Patch testing can help find the cause of allergic contact dermatitis.
On this page
- What is allergic contact dermatitis?
- Why patch tests are necessary?
- How do patch testing?
- What substances are tested?
- The test results
- Some other points about patch testing
- Photo-patch tests
- More help and information
What is allergic contact dermatitis?
This is a condition in which patches are developed dermatitis (eczema) when the skin reacts to a specific substance. Dermatitis patches are areas of skin that are itchy, red and swollen. You can also form blisters. The substance reacts to an allergen is called. You are not born with this allergy – you must have first contact with the allergen that has sensitized immune system. For reasons not fully understood, the body begins to react when contacted further. See separate leaflet called "contact dermatitis" for details.
Why patch tests are necessary?
If you know what you are allergic to you can try to avoid it, and do not need any tests. However, some people develop patches of dermatitis and the cause is unclear. Many different chemicals are produced in metals, cosmetics, creams, leather, rubber and other materials may be responsible. Patch testing can help identify the exact cause. It's not a foolproof test to find all causes of dermatitis, but often helps.
How do patch testing?
You need to be referred to a dermatologist (skin specialist). If you agree that the cause of the eruption is likely to be due to allergic contact dermatitis, arrangements can be made to return to the department of skin patch test:
- On the first day of the tests, small amounts of up to 25 or more substances are applied as small spots on skin, generally in the upper part. They are fixed with non-allergic tape.
- After two days back to the department and the patches are removed. The skin is examined to see if there is a reaction at any of the tested substances.
- After two days the skin is examined again in case you have a delayed reaction to some medication.
What substances are tested?
There is a standard set of the most common substances that cause allergic contact dermatitis. These include: Peru balsam, benzocaine, chromium, clioquinol, cobalt, epoxy resin, ethylene, formaldehyde, fragrance, imidazolidinyl urea, neomycin, nickel, paraben mixture, paraphenylenediamine, plants, p-tert-butylphenol formaldehyde resin, Quaternium-15 , rosin, rubber accelerators, wool alcohols (lanolin). You may not recognize many of these, but they are common additives to ointments, clothing, leather and other everyday materials.
Moreover, if suspected of other allergens, your skin specialist can add in other reviews. For example, chemicals found in your workplace, or your own cosmetics. You may be asked to bring small samples of these things that are added to the set of patches.
Tell your doctor if you suspect that the cause of the rash is something that were in contact with the rash when it first appeared. This can often be tested. Remember, you can become allergic to something that has been used many times before. For example, can suddenly become allergic to a component of a cosmetic choice that you have used many times before.
The test results
If you have a reaction to any of the substances, the dermatologist will be able to tell you what it is, and what materials containing the substance. They will give you advice on how to avoid the substance. Avoid substance should prevent any further outbreaks of the eruption. If no skin reaction in the patch tests, this may also be useful to rule out allergic contact dermatitis as a cause of your skin problem.
Some other points about patch testing
- Patch tests only test for allergic contact dermatitis. No other types of diagnosing allergy or food allergy, such as urticaria.
- Keep the skin area being tested dry until the end exam of the skin – which is usually four days after the patches are applied to the skin.
- While patch testing is in progress, avoid activities that make you sweat a lot.
- Patch tests are not the same as that skin tests are sometimes used to diagnose other types of allergy.
- Patch tests can not find the cause of atopic eczema.
- Keep sunlight and other sources of ultraviolet (UV) skin being tested. Therefore, maintaining a shirt when outdoors during the duration of the test.
In some people, certain substances cause an allergic skin reaction only if they are exposed to and activated by sunlight. (Usually UV light in sunlight is responsible.) This can occur if the rash appears only in the areas of skin exposed to light, such as the face, neck and back of hands.
With the photo-patch tests, two identical sets of substances is placed on the skin, as described above. One group is exposed to a UV light. The skin is examined in the usual way (after two to four days) and this may identify skin reactions to a substance when exposed to light.
More help and information
National Eczema Society
Hill House, Highgate Hill, London, N19 5NA
Tel (helpline): 0800 089 1122 Web: www.eczema.org