In vitro study with ciprofloxacin: interpretive criteria of agar diffusion test according to standards of the NCCLS and DIN.

Regression analyses to determine the correlation between minimal inhibitory concentrations (MICs) and inhibition zones produced by ciprofloxacin disks were carried out using 400 freshly isolated cultures of infective organisms (approximately 20 strains from each of 22 species). It was found in pilot studies that the correlation becomes weaker with increasing disk loads (1, 5, 10, and 30 micrograms). Disks containing 5 micrograms of ciprofloxacin were chosen for comparative studies using Mueller-Hinton agar and methods of the National Committee for Clinical and Laboratory Standards (NCCLS) as well as of the Deutsches Institut für Normung (DIN 58940). Based on preliminary MIC breakpoints (susceptible, 1 microgram/ml or less; intermediate, 2 micrograms/ml; and resistant, 4 micrograms/ml or more) and calculations from regression equations (limited to the MIC range of 0.25 to 32 micrograms/ml), the following zone interpretations using the NCCLS method are recommended: resistant, 15 mm or less; intermediate, 16 to 20 mm; and susceptible, 21 mm or more. The respective values with the DIN method are resistant, 18 mm or less; intermediate, 19 to 22 mm; and susceptible, 23 mm or more.