Allergy Patch Testing: Top Signs You Need It

Patch Testing
Patch Testing

What is Allergy Patch Testing and Its Importance?

Allergies are one of the most common chronic decisions in the U.S. It is estimated more than 50 million Americans suffer from an allergy, with it being the sixth leading cause of chronic illness in the country.

Unexpected rashes can be itchy and unsightly. Unfortunately, the cause of an allergy is not always clear. Dermatologists utilize patch testing to diagnose a contact allergy.

Patch testing can reveal the cause of an allergy and inform future treatment plans. Here’s what you need to know about patch tests for skin and how the results will be interpreted.

What is Patch Testing?

A patch test is employed to investigate and confirm whether someone has a skin allergy to a specific substance. Each patch consists of diluted allergens applied to the skin for a minimum of 48 hours.

Since reactions may not appear immediately, dermatologists will read the patch results after 72-96 hours. Patch testing in dermatology reveals an offending contact allergen by producing a red area. The area may be blistered or raised.

A patch test on the skin helps investigate allergic contact dermatitis. It has no value in diagnosing irritant dermatitis. Also, it has limited value in uncovering drug allergies.

Note that patch tests are different from skin prick tests. Dermatologists may employ a skin prick test to diagnose hay fever.

How is a Patch Test Performed?

Dermatologists will apply several allergens to provide a greater chance of eliciting an allergic response. Positive allergy patch tests are a good thing because they help uncover what is causing the reaction in the first place.

Each allergen will be applied to the back for convenience. These special tapes consist of Finn chambers, small aluminum discs consisting of each allergen. Each Finn chamber can contain up to 12 separate allergens at once.

Dermatologists are capable of testing over a hundred allergens at one time, but typically less are applied. A true patch test will only be performed when the dermatitis is inactive. Already inflamed skin cannot accurately reveal whether someone is allergic to a substance.

Those who suffer from severe and frequent allergies may already have inflamed skin. In this case, dermatologists can also apply patches to the arms or abdominal regions. Each panel is marked, so doctors can track where each allergen has been positioned.

These patches will be removed after 48 hours, with a further skin inspection performed 24-48 hours after removal.

Patients may be advised to:

  • Reduce the reaction by avoiding immunosuppressive agents before testing
  • Avoid potent topical steroids at least three days before taking a true patch test
  • Avoid bathing or showering the test area during the test
  • Avoid excessive exercise

Dermatologists will use an allergy patch test results chart to aid their interpretations. Should the test reveal no positive allergens, further tests using other allergens may be performed. With thousands of potential allergens, a single test may not be enough to show the offending allergen.

What to Expect With an Allergy Patch Test

Each round of patch testing will take 48 hours to complete. Doctors will apply the allergens and ensure they are kept in place with hypoallergenic tape. These allergens will remain in place for a minimum of 48 hours. Some allergens may take this long to elicit a reaction.

After the patches are removed, the skin will be inspected. A further follow-up appointment will be required one or two days or more after this appointment for another inspection. Expect to set aside roughly one week to receive comprehensive patch test results.

A patch test is not designed to be painful. If a patient is allergic to a substance, they will likely experience itching or burning on the skin. Patients should avoid scratching the test patch area to prevent dislodging the patches.

Furthermore, scratching further irritates the skin and worsens the itching. While it can be uncomfortable, a reaction typically poses no threat. On the other hand, those with serious allergies may experience severe itching or burning. If this occurs, patients should contact their doctor as soon as possible.

There are plenty of positive allergy patch test pictures available online from the Contact Dermatitis Institute if you want to get an idea of what a positive true test patch test looks like.

Patients are free to live their lives mainly as before during a test. Dermatologists typically advise you to refrain from any activities that may interfere with the test.

For example, swimming is always prohibited, as is full-body showering or bathing. Certain exercises should also be avoided, such as cardio workouts. Light exercise may be permitted, but only if a sweat-resistant testing material has been used.

