Patch Testing to Diagnose Contact Dermatitis

woman having an Allergy patch test

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Patch testing is a procedure used to identify causes of contact dermatitis such as can happen with exposure to poison ivy, nickel, cosmetics, perfumes, household chemicals, or latex.

Contact allergy is not an allergy in the truest sense of the word but rather an immunologic response to certain chemical substances. There are no allergic antibodies involved; instead, the body has an overly sensitive (hypersensitive) reaction to an external stimulus which causes the skin outbreak.

How a Patch Testing Is Performed

Before undergoing a patch test, the person will be asked to refrain from taking certain medications or using a tanning bed, especially on the back. The back is usually the area of skin where the patch test is performed.

The patch consists of a variety of different allergens which are applied in little dots on an adhesive sheet. Each patch is applied to the person's back and remains there for 48 hours. During this time, it is important not to get the patches wet, so showers, baths, and excessive sweating need to be avoided.

After 48 hours, the patches will be removed at the doctor's office. Before doing so, the location of each patch will be marked with an indelible surgical marker. This will provide the doctor with a reference when you return to the office for a final evaluation.

At this point, the person can bathe as normal but should avoid scrubbing the pen marks. While itching or rash can occur on the patch site, it's important not to scratch or treat it until after your final doctor's visit is complete.

The final evaluation will be performed anywhere from 72 to 96 hours after the initial placement. Any reaction will be noted by the doctor, providing insights as to which substances to avoid and the types of treatments to be considered.

Patch testing is painless and, unlike allergy tests, does not involve needles of any sort. Children can be tested once they are old enough to understand that the patches cannot be removed.

Evaluation and Side Effects

Since the goal of the patch testing is to pinpoint the cause of contact dermatitis, you can expect one or more areas of eczema or rash. A positive test may show redness, bumps, mild swelling, or even form a small blister. Some reactions may be uncomfortable but are typically mild.

Once all of the readings of the patch test are completed, a topical steroid may be used to treat any itching or rash. 

Patch testing may result in the development of a so-called memory response. This is when the body is re-exposed to an allergen after patch testing and recognizes it as something it doesn't like. This can trigger an immune response that can sometimes be bigger than the first.

While the memory response is a sign of a healthy immune system, it makes it all the more important to avoid any substance known to cause a reaction.

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