Tentative interpretive standards for disk diffusion susceptibility testing with norfloxacin (MK-0366, AM-715)

Area of Science:

• Microbiology
• Pharmacology
• Clinical Diagnostics

Background:

• Norfloxacin is an orally absorbed quinoline derivative.
• It exhibits an expanded antibacterial spectrum, including Enterobacteriaceae, Pseudomonas aeruginosa, Streptococcus faecalis, and staphylococci.
• It is structurally related to nalidixic acid.

Purpose of the Study:

• To establish interpretive zone size breakpoints for antimicrobial susceptibility testing with norfloxacin.
• To determine appropriate minimal inhibitory concentration (MIC) breakpoints for norfloxacin.
• To evaluate the suitability of a "class disk" concept for norfloxacin.

Main Methods:

• Analysis of minimal inhibitory concentration (MIC) and zone size data from 413 clinical isolates.
• Application of regression line and error rate-bounded methods.
• Comparison with related quinolone drugs.

Main Results:

• A 10-microgram disk content was favored for norfloxacin susceptibility testing.
• Proposed zone size breakpoints: ≥17 mm (susceptible), 13–16 mm (intermediate), ≤12 mm (resistant).
• Proposed MIC breakpoints: ≤16 µg/ml (susceptible), ≥32 µg/ml (resistant).

Conclusions:

• The 10-microgram norfloxacin susceptibility disk should be tested separately.
• Differences in properties argue against using a "class disk" concept for norfloxacin.
• Standardized breakpoints ensure accurate susceptibility testing.