IgE tests

Author: Dr Tim Aung, Primary Care Practitioner. Brisbane, Queensland, Australia. DermNet NZ Editor-in-Chief: A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. Copy edited by Gus Mitchell. February 2019.


What is IgE?

IgE is an abbreviation for immunoglobulin E, an antibody produced during a Type 1 hypersensitivity reaction to an allergen. See DermNet NZ's page on allergies explained. A schematic for the Type 1 reaction pathway is shown below [1].

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

What is a Type 1 hypersensitivity reaction?

Following exposure and re-exposure of antigen (allergen) in susceptible individuals, a Type 1, or immediate, allergic reaction involves antigen-presenting cells (APC), the activation of T helper (Th) cells, the stimulation of B cells which release specific IgE, and the release of various pharmacological mediators (such as histamine, cytokines, leukotrienes, and others) from mast cells and basophils. The release of the mediators causes various symptoms such as sneezing, wheezing, and weals.   

Type 1 hypersensitivity reaction

Type I, IgE-mediated hypersensitivity reaction pathway (courtesy of G Karki, Microbiologist, Kathmandu) [1].

What is an IgE test?

An IgE test is a blood test that detects circulating IgE. The test has two steps.

  1. Total IgE — the total level of IgE in the blood.
  2. Specific IgE — 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 The IgE RAST (RadioAllergoSorbent Test) has been superseded by an enzyme-linked immunosorbent assay (ELISA), fluorescent enzyme immunoassays (FEIA) and chemiluminescent immunoassays (CLIA) [3,4].

Vacutainer tubes for blood tests

The colour of the top of the tube denotes what tests it is suitable for. In this case, the gold-top tube (a serum separator tube) is used for IgE tests.

What are the indications for IgE testing?

An IgE test is indicated when a careful history and examination lead to a suspicion of Type 1 allergy.

The test can also be used for monitoring a patient with a known allergic condition, such as:

IgE tests are also useful to determine whether a patient is allergic to a specific protein, such as:

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

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

Choosing Wisely recommendations

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

  • Don’t perform an indiscriminate battery of IgE tests in the evaluation of allergy.
  • Food-specific IgE testing should not be performed without a clinical history suggestive of IgE mediated food allergy.
  • Specific IgE testing for inhalants or foods is not indicated in urticaria unless there is a clear history implicating an allergen as a provoking or perpetuating factor for the urticaria.
  • Indiscriminate batteries of IgE tests for foods are expensive and not useful, potentially leading to erroneous diagnoses and inappropriately restrictive diets.

Which specific allergens can be tested?

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

  • Aeroallergens (inhalants)
  • Indoor allergenshouse-dust mite, animal dander (cat, dog, cockroach), mould and fungal spores
  • Outdoor allergens — pollens (grasses, rye, weeds, trees), and polluted air (smoke)
  • Food allergens — milk, eggs, peanuts, tree nuts, wheat, soy, codfish, shellfish
  • Venoms — bee, wasp, hornet, ant
  • Medicines — penicillin, aspirin, and others
  • Latex, metals (especially nickel, cobalt, chromium, and zinc), household chemicals, and cosmetics.

The tests are also available as mixes of allergens. These include:

  • Food mix (egg white, milk, codfish, wheat, peanut, soybean)
  • Cereal mix (wheat, oat, corn, sesame seed, buckwheat)
  • Fruit mix (banana, pear, peach, apple)
  • Seafood mix (codfish, shrimp, mussel, tuna, salmon)
  • Grass mix (Bermuda, rye, Timothy, meadow, Johnson, Bahia)
  • Tree mix (olive, willow, eucalyptus, white pine, Melaleuca)
  • Nut mix routine (peanut, hazelnut, brazil nut, almond, coconut)
  • Nut mix extra (pecan nut, cashew nut, pistachio, walnut)
  • Mould mix (Penicillium, Aspergillus fumigatus, Cladosporium, Candida albicans, Alternaria)
  • Dust mite (Dermatophagoides)
  • Other.

