Identifying organisms directly from BacT/Alert FAN plus blood culture bottles using Vitek MS in a state-wide laboratory network

Abstract

Identifying organisms directly from positive blood culture bottles using matrix-assisted laser desorption-ionisation time-of-flight mass spectrometry (MALDI-TOF MS) has many advantages to patients, clinical services, and laboratories. However, few published methods have demonstrated good performance using the current BioMérieux culture bottles and MALDI-TOF system: BacT/Alert FAN plus and Vitek MS. The effect of transporting bottles on test performance has not been assessed for any direct-from-bottle MS method. In this study, 802 positive blood culture bottles were analysed including 234 requiring inter-laboratory transport, using a method involving protein extraction with formic acid and acetonitrile. Correct identification rates were high for Staphylococcus aureus (58/58 of new diagnostic samples), Enterococcus faecalis (27/27), Gram-negative bacilli (160/176, 90.1%), and coagulase-negative Staphylococcus species (108/132, 81.8%). Three false identifications were made, none with clinical significance. For Gram-positive cocci in pairs or chains, more correct identifications were made from bottles analysed immediately compared to transported bottles (67% vs 44%, p=0.016), and longer transport time was associated with slightly lower probability of correct identification (OR 0.984 per additional hour, p=0.040). Transportation was not associated with a difference for other organism types. This technique is a vastly more cost-effective alternative to molecular techniques for rapid identification of bacteraemia isolates, and performance is minimally affected by inter-laboratory transport of bottles at ambient temperature.