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Patch Testing

Patch testing is an essential investigation to identify specific allergens in allergic contact dermatitis, an immune-mediated skin rash caused by direct contact with an irritant.

Patch testing is based upon the principle that in sensitized individuals, T lymphocytes circulate throughout the body and are able to recreate a delayed-type hypersensitivity reaction when nonirritating concentrations of the antigen are applied to normal skin.

Patch testing is a time-consuming process that requires at least two-three visits during a specified week. Patients should avoid showering, exercising, and extremes of heat and humidity, and should be alerted that positive reactions can result in itching and discomfort.

Potent topical corticosteroids applied to the test site or oral corticosteroids ideally should be discontinued at least two weeks before patch testing.

Oral antihistamines may be continued during patch testing, as they have minimal if any effect on the mechanisms of delayed hypersensitivity. Since a positive patch test reaction is not a histamine mediated process, there is no pathophysiologic rationale to discontinue antihistamines prior to patch testing.

The patches typically are left in place for a period of two days (48 hours), which allows adequate penetration of the allergen into the skin. A second reading is critically important to distinguish irritant reactions (which fade) from true allergic reactions (which persist) and to identify allergic reactions that do not appear at the time of patch removal.

The time of the second reading generally is on day four or five. Often, a third reading is done a week later in some patients. A positive test may show redness, itching, bumps, mild swelling, and small blisters. Once all of the results are documented, the affected skin is treated with a topical steroid to alleviate possible itching and rash. Occasionally, patch test reactions can persist for a few weeks.

Studies have identified more than 4350 chemicals as contact allergens with varying potential to cause allergic contact dermatitis. However, a high proportion of allergic contact dermatitis are caused by a relatively small number of allergens commonly found in the environment.

Most frequent contact allergens include:

  • NICKEL, the most common detected allergen, which is used in batteries, buckles, coins;

  • METHYLISOTHIAZOLINONE, the “epidemic” allergen, which is widely used in cosmetics, hygiene products, and household goods;

  • BACITRACIN, an antibiotic that prevents bacterial infections;

  • BALSAM OF PERU, used in food and drink for sweet flavoring, perfumes, and medicine;

  • FORMALDEHYDE, preservative that can be found in building materials and household products;

  • URUSHIOL, a plant-based allergen found in poison ivy, poison oak, and poison sumac.

A positive reaction occurs in sensitized patients, who had at least one prior exposure to the offending substance. When the skin is re-exposed to the allergen, the cells that have already been sensitized (T-cells) overreact, leading to overproduction of inflammatory proteins (cytokines). The complex cascade response causes inflammation, itching, and rash.

Usually, it takes 2–4 days for an allergic reaction in patch testing to occur. In some cases, additional patches can be applied and removed a week later.

The North American Standard Series patch kit and T.R.U.E. Test panel are standardized panels most frequently used for patch testing in an allergy office. In the event that a standard series does not identify an offending antigen, patients may be patch tested with their own products.

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