Establishment of epidemiological cut-off values of cefotaxime-sulbactam against Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Proteus mirabilis, Serratia marcescens, Acinetobacter baumannii and Staphylococcus aureus

Objective To establish the epidemiological cut-off (ECOFF) value of cefotaxime-sulbactam against Escherichia coli , Klebsiella pneumoniae , Enterobacter cloacae , Proteus mirabilis , Serratia marcescens , Acinetobacter baumannii , and Staphylococcus aureus . Methods A total of 1,770 non-duplicate clinical isolates were collected from five laboratories in four cities in China. Minimum inhibitory concentrations (MICs) of cefotaxime-sulbactam were determined independently using the broth microdilution method according to EUCAST standards. For A. baumannii , cefotaxime-sulbactam was tested at a 2:1 ratio, whereas for the other species sulbactam was tested at a fixed concentration of 4 mg/L. ECOFFs were determined using ECOFFinder software and visual estimation following EUCAST principles. Results Statistical endpoints at 95%, 97.5%, 99%, 99.5%, and 99.9% were calculated. The final ECOFFs were assigned based on concordance between ECOFFinder-derived estimates and visual inspection, using the 99% endpoint as the primary statistical estimate. The ECOFFs were 0.25/4 mg/L for E. coli and K. pneumoniae ; 0.5/4 mg/L for E. cloacae ; 0.06/4 mg/L for P. mirabilis ; 1/4 mg/L for S. marcescens ; 4/4 mg/L for S. aureus ; and 4/2 mg/L for A. baumannii . Conclusion ECOFFs of cefotaxime–sulbactam were established for seven clinically important bacterial species. These results provide essential reference values for distinguishing wild-type from non-wild-type isolates and support future breakpoint-setting and antimicrobial resistance surveillance.