1. Patch Test relies on the principle of a type IV (delayed) hypersenstitivity reaction must be performed on a previously involved site comprises a series of small, aluminium wells containing various concentrations of the offending medication mounted on hypoallergenic tape

2. Patch Test standard occlusion time : 48 h first reading: day 2 generally 15-30 min after patch removal second reading: day 3 or 4 Results are recorded using a standardized scoring system

3. Oral Provocation check for cross-sensitivities to medications *A refractory period has been reported in fixed drug eruption; therefore, a delay before and between patch testing and oral provocation is recommended

4. Skin Biopsy Acute interface dermatitis with prominent vacuolar change and individual necrotic keratinocytes within the epidermis (X10). Interface dermatitis, vacuolar change, necrotic keratinocytes, and incontinent pigment in the dermis (X40). Diagnostic procedure of choice Generally performed during the acute stage of recurrence