Athlete's foot laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Sanjana Nethagani, M.B.B.S.[2]

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.
• Counterstains 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 very effective for tinea pedis.
• Samples that test positive with this method are re-examined using microscopy or culture methods to make a final diagnosis.