During skin prick testing, an allergy doctor (allergist) or nurse puts a tiny bit of a liquid containing an allergen into the skin by making a small scratch or prick on the skin. Allergists usually do skin testing on a person's forearm or back. The allergist then waits 15 minutes or so to see if a red, raised bump (called a wheal) forms. If it does, there might be an allergy. The allergist will use a ruler to measure the wheal and the redness around it.
If someone might be allergic to more than one thing — or if it's not clear what's triggering a person's allergy symptoms — the allergist may skin test for several different allergens at the same time.
Skin tests may itch for a little while, but this usually gets better after about 30 minutes. If your child is itchy after the test, the allergist might give them an antihistamine to take or apply a topical steroid cream to your child’s back or arm to ease the itching.
A skin test that shows up as positive to a food only means a person might be allergic to that food. The allergist may request a blood test as well, sending a small blood sample to a lab for analysis. The lab checks the blood for IgE to specific foods. Again, if there are IgE antibodies to a food, it means the person might be allergic to it.
If the results of the skin test and blood test are unclear, an allergist might do something called an oral food challenge. During an oral food challenge, a person slowly eats increasing amounts of the potential food allergen while the doctor watches for symptoms.
Because food allergies can trigger serious reactions in people, this test must be done in an allergist's office or a hospital that has access to medicines and specialists to control these reactions. Most of the time, this type of test is done to find out if someone has outgrown a known food allergy.
What Happens if a Test Shows an Allergy?
If your child does have an allergy, the allergist will work with you on a plan to keep your child safe. You’ll learn how to help your child avoid foods that aren’t safe. The doctor also will give you a prescription for emergency medicine (called epinephrine) to keep with your child all the time in case of a severe allergic reaction.