Do Blood Test Results Show Allergy Likelihood?

Published: October 25, 2013

Q: My daughter, who’s 4, was diagnosed with peanut allergy shortly before her third birthday after experiencing a rash around her mouth when she ate a peanut butter cup. Her blood test for peanut showed an antibody level of 4.85, and it was 3.82 a year later. Her allergist suggests a food challenge in a year if her blood test result drops below the 2 level.

Given that the blood test can provide a negative result when an individual has an allergy, how does a dropping number reveal that an allergy is being outgrown? It’s my understanding that the blood test results do not indicate the severity of an allergy, but just the probability that the allergy exists. It’s not easy to follow how the tests suggest that a child is outgrowing a peanut allergy.

Dr. Sharma: The interpretation of food-allergy blood test results is definitely not easy, and your confusion is shared by many parents, as well as many experts in the field!

This question highlights some of the limitations of our available testing. You are absolutely correct that IgE antibody levels correlate with the likelihood of having a reaction, but they do not necessarily predict how severe that reaction might be. For example, two people with the same exact peanut IgE level may have very different reactions to an exposure to peanut.

You are also right that, in up to 10 percent of cases, a peanut IgE level can be negative even when someone has a peanut allergy. That is why an oral food challenge is necessary to rule out a peanut allergy definitively.

Your allergist’s recommendation to proceed with an oral food challenge once your daughter’s peanut IgE level drops below 2 kU/L is based on research showing that the chance of passing a peanut challenge is greater than 50 percent at levels less than 2.

Studies have also shown that declining results on testing, while not perfect predictors, are much better indicators that a food allergy has resolved than, for example, the presence of other allergic diseases, the age at diagnosis, or the severity of prior reactions.

There is research under way to identify better tests, ones that will more accurately predict which people with a food allergy would be most likely to suffer a severe reaction. For example, a test that could separate serious peanut allergy from a milder form could help allergists decide when to pursue a food challenge.

An article in a recent issue of Allergic Living magazine described one such test, called component testing, which has recently become available for peanut allergy. A conventional blood test for peanut might give imprecise results because it measures IgE to all components of the peanut protein. In contrast, component testing measures IgE levels to several specific peanut proteins (called components).

This information might help to predict the severity of a peanut allergy since Ara h1, 2 and 3 proteins are associated with anaphylaxis, while other components are associated only with oral allergy syndrome.

OAS is a form of food allergy associated with pollen allergies; it usually causes milder allergic symptoms confined to the mouth and throat. More studies are under way to better understand component testing, and research should continue to refine our testing methods for food allergy.

Dr. Sharma is an allergist, clinical researcher and associate professor of pediatrics. He is Chief of the Division of Allergy and Immunology at Children’s National Medical Center in Washington D.C. and Director of the Food Allergy Program. He co-authors “The Food Allergy Experts” column in Allergic Living magazine. Questions submitted will be considered for answer in the magazine.

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