Reliability and structural validity of the Multidimensional Fatigue Inventory (MFI) in patients with idiopathic Parkinson's disease.

INTRODUCTION The Multidimensional Fatigue Inventory (MFI) is commonly used in patients with Parkinson's disease (PD). However, most measurement properties have not been investigated in this population. The aim of this study was to investigate internal consistency, test-retest reliability, measurement error, structural validity, and floor and ceiling effects of the MFI in PD. METHODS Patients with PD (N = 153) completed the MFI at baseline and week 3 in a randomized clinical trial. Cronbach's α, intraclass correlation coefficient (ICC), and the smallest detectable change (SDC) were calculated. Bland and Altman analysis was performed. Principal Component Analysis (PCA) was used to explore structural validity. Floor and ceiling effects were investigated. RESULTS Cronbach's α for the MFI-total and subscales ranged from 0.74 (reduced motivation (RM)) to 0.92 (MFI-total). ICC's ranged from 0.65 (mental fatigue (MF) to 0.81 (physical fatigue (PF)), SDC ranged from 4 points (PF and RM) to 15 points (MFI-total). Bland and Altman analysis showed no systematic differences between assessments. A floor effect was found for MF and ceiling effects for PF and reduced activity (RA). A four-factor model was extracted, combining general fatigue (GF) and PF as one factor. CONCLUSIONS The MFI is reliable and valid to assess fatigue in patients with PD. Clinicians and researchers interested in assessing specific aspects of fatigue should consider interpreting GF and PF as one subscale measuring physical aspects of fatigue. To establish whether the MFI can detect meaningful changes, studies on anchor-based responsiveness and the minimal important change are needed in PD.