Comparison of different methods to determine the viability of dermatophytes in skin scrapings

Abstract

One of the most prevalent infectious dermatoses seen in dermatology outpatient clinics is superficial fungal infections of the skin, hair, and nails. Filamentous fungi, dermatophytes, which invade and colonise the keratinised layers of the skin, are the main cause of skin fungal infection. Sometimes there is a discrepancy between the results of microscopy and culture for dermatophytes. If the microscopy of scrapings is positive due to presence of non-viable fungus, no growth will take place in culture. Culture is the method used to determine the presence of viable dermatophytes. However, the problem with culture is the long time, up to 4 weeks, that is required for the growth of fungus to take place. Aim of the present study is to develop a simple, rapid method(s) for identification of live dermatophytes in skin scrapings. The novel methods used for identification of live dermatophytes in skin scrapings were direct microscopic examination using periodic acid-Schiff (PAS) stain, trypan blue stain, neutral red stain, and methylene blue stain; and their results were compared with culture on Sabouraud Dextrose Agar (SDA) and Dermatophyte Test Medium (DTM). Out of a total 91 samples, fungi were isolated in 81 (89%) in SDA and 66 (72.5%) in DTM. Sixty-eight (74.7%) showed evidence of viable fungi on microscopic examination after staining with PAS and 77 (84.6%) with trypan blue. In 84 patients, the culture was positive either by SDA or DTM or both culture media, out of which 64 (76.2%) were detected viable on staining with PAS and 74 (88.1%) on staining with trypan blue. Three skin samples that failed to grow on both SDA and DTM demonstrated viable fungi on microscopic examination after staining with PAS. Four skin samples that failed to grow on both SDA and DTM demonstrated viable fungi on microscopic examination after staining with trypan blue. Taking culture positivity in SDA or DTM or both media, as the gold standard, the sensitivity of trypan blue stain was 88.1% and specificity was 57.1%. The sensitivity of PAS stain was 76.2% and specificity was 42.9%. Although culture remains the gold standard to identify viable dermatophytes; trypan blue and PAS stains, as described, may be used as simple and rapid alternatives to culture.