How to Interpret a Kirby-Bauer Disk Diffusion Test: Zone Diameter Measurement and CLSI Breakpoints

The Kirby-Bauer disk diffusion test (also called the disk diffusion antibiotic susceptibility test or antibiogram) is a standardized method for determining the susceptibility of a bacterial isolate to antimicrobial agents. Interpretation relies on measuring the diameter of the zone of inhibition around each antibiotic disk and comparing that measurement to Clinical and Laboratory Standards Institute (CLSI) breakpoint tables, which classify the isolate as Susceptible (S), Intermediate (I), or Resistant (R) to each drug. This method is most useful for rapidly growing aerobic and facultative anaerobic bacteria in routine clinical and research microbiology laboratories, providing a cost-effective, reproducible means of guiding antimicrobial therapy and monitoring resistance trends.

At a Glance

Scientific Principle of the Disk Diffusion Method

The Kirby-Bauer test relies on the diffusion of antimicrobial agents from impregnated paper disks placed on the surface of Mueller-Hinton agar that has been inoculated with a standardized bacterial suspension. As the antibiotic diffuses radially into the agar, it establishes a concentration gradient. Bacterial growth occurs where the antibiotic concentration is below the minimum inhibitory concentration (MIC) for that organism, while a clear zone of inhibition appears where the concentration exceeds the MIC. The diameter of this zone is inversely related to the MIC—larger zones indicate greater susceptibility (lower MIC), while smaller or absent zones indicate resistance (higher MIC).

The relationship between zone diameter and MIC is not perfectly linear but follows a predictable pattern established through extensive correlative studies. CLSI breakpoints are derived from population distributions of zone diameters, pharmacokinetic/pharmacodynamic data, and clinical outcome studies. As noted in the overview of CLSI M100 updates, these breakpoints are revised periodically based on new data, making it essential to use the most current edition of the standards [3].

Materials and Instrumentation Choices

Agar Medium

Mueller-Hinton agar (MHA) is the standard medium for disk diffusion testing. Key specifications include:

• Depth: 4 mm (approximately 25 mL per 100 mm plate)
• pH: 7.2–7.4 at room temperature
• Cation content: Adequate calcium and magnesium (supplemented for MHA)
• Thymidine/thymine content: Low (to avoid false susceptibility to sulfonamides and trimethoprim)

For fastidious organisms, supplemented media may be required (e.g., MHA with 5% sheep blood for Streptococcus spp., or Haemophilus Test Medium for Haemophilus influenzae). Always verify that the medium meets CLSI quality control specifications.

Antibiotic Disks

Commercially prepared disks with specified drug concentrations are essential. Disks must be stored according to manufacturer instructions—typically at -20°C or -70°C in sealed desiccated containers. Allow disks to reach room temperature before opening containers to prevent condensation. Never use expired disks or disks from containers left open for extended periods.

Inoculum Preparation

The standard inoculum is a 0.5 McFarland turbidity standard (approximately 1–2 × 10⁸ CFU/mL for most bacteria). This can be prepared using:

• A direct colony suspension method (preferred for most organisms)
• A growth method (for slow-growing organisms)

Turbidity can be adjusted using a spectrophotometer or a visual McFarland standard. Inoculum density is critical—too heavy an inoculum produces falsely small zones, while too light an inoculum produces falsely large zones.

Measurement Tools

Zone diameters are measured in millimeters using:

• Calipers (preferred for precision)
• Rulers (acceptable if calibrated)
• Automated reading systems (e.g., AI-based smartphone applications, which have shown 98% agreement with manual measurement in some studies) [2]

Automated systems can reduce inter-operator variability and are particularly valuable in resource-limited settings [2].

Critical Controls

Quality Control Strains

Each testing batch must include appropriate quality control (QC) strains with known susceptibility patterns. Common QC strains include:

• Escherichia coli ATCC 25922
• Staphylococcus aureus ATCC 25923
• Pseudomonas aeruginosa ATCC 27853
• Enterococcus faecalis ATCC 29212

Zone diameters for QC strains must fall within CLSI-established ranges. If QC results are out of range, patient or test isolate results are invalid and must be repeated.

Media and Disk Quality Control

• Verify each new lot of MHA supports adequate growth and produces expected zone sizes with QC strains
• Test each new lot of antibiotic disks with appropriate QC strains
• Document all QC results according to laboratory protocol

Inoculum Verification

Periodically verify that the 0.5 McFarland standard produces the expected colony count by performing quantitative plate counts. This is especially important when using visual turbidity standards, which can drift over time.

