What is the protocol for a patient with a history of allergic reactions, asthma, and respiratory conditions undergoing a skin prick test (SPT) for allergy diagnosis?

Skin Prick Test Protocol for Patients with Asthma and Allergic History

Skin prick testing is safe and recommended for patients with asthma and allergic conditions, but requires specific precautions including mandatory availability of emergency equipment, discontinuation of antihistamines, and avoidance of direct intradermal testing without preceding prick testing, particularly in asthmatic patients. 1, 2

Pre-Test Safety Assessment and Contraindications

Absolute Contraindications

• Do not perform SPT in patients with history of severe cutaneous adverse reactions (blistering, Stevens-Johnson syndrome, toxic epidermal necrolysis), as testing may precipitate dangerous reactions 2
• Avoid SPT in patients on β-blockers due to risk of severe, refractory anaphylaxis if systemic reaction occurs 2, 3
• Defer testing in patients with widespread eczema or severe dermatographism—use serum-specific IgE testing instead 2, 3

Critical Safety Consideration for Asthmatic Patients

• Asthma is the most commonly identified factor associated with severe reactions during allergy testing 1
• Five of six historical fatalities from intradermal inhalant testing occurred in asthmatic patients who did not receive prick testing first 1, 2
• Emergency equipment must be immediately available, including epinephrine, oxygen, and resuscitation equipment 2

Medication Management Before Testing

Must Discontinue

• Antihistamines must be stopped several days before testing (typically 3-7 days depending on specific agent) as they suppress skin test responses 1, 2, 3
• Some antidepressants (particularly tricyclics) also suppress responses and should be discontinued 1, 3
• Document all medications taken within the past week 1, 3

Can Continue

• Oral and inhaled corticosteroids do not need to be discontinued 2
• This is particularly important for asthmatic patients who require ongoing controller therapy

Testing Procedure

Technique

• Use standardized allergen extracts when available 1, 3
• Place drop of allergen extract on skin (typically volar forearm) 4, 5
• Prick skin through the drop with standardized lancet or device 2, 5
• Include positive control (histamine) and negative control (saline) to validate results 2, 3
• Read results at 15-20 minutes after placement to capture peak wheal and flare response 1, 2, 3, 5

Interpretation

• Positive result defined as mean wheal diameter ≥3 mm (compared to negative control) 2, 3, 5
• Measure the raised wheal, not just erythema—erythema alone without wheal does not constitute positive result 3
• Record measurements of both wheal and erythema for allergen and both controls 1, 3
• Positive control must demonstrate adequate wheal; if it fails, test is invalid (suggests antihistamine interference or anergy) 3

Allergen Selection Based on Clinical History

• Base allergen selection on patient's specific clinical history, including timing of symptoms, triggering exposures, geographic location, and occupational/lifestyle factors 3
• For respiratory conditions, test relevant inhalant allergens: tree/grass/weed pollens, house dust mites, animal dander, mold spores, cockroach 3, 5
• Standard European panel includes hazel, alder, birch, plane, cypress, grass mix, olive, mugwort, ragweed, Alternaria, Cladosporium, Aspergillus, cat, dog, Dermatophagoides pteronyssinus and farinae, and cockroach 5

When to Use Intradermal Testing

• Never proceed directly to intradermal testing without performing prick testing first, especially in asthmatic patients 1, 2, 3
• Use intradermal testing only when prick test is negative but clinical suspicion remains high for specific allergen 1, 2, 3
• Intradermal testing provides increased sensitivity but carries higher risk of systemic reactions 1, 3
• Positive intradermal result requires mean wheal diameter ≥5 mm at 15-20 minutes 3

Clinical Correlation and Interpretation

• Positive SPT indicates sensitization, not necessarily clinical allergy—correlation with symptoms is essential 2, 3, 4, 6, 7
• Negative predictive value is high (>95%), making negative tests useful to rule out IgE-mediated allergy 2, 3
• Positive predictive value is only 40-60%, so positive tests require clinical correlation 3
• Sensitivity and specificity of SPT typically exceed 80% for both measures 1, 2, 3

Alternative Testing When SPT Contraindicated

• Use serum-specific IgE testing when SPT cannot be performed due to: 2, 3
  • Inability to discontinue antihistamines
  • β-blocker use
  • Extensive skin disease
  • History suggesting unusually high anaphylaxis risk
• Serum testing has approximately 70-75% sensitivity compared to SPT (misses 25-30% of true allergies) 3

Common Pitfalls to Avoid

• Measuring only erythema rather than the raised wheal 3
• Performing testing while patient is on antihistamines 1, 2, 3
• Proceeding to intradermal testing without prick testing first in asthmatic patients 1, 2
• Interpreting positive tests as diagnostic without clinical correlation 2, 3, 4
• Testing patients with prominent dermatographism (will have false-positive results) 2, 3