Responsiveness of the Neck Disability Index in patients with mechanical neck disorders

PurposeReport the test-retest reliability, construct validity, minimum clinically important difference (MCID), and minimal detectable change (MDC) for the Neck Disability Index (NDI).Study design/settingCohort study of patients presenting to outpatient physical therapy clinics.Patient sampleNinety-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 MeasuresNDI and the 15-point Global Rating of Change (GRC) self-report measures.MethodsAll 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.ResultsTest-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.ConclusionsThe 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.