What are the recommended tests for diagnosing a food allergy in a patient?

Diagnostic Testing for Food Allergy

The gold standard for diagnosing food allergy is the oral food challenge (double-blind, placebo-controlled), but in clinical practice, diagnosis relies on a structured approach combining detailed clinical history, skin prick testing or allergen-specific serum IgE testing to identify potential triggers, and oral food challenges when needed for confirmation. 1, 2

Initial Diagnostic Steps

Clinical History and Physical Examination

• Obtain a detailed medical history focusing on symptoms occurring within minutes to hours after food ingestion, particularly if reactions have occurred on multiple occasions with the same food. 1, 2
• Look specifically for anaphylaxis or combinations of symptoms including laryngeal edema, wheezing, urticaria, angioedema, nausea, vomiting, diarrhea, or hypotension. 1, 3
• History and physical examination alone cannot diagnose food allergy—confirmation is mandatory because 50-90% of presumed food allergies are not true allergies. 1

Primary Laboratory Testing

Recommended Tests for IgE-Mediated Food Allergy

Skin Prick Testing (SPT)

• Perform skin prick testing as the initial technique for detecting IgE-mediated food allergies, directed by clinical history. 1, 2
• SPT assists in identifying foods provoking IgE-mediated reactions but alone cannot diagnose food allergy. 1

Allergen-Specific Serum IgE Testing

• Order allergen-specific serum IgE testing as the primary laboratory test, using modern fluorescence-labeled antibody assays such as ImmunoCAP for optimal accuracy. 1, 2
• A negative result (threshold <0.35 kUA/L) has high negative predictive value and effectively rules out IgE-mediated allergy. 2
• Higher allergen-specific IgE levels correlate with increased probability of clinical reactivity, though predictive thresholds vary by allergen. 2
• Positive results require clinical correlation—sensitization does not equal clinical allergy. 2, 4

Tests NOT Recommended

Avoid the following tests:

• Do not use intradermal testing for food allergy diagnosis. 1
• Do not measure total serum IgE routinely, as it lacks specificity for individual allergens. 1, 2, 4
• Do not use atopy patch testing (APT) in routine evaluation of non-contact food allergies. 1, 2, 4
• Do not use combinations of SPT, sIgE, and APT for routine diagnosis. 1

Confirmatory Testing

Oral Food Challenges

The double-blind, placebo-controlled food challenge is the gold standard for definitive diagnosis. 1

When to use oral food challenges:

• When history and laboratory tests are inconclusive or contradictory. 1, 5
• To confirm suspected food allergy before implementing long-term dietary restrictions. 1
• To determine if a food allergy has been outgrown. 3

Alternative challenge formats:

• Single-blind or open-food challenges may be diagnostic under specific circumstances: negative challenges rule out food allergy; positive challenges with objective symptoms that correlate with history and laboratory tests support the diagnosis. 1

Advanced Testing for Specific Scenarios

Component-Resolved Diagnostics

• Use component-resolved diagnostics (measuring IgE to specific allergen components) in specialized centers when standard tests are equivocal. 2
• Examples include Ara h 2 for peanut, Ana o 3 for cashew, and Gal d 1,2,3,5 for egg. 2
• This approach can reduce the need for oral food challenges by almost two-thirds when used in a 2-step algorithm. 2

Non-IgE-Mediated Food Allergies

Food Protein-Induced Enterocolitis Syndrome (FPIES)

• Diagnose using medical history and oral food challenge. 1
• When history indicates hypotensive episodes or multiple reactions to the same food, diagnosis may be based on history alone without challenge. 1

Eosinophilic Esophagitis (EoE)

• SPT, sIgE, and APT may help identify associated foods, but these tests alone are insufficient for diagnosis. 1
• Use elimination diets targeting 1 or a few specific foods to identify responsible foods in EoE and other non-IgE-mediated disorders. 1

Critical Pitfalls to Avoid

• Never diagnose food allergy based on laboratory tests alone without clinical correlation. 2, 4
• Do not use different laboratory assay systems interchangeably—predictive values from one system cannot be applied to another. 2, 4
• Recognize that many patients have positive tests without clinical symptoms—sensitization does not equal clinical allergy. 2, 4
• Do not accept patient or parent reports of food allergy without confirmation, as the majority are not true allergies. 1
• Avoid presumptive diagnosis based solely on history and skin/serology tests except in cases of severe anaphylaxis after isolated ingestion of a specific food. 6