Identification of Filamentous Fungi: An Evaluation of Three MALDI-TOF Mass Spectrometry Systems.

Background: Identification of filamentous fungi still poses a major challenge to laboratories. Matrix assisted laser desorption/ionisation time of flight mass spectrometry (MALDI-TOF MS) is a promising tool, since it offers low-cost and fast results. Different MALDI-TOF MS systems are available for routine laboratories. This is a comprehensive head-to-head comparison of three devices and the respective reference spectrum databases.

Materials and methods: A set of 77 pre-characterised isolates of filamentous fungi was measured (in duplicates) parallelly with three MALDI-TOF MS systems after 24 h, 48 h and 72 h of incubation: (1) Biotyper smart ('BT', Bruker Daltonics),(2) EXS2600 ('EXS', Zybio) and (3) VITEK MS PRIME ('VITEK', bioMérieux).

Results: After three measurements, no valid results at the species level ('green category') were obtained for 18%, 21% and 14% of isolates by BT, EXS and VITEK. Depending on the MALDI-TOF MS system, validity rates ranged from 58%-82% for the different time points. Correct and valid results were obtained for 82%, 73% and 81% of isolates by BT, EXS and VITEK. BT was the system that required the most duplicate measurements. EXS displayed the highest rate of misidentification events. VITEK had the highest rate of unidentified isolates, which were featured within its database.

Conclusion: Based on our experience, all three devices proved to be suitable for routine diagnostics. The timepoint of measurement had a major impact on the quality of analysis and should be considered by the user. Lowering the validity cut-off levels might increase the performance of the EXS system.