Testing for IgE Sensitization to Inhalant Allergens
Skin prick testing is the preferred first-line method for detecting IgE sensitization to inhalant allergens, with serum-specific IgE testing reserved for situations where skin testing is contraindicated. 1
Primary Testing Approach: Skin Prick Testing
Skin prick/puncture testing should be your initial diagnostic test because it offers superior sensitivity (typically exceeding 80%), lower cost, immediate results within 15-20 minutes, and allows direct observation of the body's allergic response. 1, 2, 3
Technical Execution
- Introduce allergen into the epidermis using a needle through a drop of allergen extract, gently lifting the skin. 3
- Test sites include the volar forearm or back (back is slightly more reactive, but forearm allows patient to observe reactions). 3
- A positive result requires a mean wheal diameter ≥3 mm at 15-20 minutes, measured as the raised wheal, not just the surrounding erythema. 1
- Always include positive control (histamine) and negative control (saline/glycerin) to validate test integrity. 1
Critical Medication Considerations
Discontinue antihistamines before testing as they suppress skin reactivity and invalidate results; document all medications taken within the past week. 1 Patients on β-blockers can still undergo skin testing but require special precautions due to increased anaphylaxis risk. 1
When to Use Serum-Specific IgE Testing Instead
Switch to blood-based allergen-specific IgE immunoassays in these specific scenarios: 1, 4
- Widespread eczema or severe dermatographism preventing accurate skin test interpretation
- Inability to discontinue antihistamines or other interfering medications
- Patient on β-blockers (though skin testing possible with precautions)
- History suggesting unusually high anaphylaxis risk
- Patient preference or inability to tolerate skin testing
Understanding Serum Test Limitations
Serum-specific IgE testing has approximately 70-75% sensitivity compared to skin prick testing, meaning it misses about 25-30% of true allergies. 5, 1 However, it carries no anaphylaxis risk and results are unaffected by medications. 1
Intradermal Testing: Second-Line Approach
Reserve intradermal testing for situations where skin prick testing is negative but clinical suspicion remains high. 1 Never proceed directly to intradermal testing without performing prick testing first, as five of six historical fatalities from intradermal inhalant testing occurred in asthmatics who skipped initial prick testing. 1
- Inject 0.01-0.02 mL of allergen into the dermis. 3
- Positive result requires mean wheal diameter ≥5 mm (higher threshold than prick testing). 1
- Provides increased sensitivity but carries higher systemic reaction risk. 1, 2
Selecting Which Allergens to Test
Base allergen selection on the patient's clinical history, including timing of symptoms (seasonal vs. perennial), specific triggering exposures, geographic location, and occupational/lifestyle factors. 5 Testing should be directed rather than comprehensive, as 54% of the general population tests positive to at least one allergen despite lacking clinical symptoms. 5
Common inhalant allergen categories include: 5, 3
- Tree, grass, and weed pollens (seasonal patterns)
- House dust mites (perennial symptoms)
- Animal dander (cat, dog)
- Mold spores (Alternaria, Aspergillus)
- Cockroach
Critical Interpretation Pitfalls
A positive test indicates sensitization, NOT necessarily clinical allergy. 1, 6 The negative predictive value is high (>95%), but positive predictive value is only 40-60%. 1 This means:
- Negative tests reliably exclude IgE-mediated allergy
- Positive tests require correlation with clinical symptoms and exposure history to confirm clinical relevance
Studies show positive predictive values for specific allergens can be as low as 44.7% for cat and 50.3% for ragweed in unselected populations. 5 Therefore, testing without correlating symptoms is unreliable and leads to overdiagnosis. 5
Common Errors to Avoid
- Measuring only erythema (redness) rather than the raised wheal—the wheal is the actual allergic response. 1
- Testing patients with prominent dermatographism without recognizing false-positive results (negative control helps identify this). 1
- Assuming all positive tests are clinically relevant without symptom correlation. 5, 6
- Using IgG antibody testing to identify allergen triggers—allergic rhinitis is IgE-mediated only. 7
Component-Resolved Diagnostics: Advanced Testing
Order component-resolved diagnostics (CRD) only as second-line testing when initial skin prick or whole allergen specific IgE results are equivocal, not as first-line screening. 1 CRD identifies specific allergenic proteins and can reduce unnecessary oral food challenges by nearly two-thirds, though its primary utility is in food allergy rather than inhalant allergy. 1
Special Consideration: Local Allergic Rhinitis
Some patients have allergen-specific IgE present only in nasal tissue with negative skin and serum tests, termed "local allergy." 5 If clinical suspicion remains high despite negative testing, consider referral to an allergist for specialized evaluation. 5