What methods are used to test for allergies?

How to Test for Allergies

First-Line Testing: Skin Prick Testing

Skin prick testing (SPT) should be your initial diagnostic test for suspected IgE-mediated allergies, as it offers superior sensitivity (typically >80%), immediate results within 15-20 minutes, and lower cost compared to blood testing. 1, 2

When to Order SPT

• Order SPT when clinical history suggests IgE-mediated allergic reactions with symptoms occurring within minutes to hours after allergen exposure, especially if reproducible 3
• Select specific allergens based on the patient's exposure history and geographic location—avoid ordering large general screening panels 1
• For respiratory symptoms: test aeroallergens including pollens, dust mites, molds, and animal dander 1
• For suspected food allergies: test only when history suggests IgE-mediated reactions (urticaria, angioedema, anaphylaxis within 2 hours), not for isolated gastrointestinal symptoms 4, 1

Technical Requirements for SPT

• Use standardized allergen extracts when available 1
• Always include both positive control (histamine) and negative control (diluent) with every test 1, 2
• Read results at 15-20 minutes after allergen placement 1, 2
• Measure mean wheal diameter: positive result is ≥3 mm greater than negative control 1, 2
• Document all medications taken within the past week, as antihistamines and tricyclic antidepressants suppress responses and must be discontinued before testing 1, 2

Critical Safety Requirements

• Ensure emergency equipment and epinephrine are immediately available during skin testing 1
• Never proceed directly to intradermal testing without performing prick testing first 1, 2
• Tests should be performed only by physicians with proper training in allergy and experience treating anaphylaxis 5

Second-Line Testing: Serum Allergen-Specific IgE

Order serum allergen-specific IgE testing when SPT is contraindicated or impractical, though it has lower sensitivity (70-75%) and takes days rather than providing immediate results. 1, 2

Specific Indications for Blood Testing

• Widespread skin disease (extensive eczema or severe dermatographism) 4, 1, 2
• Inability to discontinue antihistamines 1, 2
• Patient on β-blockers 1, 2
• History suggesting unusually high anaphylaxis risk 1, 2
• Uncooperative patient (particularly young children) 1

Interpretation of Serum IgE Results

• Use modern fluorescence-labeled antibody assays (such as ImmunoCAP) for optimal accuracy 3
• Negative result (<0.35 kUA/L) has high negative predictive value (>95%) and effectively rules out IgE-mediated allergy 4, 3
• Positive results indicate sensitization but require clinical correlation—positive predictive value is only 40-60% 4, 1, 3
• Higher specific IgE levels correlate with increased probability of clinical reactivity 3
• Do NOT order total serum IgE for allergy diagnosis—it lacks specificity for individual allergens 3

Advanced Testing: Component-Resolved Diagnostics (CRD)

Order CRD only as second-line testing when initial SPT or whole allergen specific IgE results are equivocal—never as first-line testing. 1, 2

• CRD measures IgE to specific allergen molecular components rather than whole allergen extracts 4, 3
• Particularly useful for peanut (Ara h 2), cashew (Ana o 3), egg (Gal d 1,2,3,5), and hazelnut allergies 3, 2
• Can reduce the need for oral food challenges by almost two-thirds when used in a 2-step algorithm 1, 3, 2
• Positive CRD to storage proteins indicates genuine sensitization with high likelihood of clinical allergy and potential for severe reactions 2

Gold Standard: Oral Food Challenge

When diagnosis remains uncertain after SPT or specific IgE testing, supervised oral food challenge is the gold standard for food allergy diagnosis, but requires mandatory referral to an allergist in a specialized setting. 1, 6

• OFCs are time-consuming and involve risk of acute allergic reactions of unpredictable severity 6
• Should be reserved for cases where diagnosis cannot be established by history and testing alone 6

Specialized Testing for Specific Scenarios

Intradermal Testing

• Provides increased sensitivity when prick testing is negative but clinical suspicion remains high 2, 7, 8
• Particularly helpful for inhalant allergens requiring additional sensitivity 2
• Positive result requires mean wheal diameter ≥5 mm at 15-20 minutes 2
• Higher risk of systemic reactions—five historical fatalities occurred in asthmatics who did not receive prick testing first 2

Nasal or Conjunctival Provocation

• Reserved for exceptional circumstances, mainly when occupational allergies are involved 4
• Nasal allergen challenge involves direct application of allergen extract solution or exposure in allergen provocation chamber 4
• Conjunctival provocation test has 89% diagnostic efficacy compared to nasal challenge 4

Tests NOT Recommended

• Atopy patch tests (APT) are not recommended for routine use in food allergy evaluation—conflicting findings and difficult interpretation due to non-specific reactions 4, 3
• Broad panel allergy testing independent of clinical history is not recommended 4

Critical Pitfalls to Avoid

• Never diagnose allergy based on testing alone—a positive test indicates sensitization, not necessarily clinical allergy; clinical allergy requires both positive test AND symptoms with exposure 1, 3, 7, 8
• Sensitization is common but not always symptomatic—more patients have positive tests than have current clinical symptoms 7, 8, 9
• Different laboratory assay systems provide significantly different measurements—predictive values from one system cannot be applied to another 3
• Measuring only erythema (redness) rather than the raised wheal is a common error—the wheal represents the actual allergic response 2
• For food allergy in children <5 years with moderate-to-severe atopic dermatitis: consider limited testing (cow's milk, eggs, wheat, soy, peanut) only if persistent disease despite optimized management OR reliable history of immediate reaction after specific food ingestion 4