Patch testing is a way of identifying whether a substance that comes in contact with the skin is causing inflammation of the skin. There are two types of contact dermatitis: irritant contact dermatitis and allergic contact dermatitis.

An irritant substance is one that would cause inflammation in almost every individual if it was applied in sufficiently high concentration for long enough. An irritant reaction is caused by the direct contact of an irritant substance with the skin and does not involve the immune system.

An allergic reaction is specific to the individual and to a substance (or a group of related substances) called an allergen. Allergy is a hypersensitivity (oversensitivity) to a particular substance, and always involves the immune system. All areas of skin that are in contact with the allergen develop the rash. The rash will disappear if you avoid contact with the substance.

Patch testing can help to differentiate between the two. The test involves the application of various test substances to the skin under adhesive tape that are then left in place for 48 hours. The skin is then examined a further 48 hours later for any response. This can help the doctor decide which allergens you are allergic to and identify those that could be aggravating your dermatitis. The doctor will then be able to advise how you can avoid the allergens.

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