When the fungus is susceptible, but the patient is not: reading antifungal susceptibility from bench to bedside

Antifungal susceptibility testing is increasingly available and is central to the management of invasive, refractory, recurrent, and breakthrough fungal infections.

However, minimum inhibitory concentrations (MICs) are not self-interpreting results. They measure in vitro antifungal activity under defined laboratory conditions, but clinical outcome is shaped by species identification, method, breakpoint or epidemiological cutoff values, availability, pharmacokinetic and pharmacodynamic exposure, infection site, fungal burden, source control, biofilm formation, and host immunity.

Non-classic fungal drug-response phenotypes, including tolerance, heteroresistance, persistence, adaptive resistance, trailing growth, paradoxical growth, and biofilm-associated survival, can further weaken the link between a susceptible MIC and therapeutic success. In this Personal View, we argue that antifungal susceptibility testing should be understood as a shared clinical-laboratory interface rather than as a standalone laboratory result.

The microbiologist makes the MIC interpretable by defining organism, method, interpretive threshold, confidence, and need for confirmation; the clinician makes it clinically useful by integrating exposure, site, source control, toxicity, interactions, and clinical trajectory.

Future reports should therefore move from isolated MICs towards structured interpretation, uncertainty flags, and interpretive guidance, while avoiding forced susceptible-resistant categorisation when evidence is incomplete.

Such reporting can strengthen stewardship, surveillance, and patient-level decisions across tertiary, routine, and resource-constrained settings, without overstating what MICs can predict alone at the bedside.