What is food allergy testing?

Food allergy testing is a way to check your body’s reaction to certain foods. Along with a medical history and physical exam, one or more of these tests may be done to see what foods you may be allergic to:

• Elimination diet
• Skin prick test
• Blood test
• Food challenge test

Why is it done?

If you have had allergic symptoms after eating certain foods, your healthcare provider may advise 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 these symptoms within a few minutes or as long as 6 hours after eating:

Mild symptoms:

• Sneezing
• Itchy, watery eyes and swollen eyelids
• Stuffy or runny nose
• Red, raised, itchy areas on the skin (hives)
• Nausea or stomach cramps

Severe symptoms:

• Trouble talking, trouble breathing, and wheezing
• Fast heartbeat
• Swelling of the lips, tongue, or throat
• Feeling very anxious or confused
• Feeling dizzy or faint
• Vomiting or diarrhea

How do I prepare for the tests?

You may need to avoid taking certain medicines, such as antihistamines or steroids, before the tests because they might affect the results. Make sure your healthcare provider knows about any medicines, herbs, or supplements that you take. Don’t stop any of your regular medicines without first checking with your healthcare provider.

Talk to your healthcare provider if you have any questions.

How are the tests done?

One or more of these tests may be done.

Elimination diet: Your healthcare provider may ask you to stop eating foods you may be allergic to for a week or two. Then you will add the foods back into your diet one at a time. This process can help connect your 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 for diagnosis.

Skin prick test: A skin prick test is often used to help identify 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 your back or arm. The skin test is ready to check in about 15 minutes. You may be allergic to the food if a red bump appears where the food extract was placed. This test by itself is not enough to diagnose a food allergy.

Blood test (RAST test): Blood tests are not done as often as skin prick tests, but they can be useful. The RAST test checks a sample of your blood for antibodies your body makes when it is trying to fight off allergy-causing substances in foods. This test by itself is also not enough for a diagnosis.

Both skin and RAST tests may show that you are allergic to a food that further tests show you are not allergic to (false positive results). You may need a food challenge test to confirm the results.

Food challenge: A food challenge test may also be used to diagnose a food allergy. The test is usually done in your healthcare 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 another food, such as 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. If you have symptoms of an allergy after the challenge that fit with your medical history and other test results, the diagnosis can be made.

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 test result may be wrong because:

• You can continue to have a positive test result for many years after you have outgrown a food allergy.
• You are allergic to something 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.

What if my test result is not normal?

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 such as:

• If you need more tests
• What kind of treatment you might need
• When you need to be tested again
• What lifestyle, diet, or other changes you might need to make

For more information contact:

• The American Academy of Allergy, Asthma and Immunology
  http://www.aaaai.org/
• Food Allergy and Anaphylaxis Network (FAAN)
  http://www.foodallergy.org