My son had his birthday party recently at an indoor play space, and a sign there got me thinking. Peanut-containing foods were prohibited ''due to the increased incidence of peanut allergies."
Anxieties about peanut allergy are understandable -- the condition can be deadly -- and some concerned parents today support banning peanut-containing foods from public places.
But the medical research suggests that severe peanut allergies are not as common as people think and are surprisingly difficult to diagnose accurately. And although, as a parent, it may seem that peanut allergies have reached epidemic proportions, the evidence is surprisingly thin.
True allergies result when the immune system mistakes innocent substances -- like dust, pet dander, and food proteins -- for harmful invaders. Almost a century ago, the scientist Carl Prausnitz injected his skin with blood from a colleague allergic to fish and got hives at the injection sites upon eating fish. Later, scientists realized that blood from allergy sufferers contains an antibody called IgE, which erroneously attracts friendly fire from the immune system and can cause runny noses, red eyes, wheezing, hives, and, rarely, shock and death.
According to Anne Munoz-Furlong, a researcher and the founder of the Food Allergy and Anaphylaxis Network, an advocacy group, today about 25 percent of parents believe that their children have food allergies, although only about 4 percent really do. A parent may suspect one after a few spit-ups or a screaming fit following a new food. Yet these are rarely true allergies. And even among children with true allergies caused by harmful IgE, only a tiny fraction will have life-threatening reactions, called anaphylaxis.
While food (and, particularly, peanut) allergies make headlines -- like the Canadian teen who died last November after kissing her boyfriend who'd eaten a peanut butter sandwich -- the Archives of Internal Medicine in 2004 reported that the average person's chance of food-induced anaphylaxis is about 4 in 100,000 per year. Roughly the same number of Americans each year die from lightning strikes as from peanut allergies.
A well-publicized household telephone survey published last year in The Journal of Allergy and Clinical Immunology suggested that rates of peanut allergies among children had doubled from 0.4 percent of the total population to 0.8 percent between 1997 and 2002. But the data were not verified by allergy tests, and it's not clear whether the numbers are meaningful. In the families surveyed in 2002, the rate of peanut allergies among children under 5 was essentially the same as the rate among 6- to 10-year-olds, indicating no sudden increase in allergic youngsters.
The only similar study of peanut allergy using clinical testing and not surveys occurred in Britain's Isle of Wight and found an increase from 0.5 percent to 1 percent of all children from 1989 to 1996. However, the study was small, and the authors said the difference was not ''statistically significant"; in other words, the difference might be due only to chance.
Studying peanut allergies is complicated since the diagnosis can be uncertain. The only 100 percent reliable way to tell if someone has a peanut allergy is to feed them peanuts or a placebo in a clinical setting to see if a reaction occurs -- a so-called food challenge. But because of the cost and the slight risk of precipitating a severe reaction, this test is not often done.
Instead, doctors usually rely on the safer skin prick test, in which a tiny dose of peanuts gets injected under the skin to see if a hive forms.
This test isn't very precise, though. According to a 2001 study from Clinical and Experimental Allergy, only 40 percent of children with even strongly positive skin tests (a hive more than 5 millimeters wide) had positive food challenges -- and of them, only half had reactions needing any treatment.
Directly measuring a child's IgE level by a blood test isn't much better. Data from a major federal health database suggest that only 1 in 6 people with peanut-specific IgE actually shows symptoms. And while extremely high levels often do mean real allergy, the converse is untrue; most children with real allergies don't have very high IgE levels.
To diagnose peanut allergy accurately, an experienced doctor carefully reviews the story of the child's reaction, then orders skin-prick and blood tests. In some cases, the data are so negative that true allergy is impossible or so positive that food challenge might be dangerous. But for those in between -- which are a lot of children -- the challenge is the only way to be certain.
The diagnosis shouldn't be made lightly. In a 2003 study, children told they were peanut allergic had more anxiety and felt more physically restricted than children with juvenile diabetes.
Though severe peanut allergies are extremely uncommon, many parents want to know how to prevent them.
One widely held theory -- endorsed by the American Academy of Pediatrics -- is that allergy might be avoided if the mother stays away from peanuts and other likely food allergens during pregnancy and breastfeeding and that children shouldn't be given them until at least 2 years of age. But supporting research is absent.
The best study, published in The Journal of Allergy and Clinical Immunology in 1995, showed that a strict maternal diet coupled with delayed introduction of allergenic foods had no impact on long-term risk of ''food allergy, atopic dermatitis, allergic rhinitis, asthma, any atopic disease, lung function, food or aeroallergen sensitization, [or] serum IgE level."
Some recent data suggest that avoiding antireflux medications like Zantac, peanut-containing diaper and breast creams, and soy-based formulas may be sensible for infants with a strong family history of peanut allergy.
Finally, despite their best attempts to avoid peanuts and carefully read labels, the average person with true peanut allergy still gets a reaction every three to five years. Yet only one in three parents of allergic children has a potentially life-saving dose of EpiPen nearby and knows how to use it. Affected children should never be without an EpiPen and someone who knows how to use it.
Dr. Darshak Sanghavi, an assistant professor of pediatrics at UMass Medical School, can be reached at www.darshaksanghavi.com.