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Allergy testing involves having a skin or blood test to find out what substance, or allergen, may trigger an allergic response in a person. Skin tests are usually done because they are rapid, reliable, and generally less expensive than blood tests, but either type of test may be used.
A small amount of a suspected allergen is placed on or below the skin to see if a reaction develops. There are three types of skin tests:
Allergy blood tests look for substances in the blood called antibodies. Blood tests are not as sensitive as skin tests but are often used for people who are not able to have skin tests.
The most common type of blood test used is the enzyme-linked immunosorbent assay (ELISA, EIA). It measures the blood level of a type of antibody (called immunoglobulin E, or IgE) that the body may make in response to certain allergens. IgE levels are often higher in people who have allergies or asthma.
Other lab testing methods, such as radioallergosorbent testing (RAST) or an immunoassay capture test (ImmunoCAP, UniCAP, or Pharmacia CAP), may be used to provide more information.
Your allergy test results may show that allergy treatment is a choice for you.
Allergy testing is done to find out what substances (allergens) may cause an allergic reaction.
The skin prick test can also be done to:
A blood test on a blood sample may be done instead of a skin prick test if a person:
Many medicines can affect the results of a skin test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take. You may need to stop taking some medicines, such as some tricyclic antidepressants and antihistamines such as cetirizine (Zyrtec), fexofenadine (Allegra), and loratadine (Claritin), before you have an allergy skin test.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will show. To help you understand the importance of this test, fill out the medical test information form( What is a PDF document? ).
The health professional doing the skin prick or intradermal test will:
An alternative skin prick method uses a device with 5 to 10 points (heads), which are dipped into bottles that contain the allergen extract. This device is pressed against the skin of the forearm or back so that all heads are pressed into the skin at the same time.
If the skin prick test is negative, you may choose to have an intradermal skin test at a later visit. A skin prick test is usually done first because the intradermal test has a greater chance of causing a severe allergic reaction.
The skin prick test and the intradermal test usually take less than an hour each.
Skin patch test
A skin patch test also uses small doses of the suspected allergen. For this test:
The health professional drawing your blood will:
The blood sample will be placed on specially treated paper and sent to a lab to determine whether antibodies to any of the allergens being tested are present. If specific antibodies are found, it may mean you are allergic to a certain allergen.
With the skin prick test and the intradermal skin test, you may feel a slight pricking sensation when the skin beneath each sample is pricked or when the needle penetrates your skin.
If you have an allergic reaction from any of the skin tests, you may have some itching, tenderness, and swelling where the allergen solutions were placed on the skin. After the testing is done, cool cloths or a nonprescription steroid cream can be used to relieve the itching and swelling.
If you are having a skin patch test and you have severe itching or pain under any of the patches, remove the patches and call your doctor.
You may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin. Some people feel a stinging pain while the needle is in the vein. But many people do not feel any pain or have only minor discomfort after the needle is positioned in the vein.
The major risk with the skin prick test or the intradermal skin test is a severe allergic reaction called anaphylaxis. Symptoms of a severe allergic reaction include itching, wheezing, swelling of the face or entire body, trouble breathing, nausea or vomiting, and low blood pressure that can lead to shock. An anaphylactic reaction can be life-threatening and is a medical emergency. Emergency care is always needed for an anaphylactic reaction. But severe allergic reaction is rare, especially with the skin prick test.
There is very little risk of a problem from having blood drawn from a vein.
A skin or blood test can tell you what substance, or allergen, may trigger an allergic response.
Skin tests work by exposing a person to suspected allergens and seeing if a reaction occurs. The results of the skin tests are available immediately after testing is done.
No raised red areas (called wheals) are created by the allergen.
A wheal created by the allergen is at least 1/8 inch (3 mm) larger than the reaction to the negative control. The larger the wheal, the more certain it is that the person is allergic to that specific allergen.
Allergy blood tests look for substances in the blood called antibodies. Results of allergy blood tests are usually available in about 7 days.
The levels of immunoglobulin E (IgE), a type of antibody, are the same as in a person who does not have allergies.
The levels of immunoglobulin E (IgE) antibodies for a particular allergen or group of allergens are above the normal level.
Reasons you may not be able to have a skin test or why the results may not be helpful include:
Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Current as of:
April 7, 2019
Author: Healthwise StaffMedical Review: E. Gregory Thompson MD - Internal MedicineAdam Husney MD - Family MedicineMartin J. Gabica MD - Family MedicineKathleen Romito MD - Family MedicineRohit K Katial MD - Allergy and Immunology
Current as of:
April 7, 2019
Medical Review:E. Gregory Thompson MD - Internal Medicine & Adam Husney MD - Family Medicine & Martin J. Gabica MD - Family Medicine & Kathleen Romito MD - Family Medicine & Rohit K Katial MD - Allergy and Immunology
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