Patch testing is useful for evaluation of allergic contact dermatitis. Allergic contact dermatitis occurs when an individual's immune system reacts to a specific allergen. An example would be a poison ivy rash.

Allergic contact dermatitis may be the primary cause of a person's eczema or it may complicate other types of eczema, making treatment more difficult. Patch testing is a way of identifying whether a substance that comes into contact with the skin is causing eczema or inflammation of the skin. Once an allergen has been identified through patch testing, avoiding the offending substance will usually result in clearance of the rash. Patch tests are not the same as skin prick or scratch tests, which are used to diagnose hay fever or food allergies. Skin prick tests have limited value for evaluating eczema.

The patch testing process (shown below) involves the application of small amounts of commonly reacting materials to your back under adhesive tape that are left in place for 48 to 72 hours.