Specific IgE tests
This is the most common blood rest also known as RAST. It is comparatively expensive tests and time consuming. The result is available usually between 7 and 10 days. However, it is very convenient and wide range of allergens can be tested. While SPT is unavailable or not suitable (e.g. could not withheld antihistamine, severe eczema) for the patient, this test can be recommended.
A small sample of blood is collected and send to the laboratory for the test performed. This test can detect and measure the allergen specific IgE antibodies. Signs and symptoms of the patient can suggest the types and numbers of the allergens could be tested for.
Interpretation of RAST: Positive result: means higher level than normal ranges which often indicate an allergy. But for the confirmation of the diagnosis, in addition to clinical history and other tests may be required as the elevated level can also mean ‘sensitised’ rather than allergic.
Negative result: means normal range of IgE specific antibody: if the result is normal it does not completely exclude allergy, however, it indicates that the patient probably do not have an IgE mediated response (true allergy) to that particular allergen. (May be other immunological mechanism involved if symptoms suggest).therefore, an expert interpretation is important.
Total IgE measurement:
This can be achieved from a blood sample, however it is not routinely recommended. The main reason it is more like informative rather diagnostic as high level of IgE cannot prove that someone is allergic as increased level of IgE can also be found in patients with eczema, parasite infestations and some rare medical conditions, plus the normal level does not exclude allergy as well.
Total eosinophil count:
Eosinophil (a white blood cell) may take part in allergic reaction and can also play a role in tissue inflammation. People with allergic rhinitis, asthma eczema often reveal with an increased level of blood eosinophil. However, it is nonspecific marker of allergy testing as other disease may cause eosinophilia such as Hodgkin’s Lymphoma, Pemphigoid, Churg-Strauss syndrome etc.