Laboratory Identification of Dermatophytes

Specimen Collection:

• Skin should be scraped from the margin of the lesion onto folded black paper.
• Hair should be plucked, not cut, from the edge of the lesion.
• Choose hairs that fluoresce under a Wood's lamp or, if none fluoresce, choose broken or scaly ones.
• Nails scrapings are obtained from the nail bed or from infected areas after the outer layers are discarded.

Direct Examination:

• A small sample of the specimen is selected for direct microscopic examination and investigated for the presence of fungal elements.
• The specimen is mounted in a small amount of potassium hydroxide or calcofluor white.
• The KOH slides are gently heated and allowed to clear for 30 to 60 minutes before examining on a light or phase contrast microscope.
• Calcofluor white slides are examined on a fluorescent microscope.

When present in the direct examination dermatophytes appear as hyaline (non-pigmented), septated elements. Hyphae rounding up into arthroconidia are diagnostic of dermatophyte involvement. Without the presence of arthroconidia the elements could also be due to a non-dermatophyte agent of onycomycosis or a small segment of a contaminating organism. When hair is involved the arthroconidia may be found on the periphery of the hair shaft (ectothrix) or within the shaft (endothrix).

Malassezia furfur infections (tinea versicolor) are diagnosed by the presence of spherical yeast cells with a single bud and a collar and short curved hyphal strands.

Culture

• Nails are scraped or minced into small pieces
• Hair is cut into short segments
• Each specimen is divided between at least two types of culture media
• The use of antibiotics will inhibit the overgrowth of bacteria and incorporation of cycloheximide will prevent the overgrowth of the rapidly growing saprophytic fungi
• The cultures are incubated at 30°C and examined frequently for 4 weeks.