Patients are advised to avoid direct sunlight and heat. Excessive sweating can loosen the tape. Plus, direct sunlight has been known to interfere with test results, especially if the panels have already been removed.

The hypoallergenic tape used by doctors is generally pretty sturdy. If a piece of tape does come loose, patients can reattach it with their own hypoallergenic tape. If a patch ultimately comes away from the skin, it can also be reattached, but take care to avoid contaminating both the patch and the skin by only taping the edge of the panels.

In the event of contamination, patients should revisit their doctor.

Adverse Reactions to Patch Testing

A positive reaction to a patch test will leave a small, itchy patch of eczema. Dermatologists can prescribe topical steroids to clear up the problem quickly. There are other adverse reactions patients need to be aware of when undergoing this form of skin allergen testing.

Common types of adverse reactions include:

  • Severe Localized Blistering – Most commonly associated with para-phenylenediamine (PPD) found within hair dye. If PPD is suspected to be the cause of the problem, a dermatologist may try to avert a severe reaction by applying low-strength PPD.
  • Post-inflammatory hyperpigmentation – Small changes in the color of the skin’s affected area may be noted.
  • Distant Flareups – The initial case of dermatitis under investigation may flare up again in a location far from the actual testing site.
  • Tape Reaction – Hypoallergenic tape will prevent severe reactions in most people, but occasionally it can lead to an allergic reaction during the test. These reactions may take up to 14 days to appear.
  • Sensitization – Some patients have reported their skin becoming more sensitive in the weeks following the beginning of a patch test.
  • Angry Back Reaction – If patches are applied when dermatitis is exceptionally active, it could cause multiple positive tests. These are difficult to interpret and may require the test to be repeated.

Any adverse reactions should be reported immediately. If severe enough, dermatologists may recommend stopping the test prematurely.

How is a Patch Test Interpreted?

After the panels are removed and the appropriate amount of time has passed, dermatologists will provide a full examination of the test area.

Each allergen will be graded on a spectrum ranging from negative to a ‘+++’ strong reaction. An alcohol swab may be applied to an impacted area because certain allergens will leave a colored residue. These residues should not be confused for an allergic reaction.

Understand that a negative result does not necessarily mean that the patient is not allergic. It could mean the reaction is endogenous or an irritant. Other tests will be required to confirm this.

A positive reaction can be interpreted in several ways, including:

  • Current Relevance – An allergen is relevant to a case of dermatitis currently under investigation.
  • Past Relevance – Someone may be allergic to something, but it is not relevant to the current case of dermatitis.
  • Future Relevance – The allergen is highly likely to be encountered going forward.
  • Uncertain Relevance – The presence of an allergen may change if a patient finds the true cause of their allergen, including through further investigation into exposure to occupational and care products.
  • Potential Cross Reaction – In some cases, patients may elicit a positive reaction to a substance within the allergenic substance. For example, a reaction may be found within PPD, a hair dye allergen, yet the reaction comes from the parabens or textile dyes.

Patients will typically receive an allergy patch test results chart to help them better understand different types of allergens and where they can be found. In many cases, patients may need to make lifestyle changes and make an effort to avoid certain substances to avoid triggering their allergic contact dermatitis again.

Most allergies cannot be cured, and doctors may not be able to explain why someone is allergic to one allergen but not another. It is a problem that must be managed through comprehensive lifestyle change and avoidance.


Patch tests are a trial-and-error process. Even if no positive test has been registered, every test provides more information on what has and hasn’t caused an allergic reaction.

Dermatologists are working to develop new treatments to help patients overcome the symptoms of allergies. Clinical trials remain ongoing to address common and uncommon allergies.

You can advance cutting-edge medical treatments by volunteering for a clinical trial in your area. The Dermatology CRO executive team at Vial has over 100 years of experience managing dermatology studies from start to finish.

To learn more about participating in a clinical trial near you, check out our study locations page.

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