Direct MALDI-TOF MS identification and EUCAST RAST from positive blood cultures: impact on turnaround time in...

Rapid identification (ID) and antibiotic susceptibility testing are vital for the treatment of bloodstream infections (BSIs). The aim of this study was to perform direct identification and rapid antimicrobial susceptibility testing (RAST), from positive blood cultures and to compare the performance of these rapid methods with standard reference methods.

Blood cultures (BC) submitted to Marmara University Pendik Training and Research Hospital Clinical Microbiology Laboratory between September 2022 - April 2023, that yielded positive signal and were determined to be monomicrobial by gram staining were selected randomly. ID was performed using MALDI-TOF MS(bioMérieux, France). Based on these results, rapid antimicrobial susceptibility testing (RAST) was applied to 103 bottles that met RAST eligibility criteria. The results of rapid ID and RAST were evaluated by comparing them with the results obtained from colony-grown isolates.

Rapid ID showed that out of 306 bottles, 281 (91.8%) were monomicrobial. High rates of correct identification were achieved for gram-negative bacilli (84.3%), gram-positive chain cocci (79.4%), and gram-positive cluster cocci (55.2%), while performance was lower for gram-positive bacilli (27.3%) and yeasts (37.5%). For RAST, the readability of inhibition zone diameters increased progressively with incubation time, reaching 85.2%, 94.5%, 100%, and 100% at 4, 6, 8, and 16-20 h, respectively. The categorical agreement of the test remained consistently high across all time points, measuring 98.35% at 4 h, 98.49% at 6 h, 98.38% at 8 h, and 98.84% at 16-20 h.

Rapid ID of microorganisms directly from positive BC bottles, combined with RAST based on EUCAST RAST criteria, demonstrates high accuracy and reliability compared to conventional methods. These rapid approaches significantly reduce the turnaround time for both identification and susceptibility results-by approximately 16-20 h and 32-48 h, respectively thereby enabling earlier targeted antimicrobial therapy and improving clinical management of BSIs.