When A Little Knowledge Can Be A Dangerous Thing

Food Allergy Blood Tests

Many parents are aware of the dangers of food allergies, however food allergy researchers are suggesting that strictly avoiding foods based on the results of an allergy test can make the problem worse.

Food allergy is common in young children. Just how common is unclear.  A recent report  in the Journal of Allergy and Clinical Immunology cited a review of over fifty studies  from around the world evaluating the prevalence of food allergy.   The result showed a disturbingly wide range of values with the frequency of milk allergy ranging from 1.2% to 17%, 0.2% to 7% for egg, 0% to 2% for peanut and fish, 0% to 10% for shellfish, and 3% to 35% for any food.

One of the problems in estimating the true frequency of food allergy is the fact that a patient’s report of food allergy is often inaccurate. In addition, currently available tests to diagnosis food allergy may give positive results in as many as 50% to 75% of patients who are able to eat the food without problems.  In other words there is a 50% false -positive rate with these tests.

Currently, the most reliable means of diagnosing food allergy is through a test known as the double-blind, placebo-controlled food challenge. This is a time-consuming investigation involving a carefully controlled administration of the suspected food as well as a placebo designed to mimic the food.   Neither the clinician performing the test, parents, or the patient knows whether the true food or the placebo was given until after the test is completed.  In this way any symptoms reported or observed during the test are unlikely to be the result of anything other than a true reaction.

Simplified versions of the food challenge test including open testing (the patient and doctor know that the food if being given) and single-blind studies (only the doctor knows) are available but still require a careful history and screening by a specialist as well as close monitoring to insure safety. In this setting, only about one third of suspected foods  result in a positive cahallenge.  Allergy blood tests are obviously much easier to perform and so common that grocery stores have offered then for a small fee as a promotion.

It is suspected that the perceived rise in food allergies in the past decade may be the result of the widespread use of these blood tests for food allergy. Most of these tests look for the presence of allergic antibodies, called immunoglobulin E (IgE) that the body produces in response to a food.   Because a positive IgE test to a food is  associated with clinically significant food allergy only half the time, many patients are misdiagnosed.

Over diagnosis of food allergy in a child, particularly to important foods such as milk, egg, and wheat, can lead to unnecessary elimination diets that can place a child at risk for malnutrition.  The fear and stress associated with a diagnosis of food allergy can be substantial.

There is also a more subtle but potentially more dangerous risk.

Many children with significant food allergies as an infant or toddler “outgrow” their allergies and are able to tolerate the foods by the time they start school.  This is particularly true with milk, eggs, soy and wheat.

Children who have foods strictly removed from the diet on the basis of an allergy test alone may be prevented from developing a natural tolerance to the food and may even have more serious reactions if the food is eaten later in life.

Researchers have suggested that a child who may not be able to tolerate large amounts of a food such as whole egg or milk, but who have been able to tolerate these foods  when extensively cooked, such as in baking, should be allowed to eat these foods even if the allergy test is positive.  In this way the immune system is given the opportunity to develop a life-long tolerance.

The most important question to answer for a parent who is concerned about food allergy in their child  is  whether the child has tolerated the food in the past.  If the child has eaten the food without obvious problems then they should be allowed to continue eating the food, regardless of blood test results.

In fact, because of the high false-positive rate in food allergy testing, a child should not have tests for food allergy unless a reaction has occurred.  Parents who are unsure and are given a positive blood test result should seek advice from an experienced allergist to  review the history and consider medically supervised food challenge testing.    Although all parents would like to know what foods to avoid before a serious allergic reaction occurs, information from a simple allergy blood test can  do more harm than good.