Microarray-based nucleic acid assay and MALDI-TOF MS analysis for the detection of gram-negative bacteria in direct blood cultures

Objectives: To compare a microarray-based identification and resistance determination system (blood culture gram-negative [BC-GN]; Nanosphere, Northbrook, IL) with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for direct blood cultures (BCs). Methods: BC-GN and MALDI-TOF MS assay results from direct BCs were compared with conventional test results after pure culture. Results: Among 124 BCs, 130 gram-negative rods (GNRs), including six cultures with mixed GNRs (117 bacteria were covered by the BC-GN panel), were detected. The BC-GN test presented 116/117 (99.1%) concordance for the identification of targeted GNRs. Among the six polymicrobial BCs, 10 targeted GNRs were correctly identified. Among the 100 BCs tested by MALDI-TOF MS, 88/106 (86.7%) GNRs were correctly identified, and 18 GNRs were not identified. Among the six polymicrobial samples, seven of 12 GNRs (58.3%) were correctly identified. Conclusions: The BC-GN assay exhibited superior performance compared with MALDI-TOF MS for the identification of targeted GNRs in direct BCs, particularly in polymicrobial samples.