Medical skin conditions

Allergy patch testing

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 such as confined to certain areas of the body or only to the hands
  • Dermatitis with unusual history such as sudden onset of severe dermatitis with no prior personal or family history of eczema

We are highly trained in performing allergy patch testing at Queensland Specialist Dermatology to investigate the cause of possible allergic contact dermatitis.

What is allergic contact dermatitis?
People with allergic contact dermatitis to certain allergens with have an allergic reaction after coming into contact with the allergen (i.e. touching an allergen, or applying it to the skin). Contrary to popular belief, allergic contact dermatitis commonly develops to products that have been used for a long time. For example, someone can become allergic to perfume or shampoo that they have used for many years. In an occupational setting, someone can be allergic to the gloves that they have used for many years.

Once allergic contact dermatitis has developed, any future exposure to the allergen will trigger further dermatitis, even in small amounts. Conversely, if the allergen leading to chronic dermatitis can be identified early, avoidance will significantly improve symptoms.

However, allergic contact dermatitis often takes days to present after initial exposure. By this time, many substances would have been in contact with the skin. This makes identifying the culprit allergen difficult. This is where allergy patch testing can help. (NB many forms of dermatitis are not due to allergy).

Allergy patch testing

What is allergy patch testing and why is it useful?
Allergy patch testing is a test to investigate possible allergens leading to contact dermatitis. It involves placing different potential allergens in rows of multiple small chambers on the back. If a reaction develops under a test chamber, it confirms allergic contact dermatitis to that reagent. Advice can then be given to prevent future exposure including where it is found, how it can be avoided and substitutes if necessary.

What does allergy patch testing involve?
Allergy patch testing involves application 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 the body takes 2-4 days to develop dermatitis after exposure to an allergen.

Usually we perform testing to the Australian Standard Series. This 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 diluted to the best concentration to demonstrate an allergic reaction without causing irritation to those who are not allergic to the material. Your dermatologist may suggest testing to additional products, especially your own products. This helps establish relevance with your own products, and can help investigate for allergy to allergens not included in the standard series.

Sixty different allergens will be applied onto six adhesive templates and then applied on your back. The back is the most appropriate place for patch testing. Exceptions are if you have active dermatitis, acne or scarring on your back. Alternative sites may be used in these circumstances. In the case for males, nursing staff may need to shave any excess hair from your back prior to the application of the adhesive patch tests.

What is the difference between allergy patch testing and skin prick tests?
Allergy patch testing is different to skin prick tests. The latter can help with the diagnosis of hay fever allergy (house dust mite, grass pollens and cat dander) and sometimes childhood eczema. However, skin prick tests have limited value for most patients with skin rashes.

The patch test
Patch test reading

Patch test reading is completed by your Dermatologist two days after the reagents are removed. Site reactions are assessed and graded as below:

Patch test scoring according to the International Contact Dermatitis Research Group. ?+: Doubtful reaction (faint, nonpalpable erythema); +: Weak reaction (palpable erythema – moderate edema or infiltrate, no or scarce papules, no vesicles); ++: Strong reaction (strong infiltrate, numerous papules, vesicles); +++: Extreme reaction (coalescing vesicles, bullae or ulceration); IR: Irritant reaction. Other symbols used for recording patch test results: – or ø: Negative (no visible change in tested area); NT: Not tested

Before your Allergy Patch testing:

  • There must be areas on your back clear of eczema (positive tests induce a small patch of eczema – it must be clear before you start).
  • Avoid topical corticosteroid to the test area (usually the back) for at least 1 week prior to the test.
  • Avoid systemic (tablet) immunosuppressants (e.g. prednisone, cyclosporine, azathioprine, methotrexate) for at least 1 week prior to the test. (Discuss with your doctor first).
  • Avoid sun to your back for 1 week prior to testing (sunlight is immunosuppressant, and will reduce the chance of finding a relevant positive result).

Risks:

  • Allergens may provoke small areas of active eczema / dermatitis which could be itchy and may require treatment with a steroid cream. Occasionally reactions may persist for a few weeks after the testing has finished. Patch tests may provoke other areas of dermatitis to recur or to appear for the first time.
  • Although hypoallergenic tape is used, occasionally people react to all areas in contact with the tape.
  • An ‘angry back’ reaction may arise, particularly in a patient with active dermatitis at the time of testing, or in someone who has multiple positive reactions. ‘Angry back’ refers to false positives to many or all of the tested allergens.
  • Rarely, sensitisation to a new allergen may occur as a result of the test – this is revealed as a reaction occurring around 10 days after the test was applied.
  • Re-testing may be required, sometimes one allergen at a time, to confirm or clarify a reaction.

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