Validation of EUCAST rapid antimicrobial susceptibility testing directly from positive blood cultures in a non-automated lab setting.

Introduction: To speed up antimicrobial susceptibility testing (AST), the European Committee on Antimicrobial Susceptibility Testing (EUCAST) proposed rapid AST (RAST), a disk diffusion method to be read after 4, 6 and 8 hours of incubation. We investigated the feasibility of implementation of RAST in a non-automated lab setting.
Materials & Methods: To this end, reference strains as well as a variety of clinical and resistant strains were used to spike sterile hemocultures (BioMérieux BACT/ALERT 3D® and Becton Dickinson BACTEC FX® systems), followed by RAST in comparison to classical long-incubation AST.
Results & Conclusion: Our results with reference strains show that reading RAST after 4 hours is frequently too soon to obtain clinical results, and that Streptococcus pneumoniae reference strain did yield readable inhibition zones in RAST when harvested from BioMérieux BACT/ALERT 3D® bottle cultures. In a wider panel of strains, Gram positives RAST results were very similar to standard AST, while with Gram negative species errors were more frequently observed, limiting clinical implementation.