Propped on his elbows and glued to an episode of Max and Ruby, my then-3-year-old son, Henry, was oblivious to the nurse marking his back with a ballpoint pen in preparation for a lengthy skin test. We were at the allergist's office looking for answers. What was causing Henry's frequent swollen eyes and breathing problems? Could an allergen be to blame for his irritability? Like any neurotic mommy-on-a-mission, I wanted a doctor to tell me what was going on and why.
The cartoon worked at distracting Henry—until the first prick. He winced through 50 more. His back looked like a messy grid of roughly 110 tiny red splotches and rubbed-away ink. Only five bumps on Henry's back puffed slightly—identified later as egg whites, sweet potatoes, almonds, crab and dust. The doctor instructed me to restrict those foods from Henry's diet and then reintroduce them one at a time during the coming month. I was confounded—all the foods were ones Henry had eaten before without incident. It didn't feel right to me, and my maternal instinct pushed me into learning all I could. The experience welcomed me to the dizzying world of food-allergy diagnosing.
Margin of Error
More than 3 million American kids have food allergies, according to the Centers for Disease Control and Prevention. It's a number that is growing rapidly—up 18 percent between 1997 and 2007. More children are being diagnosed with life-threatening food allergies than ever before. Some outgrow them. Some don't. While many doctors agree that the number of legitimate food-allergy cases is rising, many also believe the numbers may be inflated because of heightened awareness and a heavy reliance on tests that aren't 100 percent accurate. As a result, thousands of children may be misdiagnosed, and families may be unnecessarily restricting their kids' diets.
"Diagnosing food allergies is a very uncertain science," said Rauol Wolf, M.D., professor of pediatrics and director of pediatric allergy and immunology at the University of Chicago's Comer Children's Hospital. "Is it really increasing, or are people more aware and we're overdiagnosing? The answer is yes and yes."
Skin tests, such as the one given to my son, have an estimated 40 percent to 70 percent false-positive rate, meaning that the patient can have a positive result but not have any reaction after eating the food, Dr. Wolf says. That raises questions about accuracy. Children also can outgrow food allergies but still display positive reactions on a skin test. In addition, medications such as antihistamines can interfere with results. For parents hoping for clear-cut answers, skin tests are only reliable when no allergic reaction presents. "False positives are common, but false negatives are not," Dr. Wolf explains.
As an alternative to skin tests, doctors may order blood tests; however, these carry the same limitations as do skin tests. Blood testing measures a sample of blood for levels of allergy-inducing antibodies (IgE) to foods or substances, which would indicate an allergic reaction is possible. However, the antibodies alone don't cause allergic reactions, says Dr. Wolf. "Everybody can have IgE to foods, and not have a reaction when they eat them—that's why we have false positives." Blood tests merely identify the presence of IgE. Blood tests also can't distinguish between similar proteins. For instance, a child with a dairy allergy may receive high antibody levels to beef as well, without actually being allergic. A positive test correlates with real reactions less than half the time, according to the most recent research in Pediatrics, the American Academy of Pediatrics (AAP) journal.