The skin prick / scratch test (intradermal testing) is considered to be the gold-standard diagnostic test for determining environmental allergies. It is generally considered to be the most simple, quick, cheap and safe method for determining basic allergy sensitivity.
In this test, allergens such as grass, weed and tree pollens, house dust mite, cat and dog dander, mold spores and suspect foods are mixed with liquid to make a solution. A drop of each solution is then placed on the skin (the forearm or back) and then the skin below each drop is pricked with a fine needle, allowing a small amount of allergen to enter the skin. As antihistamine medications can interfere with the accuracy of allergy testing, skin testing cannot be done if antihistamines have been taken within the past 48 hours.
When the skin prick / scratch test is positive (demonstrating an immune response to the allergen), the area around the skin-prick becomes red and itchy and swells into a raised bump called a “weal”. A weal takes about 15-20 minutes to reach a maximum size, and then fades over a few hours. The more allergic the person or animal is to the allergen, the greater the swelling of the weal.
The skin prick / scratch test is considered to be more accurate for airborne allergens than for foods. In part, this may be because commercial inhalant allergen solutions are readily available, but food allergens are a little more difficult to come by and are less stable.