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The Difference Between Allergy Tests

Allergy testing is essential in the accurate diagnosis of allergies and before performing any allergy test taking a thorough clinical history by is vital as this provides the background and reasoning for requiring the allergy test.  Generally speaking, allergy tests should only be performed when the history is suggestive of allergy as a cause.  Without this, the allergy test results may be difficult to interpret or at worst, completely useless.  There are a number of types of allergy tests and here I explain about the 3 main allergy tests; skin prick testing, IgE allergy blood tests and challenge tests.

The allergy tests most commonly performed in the allergy clinic seek to identify the presence of a particular antibody called IgE.  IgE antibodies have evolved to provide immune protection particularly against parasite infections.  In many types of allergies, the immune system mistakenly makes IgE antibodies that recognise things which are harmless including foods, airborne allergens, medications and insect stings.  The simplest way to identify an IgE response to a particular allergen with allergy testing is by skin prick testing.

Skin prick testing is performed with solutions which are prepared commercially in a standardised fashion.  A drop of each solution is placed on the skin and a tiny skin prick is performed with a sterile lancet through the drop.  This introduces a tiny amount of the allergen solution to the immune cells under the surface of the skin.  If you have IgE antibodies which recognise that allergen, they will trigger the immune cells to activate and release histamine which will result in itching and redness and a small bump.

A positive result indicates the potential to be allergic to that substance although it is important to note that many people can have positive responses and absolutely no symptoms whatsoever, hence why a clinical history is so important.

In addition to using commercially prepared standardised solutions, skin prick testing can be performed with fresh foods.  This involves directly pricking the food and then pricking the skin.  This works surprisingly well for many foods and is particularly suitable where the allergen is very sensitive to being broken down during the manufacture of solutions, for example in fruit allergy.

In some cases, skin prick testing may not be possible.  Reasons can include having taken antihistamines beforehand or a skin condition which makes this difficult.  Also, commercial solutions are not always available for every allergen.  Under those circumstances, a different type of allergy test must be performed and usually a blood test is carried out and sent to the immunology laboratory for the IgE antibodies to be measured.

This takes longer than skin prick testing which provides results before the end of the appointment.

From time to time, skin prick test or IgE blood test results may be inconsistent with the clinical history.  For example, they may be negative although the clinical history is strongly suggestive of allergy.  In this situation, it is sometimes necessary to actually administer the allergen to the patient under controlled medical conditions to see if there is a reaction.  This procedure is called a “challenge test” and may take a few hours.

Depending on the severity of the suspected allergy, this may need to be performed in a special clinic setting where medications can be given in case of allergic reactions.  Challenges are also important where skin prick tests and blood tests have only partial accuracy in predicting allergy.  This is the case with medications like antibiotics for example.  It is also the case for Ibuprofen where skin tests have very little value.

Allergic reactions may also occur through other types of mechanisms.  An example would be an allergic contact dermatitis to skin care products, jewellery or hair dyes.  These types of reactions are not caused by IgE but are triggered by a different part of the immune system.  In these situations, a different type of allergy test, called a patch test, is more useful.  In the UK, this is generally more commonly performed by dermatologists although patch tests are sometimes performed with foods in children by paediatric allergists.

There is in addition a wide range of allergy “testing” performed by practitioners who operate outside the clinical specialty of allergy.  Many of these tests are unvalidated and have limited or no value in assessing triggers for symptoms.  One particular test of note is the IgG test.  IgG antibodies do not cause allergy.  Indeed, IgG antibodies can be part of the normal immune response.  IgG testing to foods in particular is therefore a highly controversial area and is not recommended by the majority of allergists.

If you have concerns of an allergy in either yourself or your child, contact Allergy London on 0203 758 9160 for allergy testing and diagnosis.

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