffusion and MIC

If disk diffusion suggests susceptible but MIC suggests resistant (or vice versa), repeat both tests. If discrepancy persists, the MIC result is generally considered more reliable for clinical decision-making. In the minocycline study, 62.5% of isolates categorized as intermediate by disk diffusion were reclassified as either susceptible or resistant when tested by broth microdilution [1], highlighting the importance of confirmatory testing for intermediate results.

Scenario 3: Organisms with Inducible Resistance

Some resistance mechanisms (e.g., inducible clindamycin resistance in staphylococci) require specific detection methods (D-zone test) that are not captured by standard breakpoint interpretation. Follow CLSI guidance for supplementary testing.

Troubleshooting Common Issues

Limitations of CLSI Breakpoints

Organism-Specific Limitations

• Not all organism-drug combinations have CLSI breakpoints: For rare organisms or novel agents, breakpoints may not exist
• Breakpoints may not apply to all infection sites: Tissue penetration may differ from serum levels; CLSI breakpoints are primarily based on systemic infections
• Fastidious organisms require special media and incubation: Breakpoints for these organisms may be less robust

Methodological Limitations

• Disk diffusion provides qualitative results: Zone diameters are continuous but interpreted categorically; isolates near breakpoints may be misclassified
• Broth microdilution is labor-intensive: Not practical for high-volume laboratories without automation
• Gradient diffusion strips may show systematic bias: As demonstrated for lefamulin, where MICs were systematically 0.49 log₂ lower than broth microdilution [5]

Temporal Limitations

• Breakpoints change over time: Laboratories must update systems and retrain staff
• Delayed implementation causes misclassification: A study found that unregulated and delayed implementation of updated CLSI breakpoints results in significant misclassification of antimicrobial susceptibility, leading to overestimation of susceptibility and masking emerging resistance [2]
• Historical susceptibility data may not be comparable: When breakpoints change, retrospective comparisons become invalid

Interpretive Limitations

• Intermediate category is ambiguous: May indicate that the drug is effective at higher doses, or that results are equivocal
• Susceptible-Dose Dependent (SDD) category: Introduced for some agents; requires understanding of dosing implications
• Breakpoints do not account for combination therapy: Most breakpoints are based on single-agent PK/PD

Documentation Requirements

For Each AST Run

• Date and time of testing
• Organism identification (species level)
• AST method (disk diffusion, broth microdilution, gradient diffusion)
• CLSI M100 edition used for interpretation
• QC strain results (must be in range)
• Raw measurements (zone diameters in mm, MIC values in µg/mL)
• Interpretive category (S, I, R, or SDD)
• Any comments (e.g., "inducible clindamycin resistance detected")

For Breakpoint Updates

• Date of implementation
• Previous and new breakpoint values
• Impact assessment on susceptibility rates
• Staff training records
• LIS update verification

Biosafety Considerations

While this article focuses on conceptual understanding, all AST procedures must be performed in accordance with institutional biosafety guidelines. Key principles from the Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition include [6]:

• Risk assessment: Determine biosafety level based on organism risk group and procedure
• Standard microbiological practices: Hand washing, no eating/drinking, proper waste disposal
• Personal protective equipment: Lab coats, gloves, eye protection as appropriate
• Work surface decontamination: Daily and after spills
• Sharps safety: Proper disposal of contaminated needles and broken glass

For recombinant or synthetic nucleic acid work (e.g., cloning resistance genes), follow NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules [7].

Most routine AST with BSL-1 or BSL-2 organisms can be performed on an open bench with standard precautions. However, when working with select agents or organisms with known aerosol transmission risk, additional containment measures are required.

Frequently Asked Questions

1. Why do CLSI breakpoints change between editions?

Breakpoints are revised when new evidence emerges from PK/PD studies, clinical outcome data, or resistance surveillance. For example, the 2025 minocycline breakpoints for A. baumannii complex were revised because previous breakpoints overestimated susceptibility, potentially leading to ineffective therapy [1]. Laboratories must monitor annual M100 updates and implement changes promptly to avoid misclassification.

