The diagnosis is made on clinical grounds. Differentiation between the above-mentioned disorders, however, may be extremely difficult or impossible at times. More than one disorder may be present at a time, such as atopic dermatitis complicated by irritant dermatitis. One should try to elicit an occupational or hobby history that may indicate a specific causative agent. A history of an atopic diathesis (atopic dermatitis, allergic asthma, allergic rhinitis) in the patient or other family members should be sought as well. If an allergen is suspected, referral to a dermatologist for skin patch testing can help determine the exact agent responsible. A fungal infection should always be considered in the differential diagnosis. A potassium hydroxide preparation can exclude this possibility. A dermatophytid eruption is another possibility. In this disorder, the hands break out in a dermatitic eruption as a result of a dermatophyte infection of the feet. Checking the patient's feet should be part of the clinical examination. Finally, psoriasis, lichen planus, pityriasis rubra pilaris, keratodermas, and autoimmune bullous diseases should be considered in the differential diagnosis. Rarely is a biopsy indicated, as it cannot differentiate between the different types of dermatitis.
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