Objectives With increasing antimicrobial resistance, rapid antimicrobial susceptibility testing (RAST) becomes important, especially in patients with bloodstream infections. EUCAST decided to develop a standardized rapid method, based on EUCAST disc diffusion, to offer susceptibility reports within 4–8 h of a positive blood culture (BC). Methods BC bottles were spiked with clinical isolates (n = 332) of the seven most relevant sepsis pathogens with a variety of resistance mechanisms. RAST was performed directly from the bottle and zones read after 4, 6 and 8 h. Several variables were investigated, including the effect of using different BC bottles and of a 0–18 h delay between a positive signal and the performance of RAST. Results For five species, most inhibition zones could be read after 4 h. The proportion of results that could be interpreted increased from 75% at 4 h to 84% after 8 h. Categorical agreement against the reference method was good, with error rates of false susceptibility of 0.2%, 0.2% and 0.2% at 4, 6 and 8 h and false resistance of 1.2%, 0.2% and 0.1% at 4, 6 and 8 h, respectively. Conclusions With the EUCAST RAST method, reliable AST results can be delivered within 4–8 h of positivity of BC bottles for seven important bloodstream infection pathogens. To reduce the occurrence of errors and to absorb the variability caused by using a non-standardized inoculum, material from different manufacturers and workflow-related delays, we have introduced an area in which interpretation is not permitted, the Area of Technical Uncertainty.

Objectives When bloodstream infections are caused by resistant bacteria, rapid antimicrobial susceptibility testing (RAST) is important for adjustment of therapy. The EUCAST RAST method, directly from positive blood cultures, was validated in a multi-laboratory study in Europe. Methods RAST was performed in 40 laboratories in northern Europe (NE) and 15 in southern Europe (SE) from clinical blood cultures positive for Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus or Streptococcus pneumoniae. Categorical results at 4, 6 and 8 h of incubation were compared with results for EUCAST standard 16–20 h disc diffusion. The method, preliminary breakpoints and the performance of the laboratories were evaluated. Results The total number of isolates was 833/318 in NE/SE. The number of zone diameters that could be read (88%, 96% and 99%) and interpreted (70%, 81% and 85%) increased with incubation time (4, 6 and 8 h). The categorical agreement was acceptable, with total error rates in NE/SE of 2.4%/4.9% at 4 h, 1.1%/3.5% at 6 h and 1.1%/3.3% at 8 h. False susceptibility at 4, 6 and 8 h of incubation was below 0.3% and 1.1% in NE and SE, respectively, and the corresponding percentages for false resistance were below 1.9% and 2.8%. After fine-tuning breakpoints, more zones could be interpreted (73%, 89% and 93%), with only marginally affected error rates. Conclusions The EUCAST RAST method can be implemented in routine laboratories without major investments. It provides reliable antimicrobial susceptibility testing results for relevant bloodstream infection pathogens after 4–6 h of incubation.