Food Allergy Testing: How It Really Works

Not everyone needs food allergy testing. Do you?

Ivor A. Emanuel, MD

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Highlights

  • Not everyone needs food allergy testing. You may already know which foods to avoid. 
  • Food allergy symptoms include gastrointestinal pain, skin reactions, chronic fatigue and more.
  • IgE blood tests are the gold standard for food allergy testing. IgG tests are not scientifically supported and should be avoided.
  • Keeping a detailed food diary and trying an elimination diet can help with diagnosis.

Testing for food allergies can be incredibly useful. But it's not as straightforward as testing for other types of allergies, like inhalant reactions to mold, dust mites, pollen or animal dander. 

In fact, I'd argue that, while most types of allergy testing involve relatively straightforward science, food allergy testing is just as much art as science. 

In this article, I'll explain what you need to know about food allergy testing, the process I use with my patients, and how home testing can help.

Who needs food allergy testing?

Not everyone needs specific testing for food allergies. For example, if your reaction to a food is obvious, an allergy test may not be required. Also, not all adverse reactions to food are true allergies. Instead, they can be the result of an intolerance or sensitivity — unfortunately, there are no good tests for these.

Think about it this way: you can probably determine on your own if an adverse reaction to dairy products is a true dairy allergy or just lactose intolerance. 

You can use lactase supplements, such as Lactaid, as an initial test, and if taking this over-the-counter product solves the problem, you’ll have your answer. You can continue eating dairy and avoid reactions by supplementing with Lactaid.

Of course, if you want to prove that a reaction is a true allergy, you can. It can be useful to do so, especially since some foods may cross-react with other foods (meaning the proteins in those foods are similar, so they produce a similar allergic reaction¹). Knowing what foods you are allergic to is important in preventing future reactions, and unfortunately the only treatment, in most cases, is to stop eating the symptom-causing food.

Certain people absolutely need allergy testing. If you have an anaphylactic reaction to a food, which is when a food sends your body into shock,² I believe it's important to consult with an allergy specialist for the appropriate allergy testing and management.

More often, I use food allergy testing when people come in with what I call ‘‘hidden food allergies.” These are cases where a person is experiencing symptoms that could be related to a food allergy, but it's not clear what food, if any, may be causing the issue. Below I discuss some of the potential symptoms of a hidden food allergy and the steps you can take to feel better.

Chronic gastrointestinal (GI) problems

Chronic GI symptoms are common indicators of food allergies. These can include the following:

  • Bloating
  • Diarrhea
  • Nausea and sometimes vomiting
  • Abdominal pain or 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 mean you need food allergy testing, but if you have these symptoms for a long period of time, it might be a good idea.

If someone has severe GI problems, I usually recommend that they see a gastroenterologist first to rule out conditions like an ulcer, Crohn's disease or celiac disease

More often, though, my patients have already seen a gastroenterologist, usually for milder symptoms that have been categorized as irritable bowel syndrome (IBS). 

IBS is a sort of catchall diagnosis that often means that although there is something wrong, the doctor can't find a specific problem. I've found that in many of these patients, the real issue turns out to be a food allergy or sensitivity.

Skin reactions

Issues like eczema and recurrent hives, known as urticaria,³ are also often caused by food allergies. Similar to GI symptoms, skin reactions to allergens are often linked to high levels of histamine production.

Eczema can be a strong sign of food allergies in children in particular, which may signal the start of what allergists call “the allergic march.” This is the progression of allergies with age, evolving into inhalant allergy reactions, causing chronic nasal allergies like hay fever and even asthma. But, as there are many other potential causes for these skin issues, children with severe skin issues should also be evaluated 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 these other causes before embarking on allergy testing.

If your physician doesn't uncover an obvious cause, food allergy testing might be useful. I often include this for people with long-term fatigue. 

A food allergy should be suspected if you have other symptoms mentioned here, and especially if you have inhalant allergies. Allergies can disrupt your sleep and exacerbate fatigue.

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 concurrent food allergies are present in these patients.

How is food allergy testing done?

Because there’s no single agreed-upon process for diagnosing food allergies, every allergy specialist has a slightly different approach. If someone has symptoms of a possible hidden food allergy, I usually start the process by having my patients take an immunoglobulin E (IgE) blood test for multiple foods. While waiting for their test results, I also ask them to keep a detailed daily food diary, and to list symptoms they have during that period.

