How can you diagnose an onion allergy?

Diagnosing Onion Allergy

Onion allergy should be diagnosed through a combination of clinical history, skin prick testing, and food-specific IgE testing, with oral food challenge being the definitive diagnostic method when necessary. 1

Diagnostic Approach

Step 1: Clinical History Assessment

• Document specific symptoms after onion consumption:
  • Timing of symptoms (typically within minutes to hours after ingestion) 1
  • Specific symptoms experienced:
    • Cutaneous: hives, flushing, angioedema (occur in majority of cases) 1
    • Respiratory: nasal congestion, throat pruritus, stridor, cough, dyspnea (up to 70% of cases) 1
    • Gastrointestinal: abdominal pain, nausea, vomiting, diarrhea (up to 40% of cases) 1
    • Cardiovascular: dizziness, tachycardia, hypotension (up to 35% of cases) 1
  • Reaction to raw vs. cooked onion (some patients react only to raw onion due to thermolabile allergens) 2
  • Quantity of onion consumed that triggered symptoms 1
  • Consistency of reactions with repeated exposures 1
  • Presence of co-factors (exercise, alcohol, NSAIDs) 1

Step 2: Allergy Testing

1. Skin Prick Test (SPT):

   • First-line diagnostic test for IgE-mediated onion allergy 1
   • A positive result is a wheal ≥3mm larger than negative control 1
   • Note: Fresh onion extract may be needed as standardized extracts are not always available 1

2. Specific IgE Blood Testing:

   • ImmunoCAP or similar technology to detect onion-specific IgE antibodies 3, 4
   • Interpretation of results:
     • Class 1 (0.35-0.69 kU/L): Low level sensitization
     • Class 2 (0.70-3.49 kU/L): Moderate sensitization
     • Class 3-4 (3.50-17.49 kU/L): High sensitization
     • Class 5-6 (>17.50 kU/L): Very high sensitization 4

Step 3: Oral Food Challenge (if needed)

• Gold standard for definitive diagnosis 1
• Should be performed:
  • When history and testing results are inconclusive
  • To confirm clinical relevance of positive test results
  • To determine if a patient has outgrown an allergy
• Must be conducted in a medical facility with:
  • Personnel experienced in performing the procedure
  • Equipment and medications available to treat anaphylaxis
  • Starting with minute amounts of onion and stopping at first symptoms 1
• Can be performed as double-blind, single-blind, or open challenge 1

Special Considerations

Cross-Reactivity

• Potential cross-reactivity with other Allium family members (garlic, leeks, chives) 5, 4
• 80% of patients with onion-specific IgE also have garlic-specific IgE 4
• Some patients may show cross-reactivity with peach, though this appears limited (~30%) 3

Thermostability of Allergens

• Some patients react only to raw onion due to thermolabile allergens 2
• Others may react to both raw and cooked onion, suggesting presence of thermostable allergens 3
• Testing with both heated and unheated onion extracts may be necessary in some cases 2

Common Pitfalls in Diagnosis

• Over-reliance on self-reported symptoms: 50-90% of presumed food allergies are not true allergies 1
• Confusing food allergy with food intolerance: Ensure symptoms are consistent with immune-mediated reaction 1
• Using large general panels of food allergens: Tests should be selected based on patient's history 1
• Misinterpreting positive test results: A positive skin test or specific IgE alone is insufficient for diagnosis without clinical correlation 1
• Failure to distinguish between sensitization and clinical allergy: Many patients have detectable IgE without clinical symptoms 1

Follow-up Management

For patients diagnosed with onion allergy:

• Provide education on complete avoidance of onion in all forms 1
• Prescribe epinephrine auto-injector for those at risk of anaphylaxis 1
• Create an emergency action plan 1
• Consider referral to allergist/immunologist for ongoing management 1
• Schedule follow-up within 1-2 weeks after any anaphylactic reaction 1