The diagnosis of food allergy should be made by a board-certified allergist, a person with specialized training in immune system responses to allergies. Diagnosis usually includes an extensive medical history, diet adjustment to eliminate suspected foods, and the use of one or more selective tests to identify specific foods that produce symptoms.

Skin Tests
The most common way to diagnose food allergies is through skin testing. Because allergy cells lie just below the surface of the skin, it takes just a tiny prick of the skin and a small drop of a concentrated food allergen to see if a red, raised bubble develops – a signal that the patient may be allergic to the substance.

The physician will make a diagnosis of food allergy only when the patient has a positive skin test to a specific allergen and the history of other reactions suggests an allergy to the same food.

Blood Tests
Not every person is a candidate for skin testing. People with a history of severe anaphylactic reactions (a life-threatening condition in which the airways swell rapidly and block breathing) or eczema (dry skin rashes) often cannot be skin tested. For these patients, the physician may use a RAST-type blood test. These tests measure the presence of food-specific IgE (allergy antibodies) in the blood. High levels of this type of antibody may indicate allergies.

RAST (radioallergosorbent)-type tests (also known as CAP-RAST, FAST, MAST, AND ELISA) are less sensitive and more expensive than skin tests and the results are not available as quickly. As with skin tests, a diagnosis of food allergies is made only when the patient has a positive blood test and a history of reactions suggesting allergy.

• Serum IgE tests measure the overall level of IgE antibodies in a patient’s blood, but do not match it with a particular food. Serum IgE can also be increased in other conditions, so if your IgE levels are high it doesn’t necessarily mean you are allergic to a food.



• Basophil histamine release (BHR) tests are highly technical tests that must be performed under the strictest laboratory conditions. They are similar to RAST tests in accurately identifying allergies.



• Cytotoxic tests study the reaction of unmetabolized (uneaten) food with a person’s white blood cells. Research studies have not found this test to be a reliable indicator of food allergy.



• FICA (food immune complex assay) tests and others that measure IgE antibodies are advertised as more reproducible, more sensitive, and more accurate in measuring immediate and delayed allergic reactions. Medical experts say that the production of these antibodies is not a reliable indicator of food allergy, since everyone makes some IgE antibodies to foods they eat.

Food Challenges
The final method used to objectively diagnose food allergy involves introducing the food to the patient while in the presence of a board-certified allergist with rescue treatment equipment immediately available. Referred to as “food challenges,” these tests are most useful if the patient has experienced a reaction to only a few suspected foods and not to other foods tested. There are three different types of food challenges.

• The open food challenge involves asking the patient to eat a food to which he has tested positive to see whether symptoms (hives, swelling, wheezing) develop.



• In the single-blind food challenge, the patient eats several different foods disguised in capsules, without knowing which capsule contains the suspected food allergen. After each capsule is swallowed, the patient is observed for signs of an allergic reaction.



• The “gold standard” food allergy test is the double-blind food challenge. As with the single-blind food challenge, the patient swallows a series of capsules over an extended time period. After each capsule, the patient is observed for signs of food allergies; however, in this case, neither the physician nor the patient knows which capsule(s) contain the food allergens. Only the medical professional who prepared the capsules has this information. This test is most often used in clinical studies.

The advantage of food challenge testing is that if the patient has a reaction only to suspected foods and not to other foods tested, it confirms the diagnosis. However, those people with a history of severe food reactions cannot be tested this way. In addition, this testing is expensive because it takes a great deal of time to perform, several medical professionals are involved, and it is usually conducted in a hospital or in medical offices with rescue equipment available in the event of a medical emergency.

As with all testing for allergies, use of antihistamines within the 72 hours prior to testing will interfere with test results. Talk to your physician about use of any medications, both prescribed and over-the-counter, when considering testing for allergies.

Reprinted from Allergy & Asthma Health magazine, Fall 1999. Medical editor for this article: Hugh Sampson, M.D.