There are several ways to test a person for food allergy: skin tests, elimination-type tests, and the RAST test.
Skin tests are common tests for most allergens, not just food.
In a skin scratch test, or patch test, a small amount of the suspected allergy-causing food is placed on the skin (on the arms or the back). The areas where the extracts were placed on the skin are scratched or pricked in order to introduce the allergen under the skin’s surface. Several allergens can be tested at the same time. Monitoring for swelling or redness on site will take around 20 minutes.
Another type is the intradermal test, which is similar to the scratch test but involves the injection of a small amount of allergen under the surface of the skin and observing the site for any sign of an allergic reaction.
Elimination-type tests are the most effective in determining food allergies.
Elimination diets are done by eliminating the suspected food from the person’s diet for several weeks, and then gradually re-introducing the foods one at a time, while observing if the person shows signs of an allergic reaction.
Placebo challenge test, or double-blind test, is another type of elimination test, wherein the person is given two capsules, one containing a suspected food allergen, and another an inert substance to act as a placebo to rule out any psychosomatic symptoms. Neither the person nor the provider knows what the substances are. Only a third party can identify the substances using a code. If more than one substance is being investigated, the test will require several sessions.
Open food challenges are non-blinded tests popular in most clinics that are not equipped to do placebo challenge tests. This is done by rubbing in traces of the offending food onto the skin, then on the eye or lip, and then on the mucous membranes in the mouth, until a substantial amount of the food is consumed without reaction. These tests are closely monitored at 15-minute intervals. If an adverse reaction occurs, the test is aborted.
Another method is the RAST (radioallergosorbent) test. This test checks for the amount of food-specific IgE antibodies in the blood, which are present if there is a true allergic reaction. RAST tests may also produce false positive results, but the advantage is that this test can be used to confirm a diagnosis for people with severe food allergy.
Pros and Cons
Skin testing is cheap, safe, and easy to do. Although a negative test is conclusive of an allergy, a positive result is not entirely accurate. The result can be false positive if an excessive amount of the food substance is introduced, thus inducing a positive reaction even in people who are not allergic. Also there is a high risk of anaphylactic reactions during the procedure because the allergens are introduced into body through the skin.
Elimination-type tests are the most effective in diagnosing food allergies. However, they may be dangerous when conducted in clinics that are not equipped for this form of testing. Only specialized allergy centers with in-house dieticians and full resuscitation equipment are prepared for these tests. The downside is that these tests are complicated, time-consuming, expensive if several foods are suspected, and of course, risky if you have never had a serious reaction.
RAST tests are more expensive. Nevertheless, these tests are totally safe, with no risk of the anaphylaxis, because there is no exposure at all to an allergen, since only a drawn sample of the patient’s blood is tested on.