Patch tests or epicutaneous tests are diagnostic procedures to diagnose contact dermatitis, which are allergic reactions mediated by cells (also called type IV reactions).
The test is performed by placing some patches with different substances (metals, cosmetic ingredients, dyes, rubber chemicals, foods, medicaments, …) on the skin of the back. These patches are removed after 48 hours, but the final reading is performed after 72-96 hours. During the first 48 hours the patches must remain in place. Care must be taken regarding baths, sweating, etc, as this can make the patches move. In severe reactions, the patient cannot stand the patches for 48 hours, and they must be removed earlier (especially patches with mercury).
After the patches are removed, the skin is examined for reactions of redness, itching, papules or vesicles. If reactions appear, the test is positive, which means that the patient is sensitised to that specific substance and must avoid it.
The number of patches depends on the suspected allergens or substances one wants to investigate. It may vary from just one or two to several dozens. The patches can be placed one by one, although there are companies that manufacture prepared patches with over 30 substances.
Some drugs, such as systemic corticosteroids or immunomodulators may change the results of the test. If you are scheduled for patch testing you should inform your doctor about all the medication you are receiving.