Assessment of fatigue in patients with fibromyalgia and chronic widespread pain. Reliability and validity of the Swedish version of the MFI-20

Objectives. Fibromyalgia ( FM) and Chronic Widespread Pain ( CWP) are common diseases in primary care and, in addition to the pain they cause, fatigue is a major problem. Fatigue is regarded as a multidimensional concept and instruments assessing fatigue should therefore cover several different dimensions. The Multidimensional Fatigue Inventory ( MFI- 20) measures five different dimensions of fatigue. The aim of the study was to investigate the test-retest reliability and convergent construct validity of the Swedish version of the MFI- 20 in female patients with FM or CWP. Methods. To investigate the convergent construct validity, 166 female patients with FM or CWP completed the Swedish version of the MFI- 20 and a Visual Analogue Scale ( VAS) measuring global fatigue. Thirty- six of the 166 patients completed the Swedish version of the MFI- 20 on two occasions, one day apart to evaluate the test-retest reliability. Results. Spearman's correlation coefficient revealed a significant association between each of the five subscales of the MFI-20 and the VAS for global fatigue. General Fatigue had the highest correlation with the VAS ( rs = 0.62, p<0.001), while Reduced Motivation had the lowest ( rs = 0.32, p<0.001). The intra- class correlation coefficients ( ICC) were satisfactory for all the five subscales of the MFI-20. Conclusion. The results of this study indicate that the Swedish version of the MFI- 20 is a reliable tool for assessing the degree of fatigue in patients with FM or CWP. This study also supports the theory that fatigue is a multidimensional concept and different aspects of fatigue should be measured separately.