What is the role of blood testing in diagnosing allergies?

Blood Testing for Allergy Diagnosis

Blood testing for allergen-specific IgE is a valid alternative to skin testing but should be reserved as second-line when skin testing cannot be performed, or used selectively based on specific clinical scenarios where it offers advantages. 1, 2

When to Use Blood Testing Instead of Skin Testing

Blood-based allergen-specific IgE testing is indicated when: 1, 2

• Widespread eczema or severe dermatographism prevents reliable skin testing 1
• Antihistamines cannot be discontinued (blood tests are unaffected by medications) 1
• Patient is on β-blockers or has comorbid conditions that preclude skin testing 1
• History suggests unusually high anaphylaxis risk (blood testing carries no risk of systemic reaction) 1
• Extensive dermatitis makes skin testing technically impossible 1

Advantages and Limitations of Blood Testing

Key Advantages: 1

• No risk of anaphylaxis during testing 1
• Not affected by antihistamines or other medications 1
• Safe for patients with contraindications to skin testing 1

Critical Limitations: 1, 2

• Less sensitive than skin prick testing (average 70-75% sensitivity vs >80% for skin tests) 2
• Requires reliable laboratory with potential for errors 1
• Different laboratory platforms yield significantly different measurements - predictive values from one system (e.g., ImmunoCAP) cannot be applied to another (e.g., Turbo-MP, Immulite) 1, 3, 4
• More expensive than skin testing 1

Critical Interpretation Principles

A positive blood test indicates sensitization only, NOT clinical allergy - correlation with clinical history is mandatory. 1, 3

• Negative predictive value exceeds 95% - undetectable specific IgE effectively rules out IgE-mediated allergy 1, 3
• Positive predictive value is less than 50% - many false positives occur 1
• Higher specific IgE levels correlate with increased probability of clinical reactivity, though predictive values vary between studies 1, 3
• Undetectable specific IgE occasionally occurs in true IgE-mediated food allergy - if history is highly suggestive, proceed to oral food challenge 1

Tests to AVOID

Do NOT order: 1

• IgG or IgG4 testing - provides no useful information for allergy diagnosis 1
• Total IgE measurement - has limited diagnostic value 1
• Standard laboratory panels of common foods without clinical suspicion 1

Recommended Diagnostic Algorithm

1. Start with skin prick testing as first-line when feasible - it is more sensitive, less expensive, and provides immediate results 1, 2

2. Use blood-based allergen-specific IgE testing when skin testing is contraindicated or impractical 1, 2

3. Order testing only for allergens suspected based on clinical history - avoid broad panels 1

4. Use modern fluorescence-labeled antibody assays (such as ImmunoCAP) rather than outdated RAST 1, 3

5. Consider Component-Resolved Diagnostics (CRD) as second-line when initial whole allergen tests are equivocal 2

6. Proceed to oral food challenge when diagnosis remains uncertain after testing, as this remains the gold standard 2, 3

Common Pitfalls to Avoid

• Testing without clinical suspicion leads to false-positive results and unnecessary dietary restrictions 1
• Interpreting positive tests as diagnostic of clinical allergy without clinical correlation 1, 3
• Comparing results between different laboratory platforms - each system uses different antigen preparations 1, 3
• Ordering IgG testing - this is not evidence-based and should never be done 1