Allergy Patch Testing

What is allergy patch testing?
Substances tested in patch testing
Patch testing schedule and procedure
When to consider allergy patch testing
When to postpone patch testing
How to interpret allergy patch test results
Risks and complications of allergy patch testing
Patch testing during pregnancy or lactation
Aftercare and managing allergic reactions
References
Further reading

What is allergy patch testing?

Allergy patch testing screens substances to determine the cause of an allergic skin reaction. Patches with different suspected irritants (allergens) are applied to a person’s back and left in place for 48 hours. The skin is then examined for any signs of a hypersensitive reaction.

Patch testing detects allergic contact dermatitis, a type IV hypersensitivity reaction. However, this allergy test cannot identify the cause of urticaria (hives) or food allergy. Prick testing, another allergy test, is used for type I hypersensitivity reactions associated with hay fever, asthma, and contact urticaria. Unlike patch testing, prick tests involve pricking the skin with a needle to test the blood, which can be read after just 20 minutes.

Substances tested in patch testing

The types of allergens tested in an allergy patch test include:

• Hair dye
• Preservatives
• Cosmetics
• Medications

Most patients use a standard set of allergens, such as those described in the European Standard Series of allergens, as these account for 85% of contact dermatitis cases. Additional allergens tailored to the patient, including materials from their home or work, may also be tested. The concentration of each substance is carefully selected to induce an allergic reaction in susceptible individuals without irritating the area of skin that is not allergic.

Patch testing also helps identify the cause and mechanisms of cutaneous adverse drug reactions (CADRs). The clinical relevance depends on the drug involved and the type of CADR investigated. These include generalized eczema, systemic contact dermatitis, maculopapular rash, acute generalized exanthematous pustulosis, and fixed drug eruption. Approximately 32-50% of drug patch tests yield positive results in patients with CADRs.

Patch testing schedule and procedure

The typical patch testing schedule involves three main steps:

• Day 1: Plastic or round aluminum square chambers containing small amounts of potential allergens are applied to the patient’s upper back and secured with hypoallergenic tape
• After 48 hours: The patches are removed, and the skin is examined for any reactions
• After 96 hours: A second evaluation is performed

Some individuals may require an additional assessment after 7 to 10 days for a more accurate diagnosis.

When to consider allergy patch testing

Patch testing is recommended in patients with:

• Suspected acute or chronic contact dermatitis, including cases triggered by occupational exposures
• Other forms of eczema or persistent dermatitis do not improve with standard treatment
• Skin or mucous membrane eruptions where delayed-type hypersensitivity is suspected, such as in delayed-type drug eruptions

These conditions highlight individuals who may benefit most from the allergy patch test process by identifying specific allergens that could be responsible for their skin reactions.

When to postpone patch testing

In certain circumstances, patch testing should be postponed to avoid unreliable results –

• Severe, widespread active dermatitis could interfere with an accurate assessment
• Dermatitis at the test site, particularly on the upper back or other areas intended for patch application
• Recent use of topical corticosteroids on test sites, as they may suppress allergic reactions temporarily. Typically, a seven-day pause is recommended to allow the skin to recover
• Recent ultraviolet (UV) exposure to the test area, such as UV light, can alter skin sensitivity and affect test results

Ensuring that these conditions are managed before testing is crucial for obtaining accurate results, as they can compromise the reliability of patch testing.

How to interpret allergy patch test results

Interpretating allergy patch test results requires experience and training. It is important to distinguish between irritation and an allergic reaction. The former occurs after the patch is removed but will reduce over the following day, whereas an allergic reaction will develop over several days and is more prominent on day five rather than immediately following patch removal. Reactions can include:

• Negative: No visible changes to the skin
• Irritant Reaction: Burn-like rash or follicular pustules
• Uncertain: A pink area under the test chamber
• Weakly Positive: Slightly elevated pink or red plaques
• Strongly Positive: Papules and vesicle
• Extreme: Blisters or ulcers

Patch tests may not always be conclusive. False negatives, in which no reaction arises despite an underlying allergy, and false positives, which indicate an allergy when there is not one, can occur. Patients can react differently to substances during a test than in everyday exposure or to the same test performed at different times. Retesting or single-allergen tests may be necessary for confirmation.

Risks and complications of allergy patch testing

While generally safe, allergy patch testing can result in:

• Itchy, active dermatitis at the test site, treated with topical steroids

• Provocation of dermatitis in other areas outside of the patch site
• Sensitivity to the adhesive tape used
• “Angry back”, usually in patients with active dermatitis before testing commences or in those who experience many strong positive reactions. This results in many false positives for the tested substances
• The generation of sensitivity to a new allergen (rare) occurs approximately 10 days after testing
• Reactivation of the original rash, with dermatitis becoming more widespread.
• Darker skin may develop dark patches that persist for many weeks/months.

These complications may interfere with the interpretation of the allergy patch test process. Therefore, retesting may be required, sometimes one allergen at a time, to confirm the results.

Medications can also interfere with the results of a patch test. Different drugs can either suppress or increase the risk of developing allergic reactions, which reduces the accuracy of the test. These drugs can include antihistamines, tricyclic antidepressants, heartburn medications, and the asthma medication omalizumab. Patients may need to stop taking medications for up to 10 days before a patch test, depending on how quickly they are cleared from the system.

Patch testing during pregnancy or lactation

Patch testing during pregnancy or lactation is generally not known to be harmful; however, dermatologists usually postpone testing during these life stages as a precaution. This helps account for potential changes in skin sensitivity, hormonal factors, and skin reactions that can arise during these periods, which may affect test outcomes.

Aftercare and managing allergic reactions

By identifying their triggers, patients can take steps to manage symptoms, such as:

• Avoid allergens.
• Using topical treatments or antihistamines.
• Making dietary changes.
• Adjusting their work or home environment.

Effective management may also include immunotherapy and collaboration with healthcare providers to develop personalized treatment plans.

References

• Patch tests: Skin Contact Allergy Tests Explained - DermNet (2024) DermNet®. Available at: https://dermnetnz.org/topics/patch-tests (Accessed: 8 January 2025).
• Allergy skin tests (2024) Mayo Clinic. Mayo Foundation for Medical Education and Research. Available at: https://www.mayoclinic.org/tests-procedures/allergy-tests/about/pac-20392895 (Accessed: 8 January 2025).
• Barbaud A. Drug patch testing in systemic cutaneous drug allergy. Toxicology 2005;209(2):209-216.
• Johansen, J. D., Aalto-Korte, K., Agner, T., Andersen, K. E., Bircher, A., Bruze, M., Cannavó, A., Giménez-Arnau, A., Gonçalo, M., Goossens, A., John, S. M., Lidén, C., Lindberg, M., Mahler, V., Matura, M., Rustemeyer, T., Serup, J., Spiewak, R., Thyssen, J. P., Vigan, M., … Uter, W. (2015). European Society of Contact Dermatitis guideline for diagnostic patch testing - recommendations on best practice. Contact dermatitis, 73(4), 195–221. https://doi.org/10.1111/cod.12432

Further Reading

• All Allergy Content
• What are Allergies?
• Different Types of Allergies
• Old Friends Hypothesis
• What is the Microbial Diversity Hypothesis?