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-,gg and 100-ag carbenicillin disks was carried out. Organisms having minimal inhibitory concentration values of 100 to 200 ,sg/ml demonstrated zones of less than 11 mm with the 50-,ug disk, resulting in very little opportunity for appropriate discrimination of results. The line of regression for the 50-,gg 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-,ag than in the 100-ag disks assayed, considerable manufacturer-to-manufacturer variability with the 50-,ug disk, and the more appropriate performance of the 100-,gg disk, lead us to conclude that the 100-,ug disk better serves the clinical test requirements for this agent than does the 50-Ag disk, which is currently the only disk available for laboratory testing.Carbenicillin (disodium-alpha-carboxybenzyl penicillin), although a relatively new antimicrobial agent, has become very much a part of the antibiotic armamentarium of the physician treating infections caused by Pseudomonas aeruginosa, Proteus species, Providencia, and, to a lesser extent, strains of Enterobacter and Escherichia coli resistant to other antibiotics (3,12,16).Susceptibility testing is now carried out in most diagnostic microbiology laboratories on a routine basis with carbenicillin. Currently, only the 50-,ug disk is approved for carbenicillin testing by the Food and Drug Administration (6, 9). Previously work was done with disks of varied antibiotic content (10-12, 17, 20, 21). However, subsequent to the advent of the Food and Drug Administration decision, a few in vitro studies were published which contain 50-,ug disk data (16,19,22,23
