[Patch test reading].

The reading of patch tests is standardised as regards both reading time (double reading at 48 h and 72 or 96 h) and the score, which is based on the elementary lesion. Some molecules are known to produce their own reactions, either irritant (chromium or cobalt) or with late expression (corticosteroids). A late positive reaction can be due to induction of sensitization or late expression of sensitization due to decreased memory or cross reaction with an allergen that has a positive reaction with a normal reading.