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The diagnosis of an allergy starts with a careful review of the patient’s symptoms, family history, and personal history, including: the age of onset, seasonal symptoms, and those that appear after exposure to animals, hay, or dust, or that develop in specific environments (e.g. home and work). Other environmental and life style factors such as pollutants, smoking, exercise, alcohol, drugs, and stress may make the symptoms worse and should be taken into consideration. Once the list of possible allergens has been narrowed, specific testing can be done.

Skin prick or scratch tests are usually done in an allergy clinic. Liquid drops of individual allergen extracts are put onto the skin (often the back) and then a small needle is used to prick or scratch through the drop into the skin. A positive test results in a small raised bump about the size of an insect bite. Reactions usually take place within 20 minutes. Skin prick tests are often used to detect airborne allergies such as pollens, dust, and moulds. Because of the potential for a severe reaction, skin prick tests are not usually used for food allergies.

If the skin prick test is negative, there is a 95% chance that you do not have an allergy to that substance. Positives are more problematic; only about 50% of those who have a positive result are actually allergic to that substance. The diagnosis of allergy will depend on whether you symptoms match up with the allergens you test positive for.

You must not have significant eczema or be taking antihistamines or certain antidepressants for several days before the skin prick test. The test must be done by a trained healthcare professional.

Allergen specific IgE testing or RAST (RadioAllergoSorbent Test), as it is known, is a laboratory test that is used to screen for allergen-specific IgE antibodies. It is often recommended when skin prick testing is not possible or when a severe allergic reaction might be anticipated. Allergen-specific IgE antibody testing involves taking a blood sample and checking for each allergen suspected. Allergens may be selected one at a time or by choosing panels such as food panels, which contain the most common adult or child food allergens, and regional weed and grass panels, which contain the most common airborne allergens in the location where the patient lives. Individual selections are very specific, for example: bumble bee versus honeybee, or egg white versus egg yolk. Your doctor will help you select the most appropriate allergens; usually a patient will only be truly allergic to a few substances (4 or less).

If a specific IgE test is negative, then chances are that you are not allergic to that substance, but a positive test must be evaluated alongside your clinical symptoms. You can have a low level and still have a severe reaction to actual exposure to the allergen or an elevated level and never experience a reaction. People who outgrow a food allergy may continue to have positive IgE test result to the food for many years.

Total IgE testing is sometimes done to look for an ongoing allergic process. It is a blood test that detects the presence of IgE protein (including allergy antibodies) but does not identify specific allergens. Conditions besides allergies can also cause it to rise.

Oral food challenges are considered the 'gold standard' for diagnosing food allergies. They require close medical supervision because reactions can be severe (life threatening anaphylaxis). Food challenges involve giving you small amounts of unmarked potential food allergens (either in a capsule or injected directly into your bloodstream) and watching for allergic reactions. Negatives are confirmed with larger meal-sized portions of food.

Elimination is another way to test for food allergies: eliminating all suspected foods from the diet, then reintroducing them one at a time to find out which one(s) are causing the problem.

Patch testing. Delayed hypersensitivity patch tests are the easiest methods of testing for 'delayed' allergies (for example allergies to rubber or nickel). A concentration of the suspected allergen is applied to the skin under a nonabsorbent adhesive patch and left for 48 hours. If burning or itching develops more rapidly, the patch is removed. A positive test consists of redness with some hardening and swelling of the skin, and, sometimes, vesicle (blister-like) formation. Some reactions will not appear until after the patches are removed, so the test sites are also checked at 72 and 96 hours.

Other tests not widely accepted as useful by the British Society for Allergy and Clinical Immunology include:

  • VEGA testing (the measurement of disordered electromagnetic currents in the body)
  • Provocation-neutralization ('Miller technique')
  • Hair analysis
  • Applied kinesiology 
  • Auriculo-cardiac pulse tests
  • Leucocytotoxic tests (‘Nutron’ and ‘ALCAT’ tests)

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