Antimicrobial susceptibility testing in European hospitals: report from the ARPAC study

Abstract

This observational study describes the antimicrobial susceptibility testing (AST) methods and interpretive criteria used in European hospitals during 2001, focusing specifically on detection of methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). Of 263 hospitals that took part in the ARPAC study, 192 submitted data on AST. Of these, 89% (n = 170) routinely used a disk-diffusion AST method, 43% (n = 82) used a semi-automated method, and 70% (n = 135) routinely determined MICs. Hospitals in southern Europe were less likely to use disk-diffusion, but were more likely to use a semi-automated method (p <0.001). In total, 173 (90%) interpreted AST results using CLSI breakpoints; 30% of these detected MRSA using unmodified CLSI disk-diffusion methods, while 35% used the unmodified CLSI agar-screening method for MRSA; 41% and 30% adhered to unmodified CLSI methodology for disk-diffusion and agar-screening, respectively, to detect VRE. Some of the modifications made may have greatly reduced the ability of the tests to detect MRSA/VRE. For example, 20% of respondents used excessively high incubation temperatures and 13% used inadequate incubation times to detect MRSA by disk-diffusion, and 28% used Mueller-Hinton agar instead of brain-heart infusion agar in VRE screening plates. The majority of respondents stated that they followed CLSI guidelines, but a high proportion had modified the CLSI methods for detecting MRSA and VRE, which may compromise clinical management and antimicrobial resistance surveillance.