What it is for
Patch testing investigates and confirms the substances that cause allergic contact dermatitis — a delayed reaction to something that touched the skin. It is described as the gold standard for diagnosing it, and it is the right test when a contact allergy is suspected in someone with recurring or long-running symptoms.
Not the same as the allergy test you may already have had. Prick tests and blood tests look for immediate, IgE-type allergies — foods, pollens, animals. Contact allergy is a slower, different mechanism, and those tests do not find it. That is why "my allergy tests came back clear" and "you might have a contact allergy" can both be true at once. See allergy testing for eczema.
How it works
- Diluted allergens are placed in small plastic or aluminium chambers on non-allergic tape and applied to the skin — usually the back, for practical reasons.
- They stay on for 48 hours.
- They are typically read again at 96 hours, because these reactions take 48–96 hours to appear.
- A positive result is a patch of dermatitis where that allergen sat — red, possibly raised, sometimes blistering.
So it is a test measured in days and multiple visits, not an afternoon. Worth knowing before you arrange time off.
When it's worth asking about
- Eczema that stays in one telling place — the eyelids, the hands, the face — especially if it is confined there.
- Eczema that improves on holiday and returns at work.
- Eczema that stopped responding to treatment that used to work.
- Long-standing atopic eczema that has changed pattern. Having atopic eczema does not protect you from also developing a contact allergy — in fact a damaged barrier lets more through.
Eyelid dermatitis is the classic example: a common cause is allergy to nail varnish or varnish remover, carried up by your own hands, and it is not something you would ever guess. If your eczema is confined to your eyelids, a referral to discuss possible allergic contact dermatitis is specifically recommended. See the face and eyelids and is it eczema or contact dermatitis?
What a result actually gives you
Identifying the allergen lets you avoid it, which is the whole point — the treatment for a contact allergy is not a stronger cream, it is removing the thing. That can mean reading ingredient lists for one substance for the rest of your life, which is a genuine adjustment, but it is a solvable problem rather than an endless one.
It's also worth saying that a negative result is information too. It closes a door you were spending energy on.
Bring the pattern that justifies the test
The case for patch testing is usually a pattern: better away from work, worse in a particular week, confined to one spot. Ollowen keeps the record that makes that pattern visible — and visible is what turns a hunch into a referral.
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Sources
- DermNet. Patch tests.
- American Academy of Dermatology. Patch testing can find what’s causing your rash.
- National Eczema Society. Contact eczema (dermatitis).
- National Eczema Society. Eczema around the eyes.
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