Azole-resistance in Aspergillus: Proposed nomenclature and breakpoints

Reports of itraconazole resistance in Aspergillus fumigatus have been more frequent since the millennium. Identifying azole resistance is critically method dependent; nevertheless reproducible methods, reflective of in vivo outcome, are now in routine use. Some isolates also have elevated MICs to posaconazole and voriconazole. Multiple mechanisms of resistance are now known to be responsible, with differing degrees of azole cross-resistance, including mutations in the Cyp51A gene at G54, L98+TR, G138, M220, G448. Establishing breakpoints for Aspergillus is probably impossible with clinical data alone for multiple reasons yet there is an urgent need to do so. We propose the following breakpoints for A. fumigatus complex using the proposed EUCAST susceptibility testing methodology: for itraconazole and voriconazole, <2 mg/L (susceptible), 2 mg/L (intermediate) and >2 mg/L (resistant); for posaconazole, <0.25, 0.5 and >0.5 mg/L respectively. We recognize that additional work will be needed to confirm these proposed breakpoints, including in vivo and clinical correlative responses. We also propose nomenclature for genotypic resistance, in the event an isolate is not cultured, typified by ITZgR, VCZgI, POSgR (G54W) indicating that the isolate has a G54W substitution with a corresponding phenotype of resistance to itraconazole and posaconazole and intermediate susceptibility to voriconazole.