Multicenter Evaluation of the New Etest Gradient Diffusion Method for Piperacillin-Tazobactam Susceptibility Testing of Enterobacterales , Pseudomonas aeruginosa , and Acinetobacter baumannii Complex

Abstract

Piperacillin-tazobactam (P/T) is a β-lactam–β-lactamase inhibitor combination frequently used in the hospital setting. Etest is a gradient diffusion method that represents an alternative to broth microdilution (BMD) for performing antimicrobial susceptibility testing. We conducted a multicenter evaluation of the performance of the new P/T Etest compared to that of BMD following U.S. Food and Drug Administration (FDA) and International Standards Organization (ISO) standard ISO 20776-2 criteria using Clinical and Laboratory Standards Institute (CLSI)-FDA and European Committee on Antimicrobial Susceptibility Testing (EUCAST) interpretive breakpoints, respectively. A total of 977 isolates (775 Enterobacterales isolates, 119 Pseudomonas aeruginosa isolates, and 83 Acinetobacter baumannii complex isolates) were tested. Overall essential agreement (EA) was 96.4% and 96.6% for Enterobacterales when FDA and ISO 20776-2 criteria, respectively, were followed. EA was 98.3% for P. aeruginosa and 91.6% for the A. baumannii complex when both the FDA and ISO criteria were followed. Applying CLSI-FDA breakpoints, categorical agreement (CA) reached 93.0%, 93.3%, and 89.2% for the Enterobacterales, P. aeruginosa, and the A. baumannii complex, respectively. Two very major errors (VMEs; 1.1%) were found among the Enterobacterales (for 2 Klebsiella pneumoniae isolates). No additional major errors (MEs) or VMEs were found. Applying EUCAST breakpoints, CA was 94.8% and 95.8% for Enterobacterales and P. aeruginosa, respectively (no breakpoints are currently available for the A. baumannii complex). No VMEs were observed among the Enterobacterales, but 2 (0.4%) MEs were found. Among the P. aeruginosa isolates, 2 (6.9%) VMEs and 3 (3.3%) MEs were observed. These errors resulted when P/T Etest MICs were 1 doubling dilution apart from the BMD MICs. In conclusion, the new P/T Etest represents an accurate tool for performing antimicrobial susceptibility testing of Enterobacterales, P. aeruginosa, and A. baumannii complex isolates with limited category errors.

Prevalence of piperacillin/tazobactam resistance in invasive Haemophilus influenzae in Germany

“…In 2015, EUCAST published a warning regarding the reliability of piperacillin/tazobactam GAD, which was partly removed for some species, but not for Hi, after the material in question was improved. 26 Therefore, our results acquired by different testing methods confirm the warning and suggest that inhomogeneous diffusion of tazobactam from test strips might result in elevated MICs. On the other hand, piperacillin diffusion from the test strips seems to be adequate.…”

Building a Better Test for Piperacillin-Tazobactam Susceptibility Testing: Would that It Were So Simple (It's Complicated)

“…Again, because of the variability of MIC testing, isolates with MIC values of 16 g/ml on initial testing may yield MIC values of 8 to 32 mg/liter on repeat testing (4). Evidence of this imprecision is found in the work of García-Fernandez et al (2), where 85 of the 239 (35%) MICs generated as a part of the precision study did not yield the same MIC when the isolate was retested. In fact, the authors had to exclude two isolates (one E. coli isolate and one Klebsiella isolate) from the analysis because a reference MIC could not be established by broth microdilution (BMD).…”

A systematic review of antimicrobial susceptibility testing as a tool in clinical trials assessing antimicrobials against infections due to gram-negative pathogens

“…At a later stage, commercial devices may be identified as having significant problems leading to very major errorsda situation where isolates resistant to a trial antimicrobial are incorrectly categorized as susceptible by the local AST device or method [24e27]. In cases where issues with commercial AST devices are identified by standards-setting bodies such as EUCAST, laboratories that continue to use these devices are at increased risk of reporting false susceptibility to one or both of the trial antimicrobials [28,29]. A clear example of this is for colistin, where many laboratories across multiple trials (AIDA, CARE, MAGIC BULLET) used commercial methods with known problems rather than recommended broth microdilution resulting in significant AST error rates for colistin [30e34].…”