There are all kinds of allergies out there, and to help make them easier to identify, your doctor might suggest allergy testing. Most doctors use skin tests because they’re less expensive than blood tests and are quick and reliable, but both kinds of tests might be used at various times.
Types of Testing
There are three types of skin tests:
- • Skin prick test: A drop of a solution with the allergen is placed on the skin, and a series of scratches or needle pricks allow it to enter the skin. If the skin develops a red, raised area, it usually signals a positive reaction to that allergen.
- • Intradermal test: A small amount of allergen is directly injected into the skin. This is a more sensitive test than the skin-prick test, but can also result in more false-positive results.
- • Skin patch test: The allergen solution is placed on a pad that’s taped to the skin for 24 to 72 hours. This test is used to detect a specific skin allergy called contact dermatitis.
The most common type of blood test is the enzyme-linked immunosorbent assay (ELISA, EIA), which measures the blood level of antibodies that the body makes in response to certain allergens. Other testing include radioallergosorbent testing (RAST) or an immunoassay capture test (ImmunoCAP).
Why They’re Done
The primary purpose of allergy tests is to determine which allergens might cause an allergic reaction in a given person. The skin prick test can also:
- • Identify inhaled allergens like pollens, molds, dust, feathers and pet dander
- • Identify likely food allergens, like eggs, milk, peanuts, nuts, fish, soy, wheat or shellfish
- • Determine whether a person has a drug allergy or might be allergic to insect venom
Cases where a blood test might be done instead of a skin prick test include:
- • Hives or other skin conditions (including eczema) can make it hard to see the results of skin testing
- • Inability to stop taking medications that might prevent or reduce a reaction to a substance even when someone is allergic. These include medications such as antihistamines or tricyclic antidepressants)
- • Severe allergic reaction (anaphylaxis)
- • Positive skin tests to many foods in the past: In these cases, ELISA testing can find out the most likely foods you might be allergic to.
What to Expect
While preparing for a skin test, ask your doctor about any medications you may need to stop taking or any other concerns you may have. For a skin prick or intradermal test, your doctor will:
- • Clean the test site with alcohol
- • Place drops of the allergen on the skin about 1 to 2 inches apart
- • Prick the skin under each drop with a needle in a skin prick test, or inject the allergen into the skin during an intradermal test
- • Check the skin after 12 to 15 minutes for any red, raised itchy areas
Both of these tests should take under an hour. In cases of skin patch tests, bandages are placed on the skin (typically the back), then left for 24 to 72 hours—a period during which you should not shower, bathe or do any activities that might make you sweat excessively. After your set period of time, your doctor will remove the patches and check for allergic reactions.
If you are getting a blood test, your doctor will:
- • Wrap an elastic around the upper arm to stop blood flow. This also makes veins below the band larger and easier to locate for the needle
- • Clean the needle site with alcohol
- • Place the needle in the vein
- • Attach a tube to the needle to fill it with blood
- • Remove the band from your arm when enough blood is collected
- • Apply a gauze pad or cotton ball as the needle is removed
- • Apply pressure to the site, then a bandage
Risks and Side Effects
Risks and side effects for skin tests may include:
- • Anaphylaxis: A severe allergic reaction. This is the major risk associated with the skin prick test or intradermal test.
- • Severe itching or pain under any patches during skin patch tests.
Risks of a blood test are generally very minor, and include:
- • Small bruise at the puncture site
- • Inflamed vein after a blood sample is taken—a rare condition called phlebitis
- • Continued bleeding, in people with bleeding disorders or for people who are taking blood-thinning medications
A skin test will usually have results available immediately after the test. Blood tests look for substances called antibodies, which signal allergen reactions. These results are usually available within about 7 days.
If you’re considering allergy testing, speak to your doctor about the right option for you.
Our Utah Valley allergy specialists diagnose and treat patients who suffer from allergic and immunologic disorders. Our board-certified physicians have over 30 years of combined experience working with pediatric and adult patients with a variety of medical problems using the most comprehensive and up-to-date medical therapies.
“Allergy Tests.” WebMD. http://www.webmd.com/allergies/allergy-tests#1
“Allergy Testing.” American Academy of Allergy, Asthma & Immunology. https://www.aaaai.org/conditions-and-treatments/library/at-a-glance/allergy-testing