Skin Prick Testing
Skin prick testing measures specific IgE attached to cells in the skin. This is probably the most commonly used allergy test and is appropriate for both inhaled and ingested (eaten) allergies.
Skin prick testing is usually the first test recommended when an allergy is suspected. The advantages are that it is a simple, quick (providing results within 15-20 minutes) and inexpensive form of testing.
It can give useful information in all forms of allergy, and is appropriate for inhaled and ingested (eaten) allergies. The test is conducted within a hospital or GP surgery by specially trained nurses or doctors.
The skin prick test introduces such a tiny amount of allergen into the skin that testing is quite safe. These tests can be carried out on all age groups, including babies, although the response will be considerably smaller than in an adult.
- Skin testing is usually carried out on the inner forearm, but if the patient has bad eczema the test can be performed on the back
- The test allergens are selected in accordance with the patient’s history
- As few as 3 or 4 or up to about 25 allergens can be tested
- The arm is coded with a marker pen for the allergens to be tested
- A drop of the allergen (extract) solution is placed by the relevant name or number
- The skin is then pricked through the drop using the tip of a lancet – this can feel a little uncomfortable but should not be painful
- The patient needs to avoid taking anti-histamines and certain other medications for 48 hours before the test
If the test is positive, the skin becomes itchy within a few minutes and then becomes red and swollen with a “wheal” in the centre (very much like the reaction to a nettle sting). The wheal has a raised edge which slowly expands to reach its maximum size in about 15 minutes, clearing for most people within an hour. However, having a reaction is not in itself a signal that someone has an allergy to that substance. The wheal needs to be over a certain size to suggest an allergy, although the size of the wheal is not an indication of how severe an allergic reaction may be.
Two control samples are included to make sure that the test has worked; one of the controls will cause a reaction in all people, and the other should not cause a reaction in anyone. This helps the nurses and doctors ensure the test has been conducted properly.
A negative response to skin prick testing usually indicates that the patient is not sensitive to that allergen. Negative reactions may occur if the patient is taking anti-histamines or medications that block the effect of histamine. For reasons which we do not yet fully understand skin prick testing with food allergens is less reliable than with some other allergens such as dust and pollens, so false negative reactions can occur. Also, in some people, the test may be negative for other reasons, which is why it is important for the results to be interpreted by a healthcare professional experienced in allergy. In some cases, the doctor may request a blood test to help clarify the results.
The person having the test is usually asked to stop taking any anti-histamine medication some days beforehand, since the results will not be reliable if anti-histamines are still in the body when the test is performed. If the patient has not stopped taking anti-histamines before their appointment they may not be able to have the skin prick test.
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Last updated: March 2012