PREDICTION OF PIPERACILLIN TAZOBACTAM SUSCEPTIBILITY AMONG ENTEROBACTERIACEAE, PSEUDOMONAS-AERUGINOSA, AND OTHER BACTERIA USING TICARCILLIN-CLAVULANIC ACID, CEFTAZIDIME, AND OTHER BROAD-SPECTRUM ANTIMICROBIAL IN-VITRO TEST-RESULTS

The ability of various in vitro beta-lactam susceptibility test results to predict the susceptibility of piperacillin-tazobactam (a new beta-lactam-beta-lactamase inhibitor combination) was assessed using more than 46,000 recent clinical isolates. The organisms were tested by reference-quality National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution procedures and interpreted by the currently published NCCLS criteria. The recommended antimicrobial tests that would accurately predict the piperacillin-tazobactam in vitro efficacy had an overall very major, false-susceptible rate of only 0.6% (less than or equal to 1.5% is acceptable). The following drug tests can be used to judge piperacillin-tazobactam activity and spectrum (low patient risk) conservatively: for Enterobacteriaceae use ticarcillin-clavulanic acid results (0.6% very major error); for Pseudomonas aeruginosa use piperacillin (0.1%) results; for enterococci use ampicillin or ampicillin-sulbactam (1.8%) results; for Haemophilus influenzae and Moraxella catarrhalis use cefotcaxime or cefuroxime or ceftriaxone (1.5%); and for staphylococci use oxacillin by NCCLS recommendations. When the piperacillin-tazobactam testing reagents become available, the direct testing of this combination should be applied to relevant clinical isolates. The pipercillin-tazobactam break points should be reassessed as indicated by the cited minimum inhibitory concentration population analysis to improve predictive accuracy; H. influenzae susceptibility modified to less than or equal to 2/4 mu g/ml and Enterococcus species susceptibility tested at less than or equal to 16/4 mu g/ml.