Patch testing may help to find the cause of allergic contact dermatitis.
What is allergic contact dermatitis?
This is a condition where you develop patches of inflammation on your skin (called dermatitis) when your skin reacts against a specific substance. The patches of dermatitis are areas of skin which are itchy, red, and scaly. They may also blister. The substance you react to is called an allergen. You are not born with this type of allergy - you must have previously come into contact with the allergen which has sensitised your immune system. Your skin then reacts with a rash when it comes into further contact. See separate leaflet called Contact Dermatitis for more details.
Why is patch testing necessary?
If you know what you are allergic to you can try to avoid it, and you do not need any tests. However, some people develop patches of dermatitis and the cause is not clear. Many different chemicals which occur in metals, cosmetics, creams, leather, rubber, and other material may be responsible. Patch testing may help to identify the exact cause. It is not a foolproof test to find every cause of dermatitis, but often helps.
How is patch testing done?
You need to be referred to a skin specialist (dermatologist). If they agree that the cause of your rash is likely to be due to allergic contact dermatitis, they may arrange for you to come back to the skin department for patch testing:
- On day one of testing, tiny amounts of up to 25 or more substances are applied as small patches to your skin. This is usually on your upper back. They are fixed on with non-allergic tape.
- After two days you return to the department and the patches are removed. The skin is examined to see if there is a reaction to any of the tested substances.
- After a further two days the skin is examined again in case you have a delayed reaction to any substance.
Which substances are tested?
There is a standard set of the most common substances which cause allergic contact dermatitis. These include:
- Balsam of Peru
- Epoxy resin
- Imidazolidinyl urea
- Paraben mix
- P-tert butylphenol
- Formaldehyde resin
- Rubber accelerators
- Wool alcohols (lanolin)
You may not recognise many of these, but they are common additives to ointments, clothes, leathers, and other everyday materials.
Also, if other allergens are suspected, your skin specialist may add in other patches. For example, chemicals found in your workplace, or your own cosmetics. You may be asked to bring in small samples of these things to be added to the set of patches.
Tell your doctor if you suspect that the cause of the rash is something you were in contact with when the rash first appeared. This can often be tested. Remember, you can become allergic to something you have used many times before. For example, you can suddenly become allergic to a component in a favourite cosmetic which you have used many times before.
The test results
If you have a reaction to any of the substances, the skin specialist will be able to tell you what it is, and what materials contain that substance. They will give you advice on how to avoid that substance. Avoiding the substance should prevent any further flare-ups of the rash. If no skin reaction occurs on patch testing then this can also be helpful to rule out allergic contact dermatitis as a cause of your skin problem.
Some other points about patch testing
- Patch testing only tests for allergic contact dermatitis. It does not diagnose other types of allergy such as food allergy or urticaria.
- Keep the area of skin being tested dry until the final skin examination - which is usually four days after the patches are put on the skin.
- While patch testing is in progress, avoid activities that cause you to sweat a lot.
- Patch tests are not the same as skin prick tests which are sometimes used to diagnose other types of allergy.
- Patch testing cannot find the cause for atopic eczema.
- Keep sunlight and other sources of ultraviolet (UV) light off the skin being tested. So, keep a shirt on when outdoors for the duration of the test.
In some people, certain substances cause an allergic reaction in the skin only if they are exposed to and triggered by sunlight. (Usually the UV light in sunlight is responsible.) This may be suspected if your rash only appears on areas of skin exposed to light, such as the face, neck and back of hands.
With photo-patch testing, two identical sets of substances are put on your skin, as described above. One set is exposed to some UV light. The skin is examined in the usual way (after two and four days) and this may identify skin reactions to a substance only when it is exposed to light.
Dr Tim Kenny
Dr Mary Harding
Prof Cathy Jackson