RAST+: pre-positivity application of EUCAST RAST using BACTEC FX growth detection plots - a proof-of-concept study

Timely antimicrobial susceptibility testing (AST) from blood cultures is crucial to optimize antimicrobial therapy.

The aim of this study is to accelerate AST results by applying EUCAST RAST method before blood culture positivity (RAST+) by utilizing Growth and Detection Plots (GDPs) generated by the BD BACTEC FX (Becton Dickinson, Sparks, MD, USA) system.

Quality control strains and 20 clinical isolates were inoculated into BACTEC™ Plus Aerobic bottles and incubated in the BD BACTEC FX system. GDPs were continuously monitored to detect the steeping phase indicative of early bacterial growth prior to signal positivity. Bottles were removed at the beginning of the steeping phase and RAST+ was performed using 300 µL inoculum. Zone diameters were interpreted according to EUCAST RAST guidelines.

The RAST+ method yielded inhibition zones within acceptable ranges both for 150 µL and 300 µL inocula for quality control strains, with improved readability at 300 µL. Of 185 antibiotic disks tested, 159 (85.9%) produced interpretable results by using a 300 µL inoculum. When compared to the reference method, RAST+ achieved a categorical agreement of 96.2%. The rates of very major, major, and minor errors were each 1.3%, indicating a high level of concordance and diagnostic reliability.

To the best of our knowledge, this is the first study to analyze GDPs in order to predict blood culture positivity. Furthermore, it is also the first to apply rapid AST directly from blood culture bottles (RAST) before the positivity signal appears. RAST+ is a proof-of-concept approach for detecting bacterial growth and performing antimicrobial susceptibility testing directly from blood cultures before they signal positive, potentially reducing turnaround times (TAT) and supporting earlier clinical decision-making.