Skin Testing - Pinpointing Your Allergic Triggers: Allergies
Content provided by the Faculty of the Harvard Medical School
Skin testing
Skin testing
Skin tests, which are designed to detect IgE antibodies, are the most commonly used testing approach because they can confirm sensitivity to a large variety of allergens.
Skin tests work by introducing a small amount of an extract of something to which you might be allergic into the superficial layer of the skin where the mast cells, coated with IgE, are located. Upon contact with an offending allergen, the IgE recognizes the allergen and triggers the mast cells to release their contents, leading to an itchy, red swelling within 10–15 minutes. Because the major substance causing this response is histamine, it is essential that you not take any antihistamines for at least 72 hours (one week for the longer-acting, newer antihistamines) before the test. Certain other drugs antagonize the effects of histamine and can make the testing useless; for example, the older tricyclic antidepressants such as amitriptyline or nortriptyline, and antiemetics such as prochlorperazine. Because of the way it works, the skin test only picks up true allergies mediated by IgE and mast cells. It will not, for example, diagnose lactose intolerance, which reflects a lack of a specific enzyme in the lining of the gut.
Prick test. This is a type of skin test in which a small amount of allergen is introduced into the skin on the back or on the inside of the arm by puncturing the skin. If you are allergic to that particular allergen, the telltale response of itchy redness and swelling will appear within minutes. The advantages of the prick test are that the test is virtually instantaneous and very specific. It will quickly show if you are allergic to grass pollen and just as quickly rule out the protein (Fel d 1) found in your cat’s dander. Because the prick test has a sensitivity of at least 95%, meaning it “misses” symptomatic allergens less than 5% of the time, many allergists rely on skin prick testing to identify the offending allergens.
A sample prick test
 For a prick test, a doctor uses a needle to lightly prick the skin so it will absorb drops of different allergens, and then examines the skin a short time later for any redness or swelling. Image courtesy of Dr. Albert L. Sheffer, Brigham and Women’s Hospital. |
Intracutaneous test. For this test, the allergen is injected under the skin — again, on the back or arm. Since this test is useful when the prick test has not confirmed sensitivity to an allergen that your history or a pattern of exposure would suggest, you might be tempted to skip over the prick test and go straight for the intracutaneous test, but don’t. With any allergy testing, there’s always a small risk that the reaction will get out of hand and result in anaphylaxis. This rare, life-threatening reaction typically occurs with intracutaneous testing when the patient has not previously undergone a prick test.
Patch test. Another type of skin test, the patch test, detects contact hypersensitivity. In this test, a chemical that might be responsible for contact dermatitis is placed on the skin under a bandage for 48 hours. During that time it is important not to get the patch wet. At the end of 48 hours, the patch is removed and the allergist or dermatologist looks for a raised, red rash, often with tiny blisters, where the patch was placed. Because the reaction is sometimes at its peak at 72 hours, you will be asked to return the next day to “read” the patch test again. Depending on which chemical or chemicals are suspected as a cause of contact dermatitis, you may receive a single patch or, more commonly, a commercially manufactured set of patches. An example is the T.R.U.E. Test, which has two separate panels of 12 chemicals, each separated from the others.
False positives. It’s important to realize that testing can reveal allergies to substances that are not causing you any symptoms. For example, you may have a positive test to ragweed but have no symptoms of allergies in the fall. This indicates that you have IgE antibodies against ragweed, but that the allergy is not severe enough to cause any symptoms. In fact, about 30%–40% of the population has positive skin prick tests to common inhaled allergens, but less than half of those with positive skin tests have allergy symptoms. The problem of false positives is even greater with intracutaneous testing, which is why it is essential to compare the test results with your history and to discuss them with your allergist.
| Last updated: | August 21, 2006 |
|---|---|
| Reviewed By: | Faculty of Harvard Medical School |
Medical content reviewed by the Faculty of the Harvard Medical School. Harvard Health Publications, Copyright © 2007 by President and Fellows of Harvard College. All rights reserved. Used with permission of StayWell.
This information is not intended to replace the advice of a doctor. By using AOL Body, you indicate that you have read, understood, and agreed to our Terms of Service, Use of Content Agreement and AOL Body Advertising Policy. Read more about our content partners.
Search
Related Articles
Where Does it Hurt?
If you're experiencing aches and pains we can help you find answers. Find out what your symptoms mean for your health.
