Your doctor may be able to diagnose contact dermatitis and identify its cause by talking to you about your signs and symptoms, questioning you to uncover clues about the trigger substance, and examining your skin to note the pattern and intensity of your rash.
Your doctor may recommend a patch test to see if you're allergic to something. This test can be useful if the cause of your rash isn't apparent or if your rash recurs often.
During a patch test, small amounts of potential allergens are applied to adhesive patches, which are then placed on your skin. The patches remain on your skin for two to three days, during which time you'll need to keep your back dry.
Your doctor then checks for skin reactions under the patches and determines whether further testing is needed.