Skin prick testing for food allergies: What to expect

The most common type of allergy skin test, skin prick testing is used to diagnose multiple allergies, including those associated with specific foods.

Dr Tshegofatso Mabelane, a family allergy specialist at Mediclinic Morningside, explains how the testing works and how to act on the results.

Finding yourself breaking out in hives, battling to breathe or facing digestive problems after eating particular foods? You may have a food allergy, an immune system reaction triggered by eating certain foods. But while you may be experiencing symptoms, how do you know for sure whether you’re, in fact, suffering from food allergies? The first port of call is generally the skin prick test.

What is a skin prick test?

The skin prick test is relatively quick with results available within 15 minutes. The doctor will label your inner forearm with possible allergens, then use a thin needle called a lancet to lightly prick your skin with the different small doses of allergens alongside these labels. A slightly different method involves placing droplets of possible allergens onto your skin and scratching the surface gently with a lancet until the droplets enter your skin. 

“A skin prick test is a way of testing for sensitisation to a substance, meaning the immune system can produce an immunoglobulin E (IgE) antibody to a ‘harmless substance’ in the environment, food or medication,” Dr Mabelane explains. While there are generally small amounts of IgE present in the blood, higher amounts signal a possible allergy but not severity of the allergy.  

“A doctor might recommend a skin prick test if you have an actual reaction to the suspected substance and would like to prove production of IgE,” Dr Mabelane continues. “If there are no reproducible symptoms in response to the tested substance, then this is just sensitisation and not allergy.”

During testing, in addition to the allergen, both positive and negative controls are applied on your skin to compare reactions and ensure the test is reliable. A positive control contains a histamine solution, which causes an itchy, raised spot (called a weal) to develop on your skin shortly after exposure. A negative control contains a saline solution, which shouldn’t cause a weal.

Tests are considered positive if a weal, which looks like a mosquito bite bump, develops on the testing site – where the allergen itself is administered.

Different types of skin allergy tests

While a skin prick test where an allergen is tested on the skin is usually done for food, environmental and medication testing, there are two other types of skin allergy tests, Dr Mabelane explains. These are:

1. Intradermal test: an allergen is injected into the dermis (the middle layer of skin) with a tiny needle. This usually tests for allergies to medication.
2. Patch testing: an allergen on a patch is placed onto the skin for up to four days. This is recommended for contact allergies such as nickel, skin care products, and so on.  

What constitutes an allergy?

“Skin prick tests have a positive predictive value of up to 95%,” Dr Mabelane explains. “That would mean if it were positive, there is sensitisation, but if the patient has symptoms, then it is a true allergy. Positive tests must therefore always go hand in hand with symptoms to be considered an allergy.”

With food, one is tested for reactions to different foods such as eggs, milk or peanuts. “The size of the reaction – in the form of a weal – to the food will determine if the response is positive. It is most likely an allergy if accompanied by symptoms,” says Dr Mabelane.

After a positive skin prick test

With food allergies, a positive skin prick test often requires further testing. “If the reaction is to food, often an oral food challenge is needed to prove what you are allergic to and in what form that food can be tolerated,” Dr Mabelane explains.

“For example, if you are allergic to egg, you may tolerate it in baked form but that can only be proven through an oral food challenge, by an allergist, under medically safe conditions.”