Histopathological discriminant criteria between lichenoid drug eruption and idiopathic lichen planus: retrospective study on selected samples.

The lesions observed in 15 idiopathic lichen planus (ILP) skin biopsies were compared with those occurring in 15 clinically relevant lichenoid drug eruptions (LDE). Various histopathological features were taken into account. Statistical analysis of results showed that none of these can be considered as a significant criterium allowing a pathognomonic differential diagnosis between ILP and LDE. Nevertheless, a strong suggestion of drug etiology can be inferred when the following histopathological signs ('LDE-related criteria') are present: focal parakeratosis, focal interruption of the granular layer, cytoid bodies in the cornified and granular layers, present in more than 50% of LDE and never in ILP.