Allergy Testing: Skin prick testing versus Blood tests
Two common tests used to aid in the diagnosis of food allergies and environmental allergies are skin prick tests and allergy blood tests. The results help you take steps to manage your child’s allergies. However, it is important that your child has been assessed thoroughly by a specialist before proceeding to testing and the results are interpreted by an allergist to avoid unnecessary dietary exclusions and lifestyle changes.
What is a skin prick test?
Usually performed on the forearm or back. The skin is cleaned, and the area is marked using a skin marker to indicate corresponding allergens. A drop of allergen extract is placed on the marked areas and a sterile lancet is used to make a small prick. This allows for a small amount of purified allergen to enter the skin.
What is your allergist looking for when a skin prick test is performed?
The test looks for a localised reaction which appears like a hive in the area pricked. This measures the release of histamine by specialised allergy cells (mast cells) that have the allergy antibody (IgE) against the individual food/aero allergen bound to their surface
How long does skin prick testing take?
Typically takes 15-20 minutes and it is done as part of your consultation.
How do you prepare your child for skin prick testing?
Avoid any oral antihistamines for at least 3 days prior to your skin prick test as these interfere with the test. Other medications that your child is on could be continued, however please check with your doctor for specific advice.
What are the common side effects of the test?
They can be slightly uncomfortable or itchy but are usually well tolerated. Local itch and swelling normally subside within 1-2 hours. More prolonged or severe swelling is not as common and could be treated with a non-sedating antihistamine, a painkiller tablet or a cool compress. Occasionally, children may feel dizzy or lightheaded and need to lie down. Severe allergic reactions are very rare.
What is an allergy blood test and what does it look at?
Allergy blood test is a test to measure the exact amount of circulating levels of the allergy antibody (IgE antibody) to specific allergens
When are they performed?
Usually performed when skin testing is not easily accessible/available or in conditions like severe generalised eczema which preclude skin prick testing.
What is the process and how long does it take?
It is a blood test, so it would require you to present to a pathology service to get your child’s bloods collected and it can take couple of weeks for the test to be reported.
What do you look for in the blood test report?
Each food/aeroallergen has reference ranges of the levels of IgE where a child is more likely to experience an allergy to the specific allergen. Generally, your report will contain phrases like negative, or moderate, high or very high, these would require interpretation by your allergist.
What is a positive test?
A positive test in the presence of a clinical history of an allergic reaction is suggestive of an IgE mediated allergy. In the absence of clinical history, a positive test could indicate a sensitisation or a false positive test. It is important that this is carefully interpreted by your allergist and a personalised management plan is provided to you.
What is a negative test?
A negative test usually means your child is no longer allergic to the specific food/ aeroallergen. However, your allergist will tailor their advice based on your child’s history.
Which test is better?
In the presence of a clinical history, both the tests perform equally well. However, there is no one size fits all and your allergist will determine the test that would is best suited for your child by considering the advantages and limitation of both types of tests.