Antifungal resistance can be defined as in vitro resistance where organisms are not inhibited by a given antifungal in standardised antifungal susceptibility tests, however the correlation between these results and clinical outcomes is often unclear. Of more concern is in vivo resistance, which is a lack of response to an antifungal treatment in a patient.

Intrinsic resistance is an inherent property of the fungus, is predictable and stable, whereas acquired resistance develops during therapy or in the environment, is unpredictable and not stable. This form of resistance is of the greatest clinical concern.

Data on antifungal resistance is relatively limited in the UK with no systematic collection of information and very patchy coverage. However, data that are available suggest that levels of resistance to both azoles and echinocandins in Candida species are low and stable. In contrast, in some parts of the country azole resistance in Aspergillus fumigatus is higher, but in most places it is low and again fairly stable.

Emerging antifungal resistance has been seen in some countries for Candida glabrata to echinocandins and for Aspergillus fumigatus to azoles, so although these are not currently a widespread problem in the UK, vigilance is required. Other organisms that have been problematic include Candida auris which can be relatively resistant to several classes of antifungals and is also an infection control challenge. Less common moulds including Fusarium, Scedosporium and mucoraceous moulds may be quite resistant to antifungals and they represent an ongoing therapeutic problem.