Patch testing is one of the most diagnostic tests performed for the detection of allergies and more specifically, to establish the causes of atopic dermatitis. It is a test in which the allergen that can be applied to sensitive skin, taking into account the most common such diseases and those that the patient is usually more exposed. The most common place where the patch test is performed is the top of the back, where a few patches on the skin are set for 48 hours. During that time you should not sunbathe or performing certain sports to prevent sweating and alter the outcome of patch testing. He spent that period, will retire and reading test will be performed, although it is not confirmed whether it has caused allergic reaction until after another 48 hours.