To diagnose food allergy a doctor must determine if the patient is having an adverse reaction to specific foods. This assessment may be made with the help of a detailed patient history, the patient's diet diary, or an elimination diet.
The doctor may ask such questions as:
- Is there a specific reaction you suspect was caused by food allergies?
- What was the timing of the reaction? Did the reaction come on quickly?
- Was allergy treatment successful?
- Is the reaction always associated with a certain food? (It may or may not be.)
- 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.
- Are there ongoing conditions (ear infections, eczema, asthma) that may be caused or worsened by food allergies?
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. Sometimes, though, the symptoms of food allergies are chronic, and don't fluctuate from meal to meal.
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, this suggests the 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.
If the patient's history, diet diary, or elimination diet suggests a specific food allergy is likely, the doctor will then use tests that can more objectively measure an allergic response to food. One of these is a scratch skin test, during which a dilute extract of the food is placed on the skin of the forearm or back. This portion of the skin is then scratched with a needle and observed for swelling or redness that would indicate a local allergic reaction. If the scratch test is positive, the patient has IgE on the skin's mast cells that is specific to the food being tested.
Skin tests are rapid, simple, and relatively safe. But except in very young patients, people often have positive skin tests to a food allergen without experiencing allergic reactions to that food. A doctor diagnoses a food allergy when a patient has a positive skin test to a specific allergen and the history of these reactions suggests an allergy to the same food.
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.
Another possibility is 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 except in very young children.
The final method used to objectively diagnose food allergy is the double-blind food challenge. This has come to be the "gold standard" of allergy testing -- although it may fail to identify causes of food-dependent, exercise-induced allergies (unless exercise is included as part of the test).
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.
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