Food patch test is reproducible, safe, and specific
My stance is that there is a place for atopy patch testing for food allergy in your practice, for a number of reasons.
First, there is a clinical need for this test: We do not have any other test for non–IgE-mediated food allergies, such as food protein–induced enterocolitis, eosinophilic esophagitis, and potentially atopic dermatitis.
Second, the test is well standardized with respect to how reagents are applied, the time until results are read, and the method of reading. In fact, standardization in these respects is better than that for skin prick testing.
One problem with atopy patch tests—I will concede—is that we don't have standardized reagents, but this is also somewhat of an issue for skin prick tests.
Third, atopy patch testing is highly reproducible, with a reproducibility rate of 94% when it is performed on the back (Acta. Derm. Venereol. 2005;85:147-51). The reproducibility rate is lower, at 69%, when the test is performed on the arms.
Fourth, testing is safe. One study among 503 children given atopy patch tests found that 2.2% experienced contact dermatitis and itching, 1.1% had a reaction to the adhesive tape, and 0.2%—a single child—had a wheezing episode (Clin. Pediatrics 2008;47:602-3).
Fifth, atopy patch testing has good diagnostic performance. If we compare it with skin prick testing for the diagnosis of IgE-mediated food allergies to milk, eggs, wheat, and peanuts, it is more specific, although somewhat less sensitive.
Positive and negative predictive values also favor the atopy patch test in children with eosinophilic esophagitis (J. Allergy Clin. Immunol. 2007;119:509-11).
Finally, both the American College of Allergy, Asthma, and Immunology (ACAAI) and the European Academy of Allergy and Clinical Immunology (EAACI) have recognized a role for atopy patch testing as an adjunctive tool in the diagnosis of food allergy.
So academies from both sides of the ocean agree—atopy patch testing has a place in your practice.
By Jonathan M. Spergel, M.D., Ph.D., associate professor of pediatrics and director of the Food Allergy Center at Children's Hospital of Philadelphia. Dr. Spergel disclosed that he has served as a speaker or consultant for GlaxoSmithKline, AstraZeneca, Schering-Plough, and Nutricia, and has received grant support from Ception and Novartis
The food patch test seldom adds information
We already have several tests that can be used for the diagnosis of food allergy: skin prick tests, serum food-specific IgE levels, and an oral food challenge. So we have to ask if atopy patch testing adds anything.
There are two main scenarios in which physicians may consider using patch testing.
In one scenario, a patient with suspected food allergy has a positive skin prick test result and a positive serum IgE level for the food. The question here is whether the patient is clinically intolerant of the food.
Among children with food-specific IgE, patch testing misses two-thirds of those who are clinically intolerant to the food in an oral challenge (J. Allergy Clin. Immunol. 2006;118:923-9). And adding these results to the results of skin prick testing and food-specific IgE allows only 0.5%-7% of children to forgo an oral food challenge.
In another scenario, a patient has symptoms or a syndrome (such as gastrointestinal symptoms or atopic dermatitis), but has negative skin prick test results and a negative serum IgE level. The question here is whether the food is causing the symptoms or syndrome.
In a study among children and adults with atopic dermatitis in remission who had negative skin prick tests and negative serum food-specific IgE, only 17% had a positive atopy patch test for the respective food (Allergy 2004;59:1318-25).
At the same time, 4%-11% of unselected children in the general population have positive food patch test results (Pediatr. Allergy Immunol. 2008;19:599-604).
Interpretation of atopy patch tests is not always straightforward. Some patients develop the angry back syndrome, which may be mistakenly called a positive result. And 8% of patients overall experience some type of adverse effect (Allergy 2006;61:1377-84).
Finally, the previously mentioned professional organizations recommend use of atopy patch testing for foods in selected cases.
In sum, atopy patch testing has not yet gained a place in the diagnosis of food allergy. It is not superior to skin prick tests or food-specific IgE, and it does not replace a properly indicated and performed oral food challenge.
By Dr. Amal H. Assa'ad, professor of clinical pediatrics and director of the Food Allergy and Eosinophilia Clinic at the Children's Hospital Medical Center in Cincinnati. She disclosed being a consultant to GlaxoSmithKline.