In the eye, conjunctivitis with a papillary or cobblestone appearance, chemosis, injection, and tearing frequently occur. Blepharitis also may occur and may be accompanied by a keratitis. This keratitis frequently is appreciated as small yellow opacities near the limbus, often described as a fine punctate keratitis. Patients whose rash occurs in an elongated or linear pattern often will have had exposure to a plant such as poison ivy or oak. Frequently encountered agents that may be responsible include drugs, soaps, lotions, cosmetics, metals, foods, dyes, preservatives, and plants.
Patch testing is the criterion standard for diagnosing contact dermatitis.
A small portion of the suspected offending agent is placed on the skin and patched for 48 hours. A low dose of the test substance is used to avoid irritation as much as possible. Allergic responses may take longer than 72 hours to become evident. Irritants will show a response in a few hours. If irritation occurs, the substance may be falsely identified, which is the major weakness of this test.