What lab tests are used to diagnose allergies?

Laboratory Tests for Allergy Diagnosis

The two primary lab tests for diagnosing allergies are allergen-specific serum IgE (sIgE) testing and skin prick testing (SPT), with sIgE blood tests being the preferred laboratory option when skin testing cannot be performed. 1

Primary Laboratory Test: Allergen-Specific Serum IgE

Allergen-specific serum IgE testing is the recommended blood test for identifying IgE-mediated allergic reactions, though it alone cannot diagnose clinical allergy without correlation to clinical history. 1, 2

Key Characteristics of sIgE Testing:

• Modern fluorescence-labeled antibody assays (such as ImmunoCAP) should be used for optimal accuracy and reproducibility. 2, 3
• A negative result (<0.35 kUA/L) has high negative predictive value (>95%) and effectively rules out IgE-mediated allergy. 2
• A positive result indicates sensitization only, not necessarily clinical allergy, and must be correlated with symptoms. 1, 2
• Higher sIgE levels correlate with increased probability of clinical reactivity, though predictive thresholds vary. 2

When to Use sIgE Blood Testing:

• When skin prick testing cannot be performed due to extensive dermatitis, dermatographism, or inability to discontinue antihistamines. 2
• As an alternative to skin testing in patients who cannot tolerate or access skin testing. 3

Skin Prick Testing (SPT)

While technically an in vivo test rather than a pure laboratory test, SPT is recommended as the initial technique for detecting IgE-mediated allergies and is often performed alongside or before blood testing. 1, 4

Important Limitations:

• SPT alone cannot be considered diagnostic of allergy—it must be correlated with clinical history. 1
• SPT is more specific but less sensitive than intradermal testing, though it correlates better with clinical sensitivity. 4

Tests NOT Recommended

Total Serum IgE:

Total serum IgE should NOT be used routinely to diagnose allergies. 1 This test lacks specificity for individual allergens and does not provide actionable diagnostic information.

Intradermal Testing:

Intradermal testing should NOT be used for routine food allergy diagnosis. 1 However, it may have limited utility in respiratory allergies when SPT is negative but clinical suspicion remains high. 5

Atopy Patch Test (APT):

The atopy patch test should NOT be used in routine evaluation of non-contact food allergies. 1

Combination Testing:

Combining SPT, sIgE, and APT is NOT recommended for routine diagnosis. 1

Advanced/Specialized Laboratory Tests

Component-Resolved Diagnostics (CRD):

Component-resolved diagnostics (measuring IgE to specific allergen components) is increasingly used in specialized centers for confirming specific allergies when standard tests are equivocal. 1

• Examples include Ara h 2 for peanut, Ana o 3 for cashew, and Gal d 1,2,3,5 for egg. 1
• CRD can reduce the need for oral food challenges by almost two-thirds when used in a 2-step algorithm. 1
• Limited global availability and lack of reference values for many populations restrict widespread use. 1

Other Specialized Tests:

• Basophil activation tests and allergen-specific IgG4 determination are research-based and available only in select specialized centers. 1

Critical Diagnostic Algorithm

1. Start with detailed clinical history: Symptoms occurring within minutes to hours after allergen exposure, especially if reproducible. 1, 2

2. Order allergen-specific serum IgE testing as the primary laboratory test, directed by clinical history. 1, 2

3. Interpret results in clinical context:

   • Negative sIgE effectively rules out IgE-mediated allergy 2
   • Positive sIgE requires clinical correlation—50-90% of presumed allergies are not actual allergies when formally tested. 1, 2

4. Consider oral food challenge if diagnosis remains uncertain, as this remains the gold standard. 1, 2

Critical Pitfalls to Avoid

• Never diagnose allergy based on laboratory tests alone—history and physical examination are essential first steps. 1
• Different laboratory assay systems provide significantly different measurements for the same serum samples, so predictive values from one system cannot be applied to another. 2
• Sensitization (positive test) does not equal clinical allergy—many patients have positive tests without symptoms. 1, 4, 6
• Do not use unvalidated or non-guideline-recommended tests that may lead to unnecessary dietary restrictions or interventions. 1