Allergy tests are used to find the substances that are causing an allergic reaction. An allergic reaction is an immune system response to a substance in the environment known as a trigger.
Who is a candidate for the procedure?
The healthcare professional may perform or order allergy tests on people with suspected allergies. Skin testing is usually not done on children younger than age 3. It may be done on older individuals to determine the triggers that are causing allergic reactions in the person. Before the procedure, a person should not take medicines, such as antihistamines, that could block a skin reaction and thus cause the test to turn out falsely negative.
How is the procedure performed?
There are many methods of allergy testing. Skin tests are used to identify specific allergens that are known to cause symptoms. The healthcare professional will select skin tests based on a person's particular history. Test solutions are made from extracts of various inhaled, ingested, or injected substances. Test solutions available include extracts from tree, grass, and weed pollens; dust mites; animal dander; insect venoms; foods; and penicillin and penicillin derivatives.
For the prick skin 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 is called an intradermal test. A small amount of the allergen is injected beneath the skin. This more sensitive test is often used when the prick test has produced a negative or uncertain result in reaction to suspected inhaled allergens.
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 under controlled conditions. If a reaction occurs, this food is the likely cause.
The best way to test for food allergies is by using an oral food challenge. This test can be used even with 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 identify the foods causing the problems.
Blood tests measure antibodies to a particular allergen in blood. One blood test for allergies is called a radioallergosorbent test, abbreviated as RAST. 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. RAST measures allergen-specific IgE. Compared to skin tests, RAST has the disadvantage of limited allergen selection and reduced sensitivity.
A newer version of a blood test is called the Immunocap. A recent study indicated that the Immunocap test was significantly more accurate than the older blood tests. Another study compared the accuracy of skin testing to blood testing for cat allergy. This study reported that skin testing and blood testing were approximately equal in accuracy.
Other blood tests, such as an eosinophil count, may be used to support an allergy diagnosis. Provocative testing is the direct application of an allergen to the eyes, nose, lungs, or gastrointestinal tract by oral administration. It may be helpful in cases where a person has had a large number of positive skin tests. Provocative testing is the only way to check for allergies to food additives. This type of testing may produce a severe allergic reaction and is used rarely.