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  • Skin Prick SetAllergies can be described as an exaggerated reaction of the immune system. This is described as a hypersensitivity to specific substances medically. Allergies can occur in different areas of the body, but mainly the respiratory system (e.g. hay fever, asthma), gastro-intestinal tract (e.g. peanut allergies) or the skin (e.g. contact dermatitis).

    Determining the cause of a person's allergy will usually start with a consultation with a specialist taking a clinical history and sometimes completing a questionnaire. Important clues can be gained by determining known triggers such as if there are certain times of the year a person is most affected, certain animals, food types or drug allergies.

    From the answers obtained by the questionnaire and history taking the clinician can then decide whether an allergy skin prick test is necessary and if testing is needed, just what allergies to test for. Sometimes it may be more appropriate to take a blood sample and request a blood test such as radioallergosorbent test (RAST) for things like atopic dermatitis, the disadvantage of this is that there may be a delay of more than a week for the results, whereas with skin prick testing the results will be known that day. There are additional investigations that can be done with blood samples and this are described below.

    If a skin prick test is indicated then the subject may need to come back on another day if they are or have been taking antihistamines as this will suppress their allergic symptoms and inhibit the skin prick results. Depending on the type of antihistamine determines how long the wait is before skin prick testing can be done.

Skin Prick Testing

    Prior to the start of the skin prick test the skin is cleaned, usually with alcohol. The forearm is generally used as a location for the drops to be placed but for children the back can be used. Small drops of allergens are placed on the skin and labelled. Additionally a positive and negative control is tested as well.

    Skin Rash TypeOnce the drops are placed a small lancet is used to prick the skin through the allergen, no blood is drawn but a slight graze is formed that allows the allergen to enter the skin. If there is an allergy, it is caused by specialised immune cells causing a hypersensitive reaction to the allergen and releases chemicals that cause inflammation. The result can be seen on the skin as a red patch on the skin called an erythema or flare or if there is more of a reaction a raised patch of inflammation is also seen called a wheal

    15 minutes after the actual skin pricks the reactions to each allergen of inspected and compared in size to the positive and negative controls. If the reaction is the same size or smaller than the negative control then the person is not allergic to that allergen. If the skin reaction is larger than the negative then the allergic reaction may be scored.

    Skin prick testing is a very simple and quick way to determine allergies. A patient's allergy can be confirmed quickly and the appropriate advice or treatment implemented. After skin prick testing the patient can usually be told what they are allergic to and with that knowledge they may be able to adopt strategies to avoid them in the future, or they can be assured that they are on the correct treatment appropriate for their allergy.

Blood Sample Testing

Blood testing is another common way to measure the potential for an allergy. Blood testing should be seen as complimentary to skin prick testing and not equivalent to it. An advantage of blood testing is that there is no risk that the test will trigger a severe reaction. Even though the likelihood of a severe anaphylaxis reaction is very remote in skin prick testing blood testing is considered the safer option. This is of note to known higher risk subjects with a life-threatening anaphylactic reaction or for those that have other destabilising conditions such as asthma or heart disease..
There are numerous blood testing methods but essentially these tests look for IgE antibodies in your blood that are specific to a certain food or other allergen. The higher the level of IgE, the more likely there is an allergy to that particular allergen.

Radioallergosorbent or RAST testing used to be the commonest blood test for helping to diagnose an allergy by using radioimmunoassay to detect specific IgE antibodies. RAST results are usually scored on a scale of 1 to 6, with 1 being absent or undetectable allergen specific IgE levels and 6 being an extremely high level of IgE. This is based on the IgE antibody concentration (
kUA/l) ranging from <0.35 kUA/l to >100 kUA/l. However, newer allergy blood tests are now available and RAST has generally been replaced with a superior test called ImmunoCAP Specific IgE Blood Test.

ImmunoCAP is now the mainastay of blodd testing and superceded the RAST test. Although similar in principle ImmunoCAP uses an enzyme instead of an isotope to detect specific IgE antibodies. ImmunoCAP is also quicker to perform and has a higher sensitivity than it's predecessor, with it's range from 0.1
kUA/l to >100 kUA/l.

Basophil Activation test is another blood test but is looking at the activation of Basophils, that will release chemical mediators if sensitised to an allergen. When activated the basophil membrane will show specific molecules (CD63 or CD203c) that allows for the identification of activation. This test is relatively  new and somewhat more laborious and so is not widely available. Additionally the number of allergens are not as plentiful as other tests.

Allergen Component Resolved Diagnostic testing is a new methodology, used mainly for food allergy diagnosis but the allergen types available are increasing. Instead of using crude allergen extracts this method uses pure allergen molecules that are acquired from natural allergen sources or produced by recombinant expression of allergen encoding cDNA's. The binding of IgE to these specific proteins can then be assessed. This can then enable the clinician to distinguish between the different phenotypes of cross-reactive proteins such as shrimp allergy with dust mite and cockroach.

There are a number of other 'Allergy Tests' that have never been fully validated, peer reviewed or proved to be safe and effective.

ELISA test. This measures the IgG and IgG4 antibodies but these is no convincing evidence to suggest this test has any diagnostic value.

Leucocytotoxic Test. Also know as Bryan's Test as it was developed by AP Black and further promoted by WT & MP Bryan. This test eluded to the fact that a patients white blood cells would swell if mixed with a sensitive allergen. The swelling was then measured and if the measurement reached a certain threshold a positive response was recorded. This test was deemed ineffective and without scientific basis according to Food and Drug Administration (FDA) in USA and to date no studies have shown any correlation between this test and allergies. The test has since been remarketed under the name of 'Nutron'.

Hair Analysis. Here, the hair is tested for levels of heavy metals and then deficiencies in Zinc, Selenium, Chromium, Magnesium and Manganese. However there is no evidence to support the theory of the relationship between heavy metals and allergy.

There are many other Alternative or Complementary Health methods of assessing allergies and caution must be taken where non-approved or anecdotal evidence is used to justify these methods.

Allergy List Link

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