Id reactions associated with stasis dermatitis are usually symmetrical and involve the forearms, thighs, legs, trunk, face, hands, neck and feet in descending order of frequency. Id reactions are usually preceded by an exacerbation of the preexisting dermatitis, induced by infection, scratching, or inappropriate therapy. Reactions have been previously reported after x-ray treatment of tinea capitis. Vesicles may be present on the hands or feet. Fingers may be tender.
Lesions are, by definition, at a site distant from the primary infection or dermatitis.
They are usually distributed symmetrically. Papulonecrotic tuberculid and some other tuberculids are now thought to be true cutaneous forms of tuberculosis and not id reactions because of the identification by polymerase chain reaction of Mycobacterium tuberculosis in lesions. Strict criteria include a proven dermatophyte infection and a positive skin test to a group-specific trichophytin antigen. Absence of fungi in the dermatophytid lesions and clearing of the dermatophytid after the fungus is eradicated are necessary to make a definitive diagnosis of a dermatophytid reaction. Small numbers of eosinophils may be present in the dermal infiltrate.