An allergy test is used to determine an allergen when the cause is unknown. Unlike people who know they have reactions to certain things, there are hundreds of allergens that are far harder to determine; this includes medicine, insect bites, environmental factors, and even food allergies.
The quickest allergy test is a skin test. This allergy test can be a simple as putting something on the skin (such as soap) and scratching or pricking the skin so the suspected cause of the problem is introduced under the skin. The person is watched for 15 or 20 minutes to determine if the site becomes red, inflamed, or itchy. In fact, when patients have allergy shots, they are asked to wait around to make sure the area doesn't show a negative reaction.
Patients who don't react to the simple allergy test but are still thought to have allergies are subjected to an allergy test that involves injecting a small amount of the allergen under the skin. A thorough test involves as many as 100 or more injections and is used more often for allergens that are found in the air, insect bites, medicine allergies, and some foods.
A food allergy test is sometimes much easier to determine, at least in terms of medical issues.
Foods are slowly eliminated from a person's diet over a period of several weeks, noting any changes in behavior or health. Then, slowly, these foods are introduced back into the diet, one at a time over the same period of several weeks and any changes are noted. This is sometimes very easy to accomplish, unless the food allergy is a nitrate or other chemical enhancement in many products.
For someone with an allergy to red dye, for instance, a food diet may only be the tip of the allergy test iceberg. For instance, red dye can be found in several edible and inedible products within the household, including soup, sausage, laundry detergent, toothpaste, popsicles, etc. Determining that a person is allergic to wheat, red dye, nitrates, or other chemicals can take quite a while, but is often easy to avoid, once the offending allergen has been isolated.
Anyone whose child has severe behavioral issues should read a wonderful book by Doris Rapp called, "The Impossible Child" The book relates the need for an allergy test to determine the relationship between offending foods and the behavior of those who are reacting to it.