When should food intolerance testing be considered?

When to Perform Food Intolerance Testing

Food intolerance testing should NOT be routinely performed, as most adverse food reactions are non-immunologic and lack validated diagnostic tests; instead, testing should be reserved for suspected IgE-mediated food allergy when there is a clear, reproducible clinical history of immediate reactions (occurring within 2 hours of ingestion) involving skin, respiratory, gastrointestinal, or cardiovascular symptoms. 1, 2

Distinguishing Food Allergy from Food Intolerance

The critical first step is understanding that "food intolerance" and "food allergy" are fundamentally different entities:

• Food allergy involves an immunologic mechanism (typically IgE-mediated) and presents with immediate symptoms including urticaria, angioedema, respiratory symptoms, gastrointestinal symptoms, or anaphylaxis occurring within minutes to 2 hours of exposure 1

• Food intolerance is non-immunologic (e.g., lactase deficiency, pharmacological effects of histamine, irritants) and does not have validated diagnostic tests in most cases 3, 4

• The number of patients who self-diagnose food intolerance far exceeds actual prevalence, leading to unnecessary dietary restrictions and nutritional deficiencies 5, 4

When Testing IS Indicated (IgE-Mediated Food Allergy)

Clinical Scenarios Warranting Testing:

In Children:

• Children under 5 years with moderate-to-severe atopic dermatitis that persists despite optimal topical treatment should undergo LIMITED testing for milk, eggs, wheat, soy, and peanuts 2, 1
• Any child with reproducible immediate reactions (within 2 hours) to specific foods involving skin, respiratory, or gastrointestinal symptoms 1, 6
• Children with symptoms during oral food challenges (70% of positive challenges in one study involved nasal symptoms) 1

In Adults and Children:

• Reproducible immediate symptoms after food exposure, particularly if involving multiple organ systems 1, 6
• History of anaphylaxis to foods (most commonly shellfish in adults; peanuts, tree nuts, fish, shellfish, milk, and eggs in children) 1
• Oral tingling or lip swelling after nut exposure, especially with comorbid asthma 6

Critical Caveat:

Do NOT perform extensive allergy testing without a clear medical history, as positive results may only reflect sensitization rather than true clinical allergy 2, 1. The negative predictive value of testing is high (>95%), but the positive predictive value is low (40-60%) 1, 2.

Recommended Testing Approach

First-Line Testing:

1. Skin Prick Testing (SPT) is preferred over serum-specific IgE testing 6, 1

   • Use commercial food extracts 1, 6
   • Interpretation: Wheal ≥8 mm is highly diagnostic; 3-7 mm requires oral food challenge for confirmation 6

2. Serum-Specific IgE Testing is appropriate when 1, 2:

   • Extensive eczematous lesions are present

   • Prominent dermatographism exists

   • Recent antihistamine use occurred

   • Interpretation: ≥15 kU/L is highly diagnostic; 0.35-14.99 kU/L is indeterminate and requires oral food challenge 6

Gold Standard Confirmation:

Double-blind, placebo-controlled food challenge (DBPCFC) is the definitive diagnostic test and should be performed when 1:

• Test results fall in the "grey area" (SPT 3-7 mm or IgE 0.35-14.99 kU/L)
• Discordance exists between clinical history and test results
• Determining if tolerance has developed over time

Important safety considerations for oral food challenges: Do NOT perform if recent anaphylaxis occurred (within 6-12 months), severe uncontrolled asthma is present, or test values indicate >95% likelihood of reaction 1, 6

When Testing Should NOT Be Performed

Avoid Testing In These Situations:

• No clear clinical history of reproducible symptoms after food exposure 2, 1
• Suspected non-IgE-mediated food intolerance (e.g., lactose intolerance, histamine sensitivity, FODMAP intolerance) - these require different diagnostic approaches, not allergy testing 5, 4
• Isolated rhinitis without other systemic symptoms in adults - food allergy rarely causes isolated rhinitis 1
• Behavioral symptoms in children - there is no objective data supporting food intolerance as a cause 7

Unvalidated Tests to Avoid:

Never use unvalidated commercial "food intolerance" tests including leukocytotoxic testing, IgG food antibody panels, or hair analysis - these lack scientific evidence and lead to unnecessary dietary restrictions 5, 4, 8

Special Populations

Food Protein-Induced Enterocolitis Syndrome (FPIES):

• This is a non-IgE-mediated condition presenting with severe vomiting and lethargy 2-3 hours after ingestion 1
• Skin testing and serum IgE are typically NEGATIVE 1
• Diagnosis is clinical, based on history and supervised oral food challenge using 0.15-0.3 g protein/kg body weight 1

Atopic Patch Testing:

• Not recommended for routine use due to difficult interpretation and conflicting evidence about utility 1

Management After Diagnosis

• A diagnostic elimination diet should last 4-6 weeks, followed by supervised oral food challenge to confirm diagnosis 2
• Avoid prolonged elimination diets without confirmed diagnosis - these cause nutritional deficiencies, growth delay, and calcium deficiency 2
• Most children develop tolerance to milk, egg, soy, and wheat over time; regular re-evaluation is necessary 1, 2
• Effective treatment for atopic dermatitis remains skin care and topical therapies, even with confirmed food allergy 1, 2