What studies and labs are typically ordered for a patient undergoing allergy testing?

Allergy Testing: Studies and Labs to Order

Initial Testing Approach

Skin prick testing (SPT) is the preferred first-line diagnostic test for allergy evaluation, directed by clinical history, due to its superior sensitivity (typically >80%), immediate results, and lower cost compared to blood testing. 1

When to Order Skin Prick Testing

• Order SPT when the clinical history suggests IgE-mediated allergic reactions (immediate symptoms within minutes to 2 hours of exposure, reproducible pattern, symptoms like urticaria, rhinitis, or anaphylaxis) 2, 3
• Select specific allergens based on the patient's exposure history and geographic location—avoid ordering large general panels 2
• Test for aeroallergens (pollens, dust mites, molds, animal dander) for respiratory symptoms 2
• Test for food allergens only when history suggests IgE-mediated food reactions (not for isolated gastrointestinal symptoms, which are non-allergic) 2

Technical Requirements for SPT

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

Alternative Testing: Serum Allergen-Specific IgE

When to Order Blood Testing Instead of SPT

Order serum allergen-specific IgE testing (not total IgE) when: 2, 1

• Widespread skin disease (severe eczema or dermatographism prevents accurate skin testing) 2, 1
• Unable to discontinue antihistamines (patient requires continuous antihistamine therapy) 1
• Patient on β-blockers (increased risk of severe reactions during skin testing) 2, 1
• History suggesting unusually high anaphylaxis risk from skin testing 2, 1
• Uncooperative patient (young children, cognitive impairment) 2

Limitations of Blood Testing

• Average sensitivity is only 70-75% compared to SPT 2
• More expensive than skin testing 2
• Results take days rather than immediate 1
• Requires reliable laboratory to avoid errors 2, 1

Advanced Testing: Component-Resolved Diagnostics (CRD)

When to Order CRD

• Order CRD only as second-line testing when initial SPT or whole allergen specific IgE results are equivocal (SPT wheal 3-8 mm or specific IgE 0.35-15 kUA/L) 2, 1
• Particularly useful for peanut allergy (Ara h 2), cashew (Ana o 3), egg (Gal d 1,2,3,5), and hazelnut (Cor a 9) 2
• Can reduce need for oral food challenges by almost two-thirds 2, 1
• Do not order as first-line testing—only when initial tests are inconclusive 1

Tests NOT to Order

Inappropriate Testing

• Total serum IgE: Does not yield helpful information for allergy diagnosis 2
• IgG or IgG4 food antibody testing: Not validated for allergy diagnosis 2
• Intradermal testing for foods: Contraindicated due to high false-positive rate and anaphylaxis risk 2
• Imaging studies (CT, MRI): No role in allergy diagnosis; history and testing are sufficient 2

Confirmatory Testing: Oral Food Challenge

When to Consider

• Order supervised oral food challenge when diagnosis remains uncertain after SPT/specific IgE testing 2, 1
• This is the gold standard for food allergy diagnosis but requires specialized setting 2
• Mandatory referral to allergist for this procedure 3

Critical Pitfalls to Avoid

Common Diagnostic Errors

• Never equate a positive test with clinical allergy—50-90% of self-reported food allergies are not confirmed on testing 2, 3
• Positive tests indicate only sensitization; clinical allergy requires both positive test AND symptoms with exposure 2, 3, 4
• Never order testing without clinical correlation—testing should be directed by history, not screening panels 2, 3
• Negative predictive value of SPT is high (>95%), but positive predictive value is only 40-60% 1

Safety Considerations

• Never proceed directly to intradermal testing without performing prick testing first—5 of 6 historical fatalities occurred in asthmatics who skipped prick testing 2, 1
• Document asthma status before testing, as asthmatics have higher risk of severe reactions 3, 1
• Ensure emergency equipment and epinephrine are immediately available during skin testing 3