Patch Testing to Diagnose Contact Dermatitis

woman having an Allergy patch test

Science Photo Library Ltd / Getty Images

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. 

Was this page helpful?

Article Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Lazzarini R, Duarte I, Ferreira AL. Patch tests. An Bras Dermatol. 2013;88(6):879-88. doi:10.1590/abd1806-4841.20132323

  2. Johnston GA, Exton LS, Mohd Mustapa MF, et al. British Association of Dermatologists' guidelines for the management of contact dermatitis 2017. Br J Dermatol. 2017;176(2):317-329. doi:10.1111/bjd.15239

  3. Moustafa M, Holden CR, Athavale P, Cork MJ, Messenger AG, Gawkrodger DJ. Patch testing is a useful investigation in children with eczema. Contact Derm. 2011;65(4):208-12. doi:10.1111/j.1600-0536.2011.01900.x

Additional Reading