PhotopatchTest

PhotopatchTest is a diagnostic procedure used to identify contact allergies to light-sensitive substances. It is a variation of the standard patch test, which is commonly used to assess reactions to chemicals on the skin. In a photopatch test, small amounts of suspected allergens are applied to the skin, typically on the back, using adhesive patches. These patches are left in place for a period, usually 24 to 48 hours. After removal, the skin is examined for any signs of reaction, such as redness, itching, or blistering.

The key difference in a photopatch test is the subsequent exposure to ultraviolet (UV) light. After the initial patch removal, the test sites are exposed to a controlled dose of UV-A light. This light activates certain substances, making them allergenic or exacerbating a pre-existing light sensitivity. The skin is then re-examined after another 24 to 48 hours to assess for delayed reactions that occur specifically in response to the combination of the allergen and UV light.

Photopatch testing is indicated when a patient presents with a skin rash that appears or worsens after sun exposure, and the suspected cause is a substance that becomes allergenic upon light exposure. These substances can include certain medications (topical or systemic), sunscreens, fragrances, dyes, and plant-derived compounds. A positive photopatch test result helps clinicians confirm the diagnosis of photoallergic contact dermatitis and guide appropriate management, which typically involves avoidance of the identified offending agent and sun protection.