Alternative Names allergy scratch tests, allergy skin tests, allergy patch tests, RAST test
Definition Allergy tests are used to find the substances that are causing an allergic reaction.
Who is a candidate for the procedure? Skin testing is usually not done on children below age 3. A person should not take drugs that could block skin reactions for a period of 6 weeks before the procedure. An antihistamine may block a skin reaction.
How is the procedure performed? There are many methods of allergy testing. Some are skin tests. One skin test is a scratch test. A small amount of the substance thought to cause the allergic reaction is placed on the skin. This substance is called an allergen. The skin is then pricked or scratched. This allows the substance to get under the skin's surface. If the person is allergic, the skin will usually get red and swell within about 20 minutes.
Another skin test injects a small amount of the allergen beneath the skin. Skin tests are particularly useful in testing for respiratory, drug, and insect bite allergies.
An elimination diet test may be used to diagnose food allergies. For this test, a person goes several weeks without eating any of the foods that may be causing the problem. Foods are returned to the diet one at a time. If allergic symptoms come back after eating a certain food, that food is probably causing the problem. If a specific food is suspected, it can be given to the person. If a reaction occurs, this food is the likely cause. The best way to test for food allergies is an oral food challenge. This can be done in small children. The suspected food is removed from the diet. After 4 to 5 days, the food should be eaten on an empty stomach. This is the best time to watch for a reaction. Parents can keep a food diary for their children. This can help find the foods causing the problems.
Another test checks the blood. This is called a RAST test: In a true allergic reaction, substances called IgE antibodies appear in the bloodstream. The body makes IgE antibodies to attack bacteria and other harmful substances. Antibodies can sometimes cause allergies. Other blood tests may also be used.
What happens right after the procedure? Specific instructions are available from a doctor. The person can usually go home after the test.
What happens later at home? If there is no response to the allergen, the test is negative. The person is not allergic to the substances tested. If a test is positive, the person has an allergy to that substance. That substance should be avoided.
What are the potential complications after the procedure? When a person is exposed to something they are allergic to, they can have a reaction. Skin testing and food testing can cause severe allergic reactions. These reactions can be quite dangerous. In skin testing, skin irritation can be severe. Skin testing should be performed under adequate supervision with access to emergency facilities.
Author: David T. Moran, MD Reviewer: eknowhow Medical Review Panel Editor: Dr John Hearne Last Updated: 12/06/2005 Contributors Potential conflict of interest information for reviewers available on request
