Ciprofloxacin disk susceptibility tests: interpretive zone size standards for 5-microgram disks

Area of Science:

• Microbiology
• Clinical Pharmacology

Background:

• Accurate antibiotic susceptibility testing is crucial for effective treatment of bacterial infections.
• Ciprofloxacin is a widely used fluoroquinolone antibiotic, necessitating reliable susceptibility testing methods.

Purpose of the Study:

• To evaluate the reliability of the 5-microgram ciprofloxacin disk diffusion susceptibility test.
• To propose tentative interpretive breakpoints for ciprofloxacin disk diffusion tests based on a large dataset.

Main Methods:

• Seven independent investigators performed disk diffusion susceptibility tests using 5-microgram ciprofloxacin disks.
• Data from 2,652 bacterial isolates were analyzed to determine test accuracy and establish breakpoints.
• Minimum Inhibitory Concentration (MIC) values were used as a reference for categorization.

Main Results:

• Tentative interpretive breakpoints were proposed: <= 15 mm (resistant, MIC > 2.0 µg/ml), 16-20 mm (intermediate, 1.0 < MIC <= 2.0 µg/ml), and >= 21 mm (susceptible, MIC <= 1.0 µg/ml).
• The disk diffusion test demonstrated high accuracy for most microorganisms tested.
• Reliability issues were noted for Streptococcus spp. and Pseudomonas maltophilia.

Conclusions:

• The proposed interpretive breakpoints for ciprofloxacin disk diffusion tests are valuable for ongoing clinical trials.
• The disk diffusion method is a reliable tool for assessing ciprofloxacin susceptibility in most bacterial pathogens.
• Further evaluation may be needed for specific organisms like Streptococcus spp. and Pseudomonas maltophilia.