What blood investigations are recommended to identify potential allergens in a patient suspected of having an allergy?

Blood Investigations for Allergy Identification

Allergen-specific IgE blood testing is the primary blood investigation for identifying allergens, but it should only be ordered when skin prick testing is contraindicated or impractical, as skin testing remains the preferred first-line diagnostic approach. 1, 2

When to Use Blood Testing Instead of Skin Testing

Blood-based allergen-specific IgE testing is indicated in specific clinical scenarios where skin testing cannot be performed: 1, 2

• Extensive dermatitis or severe dermatographism that prevents reliable skin test interpretation 3, 2
• Inability to discontinue antihistamines or other medications that suppress skin reactivity (tricyclic antidepressants) 3, 2
• Patients on β-blockers where anaphylaxis risk from skin testing is elevated 1, 2
• Uncooperative patients or very young children where blood draw is more practical than skin testing 2
• History suggesting unusually high anaphylaxis risk from skin testing 1

Primary Blood Test: Allergen-Specific IgE Immunoassay

The standard blood investigation uses fluorescence enzyme-labeled immunoassays (not the outdated RAST) to detect IgE antibodies: 3, 1

• Mechanism: Serum is incubated with allergen absorbed on a solid phase, then bound IgE is measured using labeled anti-IgE antibodies 3, 1
• Sensitivity: Approximately 70-75% compared to skin prick testing (which exceeds 80%) 1, 2
• Specificity: Similar to skin testing at approximately 90.7% for common allergens 2
• Advantages: No anaphylaxis risk, unaffected by antihistamines, safe with β-blockers 1
• Disadvantages: Less sensitive than skin testing, requires reliable laboratory, potential for inter-laboratory variability 3, 1

Selecting Which Allergens to Test

Base allergen selection strictly on clinical history—never order random screening panels: 2

• Consider age, symptoms, geographic location, and suspected exposures when selecting specific allergens 2
• For children under 5 years with moderate-to-severe atopic dermatitis, test common food allergens: cow's milk, eggs, wheat, soy, peanut 2
• For suspected inhalant allergies, select allergens based on environmental exposure patterns and symptom timing 3, 1

Interpreting Blood Test Results: Critical Limitations

A positive allergen-specific IgE result indicates sensitization, NOT clinical allergy—this distinction is essential: 1, 2, 4

• Negative predictive value: >95% (negative test reliably excludes allergy) 1, 2
• Positive predictive value: Only 40-60% (positive test requires clinical correlation) 1, 2
• Confirmation required: Positive tests must be confirmed with supervised oral food challenge (the gold standard) before implementing dietary elimination 3, 2

Additional Blood Tests for Specific Scenarios

Serum Tryptase (For Acute Reactions)

Serum tryptase confirms mast cell degranulation during suspected anaphylaxis: 3

• Timing: Draw during acute reaction and at baseline (after recovery) 3
• Interpretation: Increase above 1.2 × baseline + 2 mg/L is clinically relevant 3
• Utility: Confirms allergic mechanism, helps rule out mast cell disorders 3

Component-Resolved Diagnostics (CRD)

Order CRD only as second-line testing when initial allergen-specific IgE results are equivocal—never as first-line: 1

• CRD identifies specific allergenic proteins within whole allergen extracts 1
• Positive CRD to storage proteins indicates genuine sensitization with high likelihood of clinical allergy and severe reaction potential 1
• Can reduce need for oral food challenges by nearly two-thirds 1

Tests NOT Recommended

Do not order these blood tests for allergy diagnosis: 3

• Total serum IgE: Insufficient evidence for diagnostic utility in food or inhalant allergy 3
• Ratio of specific IgE to total IgE: Offers no advantage over specific IgE alone 3

Common Pitfalls to Avoid

• Never diagnose allergy based solely on positive blood test without clinical correlation—50-90% of presumed allergies are not confirmed 3, 2
• Do not order testing without clear clinical indication—testing should confirm suspected allergens based on history, not screen randomly 2
• Recognize inter-laboratory variability: Results from different assay systems (ImmunoCAP, Turbo-MP, Immulite) are not interchangeable, and predictive values established for one system cannot be applied to others 3
• Remember that blood testing is generally less sensitive than skin testing (70-75% vs >80%), so negative blood tests with high clinical suspicion may warrant skin testing 1, 2