Do You Have Allergies? How Testing Works

Reasons for skin vs. blood test

Do You Have Allergies? How Testing Works

Your immune system’s response to allergies can range from a mild, itchy rash to symptoms so severe that they send you straight to the emergency department. Testing can help you identify your allergy and help avoid future problems. But which kind works best — a skin test or a blood test? The answer depends on you.

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If you know what’s causing your allergy, it can help you avoid triggers and better manage the reaction, says allergist and immunologist Deepa Patadia, MD. She says both types of testing may help you find an answer.

How do allergy tests work?

While the tests are different, the goal is the same.

“Both the blood and skin allergy tests aim to detect the presence of an allergy antibody (IgE) specific for the allergen of interest,” Dr. Patadia says.

For a skin test, your doctor or a nurse will apply a possible allergen (or several of them) to a small area on your body with a device that pricks the skin. Your doctor also may draw your blood and send it to a lab to test for common allergy triggers.

What are the pros and cons of skin tests?

Pros. Skin tests are typically more sensitive than blood tests and allow your doctor to test for more allergens. And you’ll likely see a reaction quickly.

“You’ll get the results about 15 minutes after the test,” Dr. Patadia says. “So the patient and physician have that information immediately available for formulating a treatment plan.”

Cons. If you already have a rash or other skin condition, that may interfere with the testing. Some medications also may influence skin test results, she says.

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What are the pros and cons of blood tests?

Pros. Fewer factors can throw off blood test results.

“You can do blood allergy tests for patients who have skin conditions, are on some medications, or are otherwise unable to undergo skin testing,” Dr. Patadia says.

Cons. Blood allergy tests are not as sensitive as skin tests. They are typically more expensive. And, the results take time.

“Patients and physicians may have to wait for the lab results to return before making treatment decisions,” she says.

Which test is right for you?

The answer depends on you, Dr. Patadia says. A blood test, a skin test, or both may work best.

“The condition of the skin, medications and the patient’s goals, values and preferences all play a role in my testing recommendations,” she says.

You can do your part by giving your doctor as much information as possible about your symptoms. It may help to keep a diary for a week or two before your appointment.

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You’ll also want to provide a complete list of medications you are taking.

Ultimately, you and your doctor will make the best choice according to your specific medical history and situation, Dr. Patadia says. And, even then, you may not get a definitive answer right away.

False positives and false negatives

“With both methods there is potential for some false positive and false negative results,” she says. “A positive test does not always equate to an allergy and a negative test does not always indicate an absence of allergy.”

This is one reason why home allergy tests generally aren’t the answer.

“Allergy tests are just a tool in allergy diagnosis,” she explains. “Your allergist will interpret your test results in context with your clinical history to determine if you truly have allergies and to form a treatment plan with you.”

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