Diagnosing food allergies
After ruling out food intolerances and other health problems, your health care provider will use several steps to find out if you have an allergy to specific foods. This assessment will include a detailed patient history, skin and blood tests, and possibly "challenges" (eating food under the supervision of a health care provider) and elimination diets.
The detailed patient history is the most valuable technique. Your provider will ask you several questions and listen to your history of food reactions to decide if the facts go with a food allergy.
You may be such questions as:
- Is there a specific reaction you suspect was caused by food allergies?
- What was the timing of the reaction? Did the reaction come on quickly?
- Was allergy treatment successful?
- Is the reaction always associated with a certain food? (It may or may not be.)
- Did anyone else get sick? For example, if the person has eaten fish contaminated with histamine, others who ate the fish should be sick. In an allergic reaction, however, only the person allergic to the fish becomes ill.
- How much did the patient eat before experiencing a reaction? The severity of the patient's reaction is sometimes related to the amount of food the patient ate.
- How was the food prepared? Some people will have a violent allergic reaction only to raw or undercooked fish or fruits. Complete cooking of these foods destroys those allergens in the fish to which they react. If the food is cooked thoroughly, they can eat it with no allergic reaction.
- Were other foods ingested at the same time of the allergic reaction? Some foods may delay digestion and thus delay the onset of the allergic reaction.
- Are there ongoing conditions (ear infections, eczema, asthma) that may be caused or worsened by food allergies?
Sometimes your health care provider can't make a diagnosis solely on the basis of your history. In that case, you may be asked to keep diary of the contents of each meal you eat and whether you have a reaction. This gives more detail from which you and your provider can see if there is a consistent pattern in your reactions.
If your history or diet diary suggests a specific food allergy is likely, your health care provider will then use tests to confirm the diagnosis.
One of these is a scratch skin test, during which an extract of the food is placed on the skin of your lower arm. Your provider will then scratch this portion of your skin with a needle and look for swelling or redness which would be a sign of a local allergic reaction. If the scratch test is positive, it means that there is IgE on the skin's mast cells that is specific to the food being tested. Skin tests are rapid, simple, and relatively safe.
You can have a positive skin test to a food allergen, however, without having an allergic reaction to that food. A health care provider diagnoses a food allergy only when someone has a positive skin test to a specific allergen and the history of reactions suggests an allergy to the same food.
If you are extremely allergic and have severe anaphylactic reactions, your health care provider may not use skin testing because it could possibly cause a dangerous allergic reaction. Skin testing also cannot be done if you have eczema over a large portion of your body, or if you take certain medications.
In those cases, a health care provider may use blood tests such as the RAST (radioallergosorbent test) or the ELISA (enzyme-linked immunosorbent assay). These tests measure the presence of food-specific IgE in your blood. As with skin testing, positive tests do not necessarily mean you have a food allergy.
Sometimes your doctor may recommend a food challenge.
Your doctor may recommend an open food challenge. You will start eating a small amount of a suspect food, and the food will not be "hidden" in another food. If you have no reaction, you will gradually eat larger amounts until you do, or do not, have a reaction. More than 80% of the time, people taking this test do not have a reaction.
If your symptoms don’t help your doctor make a clear diagnosis, you may be asked to take a double-blind placebo controlled food challenge. This test is the gold standard of food allergy testing.
You will be given either the food that you are testing or another food that you can normally eat. However, you will not know when you are eating the food that you are being tested for. For example, if you are seeing your health care provider about an egg allergy test, you may be given bread made with eggs and bread made without. Neither your nor your health care provider will know which one has the egg. Only the person who makes the bread will. Other times, the food can be hidden in capsules to swallow, some of which contain the food suspected of causing an allergic reaction, and others containing a placebo (sugar pill). During the challenge you will swallow a capsule or eat the piece of food and be watched to see if you have a reaction. This process is repeated until you have swallowed all of the capsules or eaten all of the food. The advantage of this challenge is that subjective factors (the power of suggestion) plays no role in whether an allergic reaction is perceived. If you react only to capsules or food containing the food that is being tested, a diagnosis of food allergy is confirmed. If you react to the placebo and the capsules containing food, you are likely not allergic to that food.
This test is often performed in research studies of food allergies. The test also takes a lot of time to perform, and many food allergies are difficult to evaluate using this test. As a result, health care providers may not have proper medical staff to do this testing.
The next step some doctors use is an elimination diet. Under your health care provider's direction:
- You don't eat a food suspected of causing the allergy, such as eggs
- You then substitute another food in the case of eggs, another source of protein
Your provider can almost always make a diagnosis if the symptoms go away after you remove the food from your diet
The diagnosis is possible if you then eat the food and the symptoms come back. You should do this only when the reactions are not significant and under a health care provider's direction. Your provider can't use this technique, however, if your reactions are severe or don't happen often. If you have a severe reaction, you should not eat the food again.
Read more about food allergies:
Created by the National Institute of Allergy and Infectious Diseases. Updated and modified by A.D.A.M., Inc.
Boyce JA, Assa'ad A, Burks AW, et al. Guidelines for the diagnosis and management of food allergy in the United States: report of the NIAID-sponsored expert panel. J Allergy Clin Immunol. 2010 Dec;126(6 Suppl):S1-58