What is the process for diagnosing a food allergy?

Diagnosing Food Allergies: A Systematic Approach

The diagnosis of food allergy requires a structured approach beginning with a detailed medical history and physical examination, followed by appropriate testing including skin prick tests, specific IgE blood tests, and ultimately oral food challenges, with the double-blind placebo-controlled food challenge being the gold standard for definitive diagnosis. 1

Step 1: Initial Assessment

Medical History

A thorough medical history is essential but insufficient alone for diagnosis 1. Key questions include:

• Specific symptoms experienced
• Which food triggered symptoms and whether it caused symptoms more than once
• Quantity of food ingested
• Whether food was cooked or raw
• Timing of symptom onset relative to food exposure
• Whether the food can ever be consumed without symptoms
• Other potential factors (exercise, alcohol, NSAIDs)
• Whether symptoms occur at times unrelated to food exposure
• Treatment provided and symptom duration

Physical Examination

While no physical findings definitively diagnose food allergy, examination may reveal:

• Signs of atopic conditions (urticaria, atopic dermatitis)
• Evidence of prolonged symptoms (weight loss in eosinophilic esophagitis)
• Findings suggesting non-allergic disorders requiring further investigation

Step 2: Testing Methods

For IgE-Mediated Food Allergies

1. Skin Prick Test (SPT)

   • Recommended to identify potential IgE-mediated reactions 1
   • Cannot alone diagnose food allergy
   • Positive results indicate sensitization, not necessarily clinical allergy

2. Specific IgE Blood Tests

   • Useful for identifying foods that may provoke IgE-mediated reactions 1
   • Not diagnostic when used alone
   • Higher levels correlate with increased likelihood of clinical reactivity

3. NOT Recommended:

   • Intradermal testing (should not be used) 1
   • Total serum IgE (not useful for diagnosis) 1
   • Atopy patch testing alone (not recommended for routine evaluation) 1
   • Combination of SPT, sIgE, and atopy patch tests (not recommended for routine diagnosis) 1

For Non-IgE-Mediated Food Allergies

• Elimination diets are particularly useful for diagnosing non-IgE-mediated disorders such as:
  • Food protein-induced enterocolitis syndrome (FPIES)
  • Allergic proctocolitis
  • Heiner syndrome
  • Eosinophilic esophagitis 1

Step 3: Definitive Diagnosis

Elimination Diet

• Useful diagnostic tool, especially for non-IgE-mediated reactions
• Complete elimination of suspected food allergen for:
  • 2-4 weeks in IgE-mediated cases
  • 2-6 weeks in non-IgE-mediated cases 2

Oral Food Challenges

• Double-blind placebo-controlled food challenge (DBPCFC): Gold standard for diagnosis 1
• Open food challenge: More practical for routine clinical settings
• Should be performed under medical supervision with emergency equipment available

Important Considerations

Distinguishing Food Allergy from Food Intolerance

Food allergy is an immune-mediated response (commonly IgE-mediated), while food intolerance is a non-immunologic response that may be due to:

• Food additives (e.g., sulfates)
• Enzyme deficiencies (e.g., lactose intolerance)
• Pharmacologic effects of food chemicals 1

Confirming Self-Reported Allergies

50-90% of self-reported food allergies are not true allergies 1. All suspected food allergies must be confirmed through appropriate testing to avoid unnecessary dietary restrictions that can affect quality of life and nutrition.

Common Pitfalls to Avoid

• Overdiagnosis leading to unnecessary dietary restrictions
• Delayed diagnosis of true food allergies
• Inappropriate testing (using large general panels without clinical correlation)
• Inadequate patient/parent education 2

Special Considerations for Children

The prevalence of childhood food allergy in the United States is approximately 8% and has increased over the past decade 1. The most common food allergens are:

• Milk
• Egg
• Wheat
• Soy
• Peanut
• Tree nuts
• Finfish
• Shellfish 1

Management After Diagnosis

Once diagnosed, management includes:

1. Food avoidance education
2. Emergency medication prescription (antihistamines for mild reactions, epinephrine auto-injector for severe reactions)
3. Education on recognizing and treating allergic reactions
4. Nutritional guidance if major food groups are eliminated
5. Regular follow-up to assess for development of tolerance

When to Refer to an Allergist

Referral is indicated for:

• Anaphylaxis
• Multiple food allergies
• Significant feeding difficulties
• Persistent symptoms despite elimination diet
• Diagnostic uncertainty 2

Food allergy diagnosis requires a systematic approach that combines clinical history, appropriate testing, and often food challenges to confirm the diagnosis and guide management decisions.