Patch testing involves testing for different types of allergic reactions that are not detected during normal skin testing. The classic example is nickel allergy, but it is also used commonly for patients who may be having contact dermatitis reactions to their cosmetics or shampoo. It can also be used for further evaluation of eosinophilic esophagitis. This procedure involves placement of the patch containing the allergen on a clear area of the skin, usually the back. The patch is then removed 48 hours later and the test is read. The patch is not to be removed during this 48 hour period and should have little or no contact with water. There may be some mild itching and irritation while wearing the patch.