Definition and Overview
Allergy testing is a procedure that can either be a skin test or a blood test done to determine the substances that may be causing an allergic reaction in a person. Substances that can trigger an allergic response are called allergens. Between the two types of allergy testing, skin testing is more commonly done than blood testing because of its practicality; it is easier to do, less expensive, and its results are more accurate.
A skin test is done by putting a small amount of a certain suspected allergen on the skin. The reaction that the skin will have after the administration of the suspected allergen will be observed to determine whether or not the skin is allergic to it. The substances can be administered in three ways: through a skin prick test, intradermal skin test, and patch test.
Generally, allergy testing is helpful in determining allergy treatment plans and in diagnosing allergic conditions such as hay fever, allergic asthma, dermatitis, food allergies, penicillin allergy, bee venom allergy, and latex allergy.
Who Should Undergo and Expected Results
Anyone experiencing any allergic reaction and needs more information about it can undergo allergy testing. It is safe for people of any age, whether adult, child, or infant, unless there are specific reasons why the doctor might suggest against it. Doctors may advise against skin testing for patients who have a history of allergic reactions, are suffering from skin conditions such as eczema or psoriasis, and are taking medications such as antihistamines, antidepressants, and medication for heartburn.
The results of allergy testing can be positive, negative, or abnormal. A positive result means that the person being tested is indeed allergic to the substance used. A negative result means that the substance does not cause an allergic reaction in the patient. In some cases, however, the result may be wrong or abnormal; for example, a person may have an allergic reaction to a certain substance during the skin test, even though he is not allergic to it normally. Although skin tests have been proven to be accurate, sometimes mistakes in the process of testing may cause abnormal results. For instance, if the dose of an allergen is somehow too much, it can result in a positive outcome even if the patient is not really allergic to the substance.
Some patients may also be limited to a specific type of testing technique. Patients who cannot undergo the skin test because of their situation can take the blood test instead. Skin tests are usually best in determining airborne allergies such as allergies to pet dander, dust mites, and pollen. Allergies involving food are more complicated and might require more than a skin test. On the other hand, medicinal allergies, except allergies to penicillin, should never be observed using the skin test.
How Does the Procedure Work?
There are three different methods of conducting an allergy testing procedure. One of these three methods is the skin prick test. In this test, a small amount of a suspected allergen is placed on the forearm, upper arm, or back before the skin is pricked. Within about 15 to 20 minutes, the skin is observed to see if any kind of allergic reaction such as swelling and redness will appear. Several substances can be tested at the same time.
The intradermal skin test uses the method of injection by inserting a certain amount of allergen into the skin. Like the skin prick test, the doctor then observes the skin for any kind of allergic reaction to the substance. Usually, the intradermal skin test is used if the doctor suspects that a previous skin prick test yielded suspicious or inaccurate results.
The third technique is called the patch test. Patch testing is the most ideal kind of skin test for detecting allergies that take longer to appear or manifest an allergic reaction. In this method, possible allergens are put on the skin using patches of tape. Unlike the skin prick test and the intradermal test, the doctor observes the skin, not immediately, but after about 72 to 96 hours. The patch test is useful for people who have a skin-related allergic condition called contact dermatitis.
A blood test can also detect an allergy to a certain substance. It works by examining the amount of antibodies in the suspected or affected area of the body. The blood level of a type of antibody called the immunoglobulin E or IgE is measured when the body responds to a certain allergen. Patients with allergies or asthma often have high IgE levels.
Possible Risks and Complications
Patients should expect to experience unpleasant feelings during an allergy testing. Since allergens are being tested on the actual skin of the patient, it is almost certain that the patient will experience an allergic reaction and its symptoms, if the patient is indeed allergic to the substance in question. The symptoms of an allergic reaction range from mild to moderately serious, such as swelling and the appearance of itchy red bumps or wheals. There is also a risk of a severe and life-threatening allergic reaction, although this is very rare.
Since an allergy test is conducted safely within the confines of a hospital or clinic and under the watchful eye of an attending physician, all allergic reactions that actually occur are promptly treated. If a person is proven to be allergic to a certain substance, the reaction can either be treated or the person will be asked to avoid that substance altogether. This will help prevent the symptoms and reduce the risk of complications.
Knowing what foods or substances a person is allergic to is an important part of his health maintenance. If it is possible, the best way to prevent an allergic reaction is to find out what is causing it and to avoid it. If it cannot be avoided, treatment may ensue, and this may take two forms: relieving allergy symptoms or treating the allergy itself to reduce allergic attacks even with exposure to the said allergen.
- Bernstein IL, Li JT, Bernstein DI, Hamilton R, et al. American Academy of Allergy, Asthma and Immunology; American College of Allergy, Asthma and Immunology. Allergy diagnostic testing: an updated practice parameter. Ann Allergy Asthma Immunol. 2008;100(3 Suppl 3):S1-S148.