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Correlation of netilmicin agar dilution and disk diffusion susceptibilities
Antimicrobial Agents and Chemotherapy
|August 1, 1977
Summary
Disk diffusion testing showed good correlation with minimal inhibitory concentrations for gram-negative bacilli. However, a proposed breakpoint of <=11 mm for resistance would misclassify many resistant strains, including Pseudomonas aeruginosa, as susceptible.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Antimicrobial Susceptibility Testing
Background:
- Disk diffusion is a common method for determining antimicrobial susceptibility.
- Agar dilution provides reference minimal inhibitory concentrations (MICs).
- Accurate susceptibility testing is crucial for guiding antibiotic therapy.
Purpose of the Study:
- To evaluate the correlation between disk diffusion zone sizes and agar dilution MICs for gram-negative bacilli.
- To assess the accuracy of a proposed resistance breakpoint for disk diffusion.
Main Methods:
- A total of 283 gram-negative bacilli isolates were tested.
- Disk diffusion zone of inhibition diameters were measured.
- Agar dilution method was used to determine MICs.
- Regression analysis was performed to correlate the two methods.
Main Results:
- A strong negative correlation (r = -0.74) was observed between zone sizes and MICs.
- A breakpoint of zone size <=11 mm was suggested for resistance.
- This breakpoint would misclassify 34 of 45 resistant strains (75.6%) as susceptible.
- Notably, 27 of these misclassified strains were Pseudomonas aeruginosa.
Conclusions:
- The proposed disk diffusion breakpoint of <=11 mm is inadequate for accurately identifying resistant gram-negative bacilli.
- A significant number of resistant strains, particularly Pseudomonas aeruginosa, would be incorrectly identified as susceptible.
- Current disk diffusion breakpoints may require revision to improve accuracy in clinical settings.