Patch testing

A patch test is performed to find out if you are allergic to something that has been in regular contact with your skin.
We call these �allergens� and they can include: perfumes, cosmetics, clothing, shoes, jewellery, plants, glues, oils, and even creams used to treat skin problems.
This type of allergy usually develops over several days/weeks.
The patch test does not test for immediate allergic reactions and cannot be used to test for food allergies.

To receive your patch test, you will need to visit the hospital twice over 5 days � Monday afternoon and Friday morning (both in the Dermatology Outpatients Clinic 3B).

At the first consultation you will be seen by a Dermatologist who will decide which allergens you will be tested for.
Next you will see a nurse, who will apply a large number of plasters to your back. These plasters will contain small droplets of the allergens.
The position of the plasters is marked with a skin marker pen. Please wear an old vest, bra, or shirt, as the ink may leave marks on your clothes.
Allow about 30 minutes for this visit.
You will be instructed to carefully remove the patches at home after 48 hours with help from a friend/family member.
You can discard all patches in the bin.
The skin markings will need to be refreshed with your skin marker pen.
You must then take photographs/videos of the test area, with your mobile phone/tablet.
You must bring your device to the Friday appointment and the Dermatologist will look at the images you have taken.
At the Friday appointment the Dermatologist will take a final look at your back and will discuss the results with you. They will also create a management plan if required (allow about 30 minutes).

Sometimes it is possible to test some of your own products, if you think they are causing your skin allergy. For example: make-up, face cream, emollient, steroid creams.
Bring these with you as it is useful for the Dermatologist to see the packaging and the product ingredients.

For the first consultation please bring:

You must keep your back dry throughout the test, from Monday to Friday. Getting your back wet might stop the reaction occurring.
Do not sit in a hot bath or have a full shower. You should strip wash only.
Do not go swimming, sunbathe or exercise for the five days.
It is normal for patches to be itchy.
If possible, avoid taking antihistamines for 48 hours before, and for the duration of the test.
Try not to rub or scratch the patches, as they must not move out of place. Any loose patches should be taped over and protected.
If you do react (have a positive test result) to an allergen patch, the reactions are usually small, red, raised spots at the plaster site.
These reactions can usually be seen by the final Friday appointment, although they can occasionally appear later. If you do develop a late reaction, you can inform the Dermatology department on 0207 288 5062.
Sometimes the allergen samples might stain the skin - this is normal.
If you do not have any reactions to the patch test, this is still very helpful information. Your Dermatologist can move on to other possible reasons for your allergic reaction.

Side effects are rare, but may include:

skin reddening and itching, from positive test results - this usually disappears after a few days.
an ongoing reaction - some positive test reactions, may continue for up to a month.
a positive patch test may flare up any current or previous eczema.
an increase or decrease of skin pigmentation (lightening or darkening of the skin) at the site of positive patch tests.
infection or scarring - these are very rare.
very rarely (about one in 500 times) you may become allergic to one of the substances applied during patch testing. In practice, this does not seem to cause a problem in the long term.

Published:

10 Jul 2026

Review:

10 Jul 2026

Reference:

SC/Derm/PatchTest/06