Phototesting are used to evaluate skin abnormalities—such as rash, itching, blisters, and hives—that are either caused or exacerbated by exposure to sunlight. In standard phototests, small areas of skin are irradiated with different doses of long- and short-wave ultraviolet (UVA and UVB) and visible light, and then observed for a reaction.
The photopatch test is used when light-induced skin reactions are believed to be linked to a particular contact allergy (such as a fragrance or cosmetic).
Purpose of the Phototesting
- To confirm a diagnosis of photosensitivity (sensitivity to light) and to assess its severity
- To determine the particular wavelengths of light that elicits a skin reaction
- To evaluate the effectiveness of treatment
- To follow the development of a light-induced skin disorder over time
Who Performs It
- A technician or a nurse at a dermatologist’s office
- Some photosensitivity reactions occur only after ingestion of certain drugs or after the skin is exposed to a particular substance. If these agents are not present, a false-negative result may occur.
- Blood tests are necessary to confirm a diagnosis in some light-induced skin disorders.
Before the Phototesting
- No special preparation is needed before this procedure.
What You Experience
- The examiner shines UVA, UVB, and visible light on different small areas of skin, such as on the back or forearms. The portion of the body that is exposed to light, the type of light, and the dose and duration of exposure—ranging from a few minutes to less than one hour—vary depending on the suspected diagnosis.
- The examiner assesses the skin reaction immediately after exposure, 24 hours later, and then periodically thereafter for up to three weeks.
- The minimal erythema dose (MED) at 24 hours is noted; this is the lowest dose of UVA or UVB radiation that produces a clearly identifiable pink mark on the skin.
- When drug-induced photosensitivity is the suspected diagnosis, the testing procedure is done two times—once while you are taking the medication and again after you have discontinued it and the drug has been cleared from your system—to confirm the disappearance of photosensitivity.
Risks and Complications
- A positive reaction to phototesting may cause pain, itching, blisters, redness at the tested sites, skin pigment change, eczema flaring, infection or scarring. Fever and malaise may also occur. It should be noted, however, that side effects are rare.
After the Phototesting
- You will be instructed to avoid exposing the irradiated skin areas to sunlight until the testing process is completed (that is, after the final examination is performed).
- If any lesions or irritation develops on the exposed areas of skin, the doctor may prescribe a steroid cream or lotion to help control the reaction.
- If you develop a skin reaction at a test site after testing has been completed, let your doctor know immediately; this information is crucial for the correct diagnosis of your skin disorder.
- If you have a positive skin reaction, your doctor will try to diagnose the cause of your disorder based on your symptoms, history, and the results of other tests.
- A variety of conditions may be responsible for skin photosensitivity, including a common condition of unknown origins called polymorphous light eruption (PLE); a rare disorder called solar urticaria; and the generalized autoimmune disease systemic lupus erythematosus (SLE).
- Sensitivity to sunlight may also be a side effect of certain medications (drug-induced photosensitivity).
- If the doctor can make a definitive diagnosis, appropriate medications or ointments will be prescribed.
- If the test results are negative, the doctor may order additional tests in an effort to determine the cause of the problem.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media