Patch testing identifies substances causing allergic contact dermatitis by applying small amounts of them to the skin under dressings and reading the result after a delay. It runs across several appointments over about a week, because the reaction being looked for is slow by nature.
What is patch testing looking for?
Allergic contact dermatitis, and specifically the delayed kind of immune reaction that produces it.
That mechanism dictates the whole design of a patch test. Immune cells in the epidermis that have previously learned to recognise a substance take time to respond when they encounter it again, and the visible reaction develops over a day or more rather than within minutes. A test for it must therefore involve exposure, a wait, and a look, in that order.
This distinguishes patch testing from tests for the immediate kind of allergy, where reactions appear quickly and different methods apply. The two answer different questions, and being sent for one when you expected the other is a common source of confusion.
How is a patch test actually carried out?
Small quantities of standardised substances are applied to discs or chambers, which are fixed to the skin of the back under adhesive patches, mapped so that each position is known.
What goes onto them is not chosen at random. A standard series covers substances commonly responsible for allergic contact dermatitis, and additional series are added according to your history, occupation and exposures. You may be asked to bring your own products, which can be tested alongside, and this is one of the more valuable things you can contribute.
The patches stay in place for a set period. They are then removed and the skin is read, and it is read again later, because some reactions appear only after further time has passed. Attending every appointment matters: a reaction that develops between readings and is never seen is a result lost.
The whole sequence typically runs across about a week, with visits at the start, the middle and the end. Your clinic will give you the exact schedule.
What are the practical restrictions?
The back must stay dry while the patches are in place, so washing is restricted, swimming is out and anything that makes you sweat heavily is unhelpful, since a patch that lifts takes its result with it.
The area needs to be clear beforehand. Active dermatitis on the back makes reading impossible, and sun exposure and certain treatments applied to the area can interfere. Some medicines taken by mouth also affect results, which is why the clinic asks what you take and may give instructions in advance. Those instructions come from them and take precedence over anything written here.
Itching where reactions develop is common and scratching is the recognised difficulty. Loose clothing helps. So does knowing in advance that it happens.
How are the results interpreted?
A positive patch shows that your immune system recognises that substance. It does not, by itself, establish that the substance caused the rash you came in with.
Making that link is the clinical work following the test: comparing what came up positive against what you actually encounter, where the rash appeared, and when. A positive result to something you never meet explains nothing, and a negative result across the board is a real finding rather than a failure, since it points away from allergic contact dermatitis and towards the irritant kind or something else.
You should expect an explanation of which substances came up, where they are commonly found, and what that means for you. That information is the entire point of the exercise, and it is worth asking for it in writing, since the list of places a substance turns up is long and easy to forget.
What is worth asking?
Ask which series are being tested and whether your own products can be included. The second is frequently possible and frequently overlooked.
Ask what to do about medicines and about anything you apply to your back, well before the appointment rather than on the day.
Enquire about the schedule of visits at the outset, since a patch test fails if a reading is missed and the appointments are close together.
Where a positive result comes back, ask for the practical list: what the substance is called on labels, where it appears, and what to do with that knowledge.
What this page does not do
It gives no substance names, no series contents and no instructions about preparation, all of which come from the clinic performing it. No result is interpreted here.
Common questions
Is patch testing the same as allergy testing with a needle?
No. Skin prick tests and blood tests look for immediate allergic reactions of a different type. Patch testing looks for delayed reactions in the skin, which is a distinct mechanism requiring a completely different method and a much longer interval before reading.
Why does it take several appointments?
Because the reaction is delayed by design. Patches stay in place for a period, are then removed and read, and are read again later, since some reactions appear only after further time. A single visit would miss precisely the reactions being looked for.
Does patch testing hurt?
The application does not. What is uncomfortable is the interval: the dressings must stay dry and in place, itching where a reaction develops is common, and not scratching is the main challenge people report. Discomfort comes from the waiting rather than the procedure.
Can I wash during patch testing?
The area must stay dry while the patches are in place, so washing is restricted and swimming and heavy exercise are ruled out. The clinic will give you specific instructions when they book you in, and those instructions take precedence over anything written here.
Does a positive result explain my rash?
Not automatically. A positive patch test shows sensitisation to a substance, and whether that substance is responsible for your particular rash requires linking it to your actual exposures. That interpretation is the clinical work that follows the test rather than the test itself.
Sources
- Contact dermatitis — conditions — NHS (United Kingdom) · accessed
- Patient information on skin conditions and treatments — British Association of Dermatologists (United Kingdom) · accessed
Last updated . How this page was written.