Psychometric properties of the neck disability index and numeric pain rating scale in patients with mechanical neck pain

Objective: To examine the psychometric properties including test-retest reliability, construct validity, and minimum levels of detectable and clinically important change for the Neck Disability Index (NDI) and the numeric rating scale (NRS) for pain in a cohort of patients with neck pain. Design: Single-group repeated-measures design. Setting: Outpatient physical therapy (PT) clinics. Participants: Patients (N=137) presenting to PT with a primary report of neck pain. Interventions: Not applicable. Main Outcome Measures: All patients completed the NDI and the NRS at the baseline examination and at a follow-up. At the time of the follow-up, all patients also completed the global rating of change, which was used to dichotomize patients as improved or stable. Baseline and follow-up scores were used to determine the test-retest reliability, construct validity, and minimal levels of detectable and clinically important change for both the NDI and NRS. Results: Test-retest reliability was calculated using an intra-class correlation coefficient (ICC) (NDI ICC=.50; 95% confidence interval [CI], .25-67; NRS ICC=.76; 95% CI, .51-87). The area under the curve was .83 (95% Cl, .75-90) for the NDI score and .85 (95% Cl, .78-.93) for the NRS score for determining between stable and improved patients. Thresholds for the minimum clinically important difference (MCID) for the NDI were 19-percentage points and 1.3 for the NRS. Conclusions: Both the NDI and NRS exhibit fair to moderate test-retest reliability in patients with mechanical neck pain. Both instruments also showed adequate responsiveness in this patient population. However, the MCID required to be certain that the change in scores has surpassed a level that could be contributed to measurement error for the NDI was twice that which has previously been reported. Therefore the ongoing analyses of the properties of the NDI in a patient population with neck pain are warranted.