Skin prick tests
Skin prick tests are useful and rapid way of detecting allergic sensitisation. They can be used in the clinic to confirm the presence (or absence) of IgE antibodies directed against the test allergen.
How do skin prick tests work?
Skin prick testing introduces small quantities of the test allergen extract into the skin: binding to IgE that recognises the test allergen will lead to release of histamine which rapidly triggers enlargement of blood vessels (so increased blood flow) leading to redness, leaking of fluid from the blood vessel into the skin causing swelling or oedema, and irritation of nerve endings resulting in itching. This is the classical type 1 hypersensitivity reaction and occurs within 15 minutes of the test.
What does a positive skin test mean?
A positive skin prick tests confirms the presence of IgE to the test allergen. It is important to note that the detection of IgE antibodies to an allergen does not necessarily mean that the allergen is causing any symptoms. These tests help with diagnosis, which must include a careful history. Some patients have very sensitive skin and can show reactions that are not due the allergen. Such false positive skin tests are controlled for by also testing with a negative control solution (salt water).
What does a negative skin test mean?
Generally, a negative skin tests excludes the presence of IgE to the test allergen and rules that allergen out as a cause of allergic symptoms. However some patients will not show a reaction on skin prick testing (false negative, for example if they have taken antihistamine tablets or some antidepressant medications.
What happens during the skin testing and does it hurt?
You will be asked to stop any antihistamine medication for 48 hours before the test. Depending on the history your doctor or nurse will select which allergens to test. The test is performed on the inside of the forearm. They will mark numbers or letters to show the site of each allergen skin prick. A small drop of each allergen is then placed on the skin then each drop is pricked into the skin with a sterile sharp point or lancet (this causes a momentary pricking sensation: it is less painful than blood tests and almost always less painful than the patient expects!). Each drop is pricked with a new lancet to avoid transfer of allergen. The drops are then blotted off and the tests read at 15 minutes.
Together with the test allergens it is important to include a negative control (salt water to test for false positive reactions) and a positive control (histamine which everyone should react to). Extracts used for skin testing are carefully made to include standard amounts of allergen proteins.
A positive reaction is detected as a red itchy lump, at least 2mm larger than the negative control site.
Certain skin conditions may make it impossible to perform skin tests (for example widespread eczema).
Is there a limit to what can be tested?
Skin test solutions are available for a wide variety of allergens including dust mites, pollens, moulds, foods, some drugs and latex. However not all allergens can be tested and the doctor may then ask for a blood test or RAST to measure IgE antibodies that react to the allergen of interest.
Skin tests cannot be performed to materials that do not interact with IgE such as some medicines, plastic, water or chemicals. Some centres will perform prick-prick tests: for example to detect IgE to apple the test lancet can first be pricked into the fruit then into the skin. Whilst this can be useful it is not as reliable as using standardised extracts.
As skin tests detect IgE, they do not directly tell us which allergens are causing symptoms. For thisreason, multiple skin tests may be unhelpful, in that they can detect IgE that is not relevent. Thus there is little benefit from "allergy screening" by skin testing or blood testing for IgE to a large panel of allergens.
Are skin prick tests safe?
Using standardised extracts in this controlled manner is extremely safe. These tests should be performed by experienced personnel.
How reliable are skin tests?
Taken together with history and performed in a carefully controlled manner skin tests are very reliable in allergy diagnosis. Rates of false positive and false negative reactions are usually 1% or less though may vary according o the allergen tested. It is important to stress that skin prick tests do not diagnose whether an allergen is causing symptoms.
Who can perform allergy skin prick tests?
Because they are safe skin prick tests can be performed in specialist clinics or primary care. However in the UK they are usually only available at specialist allergy clinics where they will be performed by trained doctors or nurses. They should be used in conjunction with a careful allergy history taken by specialist personnel.
Can my medication affect skin prick test results?
Yes If you have taken antihistamine or some antidepressants these can block the effects of histamine released during the test so causing a false negative test. Steroid inhalers, nasal sprays or tablets do not interfere with skin prick tests. Your nurse or doctor should ask about this but if not please mention any medication you take.
Are there other types of skin tests for allergy?
Patch testing is used to detect skin sensitisation to chemicals, drugs perfumes and metals: this test induces a delayed reaction and is read over 48-72 hours. It is generally performed in dermatology clinics.
Blood tests for allergic sensitisation
In addition to skin prick tests it is possible to perform blood tests to detect allergic antibodies (IgE) that bind to specific allergens. These tests work by coating allergen proteins onto a surface, adding the patient’s blood then detecting IgE antibodies that have bound to the surface. Different systems are used including different detection systems, but a common test is the radioallergosorbent (RAST) test.
What are the advantages and disadvantages of RAST tests?
The advantage of blood tests is that a wider panel of test allergens is generally available and that these tests may be performed if the patient has skin diseases or has taken antihistamines. They are no more sensitive than skin prick tests and the disadvantages are that the result is not available immediately and the additional costs involved..