Patch testing is carried out on people with contact dermatitis. The test is performed by a dermatologist to find out which substances provoke an allergic reaction. Possible allergens (substances that can cause an allergic reaction) are diluted and placed on small strips or discs. The test discs are then stuck to the skin using inert (nonallergenic) tape. After 48 hours, the discs are removed and the skin underneath them is examined. A red, inflamed patch indicates a positive reaction to an allergen. The tested area is examined again 2 days later to check for delayed reactions.
Minute quantities of test substances are placed on small discs. The discs are stuck with inert tape to an inconspicuous area of the skin, usually on the back.
When the discs are removed from the skin after 48 hours, positive reactions to allergens appear as red patches. In some cases, reactions may take longer to appear.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.