The first allergy blood test was developed in the 1970s. Since specific IgE antibodies are only found in minute quantities in the blood, it posed a great challenge to developing a blood test for allergy. The first generation of allergy test involves binding allergenic extracts to a solid phase and then incubating this with the patient's serum. The binding of IgE was then detected using anti-IgE antibodies labeled with radioactivity. The quantity of the IgE can be determined by detecting the radioactivity. This RAST test involves dangerous radioactive materials and could only be performed in specially equipped laboratories.
The second generation tests involve using non-radioactive methods to detect the anti-IgE antibodies. This can be achieved through an enzyme molecule bound to the anti-IgE antibodies that catalyses a colour change, or the use of a molecule that fluoresces when excited by light of a certain wavelength. The degree of colour change or the strength of the fluorescence can be measured to determine the quantity of bound anti-IgE antibodies. This allows allergy tests to be performed in any clinical laboratory. he most accurate of these tests is the ImmunoCAP assay.
Recent developments in allergy blood tests include the use of molecular allergens. Since a crude allergen extract contains many different proteins, only a few of which can cause an allergic reaction, binding to the irrelevant proteins often result in false positive results. If we only test for protein molecules that have been proven to elicit allergic reactions, the accuracy of the tests would improve. By using genetically engineered allergen molecules, we are now able to identify patients at high risk of severe reactions to certain foods such as peanut. However, these molecules are expensive to produce, and therefore the cost of these molecular tests is high.
Another recent development is the multiplex assay. The older tests rely on the use of a solid phase (kind of like a sponge), which requires a large amount of blood for testing, and also readily binds to non-specific IgE. In general, a total IgE of more than 1000 IU/ml would result in very frequent false positive reactions. This might be the reason why some patients test positive to "everything", which is especially common in patients with atopic dermatitis. By reducing the surface area of the test reaction, one can reduce the amount of non-specific binding. The amount of allergens needed is also greatly reduced, thus lowering the cost of the test. It also allows more allergens to be tested with a given amount of blood sample. The first of these multiplex assays is the ISAC chip microarray, where the size of the reacting area is in terms of micrometers. The latest generation of multiplex assay employs nanotechnology. The size of these nanobeads is 1000-fold smaller than the chip microarray, allowing 172 allergens, half of which are molecular allergens, to be tested with less than 200uL (one-fifth of one ml) of serum. Because the reaction surface is so small, it can accurately test serum with a total IgE of up to 25,000 IU/ml.