Doctors have several tools available in the initial stages of food allergy diagnosis. Based on a history of reported symptoms, a doctor may begin the process of considering a food allergy diagnosis. People who experience hives, itchy mouth, tightening of the throat, or vomiting immediately after eating, or within two hours of eating, may be suspected of experiencing a food allergy reaction. In babies, eczema may be a pre-indicator of food allergy. As solids are introduced into the baby’s diet, signs of redness around the mouth, hives, diarrhea, vomiting, and worsening eczema symptoms should be reported to the pediatrician. Follow-up care with an allergist may be indicated.
After a thorough health history, the next step in diagnosing allergy is often a skin test. Small drops of the allergen are “pressed” into the skin and any reaction (raised red area) is measured. Skin tests, which work well for environmental allergens such as dust, pollen and grass, are often considered unreliable for food allergy. For people with eczema or other skin conditions, skin tests can give inconclusive results due to the sensitive nature of the skin.
Blood tests offer better reliability for food allergy diagnosis. Blood test results can show if IgE antibodies (the principal antibody present in allergic reactions) are present in the body, but cannot alone predict that a reaction will occur if the patient eats the suspected food. Therefore, blood test results can show there is a food allergy likely, but it doesn't show if there will be a reaction if that food is ingested.
The next step in definitive diagnosis is a food challenge, considered the gold standard for diagnosing food allergy.
What is a Food Challenge?
A food challenge is conducted in the allergist’s office under specific conditions. A food challenge takes several hours to complete. Steps are followed where small amounts of the suspected food are eaten over a period of time.
Patients can expect the allergist to begin with a physical exam. Temperature, blood pressure, respiration and overall condition of skin is noted. The next step in the challenge is to rub a small amount of the suspected food on the lips. The person waits 15 to 20 minutes to see if there is any type of reaction. If this step is successful, a tiny amount of the food is placed on the tongue. Again, there is a wait to determine if there is a reaction. Over the next hour or two, increasing amounts of the food is ingested, with a clinical determination made at each level to determine success.
If allergic symptoms develop at any time during the challenge, the challenge is stopped and appropriate medical intervention is made available. This may include an oral or topical antihistamine or epinephrine. It is important that a food challenge be conducted under a doctor’s supervision. This is not a test to try at home as there is a risk for a deadly anaphylactic reaction.
Types of Food Challenges
The most common type of challenge is conducted in the doctor's office with the doctor and patient aware that a suspected food is being used in the challenge. Some cases, such as patient anxiety or doctor's bias, may require special challenges. Often conducted in a hospital setting, these challenges are known as single and double blind tests.
In a single blind test, the patient is unaware of the food they are ingesting is a suspected food or a known safe food. This could be helpful for someone who is anxious about the test and unable to give reliable feedback about how they are feeling during the challenge. A double blind test means that the patient and doctor do not know if they are using suspected food or a known safe food. A nurse or other medical provider would be the only one with the correct information, thus allowing the doctor and patient to report clinical findings rather than hunches or feelings.
Patients and doctors together should choose the appropriate type of challenge.
Anyone who is unsure if they have a food allergy should talk to their allergist about scheduling a food challenge. If the challenge is successful in the office, the patient is encouraged to eat that food on a regular basis. If the food challenge results in allergic symptoms, patients are counseled to avoid that food in all forms.
Sources:
American Academy of Allergy, Asthma and Immunology
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