What is the purpose and procedure of a skin prick test (SPT) for diagnosing allergies?

Skin Prick Test: Purpose and Procedure for Diagnosing Allergies

Skin prick testing (SPT) is the primary diagnostic tool for confirming IgE-mediated allergic diseases by directly observing the body's reaction to specific allergens, with high sensitivity and specificity (typically over 80% for both). 1

Purpose of Skin Prick Testing

• SPT is used to determine whether a patient has specific IgE antibodies to suspected allergens, which helps diagnose allergic conditions such as allergic rhinitis, asthma, and anaphylaxis 1
• It provides direct observation of the body's reaction to specific antigens, allowing clinicians to identify sensitization to inhalants, foods, some drugs, occupational allergens, hymenoptera venom, and latex 2
• SPT results, when correlated with clinical symptoms, help guide targeted therapy and treatment decisions for allergic conditions 1
• SPT can provide physicians with useful information about the appropriate starting dose of allergens for immunotherapy 1

Procedure of Skin Prick Testing

Preparation

• Patient medications should be reviewed, as antihistamines and some antidepressants (e.g., tricyclics) may suppress skin test responses and should be discontinued a few days before testing 1
• Oral or inhaled steroids do not need to be discontinued 1
• Standardized allergen extracts should be used when available 1

Testing Method

• SPT is typically performed on the volar surface of the forearm or on the back 3
• The procedure involves:
  1. Placing a drop of allergen extract on the skin 1
  2. Pricking the skin through the drop with a standardized lancet or device 4
  3. Waiting 15-20 minutes for the reaction to develop 1
  4. Measuring the resulting wheal (raised area) and erythema (redness) 1
• Positive and negative controls (histamine solution and physiological saline, respectively) are used to validate test results 1

Interpretation

• A positive result is defined as a wheal ≥3 mm in diameter 5
• Results should be interpreted in the context of the patient's clinical history, as sensitization (positive test) does not always correlate with clinical allergy 2
• The clinician should record measurements of wheal and erythema for allergen and positive and negative controls at 15-20 minutes after placement 1

Advantages of Skin Prick Testing

• More sensitive than in vitro IgE tests in detecting allergen-specific IgE 1
• Minimally invasive and allows testing of multiple allergens in 15-20 minutes 2
• Less expensive than blood testing 1
• In children, SPT is less disturbing than venipuncture required for in vitro tests 2
• High positive predictive value (95-100%) when correlated with clinical symptoms 2

Limitations and Safety Considerations

• Systemic allergic reactions, including anaphylaxis, are possible but extremely rare 1
• May be affected by patient medications, particularly antihistamines 1
• May be difficult to perform in patients with dermatographism or severe eczema 1
• In vitro tests should be preferred if previous severe reactions are noted in the patient's clinical history 2
• More patients are sensitized (have IgE antibodies and positive skin test reactions) than have corresponding symptoms, so diagnosis requires correlation with clinical presentation 3

When to Consider Alternative Testing Methods

• For patients on β-blockers or with comorbid medical conditions that preclude skin testing, blood testing may be more appropriate 1
• Intradermal testing may be required when additional sensitivity is needed or when skin prick tests are negative despite high clinical suspicion 1
• In vitro tests are particularly useful for patients with extensive skin disease 1