[Validation of a Spanish version of the Neck Disability Index].

Background and Objective: The Neck Disability Index (NDI) is the most used scale for neck pain and disability. There is no validated Spanish version so far. This prospective study's objective was to evaluate a Spanish version's comprehension and metrics characteristics.
Material and Method: The Spanish version of the NDI was obtained by the "translation-backward translation" method. This version and the pain visual analogue scale were both administered 2 times to 48 patients with nonnespecific or posttraumatic neck pain.
Results: 16% had comprehension troubles. The cultural level had no influence on it. Items more commonly called into question were the IX (neck pain and sleeping) and the VII (neck pain and working). The scales were fulfilled in 6 min 08 s (54 s) by those with middle-high cultural level, and in 7 min 59 s (1 min 26 s) by those with low one (p < 0.001). As shown by factor analysis, our NDI version is a one-dimension scale. Internal consistence was very good (Cronbach alpha of 0.937 at the first time and 0.944 at the second one). An optimal test-retest reliability was settled by intraclass correlation coefficient (0.978) and by Bland-Altman plot. For the construct validity, correlation with the pain visual analogue Scale was 0.643 (p < 0.01) at first time, and 0.743 (p < 0.01) at second one. This was the only metric characteristic where the cultural level had any influence.
Conclusions: On reformulation of some items, our NDI Spanish version's feasibility will allow its use for future definitive validation.