A family history can help to determine whether a child is likely to have a food allergy, and various tests are used to determine which food is causing the reaction. A child may be allergic to more than one food.
Skin tests The allergy skin-prick test is the most common screening test because it is inexpensive and easy to do. It involves pricking the skin with a solution of the suspected food. A positive test will produce a small hive-like reaction, but a positive result does not always indicate a true allergy.
RAST blood tests Radioallergosorbent (RAST) laboratory tests measure the amount of food-specific IgE in the blood. IgE antibodies are made by the body in response to allergens. Once you have developed these IgE antibodies, they are constantly circulating in your blood. Therefore, this blood test can be done at any time. The greater the amount of IgE, the higher the probability the person has an allergy to that particular food. Like skin-prick tests, RAST tests are prone to false positives, meaning a positive response can occur even when the person is not allergic to the food.
Elimination and challenge The most certain way tof diagnose food allergies is with a test called a double-blind, placebo-controlled (DBPC) food challenge. In this test, capsules containing the suspected food and others containing sugar are given to a person and the reaction is observed. Because there can be a risk of a serious, life-threatening reaction, this test usually is done in a clinic or hospital.
A more common way to do this test is for the person to record what he or she eats, and monitor any reactions, which typically occur within two hours of ingestion. First, the suspected foods are eliminated from the diet for one to two weeks. Then, the foods are added back into the diet slowly under medical supervision. It's helpful to keep a food diary of everything consumed, and the amounts.
While most children outgrow food allergies, some carry them into adulthood. Teenagers need to be aware of food allergies they had as children and not assume they have outgrown them. In particular, allergies to peanuts, tree nuts, fish and shellfish usually are not outgrown.