What You Need to Know About Allergy Testing

Saira Sheikh, MD, an allergist/immunologist and rheumatologist with the UNC Allergy and Immunology Clinic and UNC Hospitals Rheumatology Specialty Clinic, answers common questions about allergy testing.

I think I have an allergy and should be tested. How is allergy testing done?

Allergy testing can be performed by skin test or blood test. Skin testing can be done by the “skin prick” method, in which a drop of the allergen is placed on the surface of the skin, which is gently scratched with a small plastic device. The second type of testing is called intradermal skin testing, in which a syringe is used to place a small amount of the allergen under the surface of the skin. Allergy testing can also be performed by drawing blood from a vein, just like any other blood draw.

Will the allergy test hurt?

The superficial scratch test doesn’t usually hurt, although some patients may experience discomfort. For blood testing, the discomfort is similar to what is associated with a routine blood draw.

If you’re exposing me to something I’m allergic to, isn’t there a chance I’ll have an allergic reaction?

There is a small theoretical risk—so it’s possible but extremely unlikely that you’ll have a systemic allergic reaction from allergy skin testing. A systemic reaction happens when inflammation spreads throughout the body. If there are any concerns related to this, blood testing can be performed instead.

How long does it take to perform an allergy test, and how long before I know the results?

Within 20 minutes after the skin allergy tests are placed, we are able to see the reaction on the surface of the skin—redness, warmth, bumps—that will indicate if you are sensitized to an allergen. Typically, skin prick testing is done on the upper back or the forearm because this yields more accurate results.

How do I know if I’m a candidate for skin testing or if I should have the blood test for allergies?

One reason you might not be a good candidate for skin testing is if you’re on medications that interfere with the testing. For example, if you’re on antihistamines, they can affect the response to allergy skin tests, thereby making these tests less accurate. When people are on antihistamines or other medications that can affect allergy testing, we ask that they stay off their allergy medicines for seven to 10 days to clear the medications from their system. In instances in which medications cannot be discontinued or skin tests cannot be performed, a blood test is more practical.

What should I tell my physician before getting tested for allergies?

When you see your allergist, it’s important to bring in a list of symptoms that you are experiencing—such as runny nose, itchy/watery eyes, sneezing, cough—anything that makes you suspect that you may have allergies, as well as what you think the potential triggers could be.

I know I can be tested for food and environmental allergies, but what about wasps or fire ants? How about checking for medication allergies?

In addition to testing for food and environmental allergies (trees, grasses, weeds, molds, dust mites, pets), you can also be tested for stinging insect hypersensitivity (honeybees, wasps, hornets, yellow jackets, fire ants) if you have a history of an allergic reaction.

The only medication for which standardized allergy testing is currently available is penicillin. However, in patients who have a suggestive history of allergy to a medication, your doctor, after obtaining further information, may be able to design custom allergy skin testing to help identify the culprit or to look for an alternative medication.

Who can get tested for allergies?

Children and adults can be tested for allergies. It’s best to consult with a health care professional trained in allergy/immunology to provide guidance for the best way to approach this.

If I have a positive skin or blood test to a food, does that mean I am allergic to the food?

A food allergy is diagnosed on the basis of a clinical history of reactivity and symptoms after exposure to that particular food, plus a positive skin or blood test. A positive test in the absence of symptoms does not necessarily mean you have a food allergy.

For example, if you eat eggs every day and have no problems or symptoms after eating them—even if by chance, you have a positive allergy test—it still does not mean you are allergic to the food. That’s why a consultation with a health care professional trained in allergy/immunology is essential in interpreting results of allergy testing.

If you think you might benefit from allergy testing, contact the UNC Allergy and Immunology Clinic at (984) 974-2645.