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Food Allergy Tests Sometimes Produce False Results

Posted Oct 28 2010 3:06pm

Our daughter's school recently called in a panic: She had broken out in hives, and her eyes were swollen and red. Since Elise who is six, was in the lunchroom at the time, we all assumed she had some kind of food allergy.

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It turns out, Elise's "allergic reaction" was no reaction at all. She was reacting to being in trouble, her red eyes and splotchy skin the result of tears and embarrassment. Her "symptoms" were gone by the time I got to school.

Still, we had two good reasons to investigate further with a common allergy test. First, Elise's 3-year-old brother is rife with food allergies. He can't eat wheat, soy, chicken, eggs or peanuts. Second, food allergies can come along at any age--sometimes ferociously.

The test results were jarring. Like her brother, she is apparently allergic to multiple foods, including wheat, eggs, pork, milk and soy. The problem: She eats all of these foods regularly with no side effects. Was she suddenly allergic to all these foods?

Maybe, but probably not. The two tests used to diagnose most food allergies aren't all that reliable. The accuracy of blood serum and skin scratch tests vary. They often result in false alarms. The same test given at different times to the same person may produce different findings.

In fact, according to one recent study , a patient who tests positive for a food allergy through these tests has less than a 50 percent chance of actually having the food allergy.

It's one of the reasons the National Institute of Allergy and Infectious Diseases (part of the National Institutes of Health) is currently refining food allergy diagnostic and management guidelines.

That's not to say these food allergy tests have zero value. Doctors can use them along with medical history and follow up tests to diagnose and treat patients. If Elise had a history of eczema, asthma or intestinal discomfort , her allergist might recommend a food challenge test, the gold standard of food allergy testing. In such a test a patient is exposed to potentially offending foods and observed in a allergist's office or at a hospital. Challenge tests come with risk (especially for patients who have shown a tendency towards anaphylaxis) and they're expensive.

( Scientists are developing another blood test which early research indicates is nearly as accurate as challenge tests. This test is potentially less expensive than a challenge test and would pose less risk.)

For now, we're skipping that additional step. We'll keep an eye on what she eats and how she feels, and we'll stay in touch with her pediatrician. But we'll wait for some better indicator to come along before putting to much stock in her test results.

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