Your doctor may decide to perform a blood test to diagnose your allergy. This may be done either by your GP or at an Allergy Centre. It involves taking a small amount of blood from a vein, usually in your arm, using a small needle, and the discomfort is minimal.
Your doctor will have decided that this form of test is better for you than a skin prick test for a number of possible reasons. It may be that skin tests would not be easy to interpret (for example if you have severe eczema), or it might be to avoid the necessity of stopping your antihistamine treatment or other medication that might interfere with a skin test, or because you might have a strong reaction to a skin test or indeed that the substance that you are allergic to is quite rare and so there is no standardized skin test.
One or two weeks after the doctor has taken the blood, you will be contacted with the results. Your doctor may talk about ‘specific IgE’ or ‘RAST; ‘Total IgE’ or ‘Histamine liberation’.
A RAST (radioallergosorbent test) is used to measure the amount of IgE to a specific allergen to which the doctor suspects you have a reaction. If it is raised then it is likely that you are allergic to this allergen. Total IgE is often raised if you suffer from an allergy, particularly eczema, however, a raised level does not necessarily mean that you are allergic and equally a ‘normal level’ does not completely rule out an allergy. Therefore, most doctors do not use this information to determine whether or not you are allergic.
Histamine liberation or histamine release tests might be used if your doctor thinks that you may be allergic to a specific product which most people do not react to. This might be because it is something specific to your work or lifestyle. In this case, your doctor will arrange for an extract to be made from the product (he may ask you to bring a sample in with you) and will then measure the amount of histamine that is produced when the cells in your blood are mixed with this product. The results will be compared with the levels of histamine released by the cells of a different person who is not sensitive to this product.
In all cases, once the blood has been taken, you will be able to go home straight after the blood has been taken and you should not have any problems associated with these tests.