Why is a patch (predisposition) test recommended before applying certain haircolors?

A patch (predisposition) test is recommended before applying haircolors that can cause allergic reactions (such as many permanent colors containing certain dyes) to check whether the client is sensitive or allergic to the product. A small amount is applied to the skin (often behind the ear or inner elbow) in advance, and the area is checked for a reaction. If a reaction occurs, the service should not be performed. The patch test protects the client from a potentially serious allergic reaction and is an important safety step.