Testing of Allergies
Skin Testing vs. Blood Testing
With skin testing, the skin is used as a “mirror” of cells in the nose or lungs.
The advantages of skin testing are that it is more sensitive and less expensive than blood tests.
The disadvantages are that patients have to stop antihistamine medications 2-5 days before testing; skin testing may rarely cause severe allergic reactions in people who are highly sensitive; and skin testing is difficult in patients with chronic skin conditions were not enough clear skin is available for the test.
The blood test, called a RAST test, measures IgE to specific allergens in the blood.
The problem with RAST testing is that most of the IgE in our bodies is firmly attached to cells in the nose, lungs, and other tissues. Very little of it is in the blood so it is difficult technically to measure such a small amount.
RAST tests are less sensitive, so we can see positive skin test results in patients with previously negative RAST tests. (false positives)
Advantages to RAST tests include the ability to measure sensitivity in patients who would be at risk of anaphylaxis from skin tests; who are unable to stop medications for the skin testing; and who have bad eczema or other skin disorders where clear skin is not available.
While any physician can order RAST tests, allergists are best trained to interpret the results, as well as to perform skin testing. Which means the doctor would have to refer patient to an allergist rather than do the test in the office.
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