Diagnosing Food Allergies

To diagnose food allergy a doctor must first determine if the patient is having an adverse reaction to specific foods. This assessment is made with the help of a detailed patient history, the patient’s diet diary, or an elimination diet.

The first of these techniques is the most valuable. The physician sits down with the person suspected of having a food allergy and takes a history to determine if the facts are consistent with a food allergy. The doctor asks such questions as:

  • What was the timing of the reaction? Did the reaction come on quickly, usually within an hour after eating the food?
  • Was allergy treatment successful? (Antihistamines should relieve hives, for example, if they stem from a food allergy.)
  • Is the reaction always associated with a certain food?
  • Did anyone else get sick? For example, if the person has eaten fish contaminated with histamine, everyone who ate the fish should be sick. In an allergic reaction, however, only the person allergic to the fish becomes ill.
  • How much did the patient eat before experiencing a reaction? The severity of the patient’s reaction is sometimes related to the amount of food the patient ate.
  • How was the food prepared? Some people will have a violent allergic reaction only to raw or undercooked fish. Complete cooking of the fish destroys those allergens in the fish to which they react. If the fish is cooked thoroughly, they can eat it with no allergic reaction.
  • Were other foods ingested at the same time of the allergic reaction? Some foods may delay digestion and thus delay the onset of the allergic reaction.

Sometimes a diagnosis cannot be made solely on the basis of history. In that case, the doctor may ask the patient to go back and keep a record of the contents of each meal and whether he or she had a reaction. This gives more detail from which the doctor and the patient can determine if there is consistency in the reactions.

The next step some doctors use is an elimination diet. Under the doctor’s direction, the patient does not eat a food suspected of causing the allergy, like eggs, and substitutes another food, in this case, another source of protein. If the patient removes the food and the symptoms go away, the doctor can almost always make a diagnosis. If the patient then eats the food (under the doctor’s direction) and the symptoms come back, then the diagnosis is confirmed. This technique cannot be used, however, if the reactions are severe (in which case the patient should not resume eating the food) or infrequent.


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