What is the patch test?
A Patch test is a procedure to diagnose allergic contact dermatitis, whereas the diagnosis of irritant contact dermatitis is based on the exclusion of an allergic component. Patch test techniques for diagnosing ACD have been used for over 100 years and the present test methods are based on the established principle of using a testing material (chemical, food, etc.).
Preparation for the patch test
It is important to prepare the skin for the patch test. Following these instructions will help you obtain more accurate and reliable results.
- Avoid exposure to sunlight for 1-2 weeks before a patch test.
- Do not use topical medications (creams and ointments) on the back and any other area where patches may be applied for at least a week before the patch test. You can continue to use these medications on areas of the body where patches will not be applied.
- You can use moisturizers on the skin until the day before the patch test.
How to patch test is performed
A patch test may involve either a skin test or a blood test. You may have to go on an elimination diet if your doctor thinks you might have a food allergy.
Skin tests are used to identify numerous potential allergens. This includes airborne, food-related, and contact allergens. The three types of skin tests are scratch, intradermal, and patch tests. Your doctor will typically try a scratch test first. During this test, an allergen is placed in liquid, then that liquid is placed on a section of your skin with a special tool that lightly punctures the allergen into the skin’s surface. You’ll be closely monitored to see how your skin reacts to the foreign substance. If there are localized redness, swelling, elevation, or itchiness of the skin over the test site, you’re allergic to that specific allergen.
If there’s a chance you’ll have a severe allergic reaction to a skin test, your doctor may call for a blood test. The blood is tested in a laboratory for the presence of antibodies that fight specific allergens. This test, called ImmunoCAP, is very successful in detecting IgE antibodies to major allergens.
The dermatologist will fill out the registration form on the second and third appointments (usually 48 and 96-hour readings). The result of each test site is recorded. The system we use is as follows:
- Negative (-)
- Irritantreaction (IR)
- Equivocal / uncertain (+/-)
- Weak positive (+)
- Strong positive (++)
- Extreme reaction (+++)
Irritating reactions include sweaty rashes, pore blisters, and burning-like reactions. Unconfirmed interactions indicate a pink area below the test chamber. Weak pulses are slightly raised pink or red plaques. Strong positives are “papular papules” and severe reactions are blisters or ulcers. Suitability depends on the size, type of dermatitis, and specific allergen. Interpreting results requires a lot of experience and training.