Patch testing is a way of identifying whether a substance that comes in contact with the skin is causing inflammation of the skin (contact dermatitis). There are two types of contact dermatitis: irritant contact dermatitis and allergic contact dermatitis. If you have a dermatitis that started recently or if you have a persistent or unusual eczema, your dermatologist may suspect you have an allergic contact dermatitis.
If you have been using a medication or cosmetic on the lower legs, hands, face, ears, eyes, anal or genital region for a while and subsequently developed dermatitis, your doctor may suspect one of these topical agents as the cause. Patch testing is the only way that your doctor can prove that a substance is inciting your dermatitis. The technique involves taping patches to the back containing numerous chemical allergens to be tested. These patches are left on the skin for 48 hours and then removed. Focal areas of dermatitis signal a positive reaction to an offending chemical. Once an allergen is identified, avoiding it should help cure your dermatitis.