A small lancet is used to gently prick the skin through a drop of fluid containing a known allergen. It is usually done on the forearm, although with young children it may be done on the back so they don't have to see what is happening. The test is not painful and results are immediately available.
In most cases, clinics have purified liquid forms of the allergen but sometimes, for example in the case of some foods, you may be asked to bring in a fresh sample. A positive reaction to the skin-prick test occurs when the skin around the needle prick becomes itchy and red with the development of a white swelling called a weal. The weal reaches its maximum size in about 15 to 20 minutes and the reaction fades within a few hours. The larger the weal, the more likely that you are allergic to the particular allergen.
Your clinical symptoms should correlate with the allergens to which you test positive, only then can an allergy be confidently diagnosed.