How is food allergy diagnosed?
A careful history and physical examination should be performed to rule out other potential causes of adverse reactions to food.
In an allergic reaction, symptoms should occur following each ingestion of the specific food. This and the resolution of symptoms with elimination of the food from the diet support a diagnosis of food allergy. The onset of symptoms may occur for up to 2 hours. The longer time until onset of symptoms with some GI reactions compared to typical (<1hr) immediate hypersensitivity reactions may be due to the need for transport of the antigen into the GI tract, processing of antigen by digestion, and absorption into the intestinal mucosa.
Symptoms of food allergy may be:
- Localized to the GI tract
- nausea
- vomiting
- diarrhea
- bloating
- pain
- Systematic
- urticaria
- angioedema
- headache
- wheezing
- hypotension
- other symptoms of anaphylaxis
Of the immunologic diagnostic procedures, skin testing is the most important if not useful. By itself, a positive skin test is not diagnostic of food allergy and must be interpreted in the context of other clinical findings. A positive skin test in the presence of a positive clinical history is highly suggestive of specific food allergy, whereas a negative skin test suggests that allergy to that specific food is unlikely. The RadioAllergoSorbent Test (RAST) may also be helpful, but its use should be limited to patients in whom skin testing cannot be properly performed and interpreted, or in those thought to be at particular risk of a severe anaphylactic reaction to skin testing.
A clinical diagnosis of food allergy can be confirmed by food challenge. If the challenge is negative, there is strong evidence against allergy to that specific food. Food challenge should be performed only in an appropriate medical setting, since life-treatening anaphylaxis may occur. The double-blinded, placebo-controlled food challenge is the ideal method for confirming food allergy. In some patients, such as those with a low probability of a positive reaction, an open challenge may be useful. If positive, then a double-blinded, placebo-controlled challenge may be necessary.