The new differential diagnostic test for the lichenoid drug eruption.

The differential diagnosis between lichenoid drug eruption (LDE) and lichen planus (LP) is difficult due to similar clinical and histological signs but important for treatment and prognosis. The purpose of this study was to propose the new diagnosis method for differentiate LDE from LP. During 2015-2018, 20 patients with confirmed LDE, 13 patients with LP and 134 controls were examined and treated at the Lenoblcenter. All enrolled patients were underwent the injection of 0.5 mL of the 2% lidocaine solution by insulin syringe into the papule with following histological examination. The formation of a blister (bulla) at the site of injection was considered a positive test result. Among LDE, 18 of 20 patients were found positive for developing blister (bulla) and two results were questionable. In 12 of 13 LP patents, bulla on the site of injection was not identified and the result of one patient was nonspecific. All control patients were negative for the proposed test. The histological sections showed that the bulla has corresponded to the separation of the epidermis from the dermis. Intracutaneous injection of 0.5 mL of lidocaine into the papule is an easy highly specific and sensitive method to differentiate LDE from LP.