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 mu g/ml were examined in dilution tests performed in agar. In addition, disks containing 100 and 10 mu g 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 mu g/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 mu g/ml. However, the differences were only twofold. Error rate-hounded analysis with the disk containing 100 and 10 mu g 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 mu g/ml. Thus, a predictor panel was useful in the identification of accurate susceptibility test for piperacillin-tazobactam.