The Copenhagen Neck Functional Disability Scale: a study of reliability and validity

To determine whether a newly developed disability scale for patients with neck pain demonstrated acceptable reliability and validity.

Testing was conducted using three different samples of patients with neck pain (n = 162). Test-retest reliability of the scale was carried out on the same day with one sample (n = 39), and between-day reliability was carried out with another (n = 21). Differential item functioning with regard to the influence of gender and age was carried out with these two patient groups, as was construct validity. Responsiveness was measured using patients participating in a clinical trial involving patients with chronic neck pain (n = 102). Additionally, scale scores were compared with a wide range of physical measurements using the patients in the clinical trial.

Short-term, between-day and postal questionnaire reliability coefficients were all extremely high. The Cronbach's alpha coefficient for internal consistency was 0.9 for the entire scale, and the coefficients for individual items were all greater than 0.88. Disability scale scores correlated strongly to pain scores as well as to doctor and patient global assessments, indicating good construct validity. Relative changes in disability scores demonstrated a moderately strong correlation to changes in pain scores after treatment. Scale scores correlated weakly to all physical measurements.

The disability scale demonstrated excellent practicality and reliability. The scale accurately reflects patient perceptions regarding functional status and pain as well as doctor's global assessment and is responsive to change over long periods of time. We feel that this scale can be a valuable tool for the assessment of patients in future clinical trials and quality of care studies.