In vitro susceptibility and provisional epidemiological cutoff values for eight antifungal agents against Trichophyton mentagrophytes and Trichophyton interdigitale : a multicenter study from China

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ABSTRACT Dermatophytosis is a highly prevalent superficial fungal infection, yet antifungal susceptibility testing results remain difficult to interpret because clinical breakpoints are unavailable, and epidemiological cutoff values (ECVs) are still limited for most dermatophyte–drug combinations. We conducted a multicenter study of 305 Trichophyton mentagrophytes and Trichophyton interdigitale isolates collected between 2019 and 2024 at six tertiary hospitals across geographically diverse regions of China. Antifungal susceptibility testing was performed using the CLSI M38 broth microdilution method under standardized quality control, and provisional ECVs were derived following CLSI M57 guidance. For T. mentagrophytes , fluconazole showed a non-unimodal MIC distribution and was not assigned a provisional ECV. The remaining seven agents yielded ECVs of 1 µg/mL for ciclopirox olamine and griseofulvin, 0.5 µg/mL for itraconazole, 0.06 µg/mL for voriconazole, 0.25 µg/mL for posaconazole and amorolfine, and 0.016 µg/mL for terbinafine. For T. interdigitale , fluconazole showed a complex non-unimodal distribution, whereas itraconazole and posaconazole showed unstable low-MIC shoulders precluding ECV estimation. The remaining five agents yielded ECVs of 1 µg/mL for ciclopirox olamine, 0.25 µg/mL for griseofulvin, 0.12 µg/mL for voriconazole and amorolfine, and 0.016 µg/mL for terbinafine. Isolates above the provisional ECVs were uncommon for most combinations (≤2.5%), except for griseofulvin against T. interdigitale (20.5%). Overall, the two species showed distinct MIC distribution profiles, underscoring the value of species-level analysis and continued surveillance. These multicenter CLSI-based data and provisional ECVs provide a contemporary baseline for species-specific interpretation of dermatophyte susceptibility data in China. IMPORTANCE Reliable interpretation of dermatophyte antifungal susceptibility testing remains limited because clinical breakpoints and epidemiological cutoff values (ECVs) are unavailable for most dermatophyte–drug combinations. This is particularly important for the Trichophyton mentagrophytes / Trichophyton interdigitale species complex, which is increasingly implicated in refractory dermatophytosis and may display species-specific susceptibility patterns. In this multicenter study, we established provisional epidemiological cutoff values for most tested species–agent combinations and showed that these two closely related species differ in MIC distribution profiles and in the detection of non-wild-type isolates. These data provide practical laboratory thresholds for recognizing isolates with reduced susceptibility, highlight the limitations of complex-level interpretation, establish a contemporary baseline for dermatophyte susceptibility surveillance in China, and may help support laboratory-informed antifungal decision-making.

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