Evaluation of an in-house rapid antimicrobial susceptibility testing (RAST) using the direct disk-diffusion method for gram-negative bacilli from positive blood cultures in a Japanese tertiary hospital.

BACKGROUND: Rapid antimicrobial susceptibility testing (RAST) by direct disk-diffusion (DD) on positive blood cultures can shorten the time to optimize therapy for bloodstream infections. Since 2014, we have implemented an in-house RAST-DD protocol for gram-negative bacilli based on CLSI M100-S23.

METHODS: Positive blood cultures processed between April and September 2024 were tested using our in-house RAST-DD method (12-antimicrobial panel including cefmetazole [CMZ]) and compared with broth microdilution interpreted according to CLSI M100-ED29. Categorical agreement (CA) and error rates-very major (VME), major (ME), and minor (mE)-were calculated. We also assessed the effects of applying updated CLSI criteria and rapid species identification, and evaluated the clinical utility of CMZ for extended-spectrum β-lactamase (ESBL)-producing Enterobacterales.

RESULTS: Among 234 g-negative isolates (206 Enterobacterales, 28 glucose non-fermenters), most agents showed high CA, but VMEs occurred for cefepime and ceftriaxone, and MEs for cefazolin and tazobactam/piperacillin among Enterobacterales. Applying CLSI M100-ED29 susceptible dose-dependent criteria and rapid identification reduced VMEs and mEs (CA 93.8%, VME 0.1%, ME 5.4%, mE 0.6%). CMZ achieved 100% CA for Enterobacterales excluding chromosomal AmpC producers and supported carbapenem-sparing treatment in several ESBL-producing Escherichia coli bacteremia cases with favorable outcomes.

CONCLUSIONS: The in-house RAST-DD method demonstrated reliable performance for gram-negative bacilli. Updating interpretive criteria and incorporating rapid identification reduced interpretive errors. Inclusion of a CMZ disk in the RAST panel may support carbapenem-sparing therapy for ESBL-producing Enterobacterales.