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Food Allergy

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Diagnosis

Photo of doctor standing in front of young patient and his parents
© iStockphoto

On this page:

Detailed History

Your healthcare professional will begin by taking a detailed medical history to find out if your symptoms are caused by an allergy to specific foods, food intolerance, or other health problems.

A detailed history is the most valuable tool used for diagnosing food allergy. Your healthcare professional will ask you several questions, including the following:

  • Did your reaction come on quickly, usually within several minutes after eating the food?
  • Is your reaction always associated with a certain food?
  • How much of this potentially allergenic food did you eat before you had a reaction?
  • Have you eaten this food before and had a reaction?
  • Did anyone else who ate the same food get sick?
  • Did you take allergy medicines, and if so, did they help? (Antihistamines should relieve hives, for example.)

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Diet diary

Sometimes your healthcare professional cannot make a diagnosis based only on your history. You may be asked to keep a diet diary containing details about the foods you eat and whether you have a reaction. Based on the diary record, you and your healthcare professional may be able to identify a consistent pattern in your reactions.

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Elimination diet

The next step some healthcare professionals use is a limited elimination diet, in which the food that is suspected of causing an allergic reaction is removed from your diet to see whether that stops your allergic reactions. For example, if you suspect you are allergic to egg, your healthcare professional will instruct you to eliminate egg from your diet. The limited elimination diet is done under the direction of your healthcare professional.

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Skin prick test

If your history, diet diary, or elimination diet suggests a specific food allergy is likely, then your healthcare professional will use the skin prick test to confirm the diagnosis.

With a skin prick test, your healthcare professional uses a needle to place a tiny amount of food extract just below the surface of the skin on your lower arm or back. If you are allergic, there will be swelling or redness at the test site. This is a positive result. It means that there are immunoglobulin E (IgE) molecules on the skin’s mast cells that are specific to the food being tested.

The skin prick test is simple and relatively safe, and results are ready in minutes.

You can have a positive skin test to a food, however, without having an allergic reaction to that food. A healthcare professional often makes a diagnosis of food allergy when someone has both a positive skin test to a specific food and a history of reactions that suggests an allergy to the same food.

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Blood test

Instead of the skin prick test, your healthcare professional can take a blood sample to measure the levels of food-specific IgE antibodies.

As with skin testing, positive blood tests do not necessarily mean that you have a food allergy. Your healthcare professionals must combine these test results with information about your history of reactions to food to make an accurate diagnosis of food allergy.

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Oral food challenge

Caution

Because oral food challenges can cause a severe allergic reaction, they should always be conducted by a healthcare professional who has experience performing them.

An oral food challenge is the final method healthcare professionals use to diagnose food allergy. This method includes the following steps:

  • Your healthcare professional gives you individual doses of various foods (masked so that you do not know what food is present), some of which are suspected of starting an allergic reaction.
    • Initially, the dose of food is very small, but the amount is gradually increased during the challenge.
  • You swallow the individual dose.
  • Your healthcare professional watches you to see if a reaction occurs.

To prevent bias, oral food challenges are often double blinded. In a true double-blind challenge, neither you nor your healthcare professional knows whether the substance you eat contains the likely allergen. Another medical professional has made up the individual doses. In a single-blind challenge, your healthcare professional knows what you are eating but you do not.

A reaction only to suspected foods and not to the other foods tested confirms the diagnosis of a food allergy.

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Last Updated February 29, 2012