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

Background: Antifungal susceptibility testing (AFST) 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, rapid adaptive states), and provide practical guidance for routine interpretation and confirmation.Sources: Narrative mini-review based on representative PubMed literature covering reference standards (CLSI/EUCAST broth microdilution), non-reference formats (gradient diffusion), commercial platforms, reproducibility and endpoint studies, and mechanistic work on tolerance, heteroresistance, and genome plasticity.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 (ECOFF/CBP/ATU) 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).