Intradermal Skin Testing: Procedure, Interpretation, and Clinical Uses

Intradermal testing (IDT) injects allergen/antigen into the dermis to evaluate immediate (IgE) or delayed (T-cell) hypersensitivity.

Procedure

1. Site: Volar forearm or upper back; clean with alcohol.
2. Inject 0.02–0.05 mL allergen extract (1:100–1:1000 w/v) via 26–30G needle, bevel up, forming a 3–4 mm bleb.
3. Controls: Histamine (positive), saline/glycerin (negative).
4. Read at 15–20 min (immediate: wheal ≥5 mm > negative + erythema ≥10 mm) and 24–48 hr (delayed: induration ≥5 mm).

Applications

• Allergy: Venom (bee/wasp), penicillin, environmental (if SPT negative but high suspicion). More sensitive than SPT.
• TB screening: Mantoux (TST) — 5 TU PPD; read induration at 48–72 hr.
• Fungal: Coccidioidin, histoplasmin (delayed).
• Drug allergy: Beta-lactams, neuromuscular blockers.

Advantages/Limitations

• Higher sensitivity than SPT; lower specificity (more false positives).
• Greater systemic reaction risk — requires observation, emergency equipment.
• Not for food allergy (high false positive).
• Performed by allergist/immunologist or trained provider.

Interpretation correlates with clinical history. Not a standalone diagnostic.