Patch Testing for Contact Dermatitis

What is a patch test, and how can it help diagnose skin conditions?

Allergy patch test
Patch testing involves the placement of various chemicals into small metal cups affixed to paper tape. The tape is then placed onto the skin of a person's back. Science Photo Library Ltd/Getty


Patch testing is used to identify causes of contact dermatitis, an immunologic reaction to a particular substance, such as poison ivy.  Contact dermatitis is not a true allergy, so the patch testing is performed in a very different way than allergy skin testing. There are no allergic antibodies involved in contact dermatitis, instead, various white blood cells entering into the skin cause the reaction.

Before undergoing a patch test, a patient may be asked to refrain from taking certain medications, or from using a tanning bed, especially on their back. The back is usually the most accommodating area of skin to perform the patch test. 

How Is Patch Testing Performed?

Various allergens are applied to patches that are then taped to the skin, with a different allergen applied to each patch. Unlike allergy skin testing, patch testing does not involve the use of needles. Each patch is applied to clean skin on the patient's back, and remains there for 48 hours. During this time, the patient has to avoid getting the tape of the patch wet, so showers, baths and excessive sweating need to be avoided.

After the 48 hours are up, the patient returns to the clinician's office for removal. Before taking the patches off the patient's back. the location of each patch is marked with a surgical marker, in case additional testing or readings are needed.

  Once the patches have been removed, the person can bathe as normal but should avoid scrubbing of the back in order to prevent removal of the ink marks.

Once the final reading of the test results are completed at 72 to 96 hours after initial placement of the patch test, the person can bathe normally.

Does Patch Testing Hurt?

No. Patch testing simply involves the placement of paper tape on the back, and does not involve the use of needles. Children can safely be patch tested, and a child is old enough for patch testing once they are old enough to understand that they cannot remove the tape themselves.

What Are the Possible Side Effects of Patch Testing?

Since the goal of the patch testing is to determine allergic reaction, at least one patch will cause a small area of contact dermatitis. A positive test may show redness, bumps, mild swelling or even form a small blister.

Since contact dermatitis involves the immune system, patch testing may result in a memory response. This means that the immune system “remembers” where it encountered a chemical to which the skin reacted. The original area of skin that reacted to a particular chemical could again get red and itchy after that same chemical was applied using patch testing, even though the patch test was placed on a different area of skin. For example, a person with contact dermatitis of the eyelids from cosmetics may notice that the rash on their eyelids gets worse after the chemical from the cosmetics is used in a patch test on their back.

The memory response is a good sign that the culprit chemical has been identified.

Once all of the readings of the patch test are completed, the patient may use a topical steroid cream on the back to reduce any rash and itching. While it can make for an uncomfortable few days, it's important to wait until the final reading of the patch test has been completed before treating the patch areas with any creams.

The topical steroid, or other prescribed anti-itch creams, also may be used on the area of the body that experiences a memory response, even if the patch test is still ongoing. Only the skin where the patch test was placed affects the results of the test.

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