Many factors are known to affect assay sensitivity; however, limited attention has been devoted to understanding whether characteristics of patients’ baseline pain impact assay sensitivity. In this study, we tested whether a combination of three baseline pain indices based on ecological momentary assessments (EMA) could detect patients with enhanced responses to placebo. The analysis was conducted with secondary data from two clinical trials in fibromyalgia patients (N=2084). For each patient, pain intensity, pain variability (individual standard deviation), and pain consistency (first-order autocorrelation) were computed from baseline EMA. A latent profile analysis (LPA) identified three subgroups of patients based on these indices. Group 1 (n=857, 41.3%) showed the lowest pain intensity levels, coupled with the highest consistency and greatest variability of pain. Group 3 (n=110, 5.3%) showed the opposite pattern, and group 2 (n=1109, 53.4%) showed intermediate levels on all pain indices. It was then tested whether the subgroups moderated treatment effects (changes in pain for active treatment versus placebo) using repeated-measures ANOVA. Treatment effects varied significantly between subgroups. Patients in group 3 demonstrated greater reduction in pain in response to placebo then those in groups 1 and 2. Further analysis showed that the removal of patients in class 3 would significantly enhance the observed treatment effect by 8-15%. In conclusion, profiles of pain characteristics derived from baseline EMA may be useful for detecting patient subgroups with enhanced placebo responses that can diminish assay sensitivity in pain clinical trials.