Conceptual Workflow for Interpretation

Step 1: Verify Test Validity

Before measuring any zones, confirm:

• The agar surface shows confluent growth (or near-confluent for fastidious organisms)
• Individual colonies are not present (indicates too light an inoculum)
• Zones are circular and clearly defined
• QC results for the testing batch are within acceptable ranges

Step 2: Measure Zone Diameters

Using reflected light (not transmitted light), measure the diameter of each zone of inhibition to the nearest millimeter. For most organisms, measure the zone including the disk diameter (typically 6 mm). Key measurement rules:

• Measure the full diameter across the center of the disk
• For swarming organisms (e.g., Proteus spp.), measure the zone ignoring the swarming edge
• For sulfonamides and trimethoprim, measure the zone ignoring fine growth within the zone (80% or more inhibition is acceptable)
• For methicillin/oxacillin with staphylococci, examine carefully for faint growth within zones (indicates heteroresistance)

Step 3: Consult Current CLSI Breakpoint Tables

Using the most current edition of CLSI M100, locate the appropriate table for the organism-drug combination. Breakpoints are organism-specific and drug-specific. For example:

• Ciprofloxacin breakpoints for Enterobacteriaceae differ from those for Pseudomonas aeruginosa
• Breakpoints may change between CLSI editions, as seen with revised ciprofloxacin breakpoints for Salmonella spp. [1]

Step 4: Assign Susceptibility Categories

Compare the measured zone diameter to the breakpoint values:

• Susceptible (S): Zone diameter ≥ the susceptible breakpoint
• Intermediate (I): Zone diameter falls between the susceptible and resistant breakpoints
• Resistant (R): Zone diameter ≤ the resistant breakpoint

Step 5: Document and Report Results

Record zone diameters, susceptibility categories, and any interpretive comments. Report results in a clear format that clinicians or researchers can use for decision-making.

Quality Checks and Validation

Internal Quality Control

• Run QC strains daily when testing is performed
• Maintain QC charts (Levey-Jennings plots) to monitor trends
• Investigate and correct any out-of-range results before reporting patient or test isolate results

External Quality Assessment

Participate in proficiency testing programs (e.g., from CAP, CDC, or WHO) at least twice yearly. These programs provide unknown isolates for testing and compare results across laboratories.

Inter-Operator Variability

Manual measurement of zone diameters is subject to inter-operator variability. Studies have shown that automated reading systems can reduce this variability while maintaining high agreement with manual measurement [2]. Laboratories should periodically assess inter-operator agreement and provide retraining when needed.

Concordance with Other Methods

Disk diffusion results generally show good concordance with broth microdilution (the reference method for MIC determination) and whole-genome sequencing-based predictions, though concordance varies by antimicrobial agent and organism [4]. When discrepancies occur, broth microdilution or gradient diffusion (e.g., Etest) can be used for confirmation.

Result Interpretation

Understanding Breakpoint Categories

Susceptible (S): The bacterial isolate is inhibited by the usually achievable concentration of the antimicrobial agent when the recommended dosage is used for the site of infection. Clinical efficacy is likely.

Intermediate (I): The bacterial isolate is inhibited by a concentration that may be achievable with higher-than-normal doses or when the drug is concentrated at the site of infection (e.g., urine). This category also serves as a buffer zone to prevent minor technical errors from causing major misclassifications.

Resistant (R): The bacterial isolate is not inhibited by the usually achievable concentrations of the agent with normal dosage schedules. Clinical efficacy is unlikely.

Special Interpretation Scenarios

Sulfonamides and Trimethoprim: Measure the zone ignoring faint growth within the zone. The zone edge is defined by 80% or more reduction in growth.

Swarming Organisms: For Proteus species, measure the zone ignoring the swarming edge. The true zone edge is where growth becomes confluent.

Methicillin/Oxacillin for Staphylococci: Examine zones carefully for faint growth or colonies within the inhibition zone. Any growth within the zone indicates heteroresistance and should be reported as resistant.

Inducible Clindamycin Resistance: For staphylococci and streptococci, perform a D-zone test (place clindamycin and erythromycin disks 15–20 mm apart). A blunted zone around clindamycin adjacent to erythromycin indicates inducible resistance.

