Skin Prick Test
As part of evaluation for your child’s allergies a skin prick test (SPT) might be required. This leaflet explains what this test involves and how it is performed.
Why does my child need a SPT?
Skin prick testing (SPT) is an accurate method to test individual substances to see if they cause an allergic reaction in the kid. Hence, this test will help us to identify which substance your child may be allergic to (also called allergen). It can also help us monitor the progress of your child’s allergies especially food allergies. It can be used test for testing food allergies and aeroallergens (allergens in the enviornment).
What substances can be tested?
The commonest substances for which SPT is performed is aeroallergens. These are the allergens which are present in the child’s environment (home, school or outside environment). The common indoor allergens are dust mites, moulds, cockroach, cat and dog dander. The most common outdoor allergens are pollens of trees, weeds and grasses.
We can also perform skin prick tests for a wide range of foods such as milk, egg, soya, nuts (peanuts, cashew nut, walnut, almond, pistachio), fish and sea food. If you think your child may have had allergies to a particular food, you can bring a small piece of it with you to the clinic. We can test that food using a prick-prick method. The food should not be mixed with any other products and packed separately.
How is Skin Allergy Testing done?
Skin testing is usually performed usually on the forearm or sometimes the upper back. A small drop of each allergen is placed on the child’s skin. The skin under the drop of allergen is then scratched with a lancet (as shown in the image).This would let the allergen under the skin surface. The drop of the allergen is then wiped off. Many allergens can be tested simultaneously. The same process is repeated. If the test is performed with the food you have brought, we will prick a piece of the food with a lancet and then scratch your child’s skin with it (‘prick to prick testing’).
After 15 minutes, we would check for the reaction. There are two types of reaction:
- A positive reaction – the skin under the drop of allergen develops a itchy raised swelling called as ‘wheal’. This is surrounded by a red area called as a ‘flare’. This is called as “wheal and flare” reaction. The reaction peaks at 15 to 20 minutes and then fades out. The readings are recorded at this time. A positive reaction means that the child is likely to be allergic to the particular allergen.
- A negative reaction – the skin under the drop of allergen does not develop a significant wheal and flare reaction. A negative reaction means that the child is not likely to be allergic to the particular allergen.
Does the skin prick testing hurt?
Skin tests usually do not hurt young children since we just scratch the upper layer of skin (also called epidermis) which do not have pain fibers. There is no bleeding since there are no blood vessels in the top layer of the skin. It might feel like a pin prick sensation. If the test is positive, the reaction might feel itchy but this settles in a while.
When are the results available?
The results of the SPT are available in 15 min.
What are the risks?
Except to the local reaction described there are usually no problems with the test. The wheal and flare may feel itchy for which some cold compresses can be used or a single dose of anti-histamine can be given. At most times nothing needs to be done. Severe allergic reactions from skin prick testing are extremely rare.