If you've ever experienced a red, itchy rash appear on your skin after using a certain lotion or even from wearing jewelry, you may be experiencing contact dermatitis. It's caused by a substance you're exposed to that irritates your skin. However, it also can be triggered by an allergic reaction.

In this Mayo Clinic Minute, Dr. Matthew Hall, a Mayo Clinic dermatologist, explains how patch testing is crucial to assess for allergic contact dermatitis.

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"Patients can become allergic to various things that they are using, such as soaps, lotions, makeup, anything that contacts the skin," says Dr. Hall.

Nickel, which is often used in costume jewelry, is the most common allergen.

But how can people know if they're having an allergic reaction to something they're putting on their skin?

"Patch testing is the test that we perform to assess for allergic contact dermatitis. It's a weeklong test. We have to see patients on Monday, Wednesday and Friday of the same week," explains Dr. Hall.

During the initial visit, the dermatologist determines possible risk factors that may be causing the contact dermatitis.

"Then, based on that, we customize a panel of allergens for each patient that are placed on these aluminum discs that are taped onto the back," says Dr. Hall.

After two days, the patient comes back to get the patches removed.

"We also have to see the patient back on Friday because it can take four to five days before we see reaction," says Dr. Hall. "So it's a weeklong commitment."

At the end of the week, patients are provided with a list of what they're allergic to.

"We also give them access to a customized database of products that are safe for them to use that do not contain the substances that they are allergic to," says Dr. Hall.

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