Diagnosis of Food Allergies
Food allergy diagnosis involves taking a comprehensive personal and family medical history (including a history of symptoms) and performing a physical examination and diagnostic tests. Before a food allergy diagnosis can be made, other medical conditions must be ruled out. Food allergies usually are diagnosed by an allergist/immunologist.
When food allergy is suspected, the patient may be instructed to keep a food diary to help determine which food is causing the allergy. A food diary involves writing down all foods that are eaten, when they are eaten, and what symptoms, if any, occur. The allergist/immunologist often can use this information to determine which food is causing the allergy.
When the physician suspects a single food allergy and the allergic reaction experienced is not severe, the patient may be instructed to avoid the suspected food for a certain length of time. Once symptoms have resolved, he or she may be asked to introduce the food back into the diet to see if an allergic reaction occurs. This is called an elimination test.
Allergy tests (e.g., skin prick test, blood tests) also may be used to help determine the food or foods to which the patient is allergic. In a skin prick test, small drops of the suspected food allergens are placed on the skin of the forearms or back and a small needle is used to lightly prick the skin through the drops. If the patient is allergic to a specific allergen, itching, swelling, and redness develop at the site of the test within 20 minutes.
In allergy blood tests (e.g., radioallergosorbent test [RAST]), a suspected food allergen is added to a blood sample and the amount of immunoglobulin E (IgE) antibodies produced in response to the allergen is measured. If the patient does not have a history of severe allergic reaction, the results of these tests may be confirmed using a food challenge.
There are several types of food challenge tests, including open food challenge, single blinded food challenge, and double-blind placebo-controlled food challenge (most reliable method for diagnosing food allergies). Food challenges are performed in a physician’s office, clinic, or hospital, and are closely monitored by an allergist/immunologist. These tests should never be performed at home or without medical supervision.
In an open food challenge, the patient knowingly consumes the food suspected of causing the allergy prepared normally (i.e., the food is not hidden). This test may be used if allergy tests for the suspected food are negative or inconclusive.
In a single blinded food challenge, the patient eats the suspected food hidden in a challenge food without knowing what he or she is eating. This test is used to reduce the patient's ability to affect the results.
A double-blind placebo-controlled food challenge is the gold standard for diagnosing food allergies. In this test, the patient eats the suspected food hidden in a challenge food with both the patient and an observer unaware of which food is being consumed. The double-blind placebo-controlled food challenge prevents the patient or the observer from influencing the results.