Prick Test – Allergy Skin Test Explained
The prick test is a widely used skin test procedure for diagnosing allergies. Small amounts of allergens are applied to the skin and briefly pricked in to detect immune reactions.
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The prick test is a widely used skin test procedure for diagnosing allergies. Small amounts of allergens are applied to the skin and briefly pricked in to detect immune reactions.
What is the Prick Test?
The prick test is a standardised diagnostic procedure used in allergology to identify allergic reactions to specific substances known as allergens. It is one of the most commonly performed tests for diagnosing type I (immediate-type) allergies, which are mediated by the antibody immunoglobulin E (IgE).
How Does the Prick Test Work?
During the prick test, small drops of various allergen solutions are placed on the inner forearm or the back. A fine lancet is then used to lightly prick the skin through each drop, allowing minimal amounts of the substance to enter the outermost layer of the skin. After approximately 15 to 20 minutes, the skin reactions are assessed.
Step-by-Step Procedure
- Cleansing and marking of the test sites on the forearm
- Application of allergen solutions along with a positive and negative control
- Pricking through each drop with a sterile single-use lancet
- Waiting period of 15 to 20 minutes
- Evaluation of skin reactions (wheal and flare response)
What Can Be Tested?
The prick test can be used to identify a wide range of allergens, including:
- Inhalation allergens: tree and grass pollen, house dust mites, animal dander, mould spores
- Food allergens: nuts, milk, egg, wheat, soy, fish
- Insect venom allergens: bee and wasp venom
- Occupational allergens: such as latex and flour
Interpreting the Results
A positive reaction appears as a wheal -- a raised, reddened swelling at the test site. This occurs because mast cells in the skin of sensitised individuals release histamine and other inflammatory substances. Results are compared against a positive control (usually histamine solution) and a negative control (saline solution). A result is considered positive when the wheal has a diameter of at least 3 mm and is larger than the negative control.
Preparation and Important Notes
Before the prick test, certain medications that may interfere with results should be discontinued. These include antihistamines (typically 3 to 7 days in advance), some antidepressants, and systemic corticosteroids. The treating physician will advise patients on the exact preparation timeline.
Risks and Side Effects
The prick test is generally very safe and well tolerated. Local reactions such as itching and redness at the test sites are common and usually resolve quickly. In rare cases, strongly sensitised individuals may experience a systemic allergic reaction. For this reason, the test is always performed under medical supervision in a setting prepared to manage allergic emergencies.
Limitations of the Prick Test
While the prick test offers high sensitivity and specificity, it does have limitations. A positive result indicates sensitisation to an allergen but does not necessarily confirm a clinically relevant allergy. Results must therefore always be interpreted in the context of the patient's medical history and, if necessary, supplemented by additional tests such as specific IgE blood tests or provocation challenges.
References
- Bousquet J. et al. - Practical guide to skin prick tests in allergy to aeroallergens. Allergy, 67(1):18-24 (2012). PubMed.
- World Allergy Organization (WAO) - Skin test guidelines for diagnosis of IgE-mediated allergy. WAO Journal (2018).
- Heinzerling L. et al. - The skin prick test - European standards. Clinical and Translational Allergy, 3(1):3 (2013). PubMed.
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