A skin test is used to determine whether a patient is allergic to certain substances, such as mold, dust mites or animal dander. Patients who experience common allergy symptoms, such as sneezing, hives, watery eyes, runny nose and itching are often tested to determine specific allergies.
Allergies occur when the body's immune system overreacts to a substance that is normally harmless (allergen), such as mold, pollen, animal dander or dust mites.
The white blood cells of an allergic individual produce an antibody called immunoglobulin E (IgE), which attaches to the allergen. This triggers the release of histamine and other inflammatory chemicals that cause allergic symptoms, such as runny nose and watery eyes.
During the test, the skin is exposed to different allergens and then observed for an allergic reaction. Qualified healthcare providers can use the test to determine whether allergy-like symptoms are immune-mediated, and they can identify the specific substance(s) that trigger the reaction. Skin testing is often used to diagnose allergic conditions, including allergic asthma, dermatitis (eczema) and hay fever.
The results of a skin test can help a doctor develop an appropriate allergy treatment plan that may include allergen avoidance, lifestyle changes to reduce allergen exposure, medication or allergy shots (immunotherapy).
WHO SHOULD NOT BE TESTED
Certain medications, including antihistamines like fexofenadine (Allegra©), diphenhydramine (like Benadryl©) and cetirizine (Zyrtec©), antidepressants like amitriptyline and doxepin (Sinequan©) and some heartburn medications like cimetidine (Tagamet©) and ranitidine (Zantac©), may interfere with test results. A qualified healthcare provider will determine if it is better to continue taking the medications or discontinue the medications temporarily, until the test is performed.
Patients who have severe skin diseases (like eczema or psoriasis) that are present on large areas of skin on the arms or back may be discouraged from taking the test. This is because the test is typically performed on the arms and back, and there may not be enough unaffected skin to perform a conclusive test.
Individuals who are extremely sensitive to suspected allergens might be advised not to undergo the test. Some patients may be so sensitive to certain substances that even trace amounts could trigger a severe allergic reaction called anaphylaxis.
PREPARING FOR THE TEST
Before a skin test is performed, a qualified healthcare provider will ask questions about the patient's medical history, including any and all allergic symptoms that occur.
A qualified healthcare provider will perform a physical examination to determine if there is an underlying medical problem triggering the reactions.
In some cases, a medical history and physical exam may provide enough information for the healthcare provider to diagnose and treat the patient. In such cases, a skin test may not be needed. However, if the cause of the symptoms remains unknown, a skin test may be recommended.
Before a skin test is scheduled, a qualified healthcare provider will need a list of all the prescription and over-the-counter medications, as well as any herbs or supplements that the patient is taking. Some agents, including antihistamines like fexofenadine (Allegra©), cetirizine (Zyrtec©) and diphenhydramine (Benadryl©) may suppress allergic reactions, thus causing false negative skin test results. Others, including antidepressants like amitriptyline and doxepin (Sinequan©) and heartburn medications like cimetidine (Tagamet©) and ranitidine (Zantac©) may increase the patient's chance of experiencing a severe allergic reaction during the test.
Different medications can stay in the body for different lengths of times after they are discontinued. Therefore, a qualified healthcare provider may ask that the patient stop taking certain mediation for 10 days or more prior to the skin test. Patients should not stop taking any drugs, herbs or supplements without consulting their healthcare providers.
HOW IT IS PERFORMED
General: A skin test is typically conducted in a healthcare provider's office. Skin tests cause minimal, if any, discomfort. The needles used barely penetrate the skin's surface and will not cause bleeding. Some tests, like the percutaneous (puncture, prick or scratch) test, detect immediate allergic reactions, which develop within minutes of exposure to an allergen. Other tests, like the epicutaneous (patch) test, detect delayed allergic reactions that develop over the course of several days. There are three main types of tests.
Percutaneous (puncture, prick or scratch) test: The percutaneous (puncture, prick or scratch) test is the most common type of skin test. During the procedure, a drop of purified allergen extract is either pricked or scratched into the skin's surface. This test is commonly performed to detect allergies to mold, pollen, pet dancer, dust mites, insect venom and penicillin.
A percutaneous test is used to detect immediate allergic reactions to as many as 40 different substances at a time. In adults, the test is usually performed on the forearm, while children are usually tested on the upper back.
The test site is first cleaned with alcohol. The healthcare provider then applies a drop of the allergen extract onto the site. A sharp instrument called a lancet is used to scratch or prick the substance into the skin's surface.
