What Are Allergies?
Allergies are hypersensitivities that involve reactions of the immune system to substances that do not cause reactions in most people. The substances that trigger the reaction are called allergens.
Allergies affect as many as 40% of people worldwide and have become increasingly common in recent decades. Examples of potential allergens include plants, insect venoms, dander from pets, mold spores, materials, foods, and drugs. They can trigger diverse symptoms that range from mild to severe.
Several types of tests can be used to determine whether a person is allergic to a specific substance. Because there are diverse kinds of allergens, appropriate testing is important to identify and manage allergies.
The Role of Allergy Testing
Allergy testing is usually diagnostic, meaning that it is done after someone has had signs of an allergic reaction. By reviewing the symptoms, a primary care physician or allergist can prescribe and perform tests for the most likely allergens. When both past reactions and testing indicate an allergy, it can enable treatment or avoidance of the allergen.
It’s uncommon to do broad screening for allergies in people who haven’t had any prior allergic reaction symptoms because some tests may have potential false positives that show sensitivity to a substance when no real allergy exists.
In people who have received treatment for allergies, testing may be used to monitor their response or to determine if they still have hypersensitivity to a substance.
Getting allergy test results
Allergy test results are usually reviewed with the doctor and in many cases with a specialist known as an allergist. Test results alone often aren’t enough to demonstrate an allergy. Instead, they are typically considered together with past patient reactions.
Depending on the test results and patient history, more than one test may be used to either confirm or rule out an allergy.
Types of Allergy Tests
Tests of the blood and skin are the standard ways of identifying potential hypersensitivities.
Allergen-specific immunoglobulin E (IgE) testing is the most common blood allergy test. IgE is a type of protein known as an antibody that is associated with allergic reactions. Certain types of IgE are related to specific allergens, and elevated levels can reflect a possible hypersensitivity.
The skin prick test is the most frequent allergy skin test. It involves placing a small drop of a potential allergen on your skin and then lightly pricking you with a tiny needle to allow the substance to get just beneath your skin. An intradermal skin test is similar but uses a larger needle to inject the substance deeper into your skin.
For these tests, doctors usually choose which allergen(s) to test for based on a review of your symptoms. When necessary, they can test for more than one allergen at the same time.
Often, no further tests are needed to determine if a person has an allergy. However, additional allergy tests, described below, may be used in certain circumstances.
Other types of blood tests include:
- Total IgE testing: This provides a count of all IgE but does not give levels for specific allergens. As a result, it is not regularly used for allergy testing.
- Complete blood count (CBC) and white blood cell differential: These tests measure levels of different blood cells, including eosinophils (a type of white blood cell) that may be elevated in people with allergies.
- Histamine and tryptase: High levels of these substances in the blood can reflect an active severe allergic reaction, including anaphylaxis, which can be life-threatening.
Another type of skin testing is called the patch test. It involves applying a possible allergen to the skin, covering the area with a patch, and checking at several intervals over the course of a few days. Patch testing is different from prick and intradermal tests because it does not break the skin. It is most often used when someone has a type of skin condition called dermatitis that develops more slowly than other allergic reactions.
Blood and skin prick tests can be used to diagnose most food allergies. Other tests for food allergies include:
- Oral food challenge: This is a structured test procedure that involves eating gradually increasing amounts of a specific food while being observed for allergic reactions.
- Food elimination: Though not a formal test, food elimination means following a specific diet that excludes one or more foods that may be causing allergies. Elimination diets generally last for at least two weeks.
Getting Tested for Allergies
Allergy testing is almost always performed in a controlled medical setting like a doctor’s office or hospital. Allergen exposure can provoke serious reactions that may be unpredictable. As a result, skin tests and oral food challenges require careful protocols to limit severe reactions and to quickly and effectively respond if they occur.
Blood testing does not involve allergen exposure and does not have the same risk of reaction. In most cases, a blood sample is taken from your arm with a needle in a medical office and then sent to a laboratory for analysis.
For both skin or blood testing, tests are normally ordered by a doctor based on your past symptoms, allowing the testing to focus on a specific allergen or set of allergens. It is important for anyone with concerns about allergies to talk to their doctor about tests that may be most beneficial in their situation.
At-home allergy testing
Allergy testing that involves direct exposure is not offered at home and should only be done in a medical office. This is an important safety measure because of the possibility of severe reactions.
