Allergy Tests

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.

Why It Is Done

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

Skin test

The skin test can 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).
• Find out whether a person may have a drug allergy or be allergic to insect venom.

Blood test

A blood test may be done instead of a skin 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. Blood tests can help find out which foods the person is most likely allergic to.

How To Prepare

Be sure to tell your doctor about all the medicines you take. You may need to stop taking some medicines such as antihistamines for a few days before you have an allergy skin test.

How It Is Done

Allergy skin tests

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 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.
• Check the skin after 12 to 15 minutes for red, raised itchy areas called wheals. If a wheal forms, it means you are possibly allergic to that allergen (this is called a positive reaction).

Another 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.

Skin patch test

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. Keep the patch area dry when you bathe. And don't do any activities that could make you sweat a lot 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.

How long the test takes

Allergy skin tests usually take less than an hour.

Allergy blood tests

A health professional uses a needle to take a blood sample, usually from the arm.

The blood sample will be placed on specially treated paper. It's then sent to a lab to find out if 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

Allergy 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.

Allergy blood tests

You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

Allergy skin tests

The major risk with the skin prick test or the intradermal skin test is a severe allergic reaction called anaphylaxis. Symptoms of this type of reaction include itching, wheezing, swelling of the face or entire body, trouble breathing, belly pain, nausea, vomiting, diarrhea, 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 this type of reaction. But severe allergic reaction is rare, especially with the skin prick test.

Allergy blood tests

There is very little chance of having a problem from this test. A small bruise may form at the site.

Results

Allergy skin tests

Allergy skin tests ^{footnote 1}

Normal (negative):	No raised red areas (called wheals) are created by the allergen.
Abnormal (positive):	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

Allergy blood tests

Normal (negative):	The levels of immunoglobulin E (IgE), a type of antibody, are the same as in a person who does not have allergies.
Abnormal (positive):	The levels of immunoglobulin E (IgE) antibodies for a particular allergen or group of allergens are above the normal level.