The Alchemy of Allergies: What Blood Test Scores Mean. . .and What They Don’t

Jan 22, 2015

IPFW Associate Professor and medical ethicist Abraham Schwab.
Credit Courtesy / Abraham Schwab

If you ever have your child’s blood tested for an allergy, there’s a chance it will be the allergen-specific IgE antibody test. This test’s results appear pretty straightforward.

For example, a score higher than 0.35 for peanuts is indicative of an allergy. When they gave my son such a test in 2010, his score was about 100 times this minimum threshold.

Given this high his score, it was easy for me to overreact. I could imagine some other kid eats a peanut butter and jelly sandwich and doesn’t wash his hands. That leaves peanut butter on the playground ladder my son uses. When my son wipes his nose, his nose starts to run. Then he has trouble breathing and I’m hoping he has his EpiPen nearby.  

And yet, since 2010, nothing has happened. No reactions at all.

So we had his blood tested again to see if maybe, just maybe, he’d outgrown the allergy. This time his score was 86—almost 250 times the minimum threshold. So why hasn’t he had a reaction in the last few years? Between baseball games, playgrounds, lunchtime at school, and play dates, he has surely been exposed to peanuts somewhere, somehow.

Perhaps I’m misunderstanding the scores. Perhaps a score 250 times the minimum threshold doesn’t mean what I think it means.

Finding out what the blood test’s scores mean takes some work. The NIH barely touches on it, WebMD only discusses “positive results” without any discussion of the numbers , the Mayo Clinic offers only that “these blood tests aren't always accurate.” Foodallergy.org and kidshealth.org say very little, and kidswithfoodallergies.org can only say that the results from blood tests have “a very high negative predictive value but a low positive predictive value.”

Digging into the more esoteric medical scholarship on allergies, you can find out a bit more. And, it turns out, there are a couple things that everyone who gets these blood tests should know.

First, the blood test results tell us nothing about the severity of an allergy.

As I mentioned, my son’s numbers went up a lot in the last few years. This increase indicates only increased likelihood he’ll have a reaction if exposed to peanuts. But they don’t tell us anything about the severity of the response. It could be digestive discomfort or it could be anaphylaxis. Because he ate peanut butter and then hives covered ¾ of his body at the age of 2, there’s good reason to believe his reaction would be severe.  But the blood test doesn’t tell us that, his clinical history does.

Second, as this set of guidelines suggests (see particularly pages S102-S104) blood tests should not be used as a stand-alone diagnostic tool in most cases. While these tests accurately identify 80-to-90 percent of allergic individuals, they may misdiagnose up to 60 percent of people who do not have a food allergy. Instead, as this Wall Street Journal article echoes, a history of allergic reactions to foods remains the only truly reliable indicator.

And this takes us to my second son. He’s six months old and won’t be exposed to peanuts for a long time. In fact, because his siblings have food allergies, he is three times as likely as the average kid to have food allergies. So he will no doubt be blood tested in the coming years. And if his blood tests suggest he has an allergy, will that mean that he actually has a food allergy? No.

But we’ll end up playing it safe all the same.

 

Abraham Schwab is an associate professor of philosophy and a medical ethicist at IPFW.

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