Patch testing refers to a specialized procedure conducted by dermatologists to determine whether an individual has a contact allergy. This involves checking if the skin reacts to a chemical allergen from the environment that is usually harmless. Common allergens that trigger contact allergies are nickel in metal jewelry, rubber chemicals, perfumes, hair dyes, and lanolin. Numerous known allergens exist and comprise both natural and synthetic substances.
Normally, patch testing is recommended when a dermatologist suspects a contact allergy or needs to rule it out. While it may occasionally be used to investigate allergies to oral or injectable drugs, it is not designed to diagnose food allergies or reactions to airborne allergens like pollen or animal fur.
Contact allergies often result in dermatitis, characterized by a red, itchy rash that can persist for days, weeks, or even longer. These reactions can sometimes go unnoticed in individuals with pre-existing conditions, including atopic eczema, a type of constitutional dermatitis.
Patch testing is performed on normal skin, often on the upper arms or upper back. Small amounts of allergens are placed on the skin using adhesive tape (or the “patches”) to check for reactions. On average, 50 to 100 allergens are tested during the procedure.
The patches remain in place for two days before being removed; at this point, the skin is examined for signs of a reaction. A follow-up examination is conducted two days later to confirm the results. A positive reaction is identified by a small, elevated, itchy swelling roughly the size of a fingernail. This process usually concludes the patch testing.
During your initial consultation, you will meet with a consultant dermatologist—a medical specialist in skin-related conditions. They will discuss your symptoms and overall health and review your medical history. In addition, they will inquire about your job and home environment in order to determine potential substances to check.
The consultant will also assess your skin for contact dermatitis signs like rash or redness. If patch testing is required, you will be scheduled for an appointment with a GP who will carry out the procedure.
However, skin patch testing may not be recommended if you:
The final diagnosis will be made depending on the symptoms and patch test outcomes.
Before appointment:
During appointment:
Bring these items with you:
Patch testing often requires three clinic visits over one week:
The first visit involves applying small, one-cm-diameter aluminum discs with diluted versions of each substance to be analyzed on your back and secured using hypoallergenic adhesive tape. The position of every disc is marked with ink. This whole process takes about two hours.
The discs remain in the skin for 48 hours, during which you may experience mild itching near the patches—this is normal.
You will go back to the clinic after 48 hours for the removal of the patches. If needed, additional patches may be placed. Sometimes, regions of your back may be exposed to ultraviolet (UV) light to test for photoallergic reactions (photo patch testing).
The ink markings will stay on your skin for two days to help identify the tested allergens.
Your final visit, typically 48 hours following patch removal, involves a review of the test results with your consultant and a discussion of potential treatment options.
To ensure accurate results and prevent interference with the patches:
During your third appointment, your consultant will carefully inspect the skin on your back to check for any reactions to irritants or allergens. If no reaction has taken place, your patch test results will be negative. On the other hand, a positive response typically appears as itching, redness, or, in some cases, elevated or blistered skin near the test site. Reactions usually become evident by the third visit, though they may occasionally take longer.
If you notice a reaction after your final appointment, take a picture of the affected area and call the hospital for further advice.
After the skin examination, your consultant will discuss your outcomes in detail and suggest potential treatment options per your needs.
Patch test is a non-invasive and usually considered a safe procedure. However, in rare cases, side effects may arise. These can include:
Choosing the right dermatologist is important, especially if you have a chronic skin condition requiring ongoing care. For this reason, our Allergy Clinic aims to provide exceptional care in a comfortable and supportive environment to ensure you feel confident and at ease throughout your treatment.
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