Food Allergy (cont.)
Melissa Conrad Stöppler, MD
Melissa Conrad Stöppler, MD
Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
William C. Shiel Jr., MD, FACP, FACR
William C. Shiel Jr., MD, FACP, FACR
Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
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How is food allergy diagnosed?
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To diagnose food allergy, a doctor first must determine if the patient is having an adverse reaction to specific foods. The doctor makes this assessment with the help of a detailed history from the patient, the patient's dietary diary, or an elimination diet. He or she then confirms the diagnosis by the more objective skin tests, blood tests, or food challenges.
History: The history usually is the most important diagnostic tool in diagnosing food allergy. The physician interviews the patient to determine if the facts are consistent with a food allergy. The doctor may ask the following questions:
Dietary diary: Sometimes, a history alone cannot determine the diagnosis. In that situation, the doctor may ask the patient to keep a record of the contents of each meal and whether reactions occurred that are consistent with allergy. The dietary diary provides more details than the oral history, so that the doctor and patient can better determine if there is a consistent relationship between a food and the allergic reactions.
Elimination diet: The next step that some doctors use is an elimination diet. Under the doctor's direction, the patient does not eat a food suspected of causing the allergy, for example, eggs, and substitutes another food, in this instance, another source of protein. If after the patient removes the food, the symptoms go away, the doctor almost always can make a diagnosis of food allergy. If the patient then resumes eating the food (still under the doctor's direction) and the symptoms return, this sequence confirms the diagnosis. The patient should not resume eating the food, however, if the allergic reactions have been severe because this re-challenge is too risky. This technique is also not suitable if the allergic reactions have been infrequent.
If the patient's history, dietary diary, or elimination diet suggests that a specific food allergy is likely, the doctor then will use tests, such as skin tests, blood tests, and a food challenge, which can more objectively confirm an allergic response to food.
Skin tests: In a scratch-the-skin test, a dilute extract of the suspected food is placed on the skin of the forearm or back. This portion of the skin then is scratched with a needle and observed for swelling or redness, which would signify a local allergic reaction to the food. A positive scratch test indicates that the patient has the IgE antibody that is specific for the food being tested on the skin's mast cells. Skin tests are rapid, simple, and relatively safe.
A person can have a positive skin test to a food allergen, however, without experiencing allergic reactions to that food. A doctor diagnoses a food allergy only when the patient has a positive skin test to a specific allergen and the history suggests an allergic reaction to the same food. In some highly allergic people, however, especially if they have had anaphylactic reactions, skin tests should not be done because they could provoke another dangerous reaction. Skin tests also cannot be done in patients with extensive eczema.
Blood tests: In those situations where skin tests cannot be done, a doctor may use blood tests such as the RAST and the ELISA. These tests measure the presence of food-specific IgE antibodies in the blood of patients, but they cost more than skin tests, and the results are not available immediately. As with positive skin tests, positive blood tests make the diagnosis of a specific food allergy only when the clinical history is compatible.
Food challenge: The double-blind food challenge has become the gold standard for objective allergy testing. (Some physicians prefer the term double-masked, rather than double-blind.) In this test, various foods, some of which are suspected of inducing an allergic reaction, are placed in individual opaque capsules. Both the patient and the doctor are blinded, so that neither of them knows which capsules contain the suspected allergens. (The capsules are prepared by another medical worker.) The patient swallows a capsule and the doctor then observes whether an allergic reaction occurs. This process is repeated with each capsule. Alternatively, the food to be tested may be disguised in another type of food to which the person is not allergic.
The advantage of a food challenge is that if the patient has an allergic reaction only to the suspected foods and not to the other foods tested, the diagnosis of food allergy is confirmed. Just as with a re-challenge after the elimination diet and with the skin tests, however, someone having a history of severe reactions should not be tested with a food challenge because of the danger of inducing another severe reaction. In addition, this procedure is expensive because it is difficult and requires a lot of time, especially for patients with multiple food allergies. This type of test must also be done under the careful supervision of a physician. Consequently, double-blind food challenges are done infrequently. They are done most commonly, however, when the doctor wishes to obtain evidence to confirm the suspicion that the patient's symptoms are not due to a food allergy. Then, additional efforts may be directed at finding the real cause of the patient's symptoms.
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