Multicenter Evaluation of the Etest Gradient Diffusion Method for Ceftolozane-Tazobactam Susceptibility Testing of Enterobacteriaceae and Pseudomonas aeruginosa.

Ceftolozane-tazobactam (C/T) is a novel beta-lactam-beta-lactamase inhibitor combination antibiotic approved by the U.S. Food and Drug Administration in 2014 for the treatment of complicated intra-abdominal infections (in combination with metronidazole) and complicated urinary tract infections. In this study, we evaluated the performance of the C/T Etest, a gradient diffusion method. C/T Etest was compared to broth microdilution (BMD) for 51 challenge isolates and 39 challenge isolates at three clinical sites. Essential agreement (EA) between the methods ranged from 47 to 49/51 (92.2 to 96.1%) for the , and categorical agreement (CA) ranged from 49 to 51/51 (96.1 to 100.0%). EA and CA for were 100% at all sites. The C/T Etest was also compared to BMD for susceptibility testing on 966 clinical isolates (793 , including 167 and 159 isolates, in addition to 173 isolates) collected at four clinical sites. EA between Etest and BMD was 96.9% for isolates and 98.8% for isolates. Within the , isolates from each species examined had >96% CA. For the clinical isolates, no very major errors were identified but two major errors were found (one for and one for ). By BMD, 47.0% of and 46.2% of challenge strains were nonsusceptible to C/T by CLSI breakpoint criteria; 8.2% of clinical isolates and 12.1% of clinical isolates were nonsusceptible to C/T by CLSI breakpoint criteria. In conclusion, Etest is accurate and reproducible for C/T susceptibility testing of and .