Minimal clinically important changes in chronic musculoskeletal pain intensity measured on a numerical rating scale

To determine the minimal clinically important difference (MCID) of changes in chronic musculoskeletal pain intensity that is most closely associated with improvement on the commonly used and validated measure of the patient's global impression of change (PGIC), and to estimate the dependency of the MCID on the baseline pain scores.

This was a prospective cohort study assessing patient's pain intensity by the numerical rating scale (NRS) at baseline and at the 3 month follow-up, and by a PGIC questionnaire. A one unit difference at the lowest end of the PGIC ("slightly better") was used to define MCID as it reflects the minimum and lowest degree of improvement that could be detected. In addition we also calculated the NRS changes best associated with "much better" (two units). In order to characterize the association between specific NRS change scores (raw or percent) and clinically important improvement, the sensitivity and specificity were calculated by the receiver operating characteristic (ROC) method. PGIC was used as an external criterion to distinguish between improved or non-improved patients.