Multicentre evaluation of a EUCAST-based agar-screening method for terbinafine and itraconazole susceptibility of Trichophyton spp.

Objectives: Resistance in Trichophyton species has become a global public health issue. Here, a 4-well agar-screening method based on the EUCAST E.Def 10.2 method was evaluated in five centres using a panel of terbinafine wildtype (WT) and non-WT Trichophyton isolates with the recently determined tentative ECOFFs of the EUCAST E.Def 11.0 method.

Methods: Forty-two Trichophyton isolates, all WT to itraconazole, 17 molecularly characterised terbinafine non-WT and 25 WT (non-WT/WT: 11/9 T. rubrum, 5/6 T. indotineae, 1/6 T. interdigitale and 0/4 T. mentagrophytes) were tested in five centres using 4-well plates containing terbinafine 0.016 mg/L and 0.125 mg/L; itraconazole 1 mg/L and drug-free agar, respectively. Plates were inoculated (25 μL, 0.5 McFarland) and incubated for 5-7 days at 25-28 °C. Visual growth comparable to drug-free control ignoring faint growth/pinpoint colonies indicated non-WT phenotype. Sensitivity and specificity in detecting Trichophyton non-WT isolates were calculated.

Results: Most isolates produced sufficient growth after 5d whereas 6-10/42 required 7d of incubation. All isolates were correctly classified as WT to itraconazole by all 5 centres. The sensitivity (median (range among centres)) in detecting terbinafine non-WT isolates was 94%-100% (95% confidence interval 79%-100%) whereas the specificity for detecting WT isolates was 100%. Sensitivity and specificity were high across different species. Among the discrepancies, 1 false WT was observed with a T. rubrum strong mutant in 1 centre. WT T. indotineae grew on terbinafine 0.016 mg/L.

Conclusions: The multicentre evaluation confirmed that the agar-screening method was sensitive and specific for detecting terbinafine non-WT Trichophyton isolates and correctly identified itraconazole WT strains.