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The diagnosis of photosensitivity typically begins with a physician consultation (usually with a dermatologist). The physician may compile a detailed medical history and ask questions about any family history of allergies or diseases. The physician may also inquire about exposure to possible irritants, current medications, over-the-counter drugs or herbal products. It is important to discuss the use of all current medications or herbal remedies even if they are not used daily.

During a subsequent physical examination, the physician will meticulously examine the skin and ask questions about when the skin reaction occurs in relation to sun exposure or other sources of ultraviolet radiation.

The physician may perform tests including:

• Allergy tests. Help identify specific allergens (substances the body mistakenly perceives to be a threat, triggering an allergic reaction). Tests include skin tests in which the skin is pricked or injected with an allergen. If a red and swollen area of the skin (e.g., rash, hive) appears at the test site, the test is positive for that allergen. Blood tests (e.g., RAST test) may also be used to measure the levels of immunoglobulin E (a protein involved in allergic reactions) in response to the introduction of a specific allergen.
  
  
• Blood tests. Tests that identify abnormally high or low levels of a given substance in the blood. Blood tests can identify underlying diseases or conditions that may be causing the skin reaction.
  
  
• Skin biopsy. Laboratory test that examines a small piece of skin that has been removed by the physician.
  
  
• Photo-testing. Test that exposes a small area of skin to measured amounts of ultraviolet light. If skin symptoms appear, the test will confirm that they are related to sun exposure. Symptoms usually appear within 30 minutes of exposure.