Over the last couple of weeks, we have discussed the importance of having a complete medical exam for a proper diagnosis of food allergies. One method of diagnosis may be through a skin scratch test (You can read the specifics of an allergy skin test, including the strengths and weaknesses of that method, here.). The next logical step in a comprehensive allergy picture is testing via a blood test.
What is an allergy blood test?
Allergy blood tests detect and measure the amount of allergen antibodies present in your blood. Naturally, when your body deems something as an invader (virus, bacteria, allergen, etc.), it makes antibodies to attack the invader. The antibodies release chemicals into the body to signal that an invader has entered your system. In the case of allergies, the symptoms that you experience are triggered by these antibody chemicals, called Immunoglobin E (IgE). Specific allergens are mixed with your blood sample and viewed for the release of these antibody chemicals.
RAST test is the most common allergy blood test known, however there are other tests, such as ELISA. One source I read said that since the introduction of the Elisa test, Rast is rarely used anymore. While allergy skin tests are the preferred method for doctors because of quick results and lower costs, there are benefits to allergy blood tests as well.
Benefits of Allergy Blood Tests
- Blood can be tested for allergens even if allergy medication is in use.
- A needle draw may be more comfortable than a longer series of skin pricks.
- Patients with severe eczema make skin testing difficult.
- As a means to support or refute skin test results.
- Safer method of testing for patients highly anaphylactic to specific allergens.
- Blood tests give more specific numbers on allergy sensitivity than skin tests. Can be used to watch for trends, particularly of outgrowing certain allergies.
Disadvantages of Allergy Blood Tests
- Cost more than skin tests and some health insurance will not cover blood tests.
- Results can take up to 2 weeks.
- Blood test results can sometimes be overly-sensitive and result in false positives. Oral challenges are useful in this situation.
- There is always the risk that a test is not sensitive enough, which results in false negatives. Again, oral challenges may be necessary to rule out an allergy.
My Son’s Experience with Allergy Blood Tests
As an infant, I requested my son have a blood test after I realized the allergist only tested him for a few food allergies and not all the most common allergies (we no longer use that doctor). She reluctantly agreed. In addition to his peanut, dairy, and egg allergies, his blood tests came back positive for wheat and almond. I believe both of those results were false positive as he quickly “grew out” of them at later skin testing, but I am not certain of this assumption. In the last year, we have resumed blood tests, in addition to skin tests, at the request of his allergist. She wants to see more specific numbers in order to start watching his trends. We saw a reduction in all his IgE numbers this fall from last spring, but they are still positive. His dairy allergy, in particular, is quite high. He will continue to get an annual blood test (and a bi-annual skin test) as long as the doctor deems it necessary.
There are many new allergy panel tests coming onto the market (panels test for 200+ allergies). I do not have a lot of experience with them as we have gone the traditional route in allergy testing. I hope to be able to address these new tests next week.
What is your experience with allergy blood tests? Have you used it solely in allergy diagnosis, as a secondary back-up test to skin testing, or not at all? Please share your experience here as we all learn from each other.