Standard Allergy testing for foods: IgE is the allergic antibody. A higher IgE level doesn’t mean your more allergic, it simply means that you’re more likely to be allergic (allergy testing for foods only tells you the risk of being allergic NOT the severity of the allergy). Without a history suggesting a true food allergy, though, no test can tell whether someone is truly allergic and there is no upper limit above which the test is 100% predictive of an allergic response.
Unconventional Allergy Testing: A number of commercial laboratories measure IgA or IgG testing for antibodies to food allergens. These tests often return some positive results and a list of foods to avoid is generated based upon this. The tests are expensive and not covered by insurance—for good reason (they are worthless!!). To explain why these tests are a waste of time requires a brief review of the immune system response to foods. Generally the immune system is trained to aggressively respond to foreign proteins to protect from infection but the immune system is also constantly challenged with foreign proteins (food and bacterial) in the diet.
So why would a person have IgA or IgG antibodies to foods? Because it is part of the normal immune response. Part of the function of the immune system of the gut is to suppress an aggressive response to food proteins or bacteria. During this process, antibodies may be generated but they are not more common in people with digestive problems. In fact, the presence of some types of IgG (IgG4) are protective and are associated with the acquisition of tolerance to a food, not allergy.
In medicine, a test has to be interpreted in the context of symptoms. Dr. Stadtmauer spoke to a reporter from PBS who summarized the topic well. Read her article about the the pitfalls of random food allergy testing.
Teuber SS, Beyer K. IgG to foods: a test not ready for prime time. Curr Opin Allergy Clin Immunol. 2007 Jun;7(3):257-8.