From the Wall Street Journal:
"Many kids whose allergies were diagnosed on the basis of blood or skin tests alone may not be truly allergic to those foods, experts say. Blood tests measure the level of antibodies, called immunogloblin E (IgE), a body makes to a particular food. But having IgE antibodies doesn't mean that a person will actually have an allergic symptom when they encounter it. Skin-prick tests are slightly more predictive, but there, too, a red wheal in response to a skin prick doesn't necessarily mean that a child will have an actual allergic reaction to that food. The only way to know for sure—short of encountering the food in real life—is with a food challenge test in a doctor's office or hospital...Experts agree that the most important part of a food-allergy diagnosis is a history: What did the child eat and what kind of reaction did he have? Even if it seems clear-cut, most doctors will also do a blood test or skin-prick test to confirm that the child has antibodies to the suspect food."
Read more from this important article from the Wall Street Journal, written by Melinda Beck.