Patch test is read after:

Patches are commonly applied on upper back for 48 hrs. Patch test First reading is taken after 48 hrs i.e at 2 days 2nd reading on day 4 i.e 96 hrs Evaluation of patch test reading: Grading Evaluation Clinical findings +- Doubt reaction Faint erythema + Weak (non-vesicular) +ve reaction Erythema, infiltration, and discrete papules ++ Strong (vesicular) +ve reaction Erythema, infiltration, papules, and vesicles. +++ Extreme (bullous) + ve reaction Intense erythema, infiltration, and coalescing vesicles.