Comparison of a glucose consumption based method with the CLSI M38-A method for testing antifungal susceptibility of Trichophyton rubrum and Trichophyton mentagrophytes.

The prevalence of dermatophytoses and the development of new antifungal agents has focused interest on susceptibility tests of dermatophytes. The method used universally for susceptibility tests of dermatophytes was published as document (M38-A) in 2002 by the Clinical and Laboratory Standards Institute (CLSI), dealing with the standardization of susceptibility tests in filamentous fungi, though not including dermatophytes especially. However, it is not a very practical method for the clinical laboratory in routine susceptibility testing. In this test, we developed a novel rapid susceptibility assay-glucose consumption method (GCM) for dermatophytes.In this study, we investigated the antifungal susceptibilities of dermatophytes to itraconazole (ITC), voriconazole (VOC), econazole nitrate (ECN) and terbinafine (TBF) by glucose consumption method (GCM), in comparison to the Clinical and Laboratory Standards Institute (CLSI) M38-A method. Twenty-eight dermatophyte isolates, including Trichophyton rubrum (T. rubrum) (n = 14) and Trichophyton mentagrophytes (T. mentagrophytes) (n = 14), were tested. In the GCM, the minimum inhibitory concentrations (MICs) were determined spectrophotometrically at 490 nm after addition of enzyme substrate color mix. For the CLSI method, the MICs were determined visually.Comparison revealed best agreement for TBF against T. mentagrophytes and T. rubrum, since MIC range, MIC50, and MIC90 were identical from two methods. However, for ITC and VOC, GCM showed wider MIC ranges and higher MICs than CLSI methods in most isolates. For ECN against T. rubrum, high MICs were tested by GCM (0.125-16 microg/ml) but not M38-A method (0.5-1 microg/ml). The overall agreements for all isolates between the two methods within one dilution and two dilutions for ITC, VOC, ECN and TBF was 53.6% and 75.0%, 57.1% and 75.0%, 82.1% and 89.3%, and 85.7 and 85.7%, respectively.Measurement of glucose uptake can predict the susceptibility of T. rubrum and T. mentagrophytes to ECN and TBF.