title: MALDI-TOF mass spectrometry following short incubation on a solid medium is a valuable tool for rapid pathogen identification from positive blood cultures.
abstract: BACKGROUND
  Rapid identification of the causative microorganism is a key element in appropriate antimicrobial therapy of bloodstream infections. Whereas traditional analysis of positive blood cultures requires subculture over at least 16-24h prior to pathogen identification by, e.g. matrix-assisted laser-desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), sample preparation procedures enabling direct MALDI-TOF MS, i.e. without preceding subculture, are associated with additional effort and costs. Hence, we integrated an alternative MALDI-TOF MS approach in diagnostic routine using a short incubation on a solid medium.
  
  
  METHODS
  Positive blood cultures were routinely plated on chocolate agar plates and incubated for 4h (37 °C, 5% CO2). Subsequently, MALDI-TOF MS using a Microflex LT instrument (Bruker Daltonics) and direct smear method was performed once per sample. For successful identification of bacteria at species level, score cut-off values were used as proposed by the manufacturer (≥ 2.0) and in a modified form (≥ 1.5 for MALDI-TOF MS results referring to Gram-positive cocci and ≥ 1.7 for MALDI-TOF MS results referring to bacteria other than Gram-positive cocci). Further data analysis also included an assessment of the clinical impact of the MALDI-TOF MS result.
  
  
  RESULTS
  Applying the modified score cut-off values, our approach led to an overall correct species identification in 69.5% with misidentification in 3.4% (original cut-offs: 49.2% and 1.8%, respectively); for Gram-positive cocci, correct identification in 68.4% (100% for Staphylococcus aureus and enterococci, 80% for beta-hemolytic streptococci), for Gram-negative bacteria, correct identification in 97.6%. In polymicrobial blood cultures, in 72.7% one of the pathogens was correctly identified. Results were not reliable for Gram-positive rods and yeasts. The approach was easy to implement in diagnostic routine. In cases with available clinical data and successful pathogen identification, in 51.1% our approach allowed an optimized treatment recommendation.
  
  
  CONCLUSIONS
  MALDI-TOF MS following 4h pre-culture is a valuable tool for rapid pathogen identification from positive blood cultures, allowing easy integration in diagnostic routine and the opportunity of considerably earlier treatment adaptation.
authors.affiliation: Department of Medical Microbiology, Ruhr-University Bochum, Universitaetsstrasse 150, Bochum, Germany; Institute of Medical Laboratory Diagnostics (IML) Bochum GmbH, Castroper Strasse 45, Bochum, Germany. Electronic address: rebekka.kohlmann@rub.de.
authors.forename: Rebekka
authors.initials: R
authors.lastname: Kohlmann
authors.affiliation: Institute of Medical Laboratory Diagnostics (IML) Bochum GmbH, Castroper Strasse 45, Bochum, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Hoffmann
authors.affiliation: Institute of Medical Laboratory Diagnostics (IML) Bochum GmbH, Castroper Strasse 45, Bochum, Germany.
authors.forename: Gabriele
authors.initials: G
authors.lastname: Geis
authors.affiliation: Department of Medical Microbiology, Ruhr-University Bochum, Universitaetsstrasse 150, Bochum, Germany; Institute of Medical Laboratory Diagnostics (IML) Bochum GmbH, Castroper Strasse 45, Bochum, Germany.
authors.forename: Sören
authors.initials: S
authors.lastname: Gatermann
mesh_terms: D001419:Bacteria / Q000737:chemistry / Q000145:classification* / Q000302:isolation & purification*; D001431:Bacteriological Techniques / Q000379:methods*; D001769:Blood / Q000382:microbiology*; D003470:Culture Media / Q000737:chemistry*; D042241:Early Diagnosis; D006801:Humans; D018805:Sepsis / Q000175:diagnosis*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization / Q000379:methods*; D013696:Temperature; D013997:Time Factors
chemical_list: D003470:Culture Media
keywords: Bacteremia; Blood culture; Clinical impact; MALDI-TOF; Mass spectrometry; Sepsis