Allergy Tests

Why It Is Done

Allergy testing is done to find out what substances (allergens) may 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).
• Find out whether a person may have a drug allergy or be allergic to 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 likely 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 allergens on your skin about 1 in. (2.5 cm) to2 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).

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:

• 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.

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 or an oral food challenge) may be needed to identify this. In an elimination diet, you avoid eating foods that may be causing an allergic reaction. In an oral food allergy challenge, you eat a variety of foods that may or may not cause an allergic reaction. Your doctor watches to see if and when a reaction occurs.
  • Can be unpleasant for children and their parents.
• 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.