Your doctor can probably make a diagnosis of polymorphous light eruption based on a physical exam and your answers to questions. Laboratory tests may be needed to confirm a diagnosis or rule out other conditions.
Your doctor will do an exam to check your general health. In particular, he or she will examine the rash and the condition of your skin at other sites.
Your doctor may order one of the following tests if a diagnosis isn't certain, the rash doesn't resolve at an expected rate, or there is any reason to suspect another condition causing the rash. Tests may include:
- Skin biopsy. Your doctor may scrape a tiny portion of the rash to remove a sample of tissue (biopsy) for examination in a lab. The biopsy is primarily used to rule out other conditions that may be causing a rash.
- Blood tests. A nurse or assistant may draw blood for laboratory tests that can rule out other conditions.
- Phototesting. You may be referred to a 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:
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- 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 of various sizes 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.
- Elmets CA. Polymorphous light eruption. http://www.uptodate.com/home. Accessed Nov. 6, 2013.
- Photosensitivity. The Merck Manuals: The Merck Manual for Health Care Professionals. http://www.merckmanuals.com/professional/dermatologic_disorders/reactions_to_sunlight/photosensitivity.html#v961913. Accessed Nov. 14, 2013.
- Honigsmann H. Polymorphous light eruption. Photodermatology, Photoimmunology & Photomedicine. 2008;24:155.
- Longo DL, et al. Harrison's Online. 18th ed. New York, N.Y.: The McGraw-Hill Companies; 2012. http://www.accessmedicine.com/resourceTOC.aspx?resourceID=4. Accessed Nov. 6, 2013.
- Sunscreen FAQs. American Academy of Dermatology. http://www.aad.org/media/background/factsheets/fact_sunscreen.htm. Accessed Nov. 14, 2013.
- Bissonnette R, et al. Influence of the quantity of sunscreen applied on the ability to protect against ultraviolet-induced polymorphous light eruption. Photodermatology, Photoimmunology & Photomedicine. 2012;28:240.
- Sun protective clothing. American Melanoma Foundation. http://www.melanomafoundation.org/prevention/clothing.htm. Accessed Nov. 14, 2013.