Comparison between the Standardized Clinical and Laboratory Standards Institute M38-A2 Method and a 2,3-Bis(2-Methoxy-4-Nitro-5-[(Sulphenylamino)Carbonyl]-2H-Tetrazolium Hydroxide-Based Method for Testing Antifungal Susceptibility of Dermatophytes

In this study, we determined the utility of a 2,3-bis(2-methoxy-4-nitro-5-[(sulfenylamino)carbonyl]-2H-tetrazolium hydroxide (XTT)-based assay for determining antifungal susceptibilities of dermatophytes to terbinafine, ciclopirox, and voriconazole in comparison to the Clinical and Laboratory Standards Institute (CLSI) M38-A2 method. Forty-eight dermatophyte isolates, including Trichophyton rubrum (n = 15), Trichophyton mentagrophytes (n = 7), Trichophyton tonsurans (n = 11), and Epidermophyton floccosum (n = 13), and two quality control strains, were tested. In the XTT-based method, MICs were determined spectrophotometrically at 490 nm after addition of XTT and menadione. For the CLSI method, the MICs were determined visually. With T. rubrum, the XTT assay revealed MIC ranges of 0.004 to > 64 mu g/ml, 0.125 to 0.25 mu g/ml, and 0.008 to 0.025 mu g/ml for terbinafine, ciclopirox, and voriconazole, respectively. Similar MIC ranges were obtained against T. rubrum by using the CLSI method. Additionally, when tested with T. mentagrophytes, T. tonsurans, and E. floccosum isolates, the XTT and CLSI methods resulted in comparable MIC ranges. Both methods revealed similar lowest drug concentrations that inhibited 90% of the isolates for the majority of tested drug-dermatophyte combinations. The levels of agreement within 1 dilution between both methods were as follows: 100% with terbinafine, 97.8% with ciclopirox, and 89.1% with voriconazole. However, the agreement within 2 dilutions between these two methods was 100% for all tested drugs. Our results revealed that the XTT assay can be a useful tool for antifungal susceptibility testing of dermatophytes.