A susceptible antifungal MIC is a starting point, not a verdict

A Personal View in Lancet Infectious Diseases argues that antifungal susceptibility testing, now more widely available, is often read as a simple susceptible or resistant label when it should not be. A minimum inhibitory concentration measures drug activity under laboratory conditions; whether the patient responds depends on species identification, the test method, the breakpoint used, drug levels achieved, the site of infection, fungal burden, source control, biofilm and the host's immunity.

The authors describe phenomena such as tolerance, heteroresistance, trailing and paradoxical growth, and biofilm-associated survival that can break the link between a susceptible MIC and cure. They propose that reports carry structured interpretation and uncertainty flags, and that the microbiologist and clinician read the result together.

This is expert opinion, not a trial. Its value at the bedside is a check against a common error: continuing a failing antifungal because the report says susceptible, when the real problem is an undrained collection, an infected line or inadequate drug exposure.

• When a patient fails antifungal therapy despite a susceptible MIC, look for an undrained source or retained line.
• Check drug levels where therapeutic monitoring is available, for example with voriconazole or posaconazole.
• Confirm species identification, since breakpoints are species-specific.
• Discuss refractory fungal infections with the microbiologist rather than relying on the printed category.

Why it matters

A susceptible label can hide the real reason a fungal infection is not responding.

Don't overread it

This is an expert opinion piece; it does not report new outcome data.

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