Histamine-Induced Itch Protocol: A Step-by-Step Guide

A 1% histamine dihydrochloride solution was used to induce chemical itch. A skin-prick paradigm was employed: 0.05 ml of the 1% histamine dihydrochloride solution was applied to the volar aspect of the right forearm, 3 cm below the wrist crease. A puncture was then made lightly using a cannula needle to aid solution penetration without causing bleeding. Finally, 0.1 ml of the 1% histamine dihydrochloride solution was applied to a 2x2 cm patch, which was then placed over the puncture site. Participants were instructed to perceive and rate the itch induced by the solution. The perception time was set at 10 seconds, followed by a 5-second interval for rating. A total of 50 consecutive ratings were collected. For the itch intensity ratings that were consistent in the last 30 trials (see supplementary materials), the average score from the first 20 ratings was considered the rating for histamine-induced itch.