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Patch testing can find what's causing your rash

If your skin reacts to something and your dermatologist suspects you’re having an allergic reaction, your dermatologist will try to find what’s causing the reaction. To a dermatologist’s trained eye, this may be obvious. A rash from poison ivy or an e-cigarette can be easy to spot.

Other causes can take time to find. More than 15,000 substances can cause an allergic skin reaction. It’s also possible that you have developed an allergic reaction to more than one substance.

If your skin continues to itch and develop rashes, your dermatologist may recommend a medical test called patch testing. Some dermatologists offer patch testing in their office. If your dermatologist does not offer patch testing, you will be referred to another doctor or medical office.

Patch testing differs from a skin prick test. When you get a skin prick test (or scratch test), your doctor checks for an immediate reaction. Skin allergies tend to develop in hours or days, so a patch test checks for this type of skin reaction.

Patch testing can find what’s causing an allergic reaction on your skin

The patches are normally applied to your back and must be left in place for a period of time.

Patch testing on woman’s back

What’s involved in patch testing?

If you have patch testing, here’s what to expect:

  1. Your dermatologist will place small amounts of allergens (what can cause an allergic reaction) on your skin and cover each allergen with a patch. The purpose is to see if any allergens cause your skin to react.

  2. You will leave the patches on your skin for 48 hours. If your skin reacts during this time, it may feel itchy or sore. Despite this, it’s essential to keep the patches on your skin so that you can get accurate results. If you remove the patches, loosen them, or get them wet, you’re likely to get inaccurate results.

  3. After 48 hours, you will return to your dermatologist’s office. Your dermatologist will remove the patches. If your skin reacted to any of the allergens, your dermatologist can tell you what caused the reaction.

  4. After 4 to 7 days, you will see your dermatologist again. It’s essential to keep this appointment because it can take time for your skin to develop an allergic reaction.

If patch testing reveals that you have one or more allergies, your dermatologist will create a treatment plan. You may need to apply medication to help the rash heal. All treatment plans include instructions to help you avoid what’s causing your skin to react.

Because so many things that touch our skin can cause contact dermatitis, it’s possible that the first round of patch testing fails to find what’s causing your skin to react. If this happens, your dermatologist may recommend testing other substances.

Many people develop an allergy to a substance that they work with often. Job-specific allergies are so common that patch testing is available for certain industries. For example, certain patches can be applied to people who work as a florist or dental technician. This is called expanded patch testing. It finds about 80% of allergens.

Possible side effects from patch testing

The patches irritate some people’s skin. If you have irritated skin, it usually clears on its own in a few days. People who have darker skin tones may see lighter or darker skin where the patches were applied to their skin. This discoloration will also clear on its own.

If you have psoriasis, the patches can trigger a psoriasis flare-up.

It’s rare to have a serious side effect from patch testing. A few people have developed an infection or life-threatening allergic reaction. When someone develops a life-threatening allergic reaction, this occurs within 30 minutes of applying the patches.

Signs of a life-threatening allergic reaction

Signs of a life-threatening allergic reaction are swelling, nausea, vomiting, and difficulty breathing. If you notice these signs, get immediate medical care.

What you do while wearing the patches affects your results

To get accurate results, you must follow the instructions given to you. If you scratch the patches or take a shower, the test will likely miss something that’s causing your skin to react.

Infographic: 5 ways to improve results from patch testing

To reduce your risk of getting inaccurate results, follow these five tips that dermatologists give their patients. Click the image below to download a PDF of the infographic.

Infographic shows you how to get the best results from patch testing | American Academy of Dermatology

Patch testing can find what’s causing your rash

The results from your patch test can help uncover what’s causing your skin to react.

Once your dermatologist knows the cause (or causes) of your contact dermatitis, your dermatologist will create a plan to help you avoid what’s causing your rash.

Some dermatologists offer extensive patch testing. This means that they can test you for many different substances that could be causing your allergic skin reaction.

To find a dermatologist who offers patch testing:

  • Go to Find a dermatologist

  • In the Search by Location box, enter your state

  • Click on Filters

  • Select Any Procedures

  • Select Patch testing

  • Click on Search

For many patients, the results from patch testing significantly improve their lives. Once you know what’s causing your itchy rash, you can avoid it. This often leads to clearing.

Getty Images

Carol R. “Are you a rash whisperer?” Dermatol World. 2019:29(1):44-9

Harris V and Smith S. “Poster 5057: Don’t brush off contact allergies in cheilitis: Modern toothbrush contact dermatitis.” J Am Acad Dermatol. 2017;76(6):AB103. No commercial support identified.

Mowad CM, Anderson B, et al. “Allergic contact dermatitis: Patient diagnosis and evaluation.” J Am Acad Dermatol. 2016;74:1029-40.

Mowad CM, Anderson B, et al. “Allergic contact dermatitis: Patient management and education.” J Am Acad Dermatol. 2016;74:1043-54.

Warshaw EM, Schlarbaum JP, et al. “Allergic reactions to tattoos: Retrospective analysis of North American Contact Dermatitis Group data, 2001-2016.” J Am Acad Dermatol. 2020 Feb;82(2):e61-e62.

Warshaw EM, Schlarbaum JP, et al. “Facial dermatitis in male patients referred for patch testing: Retrospective analysis of North American Contact Dermatitis Group Data, 1994 to 2016.” JAMA Dermatol. 2020 Jan 1;156(1):79-84.

Written by:
Paula Ludmann, MS

Reviewed by:
Bassel Hamdy Mahmoud, MD, PhD, FAAD
Elena Hawryluk, MD, PhD, FAAD
Stephen Stone, MD, FAAD

Last updated: 3/15/21