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Rapid identification and antimicrobial susceptibility testing of positive blood cultures using MALDI-TOF MS and a modification of the standardised disc diffusion test: a pilot study
  1. C Fitzgerald,
  2. P Stapleton,
  3. E Phelan,
  4. P Mulhare,
  5. B Carey,
  6. M Hickey,
  7. B Lynch,
  8. M Doyle
  1. Microbiology Laboratory, University Hospital Waterford, Waterford, Ireland
  1. Correspondence to Claire Fitzgerald, Microbiology Laboratory, University Hospital Waterford, Dunmore road, Waterford, X91W5R9 Ireland; claire.fitzgerald{at}


Aims In an era when clinical microbiology laboratories are under increasing financial pressure, there is a need for inexpensive, yet effective, rapid microbiology tests. The aim of this study was to evaluate a novel modification of standard methodology for the identification and antimicrobial susceptibility testing (AST) of pathogens in positive blood cultures, reducing the turnaround time of laboratory results by 24 h.

Methods 277 positive blood cultures had a Gram stain performed and were subcultured and incubated at 37°C in a CO2 atmosphere for 4–6 h. Identification of the visible growth was performed using matrix-assisted laser desorption time of flight mass spectrometry (MALDI-TOF MS). Taking a modified approach to the Clinical and Laboratory Standards Institute-standardised AST methodology, an inoculum density of 0.5 McFarland was prepared from the early growth for disc diffusion testing. The standard AST method was also performed on the 18–24 h culture.

Results 96% (n=73/76) of gram-negative organisms were correctly identified by MALDI-TOF MS. Comparative analysis of the rapid and standard AST results showed an overall interpretive category error rate of 7.7% (6.7% minor errors, 0.6% major errors and 0.4% very major errors). 100% of Staphylococcus aureus (n=41) and enterococcus isolates (n=9) were correctly identified after 4–6 h incubation. The overall AST categorical agreement was also 100% for these isolates.

Conclusions An incubation of 4–6 h directly from positive blood cultures allowed for both a rapid species identification and an antimicrobial susceptibility result approximately 24 h earlier than is possible using standard methodology.


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  • This work was presented in part, in poster format, at the British Society of Antimicrobial Chemotherapy Spring Meeting 2014, Irish Society of Consultant Microbiologists Spring Meeting 2014 and Biomedica 2014.

  • Handling editor Slade Jensen

  • Contributors CF, EP and PS designed the study. CF and EP performed the laboratory work. CF, EP and PS compiled and analysed the data. CF and PS wrote the manuscript. MD, EP, PM, BC, MH and BL performed critical reading and amendment of the manuscript prior to final submission.

  • Competing interests None declared.

  • Ethics approval As the intervention of this study was purely laboratory based without the involvement of patients, this study was deemed exempt from formal ethical approval. Samples were collected as part of standard care, and patients were not subjected to extra procedures or questions.

  • Provenance and peer review Not commissioned; externally peer reviewed.