Allergy Patch Testing
What is patch testing?
Allergy patch testing is a common allergy testing method used to confirm the cause of allergic contact dermatitis in a patient. Allergy patch testing is also a preferred method for identifying delayed allergic reactions that may take several days to present and are known as type IV hypersensitivity reactions.
It is an easy and topical process that involves little pain or inconvenience for a patient. All testing is done on the upper back so as not to limit a patient’s movement. The procedure involves no pricks or scratches to the skin.
Patch testing is a different procedure than skin prick testing. Skin prick testing is used by allergists to determine which foods or allergens are triggering an acute or immediate allergic reaction in a patient. Patch testing, on the other hand, is used solely to determine whether a patient’s dermatitis is caused by some substance in the patient’s environment, including metals, preservatives, medications, etc.
How does patch testing work?
Patch testing is a relatively easy and painless experience for patients. It is the quickest way to determine which environmental substances are causing contact dermatitis. Patients will then be able to eliminate these substances from daily life and reduce or eliminate the symptoms of their contact dermatitis. While patients may experience some light itching or burning from the test, the application process is easy and involves no aggravation of the skin.
In skin patch testing, the doctor applies a series of rectangular patches to the upper back. These patches adhere lightly to the skin with a tape adhesive. Each patch contains several common allergens that can trigger contact dermatitis. The patient wears these patches for three days, then returns to the doctor for initial observations on Day 3.
During this appointment, the allergist observes and marks the skin to note any preliminary skin reactions. The patient then returns for a second follow up appointment on Day 5. This appointment is used to determine whether or not the patient has had any delayed, type IV hypersensitivity reactions to an allergen.
Once the testing is completed, the allergist will determine which allergens the patient’s body reacts to, review all results with the patient, and develop a treatment plan to address the patient’s contact dermatitis.
How to prepare for patch testing
Since patch testing involves adhering testing patches to the skin, it is important that patients adequately prep the skin before their test. Patients with excess hair on the upper back should shave their back prior to the first appointment. All patients should plan on wearing loose clothing to their appointment and avoid applying any perfumes or creams to their skin on testing day.
Patients should also avoid using topical steroids on the back for two weeks before the test. Finally, patients should avoid all sun exposure for 1 to 2 weeks before testing. During the testing period, patients should continue to avoid sun and also avoid exercise, showering, and excessive sweating.
Types of results from patch testing
Patients can have any one of several different reactions and results to skin patch testing. There are three general categories of positive results: mild redness, bright red, and blistered skin. The more severe the reaction to the allergen is, the more sensitive a patient is to that substance.
Light reactions indicate some but not much sensitivity to a substance. All results can help a patient and their doctor develop a comprehensive treatment plan, including developing a list of which substances to avoid. Negative tests are typically a confirmation that a patient is not allergic to that substance. It is important to understand that patch testing often involves narrowing down a list of triggers and can be part of an ongoing process of diagnosis.
Side effects of patch testing
During testing, patients may experience some flare up of their dermatitis, along with associated aggravation and itching. These symptoms will resolve after the removal of the testing patches. It is important that patients avoid scratching the area during the testing period. Any patient who experiences extreme pain or distress during testing should contact their doctor immediately.