Food allergy testing: Complex and often misunderstood
When patients come to see an allergist with a suspected food allergy, the history is the most important piece of information in determining whether it’s truly an allergy to a suspected food.
Editor's Note: This is a follow up on a previous blog post on what food allergies are and not.
When patients come to see an allergist with a suspected food allergy, the history is the most important piece of information in determining whether it's truly an allergy to a suspected food. There must be a reasonable chronological relationship between the consumption of the food and the illness. By reasonable, I mean the timing must conform to the underlying mechanism of food allergies. The symptoms must also be consistent with a food allergy.
Otherwise, testing for a food allergy is very often unhelpful or may even be detrimental. There are two ways to test for food allergy - allergy skin testing and a blood test for specific IgE to that food. But if the symptoms and the timing are not consistent with a food allergy, then either or both of these tests may be unnecessary.
Once a physician decides that food allergy testing is indeed indicated, then he or she must decide which food to test for. Here is where the history is important. The history should guide the testing. The reason for this is that tests for food allergy carry a high level of false positives and false negatives. In other words, they are not always correct, or relevant. For example, we could get a positive test for peanut in a patient who eats peanuts every day. Does that mean he or she is allergic? No, it just means that they make IgE antibodies to peanut.
Similarly, we often see negative tests to a food when the patient clearly is allergic to that particular food. If we get a negative result on testing, does that mean that patient is not allergic? No, it just means that the test was unable to find the presence of IgE antibodies to that particular food in that individual patient at that time.
For that reason, the gold standard for determining whether a person can tolerate a food is by doing a food challenge. Now, to eliminate bias, the best test is a placebo controlled double blinded food challenge, or PCDBFC for short. Neither the patient nor the person administering the test or observing for a reaction knows what is being given. This removes all psychological or physiological bias in the test. If a patient passes the challenge, then he or she can be deemed no longer allergic to that particular food.
Sometimes, in people with multiple suspected food allergies, several challenges need to be done. These need to be done independently on different days to eliminate the possibility of late symptoms crossing over to subsequent tests. Keep in mind, even though this is the best test, even this test has pitfalls.
If the patient does not give a history consistent with a food allergy, then the above tests should not be done because they would be irrelevant. But there are also many patients who may not present with typical symptoms of food allergy who experience other symptoms, and are convinced that a particular food is causing these symptoms. If the reactions are not allergic in nature, then close observation to single out the culprit food should be done, and that food may be removed from the diet.
Are there food allergy tests that I should avoid? Yes! There are many so-called "controversial and unproven methods in allergy and immunology". These are methods that have not been validated by good scientific studies. An example of this would be the allergen-specific IgG. Unlike IgE, IgG is not so much involved in allergy. Its primary function is to protect us from infectious diseases. While some of the subclasses of this molecule may play a role in developing tolerance to food allergies, measurement the levels of allergen-specific IgG to various foods has NO KNOWN RELEVANCE to food allergy. Patients do visit laboratories that perform these tests, and the paperwork given to them frequently includes an interpretation that the positive tests are indicative of an allergy, but this is not correct. Patients should be particular wary of labs that can be performed without an order from a healthcare practitioner who has actually seen him or her.
There are other tests that are of unproven value. These include applied kinesiology, neutralization-provocation testing, hair analysis, pulse testing, electrodermal or vega testing, facial thermography, gastric juice analysis, and cytotoxicity testing. You can find additional details as to why each of these techniques is unproven on the Food Allergy, Research and Education website.
It is important to understand the reasons why these tests may in fact, be harmful. For one thing, they may cause a patient to unnecessarily avoid certain foods and become malnourished if the results come back "positive" to multiple foods, when in fact they are not. Conversely, they may lead to a patient believing that a food may be safe, if a negative test is obtained, when they may in fact be deathly allergic to that food.
Knowing this, it's best to consult with an allergist to determine if a food allergy does actually exist. Even with the rise in food allergies, there may be other reasons your child may be reacting a certain way to a particular food. An allergist can help determine how you should proceed. Also, an allergist can collaborate with your primary care doctor or other subspecialist to help you assess your history, determine appropriate tests to order, interpret any testing results, and discuss possible treatment options if you do have an adverse food reaction, allergic or not!