Evaluation of Caspofungin Susceptibility Testing by the New Vitek 2 AST-YS06 Yeast Card Using a Unique Collection ofFKSWild-Type and Hot Spot Mutant Isolates, Including the Five Most Common Candida Species

Abstract

ABSTRACTFKSmutant isolates associated with breakthrough or failure cases are emerging in clinical settings. Discrimination of these from wild-type (wt) isolates in a routine laboratory setting is complicated. We evaluated the ability of caspofungin MIC determination using the new Vitek 2 AST-Y06 yeast susceptibility card to correctly identify thefksmutants from wt isolates and compared the performance to those of the CLSI and EUCAST reference methods. A collection of 98Candidaisolates, including 31fkshot spot mutants, were included. Performance was evaluated using theFKSgenotype as the “gold standard” and compared to those of the CLSI and EUCAST methodologies. The categorical agreement for Vitek 2 was 93.9%, compared to 88.4% for the CLSI method and 98.7% for the EUCAST method. Vitek 2 misclassified 19.4% (6/31) of thefksmutant isolates as susceptible, in contrast to <4% for each of the reference methods. The overall essential agreement between the CLSI method and Vitek 2 MICs was 92.6% (88/95) but was substantially lower forfksmutant isolates (78.6% [22/28]). Correct discrimination between susceptible and intermediateCandida glabrataisolates was not possible, as the revised species-specific susceptibility breakpoint was not included in the Vitek 2 detection range (MIC of ≤0.250 to ≥4 mg/liter). In conclusion, the Vitek 2 allowed correct categorization of all wt isolates as susceptible. However, despite an acceptable categorical agreement, it failed to reliably classify isolates harboringfkshot spot mutations as intermediate or resistant, which was in part due to the fact that the detection range did not span the susceptibility breakpoint forC. glabrata.