Allergen-specific blood test for allergies

Dr Th Dhabali Singh *

Blood test for allergies helps to diagnose allergies and to monitor the effectiveness of immunotherapy treatment. An allergy test is done when one has symptoms such as hives, dermatitis, nasal congestion, red itchy eyes, asthma, or abdominal pain that the doctor suspects may be caused by an allergy.

WHAT IS BEING TESTED?

Immunoglobulin E (IgE) is a class of antibody (immune protein) associated with allergic reactions. It is normally found in very small quantities in the blood. This test measures the amount of allergen-specific IgE in the blood in order to detect an allergy to a particular substance.

IgE is an antibody that functions as a part of the body’s immune system, its defence against “intruders”. When someone with a predisposition to allergies is exposed to a potential allergen such as food, grass, or pollen grains for the first time, that person becomes sensitised.

The person’s body perceives the potential allergen as a foreign substance and produces specific IgE antibody that binds to specialised mass cells in the skin, respiratory system, and gastrointestinal tract, as well as to the basophils (a type of white blood cell) in the bloodstream. With the next exposure, these attachedIgE antibodies recognise the allergen and cause the mast and basophil cells to release histamine and other chemicals resulting in an allergic reaction that begins at the exposure site.

A total IgE test measures the overall number of IgE antibodies in the blood, while a specific IgE test measures for a response to individual allergens.

Each allergen-specific IgE antibody test performed is separate and very specific. Groupings of these tests, such as food panels (vegetarian and non-vegetarian), rhinitis/asthma panel, eczema panel, aspergillosis (mould) panel, etc.

The healthcare provider may also pick and choose selectively from a long list of individual allergens suspected of causing a person’s allergies.

While the traditional method for blood testing was the RAST (radioallergosorbent test), it has been largely replaced with newer IgE-specific immunoassay methods like ImmunoCAP.

HOW IS THE TEST USED?

The allergen-specific IgE antibody test is used to help diagnose an allergy to a specific substance or substances for a person who represents with acute or chronic allergy-like symptoms. This is especially true if symptoms are recurrent and appear to be tied to triggers, such as exposures to particular foods or environments, and if other family members are known to have allergies.

The allergen-specific IgE antibody test may also be performed to monitor immunotherapy or to see if a child had outgrown an allergy.

WHEN IS THE TEST DONE?

One or more allergen-specific antibody tests may be ordered when a person has signs or symptoms that suggest an allergy to one or more substances. Signs and symptoms may include:
o Hives
o Dermatitis
o Eczema
o Red itchy eyes
o Coughing, nasal congestion, sneezing
o Asthma
o Itching and tingling in the mouth
o Trouble breathing
o Abdominal pain or vomiting and diarrhoea

WHO ARE TYPICALLY TESTED FOR IgE SPECIFIC ALLERGY TESTS?

Anyone presenting with nasal or other allergy-like symptoms is a candidate for specific IgE testing. The tests are usually considered for persons with:
o Recurrent or chronic upper respiratory diseases (URD) like rhinitis, sinusitis, allergic-rhinitis, etc.
o Unseasonal allergy-like symptoms
o Seasonal or perennial allergy-like symptoms
o Recurrent otitis media (ear infections)
o Exogenous asthma

WHAT DOES THE RESULT MEAN?

An elevated allergen-specific IgE test indicates that the person tested has an allergy. However, the amount of specific IgE present does not necessarily predict the potential severity of a reaction. A person’s clinical history and additional medically-supervised allergy tests may be necessary to confirm an allergy diagnosis.

Negative results indicate that a person probably does not have a “true allergy”, an IgE-mediated response to the specific allergens tested. However, the results of allergy blood testing ought to be interpreted with care. False negatives and false positives can occur. Even if an IgEtest is negative, there is still a small chance that a person has an allergy.

Similarly, if the test is positive, a person may or may not ever have an actual physical allergic reaction when exposed to that substance.

Sometimes, a doctor may look at other blood tests for an indirect indication of an ongoing allergic process, including a total IgE level, complete blood count (CBC) and white blood cell differentials (specially eosinophils and basophils). Increase in these may suggest an allergy, but they may also be elevated for other reasons.