What is allergy patch testing?
Sometimes when a skin rash develops, it can be difficult to find the cause. Allergy patch testing helps pinpoint whether a specific substance is responsible. The test involves placing small amounts of chemicals on patches made of hypoallergenic adhesive paper, which are then taped to your back or upper arm.
Patch testing is typically done by dermatologists, who can determine whether your skin condition is due to an allergic reaction or another issue. At Penn Dermatology, we use a targeted patch testing approach to thoroughly identify substances causing allergic contact dermatitis.
Types of patch testing
While skin prick or scratch tests check for immediate allergic reactions, patch testing looks for delayed contact reactions. There are two main types: regular patch testing and photo patch testing.
This test checks for allergic reactions to substances that come into direct contact with your skin, like fragrances, preservatives, metals, or hair dyes. Small amounts of these allergens are placed on adhesive patches, which are applied to your back. The patches usually stay in place for 48 hours to see if a reaction develops.
This type of patch testing is used when your provider suspects that your skin reacts only after sun exposure. It helps diagnose photoallergic contact dermatitis, which happens when a substance causes a reaction only when exposed to ultraviolet (UV) light. After the patches are removed, part of the test area is exposed to light to see if a reaction occurs.
Who is a candidate for allergy patch testing?
Most adults and children can safely have allergy patch testing. You may be a good candidate if you:
- Haven’t had a severe allergic reaction in the past that could raise your risk during testing.
- Are able to temporarily stop certain medicines that might affect your results, like antihistamines, steroids, some antidepressants, or heartburn medications.
- Have a healthy patch of skin, usually on your back, where the test can be done.
How to prepare for patch testing
Knowing what to bring, how to prepare, and how the process works can help your patch testing go smoothly.
Your provider will want to know about all the products you use, including soaps, cleansers, shampoos, toothpaste, mouthwash, household cleaners, and laundry detergent. If you think your rash may be caused by something at your work, be prepared to share information about products like oils, cleansers, glues, and adhesives.
You’ll want to have the following for your first appointment:
- A list of all the products you use on your skin, nails, and hair
- Photos of product labels and ingredients lists
- Safety data sheets (SDS) for products you use at work
- Any medical records related to your rash
- Biopsy results and blood test results
Follow these instructions to help make your patch test as accurate and comfortable as possible:
- Bathing: You may shower and apply deodorant on the morning of your first appointment, but don’t shower or bathe again until after your final visit.
- Lotions: Don’t apply lotions, creams, or ointments to your back or upper arms on the day of your first test.
- Medication: You may continue using medication on other parts of your body that aren’t being tested.
- Physical activities: Avoid activities that could shift the patches or cause you to sweat.
- Hair: If you have long hair, bring a clip or tie to keep it off your back during patch placement.
- Clothing: Wear a button-down shirt and older, dark-colored clothing to your appointments, as you won’t be able to lift your arms after patch placement and skin markers may stain fabric.
Some allergic reactions take time to show up. Patch testing is designed to detect these delayed responses. The process usually takes place over several days.
- Visit 1: The patches are applied to your back (and sometimes the upper arms) and left in place for 48 hours.
- Visit 2: The patches are removed, and your skin is checked for any early signs of a reaction.
- Visit 3: After another 48 hours, your provider will review the final results to check for any delayed reactions that may have developed.
Healing after patch testing
After your test, the skin where the patches were placed may itch or burn. This is a common reaction. Over-the-counter antihistamines can help ease discomfort. If you had a positive reaction, the area might stay red, swollen, and itchy for a while. You may also get blisters or skin discoloration, which can take time to go away. If you develop a new reaction after your last appointment, take a picture and call your provider for guidance.
What are the risks of patch testing for allergies?
Patch testing is a low-risk way to identify delayed allergic reactions, but there are some possible side effects. These include mild redness, itching, or burning at the test site, which usually clears up in a few days. You might also notice some temporary changes in skin color where the patches were applied.
Less commonly, patch testing can cause flare-ups of skin conditions like eczema or, in rare cases, small blisters or even new allergies to the tested materials can result. Severe allergic reactions to patch tests are very rare but can happen. Your provider will review your medical history to decide if patch testing is right for you.
Targeted patch testing for precise relief
When you have an uncomfortable rash that isn’t getting better, you want a care provider who can not only relieve your symptoms but also find the cause. Penn Dermatology allergy experts use a targeted patch testing technique to quickly identify the substance causing your reaction. Unlike standard pre-packaged tests, our customized approach increases the chances of pinpointing the specific allergen and finding an effective treatment.
The specialists at Penn Dermatology are among the nation’s leading experts in treating skin conditions. They’re frequently honored by publications such as Philadelphia Magazine’s Top Docs™, Best Doctors in America, and America’s Top Doctors for their outstanding patient care and expertise in treating conditions like allergic contact dermatitis, eczema, and other complex rashes.
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