Azlocillin, a ureido penicillin active against Pseudomonas aeruginosa: interpretive zone standards and quality control parameters for tests with 75-mu g disks.

A nine-laboratory coordinated study was performed to establish tentative control limits for 75-mu g azlocillin disks tested against the standard control strain of Pseudomonas aeruginosa (ATCC 27853). Control limits for individual tests were 24 to 30 mm (25 to 29 mm for means of five separate tests). To establish interpretive zone standards for 75-mu g azlocillin disks, three separate laboratories each tested 93 strains of P. aeruginosa or related species. Geometric mean minimal inhibitory concentrations (MICs) were plotted against arithmetic mean zone diameters. Regression analyses were performed with zone diameter as the independent variable and also with MIC as the independent variable. The following interpretive categories were recommended: resistant, less than or equal to 14 mm (MIC, greater than 128 mu g/ml); intermediate, 15 to 17 mm (MIC, 128 mu g/ml); and susceptible, greater than or equal to 18 mm (MIC, less than or equal to 64 mu g/ml).