Food Allergy Testing Basics
Allergy testing is an important step in confirming a diagnosis of food allergy. Two important rules must be kept in mind for interpreting food allergy test results.
- First, a carefully obtained history of the suspected food allergic reaction(s) must be considered in order to determine which foods should be tested.
- Second, the magnitude of a positive food allergy test does not predict the severity of an allergic reaction, only the likelihood of its occurrence.
SKIN PRICK VERSUS SERUM IgE TESTING
Skin prick testing (SPT) remains the preferred method for confirming a food suspected of causing an allergic reaction and is also useful for excluding possible culprits in the case of an allergic reaction involving a mixture of foods.
Skin tests are usually applied to the back using a plastic pricking device which introduces food extracts into the superficial layers of the skin. Antihistamines must be withheld for several days prior to skin testing. Allergic reactivity is determined by measuring the diameter of the wheal response (similar to a mosquito bite reaction) 15-20 minutes later. Itching discomfort associated with skin testing is mild to moderate (depending on the size and number of positive responses), but typically resolves within 1 hour. Skin testing results can be reviewed and discussed with the allergist immediately after the testing procedure.
Serum IgE antibody testing for foods may also be to confirm or exclude possible food allergy culprits when skin testing is not possible. For several common foods (including milk, egg, and peanut), the level of serum IgE antibody can be especially helpful in more precisely predicting the likelihood of an allergic reaction occurring with subsequent ingestions of the food in question.
Serum IgE testing involves drawing a small amount of blood (typically 1-2 teaspoons) from a vein. The lab report typically returns to the ordering physician 3-5 days later. Antihistamines do not interfere with serum IgE testing.
MORE ADVANCED DIAGNOSTICS
When the history of an allergic reaction to a food is not so obvious (e.g., mild scattered hives that resolve spontaneously), two different kinds of specialized lab tests may be useful for better predicting the likelihood of a potentially serious allergic reaction with subsequent ingestions.
The first, called component resolved diagnostics (CRD), is a serum IgE test that looks for antibodies to specific food proteins that are likely to cause severe allergic reactions (anaphylaxis) as well as proteins that are not associated with serious reactions. These food proteins have been well characterized for peanut, cashew, hazelnut, walnut and Brazil nut. In addition, CRD can help predict if a milk or egg allergic child would be likely to tolerate baked products made with milk or egg. The results of CRD testing are most useful for guiding decisions to attempt oral food challenges in questionable cases.
Another specialized lab test to help predict the likelihood of a potentially severe allergic reaction is called basophil activation testing (BAT). This test involves measuring the response of the patient’s blood basophils exposed to increasing concentrations of the food(s) in question in a research laboratory. A positive BAT for a food suggests that the patient would likely experience a potentially serious allergic reaction by consuming that food. Thus, a positive BAT may eliminate the need to perform a potentially risky oral food challenge to answer the question of food allergy in questionable cases. Because BAT is currently performed only in research labs, insurance often does not cover the costs of these tests. In addition, current versions of BAT are a bit “fickle”, yielding inconclusive results in approximately 20% of tests.
For additional information about specialized food allergy testing, please consult your FAST OIT allergist.
Robert W. Sugerman, MD
Allergy Partners of North Texas