Use of a predictor panel to evaluate susceptibility test methods proposed for piperacillin-tazobactam

A predictor panel of clinical isolates that produce a variety of types and amounts of beta-lactamases was used to assess the accuracy of dilution and disk diffusion susceptibility tests for piperacillin-tazobactam. Combinations of piperacillin-tazobactam with a fixed ratio of 8:1 and with tazobactam held constant at 4 micrograms/ml were examined in dilution tests performed in agar. In addition, disks containing 100 and 10 micrograms of piperacillin and tazobactam, respectively, were examined in diffusion tests. Three very major discrepancies between MICs determined with an 8:1 ratio and MICs determined with tazobactam held constant at 4 micrograms/ml were noted. These involved strains that appeared to be susceptible in tests with the 8:1 ratio but resistant when tazobactam was held constant at 4 micrograms/ml. However, the differences were only twofold. Error rate-bounded analysis with the disk containing 100 and 10 micrograms of piperacillin and tazobactam, respectively, revealed low error rates, regardless of whether MICs were determined with an 8:1 ratio or tazobactam held constant at 4 micrograms/ml. Thus, a predictor panel was useful in the identification of accurate susceptibility test for piperacillin-tazobactam.