Allergy Diagnosis

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How Do Doctors Diagnose Allergies?

Do you break out in hives when a bee stings you? Or do you sneeze every time you pet a cat? If so, you may already know what some of your allergens are.

But, many times you don’t know what is causing your allergy symptoms. Make an appointment with your doctor for help.

Doctors diagnose allergies in three steps:

  • Personal and medical history. Your doctor will ask you questions to get a complete understanding of your symptoms and their possible causes. Bring your notes to help jog your memory. Be ready to answer questions about your family history, the kinds of medicines you take, and your lifestyle at home, school and work.
  • Physical exam. If your doctor thinks you have an allergy, they will pay close attention to your ears, eyes, nose, throat, chest and skin during the exam. This exam may include a lung function test to detect how well you exhale air from your lungs. You may also need an X-ray of your lungs or sinuses.
  • Tests to determine your allergens. Your doctor may do a skin test, patch test or blood test. No one test alone is able to diagnose an allergy. Test results are just one of many tools available to assist your doctor in making a diagnosis.

What Types of Tests Do Doctors Use to Diagnose Allergies?

Skin Prick Test (SPT)

Skin testing can confirm many common types of allergies. In some cases, skin tests can be the most accurate and least expensive way to confirm allergens. For prick/scratch testing, the doctor or nurse places a small drop of the possible allergen on the skin. They will then lightly prick or scratch your skin with a needle through the drop. If you are sensitive to the substance, you will develop redness, swelling and itching at the test site within 15 minutes. You may also see a “wheal,” or raised, round area, that looks like a hive. Usually, the larger the wheal, the more likely you are to be allergic to the allergen.

It is important to know:

  • A positive skin test result does not by itself diagnose an allergy.
  • A positive skin test does not predict the severity of an allergic reaction.
  • A negative skin test usually means you are not allergic.

Intradermal Skin Test

In intradermal (under the skin) testing, the doctor or nurse injects a tiny amount of allergen into the outer layer of skin. The doctor checks your skin after a set amount of time for results, like with the skin prick test. Doctors may use this test if the skin prick test results are negative but they still suspect you have allergies. A doctor may use this test for diagnosing drug or venom allergy. At this time, there are very few indications for intradermal skin testing for food allergy.

Blood Tests (Specific IgE)

If you have a skin condition or are taking medicine that interferes with skin testing, allergen blood tests may be used. They may also be used for children who may not tolerate skin testing. Your doctor will take a blood sample and send it to a laboratory. The lab adds the allergen to your blood sample and then measures the amount of antibodies your blood produces to attack the allergens. This test is called Specific IgE (sIgE) Blood Testing (previously and commonly referred to as RAST or ImmunoCAP testing). This test is a not a good screening test due to the high rates of false positive results. There is no test that can determine how severe an allergy is for someone.

Physician-Supervised Challenge Tests

In your doctor’s office, you inhale or take a tiny amount of an allergen by mouth. This test is usually done with possible medication or food allergies. A physician, usually an allergist, should supervise this test due to the risk of anaphylaxis, a severe life-threatening reaction.

Patch Test

This test determines what allergen may be causing contact dermatitis. Your doctor will place a small amount of a possible allergen on your skin, cover it with a bandage and check your reaction after 48 to 96 hours. If you are allergic to the substance, you should develop a local rash.

Medical Review October 2015.


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