Patch Testing for Eczema: How It Works and What It Can Diagnose

Eczema poses a massive public health burden in the US. It affects 16.5 million US adults and 9.5 million children, and costs about $5.3 billion annually. Understanding the pathogenesis and providing new treatments is essential to relieve this burden, and it begins with appropriate testing.

Eczema is a specific type of dermatitis, which is a general term for skin inflammation. Eczema involves inflammatory, non-infectious skin reactions caused by various factors along with a person's increased genetic sensitivity.

‍Patch testing is used to identify allergens that cause delayed allergic reactions, which might be linked to eczema flare-ups.

In this article we’ll discuss the indications and procedure for patch testing, and how it can help guide eczema management.

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What is Patch Testing?

Patch testing is a diagnostic procedure used to identify specific substances that trigger allergic contact dermatitis, a type of skin reaction that can co-occur with eczema. By detecting allergens that may be exacerbating eczema, patch testing helps guide more effective management.

Unlike skin prick tests, which are used to diagnose immediate allergic reactions (such as food or environmental allergies), patch tests assess delayed hypersensitivity reactions that develop over 48-96 hours. While skin prick tests are more sensitive, patch tests offer greater specificity in diagnosing triggering allergens.

This distinction makes patch testing particularly useful for creating targeted allergen avoidance strategies and, in some cases, for guiding immunotherapy in managing atopic eczema, especially for patients with skin flares due to environmental triggers.

How is Eczema Related to Contact Dermatitis?

Atopic eczema is a chronic inflammatory skin condition characterized by itchy, red, and dry patches, often associated with a heightened immune response and a genetic predisposition to allergies.

Atopic eczema is primarily driven by immune dysregulation which is often related to allergies, while contact dermatitis can be caused by either irritants (irritant contact dermatitis) or allergens (allergic contact dermatitis).

How is Patch Testing Different from Skin Prick Tests?

Patch testing applies environmental allergens like chemicals, metals, and cosmetics-related substances to the skin for several days to detect delayed allergic reactions. In contrast, skin prick testing introduces allergens through small punctures below the skin’s surface, with reactions observed within 15–20 minutes.

Patch testing assesses delayed, non IgE-mediated skin allergies, while skin prick tests focus on immediate, IgE-mediated reactions, often related to respiratory or food allergies.

When is Patch Testing Recommended for Eczema?

Patch testing is often recommended in cases of chronic or severe eczema, unexplained flare-ups, or when symptoms worsen unexpectedly or only in certain settings. It should also be considered in patients with suspected contact dermatitis, with or without other diagnosed skin conditions.

When standard eczema treatments such as topical steroids or moisturizers do not provide relief, it’s important to consider contact dermatitis as a potential cause. Identifying and eliminating allergens through patch testing can prevent unnecessary medication use and improve outcomes.

Patch testing can pinpoint specific allergens like metals, fragrances, or preservatives that may be triggering allergic reactions through a delayed hypersensitivity reaction, which is not assessed via skin prick tests.

One study demonstrated that in over 10% of cases of allergic contact eczema, patients were found to react to their own personal care products, illustrating the importance of assessing for contact dermatitis to identify underlying triggers.

How Patch Testing Works: The Process Explained

Patch testing is non-invasive and usually painless, although some medications and activities that involve sweating or getting the area wet need to be avoided before and during testing.

Preparation Before Testing

Before patch testing, expect to be informed about the procedure, including its purpose, benefits, and potential symptoms like itching or delayed reactions. Your doctor will want to discuss your medical history and suspected triggers.

You will be instructed to avoid activities such as showering, excessive exercise, and UV exposure for up to 15 days prior to testing, as these can interfere with the test.

Topical corticosteroids should not be applied to the test area for at least seven days prior to testing. Patients on systemic immunosuppressive treatments may need to postpone or pause testing to ensure accurate results, but this needs to be discussed with the ordering provider.

Although patch testing is not known to harm pregnant or lactating women, many dermatologists prefer to postpone testing during these periods as a precaution.

The Patch Test Procedure

Application: allergens are applied on the upper back using small chambers (like Finn chambers) with vehicles such as petrolatum or water for proper skin penetration. The chambers are secured with adhesive tape, and patients should avoid activities that may dislodge them.

First Reading (48 Hours): after 48 hours patches are removed, and initial reactions (redness, swelling, or vesicles) are recorded.

Second Reading (72-96 Hours): a follow-up reading checks for delayed reactions, evaluating redness and blistering per International Contact Dermatitis Research Group (ICDRG) criteria.

