Understanding Skin Tests: Purposes, Types & What to Expect

Skin tests assess immune reactivity to specific substances. Always interpreted by a clinician in context.

Common Types

1. Allergy Testing (Skin Prick/Intradermal): Introduce allergen extract; wheal/flare indicates IgE sensitization (allergic rhinitis, asthma, food, venom, drug allergy).
2. Tuberculin Skin Test (TST/Mantoux): Intradermal PPD; induration ≥5–15 mm (risk-based) indicates TB infection (latent or active).
3. Fungal Skin Tests: Antigens for coccidioidomycosis, histoplasmosis, blastomycosis; delayed hypersensitivity indicates exposure.
4. Patch Testing (Contact Dermatitis): Allergens on patches applied 48h; read at 48h/72h/96h for eczematous reaction (nickel, fragrance, preservatives).
5. Drug Allergy Testing: Penicillin skin testing (major/minor determinants) before administering; other drugs less validated.
6. Immunodeficiency Assessment: Candida/mumps/trichophytin skin test anergy suggests impaired cell-mediated immunity.

Interpretation Nuances

• Positive test ≠ clinical allergy (requires history correlation).
• Negative test doesn't rule out (false negatives occur).
• Medications (antihistamines, steroids) can suppress reactions.
• Systemic reactions rare but possible; emergency equipment required.