Food Allergy Testing

What is food allergy testing?

Food allergy testing is a way to check your body's reaction to certain foods. One or more of the following tests may be done:

• skin prick test
• blood test
• food challenge test
• elimination diet.

Why is it done?

If you have a history of allergic symptoms after eating certain foods, your healthcare provider may recommend that you have tests to check for food allergies. This will help you know which foods you should avoid eating to prevent an allergic reaction.

You may need to be tested for food allergies if you have some of the following symptoms shortly after eating:

• hives
• redness of the skin
• itchiness
• swelling of the lips or eyelids
• throat tightness
• wheezing or other breathing trouble
• coughing
• vomiting or diarrhea.
• fainting.

How do I prepare for the tests?

• You may need to avoid taking certain medicines before the tests because they might affect the test result. For example, you may need to stop taking any antihistamines one to several days before the tests. Make sure your healthcare provider knows about any medicines, herbs, or supplements that you are taking. Don't stop any of your regular medicines without first consulting with your healthcare provider.
• Talk to your healthcare provider if you have any questions.

How are the tests done?

One or more of the following tests may be done.

Skin prick test: A skin prick test is often used to test for food allergies. For this test, a drop of food extract is put on the skin and then the skin is pricked with a small needle through the drop of the food extract. The test can also be done with a pricking device that has been presoaked in the food extract. Only the top layer of skin is pricked. The test is usually done on the back or the arm. The skin test is ready to check in about 15 minutes. If you are allergic to the food in the extract, a red bump that looks like a mosquito bite will appear at the spot where the food extract was placed.

Intradermal skin test: A small amount of the substance being tested is injected under the skin with a needle and syringe. The intradermal test is more sensitive than the skin prick method and may be used if the results of skin prick tests are negative.

Blood test (RAST test): Blood tests are not done as often as skin prick tests, but they can be useful in certain cases. The test measures the amount of IgE antibody in your blood. The body makes this type of antibody when trying to fight off the allergy-causing substances in food (allergens). A sample of your blood is sent to a lab where tests are done with specific foods to determine if you have IgE antibodies to those foods. The test results show whether you are making antibodies to these foods and thus whether you are allergic to these foods.

Misleading results can occur with both skin and RAST tests. That is, a test may indicate that you are allergic to a food that further tests may show you are not allergic to (false positive results). A food challenge may be necessary to confirm the results.

Food challenge: If the diagnosis of food allergy is still in doubt, your healthcare provider may want you to do a food challenge test. The test is usually done in your provider's office. Sometimes it is done in the hospital. To do the test, you are given gradually increasing amounts of a food--either in colorless capsules or in a slush or pudding--while your provider watches for symptoms. This test should be done only by a trained professional who is ready to treat you if you have a serious reaction to the food. In cases of allergies that are not caused by IgE antibodies (such as some gastrointestinal allergies), a food challenge test may be the only good way to diagnose a food allergy.

Elimination diet: Your healthcare provider may want you to stop eating suspect foods for a week or two and then add the foods back into the diet one at a time. This process can help connect symptoms to specific foods. During this time, you will need to keep a record of the foods you eat and any symptoms you have. If you have had a severe reaction to foods, this method cannot be used.

How will I get the test result?

Ask your healthcare provider how you will get the result of your skin prick or blood test.

What do the test results mean?

If the skin or blood test is negative for a food, then you probably do not have an allergy to that food.

If the skin test is positive for a certain food, it may mean you are allergic to that food.

Sometimes the test can be positive even if you are not allergic to the food. The positive test result can be wrong sometimes because:

• You can sometimes continue to have a positive test result for many years to a food allergy you have outgrown.
• You are allergic to a different food or nonfood that has some components similar to the food you were tested for. For example, you might have a positive test for soy if you have peanut allergy, or a positive test to wheat if you have a grass pollen allergy.

Test results are only one part of a larger picture that takes into account your medical history and current health. Sometimes a test needs to be repeated to check the first result. Talk to your healthcare provider about your result and ask questions.

For more information contact:

• The American Academy of Allergy, Asthma and Immunology
  Web site: http://www.aaaai.org/patients/publicedmat/tips/whatisallergytesting.stm
• Food Allergy and Anaphylaxis Network (FAAN)
  Phone: (800) 929-4040
  Web site: http://www.foodallergy.org.