Can Immunoglobulin G (IgG) and Immunoglobulin E (IgE) tests definitively diagnose specific allergies?

IgG and IgE Testing for Allergy Diagnosis

IgE tests can identify potential allergens but cannot definitively diagnose allergies on their own, while IgG tests are not validated for allergy diagnosis. 1, 2

IgE Testing: Capabilities and Limitations

Allergen-specific IgE (sIgE) Testing

• sIgE tests (blood tests) are useful for identifying foods and allergens that potentially provoke IgE-mediated allergic reactions 1
• However, these tests alone are not diagnostic of allergic conditions 1
• The presence of sIgE reflects allergic sensitization but not necessarily clinical allergy 1
• A positive test without clinical symptoms must be carefully interpreted as it may represent sensitization without clinical relevance 2

Key limitations of sIgE testing:

• Different laboratory systems may provide significantly different measurements for the same samples 1
• Predictive values vary between studies and populations 1
• The American Academy of Otolaryngology-Head and Neck Surgery notes that blood tests for specific IgE have approximately 70-75% sensitivity compared to skin tests 2

Total IgE Testing

• The routine use of measuring total serum IgE should not be used to make a diagnosis of allergy 1
• Insufficient evidence exists to support total serum IgE levels as a sensitive and specific test for food allergy 1
• The American Academy of Otolaryngology-Head and Neck Surgery states that measuring total IgE has limited diagnostic value on its own 2

Clinical Correlation is Essential

• Immunotherapy should not be given to patients with positive test results for specific IgE antibodies that do not correlate with suspected triggers, clinical symptoms, or exposure 1
• The presence of specific IgE antibodies alone does not necessarily indicate clinical sensitivity 1
• The Mayo Clinic emphasizes that the presence of specific IgE without clinical symptoms is not sufficient for an allergy diagnosis 2

Proper Diagnostic Approach

Recommended testing sequence:

1. Skin prick testing is generally the preferred method of testing for specific IgE antibodies 1
2. In vitro testing for specific IgE antibodies is useful under certain circumstances (e.g., when skin tests cannot be done due to extensive dermatitis or when antihistamines cannot be discontinued) 1
3. Correlation with clinical symptoms and exposure history is mandatory 1

Interpreting results:

• Higher levels of sIgE may correlate with higher probability of clinical reactivity, but this varies by allergen and patient population 1
• The relationship between test results and clinical symptoms is stronger for inhalant allergens than for food allergens 3
• Cutoff values for specific serum IgE antibody levels may help distinguish symptomatic from asymptomatic allergy in patients with positive skin test results 4

Common Pitfalls to Avoid

• Assuming a positive IgE test automatically means clinical allergy
• Using IgG testing for allergy diagnosis (not supported by evidence)
• Relying on total IgE alone for diagnosis
• Failing to correlate test results with clinical history and symptoms
• Using different laboratory systems interchangeably without recognizing their different reference ranges

Remember that the definitive diagnosis of food hypersensitivity often requires provocation tests, as negative specific IgE does not rule out sensitization (7-18% of cases) 3.