Early Identification of Bacterial Isolates from Positive Blood Culture Bottles using MALDI-TOF MS and Early Antimicrobial Susceptibility Testing on Pediatric Oncology Patients

Background: Final identification and susceptibility results of positive blood cultures would take 24 to 48 h by conventional methods. Every hour of early treatment, could be life-saving for septic immunocompromised patients. We conducted a pilot study to determine the earliest timing for performing identification and antimicrobial susceptibility testing of microorganisms isolated, after positive blood culture bottles with the most accurate results. Materials and Methods: Positive blood culture samples were inoculated onto two blood agar plates at 37°C. The growth on one plate was used every one hour repeatedly identification by MALDI-TOF MS (bioMérieux Vitek MS IVD system) and antimicrobial susceptibility testing Vitek 2 system. A total of 600 ID and MIC testing were done (13 Gram positive and 12 Gram negative). Results: Identification of all isolates showed 100% agreement between testing initial 3-hour growth and testing over-night growth. MIC showed 100% agreement when testing at mean hours of 5.6 (SD+/- 2.5) and 9 (SD +/- 2.5) for Gram negative and positive organisms respectively. The total time needed for accurate MIC results from start of positive BC sub-culturing was 16.1 h & 22 h for Gram negative and positive organisms respectively. Conclusion: Identification of causative pathogens using MALDI-TOF MS could be achieved accurately after 3h of sub-culturing. MIC testing can be performed after 5.6 h, 9 h incubation. Applying this methodology provided final accurate rapid results and early treatment of septic patients.