Purpose: Report the test-retest reliability, construct validity, minimum clinically important difference (MCID), and minimal detectable change (MDC) for the Neck Disability Index (NDI).
Study design/setting: Cohort study of patients presenting to outpatient physical therapy clinics.
Patient sample: Ninety-one subjects with a primary complaint of neck pain, with or without concomitant upper extremity (UE) symptoms, who were participants in a randomized clinical trial.
Outcome measures: NDI and the 15-point Global Rating of Change (GRC) self-report measures.
Methods: All subjects completed the NDI at baseline and at a 3-week follow-up. Additionally, subjects completed the GRC scale, which was used to dichotomize patients into improved or stable groups. Changes in the NDI were used to assess test-retest reliability, construct validity, MCID, and MDC.
Results: Test-retest reliability was moderate for the NDI (intraclass correlation coefficient, 0.64; 95% confidence interval, 0.19-0.84). For the NDI, the MCID was 7.5 points and the MDC was 10.2 points.
Conclusions: The NDI appears to demonstrate adequate responsiveness based on statistical reference criteria when used in a sample that approximates the high percentage of patients with neck pain and concomitant UE referred symptoms. Because the MCID is within the bounds of measurement error, a 10-point change (the MDC) should be used as the MCID.