In these tests, a very small amount of an allergen (a substance suspected of causing an allergy) is applied through the skin to determine whether it will elicit an allergic reaction. Several different methods are used to prepare the skin before administering the allergen: A needle may be used to make one or more scratches in the skin (scratch test), to prick the skin and pick up the superficial layer of skin (prick test) or to make a small puncture in the skin (puncture test). Alternatively, the allergen solution may be injected between layers of the skin (intradermal test). The test is done in an effort to identify allergies to specific substances such as house dust mites, animal dander, pollens, latex, drugs or foods. A doctor, nurse or lab technician performs the test. Several factors may blunt an allergic skin test response, including dehydration and the use of certain drugs, such as antihistamines, tricyclic antidepressants and phenothiazines. This may lead to false negative results. Redness and swelling may result from skin irritation rather than allergy, leading to false positive results; this is a particular concern when testing with higher concentrations of allergens or drugs. Tell your doctor what, if any, medications you are currently taking. You may be asked to discontinue certain drugs for 3 to 7 days before the test. Long-acting antihistamines such as hydroxyzine and cetirizine should be discontinued for 2 to 3 weeks. A normal-appearing patch of skin on your forearm or back is cleansed with alcohol. The examiner then applies a solution containing a particular allergen to your skin, using the scratch, prick or puncture method. If you are having an intradermal test, the examiner will inject a lower concentration of the allergen solution under your skin. Numerous allergen solutions may be tested in a single session, with the drops placed in parallel rows about an inch apart. After about 15 minutes, the test sites are examined for a positive reaction, indicated by the presence of a raised red area called a wheal. The different test methods may be used at separate times or sequentially in a single session. The tests may be repeated if the initial findings are negative but an allergy is strongly suspected. The examiner will ask you to wait for up to 30 minutes to ensure you are not having a severe allergic reaction. Inform your doctor immediately if you experience wheezing, lightheadedness, severe itching or shortness of breath during or after the testing. Skin at the testing site(s) may itch for several hours. If you are highly allergic, you may experience swelling, particularly after intradermal testing. Antihistamines and topical steroids are often given to minimize these symptoms. Keep the skin that was used for the test clean until it heals completely. These tests are generally safe, although the intradermal test method carries a slightly higher risk of provoking a significant allergic reaction than the other methods. Very rarely, particularly sensitive individuals will experience a life-threatening condition called anaphylactic shock (characterized by symptoms such as respiratory distress, decreased blood pressure and shock). Emergency medications and equipment are kept readily available. The examiner inspects the test sites for redness and swelling. A strong skin response is regarded as evidence of an allergic sensitivity. If a specific allergy is diagnosed, your doctor may advise you to avoid the allergen, use anti-allergy medication or have a series of allergy shots to increase your tolerance. If skin tests are negative or inconclusive but an allergy is still a strong possibility, your doctor may recommend blood tests or oral provocation (challenge) tests. Last Updated: 1/15/2003
The Johns Hopkins University 1996-2003. All rights reserved. This information is not intended to provide advice on personal medical matters, nor is it intended to be a substitute for consultation | 
