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 four-well agar-screening method based on the European Committee on Antimicrobial Susceptibility Testing (EUCAST) definitive document (E.Def) 10.2 method was evaluated in five centres using a panel of terbinafine wild-type (WT) and non-WT Trichophyton isolates with the recently determined tentative epidemiological cut-offs of the E.Def 11.0 method.

METHODS: Forty-two Trichophyton isolates, all WT to itraconazole, 17 molecularly characterized 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 four-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 to 7 days at 25 to 28°C. Visual growth comparable with 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 5 days, whereas 6 to 10 of 42 isolates required 7 days of incubation. All isolates were correctly classified as WT to itraconazole by all five centres. The sensitivity (median [range among centres]) in detecting terbinafine non-WT isolates was 94% to 100% (95% CI: 79-100%), whereas the specificity for detecting WT isolates was 100%. Sensitivity and specificity were high across different species. Among the discrepancies, one false WT was observed with a T. rubrum strong mutant in one centre. WT T. indotineae grew on terbinafine 0.016 mg/L.

DISCUSSION: 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.