Estimating Minimally Important Differences for the Worst Pain Rating of the Brief Pain Inventory–Short Form

Anchor-Based Analysis

Spearman correlations between changes in the BPI-SF worst pain item and changes in potential anchors are presented in Table 2. For all potential anchors, the highest correlations with the BPI-SF worst pain rating were obtained at the baseline to week 25 interval. All potential anchors correlated significantly (P < 0.001) with the BPI-SF worst pain rating with the exception of the FACT-G Social/Family Well-Being scale. However, correlations were low (<0.30) for several potential anchors: ECOG Performance Status, FACT-B Overall QOL item, FACT-G Emotional Well-Being, and FACT-G Functional Well-Being. Therefore, the week 25 interval and the following anchors were selected for the MID analysis: BPI-SF current pain rating, EQ-5D Index score, EQ-5D Pain item, FACT-B TOI, FACT-G Physical Well-Being, and FACT-G total score. Correlation coefficients between the changes in the selected anchors and changes in the BPI-SF worst pain ratings range from 0.329–0.647.

Mean changes in the BPI-SF worst pain rating that correspond to a one-category change in anchors from baseline to week 25 are presented in Table 3. BPI-SF current pain ratings >5 and EQ-5D Index scores <0.40 were excluded from their respective analysis due to small sample sizes. A one-category increase in the anchor scores was associated with an absolute value of change in the BPI-SF worst pain item ranging from 0.26–2.42. A one-category decrease in the anchor score was associated with an absolute value of change in the BPI-SF worst pain item ranging from 0.56–3.16. Changes associated with improvement and worsening in anchors were not symmetrical, nor was there a consistent trend across anchors. For example, for the EQ-5D pain item, the magnitude of change in BPI-SF worst pain was greater for a one-category increase in the anchor than for a one-category decrease in the anchor. In contrast, for the EQ-5D Index score, the magnitude of change in BPI-SF worst pain was greater for a one-category decrease in the anchor than for a one-category increase in the anchor.

The regression of changes in anchors on changes in the BPI-SF worst pain item is shown in Table 4. Changes in each anchor are significantly (P < 0.05) associated with changes in BPI-SF worst pain rating. A one-point increase in BPI-SF current pain rating and EQ-5D Pain item is associated with a 0.817 and 1.805 increase in BPI-SF worst pain, respectively, while a one-point increase in EQ-5D Index score, FACT-B TOI, FACT-G Physical Well-Being, and FACT-G total is associated with a 3.548, 0.098, 0.163, and 0.048 decrease in BPI-SF worst pain rating, respectively. Likewise, a two-point increase in BPI-SF current pain rating and EQ-5D Pain item is associated with a 1.634 and 3.610 increase in BPI-SF worst pain, respectively, while a two-point increase in EQ-5D Index score, FACT-B TOI, FACT-G Physical Well-Being, and FACT-G total is associated with a 7.096, 0.196, 0.326, and 0.096 decrease in BPI-SF worst pain rating, respectively. The change in anchor-by-baseline anchor interaction was statistically significant only for BPI current pain and FACT-G Physical Well-Being. The interaction tests whether the anchor–BPI-SF slope differs as a function of baseline anchor score; therefore, a lack of significance suggests that the association between BPI-SF worst pain and other anchors does not differ by baseline anchor rating.