Why Not to Get a Blood Test for Allergies

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.

If your provider wants to do blood tests for allergies, make sure it's IgE testing and ask them what you're being tested for and why. Or better yet, see a board-certified allergist.

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's IgE testing and ask them what you're being tested for and why. Or better yet, see a board-certified allergist.

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Mail-in Allergy Testing

By Dr. Nicholas Cline • May 27, 2026

If you spend any time browsing the internet or social media, you’ve likely seen advertisements for mail-in allergy kits. They promise to test for hundreds of food intolerances and environmental sensitivities using just a few strands of your hair or a quick saliva swab. It sounds incredibly convenient, painless, and comprehensive. But if it sounds like a gimmick, it probably is. While these mail-in tests are heavily marketed, they often lack scientific backing and can leave patients feeling more confused than comforted. The Science of Allergy Testing. What Are We Looking For? To understand why hair and saliva tests don't work, we have to look at how the immune system reacts to allergens. When you have a true allergy, your body produces a very specific type of antibody called IgE (Immunoglobulin E) . These antibodies bind to mast cells (allergy cells), which are on high alert to protect you from perceived invaders like pollen, dust mites, or specific food proteins. The fundamental flaw with mail-in hair and saliva kits comes down to basic biology: Wrong Locations: Mast cells and IgE antibodies are not traditionally found in your hair follicles or saliva. Unreliable Results: Because these testing methods are looking in the wrong place, they are highly unstandardized and unvalidated. You are highly likely to get a flood of false positives or false negatives that are impossible to interpret accurately. What the Experts Say: Major medical organizations strongly advise against these methods. The National Institute of Allergy and Infectious Diseases (NIAID) explicitly recommends against using hair analysis for food allergy evaluations due to a lack of diagnostic value. Furthermore, the American Academy of Allergy, Asthma & Immunology (AAAAI) and the American College of Allergy, Asthma & Immunology (ACAAI) have issued strong practice guidelines stating that hair analysis should not be used to evaluate food allergies. The Big Misconception: IgE vs. IgG Many mail-in companies charge anywhere from $150 to upwards of $600 for "sensitivity" panels. When patients produce these multi-page reports, the testing is almost always checking for IgG (Immunoglobulin G) antibodies, not IgE. There is a massive difference between the two: