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Patch testing is the best diagnostic tool to evaluate patients with suspected allergic contact dermatitis. The test involves applying common allergens to the patient’s skin and assessing for a skin reaction days later.

Patch testing in the primary care setting

Patients suspected of allergic contact dermatitis may benefit greatly from having this test accessible to them. This tool may be beneficial for primary care doctors especially when no local dermatologist is available.

Recognising allergic contact dermatitis

Allergic contact dermatitis presents as an erythematous, itchy eruption, sometimes with vesicles or bulla. It can be well-demarcated to the area of exposure or diffuse and generalised. Allergic contact dermatitis is a delayed hypersensitivity reaction. It requires sensitisation of the patient to the allergen, usually a product the patient has used in the past. Patients who have suspected allergic contact dermatitis can be further evaluated with patch testing.

How to perform patch testing

Patch testing can be performed using pre-packaged chemicals in the form of a patch (such as in the T.R.U.E. test), or dispensed in chambers. These test chemicals are placed on a large area of skin, usually the patient’s back. A map or reference for where which allergen is being placed is very important for test interpretation.

The patient needs to withhold all systemic or topical steroids for at least a week before testing. Patches should not be applied on any area of skin that has active rash or sunburn. Patches can be reinforced with tape to be secured in place. Patients are instructed to keep the area dry, which means avoiding excess sweating, swimming, or showering until the test is complete.

The first follow-up

The patient returns for their first follow-up in 48 hours. Confirm that all the patches have stayed in position. Mark the placement of all the patches or chambers with a permanent skin marker. Then, the patches or chambers are removed. Document any skin reactions present. At this stage, reactions are most likely irritant contact dermatitis. Patients are then instructed to go home and continue keeping the test site dry until the final read.

The final read

The final read takes place between 72 hours to 1 week after the initial application. Assess your patient’s back and reference a map of where the patches were initially placed. Skin reactions are graded on a scale from no reaction to strong reaction as outlined in the table below.

Interpreting results

Interpreting results for clinical relevance may be challenging. Patients may react to an ingredient that they had frequent exposure to in the past, but not in their current lifestyles. For example, a person who used to work as a florist but no longer gardens may still react to parthenolide, however, parthenolide may have nothing to do with why they now have persistent eyelid dermatitis. A careful discussion with the patient to obtain a detailed history of exposure is incredibly important to assess and treat patients with allergic contact dermatitis.

After the patch test

Once triggers have been identified, patients are provided handouts about common products containing the allergen in question. For example, patient handouts are available on the American Contact Dermatitis Society website (www.contactderm.org). Tell your patients to check ingredient labels on personal care products carefully and frequently. Favourite products often undergo formulary changes from time to time.

An alternative to the traditional patch test

Not all primary care providers have access to the patch testing systems that are commercially available. You can perform a modified patch test using products the patient brought to the clinic. This modified patch test cannot identify specific ingredient triggers, and patients may still be exposed to the allergen when they switch products if the ingredient is shared.

To perform this test, patients apply their product in question twice daily for 2 weeks to a pre-determined location without rashes, such as the antecubital fossa or forearm. If a reaction occurs, they should avoid further use of the tested product.

Learn more in the Professional Diploma program in General Dermatology.

Sources

Nixon RL, Mowad CM, and Marks JG. Allergic Contact Dermatitis. In: Dermatology. 4th ed. Philadelphia, PA: Elsevier Saunders; 2012:242 – 256.