Flawed testing may be at fault
For many people with life-threatening food allergies, the simple act of choosing what to eat can be akin to navigating a landmine. Eight culprits comprise 90 percent of all allergic reactions: egg, peanut, tree nuts, wheat, fish, soybeans, shellfish and cow’s milk. Because so many allergy-causing foods are used as ingredients in other foods, avoiding the triggers can be a challenge.
According to the National Center for Chronic Disease Prevention and Health Promotion, about 4 to 8 percent of children and 2 percent of adults have food allergies, and the number is rising. From 1997 to 2007, reported food allergies increased by 18 percent among children under the age of 18.
But a new federally funded report is questioning this prevalence of allergies. Commissioned by the National Institute of Allergy and Infectious Diseases, the report states that it’s unclear if food allergies are truly increasing, or if they are simply being misdiagnosed.
“Diagnosing and Managing Common Food Allergies,” recently published in The Journal of the American Medical Association, looked at 72 different studies and compared three popular methods of diagnosis: the skin-prick test, which involves pricking the skin with an extract of the food in question and looking for swelling or redness; blood tests, which measure the presence of food-specific antibodies in the blood; and the placebo-controlled oral food challenge, in which a patient blindly ingests individual doses of various foods, some of which are suspected of causing an allergy.
The conclusion: None of the tests demonstrate any statistical superiority, and there’s a lack of criteria for proper diagnosis. Even more revealing, patients with symptoms like rash or abdominal discomfort who are diagnosed from blood or skin-prick tests alone have less than a 50 percent chance of actually having an allergy.
“Probably half of the people who report that they have a food allergy don’t,” Dr. Marc Riedl, assistant professor of clinical immunology and allergy at UCLA and one of the report’s authors, said in a telephone interview. Up to 17 percent of people self-report as having food allergies, he said, but in reality, the number is much lower.
The consequences of misdiagnoses
Why this rampant overdiagnosis? One reason is that a person can have a positive skin or blood test to a food allergen without having an allergy to that food. Cross-reactivity – such as sensitivities to ragweed pollen cross-reacting with allergies to melons – may also account for false positives in blood tests. A person who is allergic to shrimp may test positive to lobster and crayfish.
“A positive blood or skin test does not equate to a true food allergy,” Riedl said. In many cases, clinical symptoms such as acute hives, asthma or diarrhea are lacking. “People have altered their diets and lifestyles in dramatic ways, but when you review their clinical history there’s nothing to suggest they’re at risk for severe allergic reaction. Much of the time, their diagnosis is inaccurate.”
The tests are not meant to be taken as gospel. “With any test in medicine, you really need to take a patient’s history first,” Dr. Gary Stadtmauer, an allergist-immunologist in New York City, said in a telephone interview. “The problem is that some doctors use allergy tests as if they have a high predictive value. The test is imperfect. You have to make sure if it makes sense to do the test in the first place.”
Misdiagnoses of food allergies can have lasting effects, ranging from anxiety to unnecessary dietary limitations. Overdiagnoses can also trivialize the severity of life-threatening, immune-mediated food allergies, which account for about 150 deaths in the U.S. each year. Still, blood and skin tests remain the most frequently used methods of diagnosis. The food challenge method, which the report’s authors refer to as “the criterion standard” for testing, is rarely used because of its time intensiveness and risk of severe allergic reactions.
Another time-consuming method of allergy testing is to keep a food diary, see if there is a pattern of allergic reactions, and then embark on an elimination diet, removing suspected foods one by one to see if symptoms disappear. But like the food challenge, there’s a lack of standardized criteria for what constitutes an allergic reaction. As a follow-up to the report, a panel of allergy experts is drawing up guidelines for diagnosing and defining food allergies, with a draft set to be released by the end of June.