The skin prick test is used to find the cause of certain allergies.
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. Then, the skin beneath each drop is pricked with a needle. 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 positive reaction is when the skin under a drop of solution becomes red and itchy. Also, a white, raised swelling called a weal 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 negative reaction is 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 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 'Contact Dermatitis - Patch Testing'.) 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.
Further reading & references
- Food allergy in children and young people, NICE Clinical Guideline (February 2011)
Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.
Dr Tim Kenny
Dr Tim Kenny