Optimization of the score cutoff value for routine identification of Staphylococcus species by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry.

Staphylococcus species are important pathogens. We evaluated 2 score cutoffs (2.0 and 1.7) and the replicate number (a single or a duplicate test) on the identification of staphylococci using the Bruker matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS). A collection of 440 clinical isolates (11 species) and 144 reference strains (36 species) was evaluated. For clinical isolates using a cutoff of 2.0 and duplicate tests, the rates of species, genus, and unreliable identifications were 93.4%, 5.7%, and 0.9% respectively, while the respective values were 99.3%, 0.2%, and 0.5% when the cutoff was 1.7. The species identification rates were significantly higher (P<0.01) when a cutoff of 1.7 or a duplicate test was used. Similar results were obtained for reference strains. In conclusion, a cutoff of 1.7 and duplicate tests are recommended for identification of staphylococci using MALDI-TOF MS.