How do I order serum specific Immunoglobulin E (IgE) testing?

How to Order Serum Specific IgE Testing

Order serum specific IgE testing by selecting allergens based on clinical history (age, symptoms, geographic location, and suspected exposures), using fluorescence enzyme-labeled immunoassays (not outdated RAST), and understanding that results indicate sensitization rather than clinical allergy—positive tests require clinical correlation while negative tests effectively rule out IgE-mediated reactions. 1

When to Order Serum Specific IgE Testing

Order serum specific IgE instead of skin testing when:

• Extensive dermatitis or dermatographism prevents reliable skin testing 1
• Antihistamines cannot be discontinued (serum IgE is unaffected by antihistamines, unlike skin tests) 1, 2
• Patient is uncooperative or very young child where blood draw is more practical 1
• Risk of systemic reaction is a concern 1
• Confirming skin test results when clinical suspicion remains high 1

Selecting Which Allergens to Test

Base allergen selection on:

• For children <5 years with moderate-to-severe atopic dermatitis: Test cow's milk, eggs, wheat, soy, and peanut 1
• For older children/adults: Add tree nuts, shellfish, and fish based on history 1
• For aeroallergens: Select based on local environmental allergens, geographic region, and seasonal symptom patterns 1
• For occupational allergies: Test specific workplace exposures 1

Do not order total serum IgE—it has no diagnostic value for food allergy or allergic rhinitis and should not be used routinely 1

Ordering the Test

Specify the testing platform carefully:

• Use fluorescence-labeled immunoassays (ImmunoCAP/Phadia, Immulite 2000, or Turbo-MP) 1
• Never use the term "RAST"—this outdated radioallergosorbent test has been replaced by more sensitive assays 1
• Critical caveat: Results from different assay systems are NOT interchangeable—predictive values established for ImmunoCAP cannot be applied to other platforms 1, 3
• Consider allergen panels for cost-effectiveness when testing multiple common aeroallergens (sensitivity 70.8%, specificity 90.7%) 1

Interpreting Results

Understand the diagnostic limitations:

• Negative predictive value is high (>95%): Negative results effectively rule out IgE-mediated allergy 1
• Positive predictive value is low (40-60%): Positive results indicate sensitization, NOT clinical allergy 1
• Sensitivity compared to skin testing is approximately 70-75% 1
• Higher specific IgE levels correlate with greater likelihood of clinical reactivity, but cutoff values vary by allergen, age, and population 1, 4

For food allergens specifically:

• Positive tests require confirmation with supervised oral food challenge (gold standard) before dietary elimination 1
• Established 95% predictive value cutoffs exist for common foods (egg, milk, peanut) in children, but only apply to ImmunoCAP system 1
• False positives are common in patients with other food allergies or atopic dermatitis 5

Common Pitfalls to Avoid

Do not:

• Order testing without clear clinical indication—testing should confirm suspected allergens, not screen randomly 1
• Diagnose allergy based solely on positive serum IgE without clinical correlation 1, 4
• Compare results across different laboratory platforms or assay systems 1, 3
• Use total IgE measurements to diagnose or manage food allergy 1
• Assume undetectable IgE excludes allergy if history is highly suggestive—proceed to supervised challenge 1

Recognize that:

• Serum specific IgE correlates with skin testing but may not always agree (measures serum IgE vs. mast cell-bound IgE) 1
• Cross-reactive proteins and glyco-epitopes can cause false positives 1
• Component-resolved diagnostics (testing individual allergen molecules rather than extracts) may improve specificity but is not yet standard practice 1, 6