Clinical importance of changes in chronic pain intensity measured on an 11-point numerical pain rating scale

Pain intensity is frequently measured on an 11-point pain intensity numeric al rating scale (PI-NRS), where 0 = no pain and 10 = worst possible pain. H owever, it is difficult to interpret the clinical importance of changes fro m baseline on this scale (such as a 1- or 2-point change). To date, there a re no data driven estimates for clinically important differences in pain in tensity scales used for chronic pain studies. We have estimated a clinicall y important difference on this scale by relating it to global assessments o f change in multiple studies of chronic pain. Data on 2724 subjects from 10 recently completed placebo-controlled clinical trials of pregabalin in dia betic neuropathy, postherpetic neuralgia, chronic low back pain, fibromyalg ia, and osteoarthritis were used. The studies had similar designs and measu rement instruments, including the PI-NRS, collected in a daily diary, and t he standard seven-point patient global impression of change (PGIC), collect ed at the endpoint. The changes in the PI-NRS from baseline to the endpoint were compared to the PGIC for each subject. Categories of 'much improved' and 'very much improved' were used as determinants of a clinically importan t difference and the relationship to the Pl-NIRS was explored using graphs, box plots, and sensitivity/specificity analyses. A consistent relationship between the change in PI-NRS and the PGIC was demonstrated regardless of s tudy, disease type, age, sex, study result, or treatment group. On average, a reduction of approximately two points or a reduction of approximately 30 % in the PI-NRS represented a clinically important difference. The relation ship between percent change and the PGIC was also consistent regardless of baseline pain, while higher baseline scores required larger raw changes to represent a clinically important difference. The application of these resul ts to future studies may provide a standard definition of clinically import ant improvement in clinical trials of chronic pain therapies. Use of a stan dard outcome across chronic pain studies would greatly enhance the comparab ility, validity, and clinical applicability of these studies. (C) 2001 Inte rnational Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved.