Simple tests such as a skin or blood test are used to diagnose an allergy. Skin tests can determine whether a person has IgE antibodies in the skin that react to a specific allergen. However, people with widespread skin conditions like eczema should not be tested using this method. In these cases, a blood test is the more appropriate way to make a diagnosis.
People with allergy symptoms -- such as the runny nose of allergic rhinitis -- may at first suspect they have a cold, but the "cold" lingers on. Testing for allergies is the best way to find out if a person is allergic.
Tests used to help make an allergy diagnosis include skin tests and blood tests.
Skin Tests Used to Make an Allergy Diagnosis
Allergists (doctors who specialize in allergic diseases) use skin tests to determine whether a person has IgE antibodies in the skin that react to a specific allergen. The allergist will use weakened extracts from allergens such as dust mites, pollens, or molds commonly found in the local area. The extract of each allergen is injected under a person's skin or applied to a tiny scratch or puncture on the arm or back.
Skin tests are one way of measuring the level of IgE antibody in a person. With a positive reaction, a small, raised, reddened area, called a wheal (hive), with a surrounding flush, called a flare, will appear at the test site. The size of the wheal can give the doctor an important diagnostic clue, but a positive reaction does not prove that a particular allergen is the cause of symptoms.
Although such a reaction indicates that IgE antibodies to a specific allergen are present, respiratory symptoms do not necessarily result.
Skin testing is the most sensitive and least costly way to identify allergies. People with widespread skin conditions like eczema, however, should not be tested using this method.
Other diagnostic tests use a blood sample to detect levels of IgE antibody to a particular allergen. One such blood test is called the radioallergosorbent test (RAST), which can be performed when eczema is present or if a person has taken medicines that interfere with skin testing.