Diagnosing food allergies can be as complicated as the medical condition itself. Symptoms of food allergy can vary from person to person, and a single individual may not always experience the same symptoms during every reaction. Food allergic reactions can affect the skin, respiratory tract, gastrointestinal tract, and/or cardiovascular system, and people develop food allergies at various ages.
Diagnostic food allergy testing offers clues about the causes of symptoms, but it cannot determine whether someone has a food allergy with absolute certainty without a challenged study. Still, when a food allergy is suspected, it’s critically important to consult with an allergist who can determine which food allergy tests to perform, determine if food allergy exists, and counsel patients on food allergy management once the diagnosis has been established.
To make a diagnosis, allergists ask detailed questions about the history of allergy symptoms. Be prepared to answer questions about the specific foods and the quantities you consumed, the length of time that it took for symptoms to develop, the symptoms themselves, and how long they lasted. The allergist will usually order a blood test (such as an ImmunoCAP test) and/or perform a skin prick food allergy tests, which indicate whether food-specific IgE antibodies are present in your body.
Skin prick tests are conducted in a doctor’s office and provide results within 15 - 30 minutes. A nurse or the allergist administers these tests on the patient’s arm or back by pricking the skin with a sterile small probe that contains a tiny amount of the food allergen. The tests, which are not painful but can be uncomfortable, are considered positive if a wheal (resembling a mosquito bite bump) develops at the site.
The blood tests, which are less sensitive than skin prick tests, measure the amount of IgE antibody to the specific food(s) being tested. Results are typically available in about one to two weeks and are reported as a numerical value.
Your allergist will interpret these results and use them to aid in a diagnosis. While both of these diagnostic tools can signal a food allergy, neither is conclusive. A positive test result to a specific food does not always indicate that a patient will react to that food when it’s eaten. A negative test is more helpful to rule out a food allergy. Neither test, by its level of IgE antibodies or the size of the wheal, necessarily predicts the severity of a food allergic reaction.
Together with the patient’s history, an allergist may use these tests to make a food allergy diagnosis. In some cases, an allergist may wish to conduct a double-blinded, placebo-controlled oral food challenge, which is considered to be the gold standard for food allergy diagnosis. However, the procedure can be costly, time-consuming, and in some cases is potentially dangerous, so it is not routinely performed.
During an oral food challenge, the patient is fed tiny amounts of the suspected allergy-causing food in increasing doses over a period of time under strict supervision by an allergist. Emergency medication and emergency equipment must be on hand during this procedure.
Oral food challenges also may be performed to determine if a patient has outgrown a food allergy.
In 2010 the National Institute of Allergy and Infectious Diseases (NIAID) released “Guidelines for the Diagnosis and Management of Food Allergy in the United States,” a document written to provide consistent, standardized information and recommendations for clinicians. NIAID also released “What’s in It for Patients” - information for patients about the new guidelines.