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Posted on Thu, Dec 1, 2011 : 5 a.m.

Scratch test reveals triggers for allergic reaction

By Ask Dr. K


Over the past year I've suddenly developed allergies. The problem is, I don't know what I'm allergic to. My doctor wants to do a scratch test. What can I expect?


Scratch tests are often done in cases like yours, when allergy symptoms have no clear trigger. The tests usually are done by allergy specialists.

A scratch test checks for a skin reaction to substances that often cause allergies. These may include foods, molds, dust, plants or animal proteins. Your doctor will decide what allergies to test for, possibly up to a few dozen at one visit.

You should be asked not to take antihistamines, which can interfere with the test results, for three days before the test. If the doctor does not tell you that, call his or her office and check.

Here's what happens in the test: First, your doctor will drip drops of fluid in rows across the skin of your forearm. (In children the test is done on the upper back.) Each drop contains one substance, such as cat dander, that you might be allergic to. Your doctor will note where each drop of fluid is placed.

Then the doctor will make small, light scratches with a needle in the skin under each drop. This helps the substance you might be allergic to, to get under the surface of the skin. That's where the immune system can "see" it, and produce an allergic reaction against it.

That's it. It's all over, except for the waiting. And this can be difficult. You have to stay still for about 20 minutes. Some of the scratches are going to start itching, but you won't be allowed to scratch.

At the end of the waiting time, your doctor will examine each needle scratch for redness or swelling. Your doctor can tell you right away which substances caused a reaction.

A serious allergic reaction during a scratch test is extremely unlikely. It's never happened to any patient of mine that I have sent to an allergy doctor. Serious reactions include a drop in blood pressure, shortness of breath and other symptoms.

To be safe, tell your doctor if you have ever had a life-threatening allergic reaction or a serious reaction to a previous allergy test. The doctor may not want to test for that particular allergy, or may want to have special treatments all ready to go if you do start to have a serious reaction.

We have a lot more information on allergies in our Special Health Report called "What to Do About Allergies." You can find out more about it at my website.

Knowing what you are allergic to can help in at least two ways. First, you know what to avoid. Second, for some allergic substances, there are special shots -- desensitization treatments -- that can greatly reduce your allergic symptoms.

Many people don't need allergy scratch tests to know what they are allergic to. But people like you who just know they are allergic to something can benefit from the test.

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information:

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