Like Usually the skin gets red, swollen, and blisters appear. The blisters may form and break, leaving crusts and scales. Later the skin may darken and become leathery and cracked. Allergic contact dermatitis can be difficult to distinguish from other rashes. The dermatologist and patient will discuss the materials that come into contact with the person's skin at work and home to try to identify the allergen.
Most contact dermatitis is diagnosed by location of the rash.
Sometimes cause can not be identified by history and physical examination, and the dermatologist may want to perform a patch test. Patch tests are a safe and easy way to diagnose contact allergies. Small amounts of the possible allergens are applied to the skin on strips of tape and then removed after two days. An allergy shows up as a small red spot at the site of the patch. Common allergens include nickel, rubber, dyes, preservatives, fragrances, poison ivy, poison oak, and related plants.