Asthma and Allergy Skin Testing
Many people with asthma are allergic to things that they breathe in. These tiny particles that can cause allergic reactions are called "allergens" [pronounced AL'-ER-GENZ]. Examples of airborne allergens are the pollens of grasses, trees, and weeds, spores of molds, danders of cats and dogs and other furry animals, debris from cockroaches, and house dust mites. People with a tendency to allergies will usually be allergic to some but not all such allergens; different people are allergic to different things.
Breathing in Allergens Can Make Your Asthma Worse and Can Cause Asthmatic Attacks
Day to day inhalation of allergens to which you are sensitive can worsen the inflammation of the bronchial tubes in asthma (see pamphlet entitled, What is Meant by "Inflammation" in Asthma). It can make your asthma more active (meaning that you are more likely to be troubled by cough, wheeze, shortness of breath, chest tightness, or nighttime awakenings from asthma symptoms) and make you more likely to have a serious attack of asthma. In addition, heavy exposure to an allergen to which you are sensitive can bring on a sudden attack of asthma or an attack that develops a few hours after the exposure.
It May Be Helpful to Determine the Allergens to Which You Are Sensitive
You and your doctor may decide that it would be helpful to know to which allergens you are sensitive. At the Asthma Center we use this information less often for the purpose of initiating "allergy shots" (also called desensitization injections) and more often to help reduce your exposure to those things that may specifically worsen your asthma. For instance, if you are strongly allergic to the house dust mite, simple actions can be taken in the home, and especially in the bedroom, that will decrease the amount of allergen from dust mites that you breathe in, causing a likely improvement in your asthma.
Reviewing Your Own Experiences With Allergic Exposures
The process of determining your particular allergies to inhaled allergens begins by considering those exposures that have caused you to have asthmatic reactions in the past: Is your asthma worse in the Spring or Fall? Does it come on when you dust or vacuum? Does a damp, mildewy room cause you to have symptoms of asthma? These and similar questions are important to determining your asthmatic allergies and to interpreting the results of allergy testing.
Tests for Allergic Sensitivity
Besides reviewing your own past experiences, there are two principal methods to test for your sensitivity to various allergens. One involves a blood test to analyze for antibody proteins that your body may have made in reaction to particular allergens; the other involves testing for reactions in your skin to the same allergens that you might breathe in. At the Asthma Center we rely primarily on the latter (allergy skin tests) because they are more sensitive, less expensive, give immediate results, and can test a larger number of different allergens than the blood tests. This pamphlet discusses the details of this process of allergy skin testing.
How Allergy Skin Testing is Done
To test your reaction to an allergen, a drop of liquid containing the allergen in placed on your skin (generally the inside of your forearms is used). A small lance with a pinpoint is poked through the liquid into the top layer of skin. This type of skin test is called a "prick test." If you are allergic to the allergen, after about 2 minutes the skin begins to form a reaction. It becomes red, slightly swollen, and itchy: it makes a hive. The size of the hive is measured and recorded. The larger the hive, the more likely it is that you are allergic to the allergen tested.
Different Types of Allergy Skin Tests
Sometimes, if a very intense allergic sensitivity is suspected, then only a light scratch is made through the liquid that contains the allergen ("scratch test"). This way only a very little bit of the allergen is introduced into the skin. Other times, when it is particularly important to investigate sensitivity to a specific allergen, a small amount of the liquid is injected under the surface of the skin with a skinny needle ("intra-dermal test"). This method puts a little greater amount of the allergen into the skin.
The usual practice in allergy skin testing is to test all at once a group of common allergens to which one might be sensitive. At our Asthma Center we generally do 25-30 prick tests at one time. The 25-30 drops of liquid containing the various allergens are lined up in two rows on each forearm, then a prick is made through each one. Within about 5 minutes the results of the test are known.
What Are Some Possible Bad Reactions to Allergy Skin Tests?
Bad reactions to allergy skin tests are rare, but they can happen. In particular, it is possible for the small amount of allergen in the skin to set off an attack of asthma, and, even more rarely, a period of dangerously low blood pressure, called an anaphylactic reaction. Because allergy skin testing involves these risks, although very small, you will be asked to give written permission on a "consent form" prior to the testing, and you will be asked to stay under our medical observation for 1-2 hours after testing to ensure that no delayed reactions occur.
Interpreting the Results of Your Allergy Skin Tests
Interpretation of the results of the allergy skin tests will involve your doctor. The idea on which allergy skin testing for asthma is based is that if you make a reaction in your skin to a particular allergen (a "positive" result), you probably also will make a similar allergic reaction in the bronchial tubes of your lungs if you were to breathe in the same allergen. Although this is not always the case, it often is; and it is especially likely to be true if you have ever experienced asthmatic symptoms when exposed to the allergen. The opposite is also true and may provide you with equally useful information. If you do not make a reaction in your skin to the allergen (a "negative" result) and have not experienced asthmatic symptoms when you breathe in the allergen, you almost certainly do not have asthmatic sensitivity to that allergen.
Medications to Avoid Before Your Allergy Skin Testing
Finally, it is possible to get a false result on the allergy skin test if you have taken certain medications before the test that can interfere with the reaction in your skin. These medications are the group called antihistamines. Common examples of antihistamines are Benadryl®, Chlortrimeton®, Claritin®, Zyrtec®, Clarinex ®, and Allegra®. Some of these medications can affect the results of allergy skin tests for several days after you take them. As a result, you will be asked to avoid all antihistamines for several hours or days before the skin tests.