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Lundgren, Linnea, and Jeff Wald. "Allergy Treatment Basics." 03 May 2006. HowStuffWorks.com. <http://health.howstuffworks.com/allergy-treatments-ga.htm> 17 May 2008.
by Linnea Lundgren and Jeff Wald, MD
Inside This Article
A Guide to Allergy Tests
There are several kinds of allergy tests, the most common of which are skin tests and blood tests. If your doctor wants to do skin tests, you may need to schedule another appointment for them, as they take some time to administer and read. You'll also have to schedule an appointment if you've recently taken an antihistamine or other medication, as they can interfere with the results.This section explains the various types of allergy tests and what they can tell you and your doctor.
Scratch or Prick Tests
These skin tests involve applying a small diluted drop of the suspected allergen to the skin and then scratching or pricking the skin so that the allergen is absorbed. Typically, many allergens are tested at one time, and most allergists use panels based on the allergens found in their community. The arm is used for a smaller number of tests; the back is used for more extensive tests. If you are allergic to a particular allergen, you will have a reaction to it in the form of redness and/or a bump in less than 20 minutes. The doctor or an assistant will inspect the scratch areas and write down a number corresponding to the size and extent of the reaction. Skin tests aren't painful, but they can be itchy. Occasionally the arm swells.
Intradermal Tests
Another type of skin test, the intradermal test, involves injecting a minute amount of the irritant allergen under the skin. Intradermal tests are more sensitive and can better identify substances to which you have a weak allergy. Most board-certified allergists do scratch tests first and then follow up with intradermal testing. Although people often fear this kind of testing because of the needle, the needle is small and is barely slipped under the skin.
Each type of skin test has advantages and disadvantages. The scratch/prick tests let the doctor test a large number of allergens at once. Intradermal tests provide more accuracy in determining certain allergens. Both tests are typically used to determine common allergens, such as cat dander, mold spores, and weeds. They are not always valid for food or chemical allergies.
Whether you take a scratch or an intradermal test, you won't be able to leap out of the doctor's office afterward. You'll be required to wait at least 30 minutes in case you develop a stronger reaction.
Patch Tests
Patch tests are most often used to determine the source of contact dermatitis. The doctor will apply square adhesive patches, each containing potential allergens, to your back. Patients usually wear these patches for about 48 hours because reactions to them can be slow to develop. You can't take a bath or shower while the patches are in place. Like the scratch and prick tests, the area exposed to the allergen will be swollen and red if you're allergic. Some itching might occur.
Blood Tests
Blood tests are often performed on people who have had life-threatening allergic reactions or who have extensive rashes or skin problems that make skin testing impractical. If you have food allergies, blood tests can reveal the specific allergens. Called RAST (or radioallergosorbent test), the blood test measures the amount of IgE antibodies your body has produced as a result of exposure to a certain allergen. No test is perfect, and RAST can both over-diagnose and miss certain allergies.
Additionally, it is more expensive than skin testing. RAST tests can vary from lab to lab. Recent data suggest that the CAP RAST is the most reliable form of this test.
Double-Blind Food Challenge
Another way to detect food allergies is with the food challenge. The proteins found in food are hidden in gel capsules, and the patient is given those capsules as well as placebo capsules. Reactions to the various capsules are noted. This test eliminates subjective conclusions on the part of the patient or the doctor because neither knows which capsules contain the food proteins before the patient takes them (that's why it's called "double blind"). However, the test is time-consuming, complicated, and expensive, and the physician may not note delayed reactions.
A less intrusive method, although time-consuming and susceptible to patient subjectivity and error, is the trial-elimination method. A limited selection of foods are eliminated from the diet over a course of three days to three weeks. Each food is then gradually restored to the diet and any reactions are noted.
All allergy tests, while effective, can provide misleading results. The results always must be interpreted in the context of the patient's history. That's why your history and your detective work are so crucial to the proper diagnosis.
However, there are also tests you should avoid, due to their questionable effectiveness.
Allergy Tests to Avoid
Some allergy tests remain questionable in the eyes (and research) of the medical profession. Many are based on treatments whose safety and effectiveness are unproved. This section will cover a few of the allergy tests that are considered questionable. Before starting any treatment, it is best to consult your doctor.
The Cytotoxic Test
In this food allergy test, a patient's white blood cells are mixed with the food antigen. The cells are examined for any changes, which determine a food allergy. Problems: The test may show a negative result if the food wasn't eaten recently. The expertise of the technician reading the results can affect the test's accuracy. Lastly, it is expensive.
Applied Kinesiology
The allergen is placed in the hand of the patient and the tester pushes on the other arm to determine muscle response. If the person is allergic to the allergen, the tester will be able to push the arm down with little resistance. If the person isn't allergic, the tester will have great difficulty pushing the arm down.
The Pulse Test
In this test, the pulse rate is measured following exposure to a particular allergen.
The Provocation-Neutralization Test (also known as the Intradermal Provocation Technique or Sublingual Provocation and Neutralization)
The provocation part: Progressively stronger doses of an allergen are given via injection (intradermal) or under the tongue (sublingual) until symptoms appear. The neutralization part: Doses are decreased until symptoms disappear.
The IgG RAST Test
Advocates of this test claim this is another allergy response, but this has not been proved. In fact, IgG responses are often protective. This is one of the tests used to measure the effectiveness of venom immunotherapy. The meaning of the test is questionable at best and subject to misinterpretation.
While the labryinth of allergy tests is hard to get through, it can also be a tough task to choose an allergy medicine. The next section will give you the information needed to select an appropriate medication.
This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.
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Please copy/paste the following text to properly cite this How Stuff Works article:
Lundgren, Linnea, and Jeff Wald. "Allergy Treatment Basics." 03 May 2006. HowStuffWorks.com. <http://health.howstuffworks.com/allergy-treatments-ga.htm> 17 May 2008.
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