Ofloxacin susceptibility testing quality control parameters for microdilution and disk diffusion, and confirmation of disk diffusion interpretive criteria.

The susceptibilities of 221 clinical isolates to ofloxacin were tested simultaneously by broth microdilution and disk diffusion methods with commercially prepared 5-micrograms ofloxacin disks. The acceptability of the following previously proposed zone diameter breakpoints was confirmed: greater than or equal to 16 mm, susceptible; 13 to 15 mm, intermediate; less than or equal to 12 mm, resistant. On the basis of a multilaboratory collaborative study, the following are proposed as acceptable ofloxacin MIC ranges for quality control organisms: Escherichia coli ATCC 25922, 0.03 to 0.06 micrograms/ml; Staphylococcus aureus ATCC 29213, 0.12 to 0.5 micrograms/ml; Pseudomonas aeruginosa ATCC 27853 and Enterococcus faecalis ATCC 29212, 1.0 to 4.0 micrograms/ml. Ofloxacin quality control zone diameter ranges for the disk diffusion test are tentatively proposed, but variations in the performance of different lots of Mueller-Hinton agar may prove to be a serious problem for users.