Percentage of pain intensity difference on an 11-point numerical rating scale underestimates acute pain resolution

A 50% reduction in pain intensity difference (50%PID) between baseline and follow-up evaluation is commonly accepted as adequate pain relief in emergency departments (EDs). However, 50%PID seems to be problematic with the 11-point numerical rating scale (NRS) since even baseline values are more divisible by 2 (50% reduction) than odd baseline values. This study evaluated the impact of this bias and integrated time between baseline and follow-up measurements, hypothesizing that the slope of relative pain intensity difference (SRPID) is a more accurate gauge of pain relief that can decrease bias and incorporate the time component of pain relief.

A post-hoc analysis of real-time data on an adult population from an urban ED identified 3199 consecutive patients who received an analgesic, had baseline NRS > 3 and a follow-up NRS within 2 h. Primary outcome was the percentage of patients with pain relieved from the 50%PID and the 50%SRPID criteria.

Results showed that with 50%PID, even pain intensity levels on baseline NRS comprised a higher percentage of patients [60.7%; 95% confidence interval (CI): 58.8-63.0] with pain relief compared to odd pain intensity levels (51.7%; 95% CI: 48.8-54.6; p < 0.001), underestimating pain-relieved patients by 9% [95% CI: 0.05-0.13; effect size (ES) = 0.09]. The percentage of pain-relieved subjects with the 50%SRPID criteria was not affected by baseline NRS values (59.7% for whole sample; 95% CI: 58.0-61.4; ES = 0.02).

The 50%PID method with an 11-point NRS for assessing adequate pain relief is significantly biased for specific baseline pain intensity level. In the particular context of ED acute pain, the SRPID seems less biased.