OBJECTIVE:Our objective was to identify patient, study, and site factors associated with assay sensitivity in placebo-controlled neuropathic pain trials. METHODS:We examined the associations between study characteristics and standardized effect size (SES) in a database of 200 publicly available randomized clinical trials of pharmacologic treatments for neuropathic pain. RESULTS:There was considerable heterogeneity in the SESs among the examined trials. Univariate meta-regression analyses indicated that larger SESs were significantly associated with trials that had 1) greater minimum baseline pain inclusion criteria, 2) greater mean subject age, 3) a larger percentage of Caucasian subjects, and 4) a smaller total number of subjects. In a multiple meta-regression analysis, the associations between SES and minimum baseline pain inclusion criterion and age remained significant. CONCLUSIONS:Our analyses have examined potentially modifiable correlates of study SES and shown that a minimum pain inclusion criterion of 40 or above on a 0 to 100 scale is associated with a larger SES. These data provide a foundation for investigating strategies to improve assay sensitivity and thereby decrease the likelihood of falsely negative outcomes in clinical trials of efficacious treatments for neuropathic pain.