The Rast Allergy Test

The RAST allergy test is a blood test that can be used to detect the levels of antibodies to an allergen circulating in the blood. A small blood sample needs to be taken and sent to an appropriate testing laboratory so all the testing is done by the automatic machinery in the test lab. This is great from a patient's point of view because all we need to do is show up once to get a blood sample taken and then wait for the results.

The RAST allergy test is one of a number of possible allergy tests done by examining a sample of blood.

Allergies are thought of as unpleasant reactions to a trigger such as pollen or a food. Orthodox allergists define allergies as reactions which involve particular antibodies to the pollen or the pizza. There are several different types of antibody and orthodox allergists only label a reaction as an allergy if there are IgE antibodies to the allergen present in the blood. There may be other antibodies but they are ignored. Other allergists who may be called Environmentalists also look at IgE present in the blood but may still call a reaction an allergy even if IgE was absent.

In other words one Doctor may say you have an allergy but another Doctor looking at the same laboratory report and data would say that you do not have that allergy!

What the patent can do is to inform themselves and ask lots of questions.

The word, RAST is an acronym and it stands for radio allergy sorbant test. We know it's an allergy test so let's look at the reasons behind this odd sounding name.

Besides the blood sample the testing laboratory needs samples of the allergens it has to test patients for. That includes food samples and environmental irritants and pollutants too.

An extract of the allergen is put into a testing column in the lab. This is a tube with glass beads in it and the allergen sticks to the beads. A sample of the patient's blood minus the red cells is then allowed to flow into the testing column. The blood serum comes into contact with the allergen coated glass beads.

If the blood contains IgE antibodies they will stick to the allergen. That is the whole purpose of the test. The glass beads are then rinsed clean and the next stage of the test begins.

A radioactive marker or even a coloured marker liquid is then poured into the testing column where it coats the glass beads. If IgE is stuck to the beads the marker molecules will stick to them otherwise the liquid will flow through and away.

Now we have the antibodies tagged with either a colour which can be easily measured or with a radioactive molecule which can be easily measured. Using which ever method we prefer we can find out how much IgE is present and print the results in a laboratory report which goes to the allergist.

This was a pioneering method in its day. It produces what all scientists want - numbers and seems to give an objective numerical answer to whether or not and even how allergic a patient is to a particular allergen. Needless to say the method is not perfect and technological advance has refined the procedure.

Nevertheless it still provides a valid and useful alternative to the classic skin allergy test.