ABSTRACT Systemic shifts in antimicrobial resistance rates can be due to epidemiologic shifts in microbial susceptibility patterns or artifactual shifts introduced by technical biases in antimicrobial susceptibility testing (AST)—both ultimately leading to changes in antimicrobial prescribing. To reduce technical variability, quality control (QC) criteria for AST are published by manufacturers and standards organizations. However, traditional QC metrics, in isolation, are fallible. In this study, we describe a systematic shift in daptomycin AST results between 2022 and 2025 in isolates tested in two independent health systems. Comprehensive analysis of clinical isolate AST results and retrospective mining of QC data from this period revealed a subtle shift that led to a 5%–22% decrease in overall susceptibility rates for certain organisms, most notably Enterococcus faecium . As daptomycin is a key treatment option for these difficult-to-treat infections, this increase in resistance rates paralleled a decrease in prescribing daptomycin for infections with these organisms. Importantly, this trend was undetectable through routine QC processes and only became apparent through systematic review of patient data. Our findings highlight the opportunity to integrate routine patient data analysis into microbiology QC practices to enhance detection of subtle but clinically relevant changes in AST performance. IMPORTANCE In this study, we report a critical incident of technical variability using daptomycin gradient diffusion methodology that was undetectable using routine quality control metrics. More broadly, this study underscores the opportunity to incorporate additional modalities, such as clinical patient results, into a comprehensive quality assurance plan to ensure high-quality antimicrobial susceptibility testing results. Given the dynamic spread of multidrug resistance in bacteria, accurate susceptibility testing results are critical to identify and respond to shifts in local epidemiology.
Zaydman et al. (Wed,) studied this question.