Application of Total Laboratory Automation for Direct Rapid Antimicrobial Susceptibility Testing from Enterobacterales-Containing Blood Cultures

Abstract Background: Currently, EUCAST have issued guidelines for direct rapid antimicrobial susceptibility testing (RAST) on blood cultures. There were few reports on total laboratory automation (TLA) for RAST in China. Additionally, certain antibiotic discs used in China with specific concentrations lack EUCAST breakpoints. The purpose of this study is to use TLA to evaluate EUCAST RAST methods in Enterobacterales-containing blood cultures and to investigate the optimal breakpoints for a selection of antibiotics, including cefepime (EFP), cefotaxime (CTX), ceftazidime (CAZ), piperacillin-tazobactam (TZP), and cefoperazone/sulbactam (CSL). Methods: From April to August 2022, blood cultures positive for Enterobacterales (54 Escherichia coli and 60 Klebsiella pneumoniae) were analyzed. EUCAST RAST (4h, 6h, 8h) were performed using TLA and compared with Vitek 2 results. Results: EUCAST RAST's readable inhibition zone increased over time, with high categorical agreement with Vitek 2 (97.1%, 96.2%, 96.1% for E. coli , and 96.1%, 97.1%, 97.9% for K. pneumoniae) and low error rates. Based on the optimal breakpoints we defined, the categorical agreement for EFP, CTX, CAZ, and TZP against E. coli was greater than 90% at both 6 and 8 hours. However, the categorical agreement for CSL was 57.1% at 6 hours and 74.3% at 8 hours, primarily due to a higher proportion of minor errors (42.9% and 25.7%, respectively). For K. pneumoniae, the categorical agreement for all five antibiotics was greater than 90% at both time points. Conclusion: According to the CLSI-M52 standards, the detection performance of EUCAST RAST at 4h, 6h, and 8h was equivalent to that of Vitek 2. Except for ciprofloxacin in E. coli, the readable rates and categorical agreement of all antibiotics are good at 6h. For the optimal breakpoints we have established, all antibiotics except for CSL against E. coli achieve a categorical agreement of over 90%.