Allergy Tests

What are allergy tests?

Allergy tests are tests your healthcare provider can do to find out what causes your allergy symptoms. Examples of common allergy symptoms are sneezing, watery eyes, runny nose, itching, swelling, hives, stomach cramps, and diarrhea.

Why are these tests done?

Allergy testing is the most precise way to find what causes allergic symptoms. Allergy tests can help you know what substances you are allergic to. Substances that trigger allergy symptoms are called allergens. Knowing which allergens cause your symptoms can help you avoid these substances and reduce your symptoms.

Consider having allergy tests if you have allergy symptoms that you are not able to control with medicine. Your provider may recommend that you start having allergy shots. A mixture that contains the allergens identified in your tests can be prepared for the shots. Your allergies can then be treated by injecting the mixture into your skin in tiny but increasing amounts over the course of many months. Over time, the shots make you less sensitive to the allergens.

How do I prepare for these 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 3 to 7 days before the tests.
• Talk to your healthcare provider if you have any questions.

How are the tests done?

The 4 main kinds of tests used to identify allergy triggers are:

• skin tests
• blood tests
• food challenges
• elimination diets.

Skin tests: A test called the skin prick test is the most common type of skin test. For this test, a drop of allergen extract is put on the skin and then the skin is pricked with a small needle through the drop of the extract. The test can also be done with a pricking device that has been presoaked in the allergen 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 allergen in any of the extracts, a red bump that looks like a mosquito bite will appear at the spot where the extract was placed. If the prick test is negative or the test results are not clear, a similar but more sensitive test called an intradermal test may be done. For the intradermal test, a very small amount of allergen is injected under the skin.

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

Food challenges: To check for food allergies, your healthcare provider may want you to do a food challenge test. For this test, you are given gradually increasing amounts of a food 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: For another check of possible food allergies, your healthcare provider may want you to avoid eating certain foods for a few weeks to see if allergy symptoms go away. During this time, you will need to keep a record of the foods that you eat and any symptoms you have. The diet is followed until all allergic symptoms are gone. Foods are then added back to the diet one at a time. If symptoms come back, you know which foods are safe to eat and which foods to avoid.

What do the test results mean?

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

If the skin test is positive for an allergen, it may mean you are allergic to that food. However, sometimes a 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 an allergy you have outgrown.
• You are allergic to a different substance that has some components similar to the allergen 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 results and ask questions.

What happens after the tests?

An allergy specialist will interpret the results of the tests and suggest ways your allergy might be treated. Be sure to discuss your results with your healthcare provider and make sure you understand how best to care for your allergy symptoms.

For more information contact:

• The Food Allergy & Anaphylaxis Network
  Phone: 800-929-4040
  Web site: http://www.foodallergy.org
• The American Academy of Allergy, Asthma & Immunology
  Phone: 800-822-2762
  Web site: http://www.aaaai.org.