Is a susceptible antibiotic result on a Kirby-Bauer (KB) test better than a Minimum Inhibitory Concentration (MIC) report for guiding antibiotic treatment?

MIC Reports Are Superior to Kirby-Bauer Susceptibility Results for Guiding Antibiotic Treatment

MIC values provide more accurate and clinically useful information than simple susceptible/intermediate/resistant categorizations from Kirby-Bauer disc diffusion testing, and should be prioritized when available for optimizing antibiotic therapy. 1

Why MIC Values Are Superior

Precision in Dosing Optimization

• MIC values enable precise pharmacokinetic/pharmacodynamic (PK/PD) targeting, which is essential for maximizing clinical outcomes, particularly in critically ill patients 1
• For beta-lactam antibiotics, maintaining free plasma concentrations at 4-8 times the MIC for 100% of the dosing interval is associated with improved bacteriological response, higher bacterial eradication rates, and prevention of resistant subpopulation selection 1, 2
• A study of cefepime treatment for E. coli and Klebsiella infections demonstrated that when the free drug concentration to MIC ratio exceeded 7.6, bacterial eradication occurred in 100% of patients versus only 33% when below this threshold 2

Clinical Decision-Making Advantages

• MIC values directly influence clinicians' antimicrobial therapy choices and can lead to improved clinical outcomes through enhanced precision, even though direct mortality evidence is limited 1
• Kirby-Bauer disc diffusion only provides categorical interpretations (susceptible/intermediate/resistant) without the quantitative data needed for dose optimization 1, 3
• A 2025 study demonstrated that suppressing MIC values in culture reports paradoxically led to less appropriate antibiotic prescribing (42.2% appropriate pre-suppression vs 60.7% post-suppression), suggesting that when MICs are visible, prescribers may misinterpret them by assuming lower MICs always indicate better choices 4

Limitations of Kirby-Bauer Testing

Accuracy Issues

• Kirby-Bauer disc diffusion shows lower categorical agreement with the gold standard broth microdilution method compared to gradient MIC methods like E-test 1
• For carbapenem-resistant Enterobacteriaceae (CRE) susceptibility to ceftazidime-avibactam, E-test showed 96% categorical agreement with broth microdilution, while disc diffusion showed only 72% agreement 1
• Standard Kirby-Bauer testing frequently produces false intermediate or resistant results, particularly for tigecycline, misleading clinical antimicrobial therapy decisions 5

Specific Testing Failures

• A modified Kirby-Bauer method was required to improve tigecycline susceptibility testing accuracy, with standard methods showing only 44.8-52.0% categorical agreement versus 90.6-87.2% for the modified method 5
• Standard Kirby-Bauer disc diffusion missed 76% of inducible third-generation cephalosporin resistance phenotypes in Enterobacteriaceae that would otherwise be considered susceptible 6
• Automated systems like VITEK 2 and Phoenix showed major error rates of 26% and 14% respectively when testing meropenem susceptibility in CRE 1

Practical Implementation

When to Prioritize MIC Values

• Request MIC determination for critically ill patients, particularly those in intensive care units where precise PK/PD targeting is essential for survival 1
• Obtain MICs for infections at specific sites requiring precise dosing (endocarditis, bone infections, central nervous system infections) 1
• Always obtain MICs for potentially resistant organisms including Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Acinetobacter species, and Staphylococci 1

Choosing the Right MIC Method

• Broth microdilution (BMD) is the gold standard reference method but requires significant laboratory expertise 1
• E-test (gradient diffusion) is the best alternative when BMD is unavailable, showing 88-96% categorical agreement with BMD across multiple antibiotics 1
• Agar dilution methods provide 96% categorical agreement with BMD and are suitable alternatives 1
• Automated systems (VITEK 2, Phoenix) are convenient but have relatively lower accuracy and should be used cautiously for critical infections 1

Critical Caveats

Interpreting MIC Values Correctly

• Lower MIC values do not automatically mean better antibiotic choice—the achievable drug concentration at the infection site and the specific PK/PD characteristics of each antibiotic are equally important 4
• MIC values are determined under standardized in vitro conditions that may not reflect in vivo infection site conditions (pH, oxygen tension, protein binding) 1, 3
• When actual MIC is unavailable, use the epidemiological cut-off value (ECOFF) rather than clinical breakpoints for PK/PD targeting in critically ill patients 1

Quality Control Requirements

• MIC testing requires strict adherence to standardized methods (CLSI or EUCAST guidelines) including defined growth media, incubation conditions, and inoculum size 3
• Contamination during sample collection or processing by untrained staff can render MIC results unreliable, making them worse than properly performed disc diffusion 1
• Quality control testing with known reference strains is essential to ensure reproducible results 1, 3

Resource Considerations

• In resource-limited settings where skilled microbiologists are unavailable and quality control is inadequate, properly performed Kirby-Bauer testing may be more reliable than poorly executed MIC testing 1
• The complexity of MIC testing varies across healthcare facilities, and laboratories should select methods based on their specific capabilities 1