Allergy testing 

A skin prick test involves introducing a tiny amount of the suspect allergen into the skin, usually on the forearm. The test is easy to perform and causes only mild discomfort. Even babies under a year old are tested at some clinics.

Antihistamines must be avoided before the test is carried out as these may interfere with the results. Longer acting antihistamines (for example, loratadine) need five days; shorter acting ones (for example, chlorphenamine) require 48 hours, however, this should be discussed with the allergy specialist.

Suspected causes of allergy (such as foods or pollens) are mixed with liquid to make a solution. During the test, a drop of each solution is placed on the skin. Up to 10 or 12 drops of different solutions may be used in the test and the skin is marked to show which liquid has been placed where. Then the skin beneath each drop is gently pricked with a very thin needle (lancet). This is enough to let a tiny amount of solution past the top layer of the skin.

A positive test result shows you have been ‘sensitised’ to a particular allergen. The site where the allergen was introduced will become red and itchy and there will be a raised bump known as a wheal in the centre that looks like a small nettle sting.  The wheal will reach its maximum size within 15-20 minutes and usually fades within an hour. Being sensitised means your immune system has come into contact with an allergen and committed it to its memory. It does not necessarily mean you will react to the allergen. The results require careful interpretation by the doctor.

Intradermal skin tests can be used to identify or exclude allergy to certain substances such as insect venom or medicines. Antihistamines must be avoided for 72 hours before the test, and you must be in good health. The doctor will ask for your informed consent to proceed with the tests.

During the test a small amount of each substance (0.05ml) is injected directly into the skin, which raises a small blister.  If the test is positive this blister or ‘bleb’ gets bigger and is accompanied by redness and itching, which develops over a period of 20-25 minutes. The reaction will then fade after 1-2 hours. Because they are more painful than skin prick tests, they are used less often for children.

Commercial allergy testing kits, such as hair analysis, kinesiology and VEGA tests, are not recommended by the Anaphylaxis UK. We know of little scientific evidence to support them and are concerned that patients may be misled by results that are in fact false. This view is supported by the National Institute for Health and Care Excellence (NICE). A 2004 study by U.S. researchers, involving a review of published literature also supports this view.

Their report said: “To date, no complementary or alternative diagnostic procedure can be recommended as a meaningful element in the diagnostic work-up of allergic diseases.” As far as we know, nothing has changed that would alter that conclusion.