PSU Medical Minute: Food Allergies … Testing and Diagnosis
A peanut butter and jelly sandwich.
Milk and cookies.
Comfort food, or food to fear?
Food allergies affect 4 percent of adults and 5 percent of young children. Many more people suspect that they have food allergies when they may not. Before you change your diet, which may put you at risk for nutritional deficiencies or may affect your quality of life, it is important that you receive the correct diagnosis. You should meet with your physician to discuss your concerns and to undergo further evaluation.
Your doctor often will perform testing when you have certain symptoms that occur shortly after eating, most often associated with a particular food. These symptoms may include a rash, such as hives or eczema; mouth or throat itching, throat tightness or trouble breathing; vomiting, abdominal pain or diarrhea; or a drop in blood pressure.
Both allergy skin testing and allergy blood testing are commonly used to test for food allergies. Skin testing involves gently placing a small amount of a food extract under the skin using a plastic needle-like device (I work with children, so we do not use needles). If you are allergic, there will be redness and swelling at the skin test site. The testing is safe, the results are available within minutes and they can be discussed at the same office visit. With blood testing, I can look at the level of specific allergy antibody to the food that is present. However, the results from this test are not usually available the same day.
Positive tests, either by skin or by blood, only indicate the presence of allergy antibody to the food. The presence of allergy antibody alone does not mean that you will have a reaction when eating the food. There are other reasons why these tests can be positive. Your history is very important. An allergist is trained to interpret the skin testing, blood testing, and patient history to help determine if there is a true food allergy.
Also, testing alone cannot predict how you will react to a type of food. The larger the skin test reaction and the higher the amount of allergy antibody on the blood test can predict the likelihood of a reaction, but not the severity of a reaction. For example, people with larger skin test reactions to peanut are more likely to have a reaction when exposed to peanut as compared to people with small skin test reactions to peanut; however, they are not more likely to have more severe reactions.
If your history is consistent with a food allergy and the testing supports the diagnosis, then a physician supervised oral challenge—eating the food suspected of causing a problem—is often not necessary. If the diagnosis is uncertain, then an oral challenge is extremely important in determining if there is a true food allergy. Also, many children outgrow their food allergies. Food challenges are essential to see if a food allergy has been outgrown. A food challenge is the definitive test for food allergies – if a person can eat the food without any reactions, he or she is not allergic to it, no matter what the testing shows.
Tracy B. Fausnight, M.D., is division chief of Pediatric Allergy and Immunology at Penn State Hershey Children’s Hospital.