Food allergy “testing” is usually a bad idea

The Pediatric Insider

© 2015 Roy Benaroch, MD

People like tests. You get numbers, and maybe a printout, and there’s science and blood and things just feels more… serious, when testing is done. You can picture Marcus Welby  (or perhaps a more modern physician), looking solemn, declaring “We’d better run some tests.”

Are medical tests magical and mysterious, and can they unlock the secrets of life? Usually, no. And among the worst and most misunderstood tests we do are food allergy tests.

A few recent studies illustrate this well. A review of about 800 patients referred to an allergy clinic found that almost 90% of children who had been told to avoid foods based on allergy testing could in fact eat them safely. The study, bluntly titled “Food allergen panel testing often results in misdiagnosis of food allergy” also found that the positive predictive value of food allergy blood tests—the chance that a positive test accurately predicted real allergy—was 2.2%. That much, much worse than the odds if you flipped a coin, and much, much worse than your odds of winning at a casino. If someone told you that a positive test was only correct 2% of the time, would you even do the test?

What about the other way of food allergy testing, with skin scratch or prick tests? A recent study about peanut allergy made big news when it turned out to show that early peanut exposure can prevent allergy. (This isn’t new news, by the way—I’ve written about that before. But I get fewer readers than the New England Journal of Medicine.) But hidden in the methods and statistics of that paper was another gem. The authors tested all of the enrolled babies for peanut allergy, at the beginning of the study. And most of the babies who “tested positive”, whether or not they then ate peanuts, did not turn out to be allergic. A true statement from the data from that study would be: If your baby tests positive for peanut allergy, your child is probably not allergic to peanuts.

Read that sentence again. Kind of makes your brain hurt, doesn’t it?

It is true that positive-tested kids were more likely than negative tested kids to be allergic—among the group with more allergies later (those who avoided peanuts), 35% of those who had positive tests developed allergy, versus 14% who had tested negative. But still, in either case, most of the kids who tested positive did not turn out to be allergic, whatever they ate or did.

The fundamental problem, I think, is that doctors either don’t understand or can’t seem to explain the difference between sensitization and allergy. None of these tests can actually test for allergy—they test for sensitization, which is different. We gloss over that distinction, and end up giving out bad advice. People should not be told to avoid food based on the results of allergy testing alone.

Bottom line: if you child eats a food without having a reaction, he or she is not allergic, and you should not do any testing for that food as a potential allergen. You should never do broad panels of “allergy tests”—they’re much more likely to mislead and confuse than to give useful information. Any food allergy testing that is done should only look at foods that seem to have caused reactions in the past, and even then any positive testing should be confirmed by what’s called an “open challenge.” Under safe conditions, usually under an allergists’ care, give the child some of the food to eat to see what happens. That’s the only real way to “test” for allergy.

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7 Comments on “Food allergy “testing” is usually a bad idea”

  1. Betty Says:

    Thank you for this post! I have heard too many stories of parents who subject their child to these broad panel allergy tests and live their lives in fear of all the foods their kids are supposedly allergic too. In a lot of cases, these kids have never even eaten the foods they are “allergic” to. Overly cautious parenting, I would say.

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  2. Great post–I may be the only one who notices that your reference to past posts about early exposures links to not one, but three separate posts. I appreciated that touch. It seems like at least a couple times a week, I’m having to correct some sort of misinformation about allergy testing…and it’s not just the parents who don’t understand. Tests are over-ordered, the results are misinterpreted, and the outcome is that children are needlessly restricted from eating healthy foods.
    -Chad

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  3. Supermouse Says:

    Yeah…back in the early 80s, my mom took me for a panel of skin scratch tests. I have asthma, and for some reason, the doctor thought something I was eating was causing it. Based on that test, it was declared that I was allergic to corn: all of it. Corn starch, corn syrup, etc. Which is, of course, in everything. For 6 years, my mom read the ingredients on all the food she bought and I had a somewhat limited diet. Then, one day my mom gave me the wrong bread and nothing happened. Well, hallelujah, I must have outgrown the allergy!

    I suspect I was never allergic to corn and my asthma triggers were environmental (dust, mold, pollen, etc). I don’t know what my mom thinks, but I think it was a huge waste. I doubt the doctors were trying to make our lives difficult, they were probably doing their best but certainly allergy and asthma research has come a long way in the last 35yrs or so.

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  4. Dr. Roy Says:

    Asthma understanding and asthma therapy have made huge advances. Allergy…. well, we’ve got better medicines now, like non sedating antihistamines and topical steroids. But our understanding of how and why allergy develops, that’s still very much in the dark ages. Check back here in 20 years, Supermouse!

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  5. lynnawiensmd Says:

    Thanks Dr Benaroch for your insights from a pediatricians standpoint. We see patients everyday that have been told based on a “test” that their child has food allergy. The percentages vary, but a majority of children that are found to have a food allergy by testing, tolerate the food just fine after challenge. What are the exceptions? Peanut, tree nut, milk, and egg anaphylaxis should always be asked during patient histories. This is why meeting your patient and asking directed questions is so important!
    One of the reasons for this misconception is our understanding of what causes “food reactions”. IgE testing was the first to be released and that’s all we had…now we understand celiac is lymphocyte mediated; EoE is a hybrid between IgE and non-IgE mechanisms and the list goes on WITHOUT IgE causing the problem.

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  6. Lauren Says:

    Do the predictive value of the tests improve if there is reason to believe the child is allergic? For example, is it worthwhile to do the tests if a breastfed baby has an allergic reaction after its mother eats certain foods?

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  7. Dr. Roy Says:

    Lauren, the predictive value of any test always depends on pre-test probability. Here’s a (rather long) wikipedia entry about it: https://en.wikipedia.org/wiki/Pre-_and_post-test_probability

    RE: breastfed baby having an allergic reaction: it depends if the allergic reaction is IgE mediated. The only testing we have for allergies are for IgE mediated reactions (eg. hives). However, some allergies are true allergies, though they’re not mediated by IgE. For example, infant proctocolitis (mucusy, bloody stools) may be an allergic reaction to dairy ingested by a nursing mom, but it is not IgE mediated, and there is no “test” for it. Allergy testing when faced with the symptoms of proctocolitis is a bad idea.

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