Inability to control selected drugs on commercially obtained microdilution MIC panels.

As more broad spectrum antimicrobics continue to appear, many manufacturers have added drugs to their microdilution minimal inhibitory concentration (MIC) test panels at the expense of testing each drug over a broad range of twofold dilutions. Micro Scan (Mahwah, NJ) gram-positive and gram-negative 96-well microdilution panels were tested with the suggested control organisms over a period of 60 days to determine if on-scale MIC endpoints could be achieved with at least one organism per drug. Of 13 antimicrobics in the gram-positive panel, seven (clindamycin, tetracycline, cephalothin, gentamicin, nitrofurantoin, trimethaprim-sulfa, and streptomycin) could not be controlled effectively. Of 18 drugs in the gram-negative panel, six (tobramycin, trimethaprim-sulfa, cefoperazone, mezlocillin, ticarcillin, and kanamycin) similarly could not be controlled. The accuracy of results obtained when patient isolates were tested with these 13 antimicrobics could not be substantiated by control procedures as stated in the Micro Scan instructions.