Evaluation of Two SDS-Based Preparation Methods for Direct Identification and Antifungal Susceptibility Testing of Yeasts from Positive Blood Cultures.

Objective: We evaluated two SDS-based preparation methods for direct identification of yeasts using MALDI-TOF MS analysis and antifungal susceptibility testing (AFST).

Methods: A total of 149 residual monomicrobial blood culture bottles (BCs) were included. For direct identification via MALDI-TOF MS analysis from positive BCs, two in-house methods with or without a 3-hr incubation step were evaluated. 55 samples were also prepared using a Sepsityper kit. In addition, to investigate the effects of differences in Candida spp. concentrations, simulated samples were incubated and colonies were counted. Direct AFST was performed using Sensititre YeastOne.

Results: Correct identification rate was 65.8% for the saponin+0.5% SDS method and 70.5% for the 20% SDS after 3-hr incubation, respectively. For the 55 samples evaluated using the Sepsityper kit, both in-house methods showed significantly higher identification rates. For the simulated samples, Candida albicans had the lowest colony counts at the time of the positive signal in the BCs. Essential agreement was above 90% for all cases except for itraconazole using the pellet prepared by the 20% SDS method. Categorical agreement was above 90% for all cases.

Conclusion: Our newly developed preparation method using 20% SDS demonstrated superior performance to a commercial kit, and can therefore be used for direct identification and AFST.