One common skin test is a scratch test. For this test, a doctor or nurse will scratch the skin with a tiny bit of liquid extract of an allergen (such as pollen or food). Allergists usually do skin tests on a person's forearm or back. The allergist then waits 15 minutes or so to see if reddish, raised spots (called wheals) form, indicating an allergy.
If the doctor thinks someone might be allergic to more than one thing — or if it's not clear what's triggering a person's allergy — the allergist will probably skin test for several different allergens at the same time.
When a skin test shows up as positive with a certain food, that only means a person mightbe allergic to that food. In these cases, doctors may want to do additional testing.
To diagnose a food allergy for certain, an allergist might do a blood test in addition to skin testing. This involves taking a small blood sample to send to a laboratory for analysis. The lab checks the blood for IgE antibodies to specific foods. If enough IgE antibodies to a particular food are in the blood, it's very likely that the person is allergic to it.
If the results of the skin and blood tests are still unclear, though, an allergist might do something called a food challenge. During this test, the person is given gradually increasing amounts of the potential food allergen to eat while the doctor watches for symptoms.
Skin tests may itch for a while. If your child undergoes one, the allergist might give you an antihistamine or steroid cream for your child to use after the test to lessen the itching.