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Allergy tests

What are they?
Allergies (also known as ‘hypersensitivities’) are overreactions of the immune system to particular substances in some people. Hypersensitivities are grouped into four types. These classifications are based on what parts of the immune system are activated and how long it takes for a reaction to occur.
In type I hypersensitivity, a particular foreign substance (known as an 'allergen') reacts with a specific type of antibody called immunoglobulin E (IgE) – this reaction causes symptoms to appear within minutes. In type IV hypersensitivity, the allergen reacts with a specialized type of cell called a T-lymphocyte. In this case symptoms take hours to days to appear.

Type I hypersensitivities affect mainly the respiratory system (nose, throat, lungs) , the gastrointestinal system (stomach, intestines, bowel) and the skin. They occur most frequently in those with a family history of allergies. The first time a predisposed person is exposed to a potential allergen, they will not have a major reaction; instead, they will create a specific IgE antibody and become 'sensitized'.

If that person is then exposed to the allergen again, the specific IgE identifies the allergen, attaches to it, and triggers the release of chemicals, one of them is histamine, that cause allergic symptoms. These symptoms start wherever the allergen was introduced (for example in the mouth, nose or on the skin).
On the skin, an acute type I allergic reaction can cause a rash, dermatitis, and itching, while in the long term the allergy may cause atopic dermatitis and eczema. In the respiratory tract, the acute allergic reaction causes coughing, nasal congestion, sneezing, throat tightness, and, in the long term asthma. It can also cause red itchy eyes. Acute allergic reactions in the gastrointestinal system start in the mouth with swelling of the tongue and throat, followed by abdominal pain, muscle spasms, vomiting and diarrhoea.
Any severe acute allergic reaction can be potentially  life threatening, causing anaphylaxis, a reaction spread throughout the entire body that can start with agitation, a 'feeling of impending doom' pale skin (due to low blood pressure), and/or loss of consciousness (fainting). Anaphylaxis can be fatal without the rapid administration of an adrenaline or cortisone injection.

Type I allergic reactions can be variable in severity, one time causing a rash, the next time anaphylaxis. Type I allergies can be to just about anything: foods, plants, insect venoms , animal dander, dust mites, mould spores, occupational substances (for example latex), and drugs. There can also be cross-reactions, where someone allergic to grass pollen, for instance, may also react to melons and tomatoes. The most common food-related causes of severe anaphylactic reactions are peanuts, tree nuts, and shellfish.
Type IV delayed hypersensitivity reactions are most often skin reactions. Common examples include reactions to metal and jewellery. They occur when an allergen interacts with specific T lymphocytes. Type IV hypersensitivity is usually a reaction (redness, swelling, hardening of the skin, rash, dermatitis) observed at the exposure site hours to days after exposure.

What is not an allergy?
There are other reactions that can cause allergy-like symptoms but are not caused by an activation of the immune system. They range from toxic reactions that affect anyone who has sufficient exposure, such as food poisoning, to genetic conditions, such as intolerances  (for example, the inability to digest milk sugar due to lack of an enzyme, resulting in lactose intolerance) and sensitivities to things like gluten (in coeliac disease). Symptoms can also be caused by medications such as aspirin and ampicillin, food dyes, MSG (monosodium glutamate – a food flavour additive), other systematic diseases and by some psychological triggers. While these diseases and conditions may need to be investigated by your physician, they are not allergies and will not be identified during allergy testing.

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.

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 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).
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' 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.

Prevention. There is some evidence that children who were breast-fed have fewer type I and type IV hypersensitivities. It is also thought that too restricted and “hygienic” an environment may play a role in increasing allergies. Some studies have shown that infants raised on farms tend to have fewer allergies than those raised in a more allergen-free environment.

Avoidance and Elimination. Once an allergy has developed, the best way to prevent a reaction is to prevent exposure wherever possible. In the case of food, this may mean a lifetime elimination of that substance from the diet and vigilance in watching for hidden ingredients in processed and restaurant food. For example, a spatula that has touched peanut butter cookies before touching chocolate chip cookies may be contaminated enough to provoke a reaction in a peanut-sensitive person.
In the case of insects and animals, avoidance is best. In the case of airborne pollens, such as regional weeds and grasses, limiting time outside can help but may not prevent the problem. Some people try moving to another area to avoid certain local allergens; this may not be effective since people with allergies often develop new allergies to pollens or grasses in the region they move to.

Desensitization: Immunotherapy is sometimes recommended if the allergen cannot be avoided. It includes regular injections of the allergen, given in increasing doses that may “acclimatize” the body to the allergen. The injections decrease the amount of IgE antibodies in the blood and cause the body to make a protective antibody known as IgG. Because it moves across the placental barrier, IgG is important in producing immunity in an infant before birth. Immunotherapy injections can cause side effects, like a rash, and can trigger anaphylaxis. Desensitization is most effective for those with hay fever symptoms and severe insect sting allergies. Many with hay fever may have a significant reduction in their symptoms within 12 months, and it is effective in about two-thirds of those who try it. Patients may continue their injections for 3 years, then consider stopping. Some will have long-term relief; others will see their symptoms come back. Immunotherapy is not recommended for food allergens.
Short-term treatment is used for the relief of symptoms. For example, with respiratory symptoms it may include antihistamines, topical nasal steroids, and decongestants.
In the case of anaphylaxis, adrenaline injections are required. Those who have severe reactions must carry adrenaline with them at all times. Anyone who has a reaction and uses adrenaline should seek medical treatment, as follow-up treatment is often needed.