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

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.