Parents of children with a tendency to develop allergies, especially food allergies, are eager to find out exactly what their children will react to. There are plenty of good tests available, but nothing is perfect. The atopy patch test is one diagnostic tool that is still not well understood in terms of reliability but that is being studied. It may be used more often in the future, especially to help predict delayed future food allergies in children with atopic dermatitis.
What is Atopy?
The word atopy is used to refer to a genetic predisposition to have an allergic disease. These include inhalant allergies, like hay fever, but also food allergies. Also included under the term atopy are asthma and atopic dermatitis or eczema. Not all cases of the latter conditions are atopic, because they do not always involve an immune response. Anyone who has one of these atopic diseases is likely to have others.
The Atopy Patch Test
An atopy patch test (ATP) is a diagnostic tool that is similar to the skin prick test. Various allergens are applied to the skin, usually in a vehicle of petroleum jelly, and covered with a patch to hold it in place. The test takes 48 to 72 hours. It is most often used to test food allergens, although it has been used for aeroallergens, like dust mites and pollen, as well. This latter use is less common and is not well standardized. ATPs specifically test for IgE-mediated immune responses. Other types of patch tests look for other types of responses.
Why Use the ATP?
The atopy patch test is not always used to test children for food allergens, or aeroallergens, but it is being studied to determine if it could be a useful diagnostic technique. Most often ATP is used to predict food allergies in children with atopic dermatitis (also known as atopic eczema). It may be indicated for use to prove the existence of suspected food allergies, for children with severe atopic dermatitis when triggers are unknown, or for children who have multiple sensitivities but no known or proven food allergies. These same indications may also be used in cases of aeroallergens.
Reliability of Patch Tests
There is some controversy over the use of ATPs. Some studies have found that it is a useful and reliable tool, while others have found it to be unreliable. One study that was conducted by a panel of experts ultimately made the recommendation that patch tests not be used to evaluate food allergies. The panel found that ATP was less reliable than oral challenges and showed problematic variations in sensitivity and specificity.
Smaller studies have shown that the tests can be reliable and useful as part of a combination of tests. One study that involved 75 children with atopic dermatitis found that the APT was useful in diagnosing food allergies, especially in that it removed the need for a restrictive diet. Another study with just 34 children found that ATP was more sensitive and specific than the skin prick test in predicting food allergies in children with atopic dermatitis. Other studies have found that the APT is more specific, but less sensitive than skin prick tests.
APT may be a useful tool in helping to predict and diagnose allergies. However, with mixed evidence and expert recommendations, it is likely that it will not become common and research will be ongoing until there is a more conclusive answer as to the reliability of APT. If you are interested in trying a skin patch test for your child, talk to your pediatrician or allergist about the options.