Antifungal susceptibility testing across fungi: why MICs vary and methods diverge and what MIC can miss.

BACKGROUND

Antifungal susceptibility testing is central to antifungal stewardship; however, MIC results can be variable and may require careful interpretation because endpoints can be ambiguous, clinical breakpoints are lacking for many species-drug combinations, and results may differ between methods and platforms.

OBJECTIVE

To explain why fungal MICs can vary within and between methods, summarize major sources of inter-method discordance, highlight clinically relevant MIC 'blind spots' (tolerance, heteroresistance, and rapid adaptive states), and provide practical guidance for routine interpretation and confirmation.

SOURCES: Narrative minireview based on representative PubMed literature covering reference standards (CLSI/European Committee on Antimicrobial Susceptibility Testing broth microdilution), nonreference formats (gradient diffusion), commercial platforms, reproducibility and endpoint studies, and mechanistic work on tolerance, heteroresistance, and genome plasticity.

CONTENT: Variation in fungal MICs arises from both methodological and biological factors, including endpoint subjectivity, inoculum effects, incubation conditions, trailing growth, and method-specific readout rules. Discordance between methods is greatest near interpretive thresholds and in organism-drug combinations with poorly defined endpoints. Standard MIC testing may also miss clinically relevant phenotypes such as tolerance, heteroresistance, and transient adaptive resistance states.

IMPLICATIONS: MICs should be interpreted as standardized measurements with known uncertainty. Expect small, dilution-step variation; standardize inoculum preparation and reading rules; report the testing method and interpretive framework; handle near-cutoff (epidemiological cutoff/clinical breakpoint/area of technical uncertainty) results cautiously; and confirm clinically critical or discordant findings using a reference method or an independent confirmatory method. Where available and clinically relevant, targeted resistance-mechanism testing can support confirmation (e.g. FKS hotspot sequencing for echinocandin resistance in Candida; CYP51A sequencing for azole resistance in Aspergillus fumigatus).