Blood tests for allergies

Nonibala Ningthoujam *

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Allergy testing can help determine which particular substances or allergens provoke an allergic reaction. The procedure may include skin and blood tests or special diets. A wide variety of substances may cause allergic reactions in some people.

ALLERGY AND ALLERGIC REACTIONS

An allergic reaction occurs when the immune system overreacts to something, often in the environment, that’s harmless to most people. To protect the body from this perceived threat, or allergen, the immune system of an allergic person produces antibodies called Immunoglobin E (IgE).

IgE are mostly found in the lungs, skin, and mucous membranes. They cause the release of certain chemicals, including, histamine, into the bloodstream. It’s these chemicals that bring on many of the allergy symptoms that affect the person’s eyes, nose, throat, lungs, skin, or gastrointestinal tract.

Because IgE antibodies are unique to each allergen (for example, IgE produced in response to pollen differs from IgE produced after a bee sting), checking for specific variants in the blood can help determine if an allergy is present.

The allergen-specific IgE antibody test may also be performed to monitor immunotherapy (desensitisation) or to see if a child has outgrown an allergy. It can only be used in a general way, however, as the level of IgE present does not correlate to the severity of an allergic reaction and someone who has outgrown an allergy may have a positive IgE for many years afterwards.

SOME COMMON ALLERGENS

Common allergens that may be tested for by using the allergen-specific IgE test include:
o Pollen.
o Mould.
o Animal fur.
o Dust mites.
o Certain foods (including milk, eggs, mushrooms, etc.).
o Certain medications (such as penicillin).
o Insect venom (bee or wasp sting).
o Latex (found in certain balloons or hospital gloves).

The doctor may ask for a group of these tests – sometimes called a mini-screen or mini-panel – to look for antibodies against a variety of suspected allergens.

SYMPTOMS ASSOCIATED WITH ALLERGY

Symptoms associated with allergies can include:
o Skin rashes, such as eczema or hives.
o Swelling.
o Sneezing and running nose – or allergic rhinitis (hay fever).
o Teary, red, itchy eyes – or allergic conjunctivitis.
o Wheeze, cough.
o Nausea and vomiting.
o Anaphylaxis – a severe allergic reaction that causes serious breathing problems. Anaphylaxis can be fatal.

BLOOD TESTS

There are a number of blood tests for IgE available:
o Total IgE: This measures all IgE in the blood. A number of conditions, however, cause an elevated IgE. IgE levels do not necessarily relate to food allery.
o Specific IgE: This measures the amount of IgE to a specific food allergen (a protein that can cause a reaction), such as peanut or egg. However, the test can give an elevated result without the patient having any symptoms. When an elevated result is seen in conjunction with symptoms to that allergen, we can term the condition “allergy” and measures should be taken.
o IgE panels/point-of-care testing: There are panels available for a group of common allergens, e.g. food allergy panel, inhalants panel. Some IgE blood testing kits are now available for the patients to use at home.

WHAT DOES THE RESULT MEAN?

Results of allergy blood testing must be interpreted with care. Even if an IgE test is negative, there is still a small chance that a person does not have an allergy. Similarly, if the specific IgE test is positive, a person may or may not ever have an actual physical allergic reaction when exposed to that substance.

Negative results indicate that a person does not have a “true allergy”, an IgE-mediated response to the specific allergens tested.

An elevated allergen-specific IgE result indicates that the person tested likely 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.

OTHER CONSIDERATIONS

Sometimes a doctor will look at other blood tests for an indirect indication of an ongoing allergic process, including a complete blood count (CBC) and white blood cell differential (specifically, eosinophils and basophils). Increases in these test results may suggest an allergy, but they may also be elevated for other reasons.

PREPARATION

No special preparations are needed for this test.

DO ALLERGIES EVER GO AWAY?

Although children do outgrow some allergies, adults usually do not. Allergies that can cause the worst reactions, such as anaphylaxis, do not usually go away. Avoidance of the allergen and advance preparation for accidental exposure, in the form of medications, such as antihistamines, is the safest course. Immunotherapy can help decrease symptoms for some unavoidable allergies but won’t work for food.