Responsiveness and minimal important changes for the Neck Disability Index and the Neck Pain Disability Scale in Italian subjects with chronic neck pain.

Purpose: To evaluate the responsiveness and minimal important changes (MICs) for the Italian Neck Disability Index (NDI) and Neck Pain Disability Scale (NPDS) as well as which questionnaire was the most responsive in subjects with chronic neck pain (NP).

Methods: At the beginning and end of an 8-week rehabilitation programme, 200 patients completed the NDI and NPDS. After the programme, the global perceived effect (GPE) was also evaluated and collapsed to produce a dichotomous outcome (improved vs. stable). Responsiveness was calculated by distribution [effect size (ES); standardised response mean (SRM)] and anchor-based methods (ROC curves; correlations between change scores of NPDS and NDI, and GPE). ROC curves were also used to compute the best cutoff levels between improved and stable subjects (MICs).

Results: The ES was 0.66 and 0.73, and the SRM was 1.09-1.26 for the NDI and NPDS, respectively. The ROC analyses revealed AUCs of 0.96 and 0.91 for the NDI and NPDS, respectively; the MICs were 7-percentage points for the NDI (sensitivity: 98%; specificity: 81%) and 10 for NPDS (93; 83%), showing equivalent responsiveness properties. Baseline NPDS scores did not affect MIC estimate for the NPDS, while, for the NDI, higher MICs were found for patients with worst disability levels. Correlations between change scores of the NDI and NPDS and GPE were, respectively, high (0.71) and moderate (0.59).

Conclusions: The Italian NDI and NPDS were equally sensitive in detecting clinical changes in subjects with chronic NP undergoing rehabilitation. We recommend taking the MICs into account when assessing patient improvement or planning studies in this clinical context.