Your doctor can usually diagnose nickel allergy based on your skin's appearance, and a recent exposure to items that may contain nickel.
If the cause of your rash isn't apparent, however, your doctor may recommend a patch test (contact hypersensitivity allergy test). He or she may refer you to an allergy specialist (allergist) or a skin specialist (dermatologist) for this test.
During a patch test, very small quantities of potential allergens (including nickel) are applied to your skin and covered with small patches. The patches remain on your skin for two days before the doctor removes them. If you have a nickel allergy, the skin under the nickel patch will be inflamed when the patch is removed or in the days after removal of the patch.
Because of the low concentrations of allergens used, patch tests are safe even for people with severe allergies.