Imipenem susceptibility testing with a commercially prepared dry-format microdilution tray.

MICs of imipenem, concurrently generated in commercially prepared microdilution trays containing predried antibiotic dilutions (Sensititre), and in a standard agar dilution assay (as recommended by the National Committee for Clinical Laboratory Standards, Villanova, Pa.), were within +/- 1 twofold dilution for 94% of 226 bacterial isolates. Imipenem biological activity remained stable over 5 months of tray storage at room temperature against Pseudomonas aeruginosa ATCC 27853. Activity of imipenem was shown by microdilution testing with 890 clinical isolates to be high, with only 4% of isolates having MICs of greater than or equal to 16 micrograms/ml (in vitro resistance).