Food Allergy Test: What Does It Mean?

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:

 

  1. 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.
  2. 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.
  3. 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.
  4. Anaphylaxis – Rapid onset, progressive, severe symptoms involving more than one organ system that can occur with IgE mediated food allergy.
  5. 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.
  6. 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 been 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:

  1. 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.

    TAKE NOTE: "Positive" results can be wrong; "negative" results are more likely correct.
  2. 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.
  3. 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).

 

References

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 StukusDr. 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 Co-Director of the Specialty Asthma Clinic. He previously completed his pediatric residency at Nationwide Children's Hospital and his fellowship in Allergy/Immunology at the Cleveland Clinic Foundation. He is married with one 3 year old son, and a daughter due in November.

 

 

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camnicemm posted:

Any thoughts or direction on behavioral issues due to food? My now 6 year old daughter never had any stomach issues, eczema, or anything else, but she became absolutely uncontrollable (threatening to kill me, punching, kicking)after a strep infection back in February. I figured something had to have happened and had the doctor do all the gluten related bloodwork since it was just not her and her oldest brother is GFCF. Sure enough, it came back positive. The allergist put her on an elimination diet and we found she reacts just as strongly to corn - again only with behavior. Didn't show up with his tests so he thinks I'm a lunatic but about 36 hours after eating something with corn, her behavior starts going downhill and it takes over a week to get it all back out of her system. How do I find a doctor who will take me seriously and not just blow me off because he doesn't see a physical reaction?

 

I realize your post is from 2012 but in reading the comments trying to find a solution for my daughter's situation, I had to write in the event you were still having issues.
Please google PANDAS in regards to your daughter's violent behavior after having Strep throat. There is a direct link. It has been in the news a lot recently.
 
 

Hi There,

I would really appreciate your guidance. My boy who is now over 3 years has had 3 reactions to eating hake. The 1st one happened when he was 6 months old and the 2 times after happened when he was just over 1 year. His symptoms occur about 2 hours after ingestion, vomiting severely for a few hours, lethargic and non-responsive until it is all out his system. The last episode he did have runny tummy the day after.

On all 3 occasions we took him to emergency to be monitored. They gave him Zofar for vomiting under his tongue and liquid to re-hydrate.  After visiting an allergy clinic and specialist, a skin prick test was done, showing he was allergic to all fish and sea food. We were advised to have him retested at 3 years old. 

My boy turned 3 in July. We have just visited the allergy clinic again to have him retested. He had a skin prick test which came up negative and a blood test which has also come back negative.

The specialist is now suggesting we admit him into hospital to have an introduction to fish under supervision.

My questions is - is this necessary? to have to put him through another day in hospital, the trauma of having another needle in him plus the additional cost of such a procedure.

Surely if both tests have come back negative, could we not start introducing small amounts of fish at home? Or even sit in the hospital cafe and let him taste fish should a reaction occur?

Please help!!!

Many thanks

Lianne

Hi, and welcome, @AGlick0927! Hugs on the reactions and the conflicting test results on peanuts. You're definitely in the right place to get support!

Your questions are great, and it sounds like you still have some things you might want to discuss with your allergist. 

Another thing you might want to discuss with your allergist is component testing for both milk and peanuts, if you haven't already done so. Component testing can tell a doctor what particular protein in milk or peanuts a person is sensitive to, and that can help the doctor decide whether an oral challenge is appropriate. 

Since you guys are planning on doing an oral challenge, KFA has a great resource on oral food challenges

Why not post this question to our Main Support Forum so you can get more feedback from other folks in a similar situation? If you're on desktop view, just follow the link above, and click on the big green POST. On mobile, click the three horizontal lines in the upper left hand corner, and you'll see POST. Click that, and you'll be able to add your comment as a new post.  

Hello! My 2.5 year old daughter has tested positive to peanuts, tree nuts, egg and dairy on a skin test (2 different times) She has had reactions to both egg and dairy, however she has never ingested any nuts.

We just received the results of her first ever blood test and it confirmed egg and dairy, however it came back negative to all nuts! We will be scheduling an in office peanut challenge.

My question is, since I know that false negatives are more rare than false positives, should I feel optimistic that we actually have a chance at not being allergic to peanuts or tree nuts?

Additionally, our allergist is suggesting we do a BAKED milk challenge based on numbers, however this scares me because I've seen her reaction to dairy in the past and I hate to think of having to go through that again.

Thank you so much!

Hi, Lune! Since you're having symptoms when you eat food, I agree with Jen -- a board certified food allergist is definitely someone to consult. You may also want to touch base with a dermatologist as well.

Why not post this question to our Main Support Forum so you can get more feedback from other folks in a similar situation? If you're on desktop view, just follow the link above, and click on the big green POST. On mobile, click the three horizontal lines in the upper left hand corner, and you'll see POST. Click that, and you'll be able to add your comment as a new post.  

Your comment caught my eye because my DD has an allergy to apple that is in line with something called OAS (oral allergy syndrome). You can find out more about it under KFA's resource on the types of food allergies.

Also, my DD had/has eczema (worse when she was little, better now as a teen), and I have flare ups of eczema on my eyelids.

