Monday, April 27, 2015

Food Allergy Testing: Down the Rabbit Hole

“Have you guessed the riddle yet?” the Hatter said, turning to Alice again.“No, I give it up,” Alice replied: “What’s the answer?”“I haven’t the slightest idea,” said the Hatter” 

Usually, when we go to the doctor, have a blood test, roughly a week later, we receive the results and there's a diagnosis: your cholesterol is high, you have anemia, etc. These are screening tests.

Food allergy tests, both skin and blood, are not screening tests. A positive test does not mean you are allergic. In fact, both skin and blood tests are wrong about 40-60% of the time (called a false positive). False negatives are less likely, although possible, in both tests.

The only time food allergy tests should be performed is when you are having a reaction to a food. And the only way they can be interpreted properly is in the context of your history.

“That's the reason they're called lessons," the Gryphon remarked: "because they lessen from day to day.” 

So, now you have your results, and you have been given numbers, classes, wheal sizes, and the larger then numbers the more severe the allergy, right? Of course not. But, here is where the blood test numbers can come in useful (and you can eliminate some of the false positive concerns), the larger the numbers, the more likely the test is accurate. That is, the more likely you are to truly react. The likelihood that one will react is called the positive predictive value. This is from medscape here:

  • Egg: In children > 2 years of age, IgE level 7 kU/L (98% PPV); in infants ≤ 2 years, IgE level 2 kU/L (95% PPV)
  • Milk: IgE level 15 kU/L (95% PPV)
  • Peanut:IgE level 14 kU/L (95% PPV)
  • Fish: IgE level 20 kU/L (95% PPV)
  • Tree nuts: IgE level 15 kU/L (95% PPV)
  • Soy: IgE level 30 kU/L (73% PPV)
  • Wheat: IgE level 26 kU/L (74% PPV)
As you may remember, James failed his oral food challenge to hazelnuts and his Ige is 0.46 (depending on what standard you follow, positive is either >0.35 or >0.01. Our lab says above 0.35, so he is barely positive).

And, here is the lesson, barely positive can mean an anaphylactic reaction. Negative can even result in an anaphylactic reaction (rarely). The test is meant to guide the allergist with your history in diagnosis. It is not diagnostic in an of itself.

“Why, sometimes I've believed as many as six impossible things before breakfast.” 

What about IgG testing? IgG has no place in testing for allergies or intolerances. Some researchers and OIT doctors have actually begun measuring it (research wise) as a good measure, as in, as IgG rises,  your ability to tolerate the food. IgG shows that you have been exposed to the food. There have been studies showing that children who out grown their milk allergies have higher IgG milk levels and that IgG levels rise throughout OIT

“Would you tell me, please, which way I ought to go from here?""That depends a good deal on where you want to get to."

We seem to be on the cusp of the future as far as testing goes.

At Stanford, there has been news of two new tests in development. The link can explain it better than I can, but one uses a heel prick to test for 90 allergens using white blood cells. The other looks at DNA. Used together, they can also be used to determine the severity of an allergy. Now, of course, families are told to treat every allergy as potentially life threatening.

At UConn, they are also developing a food allergy test, specifically for peanut, interestingly in addition to being a food allergy test, the researchers are hoping to use the test to get a better look into the biology of a reaction.

Of course, my personal favorite, Dr. Li is developing the Basophil Activation Test (BAT). Because James's IgEs are all low, an alternate test for him, for people like him, is necessary. The idea of him undergoing another food challenge relying on a test that is, quite frankly, inadequate, is terrifying.

That said, parents are fundraising for a clinical study to help move forward Dr. Li's work on the BAT. The total is not completely updated on the page; we need slightly less than $5,000 more. If you are able to contribute anything, it would be greatly appreciated. We are trying to get the last bit raised by the end of this month.

Great news from this afternoon (April 27th) that the study has been fully funded! Thank you to everyone who has contributed and everyone who supports those with food allergies in any way.

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