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.

Irritant 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.

Allergic contact dermatitis
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.

Skin tests carry a risk that you will experience an allergic reaction when exposed to the allergens. Some sensitive persons will (rarely) experience a life-threatening anaphylactic reaction.

Penicillin (and closely related medications) are the only drugs that can be tested using skin tests. (Skin tests for other drugs are at best non-informative and can be dangerous). .