Gradient diffusion strips for detecting azole resistance in Aspergillus fumigatus sensu lato

Studies comparing gradient diffusion strips (GDSs) and the EUCAST E.Def 9.4 microdilution method are scarce, thwarted by a low number of isolates, and restricted to selected antifungal agents.We evaluated the performance of GDSs to detect azole resistance in A. fumigatus, including cryptic species.A. fumigatus sensu stricto (n = 89) and cryptic species (n = 52) were classified as susceptible or resistant to itraconazole, voriconazole, posaconazole and isavuconazole (EUCAST E.Def 9.4; clinical breakpoints v10). A. fumigatus sensu stricto azole-resistant isolates had the following cyp51A gene mutations: TRFor A. fumigatus sensu stricto, itraconazole MICs1.5 mg/L, voriconazole0.38 mg/L, posaconazole0.75 mg/L, and isavuconazole0.5 mg/L correctly separated resistant from susceptible isolates with two exceptions. Considering the aforementioned cut-off MICs, sensitivity/specificity values of GDSs to detect azole resistance were: itraconazole (97%/100%), voriconazole (97%/100%), posaconazole (97%/100%) and isavuconazole (93.3%/100%). For cryptic species isolates, voriconazole MICs1 mg/L and isavuconazole0.75 mg/L separated resistant isolates from susceptible isolates with 15 and 27 exceptions, respectively. Considering the aforementioned cut-off MICs, sensitivity/specificity values were as follows: voriconazole (68.1%/100%) and isavuconazole (25%/100%). For itraconazole and posaconazole, it was not possible to establish cut-off values.We set tentative cut-off MIC values to correctly spot resistant Aspergillus fumigatus sensu stricto isolates using GDSs. The performance against cryptic species was poor.