The Prick Skin Test

This page will describe the method of skin testing used in our office. We use prick skin testing to demonstrate or rule out an allergic reaction to allergens, such as pollens, grasses, animal dander, or moulds.

Mechanism of a positive skin test: If IgE antibody is present, the application of a small amount of the suspected allergen will produce an positive reaction i.e. a red area, the flare and a raised spot, the wheal, at the test site.

How the procedure is done.

• The skin of the forearm (or back in infants) is cleansed with alcohol and a drop of a an allergen solution is placed on the skin. The drops are placed in parallel rows about an inch apart. The skin is lightly pricked with the tip of a lancet to pick up the superficial layer of the skin (prick test).
• The test sites are examined after about 15 - 20 minutes for the presence of redness (the flare) and a raised hive (wheal).
  
• We generally do not do intradermal tests (except in special circumstances, such as penicillin testing). If there is a valid negative test by the prick method, it is unlikely that testing with the more sensitive intradermal method would detect sensitivity of a level that is clinically important. Moreover there is a significant likelihood of false positive results with the intradermal test.
• Positve skin tests are likely to continue to itch for 30 - 60 minutes after the testing. Highly allergic individuals may experience swelling of the forearm, beginning 1-2 hours after a large positive "immediate" reaction. This is known as a late-phase reaction as described in an important paper by the late  Jerry Dolovich of McMaster Medical School. This late phase reaction is prevented by applying a steroid cream to the site.
• In order to be certain that the skin is capable of displaying an allergic reaction, a positive control with a histamine solution is applied to the forearm. If the "histamine" control does not show a good positive, the interpretation of the results are in question. This can happen for several reasons but primarily because an anti-histamine medication or a medication with anti-histamine-like activity has been taken befor the testing. Antihistamines, tricyclic antidepressants, phenothiazines, and other drugs may block allergic reactions.
• False positive reactions can occur if the skin is "dermographic", that is a condition in which any pressure on the skin can raise a wheal and flare. In order to detect possible dermographism, we apply a "salt-water" negative control which should be negative at all times. If the saline control is positive, dermographism is suspected and tested for. Typically when the patient is dermographic, all allergen skin tests show some positive depending on the amount of pressure applied during the prick testing. In children, dermographism can be inherited but it commonly occurs after a "serum-sickness-like reaction" to an antibiotic, particularly cephalosporins such as Cephaclor. The dermographism can on occasion persist many weeks following the reaction to the antibiotic. With some care, it is not difficult to distinguish between dermographism and true allergic reactions.
• We consider any reaction in which the wheal measures 3 mm or more in one direction as positive. The larger the wheal, the greater the allergic sensitivity. Smaller reactions measuring maximally 2 mm in any direction are considered border-line. Reactions of this size can occur because of the pressure of the testing even in the absence of dermographism. The reproducibility of reactions of that size is poor. However in very young children, just becoming allergic to a particular allergen, the reaction may be a true positive. When they are tested again when they are older, the positive is larger. Nevertheless even infants are capable of showing large wheal and flare reactions to allergens, particularly foods such as milk, egg and peanut (Illustrated below).

Advantages of the Prick Skin test

1. simple, relatively painless, and inexpensive.
2. provides immediate results.
3. several allergens can be tested simultaneously.

Disadvantages

1. False-negative results are possible.
2. Children with severe eczema may lack an uninvolved area of skin that can be used for testing.
3. Children being treated with anti-histamine cannot be tested until they have been off the anti-histamine for a sufficient length of time, up to a few weeks for the newer anti-histamines.