What is IgE testing?

True food allergies involve an immune system reaction, which differentiates them from food intolerances or food sensitivities. 

If you're truly allergic to a food, or to inhalants like pollen, pets and dust mites, your body will produce immunoglobulin E (IgE) antibodies in response to that particular food or inhalant. IgE allergy blood tests measure the amount of IgE antibodies produced in response to specific allergens. If IgE antibodies are detected, the result is considered positive. If no IgE antibodies are detected, then the result is considered negative. 

I have a standard panel of foods that I test for, including most of the most common food allergy triggers, such as wheat, dairy, eggs, soy, peanuts, tree nuts, shellfish, fish and sesame seeds. I will add any other foods that the patient may also suspect to be a trigger. 

Some doctors may use IgE skin prick testing to look for food allergies, but I find IgE blood testing to be more reliable. 

IgE food testing can provide very useful data if it’s interpreted within the context of your medical history, and in conjunction with a food diary.

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 around the world.

You may have heard about immunoglobulin G (or IgG) testing. These tests are usually claimed to evaluate more than 100 food allergies and sensitivities. Unfortunately on multiple scientific studies, IgG testing has not been proven to be valid. It's not accepted by the mainstream scientific or medical communities as a method of diagnosing food allergies, sensitivities or intolerances.

IgG tests are appealing because they appear to provide a large amount of information with a very small blood sample, and the results often come beautifully packaged in a seemingly legitimate way, which can be deceiving. They also tend to be very expensive and are not covered by medical insurance.

IgG tests tell the user very little for a lot of money. We are seeing far too many people taking the results from a food IgG test and thinking that they have food allergies. Some people then go on restrictive diets based on these tests. The takeaway: avoid IgG testing. If you want to be tested for food allergies, always opt for IgE testing with an allergy specialist.

What is a food diary?

A food diary is a detailed record of everything a person eats. 

For this record, it's crucial to be very specific. You wouldn't just write down “hamburger” for lunch. You'd write “plain beef patty, lettuce, tomato, ketchup, whole wheat bun.” 

After each meal, you record any symptoms you're experiencing. Though it requires some effort, keeping a food diary is extremely important, as the reactions to foods will be compared to IgE lab results for a full picture of your potential food allergies or sensitivities. Often, especially if the IgE testing is negative, a food diary is the only way to determine any food triggers.

What happens after testing?

People often assume that a negative food allergy test means that item is not the cause of their symptoms. But, while the presence of a negative test is important information, a person may still have food intolerances or sensitivities. This is why cross-referencing the IgE lab results with the food diary can help figure out what's going on.

If you test positive for an allergy, are you definitely allergic to it?

No food allergy test is perfect. IgE food tests can still return false positives and negatives, and results need to be interpreted with the help of an allergy specialist.

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.

This is 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, but 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 IgE test results, but none of them may actually be the food that's causing the symptoms. On the other hand, you may have a negative test, which is common, but that doesn't mean you don’t have a food allergy or sensitivity. (In other words, just because you don’t find fingerprints at the scene doesn’t mean there wasn’t a crime!)

What is an elimination challenge?

During this phase, we remove and then reintroduce specific foods into a person's diet to see what type of reaction they have. The food allergen eliminated will be chosen strategically based on the food diary and IgE test results. We address foods one by one to determine which items are a problem. When we identify the foods that are issues, we permanently remove them from the person’s diet.

Conclusion

This is how I approach food allergy testing. As you can see, it doesn't matter so much whether you're dealing with an allergy, intolerance or sensitivity. What matters most is using certified testing techniques and working with a qualified allergy specialist to take a smart approach.

Updated on
April 12, 2022
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REFERENCES

1. American Academy of Allergy, Asthma & Immunology. Cross-Reactivity Defined. Accessed March 29, 2022.

2. American College of Allergy, Asthma & Immunology. Anaphylaxis. Accessed March 29, 2022.

3. American Academy of Allergy, Asthma & Immunology. Hives Defined. Accessed March 29, 2022.

4. American Academy of Allergy, Asthma & Immunology. Immunoglobulin E (IgE) Defined. Access March 29, 2022.

5. American Academy of Allergy, Asthma & Immunology Adverse Reactions to Foods Committee.

6. Allergy, Asthma & Clinical Immunology. CSACI Position statement on the testing of food-specific IgG. Accessed March 29, 2022.

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