Blood Testing for Allergies

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Traditional allergy testing is done via skin tests, where a small device often resembling a plastic toothpick is dipped in a well containing an allergen, like ragweed pollen.  The skin is then scratched or pricked and observed to see if the area gets red and raised.  Most allergists still feel this is the preferred method of testing, but in some cases skin tests just wont work.  In these situations, blood tests are a good alternative.

Most people refer to allergy blood tests as “RAST” tests.  The term “RAST” is sort of like kleenex or band-aid.  It started as a specific term, but has come to be generalized for all allergy blood tests.  RAST stands for Radio Allergosorbant Test.  This form of testing is actually seldom done, anymore.  These days, allergy blood tests are done via Enzyme-Linked Immunosorbant Assay, which goes by the much-easier-to-pronounce acronym, ELISA (ee-LYE-zuh).

To understand these, a little background is needed.  ELISA tests measure antibody levels.  Antibodies are proteins your body makes to help fight off infection.  There are 4 main types, IgG, IgA, IgM, and IgE.  The first three are there to help fight off bacteria and viruses.  IgE is the allergy antibody.

Each individual antibody is designed to match up with one specific thing, like a lock and key.  For example, an IgG antibody might bind to a specific part of the wall of a strep bacterium or cold virus.  In allergic people, there are IgE antibodies that bind to ragweed pollen or peanut protein.  ELISA tests measure the amount of such antibodies that are present in  the blood stream.  These are reported as a concentration (kU/L) and as a class or level, arbitrarily class 0-6.

This all brings us to a couple of important points.  First, allergy blood tests should be IgE ELISA, not IgG ELISA.  Serum IgG testing has no role in the evaluation or management of allergies, be they inhalant, food, or otherwise.  I commonly see patients who have been told they have food allergies on the basis of serum IgG tests.  Most often, physicians utilizing such tests are also touting non-standard therapies like yeast elimination, detoxification regimens or unproven expensive supplements.  Avoid these at all costs.

The second point deals with the interpretation of serum allergy tests.  The tests are easy to order- you just draw the blood and check a box.  What’s difficult is how to interpret the information.  Every test in medicine has false positives and false negatives.  With serum allergy tests, the false negative rate is very low, so they are very good at ruling out allergies.  The false positive rate is in the range of 5-10%, which is pretty good. So, if you test to 60 different things and get 5 or 6 low level positives, what does that mean?  Are they real allergies or not?  For Americans used to a black-or-white, yes-or-no, give-it-to-me-straight-Doc world, this ambiguity can be difficult to internalize.  A good allergist should be able to interpret these tests correctly and relay the information in a manner which is easy to understand.

Dr. O

2 Comments

  1. Are there any tests to determine the severity of reaction to a peanut allergy? My son recently tested 2.7 on the RAST and our doctor told us that this was not a severe allergy. He has a 4+ on the skin test. He has been given peanut butter twice and had hives both times. He was also exposed to peanuts through breast milk.

  2. The higher the absolute number on a RAST test, the more likely a person is to be truly allergic. This does not accurately predict the severity of future reactions, though. A recent study showed that in peanut allergic kids who were accidentally exposed, 90% of their reactions were similar to or milder than their initial reaction. All systemic reactions were due to ingestion, not skin exposure or inhalation. So the history regarding your son’s exposure is key here.

    Sorry I was tardy with my reply

    Dr. O

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