Identification of dermatophytes by MALDI-TOF MS technology in the clinical laboratory

Conventional dermatophyte diagnosis includes microscopic and macroscopic characterization of the fungi by a skilled mycologist following fungi cultivation for several weeks. In recent years, matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) has become useful for highly specific and sensitive identification of bacteria and yeasts from clinical samples. The current study aimed to test the influence of using a protein extraction protocol on dermatophyte identification by MALDI-TOF MS. We collected scrapings of affected skin from forty-six patients diagnosed with tinea. Each sample was cultured on four tubes of Sabouraud Dextrose Agar. After 21–28 days of incubation, fungi morphology was examined by an expert mycologist according to standard microscopic and macroscopic colony characteristics. Dermatophyte identification by MS was performed following 14 (Protocol I) or 21 (Protocol II) days of incubation with direct deposit and addition of formic acid or following an Ethanol/formic acid extraction – “EtOH/FA complete extraction protocol” after 14 days (Protocol III). We found that the EtOH/FA complete extraction protocol had a 100% correct identification rate. While identification to species level was obtained in 0% and 22% of the dermatophytes following Protocol I and Protocol II, respectively, the EtOH/FA complete extraction protocol resulted in 65% identification to the species level. Considering the straightforward performance of the EtOH/FA complete extraction protocol and the high accuracy, we recommend its implementation in routine diagnosis in clinical laboratories.