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Diagnosis of Food AllergyWhat to expect during your visit(s)
Oral ChallengeBecause many diagnostic tests can return a false positives result (positive test but no reaction when ingested) a food challenge may be necessary to confirm the results. A strong history of a reaction to a food may confirm the positive food allergy test. A positive result on a food challenge is considered proof of a food allergy.
Most food challenges are done in a controlled setting under direct doctor supervision. If the past historical food reaction was mild (i.e.- increased eczema) than the challenge may take place at home. If an oral food challenge has a risk of causing anything but the mildest of reactions, it will take place at the office. Rarely a patient with a history of a significant reaction will be challenged while in the hospital.
The suspected food is given in incrementally increasing doses until the total volume is equal to a normal serving. Each dose is given every 20-30 min and is usually split into 3-6 doses total. A Challenge is positive at the first clear sign of a reaction and no further doses will be given.
Differences Between Food Challenges and Elimination Diets: While elimination diets are better suited for narrowing down potential food allergies, or as a diagnostic tool for food intolerances, food challenges have a few advantages over elimination diets for confirming food allergies:
Allergy Skin Testing for Food Allergy
Skin Prick Testing (SPT) is the
preferred method for evaluating patients with food allergies. The
test can be done in children as young as a few months of age. Tiny
drops of purified allergen extracts are scratched into your skin's
surface (virtually pain free). The site is then monitored for the
development of redness and swelling (a positive looks like a
mosquito bite) at the site of administration over the next 15-20
minutes. A positive SPT indicates a possible association between the food being tested and a patient’s reaction to the suspected food. In general only about 50% of patients with a positive SPT will have symptoms to that food. A negative SPT is about 95% predictive (if it is negative 95% chance that there is no clinical symptoms). In some instances we will use fresh fruit or vegetables (prick to prick method) to test for certain allergies.
Blood Test for Food AllergyRAST Testing is a blood test (more painful than SPT) whereby a patient’s IgE, the allergy antibody, is measured against a specific food. This is considered slightly less reliable than the SPT because there is a higher false positive rate than the SPT (a smaller chance that a positive test will correlate with symptoms). A negative test is similar as SPT. This study is sent to a laboratory therefore the results and the results take approximately 1-2 weeks (SPT results are available in 15 min). This test is generally used when there are other factors that preclude the use of the SPT such as severe atopic dermatitis (eczema), anaphylaxis to the suspected food, patients that cannot discontinue antihistamines. RAST tests can be monitored over time. If the value decreases to an acceptable level, a food challenge can be performed under physician supervision in the office.
Atopic Patch TestingPatch testing examines for non-IgE mediated reactions. These reactions are often delayed, occurring days after ingestion of the food. Many patients with non-IgE mediated reactions have difficulty in identifying the food causing the reactions. Patch testing was first done in 1890’s for reactions to perfumes, dyes and metals. Patch testing for foods have been done since 1990's In patch testing, fresh foods are prepared and placed on aluminum chambers on the patient’s back for 48 hours.
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Food Allergy Diagnosis
Comprehensive Food Allergy Clinic of Michigan
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