Blood tests measure the amount of IgE antibody circulating in the blood. The test is carried out on a small sample of blood, usually taken from a vein in the arm in the usual way. The sample is then sent to a laboratory and the results are available in 7 to 14 days.
These tests are particularly useful when skin prick testing is impractical, for example, when the patient has extensive eczema. They may also be used for someone who cannot stop taking anti-histamine medications for any period of time, and so would not be suitable for a skin prick test. Blood tests can also be used to confirm skin prick test results, for example, before a food challenge test in hospital.
There are a number of different blood tests for IgE available:
- Total IgE: this measures all IgE in the blood; this is not usually a helpful test, as a number of conditions cause IgE to be high (including eczema) and IgE levels do not necessarily relate to food allergy.
Specific IgE (previously known as a RAST): this measures the amount of IgE to a specific food allergen, (a protein that can cause a reaction), such as peanut or egg. However, the test can give an elevated result without the patient having any symptoms (this is called “sensitisation”; it affects one-third of the population) and the elevated IgE is harmless. When an elevated result is seen in conjunction with symptoms to that allergen we can term the condition an “allergy” and measures should be taken. Therefore, specific IgE testing should only be requested against an allergen against which the patient has complained of symptoms and random testing is not recommended.