Abstract
Effects of varying in vitro susceptibility testing parameters of the broth microdilution assay on ceftazidime-avibactam MICs were determined and compared to meropenem and piperacillin-tazobactam for 9 Enterobacteriaceae and 4 Pseudomonas aeruginosa isolates. The effect of varying incubation conditions (ambient air or 5% CO2), pH of medium, medium composition (cation-adjusted Mueller Hinton Broth with and without laked horse blood and Haemophilus Test Medium), cation content of the medium, and inoculum density were tested. Most variations had no effect on ceftazidime-avibactam MIC values (no more than a 2-fold change). However, acidic pH or high inoculum resulted in 4- to 16-fold changes in MIC, which was similar to those observed for meropenem and piperacillin-tazobactam under these conditions. Overall, this study shows that slight variations in testing parameters during routine MIC testing will likely have no significant effect on ceftazidime-avibactam MIC values.
Highlights
It is important to know if small variations from standard Clinical Laboratory Standards Institute (CLSI) testing guidelines cause any significant changes in testing outcome
In medium with acidic pH, MICs for ceftazidime-avibactam were increased by 4- to
2.5% laked horse blood (LHB) had no effect on ceftazidime-avibactam MICs (Table 1), caused a 4-fold decreased piperacillin-tazobactam
Summary
These variations can have effects on the MIC when testing β-lactams, including cephalosporins and β-lactam-β-lactamase inhibitor combinations [5,6,7]. The effect of varying the broth microdilution in vitro testing parameters on the ceftazidime-avibactam Human serum albumin were previously shown to have no effect on ceftazidime-avibactam MICs [10]. In medium with acidic pH (pH 5 or 6), MICs for ceftazidime-avibactam were increased by 4- to
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