Allergy Patch Testing
At Toowong Dermatology, we are highly experienced in performing allergy patch testing for the investigation of possible allergic contact dermatitis.
Could it be allergic contact dermatitis?
Allergic contact dermatitis should be considered in certain cases of dermatitis. These include chronic dermatitis in an unusual distribution (e.g. restricted to just certain areas of the body), dermatitis with unusual history (e.g. sudden onset of severe dermatitis, or dermatitis with no prior personal or family history of eczema), and especially chronic hand dermatitis.
What is allergic contact dermatitis?
In allergic contact dermatitis, exposure to a contact allergen (i.e. touching the allergen, or applying it to the skin) induces an allergic reaction, in those who are allergic to it. Contrary to common belief, allergic contact dermatitis commonly develops to products which have been used for a long time (e.g. someone can become allergic to the same perfume they have used for years, or to a component to the same shampoo used for years; someone can become allergic to gloves he/she has used for years, an concreter can become allergic to concrete after years of use).
Once allergic contact dermatitis has developed, any future exposure to the allergen, even in small amounts, will trigger further dermatitis. Conversely, if the allergen causing a dermatitis to become chronic can be identified and avoided, the dermatitis will improve, usually significantly.
However, allergic contact dermatitis often takes days to appear after exposure, and many things will have come in contact with the skin during that time. This makes finding the culprit allergen difficult. This is where allergy patch testing can help. (NB many forms of dermatitis are not due to allergy).
What is allergy patch testing and why is it useful?
Allergy patch testing is a test used for the investigation of possible allergic contact dermatitis. It involves placing multiple small chambers on the back, each containing a different potential allergen. If a reaction develops under a test chamber, it confirms allergic contact dermatitis to that reagent. Advice can then be given including where is it found, how to avoid, and advice on substitutes if necessary.
What does allergy patch testing involve?
Allergy patch testing involves applications of reagents on a Monday, removal of these on a Wednesday, and reading by your dermatologist on a Friday. The test takes several days, as it takes the body 2-4 days to develop dermatitis after exposure to an allergen.
Usually we perform testing to the Australian Standard Series, which is an extended series covering the 60 most common allergens in Australia. Using more abbreviated series (e.g. True test series with only 28 allergens) will miss up to half of relevant positive results. Each allergen has been tested to find the best concentration to demonstrate an allergic reaction without causing irritation to those who are not allergic to the material.
Sixty different allergens will be applied onto six adhesive templates and then placed onto your back. The back is the most appropriate place for patch testing, except in the case that you have active dermatitis, acne or scarring on your back. Alternative sites may be able to be used in these circumstances. In the case for males, nursing staff may be required to shave any excessive hair from your back prior to the application of the adhesive patch tests.
Allergy patch testing is not the same as skin prick tests. The latter can help the diagnosis of hay fever allergy (house dust mite, grass pollens and cat dander), and sometimes childhood eczema. Skin prick tests have limited value for most patients with skin rashes.