Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Food Allergy

Diagnosis

Many times, the diagnosis of food allergy can be made by the history: If a child has hives or swelling of the face after eating something with peanuts, for example, they are likely allergic to peanuts. But because symptoms can be so varied, the diagnosis is sometimes harder to make. While a detailed diary of food intake and symptoms, and information about food allergies in the family, can be helpful, sometimes doctors need to do testing to be sure. There are two commonly used tests:

Skin tests
The allergy skin-prick test is the most common screening test because it is inexpensive, easy to do, and generally reliable. It involves pricking the skin with a solution of the suspected food. A positive test will produce a small hive-like reaction. The downside of this test is that it is uncomfortable – and for children with eczema or other skin conditions, the results may be difficult to interpret. And in children with severe allergies, even the small amount of the food injected into the skin can cause significant reactions.

Another problem with skin tests is that in order for them to be truly reliable, the patient can't take any antihistamines for about two weeks before the test. For children who suffer from bad hay fever or other allergies, two weeks without antihistamines may be impossible.

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.

RAST tests have the advantage of being less uncomfortable (one needle stick for the blood test instead of a bunch of pricks for the skin test) and can be done without stopping antihistamines. The downside of these tests (besides the expense) is that they can have both false positive and false negative results.

Elimination and challenge
Another way to 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. This approach would only be taken if the reaction was mild and didn't cause any trouble with breathing.