Testing for allergies Once an allergic reaction is suspected, you need to identify the exact allergen causing it. To do so, you'll probably need specific tests, as Dr Adrian Morris explains.

In this article

At the GP surgery Allergy clinics Skin-prick test

Blood tests Patch test Allergen challenge tests

Other tests Food intolerance tests Controversial tests

Your GP will take a full medical history, including your symptoms, details of when they occur and any other relevant information.

From this medical history, they'll produce a short list of suspected allergens and arrange appropriate tests. Once the allergy and allergens have been identified, they'll be able to offer treatment and advice.

If your GP's unfamiliar with allergy diagnosis, you may be referred to an NHS or independent allergy clinic for further assessment. There are only about 90 NHS allergy clinics in the UK so you may have to join a waiting list. Some clinics specialise in specific diseases, such as asthma or allergies affecting the respiratory system.

A list of NHS and private allergy clinics is available at www.specialistinfo.com.

This is the most common allergy test and, performed correctly, has a high degree of accuracy.

A small needle (lancet) is used to scratch the skin gently through a droplet of fluid containing a known allergen. In most cases, clinics use purified liquid forms of the allergen, but sometimes you may be asked to bring a fresh sample (especially if the suspected allergen is food).

The test is usually done on the forearm, although with young children it may be done on the back so they can't see what's happening. It isn't painful and results are immediately available. Children can be safely skin tested from four months of age.

A positive reaction occurs when the skin around the needle prick becomes itchy with redness and develops a white swelling called a wheal. The wheal reaches its maximum size in about 15 to 20 minutes and the reaction fades within an hour.

Wheal diameter varies from 5mm to 10mm in a positive test - the larger the wheal, the more likely that you are allergic.

Positive (histamine) and negative (water) control tests should also be used to grade skin reactivity.

An allergy can only be confidently diagnosed when your symptoms correlate with a positive test. The test can show how sensitive you are to an allergen, but can't predict how severely you'll react on exposure. These tests are cheap, safe and easy to perform, but can only test a limited number of allergens - up to 25 in one session.

A sample of your blood is sent to a specialist laboratory for a RAST (radioallergosorbent test) or CAP-RAST.

This measures the amount of specific immunoglobulin E antibodies (IgE) to inhalants and foods in your blood. IgE causes histamine to be released when you're exposed to various environmental and food allergens. The test is safer than a skin-prick test, as you're not directly exposed to the allergen.

Results are graded from grade 0 (negative) and grade 1 (weak positive) to grade 6 (strong positive), depending on the level of the allergen's specific IgE antibody in your blood. The higher the grade, the more likely it is you have an allergy to that allergen. More than 400 specific allergens can be tested for in this way.

There's a respiratory allergy screen, which tests for IgE to house dust mites, pet dander, pollens and mould spores, as well as a food allergy screening test for children, which identifies allergy to cow's milk, hen's egg, wheat, codfish, soy and peanut.

The nut screening test can identify allergy to almond, Brazil nut, hazelnut, peanut and coconut, while the seafood screen can detect allergy to various fishes, shrimp and black mussel.

Specific IgE RAST can detect other allergens including some antibiotics, latex rubber, horse hair, bee and wasp venom and practically any allergies linked to raised IgE.

RAST can't test for allergies to preservatives, food colouring and aspirin as these reactions aren't linked to IgE

RAST can't test for allergies to preservatives, food colouring and aspirin as these reactions aren't linked to IgE.

In the past, 'total IgE' was measured as an allergy indicator, but this isn't accurate as it may also be raised in parasite infections and with eczema. Total IgE may be raised in otherwise fit and healthy people with no allergies at all.

Sometimes very low levels of specific IgE are detected, which are usually 'false positive' results. The patient's medical history can help identify these.

This is used to diagnose delayed allergic reactions on the skin, such as those occurring in contact dermatitis. It can test for contact allergies to rubber, nickel, lanolin, hair dyes, cosmetics, perfume, preservatives and skin medications.

Samples of known contact chemical allergens are taped to the skin under special aluminium discs for 48 hours. After a further 24 hours, any residual reddening or blistering of the skin is assessed.

Groups of patch tests are available specifically for hairdressing chemicals and contact allergens that affect the face.

Atopy patch testing (APT) is an adapted form of patch test used to diagnose delayed hypersensitivity to foods in children. Some children show delayed reactions to foods such as cow's milk, egg, wheat and soy, with extensive eczema and oesophageal reflux, diarrhoea and vomiting.

Although less accurate than conventional skin-prick tests, APT may help identify the food allergens causing these delayed reactions.

