Reliability and responsiveness of the Norwegian version of the Neck Disability Index.

Background and aim The Norwegian version of the Neck Disability Index (NDI) has been widely used in previous studies. To our knowledge, the test-retest reliability and responsiveness of the NDI have not been investigated. Thus, the aim of the present study was to investigate the test-retest reliability and responsiveness of the Norwegian version of the NDI in neck pain patients seen in a specialized outpatient clinic. Methods This study included patients referred to the neck and back outpatient clinic at Oslo University Hospital. A total of 255 patients were included in the study, of which 42 participated in the test-retest portion of the study. The intraclass correlation coefficient (ICC) was used to assess test-retest reliability. A total of 113 patients participated in the responsiveness analyses. Based on their responses on the Global Rating Scale of Change (GRS), patients were categorized into the following groups: worsened (n = 24), unchanged (n = 7) and improved (n = 62). The minimal detectable change (MDC) for the NDI was calculated. Responsiveness was assessed by constructing a Receiver Operating Characteristic curve (ROC curve) to distinguish patients who had improved or worsened from those who remained unchanged. The minimum clinically important difference (MCID) was estimated. Results The test-retest reliability between the baseline scores and the retest NDI scores was very good (ICC = 0.84; 95% CI 0.72-0.91). The ability of the NDI to discriminate between improved and unchanged patients (responsiveness) over time was acceptable based on the ROC curve analysis (AUC = 0.70; 95% CI 0.58-0.82). The estimated MDC for the Norwegian version of the NDI is 12.3%, and the MCID is 16.6%. Conclusion The Norwegian version of the NDI proved to be an instrument with good test-retest reliability and acceptable responsiveness for assessing neck pain-related disability among neck pain patients in a specialized outpatient clinic.