Reporting Results

• Report zone diameters in millimeters
• Report susceptibility category (S, I, R)
• Include interpretive comments when appropriate (e.g., "Inducible clindamycin resistance detected")
• For intermediate results, consider recommending MIC testing for confirmation

Troubleshooting

Limitations of the Kirby-Bauer Disk Diffusion Method

Method-Specific Limitations

• Qualitative only: Provides categorical results (S, I, R) but not quantitative MIC values
• Not suitable for slow-growing organisms: Requires visible growth within 16–24 hours
• Not suitable for anaerobes: Standard method requires aerobic incubation
• Not suitable for some fastidious organisms: Requires specialized media and conditions
• Inter-operator variability: Manual measurement can vary between technicians [2]
• Agar depth sensitivity: Zone diameters are affected by agar depth; must be standardized to 4 mm

Organism-Specific Limitations

• Some organisms (e.g., Stenotrophomonas maltophilia) do not produce reliable disk diffusion results
• Inducible resistance mechanisms (e.g., macrolide-lincosamide-streptogramin B resistance) may not be detected without specific tests
• Heteroresistance (e.g., vancomycin-intermediate S. aureus) may be missed

Interpretation Limitations

• Breakpoints are population-based and may not predict individual patient outcomes
• Breakpoints change over time; using outdated breakpoints can lead to misclassification [1, 3]
• Site-specific pharmacokinetics (e.g., urine concentrations) are not reflected in standard breakpoints

Comparison with Other Methods

Disk diffusion shows variable concordance with broth microdilution and whole-genome sequencing-based predictions, depending on the antimicrobial agent and organism tested [4]. For critical clinical decisions, MIC determination by broth microdilution or gradient diffusion may be preferred.

Documentation and Record Keeping

Required Documentation

• Date and time of test setup and reading
• Organism identification and source
• Inoculum preparation method and turbidity verification
• Medium lot number and expiration date
• Antibiotic disk lot numbers and expiration dates
• QC strain results for the testing batch
• Zone diameters for each antibiotic
• Susceptibility category assignments
• Any interpretive comments or special test results
• Technician identification

QC Documentation

• Daily QC results with Levey-Jennings plots
• New lot verification records
• Proficiency testing results
• Corrective action records for out-of-range results

Record Retention

Follow institutional and regulatory requirements for record retention. Typically, AST records are retained for at least 2–5 years, though requirements vary by jurisdiction and laboratory type.

Biosafety Considerations

Risk Assessment

The Kirby-Bauer disk diffusion test is routinely performed with bacterial isolates of known or unknown pathogenicity. While teaching laboratories typically use BSL-1 organisms (e.g., E. coli K-12, S. aureus ATCC 25923), clinical and research laboratories must conduct a risk assessment based on the organism being tested [6].

BSL-1 Practices (Teaching Laboratories)

• Standard microbiological practices
• No eating, drinking, or applying cosmetics in the laboratory
• Hand washing after handling cultures
• Decontamination of work surfaces daily and after spills
• Proper waste disposal (autoclaving of contaminated materials)

BSL-2 Practices (Clinical and Research Laboratories)

All BSL-1 practices, plus:

• Limited access to laboratory areas
• Personal protective equipment (lab coat, gloves, eye protection)
• Biosafety cabinet for procedures that may generate aerosols
• Sharps precautions
• Spill response plan
• Medical surveillance as indicated by risk assessment

Decontamination

All cultures, disks, and contaminated materials must be decontaminated before disposal, typically by autoclaving at 121°C for 30 minutes. Chemical disinfection (e.g., 10% bleach) may be used for surface decontamination but is not suitable for disposal of cultures.

Recombinant or Synthetic Nucleic Acid Work

If the test involves recombinant or synthetic nucleic acid molecules (e.g., testing genetically modified organisms), follow the NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules [7]. This may require Institutional Biosafety Committee (IBC) approval and additional containment measures.

Frequently Asked Questions

1. Why do I need to use the most current CLSI breakpoints?

CLSI breakpoints are revised periodically based on new pharmacokinetic/pharmacodynamic data, clinical outcome studies, and resistance mechanism surveillance. Using outdated breakpoints can lead to misclassification of isolates as susceptible when they are actually resistant, potentially compromising patient treatment. For example, revised ciprofloxacin breakpoints for Salmonella spp. changed the zone diameter for susceptibility from ≥21 mm to ≥31 mm, reclassifying many previously "susceptible" isolates as resistant [1]. Always check the CLSI M100 edition year and use the most current version available [3].

