Criteria for the assessment of susceptibility to ceftazidime using the disc diffusion procedure

Using regression analyses, we have determined criteria for the assessment of ceftazidime using the agar diffusion test with a 10 micrograms disc. With the ICS procedure, inhibition zones of greater than or equal to 15 mm on Mueller-Hinton agar, greater than or equal to 17 mm on Iso-sensitest agar and greater than or equal to 18 mm on DST agar indicate susceptibility (MIC less than or equal to 16 mg/l). Using the Kirby-Bauer procedure, Mueller-Hinton agar and the same discs, inhibition zones of greater than or equal to 12 mm indicate susceptibility. Minor discrepancies between these results and those found in the literature are due in part to differences in methodology, in particular, however, to differences in the bacteria tested. Mean MIC values for ceftazidime on DST agar were 0.25, 2.83 and 1.0 mg/l, respectively, for the international reference strains Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923 and Pseudomonas aeruginosa ATCC 27853.