Food allergies are a common scapegoat for all kinds of health problems – but not all food allergy tests are accurate. A lot of people today buy allergy tests directly from the manufacturer and get back results showing that they’re allergic to all kinds of things. But most of these tests are based on IgG testing, a method that many different studies have shown to be totally inaccurate.
This isn’t just an academic problem. If you actually have a health problem that needs treatment, it won’t help to start avoiding some random food based on an inaccurate test claiming you’re allergic to it. In fact, it might hurt because it could distract you from finding the real cause of your problem.
In that spirit, here’s a look at which food allergy tests are actually reliable and which ones aren’t.
Food Allergies vs. Intolerances
First of all, it’s important to understand what allergies are and aren’t.
An allergy is a mis-aimed immune response. Instead of attacking a dangerous virus or bacteria, the immune system revs up all the engines and attacks…dust. Or eggs, or ragweed, or shellfish, or some other totally harmless thing. This causes the familiar allergy symptoms: rashes, itchy eyes, throat swelling, wheezing, etc.
Not all bad reactions to food are caused by food allergies. FODMAPs intolerance, lactose intolerance, fructose malabsorption, and other similar problems are serious and legitimate – but they’re not allergies. For example, people with lactose intolerance are lacking the enzyme that would allow them to digest lactose. They might feel just as sick after drinking a glass of milk as someone with a milk allergy, but they have a totally different problem.
One very common test that you can get from independent labs is an IgG test. In theory, an IgG test measures your allergic reaction to a given food by measuring whether your body is making antibodies against it. Remember how an allergic response is basically your immune system going nuts and attacking something totally harmless the way it would attack viruses and bacteria? Antibodies are the “soldiers” in that attack. IgG antibodies are one type of antibodies associated with different foods. The theory is that measuring IgG antibodies to a particular food will measure how much of an allergic “attack” your body is mounting against that food.
That’s fine in theory, but in practice, it doesn’t work. The reason is simple: IgG antibodies are the wrong antibodies to look for. IgG antibodies measure exposure to a given food, not an allergic response to it.
It’s true that your body makes IgG antibodies against the foods you eat. But those IgG antibodies aren’t involved in the food allergy response: they don’t release histamine, they don’t cause anaphylaxis, and (as far as anyone can tell, at least), they don’t cause any other actual allergic responses. They aren’t consistently associated with any chronic food allergy symptoms. Plenty of healthy people with no allergy symptoms test positive for IgG antibodies to all kinds of foods. And some people with life-threatening allergy symptoms test negative on IgG tests for that food.
It’s very common for people to go and get an IgG test and then freak out because the test shows they’re producing IgG antibodies to a huge number of different foods. But this doesn’t mean they’re actually allergic to any of those foods. It just means they’ve been eating those foods. IgG testing can’t tell you what foods you are or aren’t reacting to. There’s no reason to panic based on those tests – there’s no real reason to take them at all, because they’re measuring something totally irrelevant to allergy symptoms.
And if anyone tries to tell you that IgG tests can measure non-allergy intolerances like FODMAP intolerance laugh in their face. An IgG test measures antibodies. Measuring antibodies can’t possibly test for something like FODMAP intolerance, because FODMAP intolerance isn’t caused by the immune system attacking FODMAP carbohydrates. It's a totally different mechanism.
If IgG antibodies don’t cause the allergic response, then what does? IgE antibodies. IgE antibodies are the antibodies actually involved in (most) allergic reactions. So testing for IgE antibodies to a particular food can determine whether a person is mounting an attack on that food….kind of.
There are two main ways to test for IgE antibodies, but both of them have some problems:
- The first test is a skin prick test. In this test, the doctor puts the food on the patient’s skin and scratches the skin underneath. If the patient’s IgE response is sensitized to the food, then the IgE antibodies will set off a reaction that releases histamine and produces a wheal (a white bump in the middle of a circle of red itchy skin).
- The second type of test is a blood test. This is an alternative to a skin test: the doctor draws blood and tests it for evidence of IgE antibodies to specific foods.
Both skin prick tests and blood tests are imprecise. A few people have negative test results but a dangerous and noticeable immune response: they test negative on a skin prick test but if they eat the food, they get itching, wheezing, hives, etc. A lot more people have a false positive result. They have enough IgE antibodies to show up on a test, but when they eat the food they’re supposedly “allergic” to, they don’t have any response (no itching, wheezing, hives, etc.).
For skin pricks, 50-60% of tests give a false positives. That means about half of people who test positive on a skin prick test don’t actually have an allergic reaction to that food.
Also, IgE tests don’t test for all kinds of allergies, because not all food allergies are based on food-specific IgE response. One example is food protein-induced enterocolitis syndrome (that’s a mouthful, so call it FPIES). FPIES is a type of allergic reaction, usually to soy and milk, that doesn’t go through IgE antibodies. Since it doesn’t involve IgE antibodies, it won’t show up on an IgE test. But it’s still a food allergy with potentially dangerous symptoms. People with FPIES typically get vomiting, diarrhea, and exhaustion several hours after eating their trigger food.
IgE tests are better than IgG tests. But they’re definitely not perfect.
Oral Food Tests
The gold standard for diagnosing food allergies is the old-fashioned, un-glamorous method of “eat it and see how you feel.”
The technical name is an oral food challenge. The patient shows up to the doctor’s office for two different visits (this can be done either on two separate days, or in the morning/afternoon of the same day). At one visit, the patient gets a placebo food. At the other visit, the patient gets the allergenic food in disguise. For example, if the doctor wanted to test a child for peanut allergies, she might give the child some hamburger as a placebo food, and some hamburger with a very small amount of peanut butter in it as a test food.
If the patient reacts to the test food but not the placebo food, the allergy is confirmed. Obviously, if there's a very severe food allergy going on, this can be dangerous or even life-threatening. That’s why it’s important to do with a doctor present. It’s not the kind of test you’d want to just do on yourself or your kid at home.
This test is time-consuming but reasonably accurate.
Summing it Up
If you’re worried about food allergies, do yourself a favor and skip IgG testing completely. It isn’t a legitimate way to predict which foods are actually causing your symptoms, and it’s very likely to give you a bunch of false positive results that just make you nervous about eating totally harmless foods.
The attraction of IgG testing is that you don’t have to go to a doctor to get it done. You can buy IgG tests online or over the counter, so it’s more convenient and less expensive. But being convenient and cheap isn’t actually a benefit if the results are totally inaccurate.
IgE testing and oral food challenges are much more reliable, even though they’re not perfect. Yes, it’s kind of a pain to go to the doctor, but IgG testing just isn’t helpful at all – at least your doctor can do tests that actually work!