Allergy: Skin Testing

An allergy skin test is a test done on the skin to identify an allergen responsible for triggering an allergic reaction. There are different types of skin testing and they include:

Intradermal or intracutaneous skin testing
Prick/Puncture testing

Allergens are of different types and forms as people have different allergens triggering an allergic reaction. Allergens can be food, medications, latex, insect venoms and environmental allergens or otherwise called aeroallergens. These environmental allergens include tree pollen, grass pollen, weed pollen, furry animals and molds which can cause allergic eye symptoms, nasal symptoms and asthma symptoms. There are cases where a skin test cannot be done. For these patients, special blood tests such as RAST and ELISA would be done. They measure the presence of specific types of IgE in the blood. These tests may cost more than skin tests and results are not available immediately. Positive RAST and ELISA tests do not by themselves necessarily make the final diagnosis.

In some rare cases there could be a severe allergic reaction to skin testing. For this reason, skin testing should be completed by an experienced allergist who is well prepared to diagnose and treat a severe allergic reaction.

How you prepare

There is no special preparation to be made before a skin test is done.

How it is performed

Allergy skin testing is a non-invasive and generally well-tolerated test even by small children. A small amount of the suspected allergen is placed on the skin. The skin is gently punctured through the small drop with a sterile puncture device. This method is called the prick/puncture test. It was originally called the scratch test. With venoms, certain drugs such as penicillin and in certain cases with aeroallergens there is a second step of testing which involves injecting the allergen into the skin with a small needle. This is called an intradermal or intracutaneous test. This type of testing should never be done for foods.

A skin test is positive if a bump (wheal) and redness (flare) is formed on the skin. Skin testing is also done with a positive control (histamine) that should always cause a skin reaction, and negative control (saline) that should not cause a reaction.

A Positive test result is when the allergen causes a wheal 3mm greater than the negative control, and if the skin has a response to the histamine as well. It is important to note that individuals who are using anti-histamines cannot undergo skin testing as this blocks the histamine-mediated reaction. A positive skin test also implies that the patient has a type of antibody (IgE) on specialized cells in the skin called mast cells that release histamine to cause symptoms of an allergic reaction. A positive test raises the possibility of a true allergy but is not diagnostic or confirmatory for having a true allergy. The skin test is one component of the information an allergist uses to make an accurate diagnosis but the most important information comes from the patient’s reported symptoms.

If there is no reaction with a wheal and flare, the test is read as negative and being allergic is very unlikely.

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Below is a list of sources referenced in the Health Facts for additional reading:

https://www.mayoclinic.org/diseases-conditions
https://www.medicinenet.com/diseases_and_conditions/alpha_b.htm
https://www.healthline.com/directory/topics?ref=global
https://www.who.int/health-topics
https://www.news-medical.net/
https://www.nhs.uk/

https://www.spineuniverse.com/conditions
https://emedicine.medscape.com/
https://www.medlineplus.gov/
https://www.uclahealth.org
https://www.medicalnewstoday.com

Doctall has collated the information contained within this factsheet from several reputable sources listed here