EUCAST has defined ECOFFs and breakpoints for C. auris. The breakpoints for amphotericin B are S ≤0.001 mg/L; R>2 mg/L, thus classifying the entire wild-type population (ECOFF: 2 mg/L) as susceptible, increased exposure (“I”). Antifungal susceptibility testing of amphotericin B against C. auris using commercial tests has been associated with overestimation of resistance due to greater variability and wider MIC distributions and/or higher MICs in several studies, which are briefly summarised below. In most of these studies, a cutoff of 1 mg/L was used to classify resistance; however, this warning remains valid when applying the defined EUCAST breakpoint of 2 mg/L. Consequently, EUCAST warns against the use of the following tests for amphotericin B MIC testing of C. auris with S/R breakpoints of S ≤ 0.001 mg/L / R > 2 mg/L: Etest gradient strip, with double swab inoculation and or 48 h incubation, Vitek 2 system and Sensititre Yeast One.
Consequently, EUCAST warns against the use of the following tests for amphotericin B MIC testing of C. auris with S/R breakpoints of S ≤ 1 mg/L / R > 1 mg/L:
- Etest, (Etest, bioMerieux), gradient strip
- Mic Test Strip (MTS, Liofilchem), gradient strip
- Vitek 2 system (bioMerieux)
- Sensititre Yeast One (Thermofisher)