Patient Basics: Scratch Test for Allergies

Originally published by Harvard Health.

What is the test?

This test checks for a skin reaction to common allergy-provoking substances, such as foods, molds, dust, plants, or animal proteins. If your skin reacts to a substance, chances are that you are allergic to it.

Most people with allergy symptoms don’t need testing because they can identify their triggers and control their symptoms with medicine. Your doctor might recommend scratch testing when you have severe allergy symptoms but are not sure what is causing them. Knowing what you are allergic to can help you avoid the substance in the future, and will help your doctor determine whether you might benefit from allergy shots.

How do I prepare for the test?

Don’t take antihistamines for three days before the test, because they can interfere with the results by preventing you from reacting to a substance you would normally be allergic to. Tell your doctor if you have ever had anaphylaxis, a life threatening allergic reaction, or if you have had a serious reaction to a previous allergy test.

What happens when the test is performed?

In adults, the test is done on the forearm; in children it’s done on the upper back. (The child disrobes from the waist up and lies on his or her stomach.)

Your doctor decides what allergies are to be tested for. Some people are tested for as many as a few dozen at one visit. Individual drops of fluid are dripped in rows across the skin. The doctor uses a needle to make small light scratches in the skin under each drop, to help the skin absorb the fluid. The scratches aren’t deep enough to cause bleeding. Each drop contains proteins from a separate allergen (a substance, like ragweed pollen, that triggers allergy symptoms).Your doctor notes where each drop of fluid was placed, either by keeping a chart or by writing a code on the area of skin being tested.

For many people, the most difficult part of this test is next: You need to stay still long enough (usually about 20 minutes) to give the skin time to react. Your skin might tickle or itch during this time, but you won’t be allowed to scratch it. At the end of the waiting time, your doctor will examine each needle scratch for redness or swelling.

What risks are there from the test?

Because the allergen exposure is so small, a serious allergic reaction is extremely unlikely. For a few hours you’ll probably have some redness or irritation on the testing sites, similar to having several mosquito bites.

Must I do anything special after the test is over?

No.

How long is it before the result of the test is known?

Your doctor can tell you right away which substances caused a reaction. The next step is to see if you can prevent an allergic reaction by avoiding those substances, or relieve symptoms by taking medicine or allergy shots.