Comparison and evaluation of carbenicillin disks in diffusion susceptibility testing

A broad variety of bacterial strains, including 79 Pseudomonas aeruginosa, were studied in an in vitro evaluation of carbenicillin disk susceptibility testing. Regression analysis with both 50-mug and 100-mug carbenicillin disks was carried out. Organisms having minimal inhibitory concentration values of 100 to 200 mug/ml demonstrated zones of less than 11 mm with the 50-mug disk, resulting in very little opportunity for appropriate discrimination of results. The line of regression for the 50-mug disk intersected the ordinate at a point just above the minimal inhibitory concentration value considered to be the limit of intermediate susceptibility for Pseudomonas. These considerations, together with evidence of greater disk content variation in the 50-mug than in the 100-mug disks assayed, considerable manufacturer-to-manufacturer variability with the 50-mug disk, and the more appropriate performance of the 100-mug disk, lead us to conclude that the 100-mug disk better serves the clinical test requirements for this agent than does the 50-mug disk, which is currently the only disk available for laboratory testing.