Page 1 of 3
Allergy tests are used to determine the specific substances that cause an allergic reaction in an individual. They may also be used to determine if a group of symptoms is a true allergic reaction (involving antibodies and histamine release). Some food intolerances, in which there is an inability to digest a food because of a lack of appropriate enzymes, mimic allergies. Some drugs, such as aspirin, can cause allergy–like symptoms, but without the formation of antibodies or the release of histamine.
Skin–prick tests are usually less important than the history in the diagnosis of allergic conditions, but they often provide useful confirmatory evidence of reaction to suspected allergens. They are simple to carry out and interpret, but they do not give absolute evidence of clinically relevant allergy. A patient with asthma may, for example, show positive skin–prick reactions to allergens, which do not trigger bronchospasm on direct challenge test with allergens, which give negative skin–prick results.
The Scratch Test
It is one of the most common methods in allergy testing and it involves placement of a small amount of suspected allergy–causing substances (allergens) on the skin (usually the forearm, upper arm or the back), and then scratching or pricking the skin so that the allergen is introduced under the skin surface. The skin is observed closely for signs of a reaction, usually swelling and redness of the site. Results are usually obtained within about 20 minutes, and several suspected allergens can be tested at the same time. In the case of respiratory tract symptoms provoked by pollen, house dust mite excreta or animal dander there is a 75% agreement between the results of provocation tests and skin tests.
The concordance between skin tests and systems from ingested allergens is much lower indeed, skin–prick testing is only of limited help in the diagnosis of food allergic disease. Scratch testing is imprecise and unhelpful. Intracutaneous tests, in which allergens are injected more deeply into the skin, are reproducible and have an occasional role, but they are in general less specific, less well authenticated and more hazardous than prick tests.