Food Allergy Testing: How It Really Works — As Told By A Leading Allergy Specialist

Apr 9, 2021
Aug 16, 2021
Erica Dermer
10 minutes

Medically reviewed by

Ivor A. Emanuel, MD
Ivor A. Emanuel, MDIvor A. Emanuel, MD
Allergy & Otolaryngology Doctor
Dr Emanuel is a board-certified otolaryngologist and is considered a national expert in the field of sublingual immunotherapy and allergy blood testing. Dr Emanuel has been in practice for more than 25 years.
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Testing for food allergies can be incredibly useful. But it’s not as straightforward as testing for other types of inhalant allergies like dust mites, pet dander, mold and pollen. In fact, I’d argue that while most types of allergy testing are a relatively straightforward science, food allergy testing is more like an art. In this article, I’ll explain what you really need to know about food allergy testing, along with the process I use with my patients to test for food allergies, and how home testing can help. 

How does food allergy testing work? 

First, let’s try to understand the different types of food reactions that can happen. 

True food allergies involve a reaction in your immune system, which is what makes them different from food intolerances or food sensitivities.

If you’re truly allergic to a food, your body will produce immunoglobulin E (IgE) antibodies in response to that particular food. When a lab conducts an IgE food allergy test with your blood, they're measuring the amount of IgE antibodies in response to specific allergens. If IgE antibodies are detected, the result is considered positive. If IgE antibodies aren’t detected, then the result is considered negative. 

What’s the difference between IgE and IgG testing? 

It is important to know that IgE testing is the only scientifically validated type of food allergy blood testing. It’s the gold standard used by allergy specialists throughout the world.

You may have heard about something called IgG, or immunoglobulin G, testing, or seen it advertised. These tests usually claim that they can evaluate you for over 100+ food allergies and sensitivities from a finger-prick blood sample. The problem is, IgG testing has not been proven to be scientifically valid. It’s not accepted by the mainstream scientific or medical communities as a method of diagnosing food allergy, sensitivity, or intolerance. 

IgG tests are often appealing because they appear to provide a large amount of information and often come beautifully packaged in a seemingly legitimate way, with a nice brochure and explanation of what your results mean. They also tend to be very expensive and are not covered by medical insurance.

I was actually part of some early trials on IgG testing decades ago, and what we found was that the rates of false negatives and false positives were too high to be considered reliable. Since then, other research has confirmed that IgG testing is unreliable. We believe that if you eat a lot of a given food, your IgG levels may go up for that food. So if you frequently eat wheat, you are more likely to get a positive IgG result for wheat, but it doesn’t mean you’ll have an adverse reaction to wheat. 

IgG vs IgE: Bottom line

IgG tests tell the user very little for a lot of money. We're seeing far too many people taking the results from an IgG test and thinking that they have life-long allergies. Unfortunately, people go on restrictive diets based on these tests, despite the fact that they’re incredibly unreliable and not recommended by allergy specialists.

The takeaway: Avoid IgG testing. If you want to be tested for food allergies, always opt for IgE testing, and follow up for management with an allergy specialist. 

Who needs food allergy testing? 

Like any other medical test, not everyone needs food allergy testing. For example, if your reaction to a food is obvious, and you know you can no longer eat that food without a reaction, an allergy test may not be needed. Also not all adverse reactions to food are a true allergy. Instead they can be the result of a food intolerance or sensitivity, for which there are no laboratory tests.

For example, you can find out on your own if an adverse reaction to dairy products is a true dairy allergy or simply a lactose intolerance. You can use lactase supplements such as Lactaid as an initial test, and if this solves the problem, you have lactose intolerance. You can continue eating dairy and avoid reactions by supplementing with Lactaid.

Of course, if you want to try to prove that a reaction is a food allergy, you can. And that can be useful, especially as some foods cross-react with other foods, meaning the proteins in those foods are similar, so they produce a similar allergic reaction. But it doesn’t change the treatment, which is to stop eating the symptom-causing food(s), in most cases.

Some people really do need allergy testing, though. If you have an anaphylactic reaction to a food, which is when a food sends your body into shock, I believe it’s important to have an IgE blood allergy test and consult with an allergy specialist for appropriate management.

More often, I use food allergy testing when people come in with what I call ‘‘hidden food allergies.” These are cases when a person is experiencing symptoms that could be related to food allergy, but it’s not clear which food is causing the problem. 

Symptoms of hidden food allergy to look out for include:

Chronic GI problems

The most common indication that food allergy testing is a good idea is chronic GI symptoms

These can include:

  • Bloating
  • Diarrhea
  • Nausea and sometimes vomiting
  • Digestive discomfort 

Contact with allergens can cause an immune system reaction that releases histamines and other chemicals, resulting in inflammation that produces digestive symptoms. One bout of vomiting or diarrhea doesn’t necessarily indicate that you need food allergy testing, but if you have these symptoms for a long period of time, then testing might be a good idea. 

