Measuring fatigue among women with Sjögren's syndrome or rheumatoid arthritis: a comparison of the Profile of Fatigue (ProF) and the Multidimensional Fatigue Inventory (MFI).
BACKGROUND
Fatigue is common in both Sjögren's syndrome (SS) and rheumatoid arthritis (RA) and can restrict functioning.


AIMS
We tested the convergent validity of the Profile of Fatigue (ProF) using the Multidimensional Fatigue Inventory (MFI) in SS and RA.


METHODS
The 16-item ProF and the 20-item MFI were completed by 82 White-British women aged 35-79 years (mean 60.4 years). Thirty-four had been diagnosed with SS for a mean of 7.0 years and 48 had been diagnosed with RA for a mean of 14.5 years. The ProF measures four somatic facets of fatigue and two mental facets; the MFI contains one mental and four somatic facets. The structures of the items from both measures were tested by principal component factor analysis using varimax rotation.


RESULTS
No significant differences in fatigue were found between the women with SS or RA. Five factors explained a total of 76% of the variance of the MFI; six factors explained 94% of the variance of the ProF. Mental fatigue items from both questionnaires loaded onto separate factors from somatic fatigue items; the two original facets of mental fatigue in the ProF were replicated. The four somatic fatigue facets of the ProF were generally replicated but the somatic facets of the MFI did not replicate as clearly. Equivalent facets correlated well between the two questionnaires (r >or= 0.65).


CONCLUSIONS
Both the ProF and the MFI distinguish between somatic and mental fatigue in SS and RA but the ProF appears better at resolving somatic facets of fatigue.