Food Allergy Testing

What is food allergy testing?

Food allergy testing is a way to check the 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 your child has a history of allergic symptoms after eating certain foods, your healthcare provider may recommend that your child have tests to check for food allergies. This will help you know which foods your child should avoid eating to prevent an allergic reaction. A very few foods are responsible for most food allergies. The most common foods that cause allergies are milk, soy, eggs, peanuts, tree nuts, fish, shellfish, and wheat.

Your child may need to be tested for food allergies if your child has 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
• stomach cramps, vomiting or diarrhea
• fainting.

If possible, see your healthcare provider while the allergic reaction is occurring. This will help your provider with the diagnosis.

How should I prepare my child for an allergy test?

Your child may need to avoid taking certain medicines before the tests because they might affect the test result. For example, your child may need to stop taking any antihistamines one to several days before the tests. Make sure the healthcare provider knows about any medicines, herbs, or supplements that your child takes. Your child should not stop using any regular medicines without first consulting with your healthcare provider.

How are the tests done?

Your healthcare provider or allergist may want to do one or more of the following tests:

Skin prick tests: 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 child's back or arm. The skin test is ready to check in about 15 minutes. If your child is allergic to one of the foods, a red bump that looks like a mosquito bite will appear at the spot where the food extract was placed.

Intradermal skin test: For this test, a small amount of allergen is injected under the skin with a syringe. This test is more sensitive than the skin prick method, and can be used if the skin prick tests are negative.

Skin tests are not very painful, but they can be scary to a young child. Before the test, explain to your child what is going to happen to help calm any fears. For children who have extremely severe allergic reactions or other skin conditions such as eczema, the skin test may cause irritation or even life-threatening reactions. In this case, the RAST Test would be a safe alternative.

Blood test (RAST test): Blood tests are not done as often as skin prick tests, but they can be useful in certain cases. Blood tests are sometimes done on babies less than 1 year old because their skin does not react to the prick test as well as it does for older children. The test measures the amount of IgE antibody in the blood. The body makes this type of antibody when trying to fight off the allergy-causing substances in food (allergens). A sample of your child's blood is sent to a lab for testing. The test results show whether your child’s body is making antibodies to these foods and thus whether your child is allergic to these foods.

Food challenge: A food challenge test is considered to be the most reliable way to test for food allergies. The test is usually done in your provider’s office. Sometimes it is done in the hospital. During this test, your child is given gradually increasing amounts of the food while a healthcare provider watches for symptoms. This test should be done only by a trained professional who is ready to treat your child in case of a serious reaction. In cases of allergies that cannot be tested using a blood test (such as some gastrointestinal allergies), a food challenge test may be the only good way to make a diagnosis. The food challenge is also good way to see if your child has outgrown an allergy.

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

What do the test results mean?

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

If the skin test is positive for a certain food, it may mean your child is allergic to that food. Sometimes the test can be positive even if your child is not allergic to the food. The positive test result can be wrong sometimes because:

• Your child can sometimes continue to have a positive test result for many years to a food allergy he or she has outgrown.
• Your child is allergic to a different food or nonfood that has some components similar to the food he or she was tested for. For example your child might have a positive test for soy if she has a peanut allergy, or a positive test to wheat, if he has a grass pollen allergy.

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

For more information contact:

Food Allergy and Anaphylaxis Network (FAAN)

Web Site: http://www.foodallergy.org.

The American Academy of Allergy, Asthma and Immunology

Web Site: http://www.aaaai.org