Rapid AST directly from blood culture bottles

EUCAST has developed a method for rapid AST (reading at 4, 6 or 8h and since April 2022 also after 16-20 hours incubation) directly from positive blood culture bottles (RAST). The rationale is available in JAC.

Following the initial development, published in 2019, a clinical trial in 55 laboratories was performed and published in 2020.

These are the essential steps in the RAST method:

• direct inoculation of disk diffusion plates (MH, MH-F) using 100 - 150 µL directly from a positive blood culture bottle (the BD, bioMerieux and Thermo Fisher blood culture bottles were validated).
• no centrifugation or dilution of the inoculum - streak plates as for standard EUCAST disk diffusion.
• shortened incubation - 4, 6 and 8 hours with breakpoints adapted to each incubation time. Since 12 April 2022, it is possible to prolong incubation to 16-20 h but not beyond.
• zone diameters are read from the front of the plate after removal of the lid.
• breakpoints are specific for each species and each reading time.
• identity of species must be known prior to interpretation of AST results.
• the method is validated for the following species.
  • Escherichia coli
  • Klebsiella pneumoniae
  • Pseudomonas aeruginosa
  • Staphylococcus aureus
  • Streptococcus pneumoniae
  • Enterococcus faecalis and Enterococcus faecium
  • Acinetobacter baumannii 
• a positive blood culture bottle should be processed 0 - 18 hours after the positive signal.
• each species has its own TAB in the table and each reading time (4, 6, 8 and 16-20 hours) its own section.
• not all zone diameters can be read after 4 hours. Read again after 6 and 8 hours, or when necessary because of limited opening hours, now also after 16 - 20 h.
• read zone diameters ONLY when an obvious zone edge can be identified - otherwise re-incubate and read after 6, 8 or 16-20 hours. 
• the breakpoint table is specific for EUCAST Rapid AST - do not use the regular breakpoint table and do not use for rapid AST on an inoculum prepared from colonies.
• zone diameters which were successfully read and interpreted at one of the alternative incubation times should not be read again. 

Calibration and validation of RAST

• The EUCAST Development Laboratory makes baseline data on the relationship between zone diameters, MIC-values and ATUs publicly available.

Ongoing work (to be finalised 2024):

• Breakpoints for additional agents will be considered.
• The development of breakpoints for Salmonella enterica is ongoing

Implementing RAST from blood culture bottles

If you have implemented, or aim to implement, RAST according to the EUCAST method, please send an email stating the name, town and country of your laboratory, and tell us if you were successful or encountered difficulties. Contact EUCAST via the subject related contact form.