This Personal View in The Lancet Infectious Diseases sets out why minimum inhibitory concentrations for antifungals are not self-interpreting. The result depends on species identification, test method and the breakpoint or epidemiological cut-off used. Clinical outcome also depends on drug exposure at the site, fungal burden, source control, biofilm and host immunity.
The authors highlight behaviours that weaken the link between a susceptible MIC and success, including tolerance, heteroresistance, persistence, trailing growth, paradoxical growth and biofilm survival. They argue that laboratories should report MICs with structured interpretation and uncertainty flags, avoid forcing a susceptible or resistant category where evidence is thin, and treat the result as a joint clinical-laboratory decision.
For the ward this means a 'susceptible' candidaemia or aspergillosis result is the start of a conversation, not the end. If the patient is not improving, the questions are the line, the source, the drug level and the organism's identity, before blaming the drug.
- Confirm the species identification before acting on an antifungal MIC; breakpoints are species-specific.
- When a patient fails therapy despite a susceptible result, check source control first, including line removal.
- Consider therapeutic drug monitoring for azoles where available, since exposure varies widely.
- Discuss unusual or borderline MICs with the microbiologist rather than reading the category alone.
- Ask whether a breakpoint or an epidemiological cut-off was used; they mean different things.
Why it matters
It challenges the habit of treating a susceptible antifungal result as proof the drug will work.
Don't overread it
This is an expert opinion piece, not a study of outcomes.
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