Validation of the Spanish version of the Neck Disability Index.

Study Design: Observational prospective study.
Objective: Validate the Spanish version of the Neck Disability Index (NDI).
Summary of Background Data: The NDI is the most widely used neck pain scale in the largest number of populations and has been validated most often against multiple measurements of function, pain, and clinical signs and symptoms.
Methods: The Spanish version of the NDI was administered 2 or 3 times to 175 individuals with neck pain (including those requesting or not requesting specific healthcare at a given time and those with acute and subacute/chronic nonspecific or post-traumatic neck pain). After establishing the factorial structure by extracting the main components, the internal consistency was assessed with the Cronbach alpha method, the test-retest reliability was assessed with the Bland-Altman plot and the intraclass correlation coefficient methods, and the validity was established by calculating Pearson correlation coefficient with pain visual analogue scale values and with Northwick Park Neck Pain Questionnaire (Spanish version) values. Sensitivity to change was estimated while comparing the mean difference between the first application of the NDI score and the one after the treatment in the patients who improved, felt the same, or worsened.
Results: A single factor explained 51.6% of the variance, the Cronbach alpha score was 0.89, the intraclass correlation coefficient was 0.98, the Pearson correlation coefficient with pain visual analogue scale was r = 0.65 and with Northwick Park Neck Pain Questionnaire was r = 0.89. In the subgroup of 147 subjects in whom the sensitivity to change was studied, those who reported an improvement in neck pain showed a decrease in the NDI score of 8.20, those who felt the same showed a decrease of 0.23, and those who felt worse showed an increase of 5.55.
Conclusion: This first Spanish version of the Neck Disability Index is reliable, valid, and sensitive to change.