The NDI is a simple 10-item questionnaire used to assess patients with neck pain. The original validation was performed on 52 patients with neck pain and the test-retest on 17 whiplash patients with a 2-day interval. The SF36 measures functional ability, wellbeing and the overall health of patients. It is used in health economics to assess the health utility, gain and economic impact of medical interventions.

Objectives were to independently validate the NDI in patients with neck pain and to draw comparison between the NDI and SF36. 160 patients with neck pain attending the spinal clinic completed self-assessment questionnaires. A second questionnaire was completed in 34 patients after a period of 1-2 weeks.

The internal consistency of the NDI and SF36 was calculated using Cronbach alpha. The test-retest reliability was assessed using the Bland and Altman method and the concurrent validity between the two questionnaires was assessed using Pearson correlation.

Both questionnaires showed robust internal consistency: SF36 alpha = 0.878 (se=0.014, 95%CI=0.843 to 0.906) and NDI = 0.864 (se=0.017, 95%CI=0.825 to 0.894). The NDI had significant correlation to all eight domains of the SF36 (p<0.001). The individual scores for each of the ten items had significant correlation with the total disability score (p<0.001). The test-retest reliability of the NDI was acceptable.

We have shown irrefutably that the NDI has good reliability and validity and that it stands up well to the SF36.