Evaluation of Currently Available Laboratory Methods to Detect Terbinafine Resistant Dermatophytes Including a Gradient Strip for Terbinafine, EUCAST Microdilution E.Def 11.0, a Commercial Real-Time PCR Assay, Squalene Epoxidase Sequencing and Whole Genome Sequencing

Background: Terbinafine resistance in dermatophytes is an increasing problem worldwide. Several outbreaks of terbinafine-resistant dermatophytosis are currently occurring in India and surrounding countries, and these recent years, European countries have also been affected by this issue. Currently, antifungal susceptibility testing of dermatophytes is not routinely performed in clinical laboratories.

Objectives: Given the current situation and associated public health concerns, there is an urgent need for accurate and rapid detection of terbinafine resistance in laboratories. Therefore, we evaluated different methods currently available for the detection of terbinafine resistance in dermatophytes.

Methods: Twenty-eight strains previously identified as T. indotineae/mentagrophytes/interdigitale were concurrently characterised using terbinafine gradient strips (HiMedia), EUCAST E.Def 11.0 microdilution, the DermaGenius resistance PCR assay (PathoNostics), and SQLE sequencing. These four methods were compared to terbinafine resistance characterisation obtained by whole genome sequencing (WGS).

Results: All four evaluated methods were able to detect terbinafine resistant strains either by showing high MICs (> 0.125 μg/mL) or by detecting SQLE substitutions.

Conclusions: The gradient strips, despite questionable essential agreement with EUCAST E.Def 11.0, can be an easy, fast and cheap method to screen terbinafine resistance among dermatophytes in clinical laboratories. The DermaGenius resistance PCR assay enables rapid detection of the most common substitutions in SQLE associated with terbinafine resistance. However, its inability to precisely determine specific substitutions on SQLE or identify new ones may pose a problem in the future. These limitations can be addressed by using SQLE sequencing or whole genome sequencing (WGS).