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Dermatophytid Reaction

By

Denise M. Aaron

, MD, Dartmouth Geisel School of Medicine

Last full review/revision Sep 2021| Content last modified Sep 2021
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Dermatophytid reaction is an inflammatory reaction to dermatophytosis at a cutaneous site distant from the primary infection.

Dermatophytid (identity or id) reactions are protean; they are not related to localized growth of the fungus but rather are an inflammatory reaction to a dermatophytosis Overview of Dermatophytoses Dermatophytoses are fungal infections of keratin in the skin and nails (nail infection is called tinea unguium or onychomycosis). Symptoms and signs vary by site of infection. Diagnosis is by... read more elsewhere on the body. Lesions are typically pruritic but may manifest as

Distribution may be extensive.

Diagnosis of dermatophytid reaction is by potassium hydroxide wet mounts that are negative at the site of the id reaction and positive at the distant site of dermatophyte infection.

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A 65-year-old man comes to the office because he has had an itchy rash for the past 6 weeks that is purple in color on the flexor aspect of each wrist. On physical examination, the lesions are flat and smooth. Biopsy is performed, and lichen planus is confirmed. Which of the following laboratory studies should be done? 
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