Skin prick tests (also known as scratch tests) are an effective way of diagnosing an allergy, when interpreted by an experienced health care professional, alongside a detailed clinical history. Skin prick tests are widely used in allergy diagnosis and have been available for many years.
What is a skin prick test?
Skin prick tests are a fast and reliable way to diagnose food and/or environmental allergies. The tests are highly sensitive and because of this can yield both false positives and false negatives, hence requiring careful interpretation by a clinician with expertise in said area, in order to obtain the maximal diagnostic value.
How are skin prick tests done?
The test involves placing a small amount of allergen on the forearm, followed by a gentle scratch. The scratch is done using sterile lancets and allows the immune system cells under the skin to come into contact with the allergen.
This may cause a reaction (a small, itchy spot) which can indicate an allergy. It is essential that the test is interpreted by an expert clinician who has taken a relevant medical history to reduce risk of misdiagnosis.
Tests are carried out by either a specially trained nurse or your doctor on the day of the appointment. The results will be explained to you at the appointment itself.
What equipment is used?
An instrument called a lancet is used to do skin prick testing. These are small scalpel type devices with a double-edged blade or needle. These devices make small punctures in the skin and are disposed of afterwards. The lancet is used to scratch through allergen extract on the skin, made out of common substances which can cause allergies.
A ruler is used to measure the size of the resulting wheal to determine likelihood of an allergy.
How do skin prick tests work?
When an allergen substance is scratched into the top layer of skin, it allows the immune system to see it.
If the immune system recognises the substance as an allergen it triggers a tiny release of a chemical called histamine from mast cells that are present in the skin.
This leads to a small wheal and flare reaction, which can be measured. The presence of a reaction doesn’t mean there is definitely an allergy to the substance but the larger the wheal, the more likely there is.
What do I need to do prior to prepare for a skin prick test?
It is important that any medications containing antihistamine are avoided for 5 days prior to the test otherwise they can interfere with results.
Younger children may find this excellent clip from CBeebies ‘Get Well Soon’ a helpful explainer so they know what to expect.
Where are skin prick tests done on the body?
Usually on the forearm or in younger children on the thigh. Sometimes they are done on the back.
How long does it take to get skin prick test results?
Depending on how many tests are required, it should only take 4-5 minutes to perform the test. There is then a 10-15 minute wait before the test can be read.
What happens after my skin prick test results come back?
Once the test has been read and recorded, you will have a chance to discuss the results and what they mean with the doctor at the appointment.
Are skin tests better than blood tests to identify allergies?
Skin tests give similar levels of information an allergy may be present as IgE (previously known as RAST) blood tests. Neither test is more accurate or reliable than the other.
The benefit of a skin prick test is that it can be done at an appointment with immediate results that can then be discussed. It also allows for testing to any allergen at all.
A blood test can look for a very large number of allergens at one time, but the sample needs to be analysed in a laboratory therefore results will take longer.
A blood test can still be taken even if the patient has recently had anti-histamine medication, which can present challenges for skin pick tests.
In some cases where there is diagnostic uncertainty both tests may be required.
Neither a skin prick test or a blood test can tell us the severity of a likely allergy, as this depends on many factors at the time of the reaction itself. Some advances in blood testing (known as component testing) can be helpful to identify higher risks of severe reactions in select cases.
Do skin prick tests have any side effects?
Skin prick tests can very occasionally cause an allergic reaction. This is thought to occur only in around 1 in 10,000 tests. Such reactions are more common if a child is unwell or a large number of tests are done. A skin test should only be done where there are staff and equipment to deal with such a reaction and should not be done if the child is unwell.
How much does skin prick testing cost privately?
Skin prick tests at Allergy London are billed at £200.
What are the benefits of getting skin prick testing done privately?
Having a skin prick test with Allergy London ensures the test is done to the highest standard by experienced professionals and with the important benefit of expert interpretation.
Professor Adam Fox
Date reviewed: 2nd February 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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