If someone has severe GI problems, I usually recommend they see a gastroenterologist first to rule out GI conditions like an ulcer, Crohn's disease, or celiac disease. More often, though, my patients have already been seen by a gastroenterologist for milder symptoms that have been categorized as irritable bowel syndrome (IBS). IBS is a sort of catchall diagnosis that often means there’s something wrong, but the doctor can’t find a specific problem. I’ve found that in many of these patients, the real problem turns out to be a food allergy. 

Skin problems

Issues like eczema and recurrent hives, known as urticaria, are often caused by food allergies. Similar to GI symptoms, skin reactions to allergens are often linked to high levels of histamines. Particularly in children, eczema can be a strong sign of food allergy, and may be the start of what allergists call “The Allergic March,” whereby allergies progress with age to evolve into inhalant allergy reactions, causing chronic nasal problems like “hay fever” and even asthma. 

But, as there are also many other potential causes of these skin issues, these should be ruled out by a pediatrician or dermatologist. 

Chronic fatigue

There are many conditions that can cause chronic fatigue, such as autoimmune diseases and thyroid conditions, so it’s important to rule out other causes first with your doctor. If you don’t uncover an obvious cause, food allergy testing might be a good idea. I often include it as one of the many tests I recommend for people with long-term fatigue. A food allergy should be suspected especially if you have other inhalant allergies and other symptoms I’ve mentioned. Other possible allergic symptoms such as hives or GI problems disrupt or prevent you from sleeping, which could exacerbate fatigue. 

Chronic headaches

This issue is less commonly related to food allergies, so it’s not always a reason to test. But if you have chronic headaches and your doctor has already ruled out other causes, food allergy testing may be appropriate. Food allergies don’t directly cause headaches, but chronic headaches may be related to inhalant allergies, and sometimes food allergies are present in these patients, too.  

The art of the food allergy testing process:

Every allergist has slightly different practices when it comes to food allergy testing. That’s because there’s no single agreed-upon process for diagnosing food allergy. If someone has symptoms of a hidden food allergy, I usually start the testing process by doing two things at the same time. 

The two-pronged food allergy approach

First, I’ll have patients take an IgE food allergy blood test. 

I have a standard panel of foods that I test for that includes some of the most common allergens such as wheat, dairy, eggs, soy, peanuts, tree nuts, shellfish, fish, and sesame. Some doctors also use IgE skin prick testing to look for food allergies at this point.

In testing for the most common allergens, you’re going to catch the vast majority of food allergies. If someone tells me they’re concerned about a specific food that’s not on my standard list, I’ll add that into the panel.

At the same time, I have patients keep a detailed food diary for two weeks. 

For this, it’s crucial to be very specific. So you wouldn’t just write down “hamburger” for lunch. You’d write “plain beef patty, lettuce, tomato, ketchup, whole wheat bun.” Every few hours, you also record how you’re feeling, noting any symptoms you’re experiencing such as bloating or hives. 

The advantages of a food diary

It usually takes about two weeks to get the results of the tests, so my patients work on their food diary while we wait. Though it requires some effort, the food diary is actually really important, for two main reasons: 

It helps dispel a major misconception about food allergy testing. 

People often assume that a negative food allergy test means that food is not a cause of their symptoms. Actually, the presence of a negative food allergy test is still important information, as it then suggests a person instead may have food intolerances or sensitivities, which as I have mentioned previously, there is no good laboratory test for. From there, we can use the food diary to help figure out what those foods might be. 

IgE food testing can provide very useful data when interpreted in the context of your medical history and food diary. 

For example, if you had a reaction to shrimp and your food allergy test comes up positive for shrimp, there is a very good chance you’re allergic to it. Nevertheless, no food allergy test is perfect. Though IgE tests are the gold standard, they can still return false positives, and need to be interpreted with the help of an allergy specialist. 

The reason I bring up this second point is because rather than relying only on the test results, I use the food diary in combination with the results to create a blueprint for what we should do next.

What happens after food allergy testing? 

Whether your test result is positive or negative for a food allergy, the next step is to “prove it” through a medically supervised oral food challenge or more commonly, an elimination diet and at-home oral food challenge.

Here’s how I explain the process to my patients:

We have to think like a detective at a crime scene. We may find 20 different sets of fingerprints. Any of them could be from the guilty party, and maybe none of them are. You have to go through each person individually to rule them out as the criminal. 

Confirming a food allergy or intolerance is kind of like that. You may find fingerprints, which are the positive test results, but none of them may be the food that's causing the symptoms. On the other hand, you may have a negative test, which is very common. But that doesn't mean there's no allergy or sensitivity, just as if you don’t find any fingerprints at the crime scene, it doesn't mean there wasn’t a crime. 

Food elimination and challenge

During this phase, we remove and then reintroduce a specific food into a person’s diet to see what type of reaction they have. The food eliminated will be chosen strategically based on the food diary and/or IgE test results. We remove and reintroduce foods one by one, so we can determine which food or foods are the problem, then permanently remove them from their diet.  

The bottom line 

This is how I approach food allergy. As you can see, it ultimately doesn’t matter so much whether you’re dealing with food allergy, intolerance, or sensitivity. What matters most is using certified testing techniques and working with a qualified allergy specialist to take a smart approach to testing, identify the culprit, and then eliminate it from your diet. 

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