Skin prick testing

Skin prick testing is the most effective diagnostic test to detect IgE mediated type I allergic reactions like allergic rhinitis, atopic asthma, acute urticaria etc.

Type I hypersensitivity reactions are characterized by wheal and flare reactions to skin testing with common environmental antigens, usually with appropriate clinical history

Skin test reactivity depends on at least three separate factors:

• (1) an intact immune system;
• (2) the presence of IgE sensitized mass cells that release mediators when exposed to antigen;
• (3) and skin that can respond to histamine with the development of inflammatory response including erythema and induration

The advantages of skin prick testing advantages are that it is a simple, quick (providing results within 15-20 minutes) and inexpensive form of testing.

• can give useful information in all forms of allergy, and is appropriate for inhaled and ingested (eaten) allergies

The skin prick test introduces such a tiny amount of allergen into the skin that testing is quite safe

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
• 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

• 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