Allergy Testing

“If we can make the correct diagnosis, healing can begin.’ Andrew Weil

INTRODUCTION

Allergy testing can be a key step in the path of managing allergic disorders, however, these tests need to be utilised and interpreted correctly to avoid confusion and unnecessary avoidance.

Tests that are commonly used to identify allergies include skin prick testing and specific immunoglobulin E (IgE) blood tests.  These tests work by confirming the presence of IgE antibodies against various environmental and dietary allergens.  IgE antibodies are produced in the blood to specific allergens in a person with allergic tendency.  Not all people who produce these antibodies will experience allergy symptoms on contact with that particular allergen.  This is because the immune system is dynamic and may be able to dampen down an allergic response.  In this scenario we call the presence of the IgE antibodies innocent sensitisation- the antibodies are there but they are not producing any effects. Other individuals, however, will experience symptoms on exposure to the allergen and we then recognise this as a relevant clinical allergy.  An example would be developing a runny nose, sneezing and itching in a person with cat allergy or hives and swellings in a person with peanut allergy.  Identifying the relevant allergen enables one to institute avoidance measures.  This can be enormously beneficial and potentially life-saving if there is a risk for severe reactions.

SKIN PRICK TESTING

Skin prick testing involves placing a droplet of allergen on the skin and making a superficial prick through the skin to allow the allergen to penetrate.  The allergen will then meet the cells of the immune system and should IgE antibodies be present a wheal and flare will form on the skin, rather like a mosquito bite.  Skin prick tests are cost-effective, cause little pain and the results can be read 15 minutes after they are performed- enabling the clinician to develop a management plan during the consultation.  However, they can not be performed whilst taking antihistamines and some other medications.

BLOOD TESTS

The blood test identifies IgE antibodies present in the blood to specific allergens, including food , drug, insect venoms  and environmental allergens.  This testing does not require medications to be stopped and is safe to use in those with severe reactions to trace exposures. However, results usually take a few days to come back and suitable follow up with the patient needs to be arranged. Even more detailed blood tests include component testing in which certain clusters of proteins within an allergen are tested for in order to help interpret the result. More complex tests such as CAST tests can look for types of allergies other than IgE-mediated allergies.

INTERPRETATION OF SKIN PRICK AND BLOOD TESTS

Skin prick testing and specific IgE testing are useful in identifying an allergy to environmental allergens, like mould, house dust mite, pollens, in asthma, allergic rhinitis and to a lesser extent eczema.  They can also be used to support a diagnosis of an immediate- type (IgE mediated) reaction to food.  This kind of food reaction develops within two hours after ingestion and causes a variety of symptoms from hives to anaphylaxis.  In this scenario the results of the test should be interpreted by someone with experience.  This is because innocent sensitisation can result in the test being positive without true allergy being present.  If there is any diagnostic uncertainty an allergist will conduct a food challenge to confirm or disprove true allergy.  There is little  place for performing skin prick testing or specific IgE blood tests to foods that are a regular part of the diet.

ALLERGIES OTHER THAN IGE MEDIATED ALLERGIES

Food allergies do not always involve IgE antibodies, however.  Sometimes other parts of the immune system are involved causing, usually, gastrointestinal symptoms to develop hours to days after the offending food is consumed.  An example would be a baby who develops blood in the stools a few days after ingesting cow’s milk.  These so-called Non-IgE mediated food allergies are more difficult to diagnose as there is no single test that can conclusively prove symptoms are related to a certain food.  Often the diagnosis can only be proven by eliminating the suspected food for a time and then re-introducing it with the return of symptoms.  This should only be done under the supervision of an experienced clinician.

UNSUBSTANTIATED “ALLERGY” TESTS

Unfortunately, there are many tests offered for the diagnosis of allergies that have not been scientifically validated.  These tests often claim to identify food intolerances or sensitivities which are in fact quite different from food allergies, primarily because food intolerances are not caused by the immune system.  A widely available example is food IgG testing.  The promoters of these tests suggest that removal of foods to which an individual has high IgG antibody levels can improve a range of symptoms from eczema to fatigue and abdominal complaints.  This claim has not been substantiated and in fact, certain IgG antibodies are usually associated with food tolerance.

CONCLUSION

Useful allergy testing relies on the correct use of the right test to answer the appropriate question.  Indiscriminate testing can be costly and merely result in confusion or unnecessary dietary or lifestyle restrictions.  Should allergy testing be considered it is advisable to do so in consultation with an experienced clinician