Patch testing is the gold standard for diagnosing allergic contact dermatitis. Which of the following statements about patch testing is CORRECT?
- A Readings at 48 hours and 96 hours (or 72–96 hours) are required; late readings detect metals like gold and neomycin ✓
- B Readings are done at 48 hours only; erythema alone is a positive result
- C A 3+ (extreme/bullous) reaction at 48 hours indicates an irritant, not allergic, contact dermatitis
- D Systemic steroids do not affect patch test results and can be continued
Correct answer: A. Readings at 48 hours and 96 hours (or 72–96 hours) are required; late readings detect metals like gold and neomycin
Explanation
Patch testing requires readings at 48 hours (when patches are removed) and again at 96 hours or day 7, as some allergens (particularly metals like gold, and neomycin) give delayed positive reactions. Erythema alone at 48 hours is a doubtful (+?) or irritant reaction; a true positive shows papules/vesicles. Systemic corticosteroids at >20 mg prednisolone can suppress patch test reactions and should be discontinued if possible. A 3+ (bullous, extreme) reaction at 48 hours may indicate a strong allergen, not necessarily an irritant.
Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.