Disk susceptibility of ofloxacin, a new carboxyquinolone

Ofloxacin, a fluorinated carboxyquinolone, was tested against 485 clinical isolates, and the MICs and disk inhibitory zones were correlated. A critical zone of greater than or equal to 19 mm and an MIC of less than or equal to 2 micrograms/ml indicate susceptibility. An MIC of 4 micrograms/ml and a zone size of 16 to 18 mm is intermediate, and an MIC of greater than or equal to 8 micrograms/ml with a zone size of less than 15 mm indicates resistance. Alternatively, organisms inhibited by an MIC of less than or equal to 4 micrograms/ml with a critical zone diameter of greater than or equal to 15 mm could be considered susceptible. By either of these criteria, major errors in judging susceptibility or resistance are less than 1%.