Rapid antimicrobial susceptibility of Enterobacteriaceae by disk diffusion directly from blood culture bottles using the EUCAST RAST breakpoints.

Abstract Objectives To evaluate the disk diffusion technique after 4 and 6 h directly from positive blood culture bottles of Enterobacteriaceae using the rapid antimicrobial susceptibility test (RAST) breakpoints established by The European committee on antimicrobial susceptibility testing (EUCAST). Methods A total of 61 isolates of Escherichia coli and Klebsiella spp. were selected. The results were assessed using the RAST breakpoints (4–6 h) as well as the standard breakpoints (18 h) from EUCAST. Results The vast majority of the zone diameters of E. coli and Klebsiella spp. were optimally readable after 6 h of incubation. RAST in 6 h presented best results of CA and errors (CA = 94.4%, mE = 4.3%, ME = 0.8%, and VME = 0.4%) compared to RAST in 4 h (CA = 84.3%, mE = 13.0%, ME = 3.2%, and VME = 0.4%). The proportion of results in the ATU decreased over time: from 13.8% at 4 h to 6.8% at 6 h. According to FDA and ISO criteria, early readings at 6 h using the RAST breakpoints provided acceptable results (CA > 90%), whereas accuracy of results at 4 h was unacceptable (CA  Conclusion These data indicate that the early readings in 6 h using the RAST breakpoints for all antibiotics tested in this study (except amikacin) may be used in the clinical microbiology laboratory to anticipate the AST results of blood cultures. Early readings at 4 h (RAST breakpoints) may also be used, but not for all antibiotics.