What procedure is recommended for allergy diagnosis in a patient with asthma, respiratory conditions, and a history of allergic reactions?

Recommended Procedure for Allergy Diagnosis

Perform skin prick testing (SPT) as the first-line diagnostic procedure for this patient with asthma and allergic history, followed by intradermal testing if SPT is negative but clinical suspicion remains high. 1, 2

Primary Diagnostic Approach: Skin Prick Testing

Skin prick testing should be your initial test because it offers superior sensitivity (typically >80%), immediate results within 15-20 minutes, lower cost, and allows direct observation of the body's allergic response. 2, 3

Technical Execution of SPT

• Apply allergen extracts to the volar forearm surface and introduce them into the epidermis using a lancet, creating a wheal and flare reaction through mast cell activation and histamine release 2, 4
• Read results at 15-20 minutes, with a positive result defined as mean wheal diameter ≥3 mm compared to negative control 2
• Always include positive control (histamine) and negative control (saline) to validate test reliability 2
• Select allergens based on clinical history including timing of symptoms, geographic location, and specific triggering exposures (tree/grass/weed pollens, house dust mites, animal dander, mold spores, cockroach) 2

When SPT is Contraindicated: Use Blood Testing

Switch to allergen-specific IgE blood testing if the patient has: 2, 5

• Extensive eczema or severe dermatographism preventing reliable skin test interpretation
• Inability to discontinue antihistamines (must be stopped before SPT)
• Current β-blocker use (increases anaphylaxis risk from skin testing)
• History suggesting unusually high anaphylaxis risk

Blood testing has approximately 70-75% sensitivity compared to SPT but avoids anaphylaxis risk and is unaffected by medications 2, 5

Second-Line Testing: Intradermal Testing

If SPT is negative but clinical suspicion remains high based on history and asthma symptoms, proceed to intradermal testing for increased sensitivity. 2, 6

• Inject 0.01-0.02 mL of allergen into the dermis 4
• Positive result requires mean wheal diameter ≥5 mm at 15-20 minutes 2
• Critical safety point: Never proceed directly to intradermal testing without performing prick testing first—five historical fatalities occurred in asthmatics who received intradermal testing without prior prick testing 2
• Intradermal testing is particularly valuable for house dust mite allergy in asthma patients with negative prick tests, where it contributes significantly to diagnosis 6

Essential Interpretation Principles

A positive test indicates sensitization, NOT necessarily clinical allergy—correlation with clinical symptoms is mandatory. 1, 2

• Positive predictive value is only 40-60% (meaning many positive tests don't represent true clinical allergy) 2, 5
• Negative predictive value exceeds 95% (negative test reliably excludes allergy) 5
• In unselected populations, positive tests for cat and ragweed have positive predictive values of only 44.7% and 50.3% respectively 1
• 54% of the general US population tests positive to at least one allergen despite only ~15% having clinical allergic rhinitis 1

Critical Pitfalls to Avoid

Document all medications taken within the past week—antihistamines suppress skin test responses and invalidate results. 2

• Measuring only erythema (redness) rather than the raised wheal is a common error; the wheal represents the actual allergic response 2
• If positive control fails to produce adequate wheal, the test is invalid and indicates antihistamine interference or anergy 2
• Patients with prominent dermatographism may have false-positive results; the negative control helps identify this 2

Special Consideration for Asthma Patients

Given this patient's asthma, focus testing on inhalant allergens with particular attention to house dust mites, as exposure to indoor and outdoor allergens can worsen asthma symptoms and airway responsiveness. 1, 6

• Intradermal testing shows particular value for detecting house dust mite allergy in asthmatic patients with negative prick tests 6
• Assess for correlation between allergen exposure and asthma exacerbations to guide immunotherapy decisions 1
• Document asthma severity and control status, as this influences both testing safety and treatment planning 1

Component-Resolved Diagnostics: Second-Line Only

Order component-resolved diagnostics (CRD) only when initial SPT or allergen-specific IgE results are equivocal—never as first-line testing. 2

• CRD identifies specific allergenic proteins and can reduce need for oral food challenges by nearly two-thirds 2, 5
• Positive CRD to storage proteins indicates genuine sensitization with high likelihood of clinical allergy and potential for severe reactions 2