Patch testing isn’t a test for AD but a test for allergic contact dermatitis. In allergic contact dermatitis, it is the direct contact with an allergen which causes dermatitis, not the barrier dysfunction, inflammation and dysregulated immune reaction which is seen in AD.
July 5, 2023
From our research, when comparing the perceived value of patch testing between patients (before undergoing testing) and dermatologists, we found patients often over-valued the investigation. For patients (who hadn’t had patch testing), it provides hope that an as-yet unidentified trigger might be identified to help them cure/control their condition; while Dermatologists reported that for the average patient with AD few or no useful insights will be gained and no allergens will be identified.
What is patch testing?
The reason for the less positive outlook from the Dermatologists is patch testing isn’t a test for AD but a test for allergic contact dermatitis. You may remember that allergic contact dermatitis is a separate cause of eczema with a different underlying mechanism. In allergic contact dermatitis, it is the direct contact with an allergen which causes dermatitis, not the barrier dysfunction / inflammation / and dysregulated immune reaction which are seen in AD.
Unfortunately, to make things even more confusing allergic contact dermatitis can exist alongside AD. You may conclude that a blanket ‘test-all’ protocol would make sense but this would not only be expensive and unnecessary for the majority, but the testing process itself can also lead to individuals being sensitised to new allergens further exacerbating their condition.The crucial step therefore is identifying suitable individuals for testing.
How it works.
The principle behind the test is to expose the skin to a range of known allergens (chemicals that cause allergic reactions) and then monitor the skin to see if it causes an allergic response. For this group of allergic reactions, the response is delayed and takes days to develop.
1. Allergens are applied to the skin either as individual samples or as mixes (containing multiple allergens) and taped to the skin. The back is usually used as it is large, flat, hard to scratch and typically less affected by skin diseases.
2. Typically the patches are kept on for 48hrs and then assessed on days 2 (day of patch removal), 3/4 and 7.
3. Results are scored on a 6-point range
The 6-point scale gauges the reaction from 1 (negative reaction) to 6 (irritant reaction).
The most important part of the test is selecting the right allergens to test as there are hundreds of potential substances but individuals only have one back! Not having a positive reaction, therefore, doesn’t mean you don’t have any allergies only that you’re not allergic to what's been tested. An experienced dermatologist is able to recognise patterns in the history, areas of the body affected etc to predict relevant possible allergens to guide allergen selection.
When to carry out patch testing
Patch testing should be carried out on anyone who has suspected allergic contact dermatitis. Indicators of allergic contact dermatitis include:
1. No improvement in eczema despite appropriate treatment and avoidance of irritants
2. Cause of eczema unknown/regular relapse
3. Changes in eczema - previously well controlled, now not
4. Eczema that is worsening with treatment
5. Eczema on common allergic contact sites - face, eyelids, hands, legs, anus/vulva
6. Flares of eczema associated with work
Unfortunately, patch testing is expensive and therefore it is only offered to individuals where there is a high likelihood of an allergic element being identified. In private healthcare where users are paying, patch testing can be carried out more liberally.
Patch testing alternatives / DIY patch testing
For those of you who are into DIY. There is a test that is used by dermatologists which tests the same process but can be done at home for free. It’s called the Repeat Open Application Test and allows you to test products vs specific allergens but is a useful first step especially if you’re concerned about a specific product. The test works by applying your test product twice a day for 2 weeks over a 3x3cm area on the front side of your forearm towards your elbow. The applications should be applied twice a day for 2 weeks, or until a reaction develops. If you are highly suspicious that a product is causing a reaction you can increase the test to 4 weeks. If no reaction occurs you are unlikely to be allergic to the contents of the product in the current formulation.