Food allergy symptoms may present at any time. Upon experiencing symptoms, patients can initially present in several medical settings. Typically, in emergency departments or walk-in clinics, only medical histories and physical exams will be evaluated.
When presenting during routine primary care visits with family physicians or pediatricians, medical histories can also be obtained. Additionally, primary care practitioners may also refer to an allergist if food allergy is suspected, as allergists can conduct diagnostic testing. Accurate diagnosis of food allergies can often be difficult. Symptoms can often be misinterpreted as another type of allergy or intolerance. Therefore, it is important to implement allergy testing appropriately and to understand who to test, why to test, and what the results of those tests indicate.
The remainder of this video will discuss some of the tests available to healthcare providers and patients. If a patient presents with symptoms of an allergic reaction, they will be given a medical history interview and a physical exam. A detailed medical history often provides evidence for the type of allergic reaction caused by food and suggests which foods may be involved. A physical exam during an acute reaction can help to identify symptoms consistent with a potential food allergy. However, these assessments alone are not sufficient to diagnose food allergy. If food allergy is suspected, additional tests are needed to confirm a diagnosis.
Blood tests evaluate allergen-specific factors in serum, such as IgE, that might help identify foods that provoke allergic reactions. As an example, allergen-specific serum IgE may be used for identifying potential food allergens but is not diagnostic of food allergy. Positive results have traditionally been considered serum IgE levels greater than or equal to 0.35 kilounits per liter. However, a positive result alone indicates sensitization and does not equate to a food allergy. Serum IgE levels have been found to correlate with the likelihood of clinically significant reactions to allergy-triggering foods, particularly when supported by the patient’s history. These cutoffs have been traditionally used to predict the likelihood of true reactivity (as opposed to asymptomatic sensitization). Without a clear history of an allergic reaction, the interpretation of blood tests can be difficult. Although reliable and reproducible, results can take hours to days.
Skin prick tests performed by allergists, involve the evaluation of wheal size in response to allergen introduction at or just below the skin surface. Wheal size is correlated with likelihood of clinical allergy. Quick and simple to administer, SPTs have high sensitivity, but low specificity, which may lead to over-diagnosis.
Both blood and skin-prick testing have their pros and cons and might still be inconclusive. In that case, additional testing may be needed to confirm a diagnosis of food allergy. To confirm, an allergist may perform an oral food challenge, or OFC. The OFC is a procedure in which a food is eaten slowly, in gradually increasing amounts, under medical supervision, to accurately diagnose or rule out a true food allergy. Before each subsequent dose, the patient is evaluated for symptoms. If symptoms occur, the allergist will stop the test and give medications as needed. Otherwise, feeding continues until a meal-sized portion is eaten.
A double-blind placebo-controlled OFC is considered the gold standard of confirming food allergy. However, the test incurs some risk for anaphylaxis and requires monitoring. Accurate allergy testing is a necessary step to help patients and caregivers avoid undue issues arising from misdiagnosis.
Look for additional resources on the symptoms and presentation, management, and barriers of food allergies.
