The reliability and construct validity of the neck disability index and patient specific functional scale in patients with cervical radiculopathy

Study Design. Cohort study of patients with cervical radiculopathy undergoing physical therapy. Objectives. Examine the test-retest reliability, construct validity, and minimum levels of detectable and clinically important change for the Neck Disability Index (NDI) and Patient Specific Functional Scale (PSFS) in cohort of patients with cervical radiculopathy. Summary of Background Data. To date, no studies have investigated the psychometric properties of the NDI or PSFS in a cohort of patients with cervical radiculopathy. Methods. Thirty-eight patients with cervical radiculopathy undergoing physical therapy completed the NDI and PSFS, and Numerical Pain Rating Scale (NPRS) at the baseline examination and at a follow-up. In addition, at follow-up, patients completed a 15-point global rating of change (GROC), which was used to dichotomize patients as improved or stable. Changes in the NDI and PSFS were then used to assess test-retest reliability, construct validity, and minimal levels of detectable and clinically important change. Results. Test-retest reliability was moderate for the NDI (intraclass correlation coefficient [ICC] = 0.68; 95% confidence interval [CI], 0.30-0.90) and high for the PSFS ( ICC = 0.82; 95% CI, 0.54-0.93). The PSFS was more responsive to change than the NDI. The minimal detectable change for the NDI was 10.2 and for the PSFS 2.1. The minimally clinically important change for the NDI was 7.0 and PSFS 2.0. Conclusions. Our results suggest that the PSFS exhibits superior reliability, construct validity, and responsiveness in this cohort of patients with cervical radiculopathy compared with the NDI. Further research is needed to examine the ability of these measures to accurately reflect changes in individuals, as well as large samples of patients.