Patch testing may help to find the cause of allergic contact dermatitis.
This is a condition where you develop patches of dermatitis (eczema) when your skin reacts against a specific substance. The patches of dermatitis are areas of skin which are itchy, red, and inflamed. 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. For reasons not fully understood, your body starts to react when it comes into further contact. See separate leaflet called 'Contact Dermatitis' for more details.
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.
You need to be referred to a dermatologist (skin specialist). 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:
There is a standard set of the most common substances which cause allergic contact dermatitis. These include: balsam of Peru, benzocaine, chrome, clioquinol, cobalt, epoxy resin, ethylenediamine, formaldehyde, fragrances, imidazolidinyl urea, neomycin, nickel, paraben mix, paraphenylenediamine, plants, p-tert butylphenol, formaldehyde resin, quaternium-15, rosin, 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.
If you have a reaction to any of the substances, the dermatologist 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.
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.
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