What is Tested in a Skin Patch Test?

Date: Oct 2018

Finding out what you’re allergic to can be tricky. Our skin comes in contact with so many different things that it can be impossible to tell which ones are causing problems. If you have an allergy or a condition like eczema that is triggered by certain things then your dermatologist may suggest a skin patch test to find out what is responsible.
Skin Patch Test on Skin | Harley Street Dermatology Clinic

What is a Skin Patch Test?

A skin patch test is a way to work out exactly what is causing your symptoms. You might have it to find out what’s triggering the symptoms of a chronic skin condition or to identify the cause of an allergic reaction. The patch test procedure has been used for more than a century as it is a very clear and simple way to diagnose these problems. A large number of different allergens can be tested systematically so that we can see how your skin responds.


During the test we will be looking for an allergic response: redness, swelling and itchiness. Allergies happen when your immune systems respond to a specific trigger that it has identified as a threat. Although this reaction is very useful when the trigger is a germ that needs to be eliminated, it can cause problems if the immune system overreacts or tries to attack something harmless.


We can check for lots of different allergies during a skin patch test. The test will usually include the most common allergens, but if you’re worried about a specific product or trigger then it can be checked too. A typical patch test might include 40 different allergens, including substances that often come in contact with the skin such as metals, rubber, perfumes and ingredients from toiletries and cleaning products.


Why Have an Allergy Test?

You might think that you can just work out what’s causing your symptoms without having a test. Sometimes it is possible to identify an obvious allergy. The symptoms might have appeared just as you came in contact with something unusual. However, if you often develop symptoms or react to lots of different skincare products, then it can be a good idea to get tested.


Identifying the cause of an allergy can be difficult because:


  • There are so many possibilities that it can be impossible or incredibly time-consuming to check them all in turn. Sometimes we aren’t even aware of everything that affects our skin, such as plants in the garden or cleaning products used in our offices. You might be allergic to multiple things or to a single ingredient that happens to be in lots of different products.
  • It’s easy to make mistakes, for example if your skin happens to improve at the same time as you stop using a particular product you might believe it was the cause even if it’s just a coincidence. You could waste a lot of time and effort avoiding these products.
  • You will have to test each individual item on a case by case basis, for example every time you change your toiletries, rather than finding out which ingredient is to blame so that you can avoid all products that contain it.

Having a Skin Patch Test

The skin patch test is very straightforward. It is just a systematic way to look at how your skin responds to a range of different allergens:


  • Your back will be marked with ink so that we know exactly where each allergen is applied.
  • Sticky patches containing the allergens will be stuck on to your back. You will need to keep these on throughout the test, so you shouldn’t shower or do anything that could dislodge them.
  • You’ll come back to the clinic in a couple of days so we can check how your skin is responding. The markings and patches on your back can be re-applied as necessary.
  • The patches will be removed at your third appointment and your skin’s responses to the allergens will be assessed again.
  • We’ll rate each patch according to how your skin responds, from a negative (nothing happening) through to a strong allergic reaction.
If you do respond to any of the allergens, then you will develop symptoms that are similar to the ones we’re investigating. You might have some reddening, itchiness, blistering or an eczema flare up. The response will usually be limited to the area under a specific patch, which will tell us that you are allergic to the substance it contains.


Using Your Results

Once we have the results of your skin patch test, we’ll hopefully be able to say exactly what is causing your symptoms. You can then take steps to avoid this allergen, for example by getting rid of a plant from your garden or avoiding using products containing certain ingredients in your home. Your dermatologist will help you to understand the results and provide advice on avoiding allergens and looking after your skin. As long as you don’t come into contact with your triggers, they won’t cause any problems.


However, there are some limitations to this test. There is a small chance that you could be allergic to something that wasn’t included in the test. You could also have an allergy to something that you are eating or inhaling (like pollen), which can cause symptoms in your skin. The patch test will only look for allergens that affect you through contact with your skin.


It’s also possible that you’ll get negative results for everything we’ve tested. While this can be disappointing, it’s important to remember that ruling allergens out can be almost as useful as identifying the cause. You’ll have a list of substances that are safe for your skin and that you don’t need to avoid. We will also be able to narrow down our search for the actual cause, which can be checked with another skin patch test if necessary.


You can be allergic to all kinds of different things, so it’s understandable that they aren’t always included in the list of common allergens that we check first. Do you have any unusual allergies?

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