Establishing breakpoints for amoxicillin/clavulanate and ampicillin/sulbactam for rapid antimicrobial susceptibility testing directly from positive blood culture bottles

Introduction. In 2018, EUCAST released guidelines on rapid antimicrobial susceptibility testing (RAST) directly from positive blood culture bottles for selected bacterial species and antimicrobial agents, but not for the commonly used agents amoxicillin/ clavulanate (AMC) and ampicillin/sulbactam (SAM). Hypothesis/Gap statement. This work addresses the Enterobacterales RAST capability gap for betalactam/betalactamase inhibitor combinations. Aim. We aimed to determine RAST breakpoints for AMC and SAM for Escherichia coli and Klebsiella pneumoniae after 4 and 6 h of incubation directly from positive blood cultures. Methodology. Blood culture bottles were spiked with clinical isolates of E. coli (n=89) and K. pneumoniae (n=81). RAST was performed according to EUCAST guidelines and zones were read after 4 and 6 h. Breakpoints were defined to avoid very major errors. Results. The proportion of readable zone diameters after 4 h of incubation were 90.8% in E. coli and 85.8% in K. pneumoniae isolates. After 6 h of incubation all zone diameters could be read. The proposed breakpoints for E. coli after 6 h of incubation were ≥16 mm S (susceptible), 14–15 mm ATU (area of technical uncertainty) and <14 mm R (resistant) for AMC; ≥15 mm S, 12–14 mm ATU and <12 mm R for SAM; for K. pneumoniae these were ≥16 mm S, 14–15 mm ATU and <14 mm R for AMC; ≥13 mm S, 12 mm ATU, <12 mm R for SAM. Applying our newly set breakpoints, major errors were infrequent (2.6%). Conclusion. We propose novel AMC and SAM breakpoints for RAST directly from positive blood cultures for reading after 4 and 6 h of incubation.