Problems with measurement of the minimum clinically significant difference in acute pain in elders

Objectives: A standard value for the minimum clinically significant difference (MCSD) in pain in nonelderly ED patients has been identified and has facilitated research in this age group. It is not clear that this value is similar in older patients. The standard method for calculating the MCSD in pain is to average scores on a numerical rating scale (NRS) over contiguous time periods. This method is based on the assumption that the MCSD remains constant over time. In an earlier hypothesis-generating study of elderly patients, this assumption was not met for the arithmetic MCSD, making it difficult to identify a single benchmark for measuring efficacy in analgesia trials in elders. The proportional MCSD was more stable, suggesting that it might constitute a better measure of analgesic efficacy in elderly patients. The objective of the study was to test the hypotheses that: 1) the arithmetic MCSD in adults 65 years and older declines over time and that 2) the proportional MCSD remains constant. Methods: This was an observational, prospective, cohort study of emergency department (ED) patients ≥65 years with acute pain. Pain intensity was rated on a standard 11-point NRS upon study entry and every 30 minutes for 2 hours. The arithmetic MCSD was defined as the mean change in pain between contiguous 30-minute intervals when change in pain was described as "a little less" or "a little more." The proportional MCSD was calculated as the arithmetic MCSD divided by pain intensity at the beginning of the interval. We used generalized estimating equations (GEEs) to test trend over time. Results: A total of 214 patients were enrolled: mean (± standard deviation [SD]) age was 74 (±7.5) years, 66% were female, 63% were Hispanic, and 23% were African American. The median initial NRS was 8. The MCSD decreased 2.1 NRS units (95% confidence interval [CI] = 1.7 to 2.4) between 0 and 30 minutes, 1.4 units (95% CI = 1.0 to 1.7) between 30 and 60 minutes, 1.3 units (95% CI = 1.0 to 1.5) between 60 and 90 minutes, and 0.8 units (95% CI = 0.6 to 1.0) between 90 and 120 minutes (p < 0.001 for trend). The proportional MCSD also varied from 27% (95% CI = 23% to 32%) between 0 and 30 minutes, 19% (95% CI = 13% to 24%) between 30 and 60 minutes, 22% (95% CI = 18% to 27%) between 60 and 90 minutes, and 13% (95% CI = 9% to 18%) between 90 and 120 minutes (p < 0.001 for trend). Conclusions: Both the arithmetic and the proportional MCSD in elderly patients in acute pain declined over time. Because both measures were numerically unstable, there does not appear to be a single value for the MCSD that can be used to identify the MCSD in pain for use in analgesic efficacy trials in elderly patients. A different metric may be needed to study pain and assess comparative analgesic efficacy in elderly patients.