Direct antimicrobial susceptibility tests of bacteria and yeasts from positive blood cultures by using serum separator gel tubes and MALDI-TOF MS

Early and appropriate antimicrobial treatment can effectively reduce the mortality rate caused by bloodstream infections (BSIs) and is critical for favorable patient outcomes. In general, > 90% of positive blood cultures will show positive results within 48 h after incubation in the BACTECTM FX system. However, an additional 6-8 h are required to obtain clones of the bacterium and another 10-24 h to obtain antimicrobial susceptibility test (AST) results. In this study, direct ASTs of bacteria and yeasts from positive blood cultures were performed by using serum separator gel tubes and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). 153 BSI cases were caused by a single pathogen. The coincidence rates of genus and species identification between the direct method (from positive blood cultures) and reference method (from subcultured clones) were 86.9% and 83%, respectively. On average, 98.6% of the direct ASTs in 88 Gram-negative bacteria tested had an accurate result compared to the reference method. In Gram- positive bacteria and yeasts, the accuracy rates were 99.2% and 100%, respectively. MALDI-TOF MS combined with serum separator gel tubes can be used for rapidly identifying and performing ASTs on positive blood cultures.