If you have eczema, a patch test can help determine the substance causing your skin to have an allergic reaction.

Objects we come across in our everyday lives, such as soaps, disinfectants, and skin care products, can cause allergic skin reactions in some people. They can trigger allergy flare-ups in people with eczema.

Eczema is an inflammatory skin condition that causes itchiness, dry skin, scaly patches, rashes, and blisters. Atopic dermatitis is the most common type of eczema and can be caused by environmental, genetic, and immune system factors.

Contact dermatitis is another type of eczema that occurs when a substance to which you’re sensitive, called a contact allergen, triggers an immune system reaction or irritates your skin. Examples of common allergens include:

  • nickel, used in jewelry
  • formaldehyde, a preservative in some skin care products
  • plants like poison ivy
  • personal care products, such as soaps, shampoos, and cosmetics
  • products that cause a reaction when you’re exposed to the sun, such as sunscreen
  • paraphenylenediamine (PPD), often used in hair dye

Experts consider patch testing to be the gold standard and most effective tool for diagnosing hypersensitive skin reactions like allergic contact dermatitis.

Learn how a patch test works and how to prepare for the test. We also discuss patch test results and risks associated with the test.

woman's back with patch test being administered, eczema skin patch testShare on Pinterest
Woman’s back with patch test administered. | Getty Images

During a patch test, the healthcare professional treating you — usually a dermatologist — will place a tiny amount of a diluted allergen on a patch and apply it to your skin. Places these are typically applied include the back and upper arms.

After 48 hours, they will remove the patch and check the skin under and around it for any reactions. Another reading may take place 1 or 2 days after the patch is removed. Numerous medical professionals may apply or remove the patch, but only a dermatologist does the reading.

Doctors often recommend a patch test if your skin reacts to something or you have an allergic skin reaction, but they don’t know the exact cause.

A patch test aims to identify the particular allergen causing allergic reactions in people with skin conditions like eczema. It’s a useful tool for detecting the triggers of your skin condition.

There are between 35 and 80 allergens that are available commercially, but many more are available to you because a doctor can test for any substance you personally have.

Your doctor may only need to test for some of them.

To select the allergens they will use, your doctor will often ask you questions about the skin care products you’ve used and objects you’ve been in contact with, such as plants and latex.

They select the allergens to use in a patch test based on your history of contact with substances, your symptoms after examining you, and their suspicions.

Preparation for a patch test should start about 6 months from the time of suspected contact with a possible allergen. This is to avoid a flare of existing skin inflammation.

Preparation includes:

  • pre-test counseling, where your dermatologist will tell you about the possible side effects of the test
  • knowing you won’t exercise or wash the patch test area during the test period, as wetting the patch could dislodge it
  • avoiding topical steroids in the test site for about 1 week before the test
  • avoiding ointments or creams on the morning of the test
  • staying out of the sun during testing

Here’s how a patch test generally works:

  1. A medical professional will place small amounts of allergens on a part of your skin and cover them with patches.
  2. They will instruct that you leave the patches on for about 48 hours. You may experience itching, but should not scratch the patches or remove them.
  3. Return to the office after 48 hours. Your medical professional will check if you have any reactions and note them.
  4. Because allergic reactions don’t always develop immediately, they may ask to see you again after 4 to 7 days so that they can check for further reactions.

If the first round of test patches shows a negative result, your doctor may recommend testing other substances.

You may react to one of the allergens and feel itchy or sore during the testing. In that case, the American Academy of Dermatology (AAD) recommends not removing or loosening the patches, as that may cause inaccurate results.

If your result reads positive for one or more allergens after the testing, your medical professional will work with you to create a treatment plan. They may schedule a post-patch test counseling session to discuss what they identified as the cause of your flares and give advice on how to avoid the allergens and prevent the recurrence of symptoms.

Skin reactions during patch testing are often noticeable during the second or third visit to a medical office, but even if they occur later, it’s best to let a doctor know.

An itchy, red, or pink raised area of skin in the area the patch was applied to is often recorded as a positive result. A strong positive test may cause a blister that can last some weeks.

No skin reaction often indicates a negative result and may mean the right allergen was missed and not tested for. Additionally, it might imply that internal factors (such as a gene mutation) rather than external factors (like allergens) are the cause of your eczema flare-up.

You may experience some mild side effects during your patch test, such as:

  • skin irritation that clears within a few days
  • lighter or darker skin discoloration in the areas where the patches were applied
  • a psoriasis flare-up, if you have the condition
  • a flare-up of existing eczema
  • scarring, infection, and allergic reactions, which in rare cases can be life threatening (anaphylaxis)

Eczema is a common inflammatory skin condition that affects millions of people worldwide. One of the primary causes of flare-ups is an allergy to some substance. Because there are many such substances, health experts often use patch testing, which is helpful for isolating and identifying the allergens causing a reaction.

If a patch test shows you’re reacting to a particular allergen, your medical professional will create a personalized treatment plan for existing symptoms and guidelines on how to prevent future recurrences.