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Allergy Testing
There are several methods for allergy testing – the skin prick test, patch test and blood tests. For further information on each, click on the tabs below.
Skin prick testing is usually the first test recommended when an allergy is suspected. The advantages are that it is a simple and quick form of testing (providing results within 15-20 minutes).
Patients will be asked to avoid taking antihistamines for approximately 72 hours before their test.
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.
It is usually carried out on the inner forearm, occasionally on the back, and the allergens to be tested are decided by the patient’s history. The arm is marked and a drop of each allergen placed on the skin. The skin is then gently pricked through the drop using the tip of a lancet.
This can feel a little uncomfortable but should not be painful.
If there is a positive reaction to an allergen the skin becomes itchy within a few minutes and then becomes red and swollen with a small “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 minutes.
The weal usually disappears 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/are not present which may be causing your symptoms.
This test is performed in cases of contact dermatitis (Eczema) where allergy is suspected.
Patients are usually tested to a standard battery of allergens that include metals, fragrances, rubber, epoxy etc. Allergens specific to the patients employment e.g. hairdressing, may also be tested.
The allergens are prepared in appropriate concentrations in white soft paraffin and are then spread on to discs made of a special metal. The discs are placed on the skin, usually on the back, and are kept in place by hypoallergenic tape.
The patient is asked to keep the skin dry and the patches are left in place for 48 hours. After 48 hours the discs are removed, the skin is examined and any redness or swellings are noted. The skin is re-examined after a further 48 hours for any remaining local redness or swelling.
Sometimes the doctor may request a blood test to detect allergy. This test, known as RAST (radioallergosorbent), measures the amount of Specific IgE circulating in the blood that the immune system has produced against a suspected allergen.
Results may not be available for 3-4 weeks and are reported in Classes of 0 – 6 which indicate increasing sensitivity to an allergen. Class 0 indicates a negative result while Class 6 indicates an extremely high sensitivity.