There are three main types of allergy tests that can be taken at your local allergy clinic. An allergy test is important as it offers the person suffering with an allergy a better understanding as to what and when a certain allergen triggers their allergy symptoms. This can lead to a diagnosis of an allergic condition and help guide the person suffering with an allergy to avoid allergy triggers by practising allergen avoidance and, if necessary, make use of the appropriate allergy medication.
In an allergy test, you are exposed to a suspect allergen to see whether or not your body produces an immune response to it.
There are three main types of allergy tests:
Skin prick allergy test
This is the most common of the allergy tests and it has a high accuracy for identifying an allergy. A droplet of purified liquid allergen is placed on the skin and then a tiny disposable needle makes a scratch in the drop, so as to introduce the allergen into the body. The test is usually conducted on the forearm and it does not hurt. A positive reaction means that the skin around the prick becomes itchy and develops a white swelling (known as a wheal) within 20 minutes, fading within the hour. The doctor should ensure accuracy by using a negative control (water) and positive control (histamine, to which the skin will always produce a wheal). Up to 25 allergens can be tested in a single skin prick session. A variation is an Intradermal test, where the allergen is injected just beneath the skin.
Patch allergy test
The allergen is applied to a patch which is then placed on the skin. The patches are left on the skin for 48 hours to pick up any delayed reactions. Patch tests are also used to test for responses to irritants as well as true allergens.
Blood allergy tests
A RAST (radioallergosorbent) test, or a more sensitive and specific version called the CAP-RAST test, measures the amount of Immunoglobulin E (IgE) antibodies produced in response to allergen exposure. The test, which is done in a specialist lab to which a sample of your blood is sent, produces results graded from 0 (negative) to 6 (strongly positive). The presence of IgE is the hallmark of a true allergic response. Irritants cannot be tested for by the RAST test because they are not true allergens. There is also a blood allergy test that measures histamine, rather than IgE, that is used in the diagnosis of certain types of nettle rash (urticaria). Another allergy test, the Cellular Allergen Stimulation Test, (CAST) measures levels of leukotrienes, a type of inflammatory chemical and is used to identify certain food allergens where the response does not involve IgE.
There is also the Allergen Provocation test for airborne allergens which is only performed in a hospital. The doctor introduces the suspect allergen directly into the nose, lung or eye to see if it induces a response.
Allergy tests are a useful tool where your allergy symptoms are severe and the cause is not obvious. But it cannot tell you how severe your response to that allergen will be in a 'real world' situation. The test is only part of the diagnostic process; your doctor should also take a full history of your exposure to the allergen and symptoms. And, of course, the test cannot cure your allergy – it can merely guide you in allergen avoidance and management of symptoms. Very occasionally, an allergy test will provoke a severe allergic response. Seek medical help immediately if you experience symptoms such as swelling of the face, lips or mouth or difficulty in swallowing, rash, shortness of breath, wheezing or lightheadedness.
Ask your GP in the first instance for a referral to an allergy clinic. Not all NHS hospitals have an allergy clinic so you may need to travel/wait for an appointment. You can also get allergy tests at a private allergy clinic, but be sure you are going to an accredited allergy specialist for this (check the British Society for Allergy & Clinical Immunology listings). Tests offered online or by alternative medicine clinics are not proven and are therefore not recommended at the current time. Tests on the NHS are free but you will have to pay at a private clinic.
Patch allergy tests are available for most common allergens, like house dust mite or pet dander. Blood allergy tests can identify as many as 400 different allergens. With developments in protein analysis (allergens are proteins), it is likely that this number could be greatly expanded in the future.