Identification of pathogenic microorganisms directly from positive blood vials by matrix‐assisted laser desorption/ionization time of flight mass spectrometry

Matrix‐assisted laser desorption/ionization time of flight mass spectrometry ( MALDI ‐ TOF MS ) is a promising and fast method for identifying fungi and bacteria directly from positive blood cultures. Various pre‐treatment methods for MALDI ‐ TOF MS identification have been reported for this purpose. In‐house results for identification of bacterial colonies by MALDI ‐ TOF MS using a cut‐off score of 1.5 did not reduce the diagnostic accuracy compared with the recommended cut‐off score of 1.8. A 3‐month consecutive study of positive blood cultures was carried out in our laboratory to evaluate whether the Sepsityper™ Kit (Bruker Daltonics) with Biotyper 2.0 software could be used as a fast diagnostic tool for bacteria and fungi and whether a 1.5 cut‐off score could improve species identification compared with the recommended score of 1.8. Two hundred and fifty‐six positive blood vials from 210 patients and 19 blood vials spiked with fungi were examined. Using the cut‐off score of 1.8, 81% Gram‐negative bacteria were identified to the species level compared to 84% using a cut‐off score of 1.5. For Gram‐positive bacteria 44% were identified to the species level with a cut‐off of 1.8 compared to 55% with the value of 1.5. The overall identification rate was 63% (cut‐off 1.5) and 54% (cut‐off 1.8). Seventy‐seven per cent of fungal species were identified with both log scores. MALDI ‐ TOF MS was in this study found to be a powerful tool in fast diagnosis of Gram‐negative bacteria and fungi and to a lesser degree of Gram positives. Using 1.5 as cut‐off score increased the diagnosis for both Gram‐positives and ‐negatives bacteria.