Allergy Blood Test: The Facts

An Allergy is an immune system response to a substance that is not typically harmful to our body. These substances are called allergens. The immune system overreacts to these allergens and produces a chemical, histamine that causes allergy symptoms, mild to a life-threatening reaction (known as anaphylaxis).

Approx 20% population will suffer from allergy at sometime in their life. 

Symptom: The symptoms of an allergic reaction can vary from mild to severe. 

• Skin rashes

• Itching

• Nasal congestion 

• Sore throat

• Watery or itchy eyes

• Abdominal cramping or pain

• Difficulty in swallowing

• Nausea or vomiting

• Diarrhea

• Difficulty in breathing

• Wheezing or cough

• Dizziness

• Anxiety

• Flushing or swelling of the face or eye

• Weakness

• Unconsciousness

Sometime severe and sudden allergic reaction can develop within seconds after exposure to an allergen. This type of reaction is known as anaphylactic reaction and it is life-threatening. 

Types of allergens: There are three primary types of allergens:

• Ingested allergens are present in certain foods. More than 170 foods have been reported to cause IgE-mediated reactions.The seven most common are; cows’ milk, egg, peanut, soy, wheat, fish and cashew. 
• Inhaled allergens affect the body when they come in contact with the lungs or membranes of the nostrils or throat. Pollen is the most common inhaled allergen.
• Contact allergens must come in contact with your skin to produce a reaction. Like soap, latex etc
• Medicine allergens: There are only a limited number of medicine allergens available for testing. 

Allergy blood test: 

Allergy blood test measure a specific type of antibody (Immunoglobulin E or IgE) that your immune system produces when you come into contact with an Allergens.The higher the level of IgE, the more likely you are to have an allergy to that particular substances. 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 (Allergy Skin Test) may be necessary to confirm an allergy diagnosis.

Results of allergy blood testing must be interpreted with care. False negatives and false positives can occur. Even if an IgE test is negative, there is still a small chance that a person does 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.

The test can be run on patients of any age and regardless of skin condition (Unlike Allergy Skin test). There is no need to stop current medications prior to drawing a blood sample.

Who Should Be Tested? 

Anyone presenting with any allergy-like symptoms is a candidate for specific IgE allergy testing. However, allergy blood test should be strongly considered for patients with

• Recurrent or chronic upper respiratory disease, i.e., rhinitis, sinusitis, allergic-rhinitis etc. 
• Un-seasonal allergy-like symptoms. 
• Seasonal or perennial allergy-like symptoms. 
• Exogenous asthma and other suspected IgE mediation conditions. 

Limitations: 

• Allergy testing is not an exact science and false positives and even false negatives, are possible.
• Normal result (Normal IgE levels) do not necessarily exclude the possibility of allergy because certain allergies can be non-IgE mediated. (False Negative)
• Allergy blood tests can give false-positive results because of nonspecific binding of antibody in the assay. 
• In cases of food allergy, specific IgE antibodies may be undetectable in-spite of a convincing clinical history because these antibodies may be directed towards allergens that are revealed or altered during industrial food processing, cooking or digestion and therefore do not exist in the original food for which the patient is tested.
• Allergy test includes only common causative allergens and does not include all possible allergens.
• There is no gold-standard test for many allergic conditions. 
• The sensitivity of blood allergy testing is approximately 25% to 30% lower than that of skin testing.
• Some allergens are related, whereby a patient who is sensitive to one allergen also reacts to other similar allergens, even if they have never have been exposed them (cross-reactivity). 
• Blood tests are not likely to be clinically relevant in conditions not mediated by IgE, such as food intolerances, celiac disease, the DRESS syndrome, Stevens-Johnson syndrome, toxic epidermal necrolysis, or other types of drug hypersensitivity reactions, such as serum sickness.
• A pseudo-allergic or anaphylactoid reaction, looks like an allergic drug reaction, but it may not be allergy.  Drug contain many different substances like dyes, chemicals which can cause allergy like reaction, but it may be drug intolerance or hypersensitivity.
• True drug allergy test is IgE mediated reaction, it does not happen the first time you take a medication, much more likely to occur after your body has been exposed to the medication at least once or more. the first exposure allows body to create antibody and memory lymphocytic cells for the antigen.

Therefore, the patient’s clinical history and additional medically-supervised allergy skin tests may be necessary to confirm an allergy diagnosis.

Dr Prashant Goyal

Sources: WebMD, Lab Tests Online, Healthline