Athlete's foot laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Athlete's foot can usually be diagnosed by visual inspection of the skin, but where the diagnosis is in doubt direct microscopy of a potassium hydroxide preparation (known as a KOH test). A KOH preparation is performed on skin scrapings from the affected area. The KOH preparation has an excellent positive predictive value, but occasionally false negative results may be obtained, especially if treatment with an anti-fungal medication has already begun.

Laboratory Findings

Microscopy

• Skin scrapings from affected areas of toes, web spaces, soles including scales can be taken.*To diagnose vesicular tinea pedis, the entire roof of the bulla is removed and mounted for a high yield of fungus.
• Scrapings are stained with KOH and viewed under a microscope.
• Coounterstains may be used to better visualize the fungus such as chorazol black which is specific for chitin.
• Dimethyl sulfoxide may be used to dissolve keratin in the scrapings.

Biopsy

• A biopsy of the affected skin (i.e. a sample of the living skin tissue) can be taken for histological examination.

Culture

• A culture is usually done to identify the dermatophyte species.
• Mycobiotic agar, dermatophyte test medium, Sabouraud's agar etc are used as culture medium.

Immunochromatography

• Immunochromatography kits use a solution to extract dermatophyte antigens from a tissue sample and detect their reaction with monoclonal antibodies to these antigens.
• This method is effective for tinea unguium but not for tinea pedis.
• Samples that test positive with this method are re-examined using microscopy or culture methods to make a final diagnosis.