Comparison of Two Quinupristin–dalfopristin Susceptibility Testing Methods and Two Interpretive Criteria for <i>Enterococcus faecium</i> Bloodstream Isolates from Korean Hospitals

<i>Enterococcus faecium</i>, particularly in its multidrug-resistant forms, causes invasive nosocomial infections. Given the limited data comparing the effectiveness of the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the CLSI clinical breakpoints (CBPs) for quinupristin-dalfopristin (QD) resistance and the need to evaluate their practical application, we retrospectively investigated the susceptibility patterns of 287 <i>E. faecium</i> bloodstream isolates from Korean hospitals to QD using the updated EUCAST and CLSI CBPs and two antimicrobial susceptibility testing methods: disk diffusion (DD) and Sensititre broth microdilution (Sensititre). QD resistance rates were 5.9% (CLSI) and 18.8% (EUCAST) for DD and 22.6% (CLSI) and 28.2% (EUCAST) for Sensititre. The most prevalent QD resistance gene types among QD-resistant isolates were <i>ermB</i>+<i>msrC</i>+ or <i>ermB</i>-<i>msrC</i>+. Categorical agreement between DD and Sensititre ranged from 77.7% to 90.7%, depending on the testing method and CBPs applied. The EUCAST zone diameter CBPs more effectively help identify QD-resistant <i>E. faecium</i> isolates using the DD method than the CLSI zone diameter CBPs. In comparison, the CLSI minimum inhibitory concentration (MIC) CBPs provide more reliable results for resistance classification in the Sensititre method than EUCAST MIC CBPs. These findings would help improve clinical decision-making for treating multidrug-resistant <i>E. faecium</i> infections.