Estimating minimally important differences for the worst pain rating of the Brief Pain Inventory-Short Form

The Brief Pain Inventory-Short Form (BPI-SF) is widely used for assessing pain in clinical and research studies. The worst pain rating is often the primary outcome of interest; yet, no published data are available on its minimally important difference (MID). Breast cancer patients with bone metastases enrolled in a randomized, double-blind, phase III study comparing denosumab with zoledronic acid for preventing skeletal related events and completed the BPI-SF, FACT-B, and EQ-5 Datbaseline, week 5, and monthly through the end of the study. Anchor-and distribution-based MID estimates were computed. Data from 1,564 patients were available. Spearman correlation coefficients for anchors ranged from 0.33-0.65. Mean change scores for worst pain ratings corresponding to one-category improvement in each anchor were 0.26-1.04 for BPI-SF current pain, -1.40 to -2.42 for EQ-5D Index score, 1.71-1.98 for EQ-5D Pain item, -2.22 to -0.51 for FACT-BTOI, -1.61 to -0.16 for FACT-G Physical, and -1.31 to -0.12 for FACT-G total. Distribution-based results were ISEM = 1.6, 0.5 effect size = 1.4, and Guyatt's statistic = 1.4. Combining anchor-and distribution-based results yielded a two-point MID estimate. An MID estimate of two points is useful for interpreting how much change in worst pain is considered clinically meaningful.