How are IgE levels reported?

The total IgE reference range depends on the age of the individual (from 0–4 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 group mix or an individual allergen. The table below shows how the results are typically rated and interpreted [2,8,9].

IgE level test ratings and interpretations
Rating of specific IgE level (KUa/L)Grade/ClassInterpretation
Absent or undetectable (< 0.35) 0 Unlikely
Low (0.35–0.69) I Doubtful significance
Moderate (0.70–3.49) II Possible
High (3.50–17.49) III More possible
Very high (17.50–49.99) IV More likely
Very high (50.00–100.00) V Very likely
Extremely high (> 100.00) VI Extremely likely

Note: the table shows arbitrary international reference figures. The actual reference range and grading vary with the laboratory and are based on the method used for the test, calibration, the age of the patient, and the type of allergens.

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 malignancies.

The sensitivity of specific IgE tests ranges from 60–95% and the specificity from 30–95%, depending upon the type of allergens and the age of the patient [3]. There is a good predictive value (>90%) for food (cow’s milk, egg, fish, and peanuts), pollens (grass and trees) and dust mites. Tests for some medicines, latex, moulds and venom have poor sensitivity but greater specificity [3,4]. Note:

  • A positive test result means sensitisation 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 level of IgE may not exclude allergic disorders.
  • A result may be misleading. False positive and false negative results can be due to cross-reactivity, the age of the patient, or the type and duration of exposure to the allergen.

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

How does IgE testing compare with skin prick testing?

Skin prick testing is more specific than IgE testing and gives a rapid result (often within 30 minutes), but it requires a trained practitioner 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].

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References

  1. Karki G. Type I hypersensitivity reaction: mechanism and clinical manifestation. Available at: http://www.onlinebiologynotes.com/type-i-hypersensitivity-reaction-mechanism-and-clinical-manifestation/ (accessed 11 February 2019).
  2. Heyworth-Smith D, Campbell P. Laboratory Diagnosis of Allergy. QML pathology newsletter, Brisbane, 2017.
  3. Siles RI, Hsieh FH. Allergy blood testing: A practical guide for clinicians. Cleve Clin J Med 2011; 78: 585–92. doi: 10.3949/ccjm.78a.11023. PubMed
  4. Diaz RC, Liang J, Hallet L, Meyers AD. Diagnostic Allergy Testing: Specific IgE Testing. Updated February 29 2016. Medscape (accessed 7 Jun 2018).
  5. Don’t perform food IgE testing without a history consistent with potential IgE-mediated food allergy. Choosing Wisely. American Academy of Allergy, Asthma & Immunology. 3 March 2014. Available at: http://www.choosingwisely.org/clinician-lists/american-academy-allergy-asthma-immunology-food-ige-testing/ (accessed 5 February 2019).
  6. Food-specific IgE testing should not be performed without a clinical history suggestive of potential IgE-mediated food allergy. Australasian Society of Clinical Immunology and Allergy. Choosing Wisely. Available at: https://choosingwisely.org.nz/professional-resource/ascia/ (accessed 5 February 2019)
  7. Chapman MD. Allergen nomenclature. Clin Allergy Immunol 2008; 21: 47–58. PubMed
  8. Williams P, Sewell WA, Bunn C, Pumphrey R, Read G, Jolles S. Clinical immunology review series: an approach to the use of the immunology laboratory in the diagnosis of clinical allergy. Clin Exp Immunol 2008; 153: 10–8. doi: 10.1111/j.1365-2249.2008.03695.x. PubMed
  9. Sheldon J, Miller L. Allergy diagnosis reference guide. Clinical Biochemistry, East Kent University Hospitals, NHS. October 2014. P-6.
  10. Australasian Society of Clinical Immunology and Allergy (ASCIA). Laboratory Tests in the Diagnosis of Allergic Diseases. Updated January 2010. Available at: www.allergy.org.au/health-professionals/papers/tests-in-the-diagnosis-of-allergic-diseases (accessed 8 June 2018).

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