Skin prick testing at Oxford, The Manor Hospital

What is skin prick testing?

Skin prick testing is a simple test to help find out which substances you might be allergic to. A small amount of a suspected allergen is placed on your skin, which is then gently pricked so a tiny amount gets just under the surface, and your skin's reaction is checked a short while later.

Why might I need it?

Your doctor might suggest skin prick testing if they suspect you have an allergy, based on symptoms such as a runny nose, itchy eyes, hives, or breathing difficulties. It's commonly used to help diagnose hay fever and food allergies and can also help with conditions like latex allergies.

This test can sometimes be used for drug or insect venom allergies, so check with your own clinic which allergies they're able to test for.

What are the benefits of having it?

Skin prick testing is quick and simple, and much faster than a blood test. Results are usually ready within 15 to 20 minutes, compared with around 4 weeks for a blood test.

A negative result is generally reliable and means you're unlikely to be allergic to that substance. Your results can also help guide your treatment, such as whether to avoid a particular food, remove a pet from your home, or consider a longer course of treatment called immunotherapy.

Are there different types of test?

Skin prick testing is one of a few ways to test for allergies. Others include:

• a blood test, which is slower, usually taking about 4 weeks for results
• a supervised food challenge, where you're gradually given increasing amounts of a suspected food to see whether you react.

Your doctor will recommend whichever is most suitable for you, or a combination.

How do I prepare for the test?

You'll usually need to stop taking antihistamines before your appointment, since they can affect your results. How long depends on the type:

• short-acting antihistamines, such as chlorphenamine, are usually stopped 2 days beforehand
• longer-acting antihistamines, such as cetirizine or loratadine, are usually stopped 5 days beforehand.

Check with your clinic if you're not sure which type you're taking, or whether it's safe to stop. Don't put moisturiser, perfume, or sun cream on your arms on the day of the test, and if you have any allergic symptoms in the days before your appointment, treat them as you normally would.

What happens during the test?

1. Your arm is cleaned, then marked with a pen or numbered tape so each allergen can be tracked.
2. A small drop of each allergen solution is placed on your skin, alongside a drop of histamine, which is used to confirm the test is working properly.
3. A nurse gently pricks the skin under each drop with a sterile lancet, using a new one for each allergen, so a tiny amount gets under the surface.
4. Any excess solution is wiped away.
5. After around 15 to 20 minutes, your skin is checked for a reaction.

What happens after the test?

A positive result looks like a raised, itchy white bump surrounded by a red ring, similar to a mosquito bite. This means you're likely sensitised to that substance, though a larger reaction doesn't always mean a more severe allergy. If there's no reaction, you're unlikely to be allergic to that substance.

Your results will be discussed with you, alongside your symptoms and medical history, since the two need to be considered together. A letter is usually sent to your GP, and sometimes a blood test is arranged afterwards if the picture isn't clear.

What are the risks?

Skin prick testing is generally very safe. The most common effect is an itchy arm where you were tested, which usually settles within a few hours, though strong positive reactions can occasionally last longer.

Very occasionally, some people feel dizzy or light-headed during the test and need to lie down; staff are on hand to help if this happens. Severe allergic reactions to the testing itself are extremely rare, but you'll be monitored throughout, and if you feel unwell after you've left, contact your GP or go to your nearest emergency department.