Not all eczema is related to foods, and even if a food is a trigger for eczema, some allergists and dermatologists recommend other techniques besides avoiding that food. You can find some of those techniques here on KFA's resources for eczema

So as far as I remember I suffered with bad eczema rashes since I was little and my parents somehow managed to keep it at bay. No nuts, dairy and limited amount of citrus acidic and sugar food, I have had no issues with eczema for more than ten years and thought it went away--- I was wrong. I recently started having bad eczema flare upd around my face. Eyelids, under eyes, chins and around mouth. It's swelling, itchy and bumpy and I have no idea what is causing it. I have been eating nuts, dairies and acidic food without no problems but something (could it be environmetal?) is triggering it pretty bad. I am very curious about testing for food allergy but please let me know if you have any recommendations for different types of testing.

Also, I have been loving seafood all my life even as a kid but I found out recently that after consuming salmon or tuna my face feels little itchy. Same goes for apple and cherries. I love eating them but my mouth gets so itchy after and my lips swells. Could I have developed some kind of food allergy? And how do I manage it if I can't give up on eating them? 

Mehmet posted:

Can somebody be okay eating a food but not be okay touching it? 

My daughter (5 years old) successfully passed two oral food challenges for hazelnuts, which included eating increasing amounts of hazelnuts and also a spoon full of Nutella (this was given after she had eaten her last dose of hazelnuts). There were no symptoms or problems at all. So I (and our allergist) think it's safe to assume that she is no longer allergic to hazelnuts and we are obviously very grateful for that. 

However, whenever we give her Nutella at home and something gets stuck around her mouth or on her lips, her skin reacts. The skin turns red (not itchy though) and her lips "hurt" a little bit (that's her description, it may be itchy). This disappears quickly after we wash it off with water.

We've check all the other ingredients in Nutella but everything seems to be fine. Any idea of what could cause these reactions. Our allergist suggests that she may just have sensitive skin. I hope that's the case but why the bumps on her lips then?     

Welcome Mehmet!

May I ask when you did two food challenges?  Just curious...

Does she have any other allergies?

The redness and the lips "hurting" would have me think that she could possibly still be allergic to it.  How much is she eating at home?

As Cynthia mentioned, be sure to visit us in our Main Support Forum where others can chime in as well with their experiences.

Can somebody be okay eating a food but not be okay touching it? 

My daughter (5 years old) successfully passed two oral food challenges for hazelnuts, which included eating increasing amounts of hazelnuts and also a spoon full of Nutella (this was given after she had eaten her last dose of hazelnuts). There were no symptoms or problems at all. So I (and our allergist) think it's safe to assume that she is no longer allergic to hazelnuts and we are obviously very grateful for that. 

However, whenever we give her Nutella at home and something gets stuck around her mouth or on her lips, her skin reacts. The skin turns red (not itchy though) and her lips "hurt" a little bit (that's her description, it may be itchy). This disappears quickly after we wash it off with water.

We've check all the other ingredients in Nutella but everything seems to be fine. Any idea of what could cause these reactions. Our allergist suggests that she may just have sensitive skin. I hope that's the case but why the bumps on her lips then?     

We often mess up between food allergy and food intolerance. Food allergy usually comes on suddenly. Even a small amount of food can trigger an allergic reaction. Food allergy appens every time you eat that particular foo and can be life-threatening too. On the other hand, food intolerance generally comes on graduall and occurs when you eat a lot of that particular food. It is not life-threatening

Dr. Stukus -

Thank you for sharing this information.  I agree with much of what you had to say and wanted to know if you would ever recommend the "Gold Standard" for accommodations in school?  I have two children with close to 40 ER visits between them for allergic reactions.  One has had RAST scores over 100 to Peanuts since infancy - he is 12.  The other has RAST over 100 to Dairy and Eggs since infancy - she is 8.  They both had systemic reactions to skin testing done last summer.  They were denied accommodations in school under their 504 plan so, we went through Due Process litigation to get those accommodations.  The Dr who testified for the school district is a very well known pediatrician / advocate in the food allergy community.  She has never met my children, never performed skin testing or treated patients for food allergies.  She testified that she was not so sure that our children even had food allergies since our Allergist (with 40+ years of experience - including work at National Jewish) had failed to perform the oral food challenges on our kids.  Our Allergist testified that our children are contact, airborne and ingest to their allergens based on their history, skin and RAST tests.  Because of the pediatrician's testimony and her experience in the food allergy community, the judge ruled with her recommendation of oral and inhalation food challenges for both of our children.  The judge stated that because we failed to provide the burden of proof by submitting our children to both oral and inhalation challenges that the school district would not have to accommodate our children.  Would you ever advise a patient to undergo an oral food challenge under such a circumstance?  What are your thoughts?  Thanks!  

Missy24, sorry to hear that your son had a severe reaction to cashew. Unfortunately you can develop allergies at any time, so in his case he was not allergic before but then became allergic.  The article is referring to testing positive to a food but being able to eat it without reaction.  Some docs will run a panel to whole list of foods, even if the patient has only reacted to one.  This can lead to avoiding more foods than is necessary as there is a high chance of false positives with allergy testing.

My son ate cashews by the handful with absolutely no reaction and then went into anaphylactic shock after eating one.  How does that fit into this?  This stated that if you can eat something without reaction you aren't allergic.  He reacted to walnuts which now that we've had testing are still high but lower than both peanuts and cashews.
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