What You Need to Know about Diagnostic Allergy Testing
by David Stukus, MD
Whenever I meet with families for the first time and ask the parents whether their child has any food allergies, I often hear the following reply: “I don't know, he/she's never been tested”. This always presents a wonderful opportunity to discuss the role of diagnostic testing for food allergies, as I'd like to do in this forum.
Before we go any further, I'd like to define some common terms that you may encounter when reading about or discussing food allergies:
- Allergy – This is an immune response to a particular food. Symptoms should occur every time that food is ingested. These immune system changes fall into two categories: Immunoglobulin E (IgE) mediated and non-IgE-mediated.
- Sensitization – This is the detection of specific immunoglobulin E (IgE) through skin prick or blood testing towards a specific food, but without the development of symptoms after that food is ingested. In other words, a positive allergy test result to a food that your child has eaten without any problems, or has never eaten.
- IgE mediated hypersensitivity/allergy – Commonly referred to as “food allergy”, in which IgE antibody specific for a food is formed and attaches to the allergy cells throughout the body. Whenever that food is ingested, it causes immediate onset symptoms, usually within minutes or up to 3 hours after ingestion. Typical symptoms include hives, swelling, itchy/water nose and eyes, difficulty breathing/swallowing, vomiting, and can progress to loss of consciousness. Skin prick or blood specific IgE testing is very likely to be positive for that food.
- Anaphylaxis – Rapid onset, progressive, severe symptoms involving more than one organ system that can occur with IgE mediated food allergy.
- Non-IgE mediated reaction – This is an immunologically mediated, typically delayed-onset reaction to a particular food. This is mediated by other parts of the immune system separate from IgE, specifically T-cells. These symptoms are not immediate in onset and can occur hours to days after ingestion. Anaphylaxis is not part of this response and most symptoms involve the gastrointestinal tract, with vomiting, upset stomach, diarrhea, or blood in the stool. Skin prick or blood specific IgE testing is negative.
- Sensitivity or intolerance – This is a non-immunologic response to a certain food or foods. Symptoms occur when that food is consumed, but may be variable over time. This also most often includes gastrointestinal symptoms and does not include symptoms observed with IgE mediated reactions. Skin prick or blood specific IgE testing is negative.
When trying to determine whether a child has a food allergy, there are many steps involved. First, the most important part is taking a careful history of suspected foods, the timing and types of symptoms that occur, and any treatment that has before used to help make symptoms better. If the history is consistent with an IgE mediated allergy, then testing is often pursued. However, a good rule of thumb to remember is, if your child can eat a food without developing any symptoms, then they are unlikely to be allergic to that food. Why is that? Because the best test is actual ingestion of the food. In regards to IgE mediated allergy, you're almost always going to know if a certain food makes your child sick, and there are no 'hidden' food allergies. In many circumstances, the history is more consistent with non-IgE mediated symptoms or intolerance and skin prick or specific IgE testing is not helpful, necessary, or indicated. This is the point when many families ask, “Why don't we just do the allergy tests to find out for sure?” If only it were so easy.
Before we discuss any further, I'd like to mention something that is very important to keep in mind when discussing food allergy testing. A positive test result for food allergy is not, in and of itself, diagnostic for food allergy. These tests are best utilized to help confirm a suspicious history for IgE mediated food allergies. They have high rates of falsely elevated and meaningless results and are not useful screening tools. Some commercial laboratories offer convenient “screening panels”, in which many different foods are included. These are rarely utilized by Allergists/Immunologists, but more commonly ordered by primary care providers. This often results in falsely elevated results, along with diagnostic confusion and unnecessary dietary elimination. Ultimately, your child may have food(s) removed from their diet for no reason other than a meaningless positive test result. This may then lead to anxiety, family hardship due to food avoidance, and potentially nutritional deficiencies.
There are 3 main ways to test for IgE mediated food allergy:
- Skin Prick Testing (SPT): This involves placing a drop of allergen onto the surface of the skin, and then pricking through it to introduce the allergen into the top layer of the skin. If specific IgE antibody towards that allergen is present and attached to the allergy cells, then an itchy bump and surrounding redness (wheal/flare) should develop within 15 minutes. These tests have a high negative predictive value (when a test yields a negative result, it is very likely to be correct), but a low positive predictive value (when a test yields a positive result, it is less likely to be correct) which can result in false positive test results. Thus, it is not a good screening tool but is a very reliable test to confirm a history that is consistent with an IgE mediated food allergy.
