Which type of allergy testing applies an allergen to the skin to detect delayed allergic reactions?

Patch testing is a method used to detect delayed allergic reactions by applying small amounts of allergens to the skin, typically on adhesive patches placed on the back or another part of the body. This testing is specifically designed to identify delayed hypersensitivity, which may not manifest immediately but develops over a period of time, usually 24 to 72 hours after exposure.

The allergens in the patch test penetrate the skin and interact with the immune system, allowing healthcare professionals to assess if a sensitivity or allergy exists based on the skin's reaction. The test is highly effective for diagnosing contact dermatitis and other delayed hypersensitivity reactions.

In contrast, intradermal testing involves injecting allergens directly into the dermis and is often used for immediate hypersensitivity reactions, such as those seen in respiratory allergies. Blood testing measures the levels of specific immunoglobulin E (IgE) antibodies but does not provide localized skin responses. Provocation testing involves administering a suspected allergen to observe reactions but is more commonly used for immediate-type allergies rather than delayed reactions.