For that reason, at-home allergy tests are almost always blood tests. To take the test, you prick your finger, apply a drop of blood to an included test strip, and then send that sample to a lab. Your blood is analyzed for immunoglobulin proteins associated with different allergies.
At-home allergy tests do not confirm that you have an allergy and are not a replacement for tests ordered by a doctor. Blood tests are not 100% accurate and may detect elevated immunoglobulin levels when no actual allergy exists. As a result, any results from an at-home test should be reviewed with a doctor who can determine if follow-up testing is appropriate.
Sources and Resources
The following resources provide additional information about the symptoms, causes, and treatment of allergies:
- National Library of Medicine: Allergy
- MedlinePlus Medical Encyclopedia: Allergies
- American Academy of Allergy, Asthma & Immunology: Allergies
- Merck Manual: Overview of Allergic Reactions
A.D.A.M. Medical Encyclopedia. Allergies. Updated February 2, 2020. Accessed March 21, 2021. https://medlineplus.gov/ency/article/000812.htm
Brod BA. Patch testing. In: Fowler JA, ed. UpToDate. Updated July 28, 2020. Accessed March 21, 2021. https://www.uptodate.com/contents/patch-testing
Burks W. Diagnostic evaluation of food allergy. In: Sicherer SH, ed. UpToDate. Updated April 23, 2019. Accessed March 21, 2021. https://www.uptodate.com/contents/diagnostic-evaluation-of-food-allergy
Burks W. Patient education: Food allergy symptoms and diagnosis (Beyond the Basics). In: Sicherer SH, ed. UpToDate. Updated January 22, 2021. Accessed March 21, 2021. https://www.uptodate.com/contents/food-allergy-symptoms-and-diagnosis-beyond-the-basics
Commins SH. Food intolerance and food allergy in adults: An overview. In: Sicherer SH, ed. UpToDate. Updated May 28, 2020. Accessed March 21, 2021. https://www.uptodate.com/contents/food-intolerance-and-food-allergy-in-adults-an-overview?
Kowal K, DuBuske L. Overview of skin testing for allergic disease. In: Bochner BS, Wood RA, eds. UpToDate. Updated April 3, 2020. Accessed March 21, 2021. https://www.uptodate.com/contents/overview-of-skin-testing-for-allergic-disease
Kowal K, DuBuske L. Overview of in vitro allergy tests. In: Bochner BS, ed. UpToDate. Updated February 5, 2021. Accessed March 21, 2021. https://www.uptodate.com/contents/overview-of-in-vitro-allergy-tests
Lavine E. Blood testing for sensitivity, allergy or intolerance to food. CMAJ. 2012;184(6):666-668. doi:10.1503/cmaj.110026
MedlinePlus: National Library of Medicine. Allergy. Updated May 16, 2018. Accessed March 21, 2021. https://medlineplus.gov/allergy.html
MedlinePlus: National Library of Medicine. Food Allergy Testing. Updated July 31, 2020. Accessed March 22, 2021. https://medlineplus.gov/lab-tests/food-allergy-testing/
National Academies of Sciences, Engineering, and Medicine. Finding a Path to Safety in Food Allergy: Assessment of the Global Burden, Causes, Prevention, Management, and Public Policy. The National Academies Press; 2017. Accessed March 22, 2001. https://www.nap.edu/catalog/23658/finding-a-path-to-safety-in-food-allergy-assessment-of
Pawankar R, Holgate ST, Canonica W, Lockey RF, Blaiss MS, eds. White Book on Allergy 2013 Update. World Allergy Organization; 2013. Accessed March 21, 2021. https://www.worldallergy.org/UserFiles/file/WhiteBook2-2013-v8.pdf
Sicherer SH. Oral food challenges for diagnosis and management of food allergies. In: Wood RA, ed. UpToDate. Updated June 17, 2019. https://www.uptodate.com/contents/oral-food-challenges-for-diagnosis-and-management-of-food-allergies
Turnbull JL, Adams HN, Gorard DA. Review article: the diagnosis and management of food allergy and food intolerances. Aliment Pharmacol Ther. 2015;41(1):3-25. doi:10.1111/apt.12984
UpToDate. Patient education: Allergy skin testing (The Basics). Accessed March 22, 2021. https://www.uptodate.com/contents/allergy-skin-testing-the-basics
UpToDate. Patient education: Food allergy (The Basics). Accessed March 22, 2021. https://www.uptodate.com/contents/food-allergy-the-basics