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Diagnosing Food Allergy

Allergy image
Your doctor may ask you to keep a food journal to record what you eat and whether you have a reaction.

Allergic reactions to food can cause serious illness and, in some cases, death. Therefore, if you have a food allergy, it is extremely important for you to work with your doctor to find out what food or foods cause your allergic reaction. After ruling out food intolerances and other health problems, your doctor will use several steps to find out if you have an allergy to specific foods.

Detailed History
A detailed history is the most valuable tool for diagnosing food allergy. Your doctor will ask you several questions and listen to your history of food reactions to decide if the facts fit a food allergy.

Food Journal
Sometimes your doctor can’t make a diagnosis solely based on your history. In that case, you may be asked to record what you eat and whether you have a reaction. This food journal gives more detail from which you and your doctor can see if there is a consistent pattern in your reactions.

Elimination Diet
The next step some doctors use is an elimination diet. In this step, which is done under your doctor’s direction, certain foods are removed from your diet. You don’t eat a food suspected of causing the allergy. Your doctor 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 only under direction from your doctor. Your doctor 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.

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

Skin Test
If your history, food journal, or elimination diet suggests a specific food allergy is likely, your doctor will then use either the scratch or the prick skin test to confirm the diagnosis. Skin tests are rapid, simple, and relatively safe. During a scratch skin test, your doctor will place an extract of the food on the skin of your lower arm. He or she 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. A prick skin test is done by putting a needle just below the surface of your skin of the lower arm. Then, a tiny amount of food extract is placed under the skin.

If the scratch or prick test is positive, it means that there is IgE on the skin’s mast cells that is specific to the food being tested. You can have a positive skin test to a food allergen, however, without having an allergic reaction to that food. A doctor diagnoses a food allergy only when someone has a positive skin test to a specific allergen and when the history of reactions suggests an allergy to the same food.

However, if you are extremely allergic and have severe anaphylactic reactions, your doctor can’t use skin testing because causing an allergic reaction to the skin test could be dangerous. Skin testing also can’t be done if you have eczema over a large portion of your body.

Blood Test
Your doctor can make a diagnosis by doing a blood test as well. Blood 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 Oral Food Challenge
The final method doctors use to diagnose food allergy is a double-blind oral food challenge. Your doctor will give you capsules containing individual doses of various foods, some of which are suspected of starting an allergic reaction. Or your doctor will mask the suspected food within other foods known not to cause an allergic reaction. You swallow the capsules one at a time or swallow the masked food and are watched to see if a reaction occurs.

This type of testing is most commonly used if a doctor thinks the reaction described is not due to a specific food and wishes to obtain evidence to support this. If your doctor finds that your reaction is not due to a specific food, then additional efforts may be used to find the real cause of the reaction.


Source: National Institute of Allergy and Infectious Diseases,

This article was originally published in Coping® with Allergies & Asthma magazine, Spring/Summer 2010.