Test Overview

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.

Skin tests

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:

• Skin prick test. This test is done by placing a drop of a solution containing a possible allergen on the skin, and a series of scratches or needle pricks allows the solution to enter the skin. If the skin develops a red, raised itchy area (called a wheal), it usually means that the person is allergic to that allergen. This is called a positive reaction.
• Intradermal test. During this test, a small amount of the allergen solution is injected into the skin. An intradermal allergy test may be done when a substance does not cause a reaction in the skin prick test but is still suspected as an allergen for that person. The intradermal test is more sensitive than the skin prick test but is more often positive in people who do not have symptoms to that allergen (false-positive test results).
• Skin patch test. For a skin patch test, the allergen solution is placed on a pad that is taped to the skin for 24 to 72 hours. This test is used to detect a skin allergy called contact dermatitis.

Blood test

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. For more information, see:

Allergies: Should I Take Allergy Shots?

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	Allergies: Should I Take Allergy Shots?

Why It Is Done

Allergy testing is done to find out what substances (allergens) cause an allergic reaction.

Skin test

The skin prick test can also be done to:

• Identify inhaled (airborne) allergens, such as tree, shrub, and weed pollens, molds, dust, feathers, and pet dander.
• Identify likely food allergens (such as eggs, milk, peanuts, nuts, fish, soy, wheat, or shellfish).
• Determine whether a person may be allergic to a medicine or insect venom.

Blood test

A blood test on a blood sample may be done instead of a skin prick test if a person:

• Has hives or another skin condition, such as eczema, that makes it hard to see the results of skin testing.
• Cannot stop taking a medicine, such as an antihistamine or tricyclic antidepressant, that may prevent or reduce a reaction to a substance even when a person is allergic to the substance.
• Has had a severe allergic reaction (anaphylaxis).
• Has had positive skin tests to many foods. Enzyme-linked immunosorbent assay (ELISA) can find out the foods that a person is most allergic to.

How To Prepare

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?).

How It Is Done

Skin tests

The health professional doing the skin prick or intradermal test will:

• Clean the test site (usually on your back or arm) with alcohol.
• Place drops of the possible allergens on your skin about 1 in. (2.5 cm) to 2 in. (5 cm) apart. This allows many substances to be tested at the same time.
• Prick the skin under each drop with a needle. The needle passes through the drop and allows some of the allergen to penetrate your skin. For the intradermal test, a needle is used to inject the allergen solution deeper into the skin. See a picture of a skin prick allergy test.
• Check the skin after 12 to 15 minutes for red, raised itchy areas called wheals. If a wheal forms, it means you are allergic to that allergen (this is called a positive reaction).

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.

A skin patch test also uses small doses of the suspected allergen. For this test:

• Doses of allergens are placed on patches that look like adhesive bandages.
• The patches are then placed on the skin (usually on your back). This usually takes about 40 minutes, depending on how many patches are applied.
• You will wear the patches for 24 to 72 hours. Do not take a bath or shower or do any activities that could make you sweat excessively while you are wearing the patches. This could loosen the patches and cause them to fall off.
• The patches will be removed by your health professional, and your skin will be checked for signs of an allergic reaction.

Blood test

The health professional drawing your blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• 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 over the needle site as the needle is removed.
• Apply pressure to the site and then a bandage.

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.

How It Feels

Skin tests

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.

Blood test

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.

Risks

Skin tests

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, 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.

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.

Blood test

There is very little risk of a problem from having blood drawn from a vein.

• You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.
• In rare cases, the vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated with a warm compress applied several times daily.
• Continued bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood is drawn.

Results

A skin or blood test can tell you what substance, or allergen, may trigger an allergic response.

Skin tests

Skin tests work by exposing a person to suspected allergens and seeing if a reaction occurs. The results of the skin tests will be available immediately after testing is done.

Blood tests

Allergy blood tests look for substances in the blood called antibodies. Results of allergy blood tests are usually available in about 7 days.

What Affects the Test

Reasons you may not be able to have a skin test or why the results may not be helpful include:

• Exercise that causes excessive sweating.
• Getting a skin patch wet.
• Taking medicines such as antihistamines or tricyclic antidepressants.

What To Think About

• You may not need allergy testing if you have mild allergies that are easily controlled with medicine or lifestyle changes.
• Skin tests for allergies:
  • Are the easiest and least expensive method for identifying allergies in most people.
  • Are more reliable than blood tests for identifying common inhaled (airborne) allergies, such as pollens, dust, mold, and pet dander.
  • Cannot tell whether a person is reacting to a certain food. Further testing (such as an elimination diet) may be needed to identify this.
  • Can be unpleasant for children and their parents.
  • May take as long as 3 to 4 hours.
• Blood tests for allergies:
  • Are less sensitive than skin tests. So it is possible for people to have a positive reaction to a skin test but have a negative reaction to a blood test.
  • Are not affected by antihistamines or tricyclic antidepressants, so people do not need to stop taking these medicines while being tested.
  • May be a good choice if you have very sensitive skin or a skin problem, such as eczema, that would make it hard to see whether you have a reaction to a skin test.
  • Will not cause an allergic reaction. They may be used if you have had a serious allergic reaction in the past.
  • Cost more than skin tests.
• You may need to be tested for a latex allergy before having a major surgery if you have had frequent exposure to latex. Allergy blood tests are used for this. If you have a latex allergy, latex products should not be used during your surgery or recovery.
• Other blood tests, such as fluoro-allergosorbent testing (FAST), multiple antigen simultaneous testing (MAST), and radioallergosorbent testing (RAST), are sometimes used to check for allergens.

References

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