Food Allergy Diagnosis
An allergist/immunologist is the best qualified professional to diagnose food allergy. Diagnosis requires a carefully organized and detailed assessment of the problem. First, the allergist/immunologist will take a thorough medical history, followed by a physical examination. The allergist will inquire about detailed contents of the foods, the frequency, seasonality, severity and nature of the symptoms, and will ask about the amount of time that elapses between eating a food and any reaction.
Allergy skin tests may be helpful to determine which foods, if any, are triggering a patient’s allergic symptoms. In skin testing, a small amount of liquid extract made from the food is placed on the back or arm. In a test called a prick test, a needle is then passed through the liquid on the top layer of the skin. In some cases fresh foods may be needed for skin testing.
If the patient develops a wheal-a raised bump or small hive-within 20 minutes, this positive response indicates a possible allergy. If the patient does not develop a wheal, the test is negative. It is uncommon for someone with a negative skin test to have an IgE-mediated food allergy. Skin tests are not helpful when sensitivity to chemical food additives is suspected.
Your doctor may also use blood tests for IgE to specific foods, called RAST testing or CAP-RAST, to diagnose food allergies. In certain cases, such as severe eczema all over the body, an allergy skin test cannot be done. Your doctor may recommend a food RAST blood test to obtain the same information that can be found with a skin test. For diagnosis of milk, egg, peanut or fish allergy, the level of the CAP-RAST test may help predict future food allergy reactions to these foods. False positive results may occur with both food allergy skin testing and blood testing. Food challenges, described below, are often required to confirm the diagnosis.
The allergist/immunologist may suggest that the patient keep a food diary, which is a detailed record listing foods eaten, date, time and any symptoms that occurred after eating the food. When an allergy to a single food is suspected, the allergist may recommend eliminating the food for a time. If symptoms are relieved, the allergist/immunologist may add the food to the diet once again to further determine if it causes a reaction (however, this is never done when the patient has a history of anaphylaxis).
If the diagnosis of food allergy remains in doubt, the allergist/immunologist may recommend a “blinded” food and/or food additive challenge test. These tests are conducted in the doctor’s office, or at times, in the hospital under close observation. Usually, the suspected food or a neutral food, called a placebo, is fed to the patient in colorless capsules, or in a non-allergenic slush or pudding. Neither the patient nor the doctor knows whether the suspected food or the placebo is being eaten. This is called a “double-blind” challenge. When properly performed, these challenges are very reliable in establishing a concrete cause and effect relationship between a food and an allergy symptom.
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