Rapid determination of ceftazidime/avibactam susceptibility of carbapenemase-producing Enterobacterales directly from blood cultures: a comparative evaluation of EUCAST disc diffusion RAST and direct Etest® RAST

Objectives To evaluate the performance of two rapid antimicrobial susceptibility testing (RAST) methods to determine ceftazidime/avibactam susceptibility directly from blood cultures (BCs). Methods A total of 246 Escherichia coli or Klebsiella pneumoniae isolates were tested for ceftazidime/avibactam susceptibility directly from BC bottles using EUCAST RAST and Etest(R) RAST. Results obtained after 4, 6 and 8 h of incubation were compared with those obtained by reference broth microdilution on pure overnight subcultures. Results In total, the proportion of readable zones after 4 h of incubation was 96.7% and reached 100% after 6 and 8 h of incubation. EUCAST RAST yielded >98% of categorical agreement (CA) with all reading times. Major error (ME) and very major error (VME) rates were inferior to 3%, for each of the reading times. The proportion of results in the area of technical uncertainty (ATU) was almost similar (3.8%-4.1%) at the different reading times. DET-RAST yielded 97.5%, 98% and 99.6% of CA with readings at 4, 6 and 8 h, respectively. One (0.6%) ME was observed at each reading time, whereas five (5.9%) and four (4.5%) VMEs were observed analysing readings at 4 and 6 h, respectively. No VME was observed with readings at 8 h. Conclusions EUCAST RAST was accurate to determine ceftazidime/avibactam susceptibility of carbapenemase-producing K. pneumoniae and E. coli directly from BC bottles. DET-RAST has the advantage of determining MIC values and avoiding ATU results but showed to be an accurate method only with reading at 8 h.