Patch testing is used to identify causes of contact dermatitis. Contact dermatitis is an immunologic reaction to a particular substance; poison-oak (and poison-ivy and poison-sumac) is an example of such a reaction. Contact dermatitis is not a true allergy; therefore patch testing is performed in a very different way than allergy skin testing. There are no allergic antibodies involved in contact dermatitis; rather, various white blood cells entering into the skin cause the reaction.
How Is Patch Testing Performed?
Patch testing involves the placement of various chemicals onto the skin, usually held against the skin using a paper tape. The chemicals themselves are often contained within a small metal cup, usually smaller than a dime. Unlike allergy skin testing, patch testing does not involve the use of needles. Patch testing is available commercially in the United States as the T.R.U.E. test; some allergists and dermatologists order special chemicals from Europe or Canada to make up their own patch test panels (this is commonly done and is typically considered to be safe if the physician is trained in the performance of patch testing).
The tape (with attached chemicals contained within the metal cups) is applied to clean skin on the person’s back. The patch test remains on the skin for 48 hours. During this time, the person cannot get the tape wet; therefore, only a sponge bath can be taken, and excessive sweating should be avoided.
The patch test is removed by medical personnel after 48 hours (2 days), and an initial reading of the test is performed. A permanent or surgical marker is used to mark on the back where the tests were prior to removal, for an additional reading of the results at 72 to 96 hours (3 to 4 days) after the initial placement. Once the tape has been removed, the person can bathe as normal but should avoid scrubbing of the back in order to prevent removal of the ink marks showing where the various tests were originally placed.
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; although in my experience patch testing is not often needed for children. A child is old enough for patch testing once they are old enough to understand that they cannot remove the tape themselves; this age may vary from child to child.
What Are the Possible Side Effects of Patch Testing?
Patch testing will actually cause a small area of contact dermatitis at the site of the substance that is thought to be causing the person’s original symptoms that required the testing in the first place. Therefore, the person’s back may become very itchy under the tape; even the tape itself may cause some minor irritation and itching. Once the patch test is removed after 48 hours, the skin may become even itchier even though the tape and chemicals have been removed. 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 was placed into a patch test on the person’s 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 person may use a topical steroid on the back to reduce the resultant rash and itching. Using creams on the area of the patch test prior to the final reading may alter the results of the patch test, and should not be done unless the person is instructed to do so.
The topical steroid, or other prescribed anti-itch creams, can be used on the area of the body that experiences a memory response at any time during the test, as only the skin where the patch test was placed affects the results of the test.