Overestimation of Amphotericin B Resistance in Candida auris with Sensititre YeastOne Antifungal Susceptibility Testing: a Need for Adjustment for Correct Interpretation

Abstract

Significant variation in minimal inhibitory concentrations (MIC) has been reported for amphotericin B (AMB) and C. auris, depending on the antifungal susceptibility testing (AFST) method. Although the Sensititre YeastOne (SYO) is widely used in routine laboratory testing, data regarding its performance for the AFST of C. auris are scarce. We tested AMB against 65 C. auris clinical isolates with the SYO and the reference methodology by the Clinical and Laboratory Standards Institute (CLSI). The essential agreement (EA, ±1 dilution) between the two methods and the categorical agreement (CA) based on the Centers for Disease Control and Prevention (CDC)'s tentative breakpoint of MIC ≥ 2 mg/L were determined. The SYO wild type upper limit value (WT-UL) was determined using the ECOFFinder. The modal (range) CLSI growth inhibitory MIC was lower than the SYO colorimetric MIC [1(0.25-1) versus 2(1-8) mg/L, respectively]). The CLSI-colorimetric SYO EA was 29% and the CA was 11% (89% major errors; MaE). MaE were reduced when the SYO WT-UL of 8 mg/L was used (0% MaE). Alternatively, the use of SYO growth inhibition endpoints of MIC-1 (75% growth inhibition) or MIC-2 (50% growth inhibition) resulted in 88% CA with 12% MaE and 97% CA with 3% MaE, respectively. In conclusion, SYO overestimated AMB resistance in C. auris isolates when colorimetric MICs, as per SYO instructions and the CDC breakpoint of 2 mg/L, were used. This can be improved either by using the method-specific WT-UL MIC of 8 mg/L for colorimetric MICs or by determining growth inhibition MIC endpoints, regardless of the color. IMPORTANCE Candida auris is an emerging and frequently multidrug-resistant fungal pathogen that accounts for life-threatening invasive infections and nosocomial outbreaks worldwide. Reliable AF is important for the choice of the optimal treatment. Commercial methods are frequently used without prior vigorous assessment. Resistance to AMB was over-reported with the commercial colorimetric method Sensititre YeastOne (SYO). SYO produced MICs that were 1 to 2 twofold dilutions higher than those of the reference CLSI method, resulting in 89% MaE. MaE were reduced using a SYO-specific colorimetric wild type upper limit MIC value of 8 mg/L (0%) or a 50% growth inhibition endpoint (3%).

Keywords: Candida auris; Sensititre YeastOne; amphotericin B; antifungal susceptibility testing; epidemiological cutoff value; resistance; wild type upper limit value.

FIG 1

CLSI and Sensititre YeastOne (SYO) MIC distributions of 65 C. auris isolates by clade.

FIG 2

Distribution of colorimetric MICs (first blue well) of 205 C. auris clinical isolates from the ongoing outbreak in Greece, obtained using the Sensititre YeastOne method and analyzed using the ECOFFinder program (31).

FIG 3

Sensititre YeastOne results for 3 C. auris isolates with colorimetric MICs at (first blue well, black circle) 2 mg/L (first isolate), 4 mg/L (second isolate) and 8 mg/L (third isolate). The growth inhibition endpoints are defined as the lowest drug concentration with no growth (MIC-0, black circle), a substantial reduction of growth (approximately 75% growth inhibition, MIC-1, yellow circle), a prominent reduction of growth (approximately 50% growth inhibition, MIC-2, orange circle), and a slight reduction of growth (approximately 25% growth inhibition, MIC-3, red circle), relative to the drug-free control growth (GC).