The blood testing is an in vitro (outside the body) testing using a small amount of patient serum tested in a laboratory. The method of testing utilized is known as ELISA (Enzyme Linked Immunosorbent Assay), but is often referred to as RAST, which was the acceptable method first developed in the 1970’s using radioactive isotopes to perform an in vitro allergy test. In the late 1990’s ELISA became the more accepted methodology using enzymes in the testing procedures rather than radioactive isotopes.
This test is looking for the level of circulating IgE antibody to each of the environmental and food allergens included in the test panels. The level of IgE present indicates the degree of allergy to each specific allergen.
A small amount of a patient’s blood serum is dropped onto a microtiter laboratory plate that has been coated with each of the allergens being tested for. Through a series of chemical washes, rinses and incubation periods, a chemical reaction takes place between the serum and the allergens on the plate. This reaction results in a color transformation. Using sophisticated laboratory instruments, this color reaction is measured and compared against individual positive and negative controls that are also run for each patient. The measurement of this reaction gives a numerical scoring indicating the level, if any, of allergy to each substance.
An allergy blood test is often used because:
There is no preparation necessary to take this test. The patient does not have to fast nor do they have to stop using any anti-histamines prior to taking it. This test is looking for circulating, allergen-specific, IgE antibodies which are not effected by whether the patient has fasted or is taking anti-histamines.
A small amount of blood (one to two test tubes) is drawn and spun down and separated into serum. It is the serum portion of the blood that contains the allergen specific IgE antibodies. The test tube of serum, along with an allergy test order form is sent to the allergy testing laboratory the same day it is drawn.
The level of allergic reaction is expressed in classes ranging from 0 to 6, with 0 being the least amount of antibody found and 8 being the highest. As a general rule, the higher the class, the higher the level of circulating antibody.
0 < 0.35 Undetectable
1 0.35 – 0.70 Equivocal
2 0.71 – 3.50 Positive
3 3.51 – 17.50 Positive
4 17.51 – 50.00 Strong Positive
5 50.01 – 100.0 Strong Positive
6 > 100.01 Strong Positive
Because the blood sample must be sent to a lab for testing, it takes many days to get the results.