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.
- Component resolved IgE testing: also known as component resolved diagnosis (CRD) – this is a very recent development in specific IgE testing, whereby the laboratory can detect IgE to specific pieces of an allergen. Early evidence demonstrates that for some food allergens, such as peanut, a positive CRD to a particular part of the peanut protein may be more likely to indicate severe rather than mild allergy. Certain tests are therefore now available in specialist clinics using CRD methods.
- IgE panels / point of care testing: some IgE blood-testing kits have now become available for the patient to use at home or in a pharmacy. In general, these tests are not specific to a particular food and are therefore not easy to interpret; both false positive and false negative results are possible, and the presence of IgE in a test does not necessarily mean that the substance is actually causing the patient’s symptoms. For these reasons, these tests are not recommended.
Blood test results can take a few weeks to come back, and the amount of IgE antibodies measured in the blood needs to be above a particular level before an allergy is suspected or confirmed. The level of IgE antibodies recorded does not indicate how serious the allergy is, merely if the patient is possibly allergic or not. Interpreting the results requires care and experience and should always be done by a trained clinician in conjunction with the clinical history.