Are tree nut allergies diagnosed too often?

Many patients with a history of a single tree nut allergy are told to avoid all other tree nuts. But is that necessary? If you have a tree nut allergy and were advised to avoid other tree nuts based only on a positive blood or skin prick test, you may not be allergic to the other nuts. New research strongly suggests you should consider having an oral food challenge to properly diagnose additional nut allergies, especially if you've never had a reaction to eating those tree nuts before.

A new study in Annals of Allergy, Asthma and Immunology, the scientific publication of the American College of Allergy, Asthma and Immunology (ACAAI) showed that among people allergic to one nut who have a positive test to other tree nuts, more than half passed an oral food challenge to other tree nuts without a reaction. Passing an oral food challenge means you aren't allergic to that nut. Tree nuts include almonds, cashews, walnuts and hazelnuts, but not peanuts. The study noted that nearly none of the people allergic to peanut, but sensitized to tree nut, were clinically allergic to tree nut. This is the first study indicating that peanut allergic people may not need to avoid all nuts. 

"Too often, people are told they're allergic to tree nuts based on a blood or skin prick test," says allergist Christopher Couch, MD, ACAAI member and lead author of the study. "They take the results at face value and stop eating all tree nuts when they might not actually be allergic. We examined records of 109 people with a known tree nut allergy to an individual nut. They were tested for other tree nuts they had never eaten before using blood or skin prick tests. Despite showing a sensitivity to the additional tree nuts, more than 50 percent of those tested had no reaction in an oral food challenge." 

An oral food challenge is considered the most accurate way to diagnose food allergy. During an oral food challenge, the patient eats tiny amounts of the food in increasing doses over a period of time, followed by a few hours of observation to see if they have a reaction. An oral food challenge should only be conducted under the care of a trained, board-certified allergist. You should never do one on your own since if you are allergic, you could have a severe, life-threatening reaction.

"Previous studies suggested people with a tree nut allergy, as well as those with a peanut allergy, were at risk of being allergic to multiple tree nuts," said allergist Matthew Greenhawt, MD, chair of the ACAAI Food Allergy Committee and study co-author. "We found even a large-sized skin test or elevated blood allergy test is not enough by itself to accurately diagnose a tree nut allergy if the person has never eaten that nut. Tree nut allergy should only be diagnosed if there is both a positive test and a history of developing symptoms after eating that tree nut."

Dr. Greenhawt stressed the study did not include challenges to nuts the individual had a documented history of having a reaction to when eaten. "The practice of avoiding all peanut and tree nuts because of a single-nut allergy may not be necessary," says Dr. Greenhawt. "After an oral food challenge, people allergic to a single tree nut may be able to include other nuts in their diet."