Diagnosis of allergy is a process that requires a combination of a careful clinical history and examination together with investigations. These tests can then help to confirm the diagnosis. Typically, the investigations will either be a skin prick test or an allergy blood test which is also known as specific IgE test and in the past was often referred to as a RAST test. Allergy tests used in isolation of a careful clinical history, as a screening test, are usually unhelpful and actually can risk leading to significant over-diagnosis of allergies that are not really there.
It is therefore important that any allergy tests that are done are always interpreted carefully in the clinical context. Skin prick tests and allergy blood tests broadly aim to achieve the same thing which is an estimation of the likelihood of whether an allergy is present, but each test has its advantages and disadvantages. Skin prick tests can be done quickly in the presence of the patient and provide a result just a few minutes later. However, skin prick tests also require the patient not to have been taking any antihistamines for a few days.
On the other hand, blood tests can be done even if the patient has taken antihistamines but require a blood draw which some patients may find unpleasant and the results will not be available sometimes for a few days, which means they cannot be discussed at the same appointment. Sometimes, neither of these tests will provide clear answers and if this is the case, it is often worth considering doing the other test in order to provide further information to make the diagnosis clearer.
What do allergy blood tests identify?
The allergy blood test measures the amount of specific IgE to a particular allergen such as house dust mite, peanut, milk or a whole range of other potential allergens. Whilst the tests must always be interpreted together with the clinical history, generally speaking the higher the level of IgE to a specific substance, the more likely it is that there is a genuine allergy. Broadly speaking, skin prick tests and blood tests are equally accurate at detecting allergies but there are specific circumstances where the blood tests do have advantages, for example, in the case of component testing (see below), but likewise there are scenarios where skin testing can be more useful because testing can be done direct to certain allergens where there may not be a blood test available – this is particularly the case with more unusual allergies.
Types of allergy blood tests?
There are different types of blood tests available for allergy testing, as follows:
Specific IgE Test: This is the most common type of allergy blood test which is offered routinely by most hospitals and GP surgeries, although depending how much allergy work they do, it may take a long time for results to come back. These tests generally do not require large volumes of blood (usually 5-10ml) although the more tests, the more blood will be needed. How quickly the results are available will depend on the turnaround time of the lab they are sent to but is usually a few days. Specific IgE tests should not be confused with IgG testing which is widely available online but has been shown to have no scientific validity at all with regards to diagnosis of allergies and is not recommended.
Component Testing: This is similar to regular allergy testing in as much as it measures the amount of specific allergic antibody (IgE to a particular allergen) but with component testing these allergens are actually the component proteins that make up a particular food or plant. For example, component testing to peanut will actually return results of a number of different proteins including Ara h 2 which is a particularly sensitive marker of peanut allergy and provides better diagnostic information than a regular peanut IgE test. One of the other key benefits of component testing is that it can help differentiate between allergic reactions that happen because of a genuine allergy to the food from an allergic reaction that is happening as a result of cross-reactivity with something else such as a pollen protein. A common example would be hazelnut allergy which can be caused by genuine allergy to hazelnut in which case there is a risk of a more severe reaction or could be the result of cross-reactivity with birch pollen that would probably also be causing hay fever. If this were the case, then severe reactions are much less likely. A component test will identify which part of the hazelnut is causing the allergic reaction.
Protein Microarrays: This is a further innovation that require only a very small amount of blood but can measure IgE levels relatively reliably to a large number of allergens all in one go. Common examples of these are the ISAC or ALEX test. The advantage of these tests is that they can provide lots of information on a relatively small amount of blood and also significantly more cheaply than running each test individually. However, they cannot be tailored to the specific clinical context and will often return large numbers of results to allergens where no test was required.
At Allergy London, we have access to all of these tests and will decide with you which is most appropriate for the clinical context.
How do allergy blood tests work?
The allergy blood test requires highly specialised precision analytical equipment to measure the amount of allergic antibody (IgE) circulating in the blood against specific allergens. This level generally relates to the likelihood of allergy in as much as the larger amount of antibodies present, the more likely it is that there is a genuine allergy, but this does need to be interpreted together with the clinical history in order to provide meaningful diagnoses. It is important to recognise that IgE blood testing does not give us useful information about the severity of an allergy and a high number on the test does not equate to a likelihood of a severe reaction. That said, component allergy testing can help differentiate between different types of allergies where there may be a risk of severe reaction with one type but not with another.
How do you prepare for your blood test?
No special preparation is required and there is no need to fast or indeed be off antihistamines in the way that you do need to be for a skin prick test. For younger children, numbing cream can be applied to make the process of drawing blood less unpleasant. For the newer protein microarrays, a relatively small amount of blood is required and in the case of the ALEX test, this can sometimes be done at home using a finger prick.
What to expect at your blood test appointment?
We are usually able to do any blood tests that are required at the same visit as your consultation and will only do these if they are clinically indicated. It is important to note that the cost of blood tests is not included in the cost of your consultation and is an additional charge. This charge is not made by Allergy London but either by the facility or the laboratory that is taking it. This is a third-party relationship and therefore Allergy London is not able to negotiate discounts or get involved in any payment disputes. It is therefore extremely important that you clarify with the facility where the blood is being taken as to how this is going to be paid for and let them know of any insurance details.
How long do allergy blood tests take?
Blood tests usually only take a few minutes depending on how easy it is for the phlebotomist to find a vein. If your child prefers to have an anaesthetic cream applied beforehand, then this does need to be on for 45 minutes to one hour which will of course lengthen the time the process takes. We usually receive the results back from the laboratory within 4-5 days, but this can vary from time to time.
Are allergy blood tests accurate?
Allergy blood tests are an extremely useful tool in making a firm diagnosis of allergy but in isolation they have the potential to be misinterpreted leading to both under and over-diagnosis. As a screening tool, they can actually be very unhelpful especially in small children with eczema where there is often a lot of background noise in the test, but if carefully interpreted together with the clinical history, especially by somebody who is experienced in understanding that with different allergens, different levels of IgE can have different meanings, then they are an important part of clinical practice.
Professor Adam Fox
Date reviewed: November 2021
Adam Fox is a Professor of Paediatric Allergy with over 20 years experience in both the NHS and private sector. Professor Fox is Commercial Medical Director at Guy’s & St Thomas’ Hospitals NHS Foundation Trust and Professor of Paediatric Allergy at King’s College London and the founding Director of the KCL Allergy Academy, a postgraduate educational programme, which was a finalist at the BMJ Awards in 2018.
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