2. Can I use the Kirby-Bauer method for all types of bacteria?

No. The standard Kirby-Bauer method is validated for rapidly growing aerobic and facultative anaerobic bacteria. It is not suitable for anaerobes, slow-growing organisms (e.g., Mycobacterium spp.), or some fastidious organisms without specialized media and conditions. For organisms like Haemophilus influenzae, Neisseria gonorrhoeae, and Streptococcus pneumoniae, CLSI provides specific modified protocols with different media and incubation conditions. Always consult the CLSI M100 document for organism-specific recommendations.

3. How do I handle discrepancies between disk diffusion and MIC results?

Discrepancies between methods are not uncommon and can arise from technical errors, borderline results, or method-specific limitations. When disk diffusion and MIC results disagree:

• First, verify the purity and identity of the isolate
• Repeat both tests
• If discrepancy persists, consider the MIC result as more definitive for quantitative purposes
• For critical clinical decisions, use a reference method (broth microdilution) or gradient diffusion
• Document the discrepancy and any corrective actions taken

Studies have shown that concordance between disk diffusion and broth microdilution varies by antimicrobial agent, ranging from 42.3% to 100% in some studies [4].

4. What should I do if my QC results are out of range?

Out-of-range QC results invalidate all test results from that batch. Do not report any patient or test isolate results until the issue is resolved. Follow these steps:

1. Check for obvious errors (wrong QC strain, expired disks, incorrect incubation)
2. Repeat QC testing with fresh materials
3. If still out of range, test a different lot of medium and/or disks
4. Document all findings and corrective actions
5. If the problem persists, contact the manufacturer and CLSI for guidance

References and Further Reading

1. Girish R, Kumar A, Khan S, Dinesh KR, Karim S. Revised Ciprofloxacin Breakpoints for Salmonella: Is it Time to Write an Obituary? J Clin Diagn Res. 2013;7(12):2787-2790. PubMed — Demonstrates the clinical impact of revised CLSI breakpoints for ciprofloxacin and Salmonella.

2. Pascucci M, Royer G, Adamek J, et al. AI-based mobile application to fight antibiotic resistance. Nat Commun. 2021;12(1):1173. PubMed — Describes automated reading of disk diffusion tests using smartphone technology.

3. Schuetz AN, Ferrell A, Hindler JA, Humphries R, Bobenchik AM. Overview of changes in the Clinical and Laboratory Standards Institute Performance Standards for Antimicrobial Susceptibility Testing: M100 32nd and 33rd editions. J Clin Microbiol. 2025;63(1):e00894-24. PubMed — Summarizes key updates to CLSI M100 breakpoints and reporting guidance.

4. Snyder ER, Savitske BJ, Credille BC. Concordance of disk diffusion, broth microdilution, and whole-genome sequencing for determination of in vitro antimicrobial susceptibility of Mannheimia haemolytica. J Vet Intern Med. 2020;34(6):2698-2706. PubMed — Compares disk diffusion with other susceptibility testing methods.

5. Bubonja-Šonje M, Knežević S, Abram M. Challenges to antimicrobial susceptibility testing of plant-derived polyphenolic compounds. Arh Hig Rada Toksikol. 2020;71(4):300-311. PubMed — Discusses standardization challenges in AST for non-standard antimicrobials.

6. CDC and NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition. U.S. Department of Health and Human Services; 2020. CDC — Authoritative biosafety guidelines for microbiological laboratories.

7. National Institutes of Health. NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules. NIH Office of Science Policy — Biosafety framework for recombinant nucleic acid research.

8. National Center for Biotechnology Information. NCBI Bookshelf: Molecular Biology and Laboratory Methods. NCBI Bookshelf — Searchable collection of authoritative biomedical methods references.

Related Articles

• How to Interpret Kirby-Bauer Disk Diffusion Results Using CLSI Breakpoints
• How to Perform a Kirby-Bauer Disk Diffusion Test: Protocol and Quality Control
• How to Interpret Zone Diameter Measurements in Disk Diffusion Susceptibility Testing
• How to Calculate the Number of Bacteria Using the Kirby-Bauer Disk Diffusion Method
• Understanding CLSI Breakpoints for Disk Diffusion and MIC Interpretation
• How to Design a Reproducible Disk Diffusion Susceptibility Test Protocol