Allergy Tests

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KEY POINTS

• Allergy tests help your healthcare provider find out what causes your allergy symptoms.
• You may have skin tests or blood tests, or you may have a food challenge test or try an elimination diet. You may need to avoid taking certain medicines before the tests because they might affect the test results.
• Talk to your healthcare provider about your results and ask any questions you have.

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What are allergy tests?

An allergy is your body’s reaction to a substance that is normally harmless. With allergies, your body sees the substance as harmful or foreign, and your immune system reacts to the substance. The immune system is the body's natural defense against bacteria, viruses, and other foreign substances.

Allergy symptoms may include breathing problems, runny nose, watery eyes, itching, or a skin rash. Some allergic reactions, called anaphylaxis, can be life-threatening. Substances that can cause an allergic reaction are called allergens. Allergy tests help your provider find which allergens cause your symptoms.

The 4 main kinds of allergy tests are:

• Skin tests
• Blood tests
• Food challenges
• Elimination diets

Why are they done?

Your provider may recommend allergy tests if you have allergy symptoms that you cannot control with medicine or if your allergic reactions are severe or life-threatening. The tests check for allergies to substances such as:

• Pollen (small particles in the air from grasses, weeds, or trees)
• Mold
• Animal dander (dried skin flakes)
• Dust and dust mites (very tiny bugs)
• Latex (a liquid from rubber trees that is used in many products, like gloves and toys)
• Medicines
• Insect stings and bites
• Foods, such as shellfish, eggs, milk, nuts, and peanuts
• Chemical irritants, such as nickel, dyes in fabric, or cleaning products
• Poison oak, poison ivy, or poison sumac

Allergy tests may also be done to see how well treatment is working.

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 several days before the tests.

Talk to your healthcare provider before the tests if you have any questions.

How are the tests done?

Skin tests: The skin prick test is the most common type of skin test. For this test, a drop of allergen extract (liquid) is put on your skin and then a needle is used to scratch the surface of your skin through the drop. This lets the allergen get under your skin. The test can also be done with a pricking device that has been presoaked in the allergen extract. The test is usually done on your back or arm.

The skin test is checked in 15 minutes. If you are allergic to the allergen in any of the extracts, a small red bump will appear at the spot where the extract was placed. If the prick test is negative or the test results are not clear, you may have another test that uses a needle to inject a small amount of allergen under your skin.

Blood test (RAST test): This test checks a sample of your blood for antibodies your body may have made against different allergens. Blood tests may not be as accurate as skin tests, especially if you are trying to find what foods you may be allergic to.

Food challenge test: This test may be done to see if you have a food allergy. For this test, you will be given gradually increasing amounts of a food while your provider watches for symptoms. This test should be done only by a healthcare provider who is ready to treat you if you have a serious reaction to the food. Some stomach or intestinal allergies cannot be identified with the skin and blood tests, so a food challenge test may be the only good way to diagnose the allergy.

Elimination diet: Another way to check for food allergies is 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. You should not add foods back to your diet unless your provider tells you to do so. This can help prevent a severe reaction.

What do the test results mean?

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

If a test is positive for an allergen, it may mean you are allergic to that substance. Sometimes a test can be positive even if you are not allergic to the substance. This can happen because:

• You may have a positive test result even after you have outgrown an allergy.
• You are allergic to something similar to the allergen you were tested for. For example, you might have a positive test for soy if you are allergic to peanuts, or a positive test to wheat if you are allergic to grass pollen.

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
• What lifestyle, dietary, or other changes you might need to make

For more information contact:

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