Diagnosis

In many cases, doctors can diagnose sun allergy simply by looking at the skin. But if the diagnosis isn't clear-cut, you may need tests to help identify what's going on. These tests may include:

• Ultraviolet (UV) light testing. Also called phototesting, this exam is used to see how your skin reacts to different wavelengths of ultraviolet light from a special type of lamp. Determining which particular kind of UV light causes a reaction can help pinpoint which sun allergy you have.
• Photopatch testing. This test shows whether your sun allergy is caused by a sensitizing substance applied to your skin before you go into the sun. In the test, identical patches of common sun allergy triggers are applied directly to your skin, typically on your back. A day later, one of the areas receives a measured dose of ultraviolet rays from a sun lamp. If a reaction occurs only on the light-exposed area, it likely is linked to the substance being tested.
• Blood tests and skin samples. These tests usually aren't needed. However, your doctor may order one of these tests if he or she suspects your symptoms might be caused by an underlying condition, such as lupus, instead of a sun allergy. With these tests, a blood sample or a skin sample (biopsy) is taken for further examination in a laboratory.