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Ideally every allergy patient should be seen at least once at an NHS allergy clinic. If you are at risk of a severe reaction, this is particularly important. We recommend that you ask your GP to refer you.
For a list of NHS allergy clinics, GPs can visit the website of the British Society for Allergy and Clinical Immunology: www.bsaci.org
Diagnosis of allergy depends on a combination of four factors:
Skin prick testing is the primary tool to confirm an allergy diagnosis, and gives the most reliable results. It is suitable for any age group and is considered safe in the vast majority of cases. Even babies under a year old are tested at some clinics. A tiny prick is made with a lancet through a drop of allergen placed on the skin, usually on the forearm. A positive reaction will be indicated by itching within a few minutes. The site where the allergen was introduced then becomes red and swollen and itchy, with a raised weal in the centre that looks like a nettle sting. The weal enlarges and reaches its maximum size within 15-20 minutes, when the measurements of the weal are recorded. The reaction fades usually within an hour. A negative response usually means the patient is not sensitive to that allergen. But skin prick testing for foods may be unreliable and “false negatives” can occur. A positive response usually means you are sensitive to that allergen. However, you may have a positive skin test but suffer no symptoms when coming into contact with the allergen. In addition, skin tests may remain positive to foods and inhalants after the patient has grown out of allergic symptoms.
Blood tests are also useful in the diagnosis of allergy. These are called specific IgE blood tests. Occasionally they are called RAST tests however RAST tests have been superseded by other more modern testing techniques. Blood tests are measured in micrograms per milliliter (ug/mL) but sometimes the old numerical rating from RAST testing of 0-6 is still used.
Blood tests are not affected by antihistamines, and can be used in people with severe eczema. However they are quite costly and the results are not available immediately. Like skin tests, they are not 100% reliable.
Allergy testing for siblings
Parents of allergic children are often anxious to know whether any other child in the family has become allergic. Experts say that around seven per cent of children with peanut allergy have a brother or sister with peanut allergy. Half of those with a peanut allergy have a brother or sister with another allergy. Some clinics will test siblings of allergic children on request. Others will only test once an allergy is suspected. If you are worried, discuss this with your GP.
The NICE guidelines
The National Institute for Health and Clinical Excellence has introduced guidelines covering “Diagnosis and assessment of food allergy in children and young people in primary care and community settings”. Go to: http://guidance.nice.org.uk/CG116