In order to get accurate results, all antihistamines should be discontinued for 5-7 days before testing. A common myth is that skin prick testing is not reliable in young infants and children. Actually, skin prick testing to foods is reliable at any age if you have a history of IgE mediated food allergy. Tests may be negative in young children when they are performed for other conditions such as non-IgE mediated formula or food intolerance.
- Specific IgE (sIgE) Blood Testing (previously and commonly referred to as RAST or ImmunoCAP testing): This test measures levels of specific IgE directed towards foods in the blood. The range, depending upon the laboratory techniques, can go from 0.10 kU/L to 100 kU/L. This also has a very high negative predictive value but a low positive predictive value. Mildly elevated results are often encountered, especially in children who have other types of allergic conditions such as eczema, asthma, and allergic rhinitis. The predictive values for likelihood of an allergy being present differ with every food, but in general, the higher the level, the more likely that an IgE mediated allergy is present. This is also a very poor screening test due to the high rates of falsely elevated and meaningless results.
I've met many families whose children have been ‘screened for food allergies’ in the setting of eczema or other conditions and the report lists every food that was tested as being ‘high’, as their cutoff for reporting this is often set very low, at levels that are usually meaningless. This leads to diagnostic confusion and unnecessary dietary elimination. In addition, many laboratories will report an arbitrary class designation (a created value that is assigned to a result that has no meaning or scientific basis), along with the actual level of specific IgE obtained. This is of no clinical use and also does not help determine whether food allergy is present. It is also commonly misunderstood that higher blood test levels indicate increased ”severity”. Unfortunately there is no test that can determine severity. Individuals with higher blood (or skin) tests are at no more increased risk of anaphylaxis than someone with minimally positive tests.
TAKE NOTE: "Class Levels" are meaningless.
- Physician Supervised Oral Food Challenge (commonly referred to as IOFC on KFA):This entails consumption of gradually increasing amounts of the suspected food allergen while being supervised by a physician, usually an Allergist. If no symptoms develop that are consistent with an IgE mediated food allergy (hives, swelling, anaphylaxis), then it makes the presence of IgE directed toward that food unlikely. This is often considered the gold standard for food allergy testing, and can be considered a good way to ‘rule out’ food allergy or determine if a previously diagnosed food allergy has gone away. This is time consuming as most challenges take 4-8 hours to complete but can be a very reliable test.
TAKE NOTE: The gold standard for diagnosing a food allergy is through a physician-supervised oral food challenge.
As you can see, performing diagnostic testing for food allergies can be very complicated and requires careful consideration about what tests to order and how to interpret them. There are very few indications to perform an extensive ‘screening panel’ for food allergies. However, obtaining a careful history of what specific foods cause symptoms and then using the type of symptoms can be a helpful guide to determine whether specific IgE testing is worth pursuing, or to go in a different direction.
Lastly, a word of caution regarding other commonly used techniques (often utilized by non-board certified Allergists/Immunologists) that you may encounter. Specific IgG blood testing for foods, muscle provocation testing, acupuncture, hair/urine analysis, and applied kinesiology are not validated, standardized, or FDA approved tests for the diagnosis of food allergy or food intolerance. Use of these tests is not recommended by the American Academy of Asthma, Allergy, and Immunology, or supported by the Guidelines for the Diagnosis and Management of Food Allergy, published in 2010 (Journal of Allergy and Clinical Immunology, 126(6); supplement S1-56).
Guidelines for the Diagnosis and Management of Food Allergy, published in 2010(Journal of Allergy and Clinical Immunology, 126(6); supplement S1-56).
Dr. David Stukus is an Assistant Professor of Pediatrics in the Section of Allergy/Immunology at Nationwide Children’s Hospital in Columbus, Ohio. In addition to his interest in caring for families with food allergies and other allergic conditions, he also serves as the Director of the Complex Asthma Clinic. He currently serves as the chair of the Medical Advisory Team for Kids With Food Allergies and sits on the Board of Directors for the Asthma and Allergy Foundation of America. He previously completed his residency at Nationwide Children’s Hospital and his fellowship at the Cleveland Clinic. You can follow him on Twitter @AllergyKidsDoc.
Medical review October 2012 and April 2014.