Step 2 in Skin Allergy Testing: Intradermal Testing
If the initial prick/puncture test is negative, proceed immediately to intradermal testing using the same allergen reagents. 1
The Two-Step Skin Testing Algorithm
Skin allergy testing follows a standardized two-step approach:
Step 1: Prick/Puncture Testing
- Full-strength allergen reagents are applied first via prick/puncture technique
- This is the initial, less sensitive screening method 1
Step 2: Intradermal Testing (When Step 1 is Negative)
- Performed only if prick tests are negative 1
- More sensitive than prick testing but also more invasive 2, 3
- Involves injecting 0.02 mL of allergen into the dermis to create a 3-5 mm baseline bleb 1
Technical Details for Intradermal Testing
Proper technique is critical:
- Draw up a larger volume (0.05-0.07 mL) first
- Expel excess fluid and air bubbles to obtain exactly 0.02 mL
- Inject to produce a baseline 3-5 mm bleb 1
Reading results (15-20 minutes post-injection):
- Positive test: wheal ≥3 mm greater than negative control
- Must be accompanied by a flare ≥5 mm 1
- For intradermal tests specifically, some sources use ≥5 mm wheal criterion 4
Critical Safety Considerations
- Only trained personnel should perform skin testing with preparedness to treat rare anaphylaxis 1
- Always include appropriate positive (histamine) and negative (saline) controls 1
- Controls must react appropriately for results to be valid 1
- Intradermal testing carries higher risk than prick testing, though systemic reactions remain rare (<2% in skin test-positive patients) 1
Common Pitfall to Avoid
Do not skip directly to intradermal testing. The prick test must be performed first because it is more specific and safer, reducing unnecessary exposure to the more sensitive intradermal method 2, 3. Intradermal testing is only indicated when prick tests are negative but clinical suspicion remains high for IgE-mediated allergy.