Allergy skin tests

Overview

An allergy skin test is a procedure to help detect substances that cause a person to have an allergic reaction.

An allergy is an immune system reaction to a substance that is typically harmless. A substance that causes an allergic reaction is called an allergen. During allergy skin tests, the skin is exposed to possible allergens that a person might breathe in, touch or eat.

An allergy skin test is essentially a series of many small tests on the skin. These tests are done with one of the following:

• A small tool that pricks the skin.
• A shot with a needle, also called an intradermal test.
• A skin patch.

A healthcare professional makes a diagnosis of a specific type of allergy based on test results, symptoms, medical history and the history of exposure to possible allergens at home, work or school.

Why it's done

Your healthcare professional may order an allergy skin test to:

• Help make an allergy diagnosis.
• Find specific allergens for you to avoid.
• Find triggers that may explain why asthma or allergic reactions aren't well controlled.
• Guide treatment plans and the choice of medicines.

Allergy skin tests are used to help diagnose reactions to many possible allergens, including:

• Plant pollens.
• Pets and other animals.
• Dust mites, molds and other airborne substances.
• Food.
• Penicillin or other medicines.
• Bee venom.
• Latex, chemicals, cosmetics and other products that skin may be exposed to.

Risks

Allergy skin tests are generally safe for adults and children of all ages.

There is a small chance than an allergy skin test might cause a severe allergic reaction. This may include anaphylaxis, which threatens many organs in the body. If not treated, anaphylaxis may be life-threatening. For this reason, allergy skin tests are done in a clinic with trained staff and emergency equipment and medicines on hand.

Because the risk of a severe reaction is lowest with a skin prick test, it is the first choice. An intradermal test with a needle is not done with food and latex allergy tests because of the risk of a severe reaction.

Skin tests are not done if you have:

• A high risk for anaphylaxis.
• Cannot stop taking a medicine that may interfere with test results.
• Have an ongoing skin condition.

If an allergy skin test is not an option, your healthcare professional may recommend a blood test. Lab tests of blood samples may find immune system antibodies to certain allergens.

Another option instead of an allergy skin test is a challenge test. With this test, a tiny amount of suspected food or medicine is taken in an allergy clinic with the same precautions as used with skin testing.

How you prepare

Before recommending an allergy skin test, your healthcare professional will ask you a number of questions about:

• Your symptoms.
• The times of day you have symptoms.
• Places where you do and don't have symptoms.
• Your exposure to common allergens.
• Medicines you take to treat allergy symptoms and other medicines you take regularly.
• Your medical history and family history of allergies or asthma.
• Your work, hobbies or recreational activities.

This information is important for helping your healthcare professional interpret the results of allergy skin tests.

Medicines that interfere with tests

Some medicines can interfere with the results of allergy skin tests. Before your appointment, make a list of all medicines you take, including those that don't need a prescription.

You may be asked to stop taking some medicines for several days or longer. If stopping the medicine isn't an option, your healthcare professional will likely recommend another type of allergy test. Medicines that can affect the results of skin prick and intradermal tests include:

• Antihistamines available by prescription, such as desloratadine (Clarinex) and hydroxyzine (Vistaril).
• Antihistamines available without a prescription, such as loratadine (Alavert, Claritin), diphenhydramine, chlorpheniramine, cetirizine (Zyrtec Allergy) and fexofenadine (Allegra Allergy, Allegra Hives).
• Antihistamine nasal sprays, such as azelastine (Astepro) and azelastine with fluticasone (Dymista).
• Nighttime pain medicines with antihistamines, such as ibuprofen with diphenhydramine (Advil PM) and acetaminophen with diphenhydramine (Tylenol PM Extra Strength).
• Tricyclic antidepressants, such as nortriptyline and desipramine (Norpramin).
• H2 blockers, such as cimetidine (Tagamet HB) and famotidine (Pepcid AC), which reduce stomach acids.
• Omalizumab (Xolair), an asthma medicine that can affect allergy skin tests results for several weeks and up to six months.
• Some eczema ointments, such as tacrolimus (Protopic) and pimecrolimus (Elidel).
• Methotrexate (Jylamvo, Trexall, others), a type of immune-suppressing medicine for treating some immune system conditions and cancers.