• Allergen provocation testing - hospital medical staff perform provocation tests by introducing the suspected aero-allergen (pollen, dust mite, animal dander or mould extract) directly into the nose, lung or eye to see if they provoke an allergic reaction. This is then measured.
• Double-blind placebo-controlled food challenge (DBPCFC) - this may be performed in hospital for a suspected food allergy. The offending food is concealed in a capsule or broth and, under careful supervision, given to the patient to see if they react.

In a DBPCFC, neither doctor nor patient is aware which is the real allergen and which is the dummy (placebo), to exclude any psychological influence or bias affecting the result.

This test should only be done in specialist allergy clinics with full resuscitation equipment available. DBPCFC is the most accurate food allergy test but it's time-consuming to perform and can be dangerous in people with severe food allergies.

Sometimes shorter 'open challenges' are performed in hospital, where the patient is fully aware of what they're eating but feel 'safe' taking the suspected food in a supervised environment.

• Cellular allergen stimulation test (CAST) - measures release of inflammatory chemicals called leukotrienes in a blood sample after exposure to specific allergens. Can help to identify allergens such as preservatives (sodium benzoate, sulphites, salicylates), food colourings (tartrazine), aspirin and some medications, where the reactions don't involve IgE. It isn't 100 per cent accurate and isn't readily available in the UK.
• Histamine release (HR) - especially helpful in identifying allergic reactions that don't involve IgE and can also help diagnose idiopathic urticaria. The blood sample has to be sent to a special reference laboratory in Denmark for testing.
• Serum tryptase - histamine and tryptase are released into the bloodstream during allergic reactions. Histamine is rapidly metabolised and so is hard to measure, but tryptase can be measured in the blood for up to six hours after an allergic reaction. It can't identify the allergen, but can confirm an allergic reaction took place.

Lactose/sucrose intolerance

Lactose intolerance results from a deficiency of lactase, an enzyme in the intestine that digests the sugar found in cow's milk (lactose). Sucrose intolerance occurs when children drink excessive amounts of sweetened fruit juices, which overwhelms their sucrose digesting enzymes.

Both can be diagnosed by a hydrogen breath test or by testing for 'reducing sugars' in a (liquid) stool sample.

Gluten intolerance

Gluten, the protein found in wheat, can lead to a damaged intestinal lining owing to a delayed hypersensitivity reaction, otherwise known as coeliac disease. Families predisposed to coeliac disease should be screened for gluten intolerance, as well as people with Down's syndrome, diabetics on insulin and IgA immune deficiency.

The condition can be diagnosed by a blood test measuring IgA antibodies to intestinal wall proteins such as endomysium or tissue transglutaminases (other older tests include IgA or IgG antibodies to gliaden and bowel reticulin).

A positive test should be followed by a small bowel biopsy to confirm coeliac disease.

Some practitioners perform tests that haven't been shown to have an acceptable degree of diagnostic reliability. They shouldn't be relied on for allergy diagnostic purposes as they're of an inferior nature, according to the British Society for Allergy and Clinical Immunology and the Royal College of Pathologists.

Immunoglobulin G (IgG) - this can be measured in a blood sample and will be raised to specific foods we eat on a regular basis. This rise has no relationship to allergy. In fact, IgG may have some protective effect against allergies. The Royal College of Pathologists doesn't recommend this test as it usually implicates harmless foods , such as wheat, cow’s milk and yeast. Avoidance of these foods in adults has no health benefits and can lead to malnutrition in children.

VEGA testing – this involves measuring disordered electromagnetic currents in the body to certain substances. The test substances are kept in glass vials connected to a device while a probe measures 'disordered' readings on the patient's hand. According to a survey in the British Medical Journal, the results are unreliable.

Applied kinesiology – this tests muscle strength in the presence of various allergens held by the patient. A loss of muscle strength in the arm allegedly indicates an allergy or intolerance. The allergy antidote allows muscle strength to return. This test is unreliable and the public should be discouraged from using it, according to the British Society for Allergy and Clinical Immunology.

Hair analysis - a sample is analysed for trace element deficiencies or heavy metal toxicity. It has no allergy diagnostic value.

Provocation-neutralisation testing (Miller technique) and pulse tests - have both been found to be unreliable for diagnosing allergies.

Leucocytotoxic tests - first advocated in 1956, newer versions include Nutron and ALCAT tests, which measure changes in the blood cell size after the introduction of various food allergens (in a test tube). These tests are unreliable for diagnosing allergies or intolerances according to the European Academy for Allergology and Clinical Immunology.

Popular 'diagnoses' - usually advanced by alternative practitioners with no medical allergy training - include leaky gut syndrome, intestinal dysbiosis, chronic candidiasis, multiple chemical sensitivities and histaminosis. None of these disputed conditions has any link to allergic diseases.

This article was last medically reviewed by Dr Adrian Morris in September 2007.
First published in September 1999.

Thanks to Royal Brompton & Harefield NHS Trust for allowing BBC Health to take photos.