Diagnosing food allergies

After ruling out food intolerances and other health problems, your health care provider will use several steps to find out if you have an allergy to specific foods. This assessment will include a detailed patient history, the patient's diet diary, an elimination die, and possible skin and blood tests.

Patient History

The detailed patient history is the most valuable technique. Your provider will ask you several questions and listen to your history of food reactions to decide if the facts go with a food allergy.

You may be such questions as:

• Is there a specific reaction you suspect was caused by food allergies?
• What was the timing of the reaction? Did the reaction come on quickly?
• Was allergy treatment successful?
• Is the reaction always associated with a certain food? (It may or may not be.)
• Did anyone else get sick? For example, if the person has eaten fish contaminated with histamine, everyone who ate the fish should be sick. In an allergic reaction, however, only the person allergic to the fish becomes ill.
• How much did the patient eat before experiencing a reaction? The severity of the patient's reaction is sometimes related to the amount of food the patient ate.
• How was the food prepared? Some people will have a violent allergic reaction only to raw or undercooked fish. Complete cooking of the fish destroys those allergens in the fish to which they react. If the fish is cooked thoroughly, they can eat it with no allergic reaction.
• Were other foods ingested at the same time of the allergic reaction? Some foods may delay digestion and thus delay the onset of the allergic reaction.
• Are there ongoing conditions (ear infections, eczema, asthma) that may be caused or worsened by food allergies?

Diet Diary

Sometimes your health care provider can't make a diagnosis solely on the basis of your history. In that case, you may be asked to keep diary of the contents of each meal you eat and whether you have a reaction. This gives more detail from which you and your provider can see if there is a consistent pattern in your reactions.

Elimination Diet

The next step some doctors use is an elimination diet. Under your health care provider's direction:

• You don't eat a food suspected of causing the allergy, such as eggs
• You then substitute another food in the case of eggs, another source of protein

Your provider can almost always make a diagnosis if the symptoms go away after you remove the food from your diet

The diagnosis is confirmed if you then eat the food and the symptoms come back. You should do this only when the reactions are not significant and under health care provider direction. Your provider can't use this technique, however, if your reactions are severe or don't happen often. If you have a severe reaction, you should not eat the food again.

Skin Tests

If your history, diet diary, or elimination diet suggests a specific food allergy is likely, your health care provider will then use tests to confirm the diagnosis.

One of these is a scratch skin test, during which an extract of the food is placed on the skin of your lower arm. Your provider will then scratch this portion of your skin with a needle and look for swelling or redness which would be a sign of a local allergic reaction. If the scratch test is positive, it means that there is IgE on the skin's mast cells that is specific to the food being tested. Skin tests are rapid, simple, and relatively safe.

You can have a positive skin test to a food allergen, however, without having an allergic reaction to that food. A health care provider diagnoses a food allergy only when someone has a positive skin test to a specific allergen and the history of reactions suggests an allergy to the same food.

Blood Tests

If you are extremely allergic and have severe anaphylactic reactions, your health care provider cannot use skin testing because causing an allergic reaction could be dangerous. Skin testing also cannot be done if you have eczema over a large portion of your body.

In those cases, a health care provider may use blood tests such as the RAST (radioallergosorbent test) or the ELISA (enzyme-linked immunosorbent assay). These tests measure the presence of food-specific IgE in your blood. As with skin testing, positive tests do not necessarily mean you have a food allergy.

Double-Blind Food Challenge

The final method health care providers use to diagnose food allergy is double-blind food challenge. This testing has come to be the gold standard of allergy testing.

Your health care provider will give you individual opaque capsules containing various foods, some of which are suspected of starting an allergic reaction. You swallow a capsule and are watched to see if a reaction occurs. This process is repeated until you have swallowed all the capsules. In a true double-blind test, your health care provider is also blinded, meaning that the capsules having been made up by another medical person and your doctor does not know which capsule contains the allergen. The advantage of such a challenge is that if you react only to suspected foods and not to other foods tested, it confirms the diagnosis.

You cannot be tested this way if you have a history of severe allergic reactions. In addition, this testing is difficult because it takes a lot of time to perform and many food allergies are difficult to evaluate with this procedure. Consequently, health care providers seldom do double-blind food challenges.

Read more about food allergies:

• How allergic reactions work
• Common food allergies
• Food allergy or food intolerance?
• Treating food allergies
• Food allergies in infants and children
• Controversial issues and unproven theories

Created by the National Institute of Allergy and Infectious Diseases. Updated and modified by A.D.A.M., Inc.

Additional Reference

American Gastroenterological Association medical position statement: guidelines for the evaluation of food allergies. Gastroenterology . 2001 Mar;120(4):1023-5.

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