In patch test, common allergenssubstances known to produce an allergic reaction in susceptible individualsare applied to skin on the back, in order pinpoint the cause of recurrent skin irritation (dermatitis). Since some allergens only cause a reaction in combination with ultraviolet (UV) light, application of the potential allergens is sometimes followed by exposure to UV light; this variation is called a photopatch test.
Purpose of the Patch Test
- To determine if a substance that touches the skin is what is causing inflammation. If that is the case, the inflammation is classified as contact dermatitis.
- To determine whether dermatitischaracterized by itching, redness, and other skin irritationis caused or aggravated by a contact allergy, and to identify the substance responsible for the problem. Common culprits include perfume, cosmetics, nickel in jewelry, and latex
Who Performs Patch Tests
- A doctor or a nurse
Special Concerns about Patch Tests
- A variety of conditions may make it difficult to interpret the results. These include using very high or low concentrations of allergen extracts, impurities in the extracts, examining the test sites too early or too late, and placing patches on already inflamed skin.
- Use of high-dose oral steroids or topical steroid cream or ointment, or recent intense sun or tanning bed exposure of the test sites, may produce false-negative results.
- Even in the absence of interfering factors, the accuracy of testing may vary. You may react to an allergen during testing but not during normal exposure; conversely, you may have a negative test result and still be allergic to a substance.
Before the Patch Test
- On your initial visit to the doctor before the test, bring any items you suspect might be causing the allergic reaction, such as jewelry, perfume, cosmetics, or items of clothing.
- Do not tan outdoors or go to a tanning bed for 5 days before the test.
- Do not apply steroid cream or ointment to the skin that will be used for the test.
What You Experience
- Small amounts of the selected allergens are first applied to small disks and then taped to the skin on your back. (Usually 20 to 60 separate allergens are used.) This process takes less than 30 minutes.
- The patches should remain in place undisturbed for the next 48 hours. Because the patch sites must be kept dry, you cannot bathe or shower during this time, and you should also avoid strenuous exercise in order to prevent heavy sweating.
- If you experience severe itching or pain at any of the sites, remove the patch by cutting it out and inform your doctor immediately.
- The patches are removed in the doctor’s office, and the test sites are examined at this time and again in another 1 to 5 days.
- For a photopatch test, duplicate patches are applied to your back for each allergen and taped in place with opaque material. After 24 to 48 hours, one allergen site from each pair is uncovered and exposed to UV light, while the other site remains covered.
Risks and Complications of Patch Tests
- Mild skin irritation may occur at the test sites.
- Infrequently, the dermatitis that prompted the test may flare up, or several or many sites may turn red ("angry back"). If this occurs, the test will be discontinued.
After the Patch Test
- If some of the test sites itch, the doctor may prescribe a steroid ointment or cream for relief.
- Skin on some of the test sites may remain slightly dark for several weeks, but the discoloration will fade eventually.
Patch Test Results
- For a routine patch test, a finding of redness, bumps, blisters, or other skin abnormalities at a specific patch site suggests that you are allergic to the tested substance. Your doctor will recommend that you avoid that allergen.
- In the photopatch test, the presence of a reaction at the site that was exposed to UV light—but not at the corresponding protected site—indicates that you have a photoallergic reaction to that particular substance. Your doctor will recommend that you avoid that allergen.
- If the initial test is negative, your doctor may order additional tests in an effort to identify the cause of your dermatitis.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media