Patch testing is a process used to detect whether someone has allergic contact dermatitis. During patch testing, small amounts of chemicals are diluted and placed onto discs mounted on hypoallergenic tape and then placed on the back.
The number of patches applied is specific to each individual. There are 10 discs on each patch, and up to 10 patches are placed on the back.
Taking a history about your exposure to potential allergens at home and at work helps the doctor decide what the most likely problem substances are, and test these. It is not possible to test you against all available allergens, only the most likely.
Chemicals from the “Australian Baseline Series” are almost always used. There are 60 allergens in this series. If you have specific exposures to other chemicals at work or at home, allergens from other series may be chosen. These are ordered from the Contact Allergen Bank Australia, in Melbourne.
Most patients have to attend 3 appointments over a 5 day period. Patches are applied on a Monday and then removed after 48 hours on a Wednesday. Dr Drummond or one of the practice staff will look for any reactions at that time, and again after 4 days on the Friday, to check for late reactions. Dr Drummond will then discuss the results, management of your condition and skincare.
Patches must be kept dry during the testing. Showers must be avoided for the length of the testing (Monday through to Friday after final appointment). This is to allow reliable assessment of reactions. During testing, sweating must be avoided, as sweating can sometimes cause patches to fall off or loosen.
What if the patch testing is negative?
If this testing is negative, it may suggest that you have a different skin condition such as irritant contact dermatitis, eczema or contact urticaria (other forms of testing known as prick testing or a blood test is used to diagnose contact urticaria).
Patients are often disappointed if patch testing is negative but really this is very helpful as it means none of the products you are using are the direct cause of your dermatitis.
Skin prick testing is different from patch testing, and is used to assess type I or immediate reactions to food or airborne allergens. This type of allergy presents as urticaria (hives). Skin prick testing is done by immunologists.
What is Contact Dermatitis?
There are two main types of contact dermatitis: allergic contact dermatitis and irritant contact dermatitis.
Irritant contact dermatitis is the most common type of dermatitis but there is no specific test for it. Instead it is diagnosed based on your history of exposure to different substances, and negative patch tests. Common skin irritants are soaps and detergents, water, heat, sweating and friction.
Allergic contact dermatitis is diagnosed by patch testing. This is a delayed type hypersensitivity or allergy.
People are not born allergic to substances, but they can develop contact allergies at any time during their life. To become allergic to something, you must have been exposed to it before. The type of allergy that causes allergic contact dermatitis lasts for life. Your body has learnt to react to this substance and each time that you are exposed to it, it will produce an allergic reaction (skin rash).
The rash that develops can be treated, but there is no cure or treatment that will stop you being allergic to a substance. The best treatment is to avoid exposure to the substance in the future.
If the particular substance can be avoided, then the rash should settle within a few weeks. This will take longer if there is continued exposure, or if the initial rash was very severe or had been present for a long time. If irritant factors are not addressed, dermatitis will also persist. The skin will remain quite easily irritated for some time after it appears to be better, so good skin care is essential.
If you have a personal or family history of eczema/dermatitis, hayfever or asthma this is called atopy and means you have sensitive skin , and are more prone to develop both irritant and allergic contact dermatitis.
Patch Testing Information Sheet
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40-42 Corinna St, Phillip ACT 2606
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Drummond Dermatology acknowledges, celebrates and pays respect to all First Nations Australians on whose traditional lands we meet and work, and whose cultures are among the oldest continuing cultures in human history.