2. Can I use EUCAST breakpoints instead of CLSI breakpoints?

This article covers CLSI breakpoints exclusively. EUCAST breakpoints differ in methodology, interpretive criteria, and some categorical assignments. For cefiderocol testing, CLSI and EUCAST breakpoints produce different susceptibility rates; in one study, EUCAST breakpoints classified 94.7% of blaNDM-producing Enterobacterales as resistant while CLSI breakpoints classified only 1.8% as resistant [3]. Laboratories should follow the standard adopted by their national or institutional guidelines and not mix breakpoint systems.

3. What should I do when disk diffusion and MIC results disagree?

First, repeat both tests to rule out technical error. If the discrepancy persists, the MIC result is generally considered more reliable for clinical decision-making because it provides quantitative data. However, for some organism-drug combinations, specific guidance exists (e.g., for cefiderocol, broth microdilution is the reference method, and disk diffusion may overestimate resistance for some species) [4]. Document the discrepancy and consult the laboratory director or infectious disease specialist.

4. How do I handle isolates with zone diameters or MICs exactly at the breakpoint?

Isolates with values exactly at the susceptible breakpoint are classified as susceptible; those at the resistant breakpoint are classified as resistant. However, these "edge cases" have higher probability of misclassification due to inherent test variability. Consider repeating the test, and if the result is confirmed, report the category but note that the isolate is near the breakpoint. For clinical decision-making, these isolates may benefit from MIC determination rather than relying solely on disk diffusion.

References and Further Reading

1. Yu X, Liu Y, Du J, Hu F, Yin D. Assessment of the revision of the 2025 CLSI breakpoints for the interpretation of minocycline susceptibility for Acinetobacter baumannii complex. 2026. PubMed ID: 41685899. https://pubmed.ncbi.nlm.nih.gov/41685899/

2. Goyal N, Gangar S, Ramakrishnan B, et al. Impact of Outdated Clinical and Laboratory Standards Institute (CLSI) Breakpoint Implementation on Antimicrobial Susceptibility Interpretation: A Retrospective Analytical Study. 2026. PubMed ID: 41913829. https://pubmed.ncbi.nlm.nih.gov/41913829/

3. Duggan C, Cantillon D, Lawrie D, et al. Comparison of multiple cefiderocol susceptibility testing methods against genomic determinants of resistance in blaNDM carbapenemase producing Enterobacterales. 2026. DOI: 10.64898/2026.01.27.701980. https://doi.org/10.64898/2026.01.27.701980

4. Lin Y-T, Lin H-H, Chen C-H, et al. Interpretive agreement of susceptibility between broth microdilution and disk diffusion methods for cefiderocol, using criteria from the Clinical and Laboratory Standards Institute, European Committee on Antimicrobial Susceptibility Testing, and the Food and Drug Administration. 2026. PubMed ID: 41369533. https://pubmed.ncbi.nlm.nih.gov/41369533/

5. Tian Y, He X, Wu X, et al. Performance of gradient diffusion strip and disk diffusion versus broth microdilution for lefamulin susceptibility testing of 422 Staphylococcus aureus isolates. 2026. PubMed ID: 41575216. https://pubmed.ncbi.nlm.nih.gov/41575216/

6. CDC and NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition. U.S. Department of Health and Human Services, 2020. https://www.cdc.gov/labs/bmbl/index.html

7. National Institutes of Health. NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules. https://osp.od.nih.gov/policies/biosafety-and-biosecurity-policy/nih-guidelines-for-research-involving-recombinant-or-synthetic-nucleic-acid-molecules/

8. National Center for Biotechnology Information. NCBI Bookshelf: Molecular Biology and Laboratory Methods. https://www.ncbi.nlm.nih.gov/books/

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