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

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Abstract
Norfloxacin is a new orally absorbed quinoline derivative structurally related to nalidixic acid but showing an expanded antibacterial spectrum which includes Enterobacteriaceae, Pseudomonas aeruginosa, Streptococcus faecalis, and staphylococci, among other susceptible bacterial species. The application of the regression line and error rate-bounded methods of analysis to the minimal inhibitory concentration and zone size data collected on 413 clinical isolates favored the selection of a 10-micrograms disk content and the adoption of the following interpretive zone size breakpoints for antimicrobial susceptibility testing with norfloxacin: greater than or equal to 17 mm for susceptible, 13 to 16 mm for intermediate, and less than or equal to 12 mm for resistant categories. It is proposed that isolates with minimal inhibitory concentrations of less than or equal to 16 and greater than or equal to 32 micrograms/ml be considered susceptible and resistant to norfloxacin, respectively. Differences in the antibiotic disk contents and in vitro antibacterial spectra and pharmacokinetic properties, together with the much lower rates of cross-resistance reported between norfloxacin and related drugs, strongly argue against the use of the "class disk" concept in this instance and suggest that the 10-micrograms norfloxacin susceptibility disk should be tested separately.