Evaluation of MALDI biotyper interpretation criteria for accurate identification of nontuberculous mycobacteria

Abstract

Identification of mycobacteria by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) requires not only a good protein extraction protocol but also an adequate cutoff score in order to provide reliable results. The aim of this study was to assess the cutoff scores proposed by the MALDI-TOF MS system for mycobacterial identification. A total of 693 clinical isolates from a liquid medium and 760 from a solid medium were analyzed, encompassing 67 different species of nontuberculous mycobacteria (NTM). MALDI-TOF MS identified 558 (80.5%) isolates from the liquid medium and 712 (93.7%) isolates from the solid medium with scores of ≥1.60. Among these, four (0.7%) misidentifications were obtained from the liquid medium and four (0.5%) from the solid medium. With regard to species diversity, MALDI-TOF MS successfully identified 64 (95.5%) different species, while PCR-reverse hybridization (GenoType Mycobacterium CM and AS assays) identified 24 (35.8%) different species. With MALDI-TOF MS scores of ≥2, all isolates were correctly identified, and with scores in the range from 1.60 to 1.99, most isolates were correctly identified, except for Mycobacterium angelicum, M. parascrofulaceum, M. peregrinum, M. porcinum, and M. gastri. In conclusion, MALDI-TOF MS is a useful method for identifying a large diversity of NTM species. A score threshold of 1.60 proved useful for identifying almost all the isolates tested; only a few species required a higher score (≥2.00) to obtain a valid definitive identification.