How do I take an SPT (Skin Prick Test)?

How to Perform a Skin Prick Test (SPT)

Skin prick testing should be performed by trained healthcare professionals in a setting where anaphylactic reactions can be treated, as even though rare, systemic reactions can occur during testing. 1

Preparation for SPT

Materials Needed

• Allergen extracts for testing
• Positive control (histamine, 1 mg/mL)
• Negative control (diluent, usually phenol saline)
• Lancets or other skin puncture devices
• Alcohol swabs
• Timer
• Ruler (preferably millimeter scale)

Patient Preparation

• Patient should discontinue antihistamines before testing:
  • Short-acting antihistamines (e.g., chlorpheniramine, terfenadine): 24 hours before
  • Diphenhydramine or hydroxyzine: 4 days before
  • Long-acting antihistamines (e.g., astemizole): 3 weeks before 1
• Testing area (usually volar forearm or back) should be cleaned with alcohol and allowed to dry

SPT Procedure

1. Mark testing sites on the skin, allowing at least 2 cm between each site to prevent cross-contamination of allergens 2

2. Apply allergen extracts:

   • Place a drop of each allergen extract, positive control, and negative control on the marked sites
   • Use separate droppers for each allergen to prevent cross-contamination

3. Perform the skin prick:

   • Using a sterile lancet, prick the skin through the allergen drop at a 45-90° angle
   • Penetrate only the epidermis (upper layer of skin) without drawing blood
   • Use a new lancet for each allergen to prevent cross-contamination 1, 2

4. Blot excess solution gently with a tissue paper without cross-contamination between sites

5. Wait 15-20 minutes for reaction to develop 2

6. Read and document results:

   • Measure the wheal (raised area) and flare (redness) in millimeters
   • A positive reaction is typically defined as a wheal ≥3 mm larger than the negative control 1
   • The histamine control should be positive to ensure the test is valid and not affected by medications

Interpretation of Results

• Positive test: Wheal diameter ≥3 mm larger than negative control after 15 minutes
• Negative test: No significant wheal formation compared to negative control
• Invalid test: Negative histamine control (suggests interference from medications) 1

Safety Considerations

Risk Factors for Adverse Reactions

• Young age (<1 year): 6.28 times higher risk
• Active eczema: 16.98 times higher risk
• Female sex: 7.32 times higher risk for vasovagal reactions
• Multiple skin pricks performed on a single patient 3

Emergency Preparedness

• Always have emergency medications and equipment available
• Be prepared to treat anaphylaxis, though the risk is very low (0.12%) 3
• For high-risk patients (history of anaphylaxis, asthma, or beta-blocker therapy), consider using 100-fold dilutions of allergen extracts initially 1

Important Considerations

• SPT has excellent diagnostic value with a positive predictive value of 95-100% 2
• There is good correlation (85-95%) between SPT and in vitro specific IgE tests 2
• A positive test indicates sensitization but must be correlated with clinical symptoms, as sensitization does not always mean clinical allergy 1, 2
• For patients with negative SPT but strong clinical suspicion, intradermal testing may be considered as the next step 4

Remember that while SPT is an excellent diagnostic tool, the results must always be interpreted in the context of the patient's clinical history and symptoms.