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-5D at baseline, week 5, and monthly through the end of the study. Anchor- and distribution-based
| CHARACTERISTIC, n (%) | STUDY SAMPLE (n = 1,564) |
|---|---|
| Gender | |
| Female | 1,550 (99.1) |
| Male | 14 (0.9) |
| Age, mean years ± SD (range) | 57.2 ± 11.2 (27.1–91.2) |
| Race | |
| White | 1,265 (80.9) |
| Black | 38 (2.4) |
| Hispanic | 92 (5.9) |
| Japanese | 119 (7.6) |
| Asian | 28 (1.8) |
| Other | 22 (1.4) |
Demographic characteristics including the breakdown by gender, age, and race for the study sample are shown.
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.
Bolded correlations represent the highest correlations with anchors where correlation r ≥ 0.300.
Spearman correlation coefficients between changes in BPI-SF worst pain rating and changes in each of the 11 potential anchors that were considered are provided. The data are displayed for three intervals of time including baseline to week 5, baseline to week 13, and baseline to week 25. Using a cut point of r ≥ 0.300, only those correlations that are bolded meet the criteria of acceptability.
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.
| ANCHOR | ONE CATEGORYA INCREASE IN ANCHOR | ONE CATEGORY DECREASE IN ANCHOR |
|---|---|---|
| BPI-SF Current Pain rating | 0.26–1.04 | −0.89 to −1.66 |
| EQ 5D Index score | −2.42 to −1.40 | 0.56–1.63 |
| EQ 5D Pain item | 1.71–1.98 | −3.16 to −2.56 |
| FACT-B TOI | −2.22 to −0.51 | −0.56 to 0.77 |
| FACT-G Physical Well-Being | −1.61 to −0.16 | −0.79 to 0.46 |
| FACT-G total | −1.31 to −0.12 | −0.97 to 0.57 |
The range of mean changes in BPI-SF worst pain ratings (using the interval from baseline to week 25) for the six anchors that met the correlation criteria in Table 2 are provided. Mean changes are displayed for one-category increases and one-category decreases in anchor.
a One category (increase or decrease) represents 0.20 points for EQ-5D Index score, one point for BPI-SF current pain rating and EQ-5D pain item, three points for FACT-G Physical Well-Being, and six points for FACT-G total and FACT-B TOI.
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.
