Translation and validation of the Danish Foot Function Index (FFI-DK)

The objective of this study was to translate the Foot Function Index (FFI) for use in Danish-speaking patients with foot complaints. The FFI consists of 23 items scored on a numeric rating scale from 0 to 10. The 23 items are grouped into three subscales: pain (nine items), activity limitation (five items), and disability (nine items). The Danish FFI was developed according to the recommended forward/backward translation protocol. The data analysis included reliability [intraclass correlation coefficient (ICC) 2.1] and internal consistency (Cronbach's alpha). Excellent internal consistency was shown for the three subscales: pain (0.99), disability (0.98), and activity limitation (0.98), as for the total score (0.97). The test-retest reliability was excellent: pain subscale: ICC 0.98 [95% confidence interval (CI): 0.97-0.99]; activity limitation subscale: ICC: 0.95 (95% CI: 0.91-0.98); disability subscale: ICC 0.97 (95% CI: 0.95-0.98); total score: ICC: 0.95 (95% CI: 0.91 to 0.98). The mean difference between test and retest was below 1 point and P > 0.08. Bland-Altman plots showed no significant or clinically relevant differences from test to retest in any of the subscales or in the total score. The Danish version of the FFI was found to be valid and reliable and therefore acceptable for use in the Danish population.