Validation of a Dutch version of the Neurological Fatigue Index (NFI-MS) for patients with multiple sclerosis in the Netherlands

Fatigue is a complex, multidimensional construct. Most questionnaires measuring fatigue do not meet the standards for good measurement properties. Therefore, the Neurological Fatigue Index for multiple sclerosis (NFI-MS) is recently developed using thorough methods. The aim of this study was to develop and validate the Dutch version of the NFI-MS. The original English version of the NFI-MS was translated into Dutch using a forward-backward procedure. Reliability (intraclass correlation coefficient (ICC) and standard error of measurement (SEM)) were assessed for the summary score and the four subscale scores separately. Construct validity was assessed by testing hypotheses about expected correlations between the NFI-MS and other constructs (e.g. impact on functioning and depression) and expected differences on NFI scores between groups defined on the basis of disease type and work status. The reliability analyses (n = 118) showed that the ICC was 0.78 (NFI-MS summary) and ranged from 0.75 to 0.83 for the four subscales and the SEM was 2.5 (NFI-MS summary) and ranged from 1.1 to 2.2 for the four subscales. For construct validity (n = 136), 50 out of the 57 expected hypotheses were confirmed (88 %). The present study shows that the Dutch version of the NFI-MS is a reliable and valid self-report questionnaire for measuring fatigue in Dutch patients with MS.