What is an Allergy Test?
Allergy testing most commonly refers to a test for immediate hypersensitivity (type 1 allergy) – the type of allergy that occurs on exposure of an allergen (eg. pollen) to an IgE antibody. This type of allergic reaction is only possible when the person has already been sensitised to the allergen, and so already has the specific antibody “ready and waiting.” Allergy tests for this type of allergy are called IgE Tests and are available as either a blood IgE test or skin prick test. Allergy tests are usually for selected allergens from food or air (aerosol).
What Allergies can be tested for?
Allergy tests of the immediate allergy type may be divided into two main types: Food & Inhalant (Spread through the air). These tests may be arranged directly your GP. There is a large range of possible allergens that can be tested for – over 100 molecules from over 50 allergens. A test panel is an en bloc test of around 15 of the most common allergens in each of inhalant or food allergens. A panel can be arranged on either a blood test or skin prick test.
Random testing is more likely to give rise to false positive results. It should be emphasised that appropriate tests are best guided by the likely cause.
The specific allergy tests will depend on the clinical symptoms and the history. The following are a guide only:
- Hayfever (seasonal) – grass & tree pollen, weed & moulds.
- Asthma – pet dander, house dust mite, cockroach; consider pollen when the Asthma is seasonal.
- Severe Eczema – consider both food allergies (in kids/infants) & aerosols (inhalants). Foods most commonly implicated are cow’s milk, eggs, wheat, soy & peanuts. Inhalants that may contribute are house dust mite, pets, cockroach & pollen. The link between Eczema & Food allergy is less well defined than other types of allergy. Allergy testing often gives non significant results but still be helpful.
- Hives (Urticaria) – allergy tests may be helpful for recurrent acute urticarial. Easy diagnosis to make when a child develops a hives-type rash soon after eating peanuts, egg, fish, milk, shellfish, wheat or soy, and the IgE test is strongly positive.
- Immediate type of food allergy – but the delayed type is more common and lab testing unhelpful.
- Nuts: Almond, Cashew, Hazelnut, Peanut, Walnut
- Fruit: Banana, Mango
- Seafood: Codfish, Shrimp (prawn)
- Dairy & Eggs: Cow’s milk, Egg White
- Misc: Rice, Sesame Seed, Soybean
What do the results of the Allergy tests mean?
A completely negative blood or skin prick test makes that allergen unlikely to be the culprit – however, a negative test does not completely rule out this cause.
A positive blood or skin prick test means that there are significant amounts of antibody to that allergen. The bigger the result, the greater the chance that there is something important going on. However, it’s possible to have a very high reading to an allergen & for this not to be causing any symptoms.
Interpreting the results is often challenging and sometimes the only way to know for sure what is going on is a challenge test that is most commonly arranged by a specialist allergy clinic.
Patch testing is an indirect way of testing for delayed type hypersensitivity. This is a different type of allergy to the IgE testing and is, as its name suggests, a delayed reaction. Patch Testing is most commonly performed at a dermatology clinic.
A strongly positive result would suggest the need to avoid contact with that allergen. This often involves a close look at the ingredients to check for the allergen in various products the person might be exposed to – from cream, shampoo, & cosmetics through to plastics, cement & industrial chemicals.
Type 1 Allergy testing for immediate allergies (like hives) are sometimes used in Eczema but there are the following issues to consider.
- Many kids with Eczema are sensitised to allergens perhaps because the skin “lets through” allergens that activate the immune system & this shows up on allergy tests.
- Only a few of kids with sensitisation actually have any problem (ie. symptoms) caused by those allergens. This is more so in kids with eczema than those without eczema. So the interpretation of the results is more hit-and-miss in kids with Eczema.
- The way to confirm that the positive test to a food is relevant is a food challenge.
- Allergy testing results are reported in a sliding scale though this is most useful in kids with immediate skin reactions (hives, anaphylaxis and so on) rather than those with delayed reactions such as Eczema.