OBJECTIVES: Terbinafine resistance is increasingly reported in Trichophyton, rendering susceptibility testing particularly important in non-responding cases. We performed a multicentre evaluation of six EUCAST-based methods. METHODS: Ten laboratories susceptibility tested terbinafine, itraconazole, voriconazole and amorolfine against a blinded panel of 38 terbinafine WT and target gene mutant isolates. E.Def 9.3.1 modifications included: medium with/without addition of chloramphenicol and cycloheximide (CC), incubation at 25°C to 28°C for 5-7 days and three MIC endpoints [visually and spectrophotometrically (90%/50% inhibition)], generating 7829 MICs. Quality control (QC) strains were Aspergillus flavus ATCC 204304 and CNM-CM1813. Eyeball, ECOFFinder (where ECOFF stands for epidemiological cut-off) and derivatization WT upper limits (WT-ULs), very major errors (VMEs; mutants with MICs ≤WT-ULs) and major errors (MEs; WT isolates with MICs >WT-ULs) were determined. RESULTS: MICs fell within the QC ranges for ATCC 204304/CNM-CM1813 for 100%/96% (voriconazole) and 84%/84% (itraconazole), respectively. Terbinafine MICs fell within 0.25-1 mg/L for 96%/92%, suggesting high reproducibility. Across the six methods, the number of terbinafine MEs varied from 2 to 4 (2.6%-5.2%) for Trichophyton rubrum and from 0 to 2 (0%-2.0%) for Trichophyton interdigitale. Modes for WT and mutant populations were at least seven 2-fold dilutions apart in all cases. Excluding one I121M/V237I T. rubrum mutant and two mixed WT/mutant T. interdigitale specimens, the numbers of VMEs were as follows: T. rubrum: CC visual, 1/67 (1.5%); CC spectrophotometric 90% inhibition, 3/59 (5.1%); and CC spectrophotometric 50% inhibition, 1/67 (1.5%); and T. interdigitale: none. Voriconazole and amorolfine MICs were quite uniform, but trailing growth complicated determination of itraconazole visual and spectrophotometric 90% inhibition MIC. CONCLUSIONS: Although none of the laboratories was experienced in dermatophyte testing, error rates were low. We recommend the CC spectrophotometric 50% inhibition method and provide QC ranges and WT-ULs for WT/non-WT classification.

CIBER Enfermedades Respiratorias CIBERES Madrid Spain

Clinical Microbiology and Infectious Diseases Hospital General Universitario Gregorio Marañón Madrid Spain and Instituto de Investigación Sanitaria Gregorio Marañón Madrid Spain

Clinical Microbiology Laboratory Attikon University Hospital Medical School National and Kapodistrian University of Athens Athens Greece

Department of Clinical Medicine Copenhagen University Copenhagen Denmark

Department of Clinical Microbiology Centre for Interdisciplinary Research on Medicines University of Liège Liège Belgium

Department of Clinical Microbiology Karolinska University Hospital Stockholm Sweden

Department of Clinical Microbiology Rigshospitalet Copenhagen Denmark

Department of Laboratory Medicine and National Reference Centre for Mycosis University Hospitals Leuven Leuven Belgium

Department of Medical Mycology Vallabhbhai Patel Chest Institute University of Delhi Delhi India

Department of Microbiology Immunology and Transplantation KU Leuven Leuven Belgium

Department of Microbiology University Hospital Olomouc Czech Republic

Dipartimento di Scienze Biomediche e Sanità Pubblica Università Politecnica delle Marche Ancona Italy

Institute of Hygiene and Medical Microbiology Medical University of Innsbruck Innsbruck Austria

Malattie Infettive Ospedali Riuniti Marche Nord Pesaro Italy

Parasitology Mycology Unit Microbiology Department Georges Pompidou European Hospital University of Paris Paris France

Unit for Mycology Statens Serum Institut Copenhagen Denmark

J Antimicrob Chemother. 2022 Mar 31;77(4):1212-1213. doi: 10.1093/jac/dkac004. PubMed

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