If you’re wondering whether you have allergies, you can be tested two ways: a skin test and a blood test. Most allergists prefer skin tests – the results are immediately available, they are easier to interpret and may be more clinically relevant. That’s not to say that blood tests are bad. In the hands of someone who knows how to properly select the appropriate tests and how to interpret the data, they are an excellent tool. Unfortunately, some practitioners continue to order 1) unnecessary tests or 2) the wrong type of tests in an effort to diagnose allergies in their patients. To better explain this, a little background is necessary.
The immune system makes proteins called antibodies (Ab) or immunoglobulins (Ig) that are designed to help fight off infections. There are four main types and each has a letter name: IgA, IgG, IgM, and IgE. IgA and IgG each have different sub-types: two for IgA and four for IgG. Each of the different types of Ig has a different job. IgA is found on the lining of the airways where it helps to keep bacteria and viruses at bay. IgG circulates in the blood stream where it helps to fight bacteria and viruses that have slipped past the first lines of defense. It is the most important Ig in the overall defense from infection. IgE is the allergy antibody. It sits on the surface of allergy cells, like mast cells and basophils if you really must know, and when it comes into contact with allergens it causes the allergy cells to explode and release all of their nasty contents that make you sneeze and wheeze and blow your nose.
Igs do their work by binding to things like a bacterial cell wall, a viral capsule, or a grain of ragweed pollen. Ig binding is very selective. Each individual Ig will only bind to a very specific 3-dimensional protein structure, much like a lock and key. However, just like there are master keys that can open several locks, there are Igs that can bind to a few very different things. This is called cross-reactivity and it can often cause confusion when trying to interpret blood tests.
So what does all of this have to do with allergy blood tests? Well, when you are doing blood tests for allergies, you are measuring the amount of IgE floating around in the blood that will bind to a specific allergen: ragweed, cat, peanut, etc. Note that I said the amount of IgE, not IgG. IgG does not play a role in the allergy response, measuring the amount of IgG that binds to molds or foods will not tell you one way or the other if you have an allergy to these things. This goes for IgG sub-types such as IgG4 as well. This is a distressingly common mistake – ordering the wrong type of test. Indeed, the AAAAI saw fit to address this specifically as item number one in their Choosing Wisely initiative.
The second common mistake is indiscriminate testing: ordering unnecessary tests. The big problem with this, apart from wasting money, is that blood tests, and particularly blood tests for foods, have a fair degree of false positive tests. Often this is due to cross-reactivity, where people who are very pollen allergic will have positive blood tests to certain foods but will be able to tolerate those foods just fine. In medical parlance, this is called being sensitized but not allergic. The selection of tests should always flow from the patient history. So, if a patient’s history is not consistent with food allergies, then a provider should never order food tests. The use of food tests ordered as part of a “routine allergy panel” when the patient is being evaluated for non-food related problems, such as asthma or hay fever, is a prime example of indiscriminate testing.
So, in summary, IgE testing is the only meaningful allergy blood test and you should only be tested to things that fit with your history. If your provider wants to do blood tests for allergies, make sure it is IgE testing and ask them what you are being tested for and why. Or, better yet, see a board-certified allergist.