Patch testing for allergic contact dermatitis uses a specific concentration and vehicle to avoid false reactions. A patient is patch tested to potassium dichromate. The reading at 96 hours (D4) shows a vesicular reaction (+++) at the test site. The D4 reading is preferred over D2 (48 hours) for which allergen group?

• A Nickel sulfate
• B Fragrance mix
• C Corticosteroids — late delayed readings are essential as they may be negative at D2 ✓
• D Paraphenylenediamine (PPD)

Explanation

In patch testing, most allergens are read at D2 (48 h) and D4 (96 h). For corticosteroids (topical steroids), a late reading at D4-D7 is essential because their anti-inflammatory properties can suppress the reaction at 48 hours, causing false negatives. This is a well-recognized phenomenon where the steroid initially suppresses its own contact allergy reaction. Nickel, fragrance mix, and PPD are typically reliably positive at D2 without requiring extended reading, though D4 is still recommended for all allergens to catch late reactions.

Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.