The diagnosis of food and environmental allergies can often be made by the "history" — a detailed account of what has led to allergic symptoms in the past. Sometimes the story is not clear and allergy testing needs to be performed. There are two common ways to perform allergy testing: skin prick testing and blood tests.
No. Both methods of allergy testing are not perfect and can give false positive results (suggesting an allergy when there isn't one) or false negative results (missing an allergy that is present). The only way to know for sure if you are allergic to a food is to eat it. Therefore, all allergy testing must be interpreted together with the history of symptoms to avoid unnecessary dietary restrictions and treatment. Testing for allergies without a clinical history of allergic reaction can lead to false positives and is generally not recommended. Your allergist will determine which allergens to test your child for.
No. The results of allergy testing only provide a likelihood of whether an allergy is present or not.
Skin prick testing is usually performed on the forearm. The skin is cleaned and the area marked with the allergens to be tested. A drop of allergen extract is placed on the marked area and the skin is pricked (like a small scratch) with a sterile lancet. A very small amount of the allergen enters the skin and can trigger a local reaction. Sometimes fresh foods are used instead of allergen extracts.
There are also two control pricks:
After 15–20 minutes, the testing area is inspected for a localised reaction. This appears like a small hive or "wheal" (a raised, itchy bump). The size of the wheal is measured and recorded. Interpretation of the result depends on the wheal size, the allergen being tested, and the clinical history.
Skin prick testing can be itchy, especially if lots of allergens are tested and your child reacts to multiple allergens. Local itch and swelling usually resolve within 1–2 hours. An antihistamine can be given after the testing is finished. More prolonged swelling is uncommon and may require further doses of antihistamine. Generalised allergic reactions are very rare.
Allergy blood tests (often called RAST tests) are used to measure the amount of circulating allergy antibody (IgE antibody) to specific allergens or parts of allergens.
Blood tests are used when skin prick testing is not suitable — such as in severe eczema, when skin prick testing is not easily available, when antihistamines cannot be stopped, or when the results of the skin prick testing are not clear.
The process is identical to all other blood tests performed at a blood collection centre (pathology service). Depending on the type of test ordered, the result usually takes 1–2 weeks. No special preparation is required.
Because both false positive and false negative results can occur, interpretation of the result depends on the amount of IgE antibody detected, the allergen tested, and the clinical history.
Further information is available on the ASCIA website: www.allergy.org.au