Comparative evaluation of agar dilution and Etest against broth microdilution for ceftobiprole susceptibility testing...

Ceftobiprole, a fifth-generation cephalosporin, is effective in treating complicated Staphylococcus aureus bacteraemia, including methicillin-resistant S. aureus (MRSA). This study compared ceftobiprole minimum inhibitory concentrations (MICs) determined by broth microdilution (BMD), agar dilution (AD), and Etest for MRSA isolates from a multicentre study in China.

The in vitro activity of ceftobiprole was evaluated against 60 MRSA isolates using AD and Etest, with BMD as reference.

BMD showed 85% of MRSA isolates were susceptible to ceftobiprole (MIC₉₀ = 4 mg/L), with 95% having MICs within ±1 log₂ dilution of the EUCAST breakpoint (1-4 mg/L). Categorical agreement and essential agreement were suboptimal for both AD (85% and 85%) and Etest (81.7% and 80%). Eight of nine MRSA isolates classified as resistant by BMD were categorized susceptible by Etest, resulting in very major errors, whereas only three isolates classified as susceptible by BMD were miscategorized as resistant by Etest. Using the error-rate-bound method, high very major errors were observed for Etest (22.2%) and AD (25.0%), predominantly involving ST5 and ST239 MRSA lineages.

This study demonstrates frequent clustering of MRSA isolates within the EUCAST-defined area of technical uncertainty, defined for ceftobiprole as an MIC of 2 mg/L, corresponding to the susceptibility breakpoint and its adjacent zone, where categorization is inherently unreliable. Methodological limitations of AD and Etest further increase the risk of misclassification and potential failure to detect resistance. These findings underscore the need for optimized susceptibility testing strategies and refined interpretive guidance to support accurate clinical use of ceftobiprole.