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 physician interviews the patient to determine if the facts are consistent with a food allergy. The doctor may ask the following questions:

• What was the timing of the reaction? Did the reaction come on quickly, usually within an hour after eating the food?
• Was treatment for allergy successful? For example, if hives stem from a food allergy, antihistamines should relieve them.
• 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 can sometimes relate to the amount of the suspect food eaten.
• How was the food prepared? Some people will have a violent allergic reaction only to raw or undercooked fish. A thorough cooking of the fish destroys those allergens in the fish to which they react, so that they then can eat it with no allergic reaction.
• Were other foods eaten at the same time as the food that caused the allergic reaction? Fatty foods can delay digestion and thus delay the onset of the allergic reaction.

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.

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.

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. 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.

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