The skin prick test is used to find the cause of certain allergies.
What is allergy?
Allergies are the body's response to a substance called an allergen. Allergens themselves may not be harmful. However, in some people, things that are usually harmless can cause a reaction. Examples of allergens are:
- Certain foods.
- The house dust mite.
- Certain medicines.
- The tiny dead skin cells shed by animals (animal dander).
If you are allergic to something, your body's response can vary from very mild (like a slight itchy rash) to a severe life-threatening reaction called anaphylaxis. (See the separate leaflet called Allergy for more details.) Sometimes it is not obvious what it is that you have reacted to, and you will need a test. One type of allergy test is skin prick testing.
How is the test done?
Suspected causes of allergy (such as pollens, danders, foods, etc) are mixed with liquid to make a solution. A drop of each solution is then placed on the skin - usually the forearm. Up to 10 or 12 drops of different solutions may be placed on the skin. The skin is marked to show which liquid has been placed where. Then, the skin beneath each drop is pricked with a very thin needle (lancet). This is usually painless, as just the very surface of the skin is pricked. However, this is enough to let a tiny amount of solution into the skin.
The skin is then observed for a reaction. If a reaction occurs, it happens within 20-30 minutes.
- A reaction is considered to be 'positive' when the skin under a drop of solution becomes red and itchy. Also, a white, raised swelling called a weal (or wheal) surrounds the red central area of any skin reaction. A weal takes about 15-20 minutes to reach a maximum size, and then fades over a few hours.
- A reaction is considered to be 'negative' when the skin remains normal. This means that you are not allergic to the substance in the solution.
Note: you should not take antihistamines on the day of the test, as they may dampen any allergic response during the test.
Is skin prick testing safe?
Skin prick testing is very safe in most people, and often does not need to be done in hospital clinics. Because such a tiny amount of the allergen is used, the reaction is only very small. For most people the worst that happens is a few itchy bumps for an hour or so. Certain people would not be suitable for skin prick testing. This includes people who have had very severe allergic reactions in the past, or people with very bad asthma. Skin prick tests are also not usually done on pregnant women, people on certain medications, or those with very bad eczema.
The professionals who do skin prick testing have emergency medicines and equipment standing by for the very rare occasion when someone has a bad reaction. They can reverse the reaction if this happens.
Is skin prick testing the same as patch testing?
No. Patch testing places substances on the surface of the skin. It aims to identify skin allergies. (See separate leaflet called Patch Testing for Contact Dermatitis.) Skin prick testing checks for allergies that don't necessarily occur on the skin, such as allergies to pollen, foods, etc. It is a way of getting the suspected allergen into the body and in contact with cells of your immune system to see if they react to it.
Dr Tim Kenny
Dr Mary Harding
Dr Jacqueline Payne