Obbarius A, et al. - In view of limited information concerning the impact of characteristics of patients' baseline pain on assay sensitivity, researchers here tested if a combination of 3 baseline pain indices based on ecological momentary assessments (EMA) could aid in identifying patients with increased responses to placebo. Secondary data from 2 clinical trials in fibromyalgia patients were used to perform the analysis (N = 2,084). Based on the indices, 3 subgroups of patients were identified in a latent profile analysis. Group 1 (n = 857, 41.3%) exhibited the lowest pain intensity levels, along with the highest consistency and greatest variability of pain. The opposite pattern was recorded in group 3 (n = 110, 5.3%), and intermediate levels were recorded on all pain indices in group 2 (n = 1,109, 53.4%). Significant variation in treatment effects was observed between subgroups. Relative to patients in groups 1 and 2, those in group 3 showed greater decrease in pain in response to placebo. In addition, the removal of patients in class 3 would significantly increase the observed treatment effect by 8% to 15%. Per these findings, profiles of pain characteristics derived from baseline EMA may be valuable for identifying patient subgroups with increased placebo responses that can reduce assay sensitivity in pain clinical trials.