Food Allergies and Food Intolerance
Diagnosing Food Allergies continued...
In some extremely allergic patients who have severe anaphylactic reactions, skin testing cannot be used because it could evoke a dangerous reaction. Skin testing also cannot be done on patients with extensive eczema.
For these patients a doctor may use blood tests such as the RAST and the ELISA. These tests measure the presence of food-specific IgE in the blood of patients. These tests may cost more than skin tests, and results are not available immediately. As with skin testing, positive tests do not necessarily make the diagnosis.
The final method used to objectively diagnose food allergy is a double-blind food challenge. This testing has come to be the "gold standard" of allergy testing. Various foods, some of which are suspected of inducing an allergic reaction, are each placed in individual opaque capsules. The patient is asked to swallow a capsule and is then watched to see if a reaction occurs. This process is repeated until all the capsules have been swallowed. In a true double-blind test, the doctor is also "blinded" (the capsules having been made up by some other medical person) so that neither the patient nor the doctor knows which capsule contains the allergen.
The advantage of such a challenge is that if the patient has a reaction only to suspected foods and not to other foods tested, it confirms the diagnosis. Someone with a history of severe reactions, however, cannot be tested this way. In addition, this testing is expensive because it takes a lot of time to perform and multiple food allergies are difficult to evaluate with this procedure.
Consequently, double-blind food challenges are done infrequently. This type of testing is most commonly used when the doctor believes that the reaction a person is describing is not due to a specific food and the doctor wishes to obtain evidence to support this judgment so that additional efforts may be directed at finding the real cause of the reaction.
The Exercise-Induced Food Allergy
At least one situation may require more than the simple ingestion of a food allergen to provoke a reaction: exercise-induced food allergy. People who experience this reaction eat a specific food before exercising. As they exercise and their body temperature goes up, they begin to itch, get light-headed, and soon have allergic reactions such as hives or even anaphylaxis. The cure for exercised-induced food allergy is simple -- not eating the suspected food for a couple of hours before exercising.