Assessment of cefiderocol disk diffusion versus broth microdilution results when tested against
Acinetobacter baumannii
complex clinical isolates
Authors: Liu Yanling , Ding Li , Han Renru , Zeng Lingbing , Li Junming , Guo Yan , Hu Fupin 
Published: October 2023

The aim of this study was to evaluate the correlation between inhibitory zones and minimum inhibitory concentrations (MICs) when testing cefiderocol against
Acinetobacter baumannii
complex using disk diffusion and the broth microdilution (BMD) method according to the Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) criteria. Four-hundred and sixty-eight non-duplicated
A. baumannii
complex clinical isolates were collected from 56 hospitals of the China Antimicrobial Surveillance Network from 2019 to 2021. BMD using iron-depleted cation-adjusted Mueller-Hinton broth (ID-CAMHB) and standard disk diffusion methods were performed according to CLSI guidelines. Results were interpreted according to the CLSI and EUCAST breakpoints. Categorical agreement (CA), minor error (mE), major error (ME), and very major error (VME) were calculated for disk diffusion methods. The susceptibilities of all
A. baumannii
complex isolates by BMD were 98.7% (CLSI) and 97.6% (EUCAST). For all
A. baumannii
complex isolates, CA was 98.1% (CLSI) and 97.0% (EUCAST), with 0.9% (CLSI) and 1.9% (EUCAST) of VME, respectively. For the carbapenem-susceptible
A. baumannii
complex, the CA was 100%, with no mE or VME using both CLSI and EUCAST breakpoints. For carbapenem-resistant
A. baumannii
complex, CA was 97.5% (CLSI) and 96.2% (EUCAST), with 1.1% (CLSI) and 2.5% (EUCAST) of VME, respectively. Regarding the difficult-to-treat resistance
A. baumannii
complex isolates, CA was 97.6% (CLSI) and 95.7% (EUCAST), with 1.2% (CLSI) and 3.1% (EUCAST) of VME, respectively. Cefiderocol disk diffusion was difficult to assess in this study. Very few isolates were resistant to cefiderocol by BMD using CLSI breakpoint, and these were categorized as susceptible with the disk diffusion test. This study did, however, show that the main proportion of
A. baumannii
isolates were susceptible to cefiderocol by BMD, including carbapenem-resistant
A. baumannii
.

IMPORTANCE

Carbapenem-resistant
Acinetobacter baumannii
is a major global health concern due to its high prevalence and limited treatment options. Cefiderocol is the only novel Food and Drug Administration (FDA)-approved β-lactam agent for the salvage treatment of carbapenem-resistant
A. baumannii
infection. Currently, a commercial automated susceptibility testing panel of cefiderocol is unavailable. Both the preparation of iron-depleted cation-adjusted Mueller-Hinton broth and the performance of broth microdilution are cumbersome in routine microbiology laboratories. A disk diffusion method is convenient for cefiderocol antimicrobial susceptibility testing, but limited data are available specifically for
A. baumannii
clinical isolates. Moreover, the Clinical and Laboratory Standards Institute published revisions to the
A. baumannii
cefiderocol disk diffusion breakpoints in 2022. Hence, we evaluated the performance of cefiderocol disk diffusion compared with the reference BMD against
A. baumannii
clinical isolates, especially those with cefiderocol zone diameters ≤ 14 mm.