Interpreting Patch Test Results

Different types of reactions observed during a patch test, such as redness, bumps, and blisters, indicate varying degrees of sensitivity to specific substances.

Redness (erythema) is often the first visible sign and may suggest a weak positive reaction or irritation. Bumps (papules) typically reflect a stronger immune response, pointing toward a moderate sensitivity or allergic reaction. Blisters (vesicles) are associated with a strong positive reaction, indicating a high sensitivity or allergy to the tested substance.

Dermatologists interpret these reactions based on their severity and morphology, according to the International Contact Dermatitis Research Group (ICDRG) criteria.

The test results are classified as:

Negative (−): No visible reaction.

Doubtful (?+): Faint erythema (redness) without infiltration.

Mild positive (+): Erythema with papules (small bumps).

Strong positive (++ to +++): Increasingly intense erythema, with papules or vesicles (blisters), indicating a significant allergic reaction.

Irritant reaction (IR): Reactions that resemble allergic reactions but are marked by sharp edges or fine wrinkling, and are due to irritation from the test material or occlusion.

What is the Difference Between an Allergic Reaction and an Irritant Reaction on a Patch Test?

In patch testing, an immune reaction (allergic contact dermatitis) is a delayed Type IV hypersensitivity response that occurs 24 to 72 hours after exposure, involving T-cell activation and causing redness, swelling, papules, or vesicles.

An irritant reaction (irritant contact dermatitis) is a nonspecific response that happens quickly, within minutes to hours after contact with an irritant. It peaks rapidly and heals without immune involvement, causing burning, itching, and sometimes redness or mild swelling.

What Conditions Can Patch Testing Diagnose?

The following tests can be identified through patch testing:

Allergic Contact Dermatitis

Allergic contact dermatitis is a delayed hypersensitivity reaction where the skin reacts to small molecules, causing inflammation. Common allergens include nickel, fragrances, rubber, preservatives, and poison ivy.

Patch testing, the gold standard for diagnosing allergic contact dermatitis, helps dermatologists identify allergens and guide treatment by avoiding the offending substances.

Irritant Contact Dermatitis

Irritant contact dermatitis is a non-allergic skin response caused by direct damage to the skin from chemical or physical agents such as harsh soaps, cleaning products and solvents. It leads to inflammation without involving the immune system. Symptoms include burning, itching, and pain, often appearing soon after exposure.

Patch testing can help differentiate irritant contact dermatitis from allergic contact dermatitis. A lack of an immune reaction during patch testing suggests irritant contact dermatitis, helping guide treatment and preventive strategies focused on avoiding irritants rather than allergens.

Eczema Flare-Ups Linked to Allergens

Patch testing helps identify contact allergens that trigger eczema flare-ups, such as nickel, fragrances, or preservatives. By pinpointing these specific triggers, patients can avoid exposure and reduce the frequency and severity of flare-ups.

Patch testing results guide patients in choosing hypoallergenic products that don’t contain known triggers to prevent future eczema episodes.

What to Expect After Patch Testing

After patch testing, follow instructions carefully: keep the area dry, avoid strenuous activities, and stay out of the sun to ensure accurate results. Mild itching or discomfort at the test sites is normal.

At the follow-up, the dermatologist will review the results, explain any positive reactions, and discuss which allergens triggered a response.

If a positive result is found, allergen avoidance is key. The dermatologist will help you adjust your skincare routine, avoid products with the allergen, and suggest hypoallergenic alternatives or changes in treatment, such as topical medications.

Benefits and Limitations of Patch Testing

Patch testing offers several benefits to people suffering with overwhelming and confusing eczema outbreaks, but it also has some important limitations.

Benefits

Patch testing accurately identifies contact allergens, enabling personalized treatment plans that help manage eczema by avoiding triggers and reducing flare-ups.

Limitations

Patch testing only identifies contact allergens and cannot diagnose food or environmental allergies, or all eczema types. False positives or negatives are possible, so professional interpretation from a dermatologist is necessary for accurate diagnosis and management.

Is Patch Testing Right for You?

Patch testing is ideal for people with persistent, overwhelming or unexplained eczema flare-ups, especially if standard treatments haven't been effective. It's important to consult a healthcare professional to determine if patch testing is suitable for your specific condition and symptoms, and for proper test interpretation.

Patch testing is typically recommended for those who suspect a contact allergy is contributing to their eczema or if environmental triggers are suspected as a cause.