How Doctors Test Your Child For Food Allergies

When we suspected our son of having a food allergy, we were beyond dismayed to see things like 50-60% of positive results are not real or “false positives.” What’s the point of a test that is wrong at least half of the time? Yet when a food allergy is suspected, the first step is often an allergy test. So should you drag your child to the allergist to undergo an uncomfortable skin prick session, or not?

Things to Remember About Food Allergies

We discussed this in a post about food allergies vs food intolerances, but as a general rule, food allergies are not sneaky. If symptoms show up after you eat a food, then you may have an allergy. Conversely, if you can sometimes eat the food, or if your child can eat a food without hives/anaphylaxis vomiting or other highly noticeable symptoms, there’s probably no allergy, and maybe no intolerance either.

In the end, eating a food and seeing a reaction is the only true way to confirm a food allergy. But there are a few good reasons to go ahead with the skin prick testing or IgE blood testing:

  1. Both can, with high certainty, rule out an allergy. If it’s negative, there’s no allergy

  2. Both can help differentiate a food intolerance, which is non-immunologic, and therefore not IgE mediated, from a food allergy

  3. If there is a suspected allergy based on past history, then the tests can help confirm that

As a parent, there’s a strong urge to “just test them for everything.” But a lot of doctors recommend against that because of the high rates of false signals (also called false positives). An IgE sensitization is not the same as having an allergy. You may end up taking a bunch of foods out of your child’s diet for no reason.

What to Expect with IgE Testing

In skin prick testing, they place a drop of liquid containing the allergenic protein on the skin and then push down to prick through the top layer of skin. If your body already has the specific IgE antibody for that particular allergen, you’ll see a red puffy welt start to appear around that prick within fifteen minutes. After looking for welts and measuring the size, the doctor will wipe the area clean and apply a cortisone cream to stop any itching.

Skin Prick Test

Skin Prick Test

For IgE specific blood testing, a tube of blood is taken. The blood is sent to a lab where allergenic proteins are introduced to the blood, and the levels of specific IgE antibodies that show up are measured. You’ll get a result for each allergen tested that from 0-100 kg/ IL. Higher scores in general mean more immune reaction, so a high score would likely indicate an allergy. However you should not interpret the results without consulting a doctor. Different levels actually mean different things for each allergen, and it is quite common to see mildly elevated results all around in children who have other conditions such as eczema, asthma, or seasonal allergies. We saw this with our son when he was first diagnosed and his eczema was not controlled. When the test was repeated a year later, all the values went down significantly across the board.

IgE Blood Test Result

IgE Blood Test Result

Oral Food Challenges

A doctor supervised oral food challenge is the one definitive test of a food allergy. This test is commonly used when it seems like someone may have outgrown a food allergy,  based on IgE testing. Generally speaking, you will go to the doctor’s office and eat increasingly large amounts of the suspected allergen. This process takes four hours, so come prepared!

Oral food challenge with syrup covered french toast

Oral food challenge with syrup covered french toast

Once you have eaten a certain amount of the protein, you are considered to tolerate that allergen and therefore do not have an allergy. Why is it important to do this at a doctor’s office? Because a reaction can come on fast and you want to be right there with help if needed.


What About Food Allergy Test Kits You Can Use At Home?

Because my search history these days is all about food allergies, I see a lot of advertisements for home-based food allergy or food intolerance screening tests. As far as I’ve been able to find, none of these diagnostic tests have been cleared by the FDA. That means that they have never created and presented data showing that their tests work. Furthermore, acupuncture, hair/urine testing, and any other such testing you may have heard of is also not proven.

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