Blood Tests for Allergy Diagnosis
The primary blood test for diagnosing allergies is allergen-specific serum IgE (sIgE) testing, which identifies IgE-mediated sensitization to suspected allergens—but this test alone cannot diagnose clinical allergy and must always be correlated with clinical history. 1, 2
Primary Laboratory Test: Allergen-Specific Serum IgE
Allergen-specific serum IgE testing is the recommended blood test for identifying foods, inhalants, or other allergens that potentially provoke IgE-mediated allergic reactions. 1, 2 This test uses modern fluorescence-labeled antibody assays (such as ImmunoCAP) rather than the outdated RAST terminology, which should be abandoned. 1, 2
How to Interpret sIgE Results:
Negative results (<0.35 kUA/L) have excellent negative predictive value (>95%) and effectively rule out IgE-mediated allergy. 2, 3 If the test is negative, the patient is not allergic via IgE mechanisms. 1
Positive results indicate sensitization only—not necessarily clinical allergy. 1, 2 The presence of sIgE antibodies means the immune system has responded to the allergen, but 50-90% of presumed allergies based on positive tests alone are not actual clinical allergies. 3
Higher sIgE levels correlate with increased probability of clinical reactivity, though predictive thresholds vary by allergen, patient age, and population studied. 1, 2
Critical Diagnostic Algorithm:
Start with detailed clinical history: Look for symptoms occurring within minutes to hours after allergen exposure, especially if reproducible on multiple occasions. 1, 2, 3
Order allergen-specific sIgE testing directed by clinical history—never order broad panels without clinical justification. 1, 2, 4
Interpret results in clinical context: Negative sIgE rules out IgE-mediated allergy; positive sIgE requires correlation with symptoms before diagnosing allergy. 2, 3
Consider oral food challenge if diagnosis remains uncertain, as this remains the gold standard for definitive diagnosis. 1, 2, 3
When sIgE Blood Testing is Particularly Useful:
Serum testing is especially valuable when skin prick testing cannot be performed due to extensive dermatitis, dermatographism, or inability to discontinue antihistamines. 1, 3 Unlike skin testing, blood tests are not affected by antihistamine use and carry no risk of systemic allergic reactions. 1
Advanced Testing: Component-Resolved Diagnostics (CRD)
Component-resolved diagnostics measure IgE to specific allergen molecular components and should be ordered when initial whole allergen tests are equivocal (e.g., sIgE 0.35-15 kUA/L). 2, 4 Examples include Ara h 2 for peanut allergy and Gal d 1,2,3,5 for egg allergy. 2, 4 Using CRD in a two-step algorithm can reduce the need for oral food challenges by almost two-thirds. 2, 4
Never order CRD as a first-line test—it should follow initial sIgE testing guided by clinical history. 4
Tests That Are NOT Recommended:
Total serum IgE should not be used to diagnose allergies due to lack of specificity for individual allergens. 1, 2 While elevated total IgE is common in atopic individuals, it does not identify which allergen is causing symptoms. 1
IgG or IgG4 food testing is not recommended for assessment of food allergy. 1
Atopy patch testing should not be used in routine evaluation of non-contact food allergies due to lack of standardization and variable sensitivity/specificity. 1, 2
Critical Pitfalls to Avoid:
Never diagnose allergy based on laboratory tests alone—a positive sIgE test only indicates sensitization, not clinical allergy. 1, 2 Testing without suspicion of a symptom-provoking episode leads to misleading or inaccurate results due to false-positive findings. 1
Different laboratory assay systems provide significantly different measurements for the same serum samples, so predictive values from one system (e.g., ImmunoCAP) cannot be applied to another (e.g., Turbo-MP or Immulite). 1, 2, 3
Undetectable sIgE levels occasionally occur in patients with true IgE-mediated allergy, so when clinical history is highly suggestive, further evaluation with physician-supervised oral food challenge is necessary before declaring the patient non-allergic. 1
Special Considerations by Allergen Type:
Food Allergies:
For suspected food allergy, order sIgE testing only for foods that reasonably may have provoked the reaction based on clinical history. 1 Avoid standard panels of common foods offered by laboratories. 1 For egg allergy specifically, egg white-specific IgE is the standard blood test. 3
Inhalant Allergies:
For patients with inhalant allergies (pollen, dust mite, animal dander, mold), either skin testing or sIgE testing is appropriate as the initial diagnostic approach. 1, 5 Serum sIgE testing correlates with skin prick test results, though correlation is stronger in patients with more severe symptoms and higher total IgE. 1
Insect Venom Allergies:
Patients with anaphylactic reactions to insect stings should undergo sIgE testing or skin testing. 5, 6