Food Allergy Testing
Should my child be tested for food allergies?
A very few foods are responsible for most food allergies. Although your child could be allergic to other foods, the most common foods that cause allergies are milk, soy, eggs, peanuts, tree nuts, fish, shellfish, and wheat. You should have your child 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
- vomiting or diarrhea.
If possible, see your health care professional while the allergic reaction is occurring. This will help your provider with the diagnosis.
How is a food allergy diagnosed?
Your health care provider diagnoses a food allergy by reviewing your child's medical history. Keeping a diary of what foods your child eats before he or she has symptoms is a good way to help figure out what food is causing the problem. Your doctor or allergist may want to do one or more of the following tests: a food challenge test, an elimination diet, a skin prick test, or a blood test.
Medical history: Your child's health care provider will ask about your child's history. Your provider will want to know:
- what food you think caused the reaction
- what symptoms your child had
- how long after eating you noticed the symptoms
- if your child has had these symptoms before
- if other factors (such as exercise) are needed for the symptoms to happen.
Food challenge: Your health care provider may want your child to do a food challenge test to make sure he or she is really allergic to that food. During this test, your child is given gradually increasing amounts of the food while a health care 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 health care 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 the he eats. If your child has had a severe reaction to foods, this method cannot be used.
Skin test: When tests are needed, the skin prick test is the most common one used. A skin prick test is done by pricking the skin with a small needle through a drop of a food extract or by using a pricking device that has been presoaked in a 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.
Do not give your child any antihistamines for at least 3 days before a skin test because it can affect the test result. 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. A sample of your child's blood is sent to a lab for testing. This test measures the amount of certain antibodies (IgE antibodies) in the blood that the body makes when trying to fight off a food allergen. The lab results show the amounts of specific IgE antibodies found in the blood for certain foods.
What do the test results mean?
If the skin or blood test is negative for a food, then your child almost certainly 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. Neither the size of the reaction on the skin test or the level of IgE antibody in the blood test determines how severe your child's symptoms will be. 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.
Figuring out what the tests really mean can be quite confusing. Your child's health care provider must always look at the medical history along with the test results to help diagnose an allergy. This is especially important when your child has tested positive for several foods and eliminating all of those foods from the diet will make it difficult to provide adequate nutrition. Sometimes tests need to be repeated to check the first result.
What should I do if my child is diagnosed with a food allergy?
If your child has a food allergy, the only treatment is to have your child totally avoid all foods containing the food allergen. This can be difficult because common food allergens (such as egg, soy, milk, nuts, and wheat) are ingredients in many different foods. You will need to learn how to read food labels carefully.
You will need to tell your child's school about his or her allergy. You will also need to plan ahead to provide your child with safe foods at parties and school events.
In case your child has a severe allergic reaction, you should have emergency medicine available for your child at all times (especially if your child is allergic to peanuts or nuts). Ask your health care provider about prescribing epinephrine (such as, EpiPen) and a liquid antihistamine (such as, Benadryl or Atarax).
Peanut, tree nut, shellfish, and fish allergies are often life-long. However, other food allergies are often outgrown by children. Ask your health care provider if and when your child should be retested.
Treating a food allergy with drops or shots containing some of the allergen is called desensitization. This has not proven to be a safe or effective treatment.
For more information write to:
Food Allergy and Anaphylaxis Network (FAAN)
10400 Eaton Place, Suite 107
Fairfax, VA 22030