A new lancet is used for each new allergen to prevent allergens from mixing together.
The drops are left on the skin for about 15 minutes. Then the healthcare provider will observe the skin for reactions, such as reddening, swelling or urticaria (hives).
To determine how the immune system is reacting, two additional substances will be placed on the skin's surface. First, histamine is applied. Histamine is present in almost everyone and causes an allergic skin response. Histamine is used as a positive control, which means it is used to induce a positive reaction in the patient. Healthy patients will have an immune response to the substance. If the patient does not react to histamine, it will not be possible to interpret the test.
The second substance applied is either glycerin or saline. These substances typically cause no reaction. This is used as a negative control. If the patient reacts to this substance, he/she may have sensitive skin, and the results of the test may be inconclusive. If the results are inconclusive, a more sensitive immediate allergic reaction test, called an intracutaneous test may be performed.
Intracutaneous (intradermal) test: An intracutaneous (intradermal) test is most often performed if the suspected allergen is insect venom or penicillin. Since this test is more sensitive than a percutaneous test, it is usually performed when the percutaneous test results are inconclusive. During the procedure, the allergen extract is injected just below the surface of the skin on the arm. Then the healthcare provider will observe the site after 15 minutes for an allergic skin reaction.
Epicutaneous (patch) test: An epicutaneous (patch) test is most commonly performed to identify substances that cause contact dermatitis (skin disorder caused by exposure to allergens). Allergens, including latex, medications, fragrances, preservatives, hair dyes, metals and resins are tested because they are the most common substances to trigger the condition. During the procedure, an allergen is applied to a patch, which is then placed on the skin.
An epicutaneous (patch test) is used to identify delayed allergic reactions. During the test, the patient may be exposed to 20-30 different allergen extracts that commonly cause contact dermatitis. Caustic substances, like industrial solvents, are diluted to prevent skin damage.
Allergen extracts are applied to a patch that is worn on the arm or back for 48 hours. While the patch is on, the patient should not bathe, and he/she should try not to sweat heavily. The healthcare provider will remove the patch after 48 hours and observe the test site for allergic skin reactions.
If an allergen triggers an allergic reaction to a test, the patient will develop reddening, swelling or a raised, itchy red wheal (bump) that looks similar to a mosquito bite. The healthcare provider will measure the size of the wheal and record the results. The larger the wheal, the more severe the allergy is. The test site will be cleaned with rubbing alcohol to remove any remaining allergen extracts.
A positive skin test means the patient is allergic to a particular allergen. Larger wheals (bumps) usually indicate a higher degree of sensitivity. Wheals will resolve without treatment within a few days.
If there is no reddening, swelling or raised, itchy wheal at the test site, the results are negative, which suggest that the patient is not allergic to the particular allergen.
The accuracy of skin tests may vary. A false positive or false negative test result is possible. Other test results may be inconclusive. For instance, some patients may react positively to a substance during the test, but may not react to the allergen in everyday life.
Skin tests are most reliable when they are used to diagnose allergies caused by airborne allergens, including animal dander, dust mites and pollen. They are least reliable when they are used to diagnose allergies caused by contact.
Skin tests may produce a severe, immediate allergic reaction known as anaphylaxis, although this rarely happens during the test. Anaphylaxis is a rapid, immune-mediated (allergic), systemic reaction to allergens (like food, medication or insect stings) that the individual has previously been exposed to. Anaphylaxis is a medical emergency that requires immediate medical treatment, as well as follow-up care with an allergist or immunologist. Symptoms of anaphylaxis can vary from mild to severe and may be potentially life threatening. The most dangerous symptoms are low blood pressure, breathing difficulties, shock and loss of consciousness, all of which can be fatal. Epinephrine is a medication used to treat anaphylaxis. Administering the epinephrine as soon as possible improves the chances of survival and a quick recovery. Therefore, skin tests should be performed at a medical office that has appropriate medical emergency equipment, including epinephrine.
Patients should call their healthcare provider immediately if they experience fever, lightheadedness, shortness of breath, extensive rash, swelling of the face or mouth or difficulty swallowing 30 minutes to one hour after undergoing a skin test because they are signs of a severe allergic reaction that requires immediate medical attention.
This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).
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- Asthma and Allergy Foundation of America. www.aafa.org. Accessed March 6, 2007.
- DermNet NZ. Patch tests (contact allergy testing). http://dermnetnz.org.
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