Patch Testing Procedure
What is patch testing?
Patch testing is a specialized procedure that helps to identify the causes of allergic contact dermatitis (ACD). ACD is a type of allergic reaction of the skin. When certain substances come into contact with the skin, they may trigger a rash in some persons. An example of ACD is the rash triggered by poison ivy.
What type of substances trigger ACD?
Many different substances can trigger ACD. A common trigger is nickel in jewelry. Other triggers are ingredients in soaps and lotions, rubber chemicals, shoe chemicals, clothing chemicals, and even many all-natural substances (such as lavender or tea tree oil). Even certain medicated creams can trigger ACD (such as neomycin in Neosporin, bacitracin in Polysporin, or benzocaine in Solarcaine).
How is patch testing done?
When most people hear about skin allergy testing, they think about prick testing. This is the type of testing that is done in an allergist's office. This type of testing looks for triggers of breathing allergies, such as pollens, molds, cats and dogs.
Patch testing is a completely different kind of test.
How is patch testing performed?
Patch testing appointments are always made as two-part appointments. During the first visit we review what substances you are exposed to in your everyday life, at work, and during hobbies. This helps us determine what substances to test.
Small amounts of these substances (called allergens) are then placed on small patches. These patches are applied to the back, and reinforced with tape.
The good news is there are no needles used in patch testing. However, the patches must stay in place on the back for 48 hours.
What do I need to do during the testing?
While the patches are on your back, you need to take a few precautions. You must keep the back dry. You must limit sweating so that the patches don't come loose. After 48 hours, you will remove the tape and throw away the patches.
Even after the patches are removed, you will not be able to let your back get wet, or apply anything to your back. This allows us to get an accurate reading on your tests.
What will I do if my testing is positive?
At your second visit, we examine the back and look for any reactions. If you reacted to a substance, this is called a positive test. You may develop redness, itching, blisters, or even a scar at the site of a positive test. In some cases, a positive reaction on the back may even trigger a memory reaction, which means that other, previously involved areas may flare up again.
If you are allergic to a substance, we will discuss where the substance is found and how you may have been exposed to it. We will provide information on the allergen, and discuss how best to avoid the allergen. If the trigger ingredient is found in makeup, we will provide access to the CAMP database. This database contains over 5,000 personal care products, and can create a list of products that do not contain a particular ingredient.
Will my rash go away?
Sometimes this allergen is causing the rash. Other times, you may have an underlying skin condition, and the allergy is worsening the skin condition. For example, we see many patients with eczema, psoriasis, rosacea, and other skin conditions, where skin allergy is making the underlying condition harder to treat. Occasionally, the substance may have no role in causing or worsening your rash. Only by avoiding the substance will you find out if it is playing a role. It can take 8 weeks of avoidance before your skin recovers.
If you do not react to any allergens, then we have excluded ACD as a cause of your rash. You and your dermatologist or allergist may then discuss further treatment options for your skin condition.
What type of materials do you test for?
There are many potential causes of allergic contact dermatitis, which can make it challenging. Patch testing is used to identify the specific chemicals or substances that are causing the problem. These substances are known as "allergens".
Patch testing experts are also known as "dermatitis detectives", because they test a wide variety of allergens to narrow in on the culprit(s). As a specialist in extended and customized patch testing, Dr. Katta tests for a wide variety of allergens, which increases the likelihood of accurately diagnosing the cause of contact dermatitis.
Items that we commonly test during extended patch testing:
Preservatives in soap, shampoo, and lotions
Ingredients in medicated ointments
Dyes and chemicals in clothing
Dyes and chemicals in hair care products
Chemicals that are specific to different occupations, such as those used by dentists, machinists, hairdressers, nail technicians, and others
How is this test different from the True Test?
The True Test is a standardized set of allergens, and the same 35 allergens are used for every patient. Although it can be very helpful in diagnosing certain types of allergic reactions, such as reactions to nickel or cobalt, it may miss other potential culprits. This is why extended and customized patch testing may be recommended, especially if the True Test was negative but your physician suspects allergic contact dermatitis. With extended patch testing, we often test for 75 to 100 different allergens.