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Problems encountered in using Cefinase disks to predict in vitro penicillin susceptibility of coagulase-negative staphylococci.

The value of Cefinase (BBL Microbiology Systems, Cockeysville, M.D.) disks in predicting penicillin responses of coagulase-negative staphylococci was studied with the use of clinical isolates. The beta-lactamase activity of each isolate was determined both before and after enzyme induction for 18-24 hours with a 5-micrograms methicillin disk. Penicillin responses were determined in microdilution minimum inhibitory concentration (MIC) panels after 18 hours of incubation at 35 degrees C. Of 485 isolates tested, 338 (70%) were beta-lactamase positive, 168 (35%) before and 170 (35%) after induction. Only two (0.6%) of these isolates had a penicillin MIC of less than or equal to 0.12 microgram/mL (less than or equal to 0.12 mg/L). Of the 147 Cefinase-negative isolates, penicillin MICs of 0.12 microgram/mL or less (less than or equal to 0.12 mg/L) and 0.25 microgram/mL or more (greater than or equal to 0.25 mg/L) were obtained from 110 (75%) and 37 (25%), respectively. Although Staphylococcus saprophyticus accounted for only 30 (6%) of the total isolates, 54% of the organisms that were beta-lactamase negative with penicillin MICs of greater than or equal to 0.25 microgram/mL (greater than or equal to 0.25 mg/L) belonged to that species. A positive Cefinase test accurately predicted elevated penicillin MICs, but a negative test, especially with S. saprophyticus, was of less value.