Performance of microbial identification by MALDI-TOF MS and susceptibility testing by VITEK 2 from positive blood cultures after minimal incubation on solid media

Bloodstream infections (BSIs) are a leading cause of patient morbidity and mortality. Rapid identification of organisms from BSIs is critical for initiating targeted antimicrobial therapy. Although many methods exist for rapid identification, they do not provide detailed or definitive susceptibility information. We assessed the utility of both the VITEK MS and Bruker Biotyper MALDI-TOF mass spectrometers to identify organisms from a positive blood culture bottle after only 4 h of growth on solid media compared to identification from overnight growth using the VITEK MS. Additionally, we determined whether this limited growth could yield accurate antimicrobial susceptibility testing (AST) results compared to overnight growth using the VITEK 2 AST system. Overall, identifications using the VITEK MS and Biotyper had agreements of 127/150 (84%) and 133/150 (88%), respectively. For rapid AST, the overall categorical agreement was 1010/1017 (99.3%), where Gram-negative bacteria had concordant results for 743/750 (99.1%) organism–drug combinations and Gram-positive bacteria had concordant results for 265/267 (99.3%). Gram-negative bacteria had 4, 2, and 1 minor, major, and very major discrepancies, respectively, while Gram-positive bacteria had no minor errors, one major, and one very major discrepancy. In conclusion, organisms grown for only 4 h on solid media were accurately identified by MALDI-TOF MS and have concordant phenotypic AST profiles. This method can also be implemented using common commercial instruments, providing a way to improve upon identification and gain detailed susceptibility information without significant additional laboratory costs.