SAN DIEGO — While the number of patients diagnosed with atopic disease is on the rise, the general public might be overestimating the prevalence of food allergy, according to Dr. Jonathan M. Spergel.
At a meeting on skin disorders sponsored by Rady Children's Hospital, Dr. Spergel said that Americans perceive the prevalence of food allergy to be in the range of 20%–25%, whereas the actual prevalence ranges from 6%–8% in infants and young children and 2%–3.5% in adults.
The prevalence is higher in those with atopic dermatitis, certain pollen allergens, and latex allergies. "And [as with] all atopic diseases, the prevalence has risen," said Dr. Spergel, allergy section chief in the division of allergy and immunology at the Children's Hospital of Philadelphia. "It probably has quadrupled in the last 20 years."
Conducting a thorough history is vital and should include asking about symptoms, reproducibility, diet, concurrent exercise, use of nonsteroidal anti-inflammatory medications, and timing of reaction. "The timing is an important thing," he said. "If you eat a food and then react 12 hours later, that's not an IgE-mediated reaction. It may be a non-IgE-mediated reaction."
If you suspect an IgE-mediated allergy, broad screening should not be done without supporting history because of the high rate of false positives. Skin prick tests indicate the presence of IgE antibody but not clinical reactivity. They have a sensitivity rate of 90%, a specificity rate of 50%, and a false-positive rate of 20%–60%, depending on the individual food.
"When you do a skin prick test, look for a wheal or a flare about 15 minutes later," Dr. Spergel said. "They work very well. They're relatively painless … and they're safe. The youngest I've done it was on a 1-week-old. He was a newborn in the nursery. They gave him a bottle of milk and he got hives."
Milk, egg, and peanut account for 85% of food allergies, he said. A predictive value on whether a child will react is based on the size of the wheal from the skin test. The age of the child also affects the size. For example, 95% of children aged 0–2 years with a wheal size of 5 mm or greater will react to egg, while a wheal size of 7 mm or greater is the 95% cutoff for older children.
Unproven or experimental tests for food allergy include intradermal skin tests, "which should never be done," Dr. Spergel warned. "It has a risk of systemic reactions and death. And in people who don't have IgE reactions, there is a fair amount of false positives."
Other unproven tests include provocation/neutralization tests, cytotoxic test, applied kinesiology, hair analysis, electrodermal testing, and food-specific IgG or IgG4. "Some of these tests cost a fortune," he said. "They make some labs very rich but they don't work. Do not recommend them."
Elimination diets for 1–6 weeks might be the most useful for patients with atopic dermatitis, gastrointestinal syndromes, or other chronic diseases.
"For food allergy, the 100% test is an oral food challenge where you give a food and see what happens," Dr. Spergel said. "This may need to be supervised by a physician, and you might need emergency medications available."
If testing for specific IgE antibody is negative, reintroduce the food unless a convincing history warrants a supervised challenge. If it is positive, start the patient on an elimination diet, he said.
If the elimination diet is associated with no resolution, reintroduce the food unless a convincing history warrants a supervised challenge. If the elimination diet is associated with resolution, conduct open or single-blind challenge testing, or double-blind, placebo-controlled testing for equivocal open challenges.
Dr. Spergel disclosed that he has received grant support from Novartis and Ception Therapeutics Inc. and speaker fees from GlaxoSmithKline and Astra-Zeneca Pharmaceuticals LP.