Food Allergies and Food Intolerance
Diagnosing Food Allergies continued...
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.
Also thought to be helpful in diagnosing food allergies is an oral food challenge. If the patient or doctor fear a severe reaction, the suspected food can be given under medical supervision.
If the patient's history, diet diary, elimination diet, or oral food challenge suggests a specific food allergy is likely, the doctor may use tests that can more objectively measure an allergic response to food. One of these is a scratch puncture test, during which a dilute extract of the food is placed on the skin of the forearm or back. A skin puncture is made through the droplet with a needle and observed for swelling or redness that would indicate a local allergic reaction.
Skin tests are rapid, simple, and relatively safe although experts do not recommend making a food allergy diagnosis based on a skin test alone. A patient can have a positive skin test to a food allergen without experiencing allergic reactions to that food. A doctor diagnoses a food allergy only 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.
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.