The Skin Prick Test (SPT) is widely used in many countries around the world, including the United States, United Kingdom, Germany, Australia, France, and several others. It is a standard diagnostic tool for identifying IgE-mediated allergies and is commonly performed by allergists in clinical settings globally.
In India, with the rising prevalence of allergies, allergists in major urban centres are increasingly using SPT.
Skin prick test (SPT) is the most effective diagnostic test to detect IgE mediated type I allergic reactions like allergic rhinitis, atopic asthma, acute urticaria, food allergy etc.,
Procedure:
- One of the most common methods of allergy is the scratch test or skin prick test. The test involves placing a small amount of the suspected allergy causing substance (allergen) on the skin (usually the forearm, upper arm or the back), and then scratching or pricking the skin so that the allergen is introduced under the skin surface. Figure 1
- The location of each allergen can be marked with a pen or by using a test grid on the forearm to properly identify test results (Figure 1). Tests should be applied to the volar aspect of the forearm, at least 2 –3 cm from the wrist and the antecubital fossae. The epithelial layer of the skin should be penetrated without inducing bleeding, which can lead to false-positive results.
- Before testing the allergens, both negative and positive controls are applied. The negative control consists of the solution used to dilute the allergens (such as saline or a glycerol solution) without any allergen. The positive control, typically histamine (10mg/ml) or codeine (9% solution), is used to ensure the skin reacts as expected.
- The skin is observed closely for signs of a reaction (Wheal formation), which usually includes swelling and redness of the site. With this test, several suspected allergens can be tested at the same time and results are usually obtained within about 20 minutes.
Interpretation:
- A wheal of 3mm or greater is taken to indicate the presence of specific IgE to the allergen tested.
- The positive test indicates a raised, red, itchy bump (Wheal).
- The negative test indicates no raised, red, itchy bump (Wheal).
Indications
- Rhinitis/Rhino conjunctivitis/rhinosinusitis/allergic conjunctivitis
- Asthma, Atopic dermatitis
- Food reactions such as those manifested by anaphylaxis, immediate acute urticaria, or acute flare of eczema
- Suspected latex allergy
- Conditions in which specific IgE is considered likely to play a pathogenic role (e.g. selected cases of chronic urticaria if the history suggests an exogenous allergic cause)
- Rare disorders such as allergic bronchopulmonary aspergillosis, eosinophilic oesophagitis or eosinophilic gastroenteritis.
Contraindications/Precautions:
- Persistent severe/unstable asthma
- Pregnancy (because of the small risk of anaphylaxis with hypotension and uterine contractions)
- Babies and infants
- Patient on beta-blockers
REFERENCES
- Review article on the skin prick test –European standards
- Skin prick testing for the diagnosis of allergic disease – A manual for practitioners ASCIA
