Contact Dermatitis, Allergic vs Irritant

Workup of contact dermatitis, distinguishing allergic contact dermatitis (ACD, delayed type IV hypersensitivity) from irritant contact dermatitis (ICD, non-immunologic), and routing ACD to patch testing for allergen identification. Covers the central role of patch testing, common allergens (nickel, fragrance, preservatives, neomycin), and the principle that avoidance of the identified allergen is the definitive treatment.

Evidence tier: Guideline-derived.

Decision points

  • Does the pattern suggest allergic or irritant?
  • Has patch testing identified an allergen?

Do-not-miss pitfalls

  • History and morphology cannot reliably identify the culprit allergen in ACD, patch testing is required. "Avoid nickel" guesses without testing frequently miss the true allergen and impose useless restrictions.
  • Patch tests must be read TWICE, at 48 h and again at 72-96 h. Many relevant allergens (neomycin, corticosteroids, metals) only become positive on the delayed read; a single early read produces false negatives.
  • A positive patch test is only meaningful if clinically RELEVANT, it must correspond to an allergen the patient actually contacts where the dermatitis occurs. Acting on irrelevant positives causes needless avoidance.
  • Patch testing is NOT the same as prick/intradermal testing, contact dermatitis is delayed type IV (T-cell), not IgE-mediated. Prick testing and specific IgE have no role in diagnosing ACD.
  • Systemic contact dermatitis can occur when a patch-test-positive allergen is ingested (e.g., nickel in food, balsam of Peru in spices/foods, propylene glycol in oral medications), consider it in widespread or recalcitrant dermatitis.
  • Topical corticosteroids and antibiotics (neomycin, bacitracin) are themselves common contact allergens, a dermatitis that worsens with topical treatment may be an allergy to the treatment itself.
  • Methylisothiazolinone (a preservative in wet wipes, cosmetics, paints) has been an epidemic allergen, include it and the patient's own personal-care products in testing.
  • Occupational ACD/ICD requires workplace assessment, identifying and removing the exposure (or substituting materials) is essential; topical treatment alone fails if exposure continues.

Evidence & citations

  1. Fonacier L, Bernstein DI, Pacheco K, et al. Contact dermatitis: a practice parameter–update 2015. J Allergy Clin Immunol Pract. 2015;3(3 Suppl):S1-39. PMID 25965350
  2. Mowad CM, Anderson B, Scheinman P, et al. Allergic contact dermatitis: Patient management and education. J Am Acad Dermatol. 2016;74(6):1043-1054. PMID 27185421
  3. Johansen JD, Aalto-Korte K, Agner T, et al. European Society of Contact Dermatitis guideline for diagnostic patch testing, recommendations on best practice. Contact Dermatitis. 2015;73(4):195-221. PMID 26179009

Clinician decision support. Verify against the cited source. Not a substitute for clinical judgment. 100% on-device; no patient data is stored or transmitted.