Objectives

The European Committee on Antimicrobial Susceptibility Testing (EUCAST) recently warned about an area of technical uncertainty (ATU) of amoxicillin/clavulanate (AMX/C) disk susceptibility testing against members of the Enterobacterales. Thus, we aimed to compare the reliability of three routine methods and to evaluate the impact of the ATU.

Methods

286 Escherichia coli strains (including 159 AMX-resistant strains) were categorized for the two EUCAST AMX/C breakpoints by disk diffusion (Bio-Rad), the Phoenix automated system (Becton Dickinson) and the Etest (AES) compared to the broth microdilution reference method.

Results

By microdilution, 84.2% of strains were AMX/C-susceptible using the urinary breakpoint (MIC ≤32 mg/L) and 62.2% using the systemic breakpoint (MIC ≤8 mg/L), with 63.6% of MICs between 4 and 16 mg/L. For the systemic breakpoint, category agreement (CA) and very major error (VME) were unacceptable for the Etest (71.7% and 27.3%), disk (73.1% and 23.4% at 19-mm cut-off) and to a lesser extent for the Phoenix system (83.6% and 10.5%). For disks, an unacceptable VME rate was observed for diameters up to 22 mm, probably due to overcharged disks. For the Etest, VMEs were high at 6 mg/L (46/63) and 8 mg/L (22/29). For the urinary breakpoint, CA was more acceptable for disk (88.9%) and Etest (84.3%) but was unevaluable for Phoenix.

Conclusion

AMX/C susceptibility testing of E. coli for systemic breakpoint was unreliable with the three routine methods, explained mainly by the high prevalence (~60%) of strains with microdilution MICs around the breakpoint (8 mg/L). Our data confirmed the EUCAST 19–20-mm ATU for disk and suggest introducing ATU for Etest MIC values of 6 and 8 mg/L.

中文翻译：

---

阿莫西林/克拉维酸盐对大肠杆菌敏感性测试的技术不确定性领域：与肉汤微量稀释参考相比的自动化系统，Etest和圆盘扩散方法的分析

目标

欧洲抗菌药物敏感性测试委员会（EUCAST）最近警告了针对肠杆菌科成员的阿莫西林/克拉维酸盐（AMX / C）磁盘敏感性测试的技术不确定性（ATU）领域。因此，我们旨在比较三种常规方法的可靠性并评估ATU的影响。

方法

通过肉汤扩散（Bio-Rad），Phoenix自动化系统（Becton Dickinson）和Etest（AES），将286株大肠杆菌（包括159株抗AMX的菌株）归类为两个EUCAST AMX / C断点，与肉汤微量稀释法进行比较参考方法。

结果

通过微量稀释，使用尿分断点（MIC≤32mg / L）对AMX / C敏感的菌株为84.2％，使用全身分断点（MIC≤8mg / L）对AMX / C敏感的菌株为62.2％，其中43.6％的MIC为63.6％。毫克/升 对于系统性断点，Etest（71.7％和27.3％），磁盘（截止19 mm时为73.1％和23.4％）和较小范围的类别一致性（CA）和非常严重的错误（VME）是不可接受的凤凰系统（83.6％和10.5％）。对于磁盘，最大直径为22 mm的VME速率不可接受，这可能是由于磁盘过度充电所致。对于Etest，VME分别为6 mg / L（46/63）和8 mg / L（22/29）。对于尿液的断点，对于磁盘（88.9％）和Etest（84.3％），CA更可接受，但对于Phoenix则无法评估。

结论

用三种常规方法对大肠埃希菌的AMX / C敏感性测试是不可靠的，这主要是因为在断点附近微稀释MIC（8 mg / L）的菌株普遍存在（〜60％）。我们的数据证实了EUCAST 19–20 mm磁盘ATU，并建议针对Etest MIC值分别为6和8 mg / L引入ATU。