Factors that affect patch tests

Medicines that may affect the results of patch tests include:

• Corticosteroid ointments and pills.
• Immune-suppressing medicines.

Ultraviolet radiation from sun exposure or tanning beds also can affect the results of patch tests. You may be advised to avoid ultraviolet exposure for up to two weeks before the test.

What you can expect

Allergy skin tests are most often performed in an allergy clinic. Tests are done with medical staff, equipment and medicine on hand to treat severe reactions.

Skin prick test

Skin prick tests are the most common type of allergy test. The entire procedure often takes less than an hour.

Skin prick tests are used most often to test for allergic reactions to foods and to airborne allergens, such as dust, mold, pollens and animal dander. Your healthcare professional selects allergens based on the information you provide about your symptoms.

The tests are done on the skin of the upper back or the inside forearm. The test site is cleaned with an alcohol solution. A small dot is drawn on the skin where each allergen will be tested. Locations are numbered or labeled.

A small drop of each allergen is placed by each dot on the skin. A tiny device with a triangular point is used to prick the skin. This device is called a lancet. The prick is very shallow and does not draw blood — just enough for the allergen to get below the surface of the skin.

The pricking may be uncomfortable but not painful. A new lancet is used for each allergen.

About 15 minutes after the skin pricks, your healthcare team checks the skin for signs of allergic reactions. If you react to an allergen, you'll develop a raised, itchy bump, called a wheal. It may look and feel like a mosquito bite. The size of each wheal is measured and the results are recorded.

The marks on your skin are removed with an alcohol solution. The bumps typically go away within 30 minutes.

Intradermal test

An intradermal test uses needles to inject tiny amounts of allergens below the surface of the skin. The sites of the injections are labeled in the same way as is done with the skin prick test. The healthcare team looks at the results about 15 minutes after a shot is given.

Intradermal tests are most often used to test for allergies to:

• Penicillin and other medicines.
• Anesthetics.
• Insect venoms.

The skin prick test of these allergens has a lower chance of a severe allergic reaction. So a skin prick test is often done first. If there is no severe reaction, then the intradermal test is done. The intradermal test is generally more accurate for these allergens.

These tests also may be used if a skin prick text is negative but symptoms and other information suggest that you react to the allergen.

Patch test

Patch tests are usually done to test allergens that may cause an itchy rash, hives or other skin reactions. This condition is called contact dermatitis or allergic dermatitis.

Patch tests are most often done on the upper back. They also may be done on an outer upper arm or outer thigh.

Each allergen is applied to a patch that is put on the skin. In some cases, a test may include sheets of patches with common skin allergens. The patches are selected based on likely exposure to allergens in the home, school or workplace.

Examples of allergens tested with patches include:

• Latex.
• Metals in jewelry.
• Personal care products, makeup and fragrances.
• Plants.
• Medicines applied as ointments or creams.
• Clothing.
• Products used when working or doing hobbies, such as paints, glues and various chemicals.

The most common patch test requires at least three visits to a clinic. The patches are placed during the first visit. You then go home with instructions to keep the test site dry and protected.

The second visit is 48 hours later. The patches are removed. About 15 to 60 minutes later, your healthcare professional will look for signs of discolored, raised, irritated spots on the skin. The results are recorded. This gap in time allows the general irritation from the patch itself to lessen.

You go home again with instructions to keep the site dry. The third visit is typically 72 hours to one week later. This repeat reading is important because some allergens have a delayed reaction.

Results

If you react to an allergen in a skin test, the result is considered positive. A positive result means you may be allergic to the particular substance. A negative result means you may not be allergic to a substance.

A larger wheal or other lesion most often means that you are very sensitive to the allergen. But positive and negative results themselves are not enough to make a diagnosis. The diagnosis depends on the test results and other information you have given your healthcare professional.

For example, you may have a negative result to a cat allergen in a skin prick test, but you have allergy symptoms when you're around cats. Or you may have a positive result from a peanut allergen, but you eat peanuts without having a reaction.

To make a diagnosis of your allergies, your healthcare professional considers test results, your symptom history, your history of exposure to allergens and other factors. In some cases, your healthcare professional may order more tests.

An allergy diagnosis helps your healthcare professional work with you to make a treatment plan and to help you avoid allergens that affect you.