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Skin Prick Testing
This test measures
specific IgE attached
to cells in the skin
important in
allergies called "mast" cells.
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,
and almost any age group can be tested.
Where there has been a clear anaphylactic (shock) reaction
to a specific allergen ingested then skin testing may
not be appropriate.
These tests can be carried out on all age groups
including babies, although the response will be
considerably smaller than in an adult.
- It is usually carried out on the inner forearm, but if the patient has bad eczema the test can be performed on the back .
- Ideally the allergens to be selected should be 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 each code
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.
With a Positive Reaction to an allergen the skin becomes itchy within a few minutes and then becomes red and swollen with a "weal" in the centre (very much like the reaction to a nettle sting).
The weal has a raised edge, which slowly expands to reach its maximum size in about 15-20 minutes. The size of the weal varies with the average being 3-5 mm in diameter, and it clears for most people within an hour. The size of the weal does not indicate the severity of symptoms. The test tells us that a level of anti-bodies are /or are not present which may be causing your symptoms.
Also included in skin testing
is a negative and positive control:
The negative control is a saline (salt-water) solution,
to which a response is not expected.
If however a patient reacts to a negative control,
then this will indicate that the skin is, for what ever reason,
extremely sensitive and that the results need to be
interpreted with the utmost care.
The positive control solution contains histamine,
to which everyone is expected to react.
Failure to do so could mean that medicines the sufferer
is taking could block the response to the histamine and allergens.
Patients will be asked to avoid taking some anti histamines,
about 6 weeks prior to the test
and some other medicines for about 5-6 days.
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 medication that block the effect of histamine. Also the skin in some elderly people may not be capable of reacting.
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.
If you have difficulty obtaining the information you require please do not hesitate to contact us on 01322 619898