What are the detailed components available in allergy testing, such as specific Immunoglobulin E (IgE) tests?

Detailed Components Available in Allergy Testing

The two primary IgE-specific allergy testing modalities are skin testing (prick/puncture and intradermal techniques) and blood testing (allergen-specific IgE immunoassays), with skin testing being the preferred first-line approach due to superior sensitivity, lower cost, and immediate results. 1

Primary IgE-Specific Testing Methods

Skin Testing Components

Skin Prick/Puncture Testing is the most commonly used technique with sensitivity and specificity typically exceeding 80% for both measures. 1

• Mechanism: Introduces specific allergen into the epidermis, causing mast cell activation and histamine release, producing a wheal and flare reaction within 15-20 minutes. 1
• Advantages: Direct observation of body's reaction, more sensitive than blood testing, less expensive, and results available immediately. 1
• Disadvantages: May be affected by antihistamines and tricyclic antidepressants, rare risk of anaphylaxis (no fatalities reported with prick testing for inhalants). 1

Intradermal/Intracutaneous Testing provides increased sensitivity when prick testing is negative but clinical suspicion remains high. 1

• Use case: Particularly helpful for allergens requiring additional sensitivity or when prick test is negative with high clinical suspicion. 1
• Risk profile: Six fatalities reported historically from intradermal inhalant testing, five in asthmatics who did not receive prick testing first. 1

Blood Testing Components

Allergen-Specific IgE Immunoassays detect IgE antibodies to specific allergens using enzyme-labeled anti-IgE antibodies (RAST is now an outdated term). 1

• Mechanism: Serum is incubated with allergen absorbed on solid phase (plastic disc or bead), bound IgE is measured with labeled anti-IgE antibody. 1
• Advantages: No anaphylaxis risk, not affected by patient medications, can be used with severe eczema or dermatographism, safe for patients on β-blockers. 1
• Disadvantages: Requires reliable laboratory, potential for laboratory errors, generally less sensitive than skin testing (average 70-75% sensitivity compared to skin prick tests). 1

Component-Resolved Diagnostics (CRD)

CRD should be ordered as a second-line test when initial SPT or whole allergen sIgE results are equivocal (SPT wheal 3-8 mm or sIgE 0.35-15 kUA/L). 2

• Specific components: For peanut allergy, Ara h 2-specific IgE is the most predictive marker for clinical allergy and severe reactions. 2
• Clinical utility: Using Ara h 2 in a two-step algorithm reduces the need for oral food challenges by almost two-thirds. 2
• Interpretation: Positive CRD to storage proteins indicates genuine sensitization with high likelihood of clinical allergy and potential for severe reactions. 2

Tests NOT Recommended

IgG antibody testing and total IgE measurement have no role in diagnosing allergic rhinitis or food allergy. 1

• IgG testing: Does not yield information helpful for management of allergic rhinitis. 1
• Total IgE: Has limited diagnostic value in diagnosis of allergic rhinitis. 1

Other Diagnostic Tests (Insufficient Evidence)

The following tests have insufficient evidence for routine recommendation: 1

• Acoustic rhinometry
• Olfactory testing
• Microarray testing
• Nasal nitric oxide measurements
• Nasal allergen challenges
• Nasal smears for eosinophilia

Clinical Algorithm for Test Selection

Step 1: Begin with skin prick testing as the preferred initial diagnostic test, directed by clinical history of timing, reproducibility, and exposure patterns. 1, 2

Step 2: Use blood-based allergen-specific IgE testing when skin testing is contraindicated due to: 1

• Widespread eczema or severe dermatographism
• Inability to discontinue antihistamines
• β-blocker use
• History suggesting unusually high anaphylaxis risk

Step 3: Order CRD (such as Ara h 2 for peanut) only when initial tests are equivocal, not as first-line testing. 2

Step 4: Consider oral food challenge when diagnosis remains uncertain after testing, as this remains the gold standard. 2, 3

Critical Pitfalls to Avoid

• Never order CRD as first-line testing—always perform SPT or whole allergen sIgE first. 2
• Never order broad CRD panels without clinical justification—select components based on suspected allergen from history. 2
• Remember that positive tests indicate sensitization, not clinical allergy—50-90% of presumed food allergies based on history alone are not actual allergies when formally tested. 3
• Use standardized allergen extracts when available and document wheal/erythema measurements at 15-20 minutes with positive and negative controls. 1
• Document all medications taken within the past week, as antihistamines and tricyclic antidepressants suppress skin test responses. 1