Diagnosis

Your doctor can probably make a diagnosis of polymorphous light eruption based on a physical exam and your answers to questions. He or she may also have you undergo laboratory tests in order to confirm a diagnosis or rule out other conditions. Tests may include:

• Skin biopsy. Your doctor may remove a sample of rash tissue (biopsy) for examination in a lab.
• Blood tests. A nurse or assistant may draw blood for laboratory tests.
• Phototesting. Your doctor may refer you to a specialist in skin conditions (dermatologist) for phototesting. During the test small areas of your skin are exposed to measured amounts of UVA and UVB light to try to reproduce the problem. If your skin reacts to the UV radiation, you're considered sensitive to sunlight (photosensitive) and may have polymorphous light eruption or another light-induced disorder.

Other light-induced conditions

Your doctor may need to rule out other disorders characterized by light-induced skin reactions. These conditions include:

• Chemical photosensitivity. A number of chemicals — drugs, medicated lotions, fragrances, plant products — can induce photosensitivity. When this occurs, your skin reacts each time it's exposed to sunlight after ingesting or coming into contact with a particular chemical.
• Solar urticaria. Solar urticaria is a sun-induced allergic reaction that produces hives — raised, red, itchy welts that appear and disappear on your skin. The welts can appear within a few minutes of sun exposure and last for a few minutes to hours. Solar urticaria is a chronic condition that can last for years.
• Lupus rash. Lupus is an inflammatory disorder that affects a number of body systems. One symptom is the appearance of a discolored, bumpy rash on areas of skin exposed to sunlight, such as the face, neck or upper chest.