A recent study found that there is a large difference between testing positive for a food allergy, and having one, so we thought it would be helpful to clarify, how do you know if you have a particular allergy to a food?
Let's start by making clear that food allergies are very, very real. There is no question that lots of kids have them and that they can cause lots of trouble.
It turns out that there are essentially three types of tests available to find out if someone has an allergy to a food (or anything else).
To understand how these tests work, it will help to know a few basics about allergies.
An allergy is a reaction that happens when something you are allergic to connects with a very specific antibody, unleashing a general reaction.
To be clear on each of these elements of an allergic reaction:
- The something you are allergic to: This is usually a unique molecule, often a protein, in the food or thing one is allergic to. For example, if someone is allergic to milk, they react to just one protein in the milk. So, the whole reaction will not happen unless that protein is present.
- A specific antibody: Antibodies most distinctive property is that they only bind (connect) to one type of molecule. The body can make trillions of different antibodies, but only chooses to make many thousands. You cannot be classically allergic to something without making an antibody that attaches to that item, and only that item.
- A general reaction: Here is how the allergic reaction actually happens: the thing you are allergic to connects with the specific antibody, once that happens that connection activates the antibody to turn on the allergic reaction. No exposure to what you are allergic to, no reaction. No specific antibody to that item, no reaction.
So you need 4 things to happen to be have an allergic reaction to a food:
1. You have to eat or contact the food.
2. There has to be an antibody to that food not only present in the body, but armed and ready to ignite the reaction once the food comes by.
3. The food molecule and antibody need to connect, that complex needs to activate the allergy reaction, and then the allergic reaction happens.
The Three Tests
So with that in mind we can understand the three test for food allergy:
- Blood Test
- Skin Test
- What Happens if You Eat it Test
This test simply measures how much specific antibody to the food in question is circulating in your blood. This is not the same as measuring how allergic you are, since only antibody linked to particular cells that actually activate the allergic reaction are relevant. That is, one could have enormous quantities of antibody to shrimp, for example, circulating around in your blood, but none actually connected to the cells that cause a reaction. You could do the blood test, find a very high level of antibody, but nothing happens when you eat the shrimp. Or the other way happens too, where there is no or little antibody circulating, but lots on the relevant cells, the blood test is negative, but every time you eat shrimp you have a big reaction. Now, it does turn out that the more antibody to a food circulating, the more likely someone will react when they eat it; and, the less antibody present the less likely someone will react when eating the item, but not always.
This test actually sees if exposure to the food can create a visible allergic reaction. The problem is that it only really tells us if scratching the food molecule into the skin will cause a reaction. Again, it does turn out that the more skin reaction to a food being scratched into the skin, the more likely someone will react when they eat it; and, the less reaction in the skin the less likely someone will react when eating the item, but not always.
The What Happens if you Eat it Test
This is by far the most reliable of all three tests. And for an obvious and good reason- if you eat a lot of peanuts and you experience no reactions, then either you have no antibody to peanut molecules, or the antibodies are not activated to create a reaction, you are not allergic. And if every time you eat peanuts you develop hives, lip swelling, and wheezing, you are most definitely allergic. This test, therefore, trumps all other tests. For that particular meal, there is no error rate, you either react or you do not, that time.
In 2010 Dr. Liu et al (J Allerg Clin Immunol. 2010; 126(4): 798-806) found that about 17% of children in the United States have a positive blood test for allergy, but only 2.5% of children in the United States actually react when they eat any food.
Simply put, a person is nearly 7 time more likely to have a positive blood test for food allergy, than to actually have a food allergy.
What to do?
Given all the above, what's the best way to find out if your child is allergic to any particular food?
Here are some tips on how best to proceed:
- Guage how urgent finding out is. If your child has a very slight symptom that has only occurred once, that is clearly less urgent than if your child has trouble breathing each time something is eaten.
- If the reaction is very mild (and the best guage on severity of reaction is the degree of trouble breathing caused by the reaction), you have the option of trying to see if the symptom goes away if your child stops eating the suspected food. This is half of the "What Happens if You Eat It" test. If you are very confident that the reactions you are seeing are very mild, one can try out the other half of this test, see if any reaction happens when your child eats the suspected food. Of course, if there is no reaction on eating it, and no change in symptoms when not eating, he or she is not allergic. And, if there are allergy symptoms when he or she eats it and those symptoms go away, there is an allergy to that food present.
- If the reaction is not mild, then it becomes prudent to do some of the indirect tests noted above, namely skin and/or blood tests. These tests, as noted, are not 100% predictive of food allergy, in fact the national survey quoted above suggests the blood test will overcall a food allergy diagnosis quite a bit. But if the skin and/or blood tests are negative, it gives you a bit more ease of mind (not proof) that you can do the "What Happens If You Eat It" test, at least with less risk.
- If the reaction is severe, that is, trouble breathing in the chest or mouth/throat, occurs, then the first step needs to be elimination of the suspected food(s) from your child's diet immediately and consultation with an allergist to sort out just what is causing the reaction. Again, skin and blood tests will likely be used in this situation, too, and will again help point the way to a conclusion, but even here they cannot be the final word on what your child is allergic too.
The question of whether your child has a food allergy always will hinge on whether they actually have an allergic reaction when they eat that food.
Skin tests and blood tests measure indicators that one may or may not be allergic, they are not proof of an actual allergy sequence being active in your child.
In the case of blood tests, nearly 7 times as many children test positive as actually react.
This makes the What Happens if I Eat it Test the most reliable way to know if your child has a food allergy to that food. This test is safe when the reactions are very mild.
But, if there is any question of a reaction involving trouble breathing, an allergist should be consulted to sort out what is causing this level of reaction.
As always, we hope this information proves helpful.
To your health,
Dr. Arthur Lavin
Dr. Arthur Lavin
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