Evaluation of prolonged incubation time of 16-20 h with the EUCAST rapid antimicrobial susceptibility disc diffusion testing method.

OBJECTIVES: Antimicrobial resistance rates are continuously increasing, driving the need for rapid antimicrobial susceptibility testing (RAST) results, especially in the treatment of bloodstream infections. The EUCAST RAST method performed directly from positive blood cultures with incubation times from 4 to 8 h was developed in 2018 and is now used in many laboratories. To increase the practicality of the method, an extended incubation time of 16 and 20 h was evaluated in this study. METHOD: Blood culture bottles were spiked with clinical isolates (n = 325) of the seven most important sepsis pathogens. The EUCAST RAST method was performed, extending the incubation time to 16 and 20 h. Broth microdilution (BMD) was used as a reference, except for screening tests where standard disc diffusion or presence of resistance genes was used. RESULTS: Inhibition zones were possible to read for all species-agent combinations. For 16 and 20 h, the MIC zone diameter correlations were sufficiently similar to allow establishment of common breakpoints for the time interval of 16-20 h. The proportion of isolates in the area of technical uncertainty was, on average, 6% for all species and the number of errors were low, with <1% false-resistant and <0.5% false-susceptible results. CONCLUSIONS: This study shows that, for EUCAST RAST, prolonging the recommended incubation to 16-20 h is possible and can be used as a complement when the intended shorter incubation is not possible to achieve. The introduction of the prolonged incubation will increase the usefulness of the EUCAST RAST method in clinical laboratories with limited opening hours.