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 technique is the most valuable. 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.
- What was the timing of your reaction?
- Did your reaction come on quickly, usually within an hour after eating the food?
- Did allergy medicines help? Antihistamines should relieve hives, for example.
- Is your reaction always associated with a certain food?
- Did anyone else who ate the same food get sick? For example, if you ate fish contaminated with histamine, everyone who ate the fish should be sick.
- How much did you eat before you had a reaction? The severity of a reaction is sometimes related to the amount of food eaten.
- How was the food prepared? Some people will have a violent allergic reaction only to raw or undercooked fish. Complete cooking of the fish may destroy the allergen, and they can then eat it with no allergic reaction.
- Did you eat other foods at the same time you had the reaction? Some foods may delay digestion and thus delay the start of the allergic reaction.
Sometimes your health care provider can ’t make a diagnosis solely on the basis of your history. In that case, you may be 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.
The next step some health care providers use is an elimination diet. Under your 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 confirmed if you then eat the food and the symptoms come back. You should do this only when the reactions are not significant and under health care
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.
If your history, diet diary,or elimination diet 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 cannot use skin testing because causing an allergic reaction could be dangerous.
Skin testing also cannot be done if you have eczema over a large portion of your body.
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.
Double-Blind Food Challenge
The final method health care providers use to diagnose food allergy is double-blind food challenge.This testing has come to be the “gold standard ” of allergy testing.
- Your health care provider will give you individual opaque capsules containing various foods,some of which are suspected of starting an allergic reaction.
- You swallow a capsule and are watched to see if a reaction occurs.This process is repeated until you have swallowed all the capsules.
In a true double-blind test,your health care provider is also “blinded ” ((the capsules having been made up by another medical person).In that case your provider does not know which capsule contains the allergen.
The advantage of such a challenge is that if you react only to suspected foods and not to other foods tested,it confirms the diagnosis. You cannot be tested this way if you have a history of severe allergic reactions.
In addition,this testing is difficult because it takes a lot of time to perform and many food allergies are difficult to evaluate with this procedure.Consequently,health care providers seldom do double-blind food challenges. This type of testing is most commonly used if your health care provider thinks the reaction you describe is not due to a specific food and wishes to obtain evidence to support this. If your provider finds that your reaction is not due to a specific food, then additional efforts may be used to find the real cause of the reaction.