Testing for Food Allergies

all about allergy tests

If you think you might have food allergies, chances are, you’re interested in getting tested for them.

First, it’s important to understand the differences between allergies, sensitivities, and intolerances. If you’re not sure which one (or ones) you are suffering from, read this article.

If it sounds like allergies are your thing, here’s some more information to help you understand which type of testing might be best for you.

What types of tests are there?

Food allergies are diagnosed through skin prick tests or IgE allergy blood tests.

Skin Prick Tests:

Quick Facts:

  • Also sometimes called a scratch test.
  • Typically quick and inexpensive, performed during an office visit.
  • Cannot be on anti-histamines during testing because they will blunt the responses.

How does it work?

Your physician will place liquid solutions on your skin that contain various food antigens, then use a small needle to lightly poke those areas, allowing some of the liquid to seep under the skin and come in contact with your mast cells. (Not sure what mast cells are? Read this article.)

If you are allergic to the food or substance tested, you will develop a wheal in that area (a white bump with a red inflamed ring around it, like a mosquito bite). Generally, the larger the wheal the more likely it is that you are allergic to that substance, and if there is no wheal, you are probably not allergic.

These tests don’t really hurt & results are seen within 30 minutes.

Is it accurate?

Skin prick tests have a roughly 50% false-positive rate (when the test says you are allergic to a substance when you really aren’t), so it is necessary to take the results with a grain of salt.

These false positive occur because when the antigens are pushed under the skin, they are directly exposed to the IgE molecules in their whole form. In contrast, when you eat a food, your digestive system may break down some or all of the food antigens, changing their structure and decreasing their size to the point where your IgE molecules no longer recognize them as a threat. So while your body may react to the full form of the antigen being pricked under the skin, it may NOT react to the same protein after it has been digested & absorbed into the bloodstream when you eat the food.

False positives can also occur when the IgE molecules flag foods in the same “food family” as a food you are actually allergic to. For example, mango & cashews are in the same food family (evergreen tree family). You may only actually be allergic to cashews, but the IgE molecules may pick on the similar structure of the mango protein, and cause mast cell degranulation, even though in real life, when you eat and digest the food, it doesn’t cause a reaction.

Skin-prick tests are more useful for environmental inhalant allergens like pollen or pet dander than they are for food allergens. Additionally, extracts are not available for all food allergens.

Serum Specific IgE Allergy Blood Tests:

The second way to get tested for food allergies is through blood testing. There are different types of blood tests, so speak with your allergist to find the best one for you.

One of the newer tests, ImmunoCAP, offered by Quest Diagnostics, is an improvement upon the former ELISA & RAST techniques, and is most commonly used. ImmunoCAP uses cellulose binding discs, which have better binding capacities for the allergens than the paper & plastic discs used in ELISA & RAST. This means it has fewer false negatives (when the test says you’re not allergic but you actually are).

Perks of doing a blood test:

  • You CAN be on anti-histamines when you get tested, which makes it great for people with severe allergies or skin rashes over large areas of their bodies.
  • Since the blood is tested outside of the body, there is no risk of anaphylaxis.

How does it work?

The test is ordered by your physician or allergist, and the blood is drawn at a local lab. The blood is then shipped to a lab for processing, and the results are sent to your doctor for interpretation.

Once the results are in, you can schedule an appointment with your doctor to go over them.

Is it accurate?

While the blood test can tell you whether you have IgE antibodies to specific substances, it can’t tell you about the severity of your reaction, since the amount of antibody is not directly related to severity of symptoms. In other words, it can tell you whether your body is sensitized to certain foods, but not whether eating them will trigger a full blown allergic reaction. This could mean unnecessary dietary restriction when it is not actually needed.

To help interpret the results with more accuracy, your doctor will likely ask you for a food diary to draw connections between foods consumed & symptoms experienced. This helps avoid unnecessary dietary restriction for foods that do not actually cause symptoms.

Blood tests are slightly more accurate than skin prick test for food allergens (40% false positive rate compared to 60% on skin prick), but are still less than perfect.

For this reason, sometimes the two tests are used together to gather as much information as possible.

Other Methods:

You can also try to deduce which foods you are allergic to without skin-prick or blood testing, under the guidance of a trained physician.

1) Oral Tolerance Test

If the physician feels it is medically appropriate, they may do an oral tolerance challenge with you, in which they feed you increasing amounts of the suspected allergen under medical supervision and watch your reaction. It is CRITICAL to only do this under the direct care of a physician with an epi-pen on hand to counteract any allergic reactions.

2) Elimination Diets

Your doctor may also recommend an elimination diet, in which you stop eating the “Big 8” allergens (milk, eggs, fish, shellfish, tree nuts, peanuts, wheat, and soy) and see if your symptoms improve.

If they do, then you can assume at least 1 of those allergens was triggering your symptoms. Once the symptoms are gone, you reintroduce each allergen one at a time and watch for any reactions. If you have a reaction after reintroducing one of the Big 8 allergens, you know that you are allergic to that food & should stop consuming it. Chances are you aren’t allergic to ALL of the Big 8, so after reintroduction you will probably be able to consume some of those foods again.

While the Big 8 cover 90% of the most common food allergens, there’s still a chance that you are allergic to something else not included on this list, like, avocado, for example. This is where testing can help narrow down the picture.

Looking for an allergist?

You can ask for a referral from your physician or use the “Find an Allergist” search tool from The American Academy of Allergy, Asthma, and Immunology.

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