Immunoglobulin E tests

What is it immunoglobulin MY?

Immunoglobulin E (IgE) is a antibody produced during a type I hypersensitivity reaction to a allergen. (See DermNet NZ page Allergies explained.) A schematic for the type I reaction pathway is shown below [1].

IgE antibodies They are normally found in small amounts in the blood. A higher than normal level infers an allergic disorder. [2].

What is a type I hypersensitivity reaction?

After exposure and reexposure to a antigen (allergen) in susceptible individuals, a type I, or immediate, allergic reaction involves antigen-presenting cells, activation of T-helper (Th) cells, stimulation of B cells that release specific IgE and the release of various pharmacological mediators (such as histamine, cytokines, leukotrienesand others) of mast cells and basophils. The release of the mediators causes symptoms such as sneezing, wheezing and cure.

APC, antigen-presenting cell; IgE, immunoglobulin E; Th, T-helper [cell].
Credit: Gaurab Karki, Microbiologist Kathmandu, via Biology Notes Online.

What is an IgE test?

An IgE test is a blood test that detects circulating IgE. The test includes two types of test:

1. Total IgE test: The total level of IgE in the blood.
2. Specific IgE test: the level of specific IgE against a particular allergen.

Total IgE and specific IgE tests can be ordered at the same time or independently.

IgE levels can be measured using one of several methods. The use of the radioallergosorbent test (RAST) to measure IgE has been replaced by the use of enzyme-immunoassays (ELISA), fluorescent enzyme immunoassays (FEIA) and chemiluminescent immunoassays (CLIA) [3,4].

The color of the top of the tube indicates which tests it is suitable for. In this case, the gold-topped tube (a serum separator tube) is used for IgE testing.

What are the indications for IgE testing?

An IgE test is indicated when careful patient history taking and examination leads to suspicion of type I allergy.

An IgE test may also be used to monitor a patient with a known allergic condition, such as:

• Anaphylaxis
• Allergic rhinitis (hay fever)
• Allergic conjunctivitis
• Asthma
• Chronic rhinosinusitis

IgE testing is also useful in determining if a patient is allergic to a specific protein, such as:

• Types of food allergies (e.g., peanuts)
• house dust Mites
• Grass, grass or tree pollen
• Animal or fur dander
• Certain drugs and cosmetics.
• Molds
• The venom of a bee or wasp.

An IgE test may be used when skin prick testing is not available or appropriate; for example, in an individual with dermographism, extensive skin disease, recent use of antihistamines, or systemic steroids, or when there is concern that the prick test could cause a anaphylactic reaction.

IgE testing is rarely indicated in atopic dermatitis, allergic contact dermatitis, chronic spontaneous urticariaor angioedema As the relevance of elevated IgE is uncertain in these diseases.

Choose recommendations wisely

Although there is no absolute contraindication for IgE testing, the American Academy of Allergy, Asthma and Immunology, the Australian Society of Clinical Immunology and Allergy, and Choosing Wisely make the following recommendations [5,6].

• Do not perform an indiscriminate battery of IgE tests in allergy evaluation.
• Food-specific IgE testing should not be performed without a patient history suggesting IgE-mediated food allergy.
• Specific IgE test for inhalants or foods are not indicated in hives unless there is a clear history implying that an allergen is a factor causing or perpetuating the hives.
• Indiscriminate batteries of food IgE tests are expensive and unhelpful, which can lead to misdiagnoses and inappropriate restrictive diets.

How specific Allergens can it be tested?

The specific allergens that can be analyzed can be classified as follows [7]:

• Aeroallergens (inhalants)
• Indoor allergens: including house dust mites, animal dander (e.g., cat, dog, and cockroaches), mold, and fungal spores
• Outdoor allergens: such as pollen (from grasses, rye, weeds and trees) and polluted air (smoke)
• Food allergens, such as milk, eggs, peanuts, tree nuts, wheat, soy, cod, and shellfish.
• Poisons – including bees, wasps, hornets, ant poisons
• Dust mites (Dermatophagoides)
• Medications, such as penicillin, aspirin, and others.
• Latex
• Metals (especially nickel, cobalt, chromium and zinc)
• Household chemicals
• Cosmetic products.

IgE tests are also available to test different mixtures of allergens. These include a:

• Food mix (i.e. egg white, milk, cod, wheat, peanuts, and soy)
• Cereal mix (i.e. wheat, oats, corn, sesame seeds, and buckwheat)
• Fruit mix (i.e. banana, pear, peach, and apple)
• Seafood mix (i.e. cod, shrimp, mussels, tuna, salmon)
• Grass mix (i.e. Bermuda, rye, Timothy, prairie, Johnson and Bay grasses)
• Tree mix (i.e. olive, willow, eucalyptus, white pine, melaleuca)
• Nut Mix: Rutin (i.e. Peanut, Hazelnut, Brazil Nut, Almond, and Coconut)
• Nut mix – extra (i.e. pecan, cashew, pistachio and walnut)
• Mold mix (i.e. Penicillium, Aspergillus fumigatus, Cladosporium, Candida albicansand Alternaria)

How are IgE levels reported?

The total IgE reference range depends on the age of the individual (ranges from 0 to 4 kU/L in a newborn and from 0 to ~148 kU/L in an older child or adult).

The result of a specific IgE test is reported for a grouped mixture of allergens or an individual allergen. The following table shows how the results are typically classified and interpreted. [2,8,9].

Table. IgE Level Test Evaluations and Interpretations

This table shows arbitrary international reference figures. The actual reference range and classification varies by laboratory and is based on the method used for testing, calibration, patient age, and type of allergen.

How are IgE test results interpreted?

IgE test results should be carefully interpreted in the context of a patient's presentation. High levels of total IgE can occur in allergic conditions, parasitic infections, certain immune-related disorders, and malignant tumors.

The sensitivity of specific IgE tests varies from 60% to 95% and the specificity from 30% to 95%, depending on the type of allergen and the age of the patient. [3]. There is a good predictive value (>90%) for foods (cow's milk, eggs, fish and peanuts), pollens (grass and trees) and dust mites. Tests for some medications, latex, mold, and poison have low sensitivity but higher specificity. [3,4]. Note:

• A positive test result means sensitization to an allergen
• The indicated level of IgE may not correlate with the extent or severity of symptoms when exposed to the allergen.
• A normal IgE level may not exclude allergic disorders.
• A result can be misleading. False positive and false negative results may be due to cross reactivity, the age of the patient or the type and duration of exposure to the allergen.

Further evaluation can be performed by skin prick testing or by challenging the patient to a specific allergen in vivo.

How does the IgE test compare to the skin prick test?

The skin prick test is more specific than the IgE test and provides a rapid result (often within 30 minutes), but requires a trained professional and is not always tolerated by young children.

Specific IgE blood tests are simple and safe. They can be expensive, depending on the number of allergens tested. Caution is required when interpreting the results [10].